arXiv subjects

Boris Svistunov

Publications and source records attributed to Boris Svistunov.

At least 37 records · Page 2Linked to original sources

arXiv2021

Thermal and structural properties of topological defects in solid Helium-4: Strain induced martensitic transformation from hcp to fc orthorhombic lattice

Several experimental studies have reported thermally activated behavior of the mechanical response of solid Helium-4 which does not fit into the model of thermally activated Frenkel pairs in an ideal crystal. The purpose of the present work is to investigate how structural topological defects modify the response. Using quantum Monte Carlo Worm Algorithm, we study temperature dependence of fluctuations of the total number of particles in samples of hcp solid Helium-4 containing several types of dislocations. Such fluctuations can be described by the thermal activation law with the activation energy dependent strongly on the type of the dislocation and the crystal density. The extracted values cover a range from about 1K to 20K. It is also found that annihilation of a jog-antijog pair can produce a local region of the solid characterized by the orthorhombic symmetry. This serendipitous observation suggests that hcp solid Helium-4 can undergo a displacive phase transition into the orthorhombic crystal under uniaxial stress of about 10-15%

cond-mat.other↗

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

Quantized Magnetic Flux and the Magneto-halon Effect in a Critical Superconductor

Employing the standard worldline-vortex mapping, we conclude that at the critical temperature, superconductors demonstrate the magneto-halon effect with respect to the quantized net magnetic flux generated by a solenoid inserted into the system. The effect is a direct counterpart of the recently revealed halon effect in terms of the quantized particle charge of a static impurity in the two-dimensional U(1) quantum-critical environment. The flux-loop model (a.k.a. frozen lattice superconductor) with a quasi-solenoid perturbation proves to be the model of choice for qualitative and quantitative description of both the halon and the magneto-halon effects.

cond-mat.supr-con↗

arXiv2018

The halon: a quasiparticle featuring critical charge fractionalization

The halon is a special critical state of an impurity in a quantum-critical environment. The hallmark of the halon physics is that a well-defined integer charge gets fractionalized into two parts: a microscopic core with half-integer charge and a critically large halo carrying a complementary charge of $\pm 1/2$. The halon phenomenon emerges when the impurity--environment interaction is fine-tuned to the vicinity of a boundary quantum critical point (BQCP), at which the energies of two quasiparticle states with adjacent integer charges approach each other. The universality class of such BQCP is captured by a model of pseudo-spin-$1/2$ impurity coupled to the quantum-critical environment, in such a way that the rotational symmetry in the pseudo-spin $xy$-plane is respected, with a small local "magnetic" field along the pseudo-spin $z$-axis playing the role of control parameter driving the system away from the BQCP. On the approach to BQCP, the half-integer projection of the pseudo-spin on its $z$-axis gets delocalized into a halo of critically divergent radius, capturing the essence of the phenomenon of charge fractionalization. With large-scale Monte Carlo simulations, we confirm the existence of halons---and quantify their universal features---in O(2) and O(3) quantum critical systems.

cond-mat.quant-gas↗

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

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

Trapping Centers at the Superfluid-Mott-insulator Criticality: Transition between Charge-quantized States

Under the conditions of superfluid-Mott-insulator criticality in two dimensions, the trapping centers--i.e., local potential wells and bumps--are generically characterized by an integer charge corresponding to the number of trapped particles (if positive) or holes (if negative). Varying the strength of the center leads to a transition between two competing ground states with charges differing by $\pm 1$. The hallmark of the transition scenario is a splitting of the number density distortion, $δn(r)$, into a half-integer core and a large halo carrying the complementary charge of $\pm 1/2$. The sign of the halo changes across the transition and the radius of the halo, $r_0$, diverges on the approach to the critical strength of the center, $V = V_c$, by the law $r_0 \propto |V-V_c|^{-\tildeν}$, with $\tildeν \approx 2.33(5)$.

cond-mat.quant-gas↗

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

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

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

Rotational response of superconductors: magneto-rotational isomorphism and rotation-induced vortex lattice

The analysis of nonclassical rotational response of superfluids and superconductors was performed by Onsager (in 1949) \cite{Onsager} and London (in 1950) \cite{London} and crucially advanced by Feynman (in 1955) \cite{Feynman}. It was established that, in thermodynamic limit, neutral superfluids rotate by forming---without any threshold---a vortex lattice. In contrast, the rotation of superconductors at angular frequency ${\bf Ω}$---supported by uniform magnetic field ${\bf B}_L\propto {\bf Ω}$ due to surface currents---is of the rigid-body type (London Law). Here we show that, neglecting the centrifugal effects, the behavior of a rotating superconductor is identical to that of a superconductor placed in a uniform fictitious external magnetic filed $\tilde{\bf H}=- {\bf B}_L$. In particular, the isomorphism immediately implies the existence of two critical rotational frequencies in type-2 superconductors.

cond-mat.supr-con↗

arXiv2013

Spectral Analysis by the Method of Consistent Constraints

Two major challenges of numeric analytic continuation---restoring the spectral density, $s(ω)$, from the corresponding Matsubara correlator, $g(τ)$---are (i) producing the most smooth/featureless answer for $s(ω)$ without compromising the error bars on $g(τ)$ and (ii) quantifying possible deviations of the produced result from the actual answer. We introduce the method of consistent constraints that solves both problems.

cond-mat.stat-mech↗

arXiv2012

Bold Diagrammatic Monte Carlo technique for frustrated spin systems

Using fermionic representation of spin degrees of freedom within the Popov-Fedotov approach we develop an algorithm for Monte Carlo sampling of skeleton Feynman diagrams for Heisenberg type models. Our scheme works without modifications for any dimension of space, lattice geometry, and interaction range, i.e. it is suitable for dealing with frustrated magnetic systems at finite temperature. As a practical application we compute uniform magnetic susceptibility of the antiferromagnetic Heisenberg model on the triangular lattice and compare our results with the best available high-temperature expansions. We also report results for the momentum-dependence of the static magnetic susceptibility throughout the Brillouin zone.

cond-mat.str-el↗