
chair: Qimiao Si

09:30  10:00

Aline Ramires Neves de Oliveira
(Paul Scherrer Institute)
Driving Superconductors in Flatland: A guideline to enhance Tc
Floquet engineering has attracted significant interest given the recent developments in experimental techniques such as ultrafast spectroscopy and the potential to enhance the stability of phases of matter such as superconductivity. Here we explore how an external drive and intrinsic dissipation jointly affect superconductivity. Inspired by the fitness criterion for static superconductors, we recognize the distinct effects of external drives on superconductors based on their commutativity or anticommutativity with the superconducting order parameter within the FloquetKeldish formalism. Our proposal goes beyond standard mechanisms, such as phonon squeezing and dynamical localization. It opens the door for further studies toward drivendissipative engineering of exotic phases of complex matter in solidstate systems.

10:00  10:30

Yoshi Maeno
(Kyoto University)
\(Sr_2RuO_4\): Mystery after nearly 30 years

10:30  11:00

coffee break

11:00  11:20

Shouvik Sur
(Rice University)
Emergent flat bands from orbital selective Mott correlations
Systems with flat bands provide a rich platform for realizing correlation driven novel quantum phases with intriguing low energy properties, ranging from strange metallicity to emergent fractional excitations. When such flat bands are topologically nontrivial, they facilitate fascinating interplay between electronic topology and strong correlations. Here, we investigate flat bands in electronbased systems on lattices that realize destructive kinematic interference. Within the Hubbardmodel, we show that an effective Kondo description arises through orbital selective Mott correlations, with the flat band becoming pinned at the Fermi energy. Further, by utilizing spacegroup symmetry constrains on correlation effects, we demonstrate that symmetryenforced spectral degeneracies survive in the highly correlated bands, which is exemplified by emergent WeylKondo semimetal phases. We conclude with an overview of the interconnectedness of seemingly disparate systems, unified by correlationdriven emergent flat bands.
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Work done with Lei Chen, Fang Xie, Haoyu Hu, Silke Paschen, Jennifer Cano and Qimiao Si
L. Chen et al, arXiv:2212.08017.
L. Chen et al., to be published (2023)
H.Hu et al, Sci. Adv., in press (arXiv:2209.10396)
==

11:20  11:40

Christie Koay
(Columbia University)
Realization of a flat band lattice model in a van der Waals metal
Crystalline materials hosting dispersionless flat bands are ideal platforms to study unconventional ground states arising from strong electronelectron interactions [1–3] . While a variety of engineered platforms, such as photonic lattices [4] , cold atom traps [5,6] , and moiré superlattices [7,8], have been employed to explore their behavior, there remains a paucity of solidstate materials which intrinsically host flat bands. Here, we report the realization of a novel flat band lattice model in a squarelattice van der Waals (vdW) metal. Electrical transport, angleresolved photoemission spectroscopy (ARPES) measurements, and firstprinciples calculations reveal that the electronic structure is captured by an analog of the Lieb and Dice flat band lattice models [9,10]. While these prototypical models rely on special lattice structures to realize flat bands, here they arise from the combination of lattice geometry and atomic orbital composition. We comment on prospects of extending these insights to identify a wider family of flat band candidate materials, overlooked previously because they do not realize special lattice structures. Further, the materials remains stable even in ambient conditions down to the twodimensional (2D) limit [11], raising it as a readily accessible platform to study flat band lattice models in 2D.

11:40  12:00

Pontus Laurell
(University of Tennessee)
Reconstructing the spatial structure of quantum correlations
Quantum correlations are a fundamental property of quantum manybody states, yet a very elusive one for experiments, especially so in the case of bulk quantum materials. Here we show that the knowledge of the momentumdependent dynamical susceptibility, measured via inelastic neutron scattering, enables the systematic reconstruction of quantum correlation functions, which express the degree of quantum coherence in the fluctuations of two spins at arbitrary mutual distance. Making use of neutron scattering data on the compound KCuF3  a system of weakly coupled Heisenberg chains  and of numerically exact quantum Monte Carlo simulations, we show that quantum correlations possess a radically different spatial structure with respect to conventional correlations. Indeed they exhibit a new emergent length of quantummechanical origin  the quantum coherence length  which is finite at any finite temperature, including when the system develops longrange magnetic order upon cooling. Moreover we show theoretically that, at low temperature, quasionedimensional compounds exhibit stronger shortrange quantum correlations than systems with more strongly coupled chains, unveiling an effective form of monogamy of quantum correlations in Heisenberg spin systems.
This is work in preparation together with A. Scheie, E. Dagotto, D. A. Tennant, and T. Roscilde.

