How to demagnetize a ferromagnetic solid with a short laser pulse, and how to enhance a single peak in the harmonic spectrum: some answers given by TDDFT

"short introduction to TDDFT illustrated with two examples"

Thur.
Sep. 4

1:30pm

Aud

Leonardo DeGeorgi

Nematic-driven anisotropic electronic properties of underdoped detwinned Ba(Fe1−xCox)2As2 revealed by optical spectroscopy

Abstract:
We collect optical reflectivity data as a function of temperature across the structural
tetragonal-to-orthorhombic phase transition at TS on Ba(Fe1−xCox)2As2 for x = 0, 2.5% and 4.5%, with uniaxial and in-situ tunable applied pressure in order to detwin the sample and to exert on it an external symmetry breaking field. At T < TS, we discover a remarkable optical anisotropy as a function of the applied pressure at energies far away from the Fermi level and very much reminiscent of a hysteretic-like behavior. Such an anisotropy turns into a reversible linear pressure dependence at T≥TS. Moreover, the optical anisotropy gets progressively depleted with increasing Co content in the underdoped regime, consistent with the doping dependence of the orthorhombicity but contrary to the non-monotonic behavior observed for the dc anisotropy. Our findings bear testimony for an important anisotropy of the electronic structure and thus underscore an electronic polarization upon (pressure) inducing and entering the nematic phase.
Work in collaboration with: A. Dusza, C. Mirri, S. Bastelberger, A. Lucarelli

Thur. Sep. 4

2:30pm

Aud

Eduardo Fradkin

Intertwined Order in High Temperature Superconductors

Abstract: I will argue that the orders that are present in high temperature superconductors naturally arise with the same strength and are better regarded as intertwined rather than competing. I illustrate this concept in the context of the orders that are present in the pair-density-wave state and the phase diagrams that result from this analysis. I will also briefly discuss some recent progress in the microscopic origin of this phenomenon.

(See RMP colloquium paper posted on wiki under "Reference Materials" link)

Space group symmetry, spin-orbit coupling and the low energy effective Hamiltonian for iron based superconductor

Abstract: I will discuss a symmetry adapted low-energy effective Hamiltonian for the electronic states in the vicinity of the Fermi level in iron-based superconductors. We use Luttinger's method of invariants, expanding about the Γ and M points in the Brillouin zone corresponding to a two-iron unit cell, and then matching the coefficients of the expansion to the five- and eight-band models. This can be used to study the effects of the spin-density wave order parameters on the electronic spectrum, with and without spin-orbit coupling included. Among the results of this analysis is that, strictly speaking, the nodal spin-density wave is unstable once spin-orbit coupling is included. Similar analysis is performed for the A1g spin singlet superconducting state. Without spin-orbit coupling there is one k-independent pairing invariant near the Γ point but two near the M point. This leads to an isotropic spectral gap at the hole Fermi surface near Γ, but anisotropic near M. The relative values of these three parameters determine whether the superconducting state is s++, s+−, or nodal. Inclusion of spin-orbit coupling leads to additional mixing of spin triplet pairing, with one additional pairing parameter near Γ and one near M. This leads to an anisotropic spectral gap near both hole and electron Fermi surfaces, the latter no longer cross but rather split. (Vladimir Cvetkovic and Oskar Vafek, PRB 88, 134510 (2013))

Tue.
Sep 9

2:15pm

Aud

Wei Ku

Spin/Orbital correlation and glide translational symmetry of Fe-based superconductors

Abstract:This talk will present theoretical studies of electronic structure of Fe-based superconductors. It will first establish the basic spin/orbital correlation of the parent compound [1] that is now known to also persist in the doped superconducting samples. It will then discuss the rich magnetic correlation in the families [2], followed by our recent finding of surprising effects of nesting of the itinerant electrons [3]. Finally, the talk will discuss the novel physical effects of the glide translational symmetry of the system on the electronic structure [4] and the novel pairing structure [5], highlighting a new class of superconducting state. I will leave the topic of novel physical effects due to disordered impurities [6-9] to private discussion after the talk.

Chi-Cheng Lee, Wei-Guo Yin, and Wei Ku, Phys. Rev. Lett. 103, 267001 (2009).

Wei-Guo Yin, Chi-Cheng Lee, and Wei Ku, Phys. Rev. Lett. 105, 107004 (2010).

Yuting Tam, Daoxin Yao, and Wei Ku, unpublished.

Chia-Hui Lin, et al, Phys. Rev. Lett. 107. 257001 (2011).

Insights and Open Questions from Another Multiband Superconductor: Sr2RuO4

Abstract: Much of the interest in Sr2RuO4 arises from the possibility that it supports topological chiral p-wave superconductivity and, consequently, might support Majorana modes under certain conditions. Despite the distinct triplet pairing, Sr2RuO4 has a number of similarities to the iron superconductors, including substantial electronic correlations and magnetic fluctuations, multiple sheets of the Fermi surface arising from d-orbitals, and strong spin-orbit coupling -- all of which can impact on the superconducting state. I will give a brief introduction to and overview of Sr2RuO4, and then focus on some of the recent insights gleaned from experiments and theory, as well as on the still open questions.

Thur.
Sep.
11

2:15

Aud

Gertrud Zwicknagl

Superconductivity in Heavy Fermion Systems: Present Understanding and Recent Surprises

Abstract: Heavy Fermion metals are intermetallics containing lanthanide or actinide elements. These systems provide a multitude of unconventional superconductivity scenarios including multiple superconducting phases as well as co-existence and competiton with other ordering phenomena. Of particular interst is the fact that superconductivity occurs close to a Quantum Critical Point in many Heavy Fermion systems. I will give a brief overview of Heavy Fermion superconductors and then focus on theoretical challenges and recent surprises in Heavy Fermion physics.

Support and competition from other ordered states for high temperature superconductivity

Abstract:We show X-ray, neutron, NMR and STM results on how mass, charge and spin density wave states coexist with and influence high temperature superconductivity in two layered systems - cuprates and intercalated graphite.

Tue.
Sep.
16

2:30pm

Aud.

Kazuhiko Kuroki
Osaka U.

Are the hole Fermi surfaces missing in some of the Fe- based superconductors ?

Abstract :
Fermi-surface-nesting originated spin fluctuation has been considered as a candidate forÂ the pairing glue in the iron-based superconductors. However, there have appeared some experimental observations that the hole Fermi surfaces are not present in some of the iron-based superconductors. For instance, in hydrogen doped 1111 materials, electron doping rate reaches up to 50 %, which in a rigid band picture would wipe out the hole Fermi surfaces, but still a very high Tc is attained. In K_xFe_2-ySe_2, the ARPES experiments show that the hole bands do not intersect the Fermi level. In the present talk, we will focus on these materials and show that the reduction of the nearest neighbor hopping within the d_xy orbital due to quantum interference tends to give rise to the d_xy hole Fermi surface. For the hydrogen doped 1111 systems, this explains the high Tc in the heavily doped regime. As for K_xFe_2-ySe_2, a very recent ARPES experiment that uses a high quality sample and tuned photon energy has revealed that there actually exists a hole band originating from the d_xy orbital that intersects the Fermi level. These results suggest the importance of the d_xy hole Fermi surface in the iron-based superconductors with high Tc, although the nesting itself is not as important as expected in the
early days.

Wed.
Sep. 17

4:00pm

SSR

All participants

"QCP in Heavy Fermions, Pnictides, Organics: Fermi surface, Superconductivity, etc."

Informal discussion with Kazu on alkaline iron selenides, organized by L. Boeri and Q. Si

Thur. Sep. 18

1:30pm

Aud

James Analytis
UC Berkeley

Magnetoresistance near a quantum critical point

Abstract:
The physics of quantum critical phase transitions connects to some of the most
difficult problems in condensed mat- ter physics, including metal-insulator
transitions, frustrated magnetism and high-temperature superconductivity. Near a
quantum critical point, a new kind of metal emerges, the thermodynamic and
transport properties of which do not fit into the unified phenomenology for
conventional metalsâthe Landau Fermi-liquid theory. Studying the evolution of the
temperature dependence of these observables as a function of a control parameter
leads to the identification of both the presence and the nature of the quantum phase transition in candidate systems. In this study we measure the transport properties of BaFe2 (As1-x Px )2 below the critical temperature Tc by suppressing
superconductivity with high magnetic fields. At sufficiently low temperatures, the
resistivity of all compositions (x >0.31) crosses over from a linear to a quadratic
temperature dependence, consistent with a low-temperature Fermi-liquid ground state. As compositions with optimal Tc are approached from the overdoped side, this crossover becomes steeper, consistent with models of quantum criticality where the effective Fermi temperature TF goes to zero. These measurements also point to a unversality class of quantum-criticality which extends to the physics of heavy fermions and the cuprates.

Thur.
Sep.
18

2:30pm

Aud.

Ronny Thomale
U. Wuerzburg

New paths to time-reversal symmetry breaking superconductors

Abstract:
Since anyon superconductivity, unconventional superconducting states of matter that exhibit time reversal symmetry breaking constitute an aspired quantum state of matter which, however, has not been uniquely identified so far. I report on recent theoretical proposals and experimental findings that might allow to open new directions regarding such states, focussing on the singlet sector and not the triplet sector of superconductivity (as it has been predominantly done for Sr2RuO4 in the past years). This will involve possible non-chiral s+id pairing in the pnictides, as well as possible chiral d-wave pairing in hexagonal systems such as doped graphene, sodium doped cobaltates, and SrPtAs. I will also comment specifically on how a detailed knowledge of the SC form factor will be mandatory to relate to STM signals that might help to identify such states.

