4,729 research outputs found
Finite size effects and localization properties of disordered quantum wires with chiral symmetry
Finite size effects in the localization properties of disordered quantum
wires are analyzed through conductance calculations. Disorder is induced by
introducing vacancies at random positions in the wire and thus preserving the
chiral symmetry. For quasi one-dimensional geometries and low concentration of
vacancies, an exponential decay of the mean conductance with the wire length is
obtained even at the center of the energy band. For wide wires, finite size
effects cause the conductance to decay following a non-pure exponential law. We
propose an analytical formula for the mean conductance that reproduces
accurately the numerical data for both geometries. However, when the
concentration of vacancies increases above a critical value, a transition
towards the suppression of the conductance occurs.
This is a signature of the presence of ultra-localized states trapped in
finite regions of the sample.Comment: 5 figures, revtex
Theory of the spin-torque-driven ferromagnetic resonance in a ferromagnet/normal-metal/ferromagnet structure
We present a theoretical analysis of current driven ferromagnetic resonance
in a ferromagnet/normal-metal/ferromagnet tri-layer. This method of driving
ferromagnetic resonance was recently realized experimentally by Tulapurkar et
al. [Nature 438, 339 (2005)] and Sankey et al. [Phys. Rev. Lett. 96, 227601
(2006)]. The precessing magnetization rectifies the alternating current applied
to drive the ferromagnetic resonance and leads to the generation of a dc
voltage. Our analysis shows that a second mechanism to generate a dc voltage,
rectification of spin currents emitted by the precessing magnetization, has a
contribution to the dc voltage that is of approximately equal size for the thin
ferromagnetic films used in the experiment.Comment: 6 pages, 1 figure, final version. Changed title, updated references,
added discussions in section I
Intensity distribution of scalar waves propagating in random media
Transmission of the scalar field through the random medium, represented by
the system of randomly distributed dielectric cylinders is calculated
numerically. System is mapped to the problem of electronic transport in
disordered two-dimensional systems. Universality of the statistical
distribution of transmission parameters is analyzed in the metallic and in the
localized regimes.In the metallic regime the universality of the transmission
statistics in all transparent channels is observed. In the band gaps, we
distinguish the disorder induced (Anderson) localization from the tunneling
through the system due to the gap in the density of states. We show also that
absorption causes rapid decrease of the mean conductance, but, contrary to the
localized regime, the conductance is self-averaged with a
Gaussian distribution
Impurity-assisted Andreev reflection at a spin-active half-metal-superconductor interface
The Andreev reflection amplitude at a clean interface between a half-metallic
ferromagnet (H) and a superconductor (S) for which the half metal's
magnetization has a gradient perpendicular to the interface is proportional to
the excitation energy and vanishes at [B\'{e}ri
{\em et al.}, Phys.\ Rev.\ B {\bf 79}, 024517 (2009)]. Here we show that the
presence of impurities at or in the immediate vicinity of the HS interface
leads to a finite Andreev reflection amplitude at . This
impurity-assisted Andreev reflection dominates the low-bias conductance of a HS
junction and the Josephson current of an SHS junction in the long-junction
limit.Comment: 12 pages, 2 figure
The risk linked to ionizing radiation: an alternative epidemiologic approach.
Radioprotection norms have been based on risk models that have evolved over time. These models show relationships between exposure and observed effects. There is a high level of uncertainty regarding lower doses. Recommendations have been based on the conservative hypothesis of a linear relationship without threshold value. This relationship is still debated, and the diverse observations do not allow any definitive conclusion. Available data are contradictory, and various interpretations can be made. Here we review an alternative approach for defining causation and reconciling apparently contradictory conclusions. This alternative epidemiologic approach is based on causal groups: Each component of a causal group is necessary but not sufficient for causality. Many groups may be involved in causality. Thus, ionizing radiation may be a component of one or several causal groups. This formalization reconciles heterogeneous observations but implies searching for the interactions between components, mostly between critical components of a causal profile, and, for instance, the reasons why specific human groups would not show any effect despite exposure, when an effect would be expected
Andreev reflection at half-metal-superconductor interfaces with non-uniform magnetization
Andreev reflection at the interface between a half-metallic ferromagnet and a
spin-singlet superconductor is possible only if it is accompanied by a spin
flip. Here we calculate the Andreev reflection amplitudes for the case that the
spin flip originates from a spatially non-uniform magnetization direction in
the half metal. We calculate both the microscopic Andreev reflection amplitude
for a single reflection event and an effective Andreev reflection amplitude
describing the effect of multiple Andreev reflections in a ballistic thin film
geometry. It is shown that the angle and energy dependence of the Andreev
reflection amplitude strongly depends on the orientation of the gradient of the
magnetization with respect to the interface. Establishing a connection between
the scattering approach employed here and earlier work that employs the
quasiclassical formalism, we connect the symmetry properties of the Andreev
reflection amplitudes to the symmetry properties of the anomalous Green
function in the half metal.Comment: 13 pages, 4 figure
Photonic excess noise and wave localization
This is a theory for the effect of localization on the super-Poissonian noise
of radiation propagating through an absorbing disordered waveguide.
Localization suppresses both the mean photon current I and the noise power P,
but the Fano factor P/I is found to remain unaffected. For strong absorption
the Fano factor has the universal value 1+3f/2 (with f the Bose-Einstein
function), regardless of whether the waveguide is long or short compared to the
localization length.Comment: 3 pages including 3 figure
Voltage-probe and imaginary potential models for dephasing in a chaotic quantum dot
We compare two widely used models for dephasing in a chaotic quantum dot: The
introduction of a fictitious voltage probe into the scattering matrix and the
addition of an imaginary potential to the Hamiltonian. We identify the limit in
which the two models are equivalent and compute the distribution of the
conductance in that limit. Our analysis explains why previous treatments of
dephasing gave different results. The distribution remains non-Gaussian for
strong dephasing if the coupling of the quantum dot to the electron reservoirs
is via ballistic single-mode point contacts, but becomes Gaussian if the
coupling is via tunneling contacts.Comment: 9 pages, RevTeX, 6 figures. Mistake in Eq. (35) correcte
Conductance Distribution of a Quantum Dot with Non-Ideal Single-Channel Leads
We have computed the probability distribution of the conductance of a
ballistic and chaotic cavity which is connected to two electron reservoirs by
leads with a single propagating mode, for arbitrary values of the transmission
probability Gamma of the mode, and for all three values of the symmetry index
beta. The theory bridges the gap between previous work on ballistic leads
(Gamma = 1) and on tunneling point contacts (Gamma << 1). We find that the
beta-dependence of the distribution changes drastically in the crossover from
the tunneling to the ballistic regime. This is relevant for experiments, which
are usually in this crossover regime. ***Submitted to Physical Review B.***Comment: 7 pages, REVTeX-3.0, 4 postscript figures appended as self-extracting
archive, INLO-PUB-940607
Distributions of the Conductance and its Parametric Derivatives in Quantum Dots
Full distributions of conductance through quantum dots with single-mode leads
are reported for both broken and unbroken time-reversal symmetry. Distributions
are nongaussian and agree well with random matrix theory calculations that
account for a finite dephasing time, , once broadening due to finite
temperature is also included. Full distributions of the derivatives of
conductance with respect to gate voltage are also investigated.Comment: 4 pages (REVTeX), 4 eps figure
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