Collective and single-particle excitations in 2D dipolar Bose gases

The Berezinskii-Kosterlitz-Thouless transition in 2D dipolar systems has been studied recently by path integral Monte Carlo (PIMC) simulations [A. Filinov et al., PRL 105, 070401 (2010)]. Here, we complement this analysis and study temperature-coupling strength dependence of the density (particle-hole) and single-particle (SP) excitation spectra both in superfluid and normal phases. The dynamic structure factor, S(q,omega), of the longitudinal excitations is rigorously reconstructed with full information on damping. The SP spectral function, A(q,omega), is worked out from the one-particle Matsubara Green's function. A stochastic optimization method is applied for reconstruction from imaginary times. In the superfluid regime sharp energy resonances are observed both in the density and SP excitations. The involved hybridization of both spectra is discussed. In contrast, in the normal phase, when there is no coupling, the density modes, beyond acoustic phonons, are significantly damped. Our results generalize previous zero temperature analyses based on variational many-body wavefunctions [F. Mazzanti et al., PRL 102, 110405 (2009), D. Hufnagl et al., PRL 107, 065303 (2011)], where the underlying physics of the excitation spectrum and the role of the condensate has not been addressed.Comment: 27 pages, 15 figures, 7 table

Introduction to Configuration Path Integral Monte Carlo

In low-temperature high-density plasmas quantum effects of the electrons are becoming increasingly important. This requires the development of new theoretical and computational tools. Quantum Monte Carlo methods are among the most successful approaches to first-principle simulations of many-body quantum systems. In this chapter we present a recently developed method---the configuration path integral Monte Carlo (CPIMC) method for moderately coupled, highly degenerate fermions at finite temperatures. It is based on the second quantization representation of the $N$-particle density operator in a basis of (anti-)symmetrized $N$-particle states (configurations of occupation numbers) and allows to tread arbitrary pair interactions in a continuous space. We give a detailed description of the method and discuss the application to electrons or, more generally, Coulomb-interacting fermions. As a test case we consider a few quantum particles in a one-dimensional harmonic trap. Depending on the coupling parameter (ratio of the interaction energy to kinetic energy), the method strongly reduces the sign problem as compared to direct path integral Monte Carlo (DPIMC) simulations in the regime of strong degeneracy which is of particular importance for dense matter in laser plasmas or compact stars. In order to provide a self-contained introduction, the chapter includes a short introduction to Metropolis Monte Carlo methods and the second quantization of quantum mechanics.Comment: chapter in book "Introduction to Complex Plasmas: Scientific Challenges and Technological Opportunities", Michael Bonitz, K. Becker, J. Lopez and H. Thomsen (Eds.) Springer Series "Atomic, Optical and Plasma Physics", vol. 82, Springer 2014, pp. 153-194 ISBN: 978-3-319-05436-0 (Print) 978-3-319-05437-7 (Online

Transmission time of wave packets through tunneling barriers

The transmission of wave packets through tunneling barriers is studied in detail by the method of quantum molecular dynamics. The distribution function of the times describing the arrival of a tunneling packet in front of and behind a barrier and the momentum distribution function of the packet are calculated. The behavior of the average coordinate of a packet, the average momentum, and their variances is investigated. It is found that under the barrier a part of the packet is reflected and a Gaussian barrier increases the average momentum of the transmitted packet and its variance in momentum space.Comment: 23 pages, 5 figure

Quantum corrections to the dynamics of interacting bosons: beyond the truncated Wigner approximation

We develop a consistent perturbation theory in quantum fluctuations around the classical evolution of a system of interacting bosons. The zero order approximation gives the classical Gross-Pitaevskii equations. In the next order we recover the truncated Wigner approximation, where the evolution is still classical but the initial conditions are distributed according to the Wigner transform of the initial density matrix. Further corrections can be characterized as quantum scattering events, which appear in the form of a nonlinear response of the observable to an infinitesimal displacement of the field along its classical evolution. At the end of the paper we give a few numerical examples to test the formalism.Comment: published versio

Explicit Solution of the Time Evolution of the Wigner Function

Previously, an explicit solution for the time evolution of the Wigner function was presented in terms of auxiliary phase space coordinates which obey simple equations that are analogous with, but not identical to, the classical equations of motion. They can be solved easily and their solutions can be utilized to construct the time evolution of the Wigner function. In this paper, the usefulness of this explicit solution is demonstrated by solving a numerical example in which the Wigner function has strong spatial and temporal variations as well as regions with negative values. It is found that the explicit solution gives a correct description of the time evolution of the Wigner function. We examine next the pseudoparticle approximation which uses classical trajectories to evolve the Wigner function. We find that the pseudoparticle approximation reproduces the general features of the time evolution, but there are deviations. We show how these deviations can be systematically reduced by including higher-order correction terms in powers of $\hbar^2$.Comment: 16 pages, in LaTex, invited talk presented at the Wigner Centennial Conference, Pecs, Hungary, July 8-12, 2002, to be published in the Journal of Optics B: Quantum and Classical Optics, June 200

Solid-state active media of tunable organic-compound lasers pumped with a laser. I. An XeCl laser

Оцифровано в НБ ТГ

Quantum Dot Version of Berry's Phase: Half-Integer Orbital Angular Momenta

We show that Berry's geometrical (topological) phase for circular quantum dots with an odd number of electrons is equal to \pi and that eigenvalues of the orbital angular momentum run over half-integer values. The non-zero value of the Berry's phase is provided by axial symmetry and two-dimensionality of the system. Its particular value (\pi) is fixed by the Pauli exclusion principle. Our conclusions agree with the experimental results of T. Schmidt {\it at el}, \PR B {\bf 51}, 5570 (1995), which can be considered as the first experimental evidence for the existence of a new realization of Berry's phase and half-integer values of the orbital angular momentum in a system of an odd number of electrons in circular quantum dots.Comment: 4 pages, 2 figure

Dynamical simulation of transport in one-dimensional quantum wires

Transport of single-channel spinless interacting fermions (Luttinger liquid) through a barrier has been studied by numerically exact quantum Monte Carlo methods. A novel stochastic integration over the real-time paths allows for direct computation of nonequilibrium conductance and noise properties. We have examined the low-temperature scaling of the conductance in the crossover region between a very weak and an almost insulating barrier.Comment: REVTex, 4 pages, 2 uuencoded figures (submitted to Phys. Rev. Lett.