Special Emergent Phenomena Seminar 290S/QM Seminar Speaker Yizhuang You (UCSD) Wednesday, October 6 at 11:00 am

Time/Venue Wednesday,  October 6 at 11 am ONLY VIA ZOOM (not in person) Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 Meeting ID: 995 2349 9113 Passcode: 600704
Host Mike Zaletel
Title
Kohn-Luttinger Superconductivity and Inter-Valley Coherence in Rhombohedral Trilayer Graphene
Abstract Motivated by recent experiments on ABC-stacked rhombohedral trilayer graphene (RTG) which observed spin-valley symmetry-breaking and superconductivity, we study instabilities of the RTG metallic state to symmetry breaking orders. We find that interactions select the inter-valley coherent order (IVC) as the preferred ordering channel over a wide range, whose theoretically determined phase boundaries agree well with experiments on both the hole and electron doped sides. The Fermi surfaces near van Hove singularities admit partial nesting between valleys, which promotes both inter-valley superconductivity and IVC fluctuations. We investigate the interplay between these fluctuations and the Hunds (intervalley spin) interaction using a renormalization group approach. For antiferromagnetic Hund’s coupling, intervalley pairing appears in the spin-singlet channel with enhanced T_c, that scales with the dimensionless coupling g as T_c\sim\exp(-1/\sqrt{g}) , compared to the standard \exp(-1/g) scaling. In its simplest form, this scenario assumes a sign change in the Hund’s coupling on increasing hole doping. On the other hand, the calculation incorporates breaking of the independent spin rotations between valleys from the start, and strongly selects spin singlet over spin triplet pairing, and naturally occurs in proximity to the IVC, consistent with observations.



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Emergent Phenomena Seminar 290S/QM Seminar Speaker Michal Papaj (UC Berkeley) Wednesday, September 29 at 2:00pm

Time/Venue Wednesday, September 29 at 2 pm in 402 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title
Aspects of quantum transport in topological phases of matter
Abstract In my talk, I will cover three separate areas related to quantum transport in topological phases of matter. In the first part, I will discuss a novel Hall effect in ballistic systems that preserve time-reversal, but break inversion symmetry. In the second part, I will demonstrate how disorder can lead to an effective non-Hermitian behavior of the Dirac surface states of topological crystalline insulators, resulting in an emergence of nodal arcs and tilting of the cone. Finally, I will show how the screening supercurrent induced by external magnetic field can lead to the appearance of Fermi surface in superconductors and how it can be used to create Majorana zero modes.

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Emergent Phenomena Seminar 290S/QM Seminar Speaker Chunxiao Liu (UC Berkeley) Wednesday, September 22 at 2:00pm

Time/Venue Wednesday, September 22 at 2 pm in 402 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title
 The monitored Sachdev-Ye-Kitaev model: measurement-induced phase transition and errors
Abstract I will talk about the monitored Brownian Sachdev-Ye-Kitaev (SYK) model without and with errors [arXiv:2104.08270 and arXiv:2106.09635]. Without errors, the model exhibits a measurement-induced phase transition that can be understood as a symmetry-breaking transition of an effective Z4 magnet in the replica space. The errors describe the loss of information about the measurement outcomes and, when present, can be mapped to an emergent magnetic field in the Z4 magnet. I will discuss two cases: when errors are applied during the non-unitary evolution, the symmetry is explicitly broken independent of the measurement rate, leading to a volume-law Renyi entropy. When errors are applied at the end of the evolution, the error-induced magnetic field only exists near the boundary of the magnet and can lead to a pinning transition of domain walls that corresponds to error threshold of the quantum code prepared by the non-unitary SYK dynamics.

