arXiv subjects

Nikolay Prokof'ev

Publications and source records attributed to Nikolay Prokof'ev.

77 records · Page 5Linked to original sources

arXiv2001

Critical Point of a Weakly Interacting Two-Dimensional Bose Gas

We study the Berezinskii-Kosterlitz-Thouless transition in a weakly interacting 2D quantum Bose gas using the concept of universality and numerical simulations of the classical $|ψ|^4$-model on a lattice. The critical density and chemical potential are given by relations $n_c=(mT/2π\hbar^2) \ln(ξ\hbar^2/ mU)$ and $μ_c=(mTU/π\hbar^2) \ln(ξ_μ \hbar^2/ mU)$, where $T$ is the temperature, $m$ is the mass, and $U$ is the effective interaction. The dimensionless constant $ξ= 380 \pm 3$ is very large and thus any quantitative analysis of the experimental data crucially depends on its value. For $ξ_μ$ our result is $ξ_μ = 13.2 \pm 0.4 $. We also report the study of the quasi-condensate correlations at the critical point.

cond-mat↗

arXiv2001

Critical Temperature Shift in Weakly Interacting Bose Gas

With a high-performance Monte Carlo algorithm we study the interaction-induced shift of the critical point in weakly interacting three-dimensional $|ψ|^4$-theory (which includes quantum Bose gas). In terms of critical density, $n_c$, mass, $m$, interaction, $U$, and temperature, $T$, this shift is universal: $Δn_c(T) = - C m^3 T^2 U$, the constant $C$ found to be equal to $0.0140 \pm 0.0005$. For quantum Bose gas with the scattering length $a$ this implies $ΔT_c/T_c = C_0 a n^{1/3}$, with $C_0=1.29 \pm 0.05$.

cond-mat↗

arXiv2001

Worm algorithms for classical statistical models

We show that high-temperature expansions may serve as a basis for the novel approach to efficient Monte Carlo simulations. "Worm" algorithms utilize the idea of updating closed path configurations (produced by high-temperature expansions) through the motion of end points of a disconnected path. An amazing result is that local, Metropolis-type schemes may have dynamical critical exponents close to zero (i.e., their efficiency is comparable to the best cluster methods). We demonstrate this by calculating finite size scaling of the autocorrelation time for various (six) universality classes.

cond-mat↗

arXiv2000

Spin bath-mediated decoherence in superconductors

We consider a SQUID tunneling between 2 nearly degenerate flux states. Decoherence caused by paramagnetic and nuclear spins in the low-$T$ limit is shown to be much stronger than that from electronic excitations. The decoherence time $τ_ϕ$ is determined by the linewidth $E_o$ of spin bath states, which can be reduced by a correct choice of ring geometry and isotopic purification. $E_o$ can be measured in either field sweep or microwave absorption experiments, allowing both a test of the theory and design control.

cond-mat↗

arXiv2000

Theory of the spin bath

The quantum dynamics of mesoscopic or macroscopic systems is always complicated by their coupling to many "environmental" modes.At low T these environmental effects are dominated by localised modes, such as nuclear and paramagnetic spins, and defects (which also dominate the entropy and specific heat). This environment, at low energies, maps onto a "spin bath" model. This contrasts with "oscillator bath" models (originated by Feynman and Vernon) which describe {\it delocalised} environmental modes such as electrons, phonons, photons, magnons, etc. One cannot in general map a spin bath to an oscillator bath (or vice-versa); they constitute distinct "universality classes" of quantum environment. We show how the mapping to spin bath models is made, and then discuss several examples in detail, including moving particles, magnetic solitons, nanomagnets, and SQUIDs, coupled to nuclear and paramagnetic spin environments. We show how to average over spin bath modes, using an operator instanton technique, to find the system dynamics, and give analytic results for the correlation functions, under various conditions. We then describe the application of this theory to magnetic and superconducting systems.Particular attention is given to recent work on tunneling magnetic macromolecules, where the role of the nuclear spin bath in controlling the tunneling is very clear; we also discuss other magnetic systems in the quantum regime, and the influence of nuclear and paramagnetic spins on flux dynamics in SQUIDs.

cond-mat↗