Fri
Sep.
19

10:00am

SSR

Anyone interested

Is FeSe different?

Informal discussion about aspects of FeSe and related materials, organized by P. Hirschfeld and R. Valenti

Week 4

IRONIC14 Conference, "Strong Correlations and Superconductivity: Towards a Conceptual Framework"

Normal state behavior of bulk crystals of FeSe emerging from drastic electronic changes induced by orbitals and correlations

FeSe is one of the simplest iron-based superconductors which suffers a
structural transition not accompanied by a long-range magnetic order. I will
present angle-resolved photo-emission spectroscopy studies on bulk single
crystals FeSe which give access to the band and orbital dependent effects
that occur through the structural transition. I will complement these
studies with angle dependent quantum oscillations measurements in the normal
state at very low temperatures. These combined studies reveal a normal
two-fold symmetric electronic state comprising of at least one small hole
and electron-like Fermi surface pockets, with significant orbital-dependent
electronic correlations.

Tues.
Sep.
30

11:30am

Aud

Brian Andersen
U. Copenhagen

Emergent impurity effects and their observable consequences in Fe-based superconductors

In this talk I will discuss some of the many fascinating and
highly unusual impurity aspects of iron-based superconductors. This
includes, for example, the induction of impurity-induced long-range ordered
phases due to unconventional RKKY exchange couplings that would not be
present without the disorder. I will also discuss disorder effects in the
nematic phase above the transition temperature to the (pi, 0) magnetic state
but below the orthorhombic structural transition. The anisotropic spin
fluctuations in this region can be frozen by disorder, to create elongated
magnetic droplets whose anisotropy grows as the magnetic transition is
approached. Such states act as strong anisotropic defect potentials that
scatter with much higher probability perpendicular to their length than
parallel, although the actual crystal symmetry breaking is tiny. From the
calculated scattering potentials, relaxation rates, and conductivity in this
region we conclude that such emergent defect states are essential for the
transport anisotropy observed in experiments. Below the SDW transition the
nematogens freeze into dimer states that show many characteristics
in agreement with STM measurements.

We'll discuss the big issues, insights and questions arising from the conference last week. We'll have a presenter from five areas touched on at the conference - pnictides, heavy fermions, organic, cuprate and he-3/ruthenates - each will give a 5 min presentation, and then we'll have 15 mins discussion for each.

Presenters and bloggers:

Area Presenter Blogger
Iron based SC Luca de Medici Leni Boscones
Heavy Fermions Piers Coleman Andriy Nevidomskyy
Organics Stuart Brown Sri Raghu
Cuprate Catherine Pepin Andrey Chubukov
He-3/Str-Ruth Sri Raghu Jim Sauls.

If the presenters could put together their presentations well in advance then we'll be assured of a very interesting session. (If you really need to use powerpoint, please no more than two slides.)
Each blogger will keep a blog of the discussion. We'll aim to identify a list of key questions and issues that summarize the conference.

Thurs.
Oct. 2

10:00am

Aud

Stuart Brown
UCLA

Superconducting order parameters and phase diagrams of organic superconductors

In 1979, the first organic superconductor, (TMTSF)2PF6, was discovered. The superconducting state of this quasi-1d material emerges only upon suppression of an ambient pressure spin-density wave ground state. As for more recently found superconductors proximate to a magnetic phase, there are clear signatures for spin fluctuations in the normal state. Further, the normal state properties have been interpreted in the context of a quantum critical point associated with the magnetic order, though there are important deviations from the standard scenario. Finally, the preponderance of evidence points to an order parameter consistent with pairing mediated by spin fluctuations.

Thurs.
Oct. 2

11:00am

Aud

Jan Zaanen
Inst. Lorentz

Fermionic entanglement and the generation of global symmetry

Recently signals appeared from various sides (AdS/CFT, condensed matter experiment and -numerics) making me wonder whether a simple general principle might be at work in strongly interacting finite density fermion systems which cannot be handled by conventional technology because of the sign problem. Very different from classical- or bosonic systems this would amount to a generic capacity to generate in the infrared larger global symmetries than are present in the UV, without invoking fine tuning.

Superconducting Surprises: five decades of discovery, in both temperature and time!

Blackboard lunch talk.

Mon. Oct 6

1:30pm

Aud.

All participants

Organizational meeting

Tues.
Oct 7

10.30am

Aud

Priscila Rosa
UC Irvine

Experimentally tuning the ground state of BaFe2As2 by orbital differentiation.

The role of structural parameters in low-symmetry layered systems, such as iron
pnictides/chalcogenides (Fe-Pn/Ch), cuprates and heavy fermions, has become crucial for the understanding of their properties. In this talk, I will discuss this subject using a combination of
macroscopic and microscopic techniques to study Ba1−xEuxFe2−yMyAs2 single crystals (M = Co, Cu, Mn, Ni, and Ru). Interestingly, a close connection arises between the spin-density wave (SDW) phase suppression and local distortions in the structure. Furthermore, these changes are reflected at the Fermi surface by an increase of anisotropy and localization of the Fe 3d bands at the FeAs plane. Our results suggest that such increase in the planar (xy/x^2-y^2) orbital symmetry seems to be a favorable ingredient for the emergence of superconductivity in this class of materials.

Tues.
Oct 7

11.30am

Aud

Luca de' Medici
ESPCI, Paris

Reverse-engineering electronic correlations in Iron superconductors

I will discuss the strength of electronic correlations in the normal phase of Fe-superconductors,
showing that the agreement between a wealth of experiments and DFT+DMFT or similar
approaches supports a scenario in which strongly-correlated and weakly-correlated electrons coexist in the conduction bands of these materials. I will then reverse-engineer the realistic calculations and justify this scenario in terms of simpler behaviors easily interpreted through model results. All pieces come together to show that Hund’s coupling, besides being responsible for the electronic correlations even in absence of a very strong Coulomb repulsion is also the origin of “selective Mottness”, i.e. the decoupling of Mott physics in the various Fe orbitals, causing differentiated correlation strengths. This selective Mottness is put in perspective with the analogous differentiation of electronic correlation in k space found in the underdoped cuprates and a common phase diagram is sketched.

[1] L. de' Medici, G. Giovannetti and M. Capone, PRL 112,177001 (2014),
[2] L. de’ Medici, “Weak and strong correlations in Iron superconductors”, in “Iron-based
Superconductivity”, W. Yin and G. Xu Eds., Springer (out November 2014).

If you would like to make a specific comment/presentation, let Sri know ahead of the time -- send him a single slide, or tell him it will be blackboard:
E. Bascones slides:

Charge order and Pseudo Gap in cuprate superconductors

Emerging symmetries are one of the more potent paradigms in condensed matter theory. In this talk we will describe how anti ferromagnetic quantum fluctuations can give rise to a pseudo gap state with an emergent SU(2) symmetry relating pairing fluctuations to the charge sector. We explore the experimental consequences of such a phase and discuss relations with recently observed charge order patterns in the pseudo gap state of cuprate superconductors.

Thurs
Oct 9

11.15am

Aud

Xingjiang Zhou
National Lab for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing.

Laser ARPES on high temperature cuprate superconductors.

In this talk, after a brief introduction to the latest development of laser ARPES techniques, I will present some recent progress we have made, by utilizing the VUV-laser ARPES, in studying high temperature cuprate superconductors. These include: (1) the disappearance of nodal gap across the insulator-superconductor transition in La-Bi2201[1], (2) the evolution of nodal kink and antinodal kink in Bi2212[2], and (3) the extraction of Eliashberg functions in Bi2212.

[1]. Yingying Peng et al., Nature Communications 4 (2013) 2459.
[2]. Junfeng He et al., Phys. Rev. Lett. 111 (2013) 107005;

Week 7

Time

Place

Speaker

Title

Comments, Links, Abstracts

Mon Oct 13

1:30pm

Aud.

All participants

Organizational meeting

Tue Oct 14

10:30am

Aud.

M. Brian Maple
UC San Diego

Novel electronic phases and competing interactions in the heavy fermion compound URu2Si2

The heavy fermion compound URu2Si2 undergoes a second order transition at To = 17.5 K into an ordered phase whose identity has eluded researchers for nearly three decades. This so-called hidden order (HO) phase coexists with a type of unconventional superconductivity (SC) that is found below Tc ≈ 1.5 K. The features in the electrical resistivity, specific heat and magnetic susceptibility associated with the HO phase are reminiscent of a charge or spin density wave that forms a gap over about 40% of the Fermi surface below To, with the remainder of the Fermi surface gapped by the SC below Tc. The compound URu2Si2 has been studied extensively by means of various experimental techniques (e.g., transport, thermal, magnetic, and spectroscopic measurements), and numerous theoretical models have been proposed for the HO phase. In this talk, we describe experimental studies of URu2Si2 under high pressure, high magnetic field, and chemical substitution that have revealed extraordinary behavior and novel electronic phases that are generated by competing interactions. For example, application of pressure suppresses the SC and induces a transition from the HO phase to an antiferromagnetic (AFM) phase at 10 kbar. High magnetic fields suppress the HO phase at ~35 tesla and induce several new electronic phases at higher field, some of which exhibit non-Fermi liquid behavior. Substitution of Re results in the suppression of the SC and the HO transition, the nearby emergence of ferromagnetic (FM) order, and unique critical behavior associated with the FM phase. In contrast, substitution of Fe suppresses SC, induces a transition from the HO phase to an AFM phase, and produces a nearly two-fold increase in temperature of the HO/AFM phase boundary. The support of the US DOE, NNSA, and NSF is gratefully acknowledged.

Tue Oct 14

11:30am

Aud.

