arXiv subjects

Nikolay Prokof'ev

Publications and source records attributed to Nikolay Prokof'ev.

At least 37 records · Page 2Linked to original sources

arXiv2020

Space- and time-crystallization effects in multicomponent superfluids

We observe that space- and time-crystallization effects in multicomponent superfluids---while having the same physical origin and mathematical description as in the single-component case---are conceptually much more straightforward. Specifically, the values of the temporal and spatial periods are absolute rather than relative, and the broken translation symmetry in space and/or time can be revealed with experiments involving only one equilibrium sample. We discuss two realistic setups---one with cold atoms and another one with bilayer superconductors---for observation of space and time crystallization in two-component counterflow superfluids.

cond-mat.stat-mech↗

arXiv2018

Restoring a smooth function from its noisy integrals

Numerical (and experimental) data analysis often requires the restoration of a smooth function from a set of sampled integrals over finite bins. We present the bin hierarchy method that efficiently computes the maximally smooth function from the sampled integrals using essentially all the information contained in the data. We perform extensive tests with different classes of functions and levels of data quality, including Monte Carlo data suffering from a severe sign problem and physical data for the Green's function of the Fröhlich polaron.

stat.OT↗

arXiv2017

Implementation of the Bin Hierarchy Method for restoring a smooth function from a sampled histogram

We present $\texttt{BHM}$, a tool for restoring a smooth function from a sampled histogram using the bin hierarchy method. The theoretical background of the method is presented in [arXiv:1707.07625]. The code automatically generates a smooth polynomial spline with the minimal acceptable number of knots from the input data. It works universally for any sufficiently regular shaped distribution and any level of data quality, requiring almost no external parameter specification. It is particularly useful for large-scale numerical data analysis. This paper explains the details of the implementation and the use of the program.

stat.OT↗

arXiv2016

Dark Continuum in the Spectral Function of the Resonant Fermi Polaron

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 wavevector, $a\lesssim 1/k_{\rm F}$, 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.

cond-mat.quant-gas↗

arXiv2016

Stability of Dirac Liquids with Strong Coulomb Interaction

We develop and apply the Diagrammatic Monte Carlo technique to address the problem of stability of the Dirac liquid state (in a graphene type system) against 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 RPA treatments, with diametrically opposite conclusions. Our calculations aim at the approximations-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 $ln(L) \sim 40$), 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.

cond-mat.str-el↗

arXiv2016

Numerical analytic continuation: Answers to well-posed questions

We formulate the problem of numerical analytic continuation in a way that lets us draw meaningful conclusions about properties of the spectral function based solely on the input data. Apart from ensuring consistency with the input data (within their error bars) and the {\it a priori} and {\it a posteriori} (conditional) constraints, it is crucial to reliably characterize the accuracy---or even ambiguity---of the output. We explain how these challenges can be met with two approaches: stochastic optimization with consistent constraints and the modified maximum entropy method. We perform illustrative tests for spectra with a double-peak structure, where we critically examine which spectral properties are accessible and which ones are lost. For an important practical example, we apply our protocol to the Fermi polaron problem.

cond-mat.other↗

arXiv2016

Coulomb and electron-phonon interactions in metals

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 non-perturbative system were relying on uncontrolled approximations in the absence of small parameters. In this work, 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 non-adiabatic regimes.

cond-mat.str-el↗

arXiv2016

Spin-Ice State of the Quantum Heisenberg Antiferromagnet on the Pyrochlore Lattice

We study the low-temperature physics of the $SU(2)$-symmetric spin-$1/2$ Heisenberg antiferromagnet on a pyrochlore lattice and find fingerprint evidence for the thermal spin-ice state in this frustrated quantum magnet. Our conclusions are based on the results of bold diagrammatic Monte Carlo simulations, with good convergence of the skeleton series down to the temperature $T/J=1/6$. The identification of the spin-ice state is done through a remarkably accurate microscopic correspondence for the static structure factor between the quantum Heisenberg, classical Heisenberg, and Ising models at all accessible temperatures, and the characteristic bowtie pattern with pinch points observed at $T/J=1/6$. The dynamic structure factor at real frequencies (obtained by the analytic continuation of numerical data) is consistent with diffusive spinon dynamics at the pinch points.

