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Knap, Michael

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Knap

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Knap, Michael

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2013 - 20198

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Author's Publications: search.filters.isAuthorOfPublication.b12bd6eb-d83d-48c0-88a2-8e3806faf9cf

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Now showing 1 - 8 of 8
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    String Patterns in the Doped Hubbard Model
    (American Association for the Advancement of Science (AAAS), 2019-07-18) Ji, Geoffrey; Bohrdt, Annabelle; Xu, Muqing; Knap, Michael; Demler, Eugene; Greiner, Markus; Greif, Daniel; Chiu, Christie; Grusdt, Fabian
    Understanding strongly correlated quantum many-body states is one of the most difficult challenges in modern physics. For example, there remain fundamental open questions on the phase diagram of the Hubbard model, which describes strongly correlated electrons in solids. In this work, we realize the Hubbard Hamiltonian and search for specific patterns within the individual images of many realizations of strongly correlated ultracold fermions in an optical lattice. Upon doping a cold-atom antiferromagnet, we find consistency with geometric strings, entities that may explain the relationship between hole motion and spin order, in both pattern-based and conventional observables. Our results demonstrate the potential for pattern recognition to provide key insights into cold-atom quantum many-body systems.
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    Probing Real-Space and Time-Resolved Correlation Functions with Many-Body Ramsey Interferometry
    (American Physical Society (APS), 2013) Knap, Michael; Kantian, Adrian; Giamarchi, Thierry; Bloch, Immanuel; Lukin, Mikhail; Demler, Eugene
    We propose to use Ramsey interferometry and single-site addressability, available in synthetic matter such as cold atoms or trapped ions, to measure real-space and time-resolved spin correlation functions. These correlation functions directly probe the excitations of the system, which makes it possible to characterize the underlying many-body states. Moreover, they contain valuable information about phase transitions where they exhibit scale invariance. We also discuss experimental imperfections and show that a spin-echo protocol can be used to cancel slow fluctuations in the magnetic field. We explicitly consider examples of the two-dimensional, antiferromagnetic Heisenberg model and the one-dimensional, long-range transverse field Ising model to illustrate the technique.
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    Quantum Flutter: Signatures and Robustness
    (American Physical Society (APS), 2014) Knap, Michael; Mathy, Charles J. M.; Ganahl, Martin; Zvonarev, Mikhail; Demler, Eugene
    We investigate the motion of an impurity particle injected with finite velocity into an interacting one-dimensional quantum gas. Using large-scale numerical simulations based on matrix product states, we observe and quantitatively analyze long-lived oscillations of the impurity momentum around a nonzero saturation value, called quantum flutter. We show that the quantum flutter frequency is equal to the energy difference between two branches of collective excitations of the model. We propose an explanation of the finite saturation momentum of the impurity based on the properties of the edge of the excitation spectrum. Our results indicate that quantum flutter exists away from integrability and provide parameter regions in which it could be observed in experiments with ultracold atoms using currently available technology.
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    Dissipative Dynamics of a Driven Quantum Spin Coupled to a Bath of Ultracold Fermions
    (American Physical Society (APS), 2013) Knap, Michael; Abanin, Dmitry; Demler, Eugene
    We explore the dynamics and the steady state of a driven quantum spin coupled to a bath of fermions, which can be realized with a strongly imbalanced mixture of ultracold atoms using currently available experimental tools. Radio-frequency driving can be used to induce tunneling between the spin states. The Rabi oscillations are modified due to the coupling of the quantum spin to the environment, which causes frequency renormalization and damping. The spin-bath coupling can be widely tuned by adjusting the scattering length through a Feshbach resonance. When the scattering potential creates a bound state, by tuning the driving frequency it is possible to populate either the ground state, in which the bound state is filled, or a metastable state in which the bound state is empty. In the latter case, we predict an emergent inversion of the steady-state magnetization. Our work shows that different regimes of dissipative dynamics can be explored with a quantum spin coupled to a bath of ultracold fermions.
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    Interferometric Probes of Many-Body Localization
    (American Physical Society (APS), 2014) Serbyn, M.; Knap, Michael; Gopalakrishnan, Sarang; Papić, Z.; Yao, Norman; Laumann, C. R.; Abanin, Dmitry; Lukin, Mikhail; Demler, Eugene
    We propose a method for detecting many-body localization (MBL) in disordered spin systems. The method involves pulsed coherent spin manipulations that probe the dephasing of a given spin due to its entanglement with a set of distant spins. It allows one to distinguish the MBL phase from a noninteracting localized phase and a delocalized phase. In particular, we show that for a properly chosen pulse sequence the MBL phase exhibits a characteristic power-law decay reflecting its slow growth of entanglement. We find that this power-law decay is robust with respect to thermal and disorder averaging, provide numerical simulations supporting our results, and discuss possible experimental realizations in solid-state and cold-atom systems.
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    Far-from-Equilibrium Spin Transport in Heisenberg Quantum Magnets
    (American Physical Society (APS), 2014) Hild, Sebastian; Fukuhara, Takeshi; Schauß, Peter; Zeiher, Johannes; Knap, Michael; Demler, Eugene; Bloch, Immanuel; Gross, Christian
    We study experimentally the far-from-equilibrium dynamics in ferromagnetic Heisenberg quantum magnets realized with ultracold atoms in an optical lattice. After controlled imprinting of a spin spiral pattern with an adjustable wave vector, we measure the decay of the initial spin correlations through single-site resolved detection. On the experimentally accessible time scale of several exchange times, we find a profound dependence of the decay rate on the wave vector. In one-dimensional systems, we observe diffusionlike spin transport with a dimensionless diffusion coefficient of 0.22(1). We show how this behavior emerges from the microscopic properties of the closed quantum system. In contrast to the one-dimensional case, our transport measurements for two-dimensional Heisenberg systems indicate anomalous superdiffusion.
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    Many-Body Localization in Dipolar Systems
    (American Physical Society (APS), 2014) Yao, Norman; Laumann, C. R.; Gopalakrishnan, Sarang; Knap, Michael; Müller, M.; Demler, Eugene; Lukin, Mikhail
    Systems of strongly interacting dipoles offer an attractive platform to study many-body localized phases, owing to their long coherence times and strong interactions. We explore conditions under which such localized phases persist in the presence of power-law interactions and supplement our analytic treatment with numerical evidence of localized states in one dimension. We propose and analyze several experimental systems that can be used to observe and probe such states, including ultracold polar molecules and solid-state magnetic spin impurities.
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    Transport in Two-Dimensional Disordered Semimetals
    (American Physical Society (APS), 2014) Knap, Michael; Sau, Jay D.; Halperin, Bertrand; Demler, Eugene
    We theoretically study transport in two-dimensional semimetals. Typically, electron and hole puddles emerge in the transport layer of these systems due to smooth fluctuations in the potential. We calculate the electric response of the electron-hole liquid subject to zero and finite perpendicular magnetic fields using an effective medium approximation and a complementary mapping on resistor networks. In the presence of smooth disorder and in the limit of a weak electron-hole recombination rate, we find for small but finite overlap of the electron and hole bands an abrupt upturn in resistivity when lowering the temperature but no divergence at zero temperature. We discuss how this behavior is relevant for several experimental realizations and introduce a simple physical explanation for this effect.