Pallab Goswami
U of Maryland

Chiral pairing and Weyl quasiparticles in URu2Si2 and UPt3

Recent experiments have provided strong evidence for time reversal symmetry breaking, gapless superconducting states in both URu2Si2 and UPt3. Most likely pairing symmetries are the singlet kz(kx±iky) and the triplet kz(kx±iky)2 respectively for URu2Si2 and UPt3. Both pairings can support line nodes at the Fermi surface equator and point nodes at the intersections of the closed Fermi surfaces with the c-axis. I will show that the point nodes of these phases act as the monopoles of the Berry curvature in the momentum space and give rise to condensed matter realizations of Weyl quasiparticles. The Weyl quasiparticles can cause a large anomalous thermal Hall effect, polar Kerr effect and various magnetoelectric effects in addition to the protected Fermi arcs on the (1,0,0) and the (0,1,0) surfaces. In contrast the line node acts as a vortex loop in the momentum space and leads to the zero energy, dispersionless Andreev bound states on the (0,0,1) surface. I will suggest experiments for exploring the topological nature of the paired states, which can help in settling the long standing controversy regarding the pairing symmetry in UPt3.

Wed Oct 15

10:30am

SSR

Filip Ronning
Los Alamos National Lab

Heavy Fermion Superconductivity

Heavy fermions offer a rich playground within which to explore unconventional superconductivity. Because of the small energy scales present in heavy fermions, they are often ideal model systems through which to explore the interplay between magnetism and superconductivity, which are present in many classes of unconventional superconductors. I will give a brief survey of heavy fermion superconductivity. Some important open questions which heavy fermion superconductivity can address include: What is the mechanism for superconductivity, how does disorder influence the underlying magnetism and superconductivity, how does the crossover between localized and itinerant electronic states influence the normal state properties, what are the variety of competing electronic states, and how can we search for new superconductors. To initiate the discussion on these topics I will draw on some of the recent work at Los Alamos on cerium based heavy fermions which have explored these issues.

Wed Oct 15

11:15am

SSR

All participants

Discussion: "Superconductivity near Magnetism"

Intro: Filip Ronning

If you would like to make a specific comment/presentation, let Sri know ahead of the time -- send him a single slide, or tell him it will be blackboard:

Some Puzzles in the Specific Heat of Iron-Based Superconductors

Puzzle 1: Is the fact that the bulk transition width in the specific heat at Tc in annealed Ba(Fe1-xCox)2As2 is the *same* as that in the unannealed sample, despite an increase in Tc of 1.5 K, a clue to the involvement of a second order parameter in the bulk transition width? Puzzle 2: What is the jump in the specific heat at Tc, deltaC/Tc, telling us about the superconductivity in the iron-based superconductors? Does the Bud'ko, Ni, and Canfield deltaC ~ Tc3 still hold?

Thu Oct 16

11:30am

Aud.

Elena Bascones
CSIC, Madrid

Electronic correlations in multi-orbital systems and iron superconductors

The metallic character of iron pnictides and several iron chalcogenides has led to a strong debate on the strength of correlations in these materials. Hund’s coupling was seen to be a key player in determining these correlations and iron superconductors were named Hund metals. Hund metals were originally defined as strongly correlated systems with correlations given by Hund’s coupling which are not in close proximity to a Mott insulator and which physical properties are distinctly different from that of doped Mott insulators (Hundness as opposed to Mottness). On the other hand several authors, including myself, describe these materials as doped Mott insulators. In the first part of the talk I will address the role played by Mott physics in Hund metals and on the suppression of coherence in iron superconductors.

In the second part of the talk I will discuss the effect of correlations and doping in the optical conductivity of iron superconductors. I will focus on how the multi-orbital character of iron superconductors affects the determination of correlations from optical conductivity experiments.

Week 8

Time

Place

Speaker

Title

Comments, Links, Abstracts

Mon
Oct 20

1:30pm

Aud.

All participants

Organizational meeting

Tue
Oct 21

10:30am

Aud

Sri Raghu
Stanford U.

Enhanced pairing of quantum critical metals near d=3+1.

We discuss the dynamics of a metal tuned to a continuous quantum phase transition. A concrete realization involves a continuous phase transition to an ordered phase that preserves translational symmetry but breaks crystalline point group symmetries (i.e. a Pomeranchuk instability). In this talk, we focus mainly on the instability towards a superconducting "dome" in this system near the quantum critical point. Near d=3+1, which happens to be the upper critical dimension of this system, there are log-squared and log-cubed divergences that appear already at one-loop order in the Cooper channel. These divergences suggest that a parametrically higher superconducting scale exists in the system. We present a systematic Wilsonian renormalization group theory that incorporates these effects and demonstrates that at least near d=3+1, the pairing scale far exceeds the scale where the Landau quasiparticle is destroyed. We speculate on how to obtain a superconducting dome from a non-Fermi liquid "normal" state in the context of a simple effective field theory.

Reference:
A. Liam Fitzpatrick, S. Kachru, J. Kaplan, S. Raghu, G. Torroba, H. Wang, (to appear).

Here are a few related works (by no means an exhaustive list) that also address this problem, and have some overlap with our analysis:

1) D.T. Son Phys. Rev. D 59, 094019 (1999).
2) A. Chubukov, J. Schmalian, arXiv:0507562.
3) M. Metlitski, D. Mross, S. Sachdev, and T. Senthil, arXiv1403.3694.
4) T. Maier, D. Scalapino, arXiv:1405.5238.
5) S. Lederer, Y. Schattner, E. Berg, S. Kivelson, arXiv:1406.1193.

Tue
Oct 21

11:15am

Aud

Natalia Perkins
U. Minnesota

The evolution of magnetic order in Fe_{1+y}Te compounds with increasing interstitial iron

Abstract:We examine the influence of the excess of interstitial Fe on the magnetic properties of Fe$_{1+y}$Te compounds. Because in iron chalcogenides the correlations are stronger than in the iron arsenides, we assume in our model that some of the Fe orbitals give rise to localized magnetic moments. The remaining Fe orbitals are assumed to be itinerant. Due to the Hund's coupling between the itinerant and localized moments, the increasing amount of itinerant electrons due to excess Fe, $y$, leads to
modifications in the local moments' exchange interactions via the multi-orbital generalization of the long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. By numerically computing the RKKY corrections and minimizing the resulting effective exchange Hamiltonian, we find, in general, that the excess electrons introduced in the system change the classical magnetic ground state from a double-stripe state to an incommensurate spiral, consistent with the experimental observations. We show that these results can be understood as a result of the suppression of magnetic spectral weight
of the itinerant electrons at momenta $\left(\pi,0\right)/\left(0,\pi\right)$,
combined with the transfer of broad magnetic spectral weight from large to small momenta, promoted by the introduction of excess Fe.

Wed
Oct 22

10:30am

Aud.

All participants

Discussion: Magnetism and SC in Alkaline C60's, and much more

S. A. Kivelson and E. Abrahams: Magnetism & SC in Alkaline C60's:
blackboard, but see "reference material" section for some relevant references

G. R. Stewart: specific heat at SC transitions of iron pnictides:

For the rest of the session, we'll try a new format; you can tell what you are working on, or describe what particular topic interests you, all within the scope of the program. If you would like to make a specific comment/presentation, let Sri or Qimiao know ahead of the time -- send them a ppt (up to 2 slides), or tell them it will be blackboard:

Orbital nematicity and its interplay with magnetism and superconductivity in the iron pnictides

Motivated by the recent ARPES measurements on FeSe [1] and LiFeAs [2] families of iron-based superconductors, we have studied the orbital nematic order and its interplay with magnetism within random phase approximation (RPA), as well using a non-perturbative variational cluster approximation (VCA). We found that the electron and hole doping affect the two orders differently within the two-orbital Hubbard model. While hole doping tends to suppress both antiferromagnetism and orbital ordering, the electron doping suppresses magnetism faster, so that orbital nematicity is stabilized in the absence of magnetism for moderately high electron doping. This is reminiscent of the orbital nematic phase observed in FeSe in the absence of any magnetic ordering [1,3], as well as in overdoped BaFe2(As1-xPx)2 where ARPES finds splitting of dxz and dyz orbitals inside the superconducting phase [4]. This indicates that at least in some cases, the observed electronic nematicity may be primarily due to orbital (rather than magnetic) fluctuations.

We have also studied the symmetry and strength of the superconducting pairing in a two-orbital t-J model for iron pnictides using the slave boson mean-field theory. The resulting phase diagram consists of several competing pairing channels, including the isotropic s (A1g) channel, an anisotropic dx2-y2 (B2g) channel, and two different (s+id) pairing channels. We find that the nearest-neighbor biquadratic spin interaction ¬¬–K(SiSj)2 plays a crucial role in stabilizing the (s+id) and even pure d-wave pairing in the heavily electron- and hole-doped regimes. These findings are in qualitative agreement with recent measurements on heavily K-doped BaFe2As2 where s- and d-wave channels appear to be close competitors.

See arXiv:1404.1307 and 1408.1408 for more detail.

[1] T. Shimojima et al, arxiv:1407.1418.
[2] H. Miao et al, Phys. Rev. B 89, 220503(R) (2014).
[3] S.-H. Baek et al, arXiv: 1408.1875.
[4] T. Sonobe et al, unpublished.

