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.

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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
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.

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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.

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Special 290S/290K Quantum Materials Seminar speaker Ethan Lake (MIT) Thursday, October 27 at 4 pm in Physics South 402

Time/Venue Thursday, October 27 at 4 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 Ehud Altman
Title Renormalization group quantum circuits for 1d SPT phases
Abstract Suppose one is handed the ground state wavefunction of a local Hamiltonian. How does one determine the phase of matter that this ground state belongs to? In this talk, I will describe a way of answering this question for 1d symmetry protected topological phases using `RG quantum circuits’: unitary circuits that test whether or not their inputs belong to a particular phase. These circuits are constructed from only a small amount of universal data, and simulate renormalization group flow with the help of a simple error correction protocol. This talk is based on soon-to-appear work with Shankar Balasubramanian and Soonwon Choi.

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290S/290K Quantum Materials Seminar speaker Peter Lunts (UMD) Wednesday, October 19 at 2 pm in Physics South 402

Time/Venue Wednesday, October 19 at 2 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 Non-Hertz-Millis scaling of the antiferromagnetic quantum critical metal via scalable Hybrid Monte Carlo
Abstract Over the last decade there has been a surge of activity in studying various quantum phenomena via electronic `constructor models’ that lack the fermion sign problem. Of note are constructor models of quantum phase transitions in metallic systems, where it is easier to find such microscopic models due to the notion of universality. However, this agenda has been hampered due to the expensive scaling with system size of Determinant Quantum Monte Carlo, making it difficult to convincingly extract long-wavelength information from the critical theory.
In this talk I will introduce a Hybrid Monte Carlo (HMC) algorithm with a novel auto-tuning procedure. We apply this method to the O(3) spin-fermion model, a minimal model of the onset of antiferromagnetic order in a two-dimensional metal. Our method allows us to study unprecedentedly large systems of 80×80 sites, even at criticality. To characterize the universality class of the quantum critical point, we extract the scaling exponents and functional form of the static and zero-momentum dynamical spin susceptibility. We find a strong violation of the Hertz-Millis form, contrary to all previous results. The form that we do observe provides strong evidence that the universal scaling is actually governed by the fixed point of [Phys. Rev. X 7, 021010 (2017)], even away from perfect hot-spot nesting. Our results suggest that controlled analytical calculations of the critical theory near perfect hot-spot nesting can be used to make qualitatively correct predictions at larger nesting angles of other observables. Additionally, the HMC method we introduce is generic and can be used to study other fermionic models of quantum criticality, where there is a strong need to simulate large systems.

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290S/290K Quantum Materials Seminar speakers Hongrui Zhang and Shubhayu Chatterjee (both UCB) on Wednesday, October 12 at 2:00 pm PST in Physics South 402

Time/Venue Wednesday, October 12 at 2:00 pm PST in Physics South 402
Host Bob Birgeneau
Speaker Hongrui Zhang (Ramesh Group)
Title 
A room-temperature layered polar magnet
Abstract 
The magnets with broken inversion symmetry exhibit many novel physical properties such as magnetoelectric coupling, topological spin texture etc. Such low-symmetry layered magnetic systems are, however, scarce. In this talk, I will discuss that we designed and synthesized a room-temperature, polar magnet in van der Waals Fe2.5Co2.5GeTe2 (FCGT). In this material, we experimentally observed a Néel-type skyrmion lattice and current-induced skyrmion lattice motion at room temperature, with a threshold current density, jth =1 ×106 A/cm-2. In addition, we also realized room-temperature, current-induced magnetization self-switching in thin single-phase FCGT. This discovery of polar magnet in van der Waals materials opens the pathway for the development of next-generation spintronic devices and provides an ideal platform for studies of topological and quantum effects in 2D.
Speaker 
Shubhayu Chatterjee (Yao & Zaletel Groups)
Title 
Tunable electrical control of magnetism in chiral graphene-TMD heterostructures 
Abstract 
Electrical control of magnetism has been a longstanding goal of the spintronics community. I will discuss our ongoing theoretical work on a set of two-dimensional heterostructures – chiral multilayer graphene sandwiched inside transition metal dichalcogenides (TMD), where such control can be potentially realized via a perpendicular electric field. First, we will demonstrate that the electric field leads to enhanced low-energy density of states and promotes flavor (spin/valley) polarization. Next, we will argue that the relative alignment of graphene and TMD can be used to tune the relative signs of the induced spin-orbit coupling (SOC) in the top and bottom graphene layers. Combining these ingredients will allow us to flip either spin (or valley) degree of freedom by simply switching the direction of the electric field.

Posted in QM Seminar | Comments Off on 290S/290K Quantum Materials Seminar speakers Hongrui Zhang and Shubhayu Chatterjee (both UCB) on Wednesday, October 12 at 2:00 pm PST in Physics South 402

290S/290K Quantum Materials Seminar speaker Dan Borgnia (UCB) Wednesday, October 5 at 2 pm in Physics North 402

Time/Venue Wednesday, October 5 at 2 pm Pacific time in Physics North 402
 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Localization via Quasi-Periodic Bulk-Bulk Correspondence
Abstract Quasi-periodic systems exhibit rich spectral properties, including topological invariants, mobility edges, and localization transitions. They are one of the few examples of operators we know to have a singular continuous spectrum (a generic, but poorly understood property class of operators). I will report on a new set of tools relating quasi-periodic topology and the spectral measure for the metal-insulator transition (MIT) in the almost-Mathieu operator. By constructing quasi-periodic transfer matrix equations from the limit of rational approximate projected Green’s functions using a 2D parent Hamiltonian, we treat the metal-insulator transition like a gauge transformation and link the eigenfunction localization of the MIT to the chiral edge modes of the Hofstadter Hamiltonian. This implies the localized phase roots in a topological “bulk-bulk” correspondence, a bulk-boundary correspondence between the 1D AAH system (boundary) and its 2D parent Hamiltonian (bulk). These results have exciting consequence in the singular continuous spectrum, including applications to the Dry Ten Martini Problem.

