Berkeley CMT

Time/Venue Friday, May 26 at 11:00 am 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 Kagome chiral spin liquid in transition metal dichalcogenide moiré bilayers
Abstract In this talk, I will present results on the state at n=3/4 filling of the flat band in transition metal dichalcogenide moiré bilayers. At this filling, the system develops charge order in a kagome pattern resulting in localized spins on this lattice geometry. Starting from an extended Hubbard model description, we derive an effective spin model on the kagome lattice and find that its further neighbor spin interactions can be much less suppressed than the corresponding electron hopping strength. Using density matrix renormalization group simulations, we study its phase diagram and, for realistic model parameters relevant for WSe2/WS2, we show that this material can realize the exotic chiral spin liquid phase and the highly debated kagome spin liquid. Our work thus demonstrates that the frustration and strong interactions present in TMD heterobilayers provide a novel experimental realization of kagome spin models, representing an exciting platform to study spin liquid physics.

Shuqiu Wang (Oxford) 
Time/Venue Monday, May 22 at 11 am in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Dung-Hai Lee and Ehud Altman
TitleVisualizing the Surface States of Spin-Triplet Superconductor UTe₂
Abstract

Spin-triplet topological superconductors should exhibit many unprecedented electronic properties including fractionalized electronic states relevant to quantum information processing. Although UTe₂ may embody such bulk topological superconductivity its superconductive order-parameter ∆(k) remains unknown. Many diverse forms for ∆(k) are physically possible in such heavy fermion materials. Moreover, intertwineddensity waves of spin (SDW), charge (CDW) and pairs (PDW) may interpose, with the latter exhibiting spatially modulating superconductive order-parameter ∆(k), electron pair density and pairing energy-gap. Hence, the newly discovered CDW state in UTe₂ motivates the prospect that a PDW state may exist. To search for it, we visualize the pairing energy-gap with μeV-scale energy-resolution using superconductive STM tips. We detect three PDWs, each with peak-peak gap modulations circa 10 μeV and at incommensurate wavevectors P_(i=1,2,3) that are indistinguishable from the wavevectors Q_(i=1,2,3) of the prevenient CDW. Concurrent visualization of the UTe₂ superconductive PDWs and the non-superconductive CDWs reveals that every Pi :Qi pair exhibits a relative spatial phase δϕ≈π. From these observations and given UTe₂ as a spin-triplet superconductor_,this PDW state should be a spin-triplet pair density wave, an unprecedented states for superconductors.
Extraordinary quantum boundary conditions exist for spin-triplet superconductor with odd-parity pairing potentials. One long-predicted consequence is that quasiparticle retroreflection should generate a zero-energy surface Andreev bound state (SABS).  And, while there is clear evidence that an anomalous surface state exists in superconductive UTe₂, its electronic structure has remained inaccessible to direct visualization. Here we visualize the UTe₂ zero-energy SABS at atomic-scale and its scattering inference signature. These zero-energy SABS form a two-dimensional fluid of charge-neutral quasiparticles whose direct visualization is unique, and whose existence and scattering inference characterizes demonstrate directly that UTe₂ is an odd-parity bi-nodal superconductor.

[1] Nature. arXiv:2209.10859 (2023).
[2] Gu, Wang, Zhussupbekov, et al, in preparation (2023).