Thur
Oct 23

11:15 am

Aud

Thomas Maier
ORNL

Glide plane symmetry and gap structure in the iron-based superconductors

Abstract: Previous phenomenological treatments as well as microscopic calculations have argued that so-called eta-, or nonzero total momentum pairing states can be realized in Fe-based superconductors and possess exotic properties such as odd parity spin singlet symmetry and possible time-reversal symmetry breaking. In this talk I will discuss how Î·-pairing is an inevitable consequence when there are orbital weights at the Fermi level from
orbitals both odd and even with respect to mirror reflection in z, and that it represents a component of all usual pairing states found in previous
theoretical analyses. I will also show that, as usual, an even frequency gap for a singlet pair has even parity in the band basis and there is no time reversal symmetry breaking as a result of Î·-pairing. I will conclude by illustrating that the energy gaps deduced from spectral function leading edges correspond to those calculated in the 1-Fe zone, although the quasiparticle weights are strongly renormalized.

Week 9

Time

Place

Speaker

Title

Comments, Links, Abstracts

Mon
Oct 27

1:30pm

Aud

All Participants

Organizational Meeting

Tues
Oct 28

10:30am

Aud

Meigan Aronson
Stony Brook U/BNL

Quantum Criticality in Layered YFe2Al10

Abstract: I will present here a summary of our experimental results on metallic YFe2Al10, where the layered crystal structure is based on nearly square nets of Fe atoms. The magnetic properties are definitively two-dimensional, and the magnetic susceptibility displays a strong T→0 divergence, but only for small fields lying in the Fe-planes. No order is found for temperatures larger than 0.02 K. Our detailed work on the low temperature transport, magnetic, and thermodynamic properties has shown that diverging quantum critical fluctuations lead here to the breakdown of the standard Fermi liquid description of metals. High precision scaling of the magnetization and specific heat in YFe2Al10 reveals an underlying free energy that is quantum critical in zero field, without the need for fine-tuning by pressure or doping as required in other quantum critical systems., Augmented by a specific expression for the scaling function, whose argument is T/B^0.59, we have reproduced the observed field and temperature dependencies of the quantum critical magnetization and specific heat using a single set of critical exponents (d=z, gamma=1.4, nu z=0.59). The general success of this heuristic analysis, based on the hypothesis of hyperscaling, suggests that YFe2Al10 is a system that is below its upper critical dimension, perhaps due to its two-dimensional character. Hyperscaling generally implies universality, where the form of the free energy does not depend on the details of the system, implying that critical points can be classified into universality classes based on the values of the critical exponents themselves. We propose that YFe2Al10 is the first confirmed member of such a universality class.

Tues
Oct 28

11:30am

Aud

Steve Kivelson
Stanford

Thoughts Concerning the Origins and Implications of Charge Order in Correlated Electron Fluids

Density wave order appears in many strongly interacting systems, and there has been particular excitement recently about the experimental realization that short-range charge-density-wave (CDW) order is ubiquitous in the cuprates. To put this in context, some general aspects of the theory of CDW order will be explored, followed by a discussion of a few specific features of the CDW order seen in the cuprates. I will, in particular, stress that because CDW order does not, generically, occur in any weak coupling limit, it is typically unrelated to any aspect of Fermi surface nesting in d > 1. I will also discuss a precise sense in which the CDW order seen in the cuprates is “weak” (something that Chandra Varma has told me it is my duty to explain to the community) and I will speculate on its significance.

Wed
Oct 29

4:00pm

SSR

All Participants

Doug Scalapino
15 min+disc. Elihu Abrahams
10 min+disc Leni Bascones
5 min + disc Qimiao Si
5 min + disc Jose Rodriguez
10 min+disc Laura Fanfarillo
10 mi+disc
Jeron van den Brink
10 min+disc

Hybrid sesssion:

m* vs. x in cuprates (Ramshaw et al) m* vs. x in FeSC

m* vs. x in FeSC

m* vs x in HFSC

FeSC collective modes in strong couplng multiorbital approach to nematicity in FeSC Orbital nematicity in FeSe

What Happens Inside a Unit Cell Matters – Effects of Umklapp Processes on Correlated Materials

Abstract: Correlated materials exhibiting a number of exotic electronic states of matters have been one of the most studied subjects in condensed matter physics. A well-accepted scheme is that due to the strong short-range repulsive interactions, the quasi-particle picture, the central dogma of Fermi liquid theory, breaks down. Instead of being sharp in momentum-frequency space, the spectral weight of electrons spreads out in a wide range of frequency, which is believed to be the cause of novel temperature and frequency dependences of physical properties like DC resistivity and optical conductivity. However, a fundamental question is that since all these physical properties require a momentum-relaxing mechanism to have finite values, the interaction that breaks the momentum conservation has to be identified in order to understand the experimental results. In this talk, I will propose that since all the correlated materials are crystalline, Umklapp process is the most natural candidate for the momentum-relaxing mechanism. I will show that with the inclusion of Umklapp processese and long-range Coulomb interaction, new collective excitations leading to novel frequency dependence of optical conductivity can emerge in mid-infrared region, and these collective excitations are sensitive to superconducting phase transition. Implications for new energy scale to enhance superconducting transition temperature Tc will be discussed.

Thurs
Oct 30

11:15am

Aud

Jeroen van den Brink
IFW Dresden

Inelastic X-ray Scattering on high Tc cuprates, iron pnictides and magnetic iridat

Resonant Inelastic X-ray Scattering (RIXS) provides direct access to elementary charge, spin and orbital excitations in complex oxides. As a tech- nique it has made tremendous progress with the advent high-brilliance synchrotron X-ray sources. From the theoretical perspective the fundamental question is to precisely which low-energy correlation functions RIXS is sensitive. Depending on the experimental RIXS setup, the measured charge dynamics can include charge-transfer, phonon, d-d and orbital excitations [1]. The focus of this talk will be on RIXS as a probe of spin dynamics and superconducting gap of high-Tc cuprates and iron pnictides [2-4] and the combined magnetic and orbital modes in strongly spin-orbit coupled iridium-oxides [5-7].

[1] L. Ament, et al., Rev. Mod. Phys. 83, 705 (2011), [2] L. Braicovich, et al., Phys. Rev. Lett. 104, 077002 (2010), [3] M. Dean, et al., Nature Materials 11, 850 (2012), [4] P. Marra, et al., Phys. Rev. Lett. 110, 117005 (2013), [5] L. Ament,et al., Phys. Rev. B 84, 020403 (2011), [6] J. Kim, et al., Phys. Rev. Lett. 108, 177003 (2012), [7] H. Gretarsson, et al., Phys. Rev. Lett.110, 076402 (2013).

Week 10

Time

Place

Speaker

Title

Comments, Links, Abstracts

Tue
Nov 4

1:30pm

Aud

Bernd Buechner
IFW Dresden

FeSe: a model system for Fe based superconductors

Note unusual time due to Neutrinos conference:

The main part of the talk deals with the origin of the nematic tetragonal-to-orthorhombic transition in Fe based superconductors, which has been hotly debated since the discovery of superconductivity in pnictides. In addition, I will present our recent ARPES data on FeSe focusing on anomalous behavior at high binding energies. Comparing these experimental results to DMFT calculations allows to attribute the high energy features to a dispersive lower Hubbard band. Though there are clear deviations between model and data these findings give strong restrictions when discussing the amount of electronic correlations in Fe based superconductors.

Information on the origin of the nematic transition in FeSe is obtained from our NMR data. Since in most iron pnictide families, the nematic transition occurs very close to the antiferromagnetic instability, it has been argued that the nematicity stems from the magnetic spin fluctuations, coupled to the lattice degrees of freedom. The iron selenide FeSe is different in that the structural transition occurs at Ts ~ 90 K without any sign of antiferromagnetism, and it is therefore tempting to conclude that an alternative scenario for nematicity, based on orbital (rather than spin) degrees of freedom, takes place. To establish whether this is indeed the case, we have conducted nuclear magnetic resonance (NMR) measurements on FeSe. Below the nematic transition temperature Ts, we observe a clear splitting of the Se NMR line, which we show to be of electronic origin. Moreover, by measuring the spin-lattice relaxation rate and Knight shift, we establish unequivocally that this line splitting is not driven by a conventional spin-fluctuation driven nematic order. We furthermore establish a connection between orbital nematicity and superconductivity, showing that the two order parameters compete with each other.

Finally it is discussed, whether our findings in FeSe have implications for the interpretation of the phase diagrams of other Fe-based superconductors. In particular, it is argued that the striking difference between 1111 and the 122 systems could be connected to orbital degrees of freedom, namely the formation of orbital polarons suppressing the usual nematic phase.

Tue
Nov 4

2:30pm

Aud

Chandra Varma
UC Riverside

Local Quantum-Criticality in the Dissipative Quantum XY Model With Application to Diverse Physical Systems.

Critical fluctuations which are scale-invariant in time but local in space are proven in analytical calculations, supported by recent Monte-Carlo calculations, in the dissipative quantum XY Model with (or without) four-fold anisotropy. This is the model for the broken symmetry at Q=0 observed in under-doped cuprates, beside the superconductor to insulator transition in thin films, for which it was originally introduced. The normal state properties in the strange metal region of cuprates require scattering of fermions off such quantum crit- ical fluctuations. Remarkably, similar criticality appears consistent with the properties above the high Tc region of certain heavy-fermion compounds and of the Fe-compounds, which have antiferromagnetic quantum critical points. A canonical transformation is shown to relate a generic model with a planar anti- ferromagnetic ground state to a decorated quantum XY model, suggesting that the universality class of quantum-critical points for the two problems are the same.

