QM Seminar Speaker Tibor Rakovsky (TUM), Wednesday, October 16 at 2:00 pm in LeConte 402

Time/Venue Wednesday, October 16 at 2:00 pm in LeConte 402
Host Ehud Altman
Title tba
Abstract tba

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Special QM Seminar Speaker Alexios Michailidis (IST Austria) Friday, October 11 at 11 am in LeConte 325

Time/Venue: Friday, October 11, 11 am, LeConte 325
Host: Ehud Altman
Title: TBA
Abstract: TBA

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QM Seminar Speaker Luca Chirolli (CNR-Nano/UC Berkeley), Wednesday, October 2, 2 pm in LeConte 402

Time/Venue: Wednesday, October 2, 2 pm in LeConte 402
Host: Joel Moore
Title: TBA
Abstract: TBA

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Special QM Seminar Speaker Eyal Leviathan (Weizmann Institute), Friday, September 20, 11 am in LeConte 325

Time/Venue Friday, September 20, 11 am in LeConte 325
Host Ehud Altman
Title Bridging parton-construction and coupled-wire approaches to quantum spin liquids
Abstract Quantum spin systems that avoid symmetry-breaking order, e.g., due to geometric frustration, can instead form quantum spin liquids. These phases, characterized by fractional excitations and emergent gauge fields, are thought to be realized in an increasing number of quasi-two-dimensional materials. In my talk, I describe how a wide range of gapped, as well as gapless, spin liquids can be accessed by a generalization of the ‘coupled-wire’ technique. Our approach bridges between the microscopically oriented ‘coupled-wire’ approach and the popular field-theoretical parton constructions. In particular, we present a framework to construct microscopic Hamiltonians that realize exotic, as well as conventional, spin phases. Moreover, our method provides transparent access to subtle questions regarding the emergent gauge field, such as confinement.

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QM Seminar Speaker Roger Mong (University of Pittsburgh), Wednesday, September 18, 2 pm in LeConte 402

Time/Venue: Wednesday, September 18, 2 pm in LeConte 402
Host: Joel Moore
Title: Emergent Mode and Bound State in Single-component Fermionic Systems
Abstract: From the ubiquitous baryonic matter to superconductivity, studies of matter comprised of bound states of fermions span many branches of physics. Here, we study bound states form among fermions of the same species (in contrast to baryons and conventional Cooper pairing), in a one dimensional setting. Specifically, we study the formation of bound states in a one-dimensional, single-component Fermi chain with attractive interactions. The phase diagram, computed from DMRG, shows not only a superfluid of paired fermions (pair phase) and a liquid of fermion triplets (trion phase), but also a phase with two gapless modes. We argue that the latter phase consists of a charged and an emergent neutral mode–and that all other bound-state phases (single, pair, trion, etc.) are descendants of this two-mode-phase.

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QM Seminar Speaker Vlad Kozii (MIT/LBNL/UCB), Wednesday, September 11, 2 pm in LeConte 402

Time/Venue Wednesday, September 11 at 2 pm in LeConte 402
Host Joel Moore
Title Superconductivity in ultralow-density materials
Abstract The experimental observation of superconductivity in doped semimetals and semiconductors, where the Fermi energy is comparable to or smaller than the characteristic phonon frequencies, is not captured by the conventional theory.
We propose a mechanism for superconductivity in ultralow-density  three-dimensional Dirac materials based on the proximity to a ferroelectric quantum critical point. We derive a low-energy theory that takes into account both the strong Coulomb interaction and the direct coupling between the electrons and the soft phonon modes. We show that the Coulomb repulsion is strongly screened by the lattice polarization near the critical point even in the case of vanishing carrier density, i.e., without retardation. Using a renormalization group analysis, we further find that the system generically flows towards strong electron-phonon coupling. We then apply our results to study superconductivity in the low-density limit. We find strong enhancement of the transition temperature upon approaching the quantum critical point. The talk is mainly based on a recent work in collaboration with Jonathan Ruhman and Zhen Bi.

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QM Seminar Speaker Phillip Dumitrescu (Flat Iron Institute), Wednesday, September 4 at 2:00 pm in LeConte 402

Time/Venue Wednesday, September 4 at 2:00 pm in LeConte 402 
Host Ehud Altman
Title Long-lived interacting phases of matter and multiple time-translation symmetries in quasiperiodically driven systems
Abstract The discrete time-translational symmetry (TTS) of a periodically-driven (Floquet) system allows for the existence of novel, nonequilibrium interacting phases of matter. A canonical example is the discrete time crystal (DTC), a phase characterized by the spontaneous breaking of this TTS. We show that the presence of multiple time-translational symmetries, realized by quasiperiodically driving a system with two or more incommensurate frequencies, leads to a panoply of novel phases of matter protected by these TTSes not realizable in a static or Floquet setting. We develop new techniques to address the stability of these interacting phases in the prethermal regime.