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Carolyn Zhang (University of Chicago)
Time/Venue Monday, May 22 at 2:00 pm in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host  Ehud Altman/Alessandra Lanzara
Title Entanglement transitions in non-unitary Gaussian circuits
B We consider Gaussian quantum circuits that consist of unitary gates and post-selected weak measurements of fermion bilinears, with spatial translation symmetry and time periodicity. We show analytically that these simple models can host different kinds of measurement-induced phase transitions detected by entanglement entropy, by mapping the unitary gates and weak measurements onto Möbius transformations. We demonstrate the existence of a log-law to area-law transition, as well as an unexpected volume-law to area-law transition at a finite measurement amplitude. For the latter, we compute the critical exponent $\nu$ for the Hartley, von Neumann and Rényi entropies exactly.
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Dan Mao (Cornell)
Time/Venue Monday, May 22 at 3:00 pm in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host  Ehud Altman / Alessandra Lanzara
Title Superconducting and excitonic phase-stiffness for interacting isolated narrow bands
Abstract Inspired by the discovery of superconductivity in moir\’e materials with isolated narrow bandwidth electronic bands, I will analyze the question of what is the maximum attainable $T_c$ in interacting flat-band systems. I will focus specifically on the low-energy effective theory, where the density-density interactions are projected to the set of partially-filled flat bands. The resulting problem is inherently non-perturbative, given that the interaction energy scale can be comparable or larger than the band width of the low energy narrow bands, where the standard mean-field approximation is not applicable. Recently, we develop a Schrieffer-Wolff transformation based approach to compute the effective electromagnetic response and the superconducting phase-stiffness in terms of “projected” gauge-transformations and extend the formalism to compute the stiffness for excitonic superfluids. Importantly, our method requires neither any “wannierization” for the narrow bands of interest, regardless of their (non-)topological character, nor any knowledge of an underlying pairing symmetry, and can be set up directly in momentum-space. We use this formalism to derive upper bounds on the phase-stiffness for sign-problem-free models, where their values are known independently from numerically exact quantum Monte-Carlo computations. We also illustrate the analytical structure of these bounds for the superconducting and excitonic phase-stiffness for perfectly flat-bands with Landau-level-like wavefunctions.
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Sara Murciano (Caltech)
Time/Venue Thursday, May 25 at 11:00 am in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host  Ehud Altman/Alessandra Lanzara
Title Rise and fall of critical correlations after measurements
Abstract Quantum critical systems constitute appealing platforms for the exploration of novel measurement-induced phenomena due to their innate sensitivity to perturbations.  First, we study the impact of measurement on Ising chains using an explicit protocol, whereby correlated ancilla are entangled with the critical chain and then projectively measured. These measurements can modify the Ising order-parameter scaling dimension and catalyze order parameter condensation. In particular, the general lesson we learn is that the correlations decay faster after the ancilla is measured.  Second, we consider the system in a product state while the ancilla is an Ising critical chain, and by applying a similar measurement protocol we can “teleport” the long-range correlations from the ancilla to the system.  In both setups, we can derive numerous quantitative predictions for the behavior of correlations in selected measurement outcomes. Finally, we show that, in both protocols, observables can be averaged separately over measurement outcomes residing in distinct symmetry sectors; we demonstrate that these `symmetry-resolved averages’ reveal measurement effects even when considering standard linearly averaged observables.
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Federica Surace (Caltech)
Time/Venue Thursday, May 25 at 2:00 pm in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host  Ehud Altman/Alessandra Lanzara
Title Weak perturbations of integrable models
Abstract A quantum integrable system slightly perturbed away from integrability is typically expected to thermalize on timescales of order τ∼λ^(-2), where λ is the perturbation strength. We here study classes of perturbations that violate this scaling, and exhibit much longer thermalization times τ∼λ^(-2k) where k>1 is an integer.  Systems with these “weak integrability breaking” perturbations have an extensive number of quasi-conserved quantities that commute with the perturbed Hamiltonian up to corrections of order λ^k. We demonstrate a systematic construction to obtain families of such weak perturbations of a generic integrable model for arbitrary k. We then apply the construction to various models, including the Heisenberg, XXZ, and XYZ chains, the Hubbard model, models of spinless free fermions, and the quantum Ising chain. Our analytical framework explains the previously observed evidence of weak integrability breaking in the Heisenberg and XXZ chains under certain perturbations.