Visualization of Heavy Fermion Formation through Scanning Tunneling Microscopy

Intermetallic rare earth and actinide compounds containing Cerium, Ytterbium, Uranium, etc. have attracted interest ever since their discovery more than 3 decades ago. While the initial interest was spurred by the unusual Fermi liquid properties observed in some of these compounds, recent interest has been focusing on the unconventional nature in which the Fermi liquid state is destroyed at a zero-temperature phase transition and the accompanying emergence of novel states of matter. It has been realized that one of the central issues is the fate of the Kondo effect at criticality. One of the most important techniques that helped shaping our understanding of Kondo screening processes and their interplay with nonlocal correlations, both magnetic and superconducting, has been tunneling spectroscopy with its ability to give local, microscopic information that directly relates to the one-particle Green’s function. In this talk I'll present recent STM results on YbRh_2Si_2 [1,2] and the unconventional superconductor CeCoIn_5 [3] and discuss how the STM can help sheding light on how superconductivity forms in heavy fermion systems.

[1] S. Ernst, S. Kirchner, C. Krellner, C. Geibel, G. Zwicknagl, F. Steglich
& S. Wirth, Nature 474, 362 (2011)
[2] S. Seiro, S. Kirchner, S. Hartmann, C. Krellner, C. Geibel, Q. Si,
F. Steglich, and S. Wirth, in preparation
[3] S. Wirth, Y. Prots, M. Wedel, S. Ernst, S. Kirchner, Z. Fisk,
J. D. Thompson, F. Steglich, and Y. Grin,
J. Phys. Soc. Jpn. 83 (2014).

Thurs
Nov 6

11:15am

Aud

Andrey Chubukov
U. Minnesota

Quantum-critical superconductivity

I will discuss our recent work(s) on superconductivity near quantum-critical points in two dimensions.
This will be a follow-up of the talk presented by Sri. He focused on SC near D=3. I will extend the
analysis to the cases when bosonic propagator diverges near a QCP by a power-law rather than logarithmically.

Week
11

Time

Place

Speaker

Title

Comments, Links, Abstracts

Mon, Nov 10

10.30am

Aud

Premala Chandra
(Rutgers CMT)

A New Broken Symmetry: Hidden (Hastatic) Order in URu2Si2

The development of collective long-range order by means of phase transitions occurs by the spontaneous breaking of fundamental symmetries. Magnetism is a consequence of broken time-reversal symmetry, whereas superfluidity results from broken gauge invariance. The broken symmetry that develops below 17.5 kelvin in the heavy-fermion compound URu2Si2 has long eluded such identification. Here we show that the recent observations of Ising quasiparticles in URu2Si2 results from a spinor order parameter that breaks double time-reversal invariance, mixing states of integer and half-integer spin. Such "hastatic" order hybridizes uranium-atom conduction electrons with Ising 5f2 states to produce Ising quasiparticles; it accounts for the large entropy of condensation and the magnetic anomaly observed in torque magnetometry. Hastatic order predicts a tiny transverse moment in the conduction-electron sea, a collosal Ising anisotropy in the nonlinear susceptibility anomaly and a resonant, energy-depedent nematicity in the tunnelling density of states. We also discuss the microscopic origin of hastatic order, identifying it as a fractionalization of three-bod body bound-states into integer spin fermions and half-integer spin bosons.

Work done with Piers Coleman and
Rebecca Flint.

References:
PC, P. Coleman and R. Flint,
Nature 493, 421 (2013).
arXiV: 1404.5920

Monday
Nov 10

11:15am

Aud

Inna Vishik

Momentum space imaging of cuprate superconductors

A comprehensive phenomenology is an important starting point for microscopic understanding in complex materials such as the cuprate high temperature superconductors. We have performed a high-resolution laser ARPES study as a function of temperature and momentum, spanning most of the doping range where Bi2Sr2CaCuO8+d (Bi-2212) can be grown. We studied the components of the nodal Fermi velocity both tangential (vD) to and perpendicular (vF) to the Fermi surface at the node using laser ARPES. In measurements of vF, we observe a diverging effective mass approaching p=0.08. In measurements of vD, we find three distinct phase regions inside the superconducting dome. These are interpreted as the regime where superconductivity exists alone (p>0.19), the regime where superconductivity exists with the pseudogap (0.08<p<0.19), and the regime where the Fermi surface is gapped at every momentum by an order of non-superconducting origin (p<0.08). These results will be discussed in the context of recent experimental and theoretical works.

Piers Coleman reprises his Oscar-winning performance in "Sideways" (photo: Sri Raghu)

Peter Hirschfield discussing challenging questions about superconductivity. (Credit: Piers Coleman).

Peter Hirschfeld discussing challenging questions about superconductivity (photo: Piers Coleman)

Thurs
Nov 13

10:30am

Aud

Igor Mazin
Naval Research Lab

Doping a spin liquid: Theoretical prediction of a strongly correlated (superconducting?) Dirac metal

Herbertsmithite ZnCu3(OH)6Cl2 is essentially the only real-world realization of the ideal single-orbital Kagome model. Being half-doped, it is a Mott insulator. In the nn p-d TB model, it maps exactly onto a single s-orbital Kagome Hamiltonian, in particular, exhibits topologically protected Dirac points (DP) at the 4/3 doping. We propose to achieve this doping by substituting Ga for Zn. Such Ga-herbertsmithite (GHS) would be the first material with strongly correlated Dirac electrons. We have investigated GHS by means of DFT, TB-DCA and the Slave Bosons approachs and searched for Mott and/or charge order instabilities, and found that it remains metallic and uniform, retaining the DPs.
Such a metal with strongly correlated DP electrons would have rather unique topological, magnetic and transport properties. In particular, we show analytically and using fRG that when back-doped with Zn, GHS would harbor unconventional spin-fluctuation driven superconductivity which by symmetry must be f-wave of the +-+-+- type.

Thurs
Nov 13

11:15am

Aud

Silke Biermann
Ecole Polytechnique

Spectral properties of iron pnictides from first principles -- Where do we stand?

Besides posing fundamental questions about the relation of
superconductivity, magnetism and bad metal behavior, iron
pnictide materials have also provided an important new
playground for dynamical mean field (DMFT)-based electronic
structure calculations. We review in particular recent efforts
of improving the interface between DMFT and electronic structure
theory. This comprises first principles calculations of the
effective Hubbard interactions, as well as incorporating dynamical
screening effects into the DMFT description [1].
An interesting application concerns BaFe2As2 [2], where a
remarkable doping-dependence of the coherence properties was
revealed (confirmed -- among others -- by ARPES measurements
on BaCo2As2 [3]).

Week 12

Time

Place

Speaker

Title

Comments, Links, Abstracts

Tues
Nov 18

10am

Aud

Stephen Wilson
UCSB

Electron-doping the spin-orbit Mott phase of Sr3Ir2O7

A growing body of theoretical work has predicted multiple routes to fundamentally new electronic states realized upon perturbing a form Mott state stabilized by the cooperative interplay of strong spin-orbit coupling and on-site Coulomb U. A variety of materials classes have been proposed to manifest this spin-orbit (or Jeff=1/2) Mott state, yet very little is known regarding how the properties of their antiferromagnetic, insulating, parent states evolve upon carrier substitution. One case of interest are the seminal examples of spin-orbit Mott states found in the n=1 and n=2 members of the Ruddlesden-Popper series of strontium iridates. Here predictions of novel states realized upon doping (such as high temperature superconductivity) have been recently complemented by experimental reports of exotic phases ranging from Fermi arcs to unconventional metals with negative compressibility. In this talk, I will present our working picture of the electronic phase diagram of one such spin-orbit Mott phase, (Sr1-xLax)3Ir2O7. The nature of the metallic state realized via electron-doping this spin-orbit Mott phase will be discussed.

Tues
Nov 18

11am

Aud

Young-June Kim
U. Toronto

Local moment physics in doped CaFe2As2: A dual description

Understanding magnetism is at the heart of iron-based superconductivity research. Since there are multiple orbitals involved in the electronic structure of iron pnictides and chalcogenides, both local and itinerant descriptions seem to be necessary. Many experimental and theoretical studies support this viewpoint, including our recent x-ray emission spectroscopy (XES) studies. The Fe Kbeta XES is a fast, local probe that is bulk-sensitive and couples directly to the d-electron moment. This is particularly useful for studying paramagnetic phases of iron superconductors. We found that local magnetic moments are doping- and temperature-independent in a wide range of Fe based superconductors. The only exceptions are CaFe2As2 samples doped with rare-earths or phosphorous. These materials go through so-called collapsed tetragonal transition at low temperatures, in which the c-lattice constant shrinks by almost 10% and magnetic moments are quenched. However, even well above this transition, the c lattice constant and the local moment size are found to exhibit dramatic temperature dependences. We present dual viewpoints to explain the observed temperature dependence, and demonstrate that considerable structure-magnetism correlation exists in this compound.

Significant Tc enhancement in FeSe films on SrTiO3 due to anomalous interfacial mode coupling

Very recently a system composed of a single unit cell thick iron selenide film (1UC FeSe) grown on SrTiO3 (STO) substrate has emerged under the spotlight. In situ measurements of these films show a superconducting-like energy gap opening near the boiling point of liquid nitrogen (77K), considerably higher than that of any other iron based superconductor. Here, I will discuss high
resolution angle resolved photoemission spectroscopy (ARPES) results which reveal an unexpected and unique characteristic of the 1UC FeSe/STO system: each energy band of the FeSe film is exactly replicated at a fixed energy separation. Such a phenomenon
implies the presence of bosonic modes which, after being absorbed or emitted, scatter the electrons in a fashion more forwardly
focused than anticipated. The energy separation between the replica bands implies the modes are oxygen optical phonons in STO.
Phonons which forwardly scatter have the unusual benefit of helping superconductivity in most channels including those mediated
by spin fluctuations. The experimental correlation between the high gap opening temperature and the replica bands plus our
estimate of the effects of electron-phonon coupling on Cooper pairing enable us to conclude that the interaction of the FeSe
electron and this particular phonon is responsible for raising the superconducting gap opening temperature in 1UC FeSe/STO. This
discovery suggests a pathway to engineer high temperature superconductors.

