1,274 research outputs found
Coulomb drag at zero temperature
We show that the Coulomb drag effect exhibits saturation at small
temperatures, when calculated to the third order in the interlayer
interactions. The zero-temperature transresistance is inversely proportional to
the third power of the dimensionless sheet conductance. The effect is therefore
the strongest in low mobility samples. This behavior should be contrasted with
the conventional (second order) prediction that the transresistance scales as a
certain power of temperature and is almost mobility-independent. The result
demonstrates that the zero-temperature drag is not an unambiguous signature of
a strongly-coupled state in double-layer systems.Comment: 4 pages, 2 figure
On steady-state currents through nano-devices: a scattering-states numerical renormalization group approach to open quantum systems
We propose a numerical renormalization group (NRG) approach to steady-state
currents through nano-devices. A discretization of the scattering-states
continuum ensures the correct boundary condition for an open quantum system. We
introduce two degenerate Wilson chains for current carrying left and
right-moving electrons reflecting time-reversal symmetry in the absence of a
finite bias . We employ the time-dependent NRG to evolve the known
steady-state density operator for a non-interacting junction into the density
operator of the fully interacting nano-device at finite bias. We calculate the
temperature dependent current as function of and applied external magnetic
field using a recently developed algorithm for non-equilibrium spectral
functions.Comment: 4 pages, 6 figure
Dilepton production from non-equilibrium hot hadronic matter
It is investigated under which conditions an adiabatic adaption of the
dynamic and spectral information of vector mesons to the changing medium in
heavy ion collisions, as assumed in schematic model calculations and
microscopic transport simulations, is a valid assumption. Therefore time
dependent medium modifications of low mass vector mesons are studied within a
non-equilibrium quantum field theoretical description. Timescales for the
adaption of the spectral properties are given and non-equilibrium dilepton
yields are calculated, leading to the result that memory effects are not
negligible for most scenarios.Comment: 6 pages, 4 figures, To appear in the proceedings of the 43rd
International Winter Meeting on Nuclear Physics, Bormio, Italy, 13 Mar - 20
Mar 200
Nonequilibrium mesoscopic conductance fluctuations
We investigate the amplitude of mesoscopic fluctuations of the differential
conductance of a metallic wire at arbitrary bias voltage V. For non-interacting
electrons, the variance increases with V. The asymptotic large-V
behavior is \sim V/V_c (where eV_c=D/L^2 is the Thouless energy),
in agreement with the earlier prediction by Larkin and Khmelnitskii. We find,
however, that this asymptotics has a very small numerical prefactor and sets in
at very large V/V_c only, which strongly complicates its experimental
observation. This high-voltage behavior is preceded by a crossover regime,
V/V_c \lesssim 30, where the conductance variance increases by a factor \sim 3
as compared to its value in the regime of universal conductance fluctuations
(i.e., at V->0). We further analyze the effect of dephasing due to the
electron-electron scattering on at high voltages. With the Coulomb
interaction taken into account, the amplitude of conductance fluctuations
becomes a non-monotonic function of V. Specifically, drops as 1/V
for voltages V >> gV_c, where g is the dimensionless conductance. In this
regime, the conductance fluctuations are dominated by quantum-coherent regions
of the wire adjacent to the reservoirs.Comment: 14 pages, 4 figures. Fig.2 and one more appendix added, accepted for
publication in PR
One-Electron Ionization of Multielectron Systems in Strong Nonresonant Laser Fields
We present a novel approach to calculating strong field ionization dynamics
of multielectron molecular targets. Adopting a multielectron wavefunction
ansatz based on field-free ab initio neutral and ionic multielectron states, a
set of coupled time-dependent single-particle Schroedinger equations describing
the neutral amplitude and continuum electron are constructed. These equations,
amenable to direct numerical solution or further analytical treatment, allow
one to study multielectron effects during strong field ionization, recollision,
and high harmonic generation. We apply the method to strong field ionization of
CO_2, and suggest the importance of intermediate core excitation to explain
previous failure of analytical models to reproduce experimental ionization
yields for this molecule.Comment: 25 pages, 6 figure
Condensation of Cavity Polaritons in a Disordered Environment
