**Time/Venue** Wednesday, January 18 at 2:00 pm Pacific Time in Physics South 402 and via Zoom:

https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704**Host **Joel Moore**Title** Quantum geometry: Quantum route to optical properties of materials**Abstract** The discoveries of a larger number of quantum materials in recent years raise the question of how to employ them in scientific research and technological applications. Designing quantum material properties on demand is a big challenge. Toward this goal, a crucial preliminary step is to identify which key characteristic property of the quantum wave functions relates to the target physical response property. One exciting direction emerging in the field of topological materials is to use the geometry of quantum wave functions – the so-called quantum geometry. This approach has been successful in characterizing various novel electric properties of materials. In this seminar, I will first explain why quantum geometry is a natural concept for characterizing the quantum-ness of response properties. I will then explain a major issue of applying the idea of quantum geometry to resonant optical responses of materials and talk about our resolution to the issue by focusing on electric-dipole transitions. Finally, I will talk about intriguing optical electromagnetic multipole responses beyond electric-dipole transitions due to quantum geometry.

## 290S/290K Quantum Materials Seminar speaker Junyeong Ahn (Harvard), Wednesday, January 18, 2023 at 2 pm in Physics South 402

## 290S/290K Quantum Materials Special. Seminar speaker Ewelina Hankiewicz (Würzburg University), Tuesday, January 17 at 2 pm in Physics South 402

**Time/Venue** Tuesday, January 17 at 2:00 pm Pacific Time in Physics South 402 and via Zoom:

https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704**Host **Joel Moore**Title** Quantum anomalies in topological materials and spin chains**Abstract** Recent theoretical and experimental advances allow for an observation of signatures of quantum anomalies in non-interacting condensed matter systems. Quantum anomalies violates one of the classical symmetries and bridge between condensed matter and high- energy physics.

The possibility of observation of signatures of the parity anomaly (failure of the existence of single Dirac fermion in two spatial dimensions characterized by broken parity symmetry) in Dirac-like materials [1,2] is especially interesting. Using effective field theories and analyzing band structures in external out-of-plane magnetic fields (orbital field), we show that topological properties of quantum anomalous Hall (QAH) insulators are related to the parity anomaly [2]. Moreover, we showed together with experimentalists a novel transition from -1 to 1 Hall plateau, caused by scattering processes between counter-propagating quantum Hall and QAH edge states related to the parity anomaly [3]. This model can be extended easily to **any three-dimensional topological insulators (TIs) or magnetic 3D TIs with odd numbers of surface states propagating in transport. **

Further, we turn our attention to the conformal anomaly in one-dimensional spin systems. The conformal anomaly signals itself in a breaking of scale invariance by quantum effects, visible in multi-point functions of the energy-momentum tensor. We relate the variance of the on-site static magnetization that could be observed in neutron scattering experiments to the conformal anomaly in these systems. This paves a path to observe quantum anomalies in strongly interacting spin systems [4].

[1] F. D. M. Haldane, *Phys. Rev. Lett.*** 61**, 2015 (1988).[2] J. Böttcher, C. Tutschku, L. W. Molenkamp, and E. M. Hankiewicz *Phys. Rev. Lett. ***123,** 226602 (2019); C. Tutschku, F. S. Nogueira, C. Northe, J. van den Brink, and E. M. Hankiewicz *Phys. Rev. B***102**, 205407 (2020); C. Tutschku, J. Böttcher, R. Meyer, and E. M. Hankiewicz *Phys. Rev. Research***2**, 033193 (2020).

[3] S. Shamim, P. Shekhar, W. Beugeling, J. Böttcher, A. Budewitz, J.-Benedikt Mayer, L. Lunczer, E. M. Hankiewicz, H. Buhmann, L. W. Molenkamp,*Nat. Commun. ***13**, Article number: 2682 (2022).

[4] C. Northe, C. Zhang, S. Galeski and E.M. Hankiewicz, arXiv:2210.07972 (2022).

## Special 290S/290K Quantum Materials Seminar speaker Lukas Homeier (LMU Munich) Friday, December 16 at 2 pm in 402 Physics South

**Time/Venue** Friday, December 16 at 2:00 pm in Pacific time in 402 Physics South and via Zoom:

https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704**Host** Dan Borgnia, Moore Group**Title** Realistic scheme for quantum simulation of Z2 lattice gauge theories with dynamical matter in (2+1)D**Abstract** Gauge fields coupled to dynamical matter are a ubiquitous framework in many disciplines of physics, ranging from particle to condensed matter physics, but remain challenging to implement robustly in large-scale quantum simulators. Here we propose a realistic scheme for Rydberg atom array experiments in which a Z2 gauge structure with dynamical charges emerges from only local two-body interactions and one-body terms in two spatial dimensions. The scheme enables the experimental study of a variety of models, including (2+1)D Z2 lattice gauge theories coupled to different types of dynamical matter and quantum dimer models on the honeycomb lattice, for which we derive effective Hamiltonians. We discuss ground-state phase diagrams of the experimentally most relevant effective Z2 lattice gauge theories with dynamical matter featuring various confined and deconfined, quantum spin liquid phases. Further, we present selected probes with immediate experimental relevance, including signatures of disorder-free localization as well as a thermal deconfinement transition of two charges. [arXiv:2205.08541]