Thurs
Nov 20

11:15am

Aud

Doug Scalapino
UCSB

Pairing in the bilayer Hubbard model: a connection between the cuprates and the Fe-based superconductors

Abstract: The pairing symmetry of the doped bi-layer Hubbard model changes from B1g to A1g as the hopping t_{perp} between the layers increases. It provides an interesting model for studying the relationship between the pair structure and
the underlying pairing interaction as well as identifying the basic correlations which are responsible for pairing.

Correlated materials exhibiting a number of exotic electronic states of matters have been one of the most studied subjects in condensed matter physics. A well-accepted scheme is that due to the strong short-range repulsive interactions, the quasi-particle picture, the central dogma of Fermi liquid theory, breaks down. Instead of being sharp in momentum-frequency space, the spectral weight of electrons spreads out in a wide range of frequency, which is believed to be the cause of novel temperature and frequency dependences of physical properties like DC resistivity and optical conductivity. However, a fundamental question is that since all these physical properties require a momentum-relaxing mechanism to have finite values, the interaction that breaks the momentum conservation has to be identified in order to understand the experimental results. In this talk, I will propose that since all the correlated materials are crystalline, Umklapp process is the most natural candidate for the momentum-relaxing mechanism. I will show that with the inclusion of Umklapp processese and long-range Coulomb interaction, new collective excitations leading to novel frequency dependence of optical conductivity can emerge in mid-infrared region, and these collective excitations are sensitive to superconducting phase transition. Implications for new energy scale to enhance superconducting transition temperature Tc will be discussed.

Go to current week

Week 1TimePlace*SpeakerTitleComments, Links,...HolidayAll participantsOrganizational MeetingAll participantsGeneral discussion: "Similarities and differences between Cuprates, Pnictides/Chalgogenides,and other superconductors"Hirschfeld intro slides:

Sep. 4

Hardy GrossLASERS14

talk

How to demagnetize a ferromagnetic solid with a short laser pulse, andhow to enhance a single peak in the harmonic spectrum: some answers given by TDDFTSep. 4

Leonardo DeGeorgiNematic-driven anisotropic electronic properties of underdoped detwinnedBa(Fe1−xCox)2As2 revealed by optical spectroscopyWe collect optical reflectivity data as a function of temperature across the structural

tetragonal-to-orthorhombic phase transition at TS on Ba(Fe1−xCox)2As2 for x = 0, 2.5% and 4.5%, with uniaxial and in-situ tunable applied pressure in order to detwin the sample and to exert on it an external symmetry breaking field. At T < TS, we discover a remarkable optical anisotropy as a function of the applied pressure at energies far away from the Fermi level and very much reminiscent of a hysteretic-like behavior. Such an anisotropy turns into a reversible linear pressure dependence at T≥TS. Moreover, the optical anisotropy gets progressively depleted with increasing Co content in the underdoped regime, consistent with the doping dependence of the orthorhombicity but contrary to the non-monotonic behavior observed for the dc anisotropy. Our findings bear testimony for an important anisotropy of the electronic structure and thus underscore an electronic polarization upon (pressure) inducing and entering the nematic phase.

Work in collaboration with: A. Dusza, C. Mirri, S. Bastelberger, A. Lucarelli

Eduardo FradkinIntertwined Order in High Temperature Superconductors(See RMP colloquium paper posted on wiki under "Reference Materials" link)

Week 2TimePlace*SpeakerTitleComments, Links,...All participantsOrganizational meetingOskarVafekSpace group symmetry, spin-orbit coupling and the low energy effective Hamiltonian for iron based superconductorSep 9

Wei KuSpin/Orbital correlation and glide translational symmetry of Fe-based superconductors103, 267001 (2009).105, 107004 (2010).et al, Phys. Rev. Lett.107. 257001 (2011).et al, arXiv:1403.3687109, 147003 (2012).et al, Phys. Rev. Lett.108, 207003 (2012).et al, Phys. Rev. Lett.110. 037001 (2013).et al, Phys. Rev. B89, 020501(R) (2014).All participants(Andriy Nevidomskyy,Discussion leader)

Nematicity: spin vs. orbit"Maiti Blog

Sep. 11

Catherine KallinInsights and Open Questions from Another Multiband Superconductor: Sr2RuO4Sep.

11

GertrudZwicknaglSuperconductivity in Heavy Fermion Systems: Present Understanding and Recent SurprisesWeek 3TimePlace*SpeakerTitleComments, Links,...TBAAll participantsOrganizational meetingGabe AeppliPSI

Support and competition from other ordered states for high temperature superconductivitySep.

16

Kazuhiko KurokiOsaka U.

Are the hole Fermi surfaces missing in some of the Fe- based superconductors ?Fermi-surface-nesting originated spin fluctuation has been considered as a candidate forÂ the pairing glue in the iron-based superconductors. However, there have appeared some experimental observations that the hole Fermi surfaces are not present in some of the iron-based superconductors. For instance, in hydrogen doped 1111 materials, electron doping rate reaches up to 50 %, which in a rigid band picture would wipe out the hole Fermi surfaces, but still a very high Tc is attained. In K_xFe_2-ySe_2, the ARPES experiments show that the hole bands do not intersect the Fermi level. In the present talk, we will focus on these materials and show that the reduction of the nearest neighbor hopping within the d_xy orbital due to quantum interference tends to give rise to the d_xy hole Fermi surface. For the hydrogen doped 1111 systems, this explains the high Tc in the heavily doped regime. As for K_xFe_2-ySe_2, a very recent ARPES experiment that uses a high quality sample and tuned photon energy has revealed that there actually exists a hole band originating from the d_xy orbital that intersects the Fermi level. These results suggest the importance of the d_xy hole Fermi surface in the iron-based superconductors with high Tc, although the nesting itself is not as important as expected in the

early days.

Sep. 17

All participantsQCP in Heavy Fermions, Pnictides, Organics: Fermi surface, Superconductivity, etc."Bring single slide if you would like to make a specific comment/presentation:

H. v. Löhneysen;

Rafael Fernandes;

Discussion blog by R. Thomale

Anybody interested is welcomeInformal discussion with K. KurokiJames AnalytisUC Berkeley

Magnetoresistance near a quantum critical pointThe physics of quantum critical phase transitions connects to some of the most

difficult problems in condensed mat- ter physics, including metal-insulator

transitions, frustrated magnetism and high-temperature superconductivity. Near a

quantum critical point, a new kind of metal emerges, the thermodynamic and

transport properties of which do not fit into the unified phenomenology for

conventional metalsâthe Landau Fermi-liquid theory. Studying the evolution of the

temperature dependence of these observables as a function of a control parameter

leads to the identification of both the presence and the nature of the quantum phase transition in candidate systems. In this study we measure the transport properties of BaFe2 (As1-x Px )2 below the critical temperature Tc by suppressing

superconductivity with high magnetic fields. At sufficiently low temperatures, the

resistivity of all compositions (x >0.31) crosses over from a linear to a quadratic

temperature dependence, consistent with a low-temperature Fermi-liquid ground state. As compositions with optimal Tc are approached from the overdoped side, this crossover becomes steeper, consistent with models of quantum criticality where the effective Fermi temperature TF goes to zero. These measurements also point to a unversality class of quantum-criticality which extends to the physics of heavy fermions and the cuprates.

Sep.

18

Ronny ThomaleU. Wuerzburg

New paths to time-reversal symmetry breaking superconductorsSince anyon superconductivity, unconventional superconducting states of matter that exhibit time reversal symmetry breaking constitute an aspired quantum state of matter which, however, has not been uniquely identified so far. I report on recent theoretical proposals and experimental findings that might allow to open new directions regarding such states, focussing on the singlet sector and not the triplet sector of superconductivity (as it has been predominantly done for Sr2RuO4 in the past years). This will involve possible non-chiral s+id pairing in the pnictides, as well as possible chiral d-wave pairing in hexagonal systems such as doped graphene, sodium doped cobaltates, and SrPtAs. I will also comment specifically on how a detailed knowledge of the SC form factor will be mandatory to relate to STM signals that might help to identify such states.

Sep.

19

Anyone interestedIs FeSe different?Week4Week 5TimePlace*SpeakerTitleComments, Links...All participantsSep.

30

AmaliaColdeaU. Oxford

Normal state behavior of bulk crystals of FeSe emerging from drastic electronic changes induced by orbitals and correlationsstructural transition not accompanied by a long-range magnetic order. I will

present angle-resolved photo-emission spectroscopy studies on bulk single

crystals FeSe which give access to the band and orbital dependent effects

that occur through the structural transition. I will complement these

studies with angle dependent quantum oscillations measurements in the normal

state at very low temperatures. These combined studies reveal a normal

two-fold symmetric electronic state comprising of at least one small hole

and electron-like Fermi surface pockets, with significant orbital-dependent

electronic correlations.

Sep.

30

Brian AndersenU. Copenhagen

Emergent impurity effects and their observable consequences inFe-based superconductorshighly unusual impurity aspects of iron-based superconductors. This

includes, for example, the induction of impurity-induced long-range ordered

phases due to unconventional RKKY exchange couplings that would not be

present without the disorder. I will also discuss disorder effects in the

nematic phase above the transition temperature to the (pi, 0) magnetic state

but below the orthorhombic structural transition. The anisotropic spin

fluctuations in this region can be frozen by disorder, to create elongated

magnetic droplets whose anisotropy grows as the magnetic transition is

approached. Such states act as strong anisotropic defect potentials that

scatter with much higher probability perpendicular to their length than

parallel, although the actual crystal symmetry breaking is tiny. From the

calculated scattering potentials, relaxation rates, and conductivity in this

region we conclude that such emergent defect states are essential for the

transport anisotropy observed in experiments. Below the SDW transition the

nematogens freeze into dimer states that show many characteristics

in agreement with STM measurements.