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QM Seminar Speaker Ferdinand Evers (Regensburg University), Wednesday, August 7 at 2:00 pm in LeConte 402

Time/Venue: 2 pm in LeCone 402
Host: Ehud Altman
Title: Hydrodynamics and Many-Body Localization — Numerical Studies of the Spin-1/2 Heisenberg chain
Abstract: 

Numerical investigations of model systems, such as the XXZ-model and its equivalent fermion representation, provide a major source of insight into the intricate interplay of Anderson localization and many-body interactions. Such studies pose major computational challenges, however; the available system sizes and observation times are quite limited. As a consequence, finite size effects are pronounced and predictions for the long-time large-distance dynamics are challenging
Considering the spin-autocorrelation function as the main observable, the talk will provide an overview with respect to finite size effects and transient dynamics. The first part of the talk will report a numerical study of the clean Heisenberg chain. We compute the scaling function at the isotropic point and confirm a previous conjecture concerning KPZ-scaling. The second part of the talk is devoted to many-body localisation. We observe very large finite-size effects in the thermal and, perhaps unexpectedly, also in the localised phase. While the computational parameter window allows to study transient phenomena, the asymptotic regime appears to be very hard to reach. Our observations are qualitatively similar for fully random and quasi-periodic potentials, which appears to be hard to reconcile with traditional scenarios of dynamics based on rare-region (Griffiths) physics.
The talk is based on common work with Felix Weiner, Bruno Lang and Soumya Bera.

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Special QM Seminar Speaker Yuri Minoguchi (Vienna University of Technology), Tuesday, July 16 at 11:00 am in 325 LeConte

Time/Venue:  Tuesday, July 16 at 11:00 am in 325 LeConte
Host: Norman Yao with Soonwon Choi
Title: Environment Induced Rabi Oscillations in the Boson-Boson Model
Abstract: We analyze the strong-coupling dynamics of a driven harmonic oscillator whose energy is mod- ulated by a continuum of other bosonic modes. This type of system-bath interaction appears, for example, in optomechanical or equivalent circuit QED setups, where the frequency of a confined photonic mode depends linearly on a fluctuating boundary. Compared to the canonical spin-boson model, where coupling to bath modes only leads to decoherence, the role of the environment in such systems is more complex, since it also provides the only source of nonlinearity. We show that even for an unstructured bath, these environment-induced nonlinearities can dominate over decoherence pro- cesses resulting in Rabi oscillations and the formation of highly non-classical states. These findings provide important insights into the non-Markovian dynamics of higher-dimensional open quantum systems and for realizing few-photon optical nonlinearities through strong interactions with a bath.

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QM Seminar speaker Arzhang Ardavan (The Clarendon Laboratory, Oxford), Wednesday, July 10

Time/Venue:  Wednesday, July 10 at 2:00 pm in LeConte 402
Title: Electrical control of quantum spins
Abstract: Magnetic fields are challenging to localize to short length scales because their sources are electrical currents. Conversely, electric fields can be applied using electrostatic gates on scales limited only by lithography. This has important consequences for the design of spin-based information technologies: while the Zeeman interaction with a magnetic field provides a convenient tool for manipulating spins, it is difficult to achieve local control of individual spins on the length scale anticipated for useful quantum technologies. This motivates the study of electric field control of spin Hamiltonians [1].Mn2+ defects in ZnO exhibit extremely long spin coherence times and a small axial zero-field splitting. Their environment is inversion-symmetry-broken, and the zero-field splitting shows a linear dependence on an externally-applied electric field. This control over the spin Hamiltonian offers a route to controlling the phase of superpositions of spin states using d.c. electric field pulses, and to driving spin transitions using microwave electric fields [2].
Experiments on Mn defects in ZnO provide insights into how to achieve manipulation of individual spins on surfaces using a scanning tunnelling microscope. A high-frequency voltage applied to the tip can drive electron spin resonance in Fe atoms on MgO surfaces via modulation of the crystal field experienced by the Fe atom [3].
It has been proposed theoretically that frustrated exchange-coupled molecular clusters might offer sensitivity to externally-applied electric fields [4]. Experiments on an antiferromagnetically-coupled Cu3 compound reveal a small linear electric field effect. A comparable sensitivity is exhibited by the heterometallic = 1 antiferromagnetic ring Cr7Mn, but no effect is found for the S = 1/2 Cr7Ni [5].

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