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Emergent Phenomena Seminar 290S/QM Seminar Speaker, Samuel Garratt (UC Berkeley) Wednesday, September 15 at 2:00pm

Time/Venue Wednesday, September 15 at 2 pm in 402 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title
Local resonances and parametric level dynamics in the many-body localised phase
Abstract I will talk about resonances in the many-body localised (MBL) phase of disordered quantum spin chains, following arXiv:2107.12387. The standard theory of the MBL phase is based on the existence of local integrals of motion (LIOM), and eigenstates of the time evolution operator can be described as LIOM configurations. Resonances between LIOM configurations arise under variations of the disorder, and correspond to avoided level crossings. This parametric approach provides a way to describe resonances in terms of standard properties of non-resonant LIOM. As examples, I will show how to calculate correlations of the level density, and distributions of matrix elements of local observables.

Snacks to follow

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Emergent Phenomena Seminar 290S/QM Seminar Speaker, Assa Auerbach (KITP) Wednesday, September 8 at 2:00pm

Time/Venue Wednesday, September 8 at 2 pm in 402 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title
 The Hall effect: what moves in a metal or a superconductor?
Abstract The Hall resistivity has long been used to identify the ‘’moving parts’’ of electrical and thermal conductivity.
However, Boltzmann transport theory has failed to explain some intriguing ‘’Hall anomalies’’ in strongly correlated metals, superconductors, and large thermal Hall effect in insulators. Recent reformulations of the Kubo Hall conductivity and Hall coefficient and a linear response theory of flux flow in superconductors, helps us understand the Hall anomalies in these systems.

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Emergent Phenomena Seminar 290S/QM Seminar Speaker, Elizabeth Dresselhaus (UC Berkeley) Wednesday, September 1 at 2:00pm

Time/Venue Wednesday, September 1 at 2 pm in 402 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title
 Numerical Evidence for Marginal Scaling at the Integer Quantum Hall Transition
Abstract The integer quantum Hall transition (IQHT) is one of the most mysterious members of the family of Anderson transitions. Since the 1980s, the scaling behavior near the IQHT has been vigorously studied in experiments and numerical simulations. Despite all efforts, it is notoriously difficult to pin down the precise values of critical exponents, which seem to vary with model details and thus challenge the principle of universality. Recently, M. Zirnbauer [Nucl. Phys. B 941, 458 (2019)] has conjectured a conformal field theory for the transition, in which linear terms in the beta-functions vanish, leading to a very slow flow in the fixed point’s vicinity which we term marginal scaling. In this work, we provide numerical evidence for such a scenario by using extensive simulations of various network models of the IQHT at unprecedented length scales. At criticality, we show that the finite-size scaling of the disorder averaged longitudinal Landauer conductance is consistent with its recently predicted fixed-point value and a third-order expansion of RG beta functions. In the future, our numerical findings can be checked with analytical results from the conformal field theory. Away from criticality we describe a mechanism that could account for the emergence of an effective critical exponent ν_eff, which is necessarily dependent on the parameters of the model. We further support this idea by numerical determination of νeff in suitably chosen models.

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Special QM Seminar Speaker, Ramanjit Sohal (University of Illinois at Urbana-Champaign) Friday, January 8 at 2:00 pm

Time/Venue Friday, January 8 at 2:00 pm Pacific Time via this Zoom link
Host Mike Zaletel
Title Intertwined topological and broken symmetry orders in fractional Chern insulators
Abstract Recent advances in the engineering of topological band structures in solid-state and cold-atom systems have raised the possibility of realizing fractional Chern insulators (FCIs), lattice analogues of fractional quantum Hall (FQH) states formed in Chern bands. It is thus of experimental interest to understand how the underlying lattice can drive new physics in these states. To that end, we investigate FCI states arising from pairing of composite fermions, analogous to the Moore-Read FQH state, in the square-lattice Hofstadter model. We find that magnetic translation symmetry favors finite-momentum pairing which, as in pair-density wave theories of the cuprates, can lead to concomitant charge-density wave order. These novel paired FCI states are thus characterized by the simultaneous emergence – or intertwining – of topological and broken symmetry orders. We obtain mean-field phase diagrams exhibiting a rich array of these striped topological phases, establishing paired FCI states as ideal platforms to investigate the phenomenon of intertwined orders.

