Berkeley CMT

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.

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. B102, 205407 (2020); C. Tutschku, J. Böttcher, R. Meyer, and E. M. Hankiewicz Phys. Rev. Research2, 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).

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]

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.

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)

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.

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.

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
Abstract 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.
Speaker Luke Pritchard Cairns (Analytis Group)
Title Tracking the evolution from isolated dimers to many-body entanglement in NaLuxYb1−xSe2
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.

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.