25,149 research outputs found
Luttinger liquid behavior in weakly disordered quantum wires
We have measured the temperature dependence of the conductance in long
V-groove quantum wires (QWRs) fabricated in GaAs/AlGaAs heterostructures. Our
data is consistent with recent theories developed within the framework of the
Luttinger liquid model, in the limit of weakly disordered wires. We show that
for the relatively small amount of disorder in our QWRs, the value of the
interaction parameter g is g=0.66, which is the expected value for GaAs.
However, samples with a higher level of disorder show conductance with stronger
temperature dependence, which does not allow their treatment in the framework
of perturbation theory. Trying to fit such data with perturbation-theory models
leads inevitably to wrong (lower) values of g.Comment: 4 pages, 4 figure
The phase-dependent linear conductance of a superconducting quantum point contact
The exact expression for the phase-dependent linear conductance of a weakly
damped superconducting quantum point contact is obtained. The calculation is
performed by summing up the complete perturbative series in the coupling
between the electrodes. The failure of any finite order perturbative expansion
in the limit of small voltage and small quasi-particle damping is analyzed in
detail. In the low transmission regime this nonperturbative calculation yields
a result which is at variance with standard tunnel theory. Our result predicts
the correct sign of the quasi-particle pair interference term and exhibits an
unusual phase-dependence at low temperatures in qualitative agreement with the
available experimental data.Comment: 12 pages (revtex) + 1 postscript figure. Submitted to Phys. Rev. Let
Semiclassical theory of spin-polarized shot noise in mesoscopic diffusive conductors
We study fluctuations of spin-polarized currents in a three-terminal
spin-valve system consisting of a diffusive normal metal wire connected by
tunnel junctions to three ferromagnetic terminals. Based on a spin-dependent
Boltzmann-Langevin equation, we develop a semiclassical theory of charge and
spin currents and the correlations of the currents fluctuations. In the three
terminal system, we show that current fluctuations are strongly affected by the
spin-flip scattering in the normal metal and the spin polarizations of the
terminals, which may point in different directions. We analyze the dependence
of the shot noise and the cross-correlations on the spin-flip scattering rate
in the full range of the spin polarizations and for different magnetic
configurations. Our result demonstrate that noise measurements in
multi-terminal devices allow to determine the spin-flip scattering rate by
changing the polarizations of ferromagnetic terminals.Comment: 12 pages, 5 figure
Outlaw Community Innovations
Recent studies of outlaw communities provide qualitative evidence of their existence and the organisation of the underlying innovation processes. We provide descriptive results from a large scale survey of two online outlaw communities focussing on Microsoft's XBox. In line with previous findings, we identify two types of participants in outlaw communities - user innovators and adopters. Based on 2,256 responses, we find that users modify their XBox mainly to be able to increase the set of available functions of their XBox. Users are also motivated to modify their XBox for the sake of having fun and to conduct pirate behaviour. Finally, the results from our survey suggest that user innovators are largely intrinsically motivated by fun and the intellectual stimulation of writing code for homebrew software
The Multidimensional Media Literacy & Engagement Framework: A Tool for Fostering Informed Civic Participation
Teaching media literacy helps students interpret media messages accurately and supports their informed civic participation
Center of mass and relative motion in time dependent density functional theory
It is shown that the exchange-correlation part of the action functional
in time-dependent density functional theory , where
is the time-dependent density, is invariant under the
transformation to an accelerated frame of reference , where is an arbitrary
function of time. This invariance implies that the exchange-correlation
potential in the Kohn-Sham equation transforms in the following manner:
. Some of the
approximate formulas that have been proposed for satisfy this exact
transformation property, others do not. Those which transform in the correct
manner automatically satisfy the ``harmonic potential theorem", i.e. the
separation of the center of mass motion for a system of interacting particles
in the presence of a harmonic external potential. A general method to generate
functionals which possess the correct symmetry is proposed
Fractional Fokker-Planck Equation for Fractal Media
We consider the fractional generalizations of equation that defines the
medium mass. We prove that the fractional integrals can be used to describe the
media with noninteger mass dimensions. Using fractional integrals, we derive
the fractional generalization of the Chapman-Kolmogorov equation (Smolukhovski
equation). In this paper fractional Fokker-Planck equation for fractal media is
derived from the fractional Chapman-Kolmogorov equation. Using the Fourier
transform, we get the Fokker-Planck-Zaslavsky equations that have fractional
coordinate derivatives. The Fokker-Planck equation for the fractal media is an
equation with fractional derivatives in the dual space.Comment: 17 page
A density-functional approach to fermionization in the 1D Bose gas
A time-dependent Kohn-Sham scheme for 1D bosons with contact interaction is
derived based on a model of spinor fermions. This model is specifically
designed for the study of the strong interaction regime close to the Tonks gas.
It allows us to treat the transition from the strongly-interacting
Tonks-Girardeau to the weakly-interacting quasicondensate regime and provides
an intuitive picture of the extent of fermionization in the system. An
adiabatic local-density approximation is devised for the study of
time-dependent processes. This scheme is shown to yield not only accurate
ground-state properties but also overall features of the elementary excitation
spectrum, which is described exactly in the Tonks-gas limit.Comment: 15 pages, 3 figures, misprints (of published version) correcte
Semi-classical spectrum of integrable systems in a magnetic field
The quantum dynamics of an electron in a uniform magnetic field is studied
for geometries corresponding to integrable cases. We obtain the uniform
asymptotic approximation of the WKB energies and wavefunctions for the
semi-infinite plane and the disc. These analytical solutions are shown to be in
excellent agreement with the numerical results obtained from the Schrodinger
equations even for the lowest energy states. The classically exact notions of
bulk and edge states are followed to their semi-classical limit, when the
uniform approximation provides the connection between bulk and edge.Comment: 17 pages, Revtex, 6 figure
Optical excitations in a non-ideal Bose gas
Optical excitations in a Bose gas are demonstrated to be very sensitive to
many-body effects. At low temperature the momentum relaxation is provided by
momentum exchange collisions, rather than by elastic collisions. A collective
excitation mode forms, which in a Boltzmann gas is manifest in a collision
shift and dramatic narrowing of spectral lines.
In the BEC state, each spectral line splits into two components. The doubling
of the optical excitations results from the physics analogous to that of the
second sound. We present a theory of the line doubling, and calculate the
oscillator strengths and linewidth.Comment: 5 pages, 3 eps figure
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