08:45  09:00

Roderich Moessner (MPIPKS) & scientific coordinators
Opening


Chair: Jonathan Ruhman

09:00  09:45

Sergey Borisenko
(Leibniz Institute for Solid State and Materials Research (IFW) Dresden)
Nematicity and topology in ironbased superconductors from ARPES
I will overview our recent ARPES results on this topic. In particular, the data on LiFeSe, Ba122 and FeSeTe will be discussed.

09:45  10:30

Takasada Shibauchi
(University of Tokyo)
Electronic nematicity in the pseudogap state in cuprates
In the quest to understand the superconductivity in cuprates, a key issue is the nature of the enigmatic pseudogap region of the phase diagram. An especially important question is whether the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries below the onset temperature $T^∗$. We use torquemagnetometry and elastoresistance measurements to test the rotational symmetry breaking in the pseudogap state in single crystals of YBa$_2$Cu$_3$O$_y$ (YBCO) and Bi$_2$Sr$_2$CaCu$_2$O$_8+\delta$ (Bi2212). In YBCO, anisotropic susceptibility within the ab planes was measured by the torque magnetometry with exceptionally high precision [1]. The inplane anisotropy displays a significant increase with a distinct kink at the pseudogap onset temperature $T^∗$, showing a remarkable scaling behavior with respect to $T/T^∗$ in a wide doping range. Our systematic analysis reveals that the rotational symmetry breaking sets in at $T^∗$ in the limit where the effect of orthorhombicity is eliminated. In Bi2212, the nematic susceptibility that describes the electronic anisotropy response to a uniaxial strain was measured by elastoresistance above $T^∗$ [2]. We find that the nematic susceptibility increases upon cooling with a kink anomaly at $T^∗$, evidencing a secondorder transition with neamtic fluctuations. These results provide thermodynamic evidence that the pseudogap in cuprates accompanies an electronic nematic order, which differs from the recently reported chargedensitywave transition that accompanies translational symmetry breaking.
[1] Y. Sato {\it et al.}, Nat. Phys. {\bf 13}, 1074 (2017). [2] K. Ishida {\it et al.}, preprint.

10:30  11:00

Coffee break

11:00  11:45

Rafael Fernandes
(University of Minnesota, Minneapolis)
Vestigial electronic orders in correlated systems: nematicity and beyond
A hallmark of the phase diagrams of correlated electronic systems is the existence of multiple electronic ordered states. In many cases, they cannot be simply described as independent competing phases, but instead display a complex intertwinement. A prime example of intertwined states is the case of primary and vestigial phases. While the former is characterized by a multicomponent order parameter, the fluctuationdriven vestigial state is characterized by a composite order parameter formed by higherorder, symmetrybreaking combinations of the primary order parameter. This concept has been widely employed to elucidate nematicity in both ironbased and cuprate superconductors. In this talk, I will present a grouptheoretical framework, supplemented by microscopic calculations, that extends this notion to a variety of phases, providing a general classification of vestigial orders of unconventional superconductors and densitywaves. Electronic states with scalar and vector chiral order, spinnematic order, Isingnematic order, timereversal symmetrybreaking order, and algebraic vestigial order emerge from this simple underlying principle. I will present a rich variety of possible phase diagrams involving the primary and vestigial orders, and discuss possible realizations of these exotic composite orders in different materials.

11:45  12:30

Alain Sacuto
(University Paris Diderot)
Energy Scale of Charge Density Wave in Cuprates
The cuprate high temperature superconductors develop spontaneous charge density wave (CDW) order below a temperature $T_{\rm CDW}$ and over a wide range of hole doping ($p$). An outstanding challenge in the field is to understand whether this modulated phase is related to the more exhaustively studied pseudogap and superconducting phases [1,2].
To address this issue, it is important to extract the energy scale $\Delta_{\rm CDW}$ associated with the CDW order, and to compare it with the pseudogap (PG) $\Delta_{\rm PG}$ and with the superconducting gap $\Delta_{\rm SC}$. However, while $T_{\rm CDW}$ is wellcharacterized from earlier work, little is known about $\Delta_{\rm CDW}$ until now. Here, we report the extraction of $\Delta_{\rm CDW}$ for several cuprates using electronic Raman spectroscopy. Crucially, we find that upon approaching the parent Mott state by lowering $p$, $\Delta_{\rm CDW}$ increases in a manner similar to the doping dependence of $\Delta_{\rm PG}$ and $\Delta_{\rm SC}$ [3].
This reveals that the above three phases have a common microscopic origin.
In addition, we find that $\Delta_{\rm CDW} \approx \Delta_{\rm SC}$ over a substantial doping range, which suggests that CDW and superconducting phases are intimately related, for example they may be intertwined or connected by an emergent symmetry [1,46]
[1] E. Fradkin et al., Rev. Mod. Phys. 87, 457 (2015).
[2] B. Keimer et al., Nature 518, 179 (2015).
[3] B. Loret et al., arXiv:1808.08198 accepted in Nature Physics
[4] K. B. Efetov et al., Nat. Phys. 9, 442 (2013).
[5] S. Sachdev et al., Phys. Rev. Lett. 111, 027202 (2013).
[6] Y. Wang et al., Phys. Rev. Lett. 114, 197001 (2015).

