478 research outputs found
Ambipolar Nernst effect in NbSe
The first study of Nernst effect in NbSe reveals a large quasi-particle
contribution with a magnitude comparable and a sign opposite to the vortex
signal. Comparing the effect of the Charge Density Wave(CDW) transition on Hall
and Nernst coefficients, we argue that this large Nernst signal originates from
the thermally-induced counterflow of electrons and holes and indicates a
drastic change in the electron scattering rate in the CDW state. The results
provide new input for the debate on the origin of the anomalous Nernst signal
in high-T cuprates.Comment: 5 pages including 4 figure
Integral equation for inhomogeneous condensed bosons generalizing the Gross-Pitaevskii differential equation
We give here the derivation of a Gross-Pitaevskii--type equation for
inhomogeneous condensed bosons. Instead of the original Gross-Pitaevskii
differential equation, we obtain an integral equation that implies less
restrictive assumptions than are made in the very recent study of Pieri and
Strinati [Phys. Rev. Lett. 91 (2003) 030401]. In particular, the Thomas-Fermi
approximation and the restriction to small spatial variations of the order
parameter invoked in their study are avoided.Comment: Phys. Rev. A (accepted
The onset of the vortex-like Nernst signal above Tc in La_{2-x}Sr_xCuO_4 and Bi_2Sr_{2-y}La_yCuO_6
The diffusion of vortices down a thermal gradient produces a Josephson signal
which is detected as the vortex Nernst effect. In a recent report, Xu et al.,
Nature 406, 486 (2000), an enhanced Nernst signal identified with vortex-like
excitations was observed in a series of La_{2-x}Sr_xCuO_4 (LSCO) crystals at
temperatures 50-100 K above T_c. To pin down the onset temperature T_{\nu} of
the vortex-like signal in the lightly doped regime (0.03 < x < 0.07), we have
re-analyzed in detail the carrier contribution to the Nernst signal. By
supplementing new Nernst measurements with thermopower and Hall-angle data, we
isolate the off-diagonal Peltier conductivity \alpha_{xy} and show that its
profile provides an objective determination of T_{\nu}. With the new results,
we revise the phase diagram for the fluctuation regime in LSCO to accomodate
the lightly doped regime. In the cuprate Bi_2Sr_{2-y}La_yCuO_6, we find that
the carrier contribution is virtually negligible for y in the range 0.4-0.6.
The evidence for an extended temperature interval with vortex-like excitations
is even stronger in this system. Finally, we discuss how T_{\nu} relates to the
pseudogap temperature T* and the implications of strong fluctuations between
the pseudogap state and the d-wave superconducting state.Comment: 10 pages, 10 figure
Heat kernel of integrable billiards in a magnetic field
We present analytical methods to calculate the magnetic response of
non-interacting electrons constrained to a domain with boundaries and submitted
to a uniform magnetic field. Two different methods of calculation are
considered - one involving the large energy asymptotic expansion of the
resolvent (Stewartson-Waechter method) is applicable to the case of separable
systems, and another based on the small time asymptotic behaviour of the heat
kernel (Balian-Bloch method). Both methods are in agreement with each other but
differ from the result obtained previously by Robnik. Finally, the Balian-Bloch
multiple scattering expansion is studied and the extension of our results to
other geometries is discussed.Comment: 13 pages, Revte
A Variational Procedure for Time-Dependent Processes
A simple variational Lagrangian is proposed for the time development of an
arbitrary density matrix, employing the "factorization" of the density. Only
the "kinetic energy" appears in the Lagrangian. The formalism applies to pure
and mixed state cases, the Navier-Stokes equations of hydrodynamics, transport
theory, etc. It recaptures the Least Dissipation Function condition of
Rayleigh-Onsager {\bf and in practical applications is flexible}. The
variational proposal is tested on a two level system interacting that is
subject, in one instance, to an interaction with a single oscillator and, in
another, that evolves in a dissipative mode.Comment: 25 pages, 4 figure
Prediction of Anisotropic Single-Dirac-Cones in BiSb Thin Films
The electronic band structures of BiSb thin films can be
varied as a function of temperature, pressure, stoichiometry, film thickness
and growth orientation. We here show how different anisotropic
single-Dirac-cones can be constructed in a BiSb thin film for
different applications or research purposes. For predicting anisotropic
single-Dirac-cones, we have developed an iterative-two-dimensional-two-band
model to get a consistent inverse-effective-mass-tensor and band-gap, which can
be used in a general two-dimensional system that has a non-parabolic dispersion
relation as in a BiSb thin film system
Quantum transport through mesoscopic disordered interfaces, junctions, and multilayers
The study explores perpendicular transport through macroscopically
inhomogeneous three-dimensional disordered conductors using mesoscopic methods
(real-space Green function technique in a two-probe measuring geometry). The
nanoscale samples (containing atoms) are modeled by a tight-binding
Hamiltonian on a simple cubic lattice where disorder is introduced in the
on-site potential energy. I compute the transport properties of: disordered
metallic junctions formed by concatenating two homogenous samples with
different kinds of microscopic disorder, a single strongly disordered
interface, and multilayers composed of such interfaces and homogeneous layers
characterized by different strength of the same type of microscopic disorder.
