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.

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

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.

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

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

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

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)

Posted in QM Seminar | Comments Off on 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

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

290S/290K Quantum Materials Seminar speakers Nathan Giles-Donovan and Yu-Ping Lin (both UC Berkeley) on Wednesday, September 14 at 2:00 pm in Physics South 402

Time/Venue Wednesday, September 14 at 2:00 pm PST in Physics South 402 (in person only)
Host Bob Birgeneau
Speaker  Nathan Giles-Donovan (Birgeneau Group)
Title Characterization of Coupled Orders in Multiferroic Dielectric Oxides
Abstract  In general, coupling and competition of different orders result in materials which exhibit many intriguing properties and can result in thought-provoking physics. Materials with coupled ferroic orders are central to developing transduction devices due to their ability to convert between different forms of energy. This is seen in both the case of piezoelectric/ferroelectrics which are used in ultrasonic applications and in multiferroics which have the potential to enable hybrid magnetoelectric devices independently controllable using magnetic and electric fields. However, due to this necessary and often complex interplay of orders, the microscopic mechanisms present in these materials are typically not well-understood. In this talk I will present some research highlights from a study into the fundamental behaviour of novel ferroelectric and multiferroic materials. Firstly, polarized neutrons were used to characterise the magnetic ground state of Cu3Nb2O8, addressing an issue in the literature regarding the ordering mechanism. Secondly, muon techniques were used to study the magnetic structure and composition of the relaxor Pb(Fe1/2Nb1/2)O3 which provided insight into the role of disorder and random fields.
Speaker  Yu-Ping Lin (CMT Group)
Title  Band geometry from position-momentum duality
Abstract  I propose a simple viewpoint of band geometry by analogizing to the free-particle theory. Specifically, I deduce that the quantum metric trace is a momentum-space “dual kinetic energy” under the Berry gauge field. This “position-momentum duality” allows us to borrow the knowledge of free-particle motion under the magnetic field. The possibility of “dual energy quantization” arises in the quantum metric trace. Although a simple quantization rule is generally unavailable due to the nonuniform Berry flux, a lowest-Landau-level interpretation applies to the lower bound by the Berry flux. I further present some examples of interesting quantization rules at the band crossings with approximate rotation symmetries.

Posted in QM Seminar | Comments Off on 290S/290K Quantum Materials Seminar speakers Nathan Giles-Donovan and Yu-Ping Lin (both UC Berkeley) on Wednesday, September 14 at 2:00 pm in Physics South 402

290S/290K Quantum Materials Seminar Speaker Georgios Varnavides (UC Berkeley) Wednesday, September 7 at 2 pm in Physics South 402

Time/Venue Wednesday, September 7 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: Spatially-Resolved Transport Framework for Electron Hydrodynamics in Crystalline Solids
Abstract: Recent advances in spatially-resolved transport measurements have revealed that electrons in condensed matter can flow collectively, exhibiting fluid phenomena such as channel flow and vortices. These observations confirm theoretical predictions over half a century old, violating textbook descriptions which treat electron collisions akin to balls in a pinball machine, and hold promise in designing energy-efficient nanoelectronic devices.
The first part of this talk will introduce electron hydrodynamic flow, highlighting recent experimental observations in WTe2 and PdCoO2,  and illustrate the novel electronic transport phenomena introduced by preferred directions in crystalline solids, including anisotropic and non-dissipative viscous contributions.
In the second part of the talk, I will introduce a new spatially-resolved transport framework, and use it to investigate an intriguing conundrum. Namely, that despite the microscopic scattering differences between materials exhibiting electron hydrodynamics, the experimentally-accessible current densities all share remarkable similarities. By projecting the system’s conserved quantities, physically-plausible collision operators respecting crystal symmetries are constructed and used to quantify the variability of the macroscopic current density observables statistically.
I will conclude by presenting ongoing work at Berkeley, to develop new electron microscopy imaging modalities to image these non-uniform current densities with nanoscale spatial resolution, as-well as outlining future theoretical work in introducing the role of band-topology in these electron flows.


Posted in QM Seminar | Comments Off on 290S/290K Quantum Materials Seminar Speaker Georgios Varnavides (UC Berkeley) Wednesday, September 7 at 2 pm in Physics South 402

290S/290K Quantum Materials Seminar Speaker Zhehao Dai (UC Berkeley) , Wednesday, August 31 at 2 pm in Physics South 402

Time/Venue Wednesday, August 31 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 2D fermionic isometric tensor networks, ground states and dynamics
Abstract It is hard to simulate a generic quantum manybody system on a classical computer. However, it is not necessarily the case for ground states and low-energy excited states of local Hamiltonians, things we care about most in condensed matter physics. These states have particularly low entanglement entropy, ideal for a tensor network representation. I will give an informal talk on recent progress on 2D isometric tensor networks, in particular, an efficient algorithm for finding ground states and simulating the low-energy dynamics of correlated electrons.

Posted in QM Seminar | Comments Off on 290S/290K Quantum Materials Seminar Speaker Zhehao Dai (UC Berkeley) , Wednesday, August 31 at 2 pm in Physics South 402

QM Seminar Speaker Ahmed Abouelkomsan (Stockholm University), Wednesday, July 27 at 2:00 pm Pacific Time

Time/Venue Wednesday, July 27 at 2:00 pm Pacific Timein Physics South 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Shubhayu Chatterjee/Joel Moore
Title Moiré Superlattices at Fractional Band Fillings:  Fractional Chern Insulators, Particle-Hole duality, and Quantum Geometry
Abstract We consider the core problem of Coulomb interactions within fractionally filled Moiré flat bands and demonstrate that the dual description in terms of holes, which acquires a non-trivial hole-dispersion, provides key physical intuition and enables more understanding for this strongly correlated problem. We find that the single-hole dispersion has a profound impact on the phase diagram in experimentally relevant examples such as ABC stacked trilayer and twisted bilayer graphene aligned with boron nitride. Such non-trivial particle-hole asymmetry which is generally absent in continuum Landau levels stems from the non-trivial underlying quantum geometry of the Moiré bands. In addition, we predict both twisted bilayer graphene aligned with boron nitride and gate-tunable twisted double bilayer graphene to be versatile platforms for the realization of fractional Chern insulator states like spin-singlet Halperin states and spin-polarized states in bands with Chern number C =1 and C = 2 at zero external magnetic fields.”

[1] Phys. Rev. Lett. 124, 106803 (2020)
[2] Phys. Rev. Lett. 126, 026801 (2021)
[3] arXiv:2202.10467

Posted in QM Seminar | Comments Off on QM Seminar Speaker Ahmed Abouelkomsan (Stockholm University), Wednesday, July 27 at 2:00 pm Pacific Time