cond-mat.str-el↗

arXiv2016

Shifted-Action Expansion and Applicability of Dressed Diagrammatic Schemes

While bare diagrammatic series are merely Taylor expansions in powers of interaction strength, dressed diagrammatic series, built on fully or partially dressed lines and vertices, are usually constructed by reordering the bare diagrams, which is an a priori unjustified manipulation, and can even lead to convergence to an unphysical result [Kozik, Ferrero and Georges, PRL 114, 156402 (2015)]. Here we show that for a broad class of partially dressed diagrammatic schemes, there exists an action $S^{(ξ)}$ depending analytically on an auxiliary complex parameter $ξ$, such that the Taylor expansion in $ξ$ of correlation functions reproduces the original diagrammatic series. The resulting applicability conditions are similar to the bare case. For fully dressed skeleton diagrammatics, analyticity of $S^{(ξ)}$ is not granted, and we formulate a sufficient condition for converging to the correct result.

cond-mat.stat-mech↗

arXiv2016

Quantum Walk in Degenerate Spin Environments

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 ultra-cold atoms in an optical lattice. At the short-to-intermediate timescale that we can access using a stochastic-series-type numeric scheme, the propagation turns out to be distinctly non-diffusive with the proba- bility 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.

cond-mat.quant-gas↗

arXiv2016

Superfluid--Insulator Transition in Strongly Disordered One-dimensional Systems

We present an asymptotically exact renormalization-group theory of the superfluid--insulator transition in one-dimensional disordered systems, with emphasis on an accurate description of the interplay between the Giamarchi--Schulz (instanton--anti-instanton) and weak-link (scratched-XY) criticalities. Combining the theory with extensive quantum Monte Carlo simulations allows us to shed new light on the ground-state phase diagram of the one-dimensional disordered Bose-Hubbard model at unit filling.

cond-mat.str-el↗

arXiv2015

The Massive Goldstone (Higgs) mode in two-dimensional ultracold atomic lattice systems

We discuss how to reveal the massive Goldstone mode, often referred to as the Higgs amplitude mode, near the Superfluid-to-Insulator quantum critical point (QCP) in a system of two-dimensional ultracold bosonic atoms in optical lattices. The spectral function of the amplitude response is obtained by analytic continuation of the kinetic energy correlation function calculated by Monte Carlo methods. Our results enable a direct comparison with the recent experiment [M. Endres, T. Fukuhara, D. Pekker, M. Cheneau, P. Schauß, C. Gross, E. Demler, S. Kuhr, and I. Bloch, Nature 487, 454-458 (2012)], and demonstrate a good agreement for temperature shifts induced by lattice modulation. Based on our numerical analysis, we formulate the necessary conditions in terms of homogeneity, detuning from the QCP and temperature in order to reveal the massive Goldstone resonance peak in spectral functions experimentally. We also propose to apply a local modulation at the trap center to overcome the inhomogeneous broadening caused by the parabolic trap confinement.

cond-mat.quant-gas↗

arXiv2015

p-wave Superfluidity by Spin-Nematic Fermi Surface Deformation

We study attractively interacting fermions on a square lattice with dispersion relations exhibiting strong spin-dependent anisotropy. The resulting Fermi surface mismatch suppresses the s-wave BCS-type instability, clearing the way for unconventional types of order. Unbiased sampling of the Feynman diagrammatic series using Diagrammatic Monte Carlo methods reveals a rich phase diagram in the regime of intermediate coupling strength. Instead of a proposed Cooper-pair Bose metal phase [A. E. Feiguin and M. P. A. Fisher, Phys. Rev. Lett. 103, 025303 (2009)] we find an incommensurate density wave at strong anisotropy and two different p-wave superfluid states with unconventional symmetry at intermediate anisotropy.