12:00  12:30

Elena Hassinger
(TU Dresden)
Interplay of ordered states in the locally noncentrosymmetric superconductor \(CeRh_2As_2\)
Local inversionsymmetry breaking in a crystal combined with strong Rashba interaction can lead to a fieldinduced transition from an even to an oddparity superconducting state, when the orbital limit is large due to correlations [1]. The first realisation of this phenomenology is likely found in the locally noncentrosymmetric CeRh$_2$As$_2$ in which two superconducting states (SC1 and SC2) occur in a magnetic field along the crystallographic c axis [2,3]. The superconducting state with $T_\mathrm{c} = 0.3$ K develops out of a possible quadrupoledensitywave order (QDW) setting in at $T_\mathrm{0} = 0.5$ K [4]. Furthermore, signatures of antiferromagnetism (AF) are observed below $T_\mathrm{c}$ by NQR [5]. The low ordering temperatures and nonFermiliquid behavior suggest the proximity to a quantum critical point but the role of QDW and AF orders for superconductivity remains unclear.
In this talk I will give a comprehensive overview of experimental results on this unique material including the discovery [2], the strong anisotropy of the phase diagram in different field directions [2,3,4,6], as well as the response to hydrostatic pressure. These results support the even to oddparity transition scenario for the superconducting states and suggest a weak competitive coupling between the QDW and SC order parameters [6]. I will also discuss the relevance of quantum criticality and the roles of tuned interactions based on results from recent highpressure experiments.
[1] T. Yoshida et al., Phys. Rev. B 86, 134514 (2012).
[2] S. Khim & J. Landaeta et al., Science 373, 1012–1016 (2021).
[3] J. Landaeta et al., Phys. Rev. X 12, 031001 (2022).
[4] D. Hafner et al., Phys. Rev. X 12, 011023 (2022).
[5] K. Kibune et al., Phys. Rev. Lett. 128, 057002 (2022).
[6] K. Semeniuk et al., arXiv:2301.09151 (2023).

12:30  14:00

Lunch


chair: Hidenori Takagi

14:00  14:30

Peter Abbamonte
(University of Illinois UrbanaChampaign)
Observation of Pines' demon in \(Sr_2RuO_4\)
The characteristic excitation of a metal is its plasmon, which is a quantized sound wave in its valence electron density. In 1965, David Pines predicted that a distinct type of plasmon, which he named a "demon," could exist in multiband metals that contain more than one species of charge carrier. Unlike conventional plasmons, demons are acoustic excitations, meaning they are ``massless," i.e., their energy tends toward zero as the momentum q>0. So demons may play a central role in the lowenergy physics of multiband metals. However, demons are neutral excitations that do not couple to light, so they have never been observed experimentally, at least in a 3D material.
In this talk I will present the discovery of a demon in the multiband metal Sr$_2$RuO$_4$. Formed of electrons in the $\beta$ and $\gamma$ bands, the demon is gapless with critical momentum $q_c$ = 0.08 reciprocal lattice units and room temperature velocity v = 1.065(120)$\times 10^5$ m/s. Our observation confirms a 67year old prediction and may have implications for multicompinent Fermi systems ranging from vdW materials to cold atomic gases to neutron starts.
*A. A. Husain, et al., arxiv:2007.06670 (to appear in Nature)

14:30  15:00

HaeYoung Kee
(University of Toronto)
Multipolar Spin Liquids in 5d2 Honeycomb Mott Insulators
The Kitaev model, characterized by bonddependent Ising spin interactions among spinorbit entangled dipole moments in Mott insulators, offers a new approach to quantum spin liquids.
Such bonddependent interactions are not limited to Kramers doublets. In the 5d^2 Mott insulators with strong spin orbit coupling, the nonKramers doublet hosts quadrupole and octupole moments while lacking a dipole moment. After a quick review of multipolar physics in double perovskites, I will present a microscopic theory of the multipolar interactions that exhibit bonddependent quadrupolequadrupole interactions. Remarkably, these interactions on the honeycomb lattice take the form of the extended Kitaev model including the bonddependent offdiagonal and Heisenberg interactions. I will discuss a way to realize the exactly solvable Kitaev limit in honeycomb Mott insulators.

15:00  15:30

Martin Mourigal
(Georgia Institute of Technology)
Tetravalent Praseodymium: from Kitaev magnets to molecular spin qubits

15:30  16:00

coffee break

16:00

Poster flash presentations & poster session  focus on odd poster numbers

18:30  19:30

Dinner