A model for direct two band excitons in a disordered quantum well coupled to
light in a cavity is investigated. In the limit in which the exciton density is
high, we assess the impact of weak `pair-breaking' disorder on the feasibility
of condensation of cavity polaritons. The mean-field phase diagram shows a
`lower density' region, where the condensate is dominated by electronic
excitations and where disorder tends to close the condensate and quench
coherence. Increasing the density of excitations in the system, partially due
to the screening of Coulomb interaction, the excitations contributing to the
condensate become mainly photon-like and coherence is reestablished for any
value of disorder. In contrast, in the photon dominated region of the phase
diagram, the energy gap of the quasi-particle spectrum still closes when the
disorder strength is increased. Above mean-field, thermal, quantum and
fluctuations induced by disorder are considered and the spectrum of the
collective excitations is evaluated. In particular, it is shown that the angle
resolved photon intensity exhibits an abrupt change in its behaviour, going
from the condensed to the non-condensed region.Comment: 17 pages, 9 eps figures; published versio
Conventional character of the BCS-BEC cross-over in ultra-cold gases of 40K
We use the standard fermionic and boson-fermion Hamiltonians to study the
BCS-BEC cross-over near the 202 G resonance in a two-component mixture of
fermionic 40K atoms employed in the experiment of C.A. Regal et al., Phys. Rev.
Lett. 92, 040403 (2004). Our mean-field analysis of many-body equilibrium
quantities shows virtually no differences between the predictions of the two
approaches, provided they are both implemented in a manner that properly
includes the effect of the highest excited bound state of the background
scattering potential, rather than just the magnetic-field dependence of the
scattering length. Consequently, we rule out the macroscopic occupation of the
molecular field as a mechanism behind the fermionic pair condensation and show
that the BCS-BEC cross-over in ultra-cold 40K gases can be analysed and
understood on the same basis as in the conventional systems of solid state
physics.Comment: 16 pages, 10 eps figures; final versio
Bose--Hubbard Models Coupled to Cavity Light Fields
Recent experiments on strongly coupled cavity quantum electrodynamics present
new directions in "matter-light" systems. Following on from our previous work
[Phys. Rev. Lett. 102, 135301 (2009)] we investigate Bose-Hubbard models
coupled to a cavity light field. We discuss the emergence of photoexcitations
or "polaritons" within the Mott phase, and obtain the complete variational
phase diagram. Exploiting connections to the super-radiance transition in the
Dicke model we discuss the nature of polariton condensation within this novel
state. Incorporating the effects of carrier superfluidity, we identify a
first-order transition between the superradiant Mott phase and the single
component atomic superfluid. The overall predictions of mean field theory are
in excellent agreement with exact diagonalization and we provide details of
superfluid fractions, density fluctuations, and finite size effects. We
highlight connections to recent work on coupled cavity arrays.Comment: 16 pages, 17 figure
Multiphoton Processes in Driven Mesoscopic Systems
We study the statistics of multi-photon absorption/emission processes in a
mesoscopic ring threaded by an harmonic time-dependent flux . For this
sake, we demonstrate a useful analogy between the Keldysh quantum kinetic
equation for the electrons distribution function and a Continuous Time Random
Walk in energy space with corrections due to interference effects. Studying the
probability to absorb/emit quanta per scattering event, we
explore the crossover between ultra-quantum/low-intensity limit and
quasi-classical/high-intensity regime, and the role of multiphoton processes in
driving it.Comment: 6 pages, 5 figures, extended versio
Spin-Orbit Scattering and Time-Reversal Symmetry: Detection of a Spin by Tunneling
We consider the possibility of detecting spin precession in a magnetic field
by nonequilibrium transport processes. We find that time reversal symmetry
imposes strong constraints on the problem. Suppose the tunneling occurs
directly between systems at two different chemical potentials, rather than
sequentially via a third system at an intermediate chemical potential. Then,
unless the magnetic fields are extremely strong or spin polarized electrons are
used, the periodic signal in the current results from beating together two
different precession frequencies, so that observing a signal near the Larmor
frequency in this case requires having some cluster with a factor close to
zero.Comment: 4 pages, 1 figur
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