362 research outputs found
Comment on "Creating artificial magnetic fields for cold atoms by photon-assisted tunneling" by Kolovsky A.R
We comment briefly on the scheme proposed in EPL 93, 20003 (2011) to produce
synthetic gauge fields by means of photon-assisted tunneling.Comment: 2 pages, EPL forma
Conductances in normal and normal-superconductor structures
We study theoretically electronic transport through a normal metal--
superconductor (NS) interface and show that more than one conductance may be
defined, depending on the pair of chemical potentials whose difference one
chooses to relate linearly to the current. We argue that the situation is
analogous to that found for purely normal transport, where different
conductance formulae can be invoked. We revisit the problem of the "right"
conductance formula in a simple language, and analyze its extension to the case
of mesoscopic superconductivity. The well-known result that the standard
conductance of a NS interface becomes 2 (in units of ) in the
transmissive limit, is viewed here in a different light. We show that it is not
directly related to the presence of Andreev reflection, but rather to a
particular choice of chemical potentials. This value of 2 is measurable because
only one single-contact resistance is involved in a typical experimental setup,
in contrast with the purely normal case, where two of them intervene. We
introduce an alternative NS conductance that diverges in the transmissive limit
due to the inability of Andreev reflection to generate a voltage drop. We
illustrate numerically how different choices of chemical potential can yield
widely differing I--V curves for a given NS interface.Comment: Minor changes have been introduced and several references have been
added, 12 pages, submitted to special issue of ``Superlattices and
Microstructures
Generation of uniform synthetic magnetic fields by split driving of an optical lattice
We describe a method to generate a synthetic gauge potential for ultracold
atoms held in an optical lattice. Our approach uses a time-periodic driving
potential based on two quickly alternating signals to engineer the appropriate
Aharonov-Bohm phases, and permits the simulation of a uniform tunable magnetic
field. We explicitly demonstrate that our split driving scheme reproduces the
behavior of a charged quantum particle in a magnetic field over the complete
range of field strengths, and obtain the Hofstadter butterfly band-structure
for the Floquet quasienergies at high fluxes.Comment: 5 pages, 3 eps figure
Coherent ratchets in driven Bose-Einstein condensates
We study the response of a Bose-Einstein condensate to an unbiased periodic
driving potential. By controlling the space and time symmetries of the driving
we show how a directed current can be induced, producing a coherent quantum
ratchet. Weak driving induces a regular behavior that is strongly governed by
the interparticle interaction. Breaking both space and time symmetries is
required to produce current flow. For strong driving the behavior becomes
chaotic. The resulting effective irreversibility renders the space asymmetry
sufficient to produce the ratchet effect, although the system is completely
coherent.Comment: 5 pages, 4 eps figures. Minor changes, this version to be published
in PR
Controlled generation of coherent matter-currents using a periodic driving field
We study the effect of a strong, oscillating driving field on the dynamics of
ultracold bosons held in an optical lattice. Modeling the system as a
Bose-Hubbard model, we show how the driving field can be used to produce and
maintain a coherent atomic current by controlling the phase of the intersite
tunneling processes. We investigate both the stroboscopic and time-averaged
behavior using Floquet theory, and demonstrate that this procedure provides a
stable and precise method of controlling coherent quantum systems.Comment: 4.1 pages, 4 eps figure
Variational approach to the excitonic phase transition in graphene
We analyze the Coulomb interacting problem in undoped graphene layers by
using an excitonic variational ansatz. By minimizing the energy, we derive a
gap equation which reproduces and extends known results. We show that a full
treatment of the exchange term, which includes the renormalization of the Fermi
velocity, tends to suppress the phase transition by increasing the critical
coupling at which the excitonic instability takes place.Comment: 4 page
Comment on ``Phase and Phase Diffusion of a Split Bose-Einstein Condensate''
Recently Javanainen and Wilkens [Phys. Rev. Lett. 78, 4675 (1997)] have
analysed an experiment in which an interacting Bose condensate, after being
allowed to form in a single potential well, is "cut" by splitting the well
adiabatically with a very high potential barrier, and estimate the rate at
which, following the cut, the two halves of the condensate lose the "memory" of
their relative phase. We argue that, by neglecting the effect of interactions
in the initial state before the separation, they have overestimated the rate of
phase randomization by a numerical factor which grows with the interaction
strength and with the slowness of the separation process.Comment: 2 pages, no figures, to appear in Phys. Rev. Let
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