Berkeley CMT

Time/Venue Thursday, March 30 at 2 pm in Pacific time in Physics South 325 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Bartholomew Andrews, Zaletel Group
Title s-wave paired composite fermions in quantum Hall bilayers. 
Abstract The half-filled Landau level is well described by a Fermi
liquid of composite fermions: two flux quanta attached to each electron
to form a composite object that feels no magnetic field. We study the
scenario where two half-filled Landau levels are brought closer and
closer together. This results in pairing between the composite fermions.
Using a trial wavefunction for s-wave pairing of composite fermions and
comparing to exact diagonalization, we deduce that the system undergoes
a BEC-BCS crossover as the distance between the two half-filled Landau
levels varies.

Time/Venue Wednesday, March 29 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 Aaron Szasz, Moore Research Group
Title Momentum creates long-range entanglement and quantum anomalies
Abstract In this talk, I will show how crystalline momentum holds a deep connection to quantum entanglement and quantum anomalies. First I will demonstrate that a quantum state in a lattice spin (boson) system must be long-range entangled if it has non-zero crystalline momentum [1]. The statement can also be generalized to fermion systems, and all dimensions. I will present several non-trivial consequences that follow from our theorem:
1.  Several different types of Lieb-Schultz-Mattis-Oshikawa-Hastings theorems, including a previously unknown version involving only a discrete symmetry;
2.  A gapped topological order (in space dimension d>1) must weakly break translation symmetry if one of its ground states on torus has nontrivial momentum – this generalizes the familiar physics of Tao-Thouless;
3.  Quantum anomalies associated to momentum are described by a Chern-Simons term that emerges in physical systems such as the time-reversal invariant Weyl semimetals [2].
I conclude by presenting potential measurable effects in semimetal systems, and possible generalisations of this statement.
[1] Lei Gioia, Chong Wang, Phys. Rev. X 12, 031007 (2022)
[2] Lei Gioia, Chong Wang, A. A. Burkov, Phys. Rev. R 3, 043067 (2021)

Time/Venue Wednesday, March 22 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 When is a phase diagram not a phase diagram? Lessons learned from Sr₂RuO₄
Abstract I will review recent developments in the experimental study of the thermodynamic properties of the superconducting state of one of the cleanest of all known unconventional superconductors, Sr₂RuO₄.  After nearly two decades in which its superconducting state was thought to have a time-reversal symmetry breaking, odd parity order parameter of the form p_x ± ip_y, recent NMR work has shown beyond reasonable doubt that its parity is in fact even.  The focus of attention has now moved to whether or not it has a two-component order parameter and whether or not time reversal symmetry is broken.
Investigating these issues has raised a number of uncomfortable questions of a generality that goes beyond any physics specific to superconducting Sr₂RuO₄.  One of the most common experimental activities in the fields of correlated electron physics and unconventional superconductivity is the construction of ‘phase diagrams’.  What is often being done in reality is taking some two-dimensional cut through a higher-dimensional phase space and plotting on it the positions of a series of experimentally observed anomalies.  By referring to the results as ‘phase diagrams’, we make the implicit assumption that the boundaries of bulk thermodynamic phases have been identified.  Based on our findings in strain-tuned Sr₂RuO₄, I will show that this implicit assumption should be treated with extreme caution, in relation not only to this particular material but likely to many others as well.

Time/Date Wednesday, March 22 at 11 am in 402 Physics South and via ia Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Chiral anomalies in hydrodynamics
Abstract Axial-current anomaly is the fundamental property of quantum field theory with Dirac fermions coupled to a vector gauge field. In quantum electrodynamics, for example, the anomaly states that the divergence of the axial current does not vanish as it would follow from the classical Dirac equation but is controlled by the electromagnetic field. 
Recently and quite remarkably the axial-current anomaly has been identified in the hydrodynamic of a classical ideal fluid as a kinematic property of the Euler equation. In  fluid, the axial current is closely related to the fluid helicity and  (in appropriate units) is identical to that of the axial current of quantum Dirac fermions in QED. The talk is based on the joint works with A. Abanov.

Time/Venue Tuesday, March 21 at 11:00 am in 325 Physics South and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Chris Waechtler / Joel Moore
Title Strongly correlated effects in quantum impurity systems with NRG-MPS methods.
Abstract The characterization of strongly-correlated effects in quantum impurity systems (QIS) is particularly challenging due to the infinite size of the environment and the inability of local correlators to capture the build-up of long-ranged entanglement in the system. Here, we devise an entanglement-based observable – the purity of the impurity – as a witness for the formation of strong correlations. In combination with the Numerical Renormalisation Group (NRG) and Variational Matrix Product States (VMPS) method for QIS, we showcase the utility of our scheme when exactly solving (i) all-electronic dot–cavity, (ii) all-electronic double-dot-cavity, and (iii) graphene quantum dot devices. In (i), we identify how the conducting dot-lead Kondo singlet is quenched by an insulating intra-impurity singlet formation. In (ii), we identify a cavity-mediated Ruderman–Kittel–Kasuya–Yosida-like interaction, where the two dots hybridizes forming a singlet. This dot-dot hybridization competes with a nonlocal Kondo effect, where the double-dot-cavity impurity forms a singlet spanning over the full device. In (iii), we identify SU(2) and SU(4) dot-leads Kondo effects, as well as valley-valley intradot singlet formation. With the generalization of the NRG-VMPS method to out-of-equilibrium scenarios, we advance an extensive framework for the study of many-body effects in QIS.

