42 research outputs found
The decay of excited He from Stochastic Density-Functional Theory: a quantum measurement theory interpretation
Recently, time-dependent current-density functional theory has been extended
to include the dynamical interaction of quantum systems with external
environments [Phys. Rev. Lett. {\bf 98}, 226403 (2007)]. Here we show that such
a theory allows us to study a fundamentally important class of phenomena
previously inaccessible by standard density-functional methods: the decay of
excited systems. As an example we study the decay of an ensemble of excited He
atoms, and discuss these results in the context of quantum measurement theory.Comment: 4 pages, 2 figure
Self-consistent fragmented excited states of trapped condensates
Self-consistent excited states of condensates are solutions of the
Gross-Pitaevskii (GP) equation and have been amply discussed in the literature
and related to experiments. By introducing a more general mean-field which
includes the GP one as a special case, we find a new class of self-consistent
excited states. In these states macroscopic numbers of bosons reside in
different one-particle functions, i.e., the states are fragmented. Still, a
single chemical potential is associated with the condensate. A numerical
example is presented, illustrating that the energies of the new, fragmented,
states are much lower than those of the GP excited states, and that they are
stable to variations of the particle number and shape of the trap potential.Comment: (11 pages 2 figures, submitted to PRL
Simulation of a stationary dark soliton in a trapped zero-temperature Bose-Einstein condensate
We discuss a computational mechanism for the generation of a stationary dark
soliton, or black soliton, in a trapped Bose-Einstein condensate using the
Gross-Pitaevskii (GP) equation for both attractive and repulsive interaction.
It is demonstrated that the black soliton with a "notch" in the probability
density with a zero at the minimum is a stationary eigenstate of the GP
equation and can be efficiently generated numerically as a nonlinear
continuation of the first vibrational excitation of the GP equation in both
attractive and repulsive cases in one and three dimensions for pure harmonic as
well as harmonic plus optical-lattice traps. We also demonstrate the stability
of this scheme under different perturbing forces.Comment: 7 pages, 15 ps figures, Final version accepted in J Low Temp Phy
Stability of excited states of a Bose-Einstein condensate in an anharmonic trap
We analyze the stability of non-ground nonlinear states of a Bose-Einstein
condensate in the mean field limit in effectively 1D (``cigar-shape'') traps
for various types of confining potentials. We find that nonlinear states
become, in general, more stable when switching from a harmonic potential to an
anharmonic one. We discuss the relation between this fact and the specifics of
the harmonic potential which has an equidistant spectrum
Stochastic time-dependent current-density functional theory: a functional theory of open quantum systems
The dynamics of a many-body system coupled to an external environment
represents a fundamentally important problem. To this class of open quantum
systems pertains the study of energy transport and dissipation, dephasing,
quantum measurement and quantum information theory, phase transitions driven by
dissipative effects, etc. Here, we discuss in detail an extension of
time-dependent current-density-functional theory (TDCDFT), we named stochastic
TDCDFT [Phys. Rev. Lett. {\bf 98}, 226403 (2007)], that allows the description
of such problems from a microscopic point of view. We discuss the assumptions
of the theory, its relation to a density matrix formalism, and the limitations
of the latter in the present context. In addition, we describe a numerically
convenient way to solve the corresponding equations of motion, and apply this
theory to the dynamics of a 1D gas of excited bosons confined in a harmonic
potential and in contact with an external bath.Comment: 17 pages, 7 figures, RevTex4; few typos corrected, a figure modifie
Hydrodynamic approach to transport and quantum turbulence in nanoscale conductors
The description of electron-electron interactions in transport problems is
both analytically and numerically difficult. Here we show that a much simpler
description of electron transport in the presence of interactions can be
achieved in nanoscale systems. In particular, we show that the electron flow in
nanoscale conductors can be described by Navier-Stokes type of equations with
an effective electron viscosity, i.e., on a par with the dynamics of a viscous
and compressible classical fluid. By using this hydrodynamic approach we derive
the conditions for the transition from laminar to turbulent flow in nanoscale
systems and discuss possible experimental tests of our predictions.Comment: 9 pages, 1 figure, Late
Order parameter for the dynamical phase transition in Bose-Einstein condensates with topological modes
In a trapped Bose-Einstein condensate, subject to the action of an
alternating external field, coherent topological modes can be resonantly
excited. Depending on the amplitude of the external field and detuning
parameter, there are two principally different regimes of motion, with mode
locking and without it. The change of the dynamic regime corresponds to a
dynamic phase transition. This transition can be characterized by an effective
order parameter defined as the difference between fractional mode populations
averaged over the temporal period of oscillations. The behavior of this order
parameter, as a function of detuning, pumping amplitude, and atomic
interactions is carefully analyzed. A special attention is payed to numerical
calculations for the realistic case of a quadrupole exciting field and the
system parameters accessible in current experiments
Resonant Generation of Topological Modes in Trapped Bose Gases
Trapped Bose atoms cooled down to temperatures below the Bose-Einstein
condensation temperature are considered. Stationary solutions to the
Gross-Pitaevskii equation (GPE) define the topological coherent modes,
representing nonground-state Bose-Einstein condensates. These modes can be
generated by means of alternating fields whose frequencies are in resonance
with the transition frequencies between two collective energy levels
corresponding to two different topological modes. The theory of resonant
generation of these modes is generalized in several aspects: Multiple-mode
formation is described; a shape-conservation criterion is derived, imposing
restrictions on the admissible spatial dependence of resonant fields; evolution
equations for the case of three coherent modes are investigated; the complete
stability analysis is accomplished; the effects of harmonic generation and
parametric conversion for the topological coherent modes are predicted. All
considerations are realized both by employing approximate analytical methods as
well as by numerically solving the GPE. Numerical solutions confirm all
conclusions following from analytical methods.Comment: One reference modifie
Shot noise suppression at room temperature in atomic-scale Au junctions
Shot noise encodes additional information not directly inferable from simple
electronic transport measurements. Previous measurements in atomic-scale metal
junctions at cryogenic temperatures have shown suppression of the shot noise at
particular conductance values. This suppression demonstrates that transport in
these structures proceeds via discrete quantum channels. Using a high frequency
technique, we simultaneously acquire noise data and conductance histograms in
Au junctions at room temperature and ambient conditions. We observe noise
suppression at up to three conductance quanta, with possible indications of
current-induced local heating and noise in the contact region at high
biases. These measurements demonstrate the quantum character of transport at
room temperature at the atomic scale. This technique provides an additional
tool for studying dissipation and correlations in nanodevices.Comment: 15 pages, 4 figures + supporting information (6 pages, 6 figures
Regulating entanglement production in multitrap Bose-Einstein condensates
A system of traps is considered, each containing a large number of
Bose-condensed atoms. This ensemble of traps is subject to the action of an
external modulating field generating nonequilibrium nonground-state
condensates. When the frequency of the modulating field is in resonance with
the transition frequency between two different topological coherent modes, each
trap becomes an analog of a finite-level resonant atom. Similarly to the case
of atoms in an electromagnetic resonant field, one can create entanglement
between atomic traps subject to a common resonant modulating field generating
higher coherent modes in each of the traps. A method is suggested for
regulating entanglement production in such a system of multitrap and multimode
Bose-Einstein condensates coupled through a common resonant modulating field.
Several regimes of evolutional entanglement production, regulated by
manipulating the external field, are illustrated by numerical calculations.Comment: Latex file, 3 figure