Time/Venue Friday, May 19 at 11 am in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Johannes Mitscherling / Joel Moore
Title Taming energy dissipation in driven topological systems
Abstract In this talk, I will discuss energy dissipation and heating in slowly driven quantum systems, focusing on topological driving schemes. In the first part of my talk, I will present a system in which many-body dynamics leads to the emergence of a quasi-steady state with a high entropy density and yet robust topological transport. I will explain the mechanisms behind this phenomenon and demonstrate the emergence of the quasi-steady state on an exactly solvable strongly coupled fermionic model. In the second part of my talk, I will show that the dissipation of energy in nearly adiabatic quantum systems is linked to the quantum geometry of the problem. Interestingly, this result implies a topological bound on the energy dissipation rate in a class of topological systems. Our findings uncover new connections between topology and dissipation in slowly driven quantum systems, shedding light on their fundamental properties and potential for practical applications, such as the development of optimized driving protocols for topological drives.

Time/Venue Thursday, May 18 at 11:00 pm in 402 Physics South and via Zoom:

https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman
Title Quantum impurities in cold atoms and beyond
Abstract The problem of an impurity interacting with an environment of quantum objects is one of the most paradigmatic in the physics of quantum many-body systems. A central concept in this problem is the emergence of quasiparticle excitations called polarons, which may feature very different properties depending on the nature of the environment and of the interactions. Several theoretical aspects, such as the relation of impurity problems with particle-imbalance-driven phases (e.g., FFLO), as well as experimental aspects related to the detection of polarons, are still objects of intensive research.
In this talk, I will discuss experimental and theoretical progress on studying polarons in ultracold atomic systems. I will review spectroscopic methods in ultracold quantum gases with impurities and introduce a new protocol for the detection of molarons, composite quasiparticles of polaronic character.
Furthermore, I will discuss related polaronic phenomena encountered in two-dimensional semiconductors, presenting recent experimental results.

Time/VenueWednesday, May 17 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 Local Topological Markers in Odd Spatial Dimensions and Their Application to Amorphous Topological Matter
Abstract Local topological markers, topological invariants evaluated by local expectation values, are valuable for characterizing topological phases in materials lacking translation invariance. The Chern marker—the Chern number expressed in terms of the Fourier transformed Chern character—is an easily applicable local marker in even dimensions, but there are no analogous expressions for odd dimensions. We provide general analytic expressions for local markers for free-fermion topological states in odd dimensions protected by local symmetries: a Chiral marker, a local Z marker which in case of translation invariance is equivalent to the chiral winding number, and a Chern-Simons marker, a local Z2 marker characterizing all nonchiral phases in odd dimensions.
We achieve this by introducing a one-parameter family PJ of single-particle density matrices interpolating between a trivial state and the state of interest. By interpreting the parameter J as an additional dimension, we calculate the Chern marker for the family PJ . We demonstrate the practical use of these markers by characterizing the topological phases of two amorphous Hamiltonians in three dimensions: a topological superconductor (Z classification) and a topological insulator (Z2 classification).

Time/Venue Thursday, May 11 at 11:00 pm in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman 
Title Exact Quantum Algorithms to Recognize Quantum Phases of Matte
Abstract In this talk, we explore the relationship between renormalization group (RG) flow and error correction by constructing quantum algorithms that exactly recognize 1D symmetry-protected topological (SPT) phases protected by finite internal Abelian symmetries. For each SPT phase, our algorithm runs a quantum circuit which emulates RG flow: an arbitrary input ground state wavefunction in the phase is mapped to a unique minimally-entangled reference state, thereby allowing for efficient phase identification. This construction is enabled by viewing a generic input state in the phase as a collection of coherent `errors’ applied to the reference state, and engineering a quantum circuit to efficiently detect and correct such errors. Importantly, the error correction threshold is proven to coincide exactly with the phase boundary. We discuss the implications of our results in the context of condensed matter physics, machine learning, and near-term quantum algorithms.