This allows us to: contrast resistor model (semiclassical) approach with fully
quantum description of dirty mesoscopic multilayers; study the transmission
properties of dirty interfaces (where Schep-Bauer distribution of transmission
eigenvalues is confirmed for single interface, as well as for the stack of such
interfaces that is thinner than the localization length); and elucidate the
effect of coupling to ideal leads (``measuring apparatus'') on the conductance
of both bulk conductors and dirty interfaces When multilayer contains a
ballistic layer in between two interfaces, its disorder-averaged conductance
oscillates as a function of Fermi energy. I also address some fundamental
issues in quantum transport theory--the relationship between Kubo formula in
exact state representation and ``mesoscopic Kubo formula'' (which gives the
zero-temperature conductance of a finite-size sample attached to two
semi-infinite ideal leads) is thoroughly reexamined by comparing their answers
for both the junctions and homogeneous samples.Comment: 18 pages, 17 embedded EPS figure
Simulation of dimensionality effects in thermal transport
The discovery of nanostructures and the development of growth and fabrication
techniques of one- and two-dimensional materials provide the possibility to
probe experimentally heat transport in low-dimensional systems. Nevertheless
measuring the thermal conductivity of these systems is extremely challenging
and subject to large uncertainties, thus hindering the chance for a direct
comparison between experiments and statistical physics models. Atomistic
simulations of realistic nanostructures provide the ideal bridge between
abstract models and experiments. After briefly introducing the state of the art
of heat transport measurement in nanostructures, and numerical techniques to
simulate realistic systems at atomistic level, we review the contribution of
lattice dynamics and molecular dynamics simulation to understanding nanoscale
thermal transport in systems with reduced dimensionality. We focus on the
effect of dimensionality in determining the phononic properties of carbon and
semiconducting nanostructures, specifically considering the cases of carbon
nanotubes, graphene and of silicon nanowires and ultra-thin membranes,
underlying analogies and differences with abstract lattice models.Comment: 30 pages, 21 figures. Review paper, to appear in the Springer Lecture
Notes in Physics volume "Thermal transport in low dimensions: from
statistical physics to nanoscale heat transfer" (S. Lepri ed.
Spin injection into a ballistic semiconductor microstructure
A theory of spin injection across a ballistic
ferromagnet-semiconductor-ferromagnet junction is developed for the Boltzmann
regime. Spin injection coefficient is suppressed by the Sharvin
resistance of the semiconductor , where is the
Fermi-surface cross-section. It competes with the diffusion resistances of the
ferromagnets , and in the absence of contact
barriers. Efficient spin injection can be ensured by contact barriers. Explicit
formulae for the junction resistance and the spin-valve effect are presented.Comment: 5 pages, 2 column REVTeX. Explicit prescription relating the results
of the ballistic and diffusive theories of spin injection is added. To this
end, some notations are changed. Three references added, typos correcte
Boundary scattering and weak localization of electrons in a magnetic field
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
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