12:30  13:30

Lunch break

13:30  14:00

Discussions


Chair: Erez Berg

14:00  14:45

Mathias Scheurer
(Harvard University)
Gauge theories for the cuprate superconductors
As a consequence of strong correlations, many aspects of the complex phase diagram of the cuprate hightemperature superconductors are still under debate. In the first part of the talk, I will discuss a nonAbelian gauge theory that we propose [1] as an effective field theory for the cuprates near optimal doping. In this theory, spindensitywave order is fractionalized into Higgs fields while all lowenergy fermionic excitations are electronlike and gauge neutral. The conventional Fermiliquid state corresponds to the confining phase of the theory at large doping and there is a quantum phase transition to a Higgs phase, describing the pseudogap, at low doping. It will be shown that the topological order of the pseudogap state is very naturally intertwined with chargedensitywave, Isingnematic, and scalar spinchirality order. We will also discuss the deconfined quantum criticality in the limit of large numbers of Higgs flavors. The second part of the talk deals with the thermal Hall effect in related gauge theories of the squarelattice antiferromagnet [2,3]. I will show that these approaches can yield a sizeable thermal Hall response, similar to what has recently been seen in experiment [4].
[1] Phys. Rev. B 99, 054516 (2019).
[2] Phys. Rev. B 99, 165126 (2019).
[3] arXiv:1903.01992 (2019).
[4] arXiv:1901.03104 (2019).

14:45  15:30

Yuxuan Wang
(University of Florida, Gainesville)
A solvable random model for the nonFermi liquid pairing problem
We show that a random interacting model exhibits solvable nonFermi liquid behavior and exotic pairing behavior. The model describes the random Yukawa coupling between $N$ quantum dots each hosting $M$ flavors of fermions and $N^2$ bosons that becomes critical at low energies.
This diagrammatic expansion is controlled by $1/MN$, and the results become exact in a large$M$, large$N$ limit. We find that pairing only develops within a region of the $(M,N)$ plane  even though the pairing interaction is strongly attractive, the incoherence of the fermions can spoil the forming of Cooper pairs{, rendering the system a nonFermi liquid down to zero temperature}. By solving the Eliashberg equation and renormalization group analysis, we show that the transition into the pairing phase exhibits KosterlitzThouless quantumcritical behavior.

15:30  16:00

Richard Greene (via Skype)
(University of Maryland, College Park)
Strange metal transport properties of an electrondoped cuprate
I report measurements of resistivity, Hall Effect, magnetoresistance and thermopower in the electrondoped cuprate La2xCexCuO4 for 0.19≥ x ≥0.08 as a function of temperature. The new and unconventional results are:
1) The normal state magnetoresistance exhibits an anomalous linearinH behavior [1] at the same doping and temperature where a linearinT resistivity was previously observed for H>Hc2 [2], i.e. above the Fermi surface reconstruction at x =0.14 up to the end of the superconducting dome (x ~ 0.175). For doping above the dome conventional Fermi liquid behavior is found (but, with intrinsic Ferromagnetism below 4KarXiv:1902.11235).
2) The normal state Seebeck coefficient, S/T, exhibits an unconventional low temperature –lnT dependence at the same doping where linearinT and linearinH resistivity is found [3]. Conventional S/T = constant behavior is found above the superconducting dome.
3) The normal state resistivity above Tc, from 80 K to 400 K, follows an anomalous ~A(x)T2 behavior at zero field for all doping(x), with no indication of a MIR limit.
Fermi liquid theory cannot explain any of these results. Moreover, the magnitude of the anomalous magnetoresistance and thermopower scales with Tc, suggesting that the origin of the superconductivity is correlated with the anomalous normal state properties.
1. T. Sarkar et al., Sci. Adv. 2019
2. K. Jin et al. , Nature 476, 73 (2012)
3. P. R. Mandal et al., PNAS 116, 5991 (2019)
4. T. Sarkar, R. L. Greene, and S. Das Sarma, Phys. Rev. B 98, 224503 (2018)

16:00  16:30

Coffee break


Chair qctces19colloquium: Falko Pientka (MPIPKS)

16:30  17:30

Jörg Schmalian
(Karlsruhe Institute of Technology (KIT))
qctces19colloquium: Critical and incoherent electron systems and superconductivity within the SachdevYeKitaev approach
The established and highly successful description of metals is based on Landau’s Fermi liquid theory. The relevant phase space for electronelectron collisions is determined by the Pauli blocking of a degenerate Fermi gas due to its Fermi surface. In some strongly correlated systems narrow bands form and the energetics of the system at elevated energies or temperatures becomes dominated by strong interactions and is no longer restricted by Fermisurface phasespace effects. To develop a wellcontrolled approach of this regime is an important question in the theory of strongly correlated electrons.
In this talk I will give an overview over the description of incoherent and critical electronic systems using the SachdevYeKitaev approach. We consider versions of the model where electrons interact with each other, with boson collective modes, and even with phonons. We also comment on the very direct relation to holographic approaches to strongly coupled quantum theories. Finally we address the emergence of superconductivity in such an incoherent metal and show that pairing and superconductivity of a fully incoherent electronic system is allowed and leads to a pairing state with high transition temperature but low condensation energy.


Chair discussion session: Erez Berg

17:30  19:00

Discussions

19:00  20:00

Dinner

20:00  21:00

Informal discussions
