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  • 1. Carlstrom, Johan
    et al.
    Prokofiev, Nikolay
    KTH, School of Engineering Sciences (SCI), Theoretical Physics. University of Massachusetts, United States; Kurchatov Institute, Russian Federation.
    Svistunov, Boris
    Quantum Walk in Degenerate Spin Environments2016In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 116, no 24, article id 247202Article in journal (Refereed)
    Abstract [en]

    We study the propagation of a hole in degenerate (paramagnetic) spin environments. This canonical problem has important connections to a number of physical systems, and is perfectly suited for experimental realization with ultracold atoms in an optical lattice. At the short-to-intermediate time scale that we can access using a stochastic-series-type numeric scheme, the propagation turns out to be distinctly nondiffusive with the probability distribution featuring minima in both space and time due to quantum interference, yet the motion is not ballistic, except at the beginning. We discuss possible scenarios for long-term evolution that could be explored with an unprecedented degree of detail in experiments with single-atom resolved imaging.

  • 2. Goulko, Olga
    et al.
    Mishchenko, Andrey S.
    Prokofiev, Nikolay
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Svistunov, Boris
    Dark continuum in the spectral function of the resonant Fermi polaron2016In: PHYSICAL REVIEW A, ISSN 2469-9926, Vol. 94, no 5, article id 051605Article in journal (Refereed)
    Abstract [en]

    We present controlled numerical results for the ground-state spectral function of the resonant Fermi polaron in three dimensions. We establish the existence of a "dark continuum"-a region of anomalously low spectral weight between the narrow polaron peak and the rest of the spectral continuum. The dark continuum develops when the s-wave scattering length is of the order of the inverse Fermi wave vector, a less than or similar to 1/k(F), i.e., in the absence of a small interaction-related parameter when the spectral weight is not expected to feature a near-perfect gap structure after the polaron peak.

  • 3. Pollet, L.
    et al.
    Kiselev, M. N.
    Prokof'Ev, Nikolay V
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Svistunov, B. V.
    Grassmannization of classical models2016In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 18, no 11, article id 113025Article in journal (Refereed)
    Abstract [en]

    Applying Feynman diagrammatics to non-fermionic strongly correlated models with local constraints might seem generically impossible for two separate reasons: (i) the necessity to have a Gaussian (non-interacting) limit on top of which the perturbative diagrammatic expansion is generated by Wick's theorem, and (ii) Dyson's collapse argument implying that the expansion in powers of coupling constant is divergent. We show that for arbitrary classical lattice models both problems can be solved/circumvented by reformulating the high-temperature expansion (more generally, any discrete representation of the model) in terms of Grassmann integrals. Discrete variables residing on either links, plaquettes, or sites of the lattice are associated with the Grassmann variables in such a way that the partition function (as well as all correlation functions) of the original system and its Grassmann-field counterpart are identical. The expansion of the latter around its Gaussian point generates Feynman diagrams. Our work paves the way for studying lattice gauge theories by treating bosonic and fermionic degrees of freedom on equal footing. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

  • 4. Tupitsyn, I. S.
    et al.
    Prokofiev, Nikolay
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Stability of Dirac Liquids with Strong Coulomb Interaction2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 118, no 2, article id 026403Article in journal (Refereed)
    Abstract [en]

    We develop and apply the diagrammatic Monte Carlo technique to address the problem of the stability of the Dirac liquid state (in a graphene-type system) against the strong long-range part of the Coulomb interaction. So far, all attempts to deal with this problem in the field-theoretical framework were limited either to perturbative or random phase approximation and functional renormalization group treatments, with diametrically opposite conclusions. Our calculations aim at the approximation-free solution with controlled accuracy by computing vertex corrections from higher-order skeleton diagrams and establishing the renormalization group flow of the effective Coulomb coupling constant. We unambiguously show that with increasing the system size L (up to ln(L)∼40), the coupling constant always flows towards zero; i.e., the two-dimensional Dirac liquid is an asymptotically free T=0 state with divergent Fermi velocity.

  • 5. Tupitsyn, Igor S.
    et al.
    Mishchenko, Andrey S.
    Nagaosa, Naoto
    Prokof'ev, Nikolay
    KTH, School of Engineering Sciences (SCI), Theoretical Physics. University of Massachusetts, United States.
    Coulomb and electron-phonon interactions in metals2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 15, article id 155145Article in journal (Refereed)
    Abstract [en]

    An accurate and consistent theory of phonons in metals requires that all long-range Coulomb interactions between charged particles (electrons and ions) be treated on equal footing. So far, all attempts to deal with this nonperturbative system were relying on uncontrolled approximations in the absence of small parameters. In this paper, we develop the diagrammatic Monte Carlo approach for a two-component Coulomb system that obtains the solution to this fundamental problem in an approximation-free way by computing vertex corrections from higher-order skeleton graphs. The feasibility of the method is demonstrated by calculating the spectrum of longitudinal acoustic phonons in a simple cubic lattice, determining their sound velocity, and obtaining the phonon spectral densities by analytic continuation of the Matsubara-Green's functions. Final results are checked against the lowest-order fully self-consistent GW approximation in both adiabatic and nonadiabatic regimes.

  • 6. Van Houcke, Kris
    et al.
    Tupitsyn, Igor S.
    Mishchenko, Andrey S.
    Prokof'ev, Nikolay V.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics. Kurchatov Institute, Russia.
    Dielectric function and thermodynamic properties of jellium in the GW approximation2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 19, article id 195131Article in journal (Refereed)
    Abstract [en]

    The fully self-consistent GW approximation is an established method for electronic structure calculations. Its most serious deficiency is known to be an incorrect prediction of the dielectric response. In this work, we examine the GW approximation for the homogeneous electron gas and find that problems with the dielectric response are drastically improved by enforcing the particle-number conservation law in the polarization function. We also find that previously reported data for the ground-state energy contradict each other well outside of reported error bounds. Some of these results created a false impression of how accurate the fully self-consistent GW approximation is. Our two independent implementations of the GW method agree with the data plotted in X.-Z. Yan [Phys. Rev. E 84, 016706 (2011)], thus confirming only that data set. We also present values for other key Fermi-liquid properties.

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