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Special 290S/290K Quantum Materials Seminar speaker Nathanan Tantivasadakarn (Harvard) Tuesday, October 4 at 11 am in Physics North 402

Time/VenueTuesday, October 4 at 11 am in Pacific time in Physics North 402
 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman
Title Topological order from finite-depth circuits and measurements: from theory to quantum devices
Abstract A fundamental distinction between many-body quantum states are those with short- and long-range entanglement (SRE and LRE). The latter, such as cat states, topological order, or critical states cannot be created by finite-depth circuits. Remarkably, examples are known where LRE is obtained by performing single-site measurements on SRE states such as preparing the toric code from measuring a sublattice of a 2D cluster state. I will present a general framework of how and why these known protocols give rise to long range entanglement based on interpreting the cluster state measurement as implementing the non-local Kramers-Wannier transformation. This provides a scalable and practical way to “gauge” a symmetry in finite time, and moreover allows us to go beyond the preparation of stabilizer states. In addition, we find a complexity hierarchy on long-range entangled states based on the minimal number of measurement layers required to create the state. I will argue that certain phases of matter cannot be prepared using any finite number of layers, while remarkably certain non-Abelian topological orders can be prepared in a single round of measurement. As an application, I will outline how current NISQ devices, ranging from Rydberg atom arrays to Google’s quantum processors, can scalably prepare a large class of exotic phases such as non-Abelian topological order and even fracton phases.
This talk is based on 2112.01519, 2112.03061, 2209.03964, and 2209.06202

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Special time: 290S/290K Quantum Materials Seminar Speaker seminar speaker Sebastian Paeckel (University of Munich) Thursday, September 29 at 2 pm in 402 Physics South

Time/Venue Thursday, September 29 at 2 pm Pacific time in 402 Physics South and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Mike Zaletel
Title Tensor network methods for open electron-phonon systems: Bipolarons in the presence of dissipation
Abstract Studying the interplay between electrons and phonons recently has seen a remarkable revival, driven by both methodical progress, as well as fascinating new physical insights, such as the possibility of light-bipolaron induced superconductivity [1,2] or the enhancement of transport properties [3]. Here, investigating the effects of environments (dissipative, thermal, driven) coupled to the phonon system is a crucial but enormously challenging problem, which on the one hand is important to understand the validity of effective, isolated models, while on the other hand allows for tailored manipulations of the phononic state. Recently, we developed a new toolbox of tensor network methods which are designed to allow an efficient treatment of electron-phonon systems in- and out-of equilibrium as well as coupled to an environment. Exhibiting a speed-up of significantly more than an order of magnitude, we are able to implement open quantum system techniques, which previously had been way to expensive to be of proper use for studying large system sizes by means of tensor network methods.
In this talk I introduce the developed tools and give a brief overview of the current state, their potential and limitations. Furthermore, I discuss a first application, namely the effect of a dissipative environment, coupled to the Hubbard-Holstein model, putting the emphasis on a previously reported enhancement of the metallic phase.
[1]  Phys. Rev. Lett. 121, 247001 (2021)
[2]  https://doi.org/10.48550/arXiv.2203.07380
[3] Scientific Reports volume 7, 3774 (2017)

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Special 290S/290K Quantum Materials Seminar speaker Michael Buchhold (U of Cologne) Friday, September 16 at 2 pm in Physics North 402

Time/Venue Friday, September 16 at 2 pm in Pacific time in Physics North 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Measurement-Induced Phase Transitions of Fermions: Phenomenology, Effective Theory and Strategies to Reveal Them
Abstract A wave function exposed to measurements undergoes pure state dynamics, with deterministic unitary and probabilistic measurement-induced state updates. For many-particle systems, the competition of these different elements gives rise to a scenario similar to quantum phase transitions, which are visible in the entanglement structure of the wave functions. They are masked, however, in standard quantum mechanical observables due to the randomness of measurement outcomes. We study the dynamics of locally measured free fermions in (1+1) dimensions undergoing a measurement-induced phase transition. We strengthen the analogy between this transition and ground state quantum phase transitions by examining a replica field theory for the n-th moment of the measured wave function: the phase transition corresponds to a macroscopic change in the dark state wave function of a non-Hermitian Hamiltonian. In a second step, we introduce a general strategy to make measurement-induced transitions observable. It relies on breaking the measurement degeneracy explicitly by steering the system towards a chosen representative state. This strategy introduces a unique dark or absorbing state and creates a link of measurement-induced phase transitions to new forms of quantum absorbing state transitions, which can be detected by standard means via a local order parameter.

Posted in QM Seminar | Comments Off on Special 290S/290K Quantum Materials Seminar speaker Michael Buchhold (U of Cologne) Friday, September 16 at 2 pm in Physics North 402