Oct. 1

All participantsPresenters and bloggers:

Area Presenter Blogger

Iron based SC Luca de Medici Leni Boscones

Heavy Fermions Piers Coleman Andriy Nevidomskyy

Organics Stuart Brown Sri Raghu

Cuprate Catherine Pepin Andrey Chubukov

He-3/Str-Ruth Sri Raghu Jim Sauls.

If the presenters could put together their presentations well in advance then we'll be assured of a very interesting session. (If you really need to use powerpoint, please no more than two slides.)

Each blogger will keep a blog of the discussion. We'll aim to identify a list of key questions and issues that summarize the conference.

Oct. 2

StuartBrownUCLA

Superconducting order parameters and phase diagrams of organic superconductorsOct. 2

JanZaanenInst. Lorentz

Fermionic entanglement and the generation of global symmetrysign problem. Very different from classical- or bosonic systems this would amount to a generic capacity to generate in the infrared larger global symmetries than are present in the UV, without invoking fine tuning.

Week 6TimePlace*SpeakerTitleComments, Links, abstractsPiers ColemanRutgers CMT,

Royal Holloway, UL

Superconducting Surprises: five decades of discovery, in both temperature and time!All participantsOrganizational meetingOct 7

Priscila RosaUC Irvine

Experimentally tuning the ground state of BaFe2As2 by orbital differentiation.pnictides/chalcogenides (Fe-Pn/Ch), cuprates and heavy fermions, has become crucial for the understanding of their properties. In this talk, I will discuss this subject using a combination of

macroscopic and microscopic techniques to study Ba1−xEuxFe2−yMyAs2 single crystals (M = Co, Cu, Mn, Ni, and Ru). Interestingly, a close connection arises between the spin-density wave (SDW) phase suppression and local distortions in the structure. Furthermore, these changes are reflected at the Fermi surface by an increase of anisotropy and localization of the Fe 3d bands at the FeAs plane. Our results suggest that such increase in the planar (xy/x^2-y^2) orbital symmetry seems to be a favorable ingredient for the emergence of superconductivity in this class of materials.

Oct 7

Luca de' MediciESPCI, Paris

Reverse-engineering electronic correlations in Iron superconductorsshowing that the agreement between a wealth of experiments and DFT+DMFT or similar

approaches supports a scenario in which strongly-correlated and weakly-correlated electrons coexist in the conduction bands of these materials. I will then reverse-engineer the realistic calculations and justify this scenario in terms of simpler behaviors easily interpreted through model results. All pieces come together to show that Hund’s coupling, besides being responsible for the electronic correlations even in absence of a very strong Coulomb repulsion is also the origin of “selective Mottness”, i.e. the decoupling of Mott physics in the various Fe orbitals, causing differentiated correlation strengths. This selective Mottness is put in perspective with the analogous differentiation of electronic correlation in k space found in the underdoped cuprates and a common phase diagram is sketched.

[1] L. de' Medici, G. Giovannetti and M. Capone, PRL 112,177001 (2014),

[2] L. de’ Medici, “Weak and strong correlations in Iron superconductors”, in “Iron-based

Superconductivity”, W. Yin and G. Xu Eds., Springer (out November 2014).

All participantsDiscussion: "Single-layer FeSe"Blog by T. Maier:

All participantsDiscussion: "Nature of Magnetism in the Pnictides"A. Chubukov slides:

Q. Si slides:

If you would like to make a specific comment/presentation, let Sri know ahead of the time -- send him a single slide, or tell him it will be blackboard:

E. Bascones slides:

Blog by Luca de' Medici:

Oct 9

Catherine PepinCEA Saclay, ParisCharge order and Pseudo Gap in cuprate superconductorsOct 9

Xingjiang ZhouNational Lab for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing.

Laser ARPES on high temperature cuprate superconductors.[1]. Yingying Peng et al., Nature Communications 4 (2013) 2459.

[2]. Junfeng He et al., Phys. Rev. Lett. 111 (2013) 107005;

Week 7TimePlaceSpeakerTitleComments, Links, AbstractsAll participantsOrganizational meetingM. Brian MapleUC San Diego

Novel electronic phases and competing interactions in the heavy fermion compound URu2Si2Pallab GoswamiU of Maryland

Chiral pairing and Weyl quasiparticles in URu2Si2 and UPt3Filip RonningLos Alamos National Lab

Heavy Fermion SuperconductivityAll participantsDiscussion: "Superconductivity near Magnetism"If you would like to make a specific comment/presentation, let Sri know ahead of the time -- send him a single slide, or tell him it will be blackboard:

Brian Maple:

Greg Stewart:

Blog (I. Vekhter)

Greg StewartU of Florida

Some Puzzles in the Specific Heat of Iron-Based Superconductors3 still hold?Elena BasconesCSIC, Madrid

Electronic correlations in multi-orbital systems and iron superconductorsIn the second part of the talk I will discuss the effect of correlations and doping in the optical conductivity of iron superconductors. I will focus on how the multi-orbital character of iron superconductors affects the determination of correlations from optical conductivity experiments.

Week8TimePlaceSpeakerTitleComments, Links, AbstractsOct 20

All participantsOrganizational meetingOct 21

Sri RaghuStanford U.

Enhanced pairing of quantum critical metals near d=3+1.Reference:

A. Liam Fitzpatrick, S. Kachru, J. Kaplan, S. Raghu, G. Torroba, H. Wang, (to appear).

Here are a few related works (by no means an exhaustive list) that also address this problem, and have some overlap with our analysis:

1) D.T. Son Phys. Rev. D 59, 094019 (1999).

2) A. Chubukov, J. Schmalian, arXiv:0507562.

3) M. Metlitski, D. Mross, S. Sachdev, and T. Senthil, arXiv1403.3694.

4) T. Maier, D. Scalapino, arXiv:1405.5238.

5) S. Lederer, Y. Schattner, E. Berg, S. Kivelson, arXiv:1406.1193.

Oct 21

Natalia PerkinsU. Minnesota

The evolution of magnetic order in Fe_{1+y}Te compounds with increasing interstitial ironmodifications in the local moments' exchange interactions via the multi-orbital generalization of the long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. By numerically computing the RKKY corrections and minimizing the resulting effective exchange Hamiltonian, we find, in general, that the excess electrons introduced in the system change the classical magnetic ground state from a double-stripe state to an incommensurate spiral, consistent with the experimental observations. We show that these results can be understood as a result of the suppression of magnetic spectral weight

of the itinerant electrons at momenta $\left(\pi,0\right)/\left(0,\pi\right)$,

combined with the transfer of broad magnetic spectral weight from large to small momenta, promoted by the introduction of excess Fe.

Oct 22

All participantsDiscussion:Magnetism and SC in Alkaline C60's, and much moreblackboard, but see "reference material" section for some relevant references

G. R. Stewart: specific heat at SC transitions of iron pnictides:

For the rest of the session, we'll try a new format; you can tell what you are working on, or describe what particular topic interests you, all within the scope of the program. If you would like to make a specific comment/presentation, let Sri or Qimiao know ahead of the time -- send them a ppt (up to 2 slides), or tell them it will be blackboard:

A. Chubukov:

L. Balents:

Q. Si:

Blog: (C. Kallin)

Oct 23

Andriy NevidomskyyRice

Orbital nematicity and its interplay with magnetism and superconductivity in the iron pnictidesWe have also studied the symmetry and strength of the superconducting pairing in a two-orbital t-J model for iron pnictides using the slave boson mean-field theory. The resulting phase diagram consists of several competing pairing channels, including the isotropic s (A1g) channel, an anisotropic dx2-y2 (B2g) channel, and two different (s+id) pairing channels. We find that the nearest-neighbor biquadratic spin interaction ¬¬–K(SiSj)2 plays a crucial role in stabilizing the (s+id) and even pure d-wave pairing in the heavily electron- and hole-doped regimes. These findings are in qualitative agreement with recent measurements on heavily K-doped BaFe2As2 where s- and d-wave channels appear to be close competitors.

- See arXiv:1404.1307 and 1408.1408 for more detail.

[1] T. Shimojima et al, arxiv:1407.1418.[2] H. Miao et al, Phys. Rev. B 89, 220503(R) (2014).

[3] S.-H. Baek et al, arXiv: 1408.1875.

[4] T. Sonobe et al, unpublished.

Oct 23

Thomas MaierORNL

Glide plane symmetry and gap structure in the iron-based superconductorsorbitals both odd and even with respect to mirror reflection in z, and that it represents a component of all usual pairing states found in previous

theoretical analyses. I will also show that, as usual, an even frequency gap for a singlet pair has even parity in the band basis and there is no time reversal symmetry breaking as a result of Î·-pairing. I will conclude by illustrating that the energy gaps deduced from spectral function leading edges correspond to those calculated in the 1-Fe zone, although the quasiparticle weights are strongly renormalized.

Week 9TimePlaceSpeakerTitleComments, Links, AbstractsOct 27

All ParticipantsOrganizational MeetingOct 28

Meigan AronsonStony Brook U/BNL

Quantum Criticality in Layered YFe2Al10Oct 28

Steve KivelsonStanford

Oct 29

All ParticipantsDoug Scalapino15 min+disc.