Title Time/Venue Wednesday, May 10 at 11 am in 325 Physics South and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meet Joel Moore
Title A topological principle for photovoltaics
Abstract For a non-centrosymmetric semiconductor driven out of equilibrium, the steady photovoltaic current is contributed by real-space displacements (or ‘shifts’) of quasiparticles as they transit between Bloch states. These transitions are of three intrinsic types: (i) optical excitation, (ii) phonon-induced intraband relaxation, and (iii) radiative recombination of electron-hole pairs. The phonon- and recombination-induced shifts have been ignored in all Kubo-type perturbative and Floquet theories, but can dominate over the excitation-induced shift by orders of magnitude. Such dominance originates from time-reversal-invariant, intraband Berry curvature inducing an anomalous shift, as well as topological singularities in the interband Berry phase.
With certain crystallographic symmetry, the photo-excited electron-hole separation (or ‘shift vector’) becomes a topological invariant, which is quantized to Bravais-lattice vectors of the underlying crystal. A new principle emerges to maximize the shift current: namely, to maximize a topological invariant.

Time/Venue Wednesday, May 10 at 2:00 pm PST in Physics South 402
Host Bob Birgeneau
Speaker 1  Peter Meisenheimer (Ramesh Group)
Title  Electrostatic control of ordering in polar topologies
Abstract  Complex topological configurations are fertile ground for exploring emergent phenomena and exotic phases in condensed-matter physics. Topological solitons such as magnetic skyrmions have long drawn attention as stable quasi-particle-like objects, but the recent discovery of polar vortices and skyrmions in ferroelectric oxide superlattices has opened the door for new length scales and electric-field manipulation. Functional phenomena can be distinct from those of normal ferroelectrics, with properties such as collective dynamics, chirality, and negative capacitance. Topologically nontrivial ferroelectric textures are uniquely possible and manipulable by careful control of thin film boundary conditions, allowing for the exploration of phase space and order-disorder transitions with atomic precision. Here, we show controlled ordering of polar skyrmions enabled through electrostatic coupling, controlled by the ferroelectric proximity effect in incipient ferroelectric SrTiO₃. 
Speaker 2  Lizzy Dresselhaus (Moore Group)
Title  Flat band robust to amorphous disorder and localization beyond the standard paradigm in a photonics-inspired topological system
Abstract  Emerging experimental platforms use amorphousness, a controlled form of disorder, to tailor material properties. We study how this type of disorder induces Anderson localization in a class of 2D models recently realized in photonics systems. Focusing on a family of amorphized kagome tight-binding models we explore the applicability of the standard paradigm of Anderson localization to these models. We find the agreement with the standard paradigm to depend on the symmetry class within the family, set by a tunable synthetic magnetic field. For this analysis, we adapt the participation ratio and level spacing statistics approaches to identify metallic and localized energy regions.  Additionally we find that, unlike for on-site disorder, the flat bands innate to kagome systems survive under amorphousness. We explain how this phenomenon arises from the cooperation between the structure of the flat band states and the geometry of the amorphous graph.

Time/Venue Wednesday, May 3 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 Competing magnetic and nematic phases of a highly-frustrated spin-1 triangular lattice model
Abstract I will discuss my work (2206.04087) on a highly frustrated spin-1 model on the triangular lattice, with nearest- and next-nearest-neighbor antiferromagnetic S.S interactions and nearest-neighbor (S.S)^2 interactions. Using DMRG, we find three magnetically ordered phases, namely 120∘ spiral order, stripe order, and tetrahedral order, as well as two spin nematic phases: ferroquadrupolar and antiferroquadrupolar. While our data could be consistent with a spin liquid phase between the 120∘ spiral and antiferroquadrupolar orders, the more likely scenario is a direct continuous transition between these two orders.

Time/Venue Tuesday, May 2 at 11 am in tba conference room and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Host Ehud Altman
Title Universal Lindblad Equation for Open Quantum Systems
Abstract We develop a Markovian master equation in the Lindblad form that enables the efficient study of a wide range of open quantum many-body systems that would be inaccessible with existing methods. The validity of the master equation is based entirely on properties of the bath and the system-bath coupling, without any requirements on the level structure of the system. The master equation is derived rigorously using aMarkov approximation distinct from earlier approaches.  We discuss how our method can be applied to static or driven quantum many-body systems, and illustrate its power through numerical simulation of a spin chain that would be challenging to treat by existing methods.