Time/Venue Friday, March 17 at 2 pm 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 Research Group
Title Topological pumping in optical quasiperiodic lattices
Abstract Cold atoms have become a rich platform for realising topological phases of matter, characterised by finite Chern numbers. In particular, quasiperiodic lattices can exhibit up to infinite Chern numbers, inherited from higher-dimensional parent Hamiltonians. Here, we design an experimentally realizable pumping protocol for optical quasicrystals that leads to bulk currents of topological nature, and harness it to measure the hierarchy of increasing Chern numbers. Expanding the configuration space representation developed in [1],  we offer a simple explanation of the fractal energy spectrum based on resonances between increasingly distant sites as well as a general way of understanding the adiabatic pumping of optical quasicrystals. The pumping protocol is illustrated in the case of the one-dimensional Aubry-André model, but is general and applicable for different quasiperiodic lattices, and higher dimensions.
In addition, I will present recent results about the experimental observation of the Bose glass phase in an optical quasicrystal with ultracold atoms [2], and some new preliminary results about quench dynamics between the Bose glass and the superfluid phases.

[1] E. Gottlob, U. Schneider, “Hubbard Models for Quasicrystalline Potentials” arXiv:2210.05691
[2]J-C Yu et al., “Observing the two-dimensional Bose glass in an optical quasicrystal” arXiv:2303.00737

Time/Venue Friday, March 17 at 10:00 am in CQCS conference room (101 Campbell Hall) and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore/Ehud Altman
Title Quantum criticality and topology under weak measurement and decoherence
 Abstract We discuss the effect of decoherence/weak measurement on strongly entangled quantum many-body systems. Decoherence or WM turn a pure state into a mixed state density matrix, and as was pointed out recently, the effect of decoherence can be mathematically mapped to a boundary problem. There are two classes of quantum many-body systems with well-known nontrivial boundary effects: the symmetry protected topological states, and quantum critical points. For the SPT states, the effect of decoherence is mathematically equivalent to turning on nonlocal interactions between physical topologically protected boundary states, and we design the so-called type-I and type-II “strange correlator” to diagnose the mixed state density matrix. We demonstrate that usually the type-II strange correlator still “remembers” the information of the SPT even after decoherence. We also consider 2+1d Wilson-Fisher fixed point under decoherence. The boundary effect of 2+1d quantum critical points have attracted a lot of theoretical and numerical efforts in the last few years, and we demonstrate that under decoherence we may observe some exotic physics such as the “extraordinary-log” physic that was proposed recently.

Time/Venue Thursday, March 16 at 11 am in Physics North 251 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Host Mike Zaletel
Title Quantum Spin Lakes: NISQ-Era Spin Liquids from Non-Equilibrium Dynamics
Abstract While many-body quantum systems can in principle host exotic
quantum spin liquid (QSL) states, realizing them as ground states in
experiments can be prohibitively difficult. In this talk, we show how
non-equilibrium dynamics can provide a streamlined route toward
creating QSLs. In particular, we show how a simple Hamiltonian
parameter sweep can dynamically project out condensed anyons from a
family of initial product states (e.g. dynamically  “un-Higgs”),
yielding a QSL-like state. We christen such states “quantum spin
lakes” which, while not thermodynamically large QSLs, enable their
study in NISQ-era quantum simulators. Indeed, we show that this
mechanism sheds light on recent experimental and numerical
observations of the dynamical state preparation of the ruby lattice
spin liquid in Rydberg atom arrays. Time permitting, we will discuss
how our theory motivates a tree tensor network-based numerical
tool—reliant on our theory—that quantitatively reproduces the
experimental data two orders of magnitude faster than conventional
brute-force simulation methods. Finally, we will highlight that even
spin liquid states that are unstable in equilibrium—namely, 2 + 1D
U(1) spin liquid states—can be robustly prepared by non-equilibrium
dynamics.

Time/Venue Wednesday, March 15 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 Joel Moore
Title Topological amorphous solids:  from flat-bands to chiral spin liquids
Abstract Topological materials are often predicted using crystal symmetries, although they don’t rely on them to exist. Our methodology thus excludes all amorphous materials, which are ubiquitous in technology and can display properties beyond those of crystals. In this talk I will present recent theoretical progress and experimental signatures of amorphous topological solids. I will discuss how to predict them efficiently, and their potential to host novel and controllable topological phenomena, featuring flat-bands and gapped chiral spin-liquids.

Time/Date Wednesday, March 15 at 11:00 am PST in Physics South 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
(Please note: we will enter 402 after a class ends at 2 pm.)
Host Joel Moore
Title Probing Band Topology in Theory & RIXS Experiment
Abstract Since the discovery of the topological band insulator, “topology” has become the most important information of the many-body ground states in condensed matter physics. Despite its importance, it is not always straightforward to detect the information both in theory and experiment. This problem becomes particularly serious when the strong correlation presents. In the first part of this talk, we will present our novel detection scheme for the topology, applicable even when there are significant correlations. We demonstrate the performance of our scheme on several prototypical topological insulators such as Chern insulator, Su-Schrieffer-Heeger chain, multipolar insulators, boundary-polarization model, and chiral hinge insulators[1-4]. In the second part of this talk, we will show that some of these topological insulators leave definitive fingerprints in resonant inelastic X-ray scatterings (RIXS), which enables the unambiguous experimental detection of the topological band indices [5].

Ref:
[1] Kang, Shiozaki, GYC, Phys. Rev. B (2019)
[2] Lee, GYC, Kang, Phys. Rev. B (2022)
[3] Park, Kim, GYC, Lee, Phys. Rev. Lett (2019) 
[4] Kang, Lee, GYC, Phys. Rev. Lett.  (2021) 
[5] Lee, Jin, Kang, Kim, GYC, Phys. Rev. B (2023)