Elihu Abrahams10 min+disc

Leni Bascones5 min + disc

Qimiao Si5 min + disc

Jose Rodriguez10 min+disc

Laura Fanfarillo10 mi+disc

Jeron van den Brink

10 min+disc

Hybrid sesssion:m* vs. x in cuprates (Ramshaw et al)m* vs. x in FeSCm* vs. x in FeSC

m* vs x in HFSCFeSC collective modes in strong couplngmultiorbital approach to nematicity in FeSCOrbital nematicity in FeSeAbrahams:

references will be uploadedBascones:

Rodriguez:

Fanfarillo:

Blog by L. Bascones:

Oct.30

Wei-Cheng LeeSUNY-Binghamton

What Happens Inside a Unit Cell Matters – Effects of Umklapp Processes on Correlated MaterialsOct 30

Jeroen van den BrinkIFW Dresden

Inelastic X-ray Scattering on high Tc cuprates, iron pnictides and magnetic iridat[1] L. Ament, et al., Rev. Mod. Phys. 83, 705 (2011),

[2] L. Braicovich, et al., Phys. Rev. Lett. 104, 077002 (2010), [3] M. Dean, et al., Nature Materials 11, 850 (2012),

[4] P. Marra, et al., Phys. Rev. Lett. 110, 117005 (2013),

[5] L. Ament,et al., Phys. Rev. B 84, 020403 (2011),

[6] J. Kim, et al., Phys. Rev. Lett. 108, 177003 (2012),

[7] H. Gretarsson, et al., Phys. Rev. Lett.110, 076402 (2013).

Week10TimePlaceSpeakerTitleComments, Links, AbstractsNov 4

Bernd BuechnerIFW Dresden

FeSe: a model system for Fe based superconductorsNote unusual time due to Neutrinos conference:The main part of the talk deals with the origin of the nematic tetragonal-to-orthorhombic transition in Fe based superconductors, which has been hotly debated since the discovery of superconductivity in pnictides. In addition, I will present our recent ARPES data on FeSe focusing on anomalous behavior at high binding energies. Comparing these experimental results to DMFT calculations allows to attribute the high energy features to a dispersive lower Hubbard band. Though there are clear deviations between model and data these findings give strong restrictions when discussing the amount of electronic correlations in Fe based superconductors.

Information on the origin of the nematic transition in FeSe is obtained from our NMR data.

Since in most iron pnictide families, the nematic transition occurs very close to the antiferromagnetic instability, it has been argued that the nematicity stems from the magnetic spin fluctuations, coupled to the lattice degrees of freedom. The iron selenide FeSe is different in that the structural transition occurs at Ts ~ 90 K without any sign of antiferromagnetism, and it is therefore tempting to conclude that an alternative scenario for nematicity, based on orbital (rather than spin) degrees of freedom, takes place. To establish whether this is indeed the case, we have conducted nuclear magnetic resonance (NMR) measurements on FeSe. Below the nematic transition temperature Ts, we observe a clear splitting of the Se NMR line, which we show to be of electronic origin. Moreover, by measuring the spin-lattice relaxation rate and Knight shift, we establish unequivocally that this line splitting is not driven by a conventional spin-fluctuation driven nematic order. We furthermore establish a connection between orbital nematicity and superconductivity, showing that the two order parameters compete with each other.

Finally it is discussed, whether our findings in FeSe have implications for the interpretation of the phase diagrams of other Fe-based superconductors. In particular, it is argued that the striking difference between 1111 and the 122 systems could be connected to orbital degrees of freedom, namely the formation of orbital polarons suppressing the usual nematic phase.

Nov 4

Chandra VarmaUC Riverside

Local Quantum-Criticality in the Dissipative Quantum XY Model With Application to Diverse Physical Systems.Nov 5

All ParticipantsDiscussion slides: (Qimiao Si)

Blog of discussion (Andriy Nevidomskyy)

Nov 6

Stefan KirchnerMPI-PKS Dresden

Zhejiang U.

Visualization of Heavy Fermion Formation through Scanning Tunneling Microscopy[1] S. Ernst, S. Kirchner, C. Krellner, C. Geibel, G. Zwicknagl, F. Steglich

& S. Wirth, Nature 474, 362 (2011)

[2] S. Seiro, S. Kirchner, S. Hartmann, C. Krellner, C. Geibel, Q. Si,

F. Steglich, and S. Wirth, in preparation

[3] S. Wirth, Y. Prots, M. Wedel, S. Ernst, S. Kirchner, Z. Fisk,

J. D. Thompson, F. Steglich, and Y. Grin,

J. Phys. Soc. Jpn. 83 (2014).

Nov 6

Andrey ChubukovU. Minnesota

Quantum-critical superconductivityThis will be a follow-up of the talk presented by Sri. He focused on SC near D=3. I will extend the

analysis to the cases when bosonic propagator diverges near a QCP by a power-law rather than logarithmically.

Week11

TimePlaceSpeakerTitleComments, Links, AbstractsPremala Chandra(Rutgers CMT)

A New Broken Symmetry: Hidden (Hastatic) Order in URu2Si2Work done with Piers Coleman and

Rebecca Flint.

References:

PC, P. Coleman and R. Flint,

Nature 493, 421 (2013).

arXiV: 1404.5920

Nov 10

Inna VishikMomentum space imaging of cuprate superconductorsNov 12

All ParticipantsSynthesis IPiers Coleman reprises his Oscar-winning performance in "Sideways" (photo: Sri Raghu)

Peter Hirschfeld discussing challenging questions about superconductivity (photo: Piers Coleman)

Nov 13

Igor MazinNaval Research Lab

Doping a spin liquid: Theoretical prediction of a strongly correlated(superconducting?) Dirac metalSuch a metal with strongly correlated DP electrons would have rather unique topological, magnetic and transport properties. In particular, we show analytically and using fRG that when back-doped with Zn, GHS would harbor unconventional spin-fluctuation driven superconductivity which by symmetry must be f-wave of the +-+-+- type.

Nov 13

Silke BiermannEcole Polytechnique

Spectral properties of iron pnictides from first principles-- Where do we stand?superconductivity, magnetism and bad metal behavior, iron

pnictide materials have also provided an important new

playground for dynamical mean field (DMFT)-based electronic

structure calculations. We review in particular recent efforts

of improving the interface between DMFT and electronic structure

theory. This comprises first principles calculations of the

effective Hubbard interactions, as well as incorporating dynamical

screening effects into the DMFT description [1].

An interesting application concerns BaFe2As2 [2], where a

remarkable doping-dependence of the coherence properties was

revealed (confirmed -- among others -- by ARPES measurements

on BaCo2As2 [3]).

Week 12TimePlaceSpeakerTitleComments, Links, AbstractsNov 18

Stephen WilsonUCSB

Electron-doping the spin-orbit Mott phase of Sr3Ir2O7Nov 18

Young-June KimU. Toronto

Local moment physics in doped CaFe2As2: A dual descriptionNov 19

All participantsSynthesis IIAndriyy Nevidomskyy slides on nematicity issues in FeSC.

Doug Scalapino slides of moment formation in Hubbard model

Qimiao Si slides of importance of linear resistivity

Nov 20

Tom DevereauxStanford

Significant Tc enhancement in FeSe films on SrTiO3 due to anomalous interfacial mode couplingresolution angle resolved photoemission spectroscopy (ARPES) results which reveal an unexpected and unique characteristic of the 1UC FeSe/STO system: each energy band of the FeSe film is exactly replicated at a fixed energy separation. Such a phenomenon

implies the presence of bosonic modes which, after being absorbed or emitted, scatter the electrons in a fashion more forwardly

focused than anticipated. The energy separation between the replica bands implies the modes are oxygen optical phonons in STO.

Phonons which forwardly scatter have the unusual benefit of helping superconductivity in most channels including those mediated

by spin fluctuations. The experimental correlation between the high gap opening temperature and the replica bands plus our

estimate of the effects of electron-phonon coupling on Cooper pairing enable us to conclude that the interaction of the FeSe

electron and this particular phonon is responsible for raising the superconducting gap opening temperature in 1UC FeSe/STO. This

discovery suggests a pathway to engineer high temperature superconductors.

Nov 20

Doug ScalapinoUCSB

Pairing in the bilayer Hubbard model: a connection between the cuprates and the Fe-based superconductorsthe underlying pairing interaction as well as identifying the basic correlations which are responsible for pairing.

*Aud=Auditorium, SSR=Small Seminar Room, MSR=Main Seminar Room FR=Founders' Room**⬆TOP

Correlated materials exhibiting a number of exotic electronic states of matters have been one of the most studied subjects in condensed matter physics. A well-accepted scheme is that due to the strong short-range repulsive interactions, the quasi-particle picture, the central dogma of Fermi liquid theory, breaks down. Instead of being sharp in momentum-frequency space, the spectral weight of electrons spreads out in a wide range of frequency, which is believed to be the cause of novel temperature and frequency dependences of physical properties like DC resistivity and optical conductivity. However, a fundamental question is that since all these physical properties require a momentum-relaxing mechanism to have finite values, the interaction that breaks the momentum conservation has to be identified in order to understand the experimental results. In this talk, I will propose that since all the correlated materials are crystalline, Umklapp process is the most natural candidate for the momentum-relaxing mechanism. I will show that with the inclusion of Umklapp processese and long-range Coulomb interaction, new collective excitations leading to novel frequency dependence of optical conductivity can emerge in mid-infrared region, and these collective excitations are sensitive to superconducting phase transition. Implications for new energy scale to enhance superconducting transition temperature Tc will be discussed.