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Special QM Seminar Speaker Oles Shtanko (JQI and QuICS, UMD College Park) Friday, December 4 at 2:00 pm

Time/Venue Friday, December 4 at 2:00 pm Pacific Time via this Zoom link:
https://berkeley.zoom.us/j/93181359950
Host 
Norman Yao
Title Complexity Transitions in Open Many-Body Systems
Abstract Under the environment’s action, open quantum many-body systems are often transitioning into a classical regime described by a polynomial number of parameters and simulable by a classical algorithm. Understanding the conditions of this transition is crucial for quantum computing and classifying open quantum phases. In my talk, I will explore this phenomenon using two different measures: computational hardness and quantum entanglement. I will propose free fermions with quadratic jump operators as a promising candidate for the complexity transition. I also show that, under an anticoncentration assumption, the transition in entanglement in monitored random circuits maps to a disordered classical phase transition.

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Special QM Seminar Speaker, Leo Zhou (Harvard) Thursday, December 3 at 4:00 pm

Time/Venue Thursday, December 3 at 4:00 pm Pacific Time via this Zoom link
Host Norman Yao
Title Quantum Simulation and Optimization in Near-Term Quantum Computers
Abstract: Quantum simulation and optimization are two of the most promising applications of near-term quantum computers. In fact, there are already many experiments where analog simulations of quantum models are implemented to probe interesting physical phenomena. Some experiments have also begun testing performance of quantum algorithms for optimization.
In this two-part talk, I will first describe our study of the resource required to simulate a quantum Hamiltonian by another whose underlying interaction graph is simpler. We show a surprising result that unlike the classical setting, reducing the graph degree to a constant is impossible in general unless the interaction energy diverges with system size n. Instead, we develop a new construction where such degree-reduction becomes possible using O(poly(n)) interaction energy, which is exponentially better than what was known previously. In the second part, I will discuss our recent insights into the performance and mechanism of a general-purpose Quantum Approximate Optimization Algorithm (QAOA). We develop a parameter-optimization procedure for the QAOA that is exponentially more efficient than standard strategies and reveal a mechanism of the algorithm to exploit non-adiabatic operations. We also analyze the typical-case performance of the QAOA on the Sherrington-Kirkpatrick spin glass problem and find that it can outperform the classical semi-definite programming algorithm. Implementations in experiments are also discussed.
References:
[1] D. Aharonov and LZ, “Hamiltonian sparsification and gap-simulation,”arXiv:1804.11084.
[2] LZ, S.T. Wang, S. Choi, H. Pichler, and M.D. Lukin, “Quantum Approximate Optimization Algorithm: Performance, Mechanism, and Implementation on Near-Term Devices,”arXiv:1812.01041.
[3] E. Farhi, J. Goldstone, S. Gutmann, and LZ, “The Quantum Approximate Optimization Algorithm and the Sherrington-Kirkpatrick Model at Infinite Size,”arXiv:1910.08187.

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Special QM Seminar Speaker Daniel Ranard (Stanford) Wednesday, December 2 at 10:00 am

Time/Venue Wednesday, December 2, at 10:00 am Pacific Time via this Zoom link
Host Michael Zaletel
Title Emergent classicality in the dynamics of large systems
Abstract In a quantum measurement process, classical information about the measured system spreads through the environment. In contrast, quantum information about the system becomes inaccessible to local observers. In this talk, I will present a result about quantum channels indicating that an aspect of this phenomenon is completely general. We show that for any evolution of the system and environment, for everywhere in the environment excluding an O(1)-sized region we call the “quantum Markov blanket,” any locally accessible information about the system must be approximately classical, i.e. obtainable from some fixed measurement. The result strengthens the earlier result of arXiv:1310.8640, allowing applications to understanding and simulating many-body systems.
Based on: arXiv:2001.01507 with Xiao-Liang Qi

Posted in QM Seminar | Comments Off on Special QM Seminar Speaker Daniel Ranard (Stanford) Wednesday, December 2 at 10:00 am