## 290S/290K Quantum Materials Seminar speakers postponed: no seminar Wednesday, December 14 at 2:00 pm in Physics South 402

## Special 290S/290K Quantum Materials Seminar speaker Matthew Powell (UC Irvine) Tuesday, December 6 at 3:30 pm in 402 Physics South

**Time/Venue** Tuesday, December 6 at 3:30 pm in Pacific time in 402 Physics South and via Zoom:

https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704**Host **Joel Moore**Title** Continuity of the Lyapunov exponent for analytic multi-frequency quasi-periodic cocycles**Abstract** The purpose of this talk is to discuss our recent work on multi-frequency quasi-periodic cocycles, establishing continuity (both in cocycle and jointly in cocycle and frequency) of the Lyapunov exponent for non-identically singular cocycles. Analogous results for one-frequency cocycles have been known for over a decade, but the multi-frequency results have been limited to either Diophantine frequencies (continuity in cocycle) or $SL(2,\C)$ cocycles (joint continuity). We will discuss the history of this problem and the main points of our argument, which extends earlier work of Bourgain.

## Special 290S/290K Quantum Materials Seminar speaker Thomas Scaffidi (UC Irvine) Tuesday, December 6 at 2 pm in 402 Physics South

**Time/Venue** Tuesday, December 6 at 2:00 pm Pacific Time in Physics South 402 and via Zoom:

https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704**Host** Ehud Altman**Title **Spread and erase — How electron hydrodynamics can eliminate the

Landauer-Sharvin resistance**Abstract** What is the ultimate limit of conductance of a metallic device of

lateral size W? In the ballistic limit, the answer is the

Landauer-Sharvin conductance, which is associated with an abrupt

reduction of the number of conducting channels when going from the

contacts to the device. However, the ballistic limit is not always the

best-case scenario, since adding strong electron-electron scattering can

take electrons to a viscous regime of transport for which

“super-ballistic” flows were recently studied. In this talk, we will

show that by a proper choice of geometry which resembles a “wormhole”,

it is possible to spread the Landauer-Sharvin resistance throughout the

bulk of the system, allowing its complete elimination by electron

hydrodynamics. This effect arises due to the interplay between geometry

and strong electron-electron scattering, which allows for a net transfer

of carriers from reflected to transmitted channels. Finally, we will

discuss a recent experiment in a Corbino geometry which realizes one

half of this “wormhole” geometry

Refs:

Theory: Phys. Rev. Lett. 129, 157701 (2022)

Experiment: Nature 609, 276–281 (2022)

## 290S/290K Quantum Materials Seminar speaker Ashley Cook (Max Planck Institute for the Physics of Complex Systems) Wednesday, November 16 at 2 pm in Physics South 402

**Time/Venue** **Wednesday, November 16 at 2 pm in **Pacific time in Physics South 402 and via Zoom:

https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704**Host **Joel Moore **Title **Topological skyrmion phases of matter

Title: Topological skyrmion phases of matter

Abstract: Symmetry-protected topological phases of matter have long been classified based on mappings from the full Brillouin zone to the space of projectors onto occupied states. This is the basis of the ten fold way classification scheme. Here, we show effectively non-interacting topological phases are associated with mappings from the Brillouin zone to the space of other observables. We specifically consider topological phases of the spin degree of freedom of the ground state, protected by a generalized particle-hole symmetry, which are independent of the topological phases of the full set of degrees of freedom of the ground state. We show these phases realize distinctive momentum-space skyrmionic spin textures, and exhibit exotic bulk-boundary correspondence, which we characterize in detail by introducing the symmetry-enriched partial trace and symmetry-enriched slab entanglement spectrum. Non-trivial spin skyrmion number corresponds to additional spin-momentum-locking at the edge in slab geometries, and topologically-protected gapless boundary modes even when the projector topological invariant is trivial. We present recipes for constructing myriad toy models of these phases, and also explore consequences of this physics in transition metal oxide superconductors as the generalized particle-hole symmetry occurs in centrosymmetric superconductors. When spin is not conserved due to non-negligible atomic spin-orbit coupling, we find two kinds of topological phase transitions are possible. The second kind occurs without the closing of the minimum direct bulk energy gap while respecting the symmetry protecting the topological phase, due to the minimum spin magnitude going to zero somewhere in the Brillouin zone. This type-II topological phase transition serves as the first-known contradiction of the flat-band limit assumption, widely-used since the early 1980’s.