cond-mat.quant-gas↗

arXiv2014

Diagrammatic Monte Carlo method for many-polaron problems

We introduce the first bold diagrammatic Monte Carlo approach to deal with polaron problems at finite density non-perturbatively, i.e., by including vertex corrections to high orders. Using Holstein model on a square lattice as a prototypical example, we demonstrate that our method is capable of providing accurate results in the thermodynamic limit in all regimes from renormalized Fermi-liquid to single polarons, across the non-adiabatic region where Fermi and Debye energies are of the same order of magnitude. By accounting for vertex corrections the accuracy of theoretical description is increased by orders of magnitude relative to the lowest-order self-consistent Born approximation employed in most studies. We also find that for electron-phonon coupling typical for real materials, the quasiparticle effective mass increases and the quasiparticle residue decreases with increasing the system density.

cond-mat.str-el↗

arXiv2014

Quantum Plasticity and Supersolid Response in Helium-4

We argue that the three key phenomena recently observed in solid $^4$He---mass supertransport, anomalous isochoric compressibility (syringe effect), and giant plasticity---are closely linked to each other through the physics of an interconnected network of tilted quantum-rough dislocations. As immediate implications of this connection several predictions follow: In the absence of $^3$He impurities, the syringe effect and giant plasticity persist down to $T=0$; the dynamical low-frequency syringe and giant-plasticity responses are dispersionless; and similarly to giant plasticity but without direct relationship to the supertransport along the dislocation cores, $^3$He impurities should suppress the syringe effect partially or completely at appropriately low temperatures.

cond-mat.other↗

arXiv2014

Critical Exponents of the Superfluid-Bose Glass Transition in Three-Dimensions

Recent experimental and numerical studies of the critical-temperature exponent $ϕ$ for the superfluid-Bose glass universality in three-dimensional systems report strong violations of the key quantum critical relation, $ϕ=νz$, where $z$ and $ν$ are the dynamic and correlation length exponents, respectively, and question the fundamental concepts underlying quantum critical phenomena. Using Monte Carlo simulations of the disordered Bose-Hubbard model, we demonstrate that previous work on the superfluid-to-normal fluid transition-temperature dependence on chemical potential (or magnetic field, in spin systems), $T_c \propto (μ-μ_c)^ϕ$, was misinterpreting transient behavior on approach to the fluctuation region with the genuine critical law. When the model parameters are modified to have a broad quantum critical region, simulations of both quantum and classical models reveal that the $ϕ=νz$ law [with $ϕ=2.7(2)$, $z=3$, and $ν= 0.88(5)$] holds true, resolving the $ϕ$-exponent "crisis".

cond-mat.str-el↗

arXiv2014

Universal Conductivity in a Two-Dimensional Superfluid-to-Insulator Quantum Critical System

We compute the universal conductivity of the (2+1)-dimensional XY universality class, which is realized for a superfluid-to-Mott insulator quantum phase transition at constant density. Based on large-scale Monte Carlo simulations of the classical (2+1)-dimensional $J$-current model and the two-dimensional Bose-Hubbard model, we can precisely determine the conductivity on the quantum critical plateau, $σ(\infty)=0.359(4)σ_Q$ with $σ_Q$ the conductivity quantum. The universal conductivity curve is the standard example with the lowest number of components where the bottoms-up AdS/CFT correspondence from string theory can be tested and made to use [R. C. Myers, S. Sachdev, and A. Singh, Phys. Rev. D83, 066017 (2011)]. For the first time, the shape of the $σ(iω_n)- σ(\infty)$ function in the Matsubara representation is accurate enough for a conclusive comparison and establishes the particle-like nature of charge transport. We find that the holographic gauge/gravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation.

cond-mat.str-el↗

arXiv2013

Universal Properties of the Higgs Resonance in (2+1)-Dimensional U(1) Critical Systems

We present spectral functions for the magnitude squared of the order parameter in the scaling limit of the two-dimensional superfluid to Mott insulator quantum phase transition at constant density, which has emergent particle-hole symmetry and Lorentz invariance. The universal functions for the superfluid, Mott insulator, and normal liquid phases reveal a low-frequency resonance which is relatively sharp and is followed by a damped oscillation (in the first two phases only) before saturating to the quantum critical plateau. The counterintuitive resonance feature in the insulating and normal phases calls for deeper understanding of collective modes in the strongly coupled (2+1)-dimensional relativistic field theory. Our results are derived from analytically continued correlation functions obtained from path-integral Monte Carlo simulations of the Bose-Hubbard model.

cond-mat.stat-mech↗