10,514 research outputs found
Microwave stabilization of edge transport and zero-resistance states
Edge channels play a crucial role for electron transport in two dimensional
electron gas under magnetic field. It is usually thought that ballistic
transport along edges occurs only in the quantum regime with low filling
factors. We show that a microwave field can stabilize edge trajectories even in
the semiclassical regime leading to a vanishing longitudinal resistance. This
mechanism gives a clear physical interpretation for observed zero-resistance
states
Quantum discord and the power of one qubit
We use quantum discord to characterize the correlations present in the
quantum computational model DQC1, introduced by Knill and Laflamme [Phys. Rev.
Lett. 81, 5672 (1998)]. The model involves a collection of qubits in the
completely mixed state coupled to a single control qubit that has nonzero
purity. The initial state, operations, and measurements in the model all point
to a natural bipartite split between the control qubit and the mixed ones.
Although there is no entanglement between these two parts, we show that the
quantum discord across this split is nonzero for typical instances of the DQC1
ciruit. Nonzero values of discord indicate the presence of nonclassical
correlations. We propose quantum discord as figure of merit for characterizing
the resources present in this computational model.Comment: 4 Pages, 1 Figur
Quasi-equilibrium optical nonlinearities in spin-polarized GaAs
Semiconductor Bloch equations, which microscopically describe the dynamics of
a Coulomb interacting, spin-unpolarized electron-hole plasma, can be solved in
two limits: the coherent and the quasi-equilibrium regime. These equations have
been recently extended to include the spin degree of freedom, and used to
explain spin dynamics in the coherent regime. In the quasi-equilibrium limit,
one solves the Bethe-Salpeter equation in a two-band model to describe how
optical absorption is affected by Coulomb interactions within a
spin-unpolarized plasma of arbitrary density. In this work, we modified the
solution of the Bethe-Salpeter equation to include spin-polarization and light
holes in a three-band model, which allowed us to account for spin-polarized
versions of many-body effects in absorption. The calculated absorption
reproduced the spin-dependent, density-dependent and spectral trends observed
in bulk GaAs at room temperature, in a recent pump-probe experiment with
circularly polarized light. Hence our results may be useful in the microscopic
modelling of density-dependent optical nonlinearities in spin-polarized
semiconductors.Comment: 7 pages, 6 figure
Temperature Profiles of Accretion Disks around Rapidly Rotating Neutron Stars in General Relativity and Implications for Cygnus X-2
We calculate the temperature profiles of (thin) accretion disks around
rapidly rotating neutron stars (with low surface magnetic fields), taking into
account the full effects of general relativity. We then consider a model for
the spectrum of the X-ray emission from the disk, parameterized by the mass
accretion rate, the color temperature and the rotation rate of the neutron
star. We derive constraints on these parameters for the X-ray source Cygnus X-2
using the estimates of the maximum temperature in the disk along with the disk
and boundary layer luminosities, using the spectrum inferred from the EXOSAT
data. Our calculations suggest that the neutron star in Cygnus X-2 rotates
close to the centrifugal mass-shed limit. Possible constraints on the neutron
star equation of state are also discussed.Comment: 18 pages, 9 figs., 2 tables, uses psbox.tex and emulateapj5.sty.
Submitted to Ap
Dynamical mean field theory for strongly correlated inhomogeneous multilayered nanostructures
Dynamical mean field theory is employed to calculate the properties of
multilayered inhomogeneous devices composed of semi-infinite metallic lead
layers coupled via barrier planes that are made from a strongly correlated
material (and can be tuned through the metal-insulator Mott transition). We
find that the Friedel oscillations in the metallic leads are immediately frozen
in and don't change as the thickness of the barrier increases from one to
eighty planes. We also identify a generalization of the Thouless energy that
describes the crossover from tunneling to incoherent Ohmic transport in the
insulating barrier. We qualitatively compare the results of these
self-consistent many-body calculations with the assumptions of
non-self-consistent Landauer-based approaches to shed light on when such
approaches are likely to yield good results for the transport.Comment: 15 pages, 12 figures, submitted to Phys. Rev.
The spin-double refraction in two-dimensional electron gas
We briefly review the phenomenon of the spin-double refraction that
originates at an interface separating a two-dimensional electron gas with
Rashba spin-orbit coupling from a one without. We demonstrate how this
phenomenon in semiconductor heterostructures can produce and control a
spin-polarized current without ferromagnetic leads
A pertubative approach to the Kondo effect in magnetic atoms on nonmagnetic substrates
Recent experimental advances in scanning tunneling microscopy make the
measurement of the conductance spectra of isolated and magnetically coupled
atoms on nonmagnetic substrates possible. Notably these spectra are
characterized by a competition between the Kondo effect and spin-flip inelastic
electron tunneling. In particular they include Kondo resonances and a
logarithmic enhancement of the conductance at voltages corresponding to
magnetic excitations, two features that cannot be captured by second order
perturbation theory in the electron-spin coupling. We have now derived a third
order analytic expression for the electron-spin self-energy, which can be
readily used in combination with the non-equilibrium Green's function scheme
for electron transport at finite bias. We demonstrate that our method is
capable of quantitative description the competition between Kondo resonances
and spin-flip inelastic electron tunneling at a computational cost
significantly lower than that of other approaches. The examples of Co and Fe on
CuN are discussed in detail
Relevance of Induced Gauge Interactions in Decoherence
Decoherence in quantum cosmology is shown to occur naturally in the presence
of induced geometric gauge interactions associated with particle production.A
new 'gauge '-variant form of the semiclassical Einstein equations is also
presented which makes the non-gravitating character of the vacuum polarisation
energy explicit.Comment: 10 pages, LATEX, IC/94/16
Transport properties of a quantum wire: the role of extended time-dependent impurities
We study the transport properties of a quantum wire, described by the
Tomonaga-Luttinger model, in the presence of a backscattering potential
provided by several extended time-dependent impurities (barriers). Employing
the B\" uttiker-Landauer approach, we first consider the scattering of
noninteracting electrons () by a rectangular-like barrier and find an
exact solution for the backscattering current, as well as a perturbative
solution for a weak static potential with an arbitrary shape. We then include
electron-electron interactions and use the Keldysh formalism combined with the
bosonization technique to study oscillating extended barriers. We show that the
backscattering current off time-dependent impurities can be expressed in terms
of the current for the corresponding static barrier. Then we determine the
backscattering current for a static extended potential, which, in the limit of
noninteracting electrons (), coincides with the result obtained using the
B\" uttiker-Landauer formalism. In particular, we find that the conductance can
be increased beyond its quantized value in the whole range of repulsive
interactions already in the case of a single oscillating extended
impurity, in contrast %contrary to the case of a point-like impurity, where
this phenomenon occurs only for .Comment: 9 pages, 5 figure
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