## Special 290S/290K Quantum Materials Seminar speaker Nikita Astrakhantsev (Uni Zürich) Monday, November 14 at 11 am in Physics South 325

**Time/Venue** **Monday, November 14 at 11 am in **Physics South 325

and via Zoom:

https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704**Host** Joel Moore **Title **Spin-liquid states on the pyrochlore lattice and Rydberg atoms

simulator**Abstract** The XXZ model on the three-dimensional frustrated pyrochlore

lattice describes a family of rare-earth materials showing signatures of

fractionalization and no sign of ordering in the neutron-scattering

experiments. The phase diagram of such XXZ model is believed to host

several spin-liquid states with fascinating properties, such as emergent

U(1) electrodynamics with emergent photon and possible

confinement-deconfinement transition. Unfortunately, numerical studies

of such lattice are hindered by three-dimensional geometry and absence

of obvious small parameters.

In this talk, I will present my work [Phys. Rev. X 11, 041021] on the

variational study of the pyrochlore XXZ model using the RVB-inspired and

Neural-Network-inspired ansätze. They yield energies better than known

results of DMRG at finite bond dimension. With these wave functions, we

study the properties of frustrated phase at the Heisenberg point, and

observe signatures of long-range dimer correlations.

Lastly, I will sketch the prospects of using the Programmable Rydberg

Simulator platform for the study of these spin-liquid states. I will

construct two possible embeddings of the pyrochlore XXZ model onto the

Rydberg atoms simulator, employing the notion of spin ice and

perturbative hexagon flip processes.

## 290S/290K Quantum Materials Seminar speakers Yi Lin and Luke Pritchard Cairns (both UCB) on Wednesday, November 9 at 2:00 pm in Physics South 402

**Time/Venue** Wednesday, November 9 at 2:00 pm PST in Physics South 402**Host** Bob Birgeneau**Speaker **Yi Lin (Lanzara Group)**Title **Mapping Ultrafast Excitonic Phenomena in 2D Materials by Using Time-Resolved ARPES

**Excitonic correlations between electrons and holes play the key role for understanding the optical phenomena, many-body interactions and exotic phase transitions in semiconducting materials and excitonic insulators, which have been conventionally studied by optical spectroscopies. In this talk, we present our experimental and theoretical work toward probing ultrafast excitonic phenomena in 2D materials by using extreme-UV time- and angle-resolved photoemission spectroscopy (XUV-trARPES). By photoemitting the electrons from different types of correlated electron-hole gases in the material, we made ultrafast movies of electronic band structure impacted by the excitonic correlations. Our results visualized a non-resonant exciton formation process and revealed exciton-driven band renormalization effects, featuring surprising bandgap opening and band flattening. Our work demonstrates a new pathway for studying excitonic many-body phenomena by using photoemission techniques under a single-particle narrative fully in energy, momentum and time.**

Abstract

Abstract

**Luke Pritchard Cairns (Analytis Group)**

Speaker

Speaker

**Tracking the evolution from isolated dimers to many-body entanglement in NaLuxYb1−xSe2**

Title

Title

**Abstract**NaYbSe2 belongs to a recently discovered class of Yb delafossites, the majority of which have been shown to exhibit the hallmarks of a quantum spin liquid (QSL) – for example an absence of long-range magnetic order and a large anomalous heat capacity at the lowest measurable temperatures. However, given the small regions of parameter space QSL states can occupy for the triangular lattice, it seems unlikely that such a varied collection of compounds should all satisfy the constraints. In this study we look to diverge from the typical investigation of a QSL, and have instead doped NaYbSe2 with non-magnetic Lu. In this way, we are able to investigate the evolution of magnetic correlations, from isolated dimers, through the percolation transition and building towards the highly-correlated many-body entangled state in NaYbSe2. We hope that this might shed light on the proposed QSL state in this compound, and also the broader class of materials.

## 290S/290K Quantum Materials Seminar speaker Thais Victa Trevisan (new to UCB!) Wednesday, November 2 at 2 pm in Physics South 402

**Time/Venue** Wednesday, November 2 at 2 pm in Pacific time in 402 Physics South and via Zoom:

https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704**Host **Joel Moore**Title **Tuning magnetic symmetries and topology with bicircular light**Abstract** Light-matter interaction is a powerful tool to manipulate the electronic properties of materials. Coherent light can be carefully tailored to break selected symmetries and stabilize phases of matter otherwise absent. This technique is called Floquet engineering and has received increasing attention in condensed matter physics in the past few years. In this talk, we theoretically analyze the effects of shining a bicircular light (BCL) on Dirac semimetals and topological insulators. BCL consists of a superposition of two circularly polarized light beams with frequencies that are integer multiples of each other. The resulting electric field traces a rose curve whose shape and orientation can be controlled by light parameters. A distinctive feature of BCL is its capability to simultaneously break time-reversal and spatial inversion symmetry, which leads to the realization of sought-after magnetic topological states. Another outcome of BCL driving is a dynamical tunability of the position and energy of topologically protected nodes of the band structure, resulting in a controlled modulation of the gyrotropic magnetic effect.