Emergent Phenomena Seminar 290S/QM Seminar Speaker Thomas Scaffidi (U Toronto) Wednesday, December 8 at 2:00pm

Time/Venue Wednesday, December 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
 TBD
Abstract TBD

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Emergent Phenomena Seminar 290S/QM Seminar Speaker Binghai Yan (Weizmann Institute) Wednesday, October 27 at 2:00 pm

Time/Venue Wednesday, October 27 at 2 pm in 402 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman
Title
 Band structure topology and nonlinear effects in quantum materials
Abstract
 Topological quantum materials provide fascinating platforms to investigate band structure topology and explore its impacts on material properties. They also triggered renewed interest in the bulk photovoltaic effect (BPVE), where a homogenous material (instead of a p-n junction) rectifies light into a dc current. For example, nonmagnetic Weyl semimetals (e.g., TaAs and WTe2) are found to exhibit giant photocurrent and second harmonic generation. Recently, we proposed that magnetism can lead to unexpected, even stronger nonlinear effects [1]. For example, linearly polarized light can generate an injection current in a magnet [2], which is forbidden by symmetry in ordinary materials. Surprisingly, the magnetic indued photocurrent does not vanish even in the subgap regime (hν < Eg) [3]. In the dc electric field limit, related nonlinear transport remains and results in the unidirectional magnetoresistance, i.e., a diode controlled by magnetism or magnetic field. The diode rectification can be significantly enhanced by flat bands for example in magic angle twisted bilayer graphene [4,5].
References: [1] Phys. Rev. Research 2, 033100 (2020). [2] Nature Commun. 10,3783 (2019). [3] Phys. Rev. Lett. 125, 227401(2020). [4] The Innovation 2 (1), 100085(2021). [5] arXiv:2101.07539

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Emergent Phenomena Seminar 290S/QM Seminar Speaker Yaodong Lee (UCSB) Wednesday, October 20 at 2:00 pm

Time/Venue Wednesday, October 20 at 2 pm in 402 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman
Title Robust decoding in monitored dynamics of open quantum systems with Z_2 symmetry
Abstract We explore a class of “open” quantum circuit models with local decoherence (“noise”) and local projective measurements, each respecting a global Z_2 symmetry. The model supports a spin glass phase where the Z_2 symmetry is spontaneously broken, a paramagnetic phase characterized by a divergent susceptibility, and an intermediate “trivial” phase. Within the spin glass phase the circuit dynamics can be interpreted as a quantum repetition code, with each stabilizer of the code measured stochastically at a finite rate, and the decoherences as effective bit-flip errors. Motivated by the geometry of the spin glass phase, we devise a novel decoding algorithm for recovering an arbitrary initial qubit state in the code space, assuming knowledge of the history of the measurement outcomes, and the ability of performing local Pauli measurements and gates on the final state. For a circuit with L^d qubits running for time T, the time needed to execute the decoder scales as O(L^d T) (with dimensionality d). With this simple decoder in hand, we find that the information of the initial encoded qubit state can be retained (and then recovered) for a time logarithmic in L for a 1d circuit, and for a time at least linear in L in 2d below a finite error threshold. We also outline a connection of the simple decoder to correlation functions in a random bond Ising model, which leads to an improved decoder that has a finite threshold in both 1d and 2d, both for T linear in L. The improved decoder has a time complexity O(L^{d+1} T), thus preferable as compared to existing ones based on a “perfect matching” of error defects.

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Emergent Phenomena Seminar 290S/QM Seminar Speaker Xianzhe Chen (University of Tokyo) Wednesday, October 13 at 2:00 pm

Time/Venue Wednesday, October 13 at 2 pm in 402 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 Meeting ID: 995 2349 9113 Passcode: 600704Host Ehud Altman
Title
Magnetic spin Hall effect in collinear antiferromagnet
Abstract The discovery of the spin Hall effect enabled the efficient generation and manipulation of the spin current. More recently, the magnetic spin Hall effect was observed in non-collinear antiferromagnets, where the spin conservation is broken due to the non-collinear spin configuration. This provides a unique opportunity to control the spin current and relevant device performance with controllable magnetization. Here, we report a magnetic spin Hall effect in a collinear antiferromagnet, Mn2Au. The spin currents are generated at two spin sublattices with broken spatial symmetry, and the antiparallel antiferromagnetic moments play an important role. Therefore, we term this effect the ‘antiferromagnetic spin Hall effect’. The out-of-plane spins from the antiferromagnetic spin Hall effect are favourable for the efficient switching of perpendicular magnetized devices, which is required for high-density applications. The antiferromagnetic spin Hall effect adds another twist to the atomic-level control of spin currents via the antiferromagnetic spin structure.

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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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