1,688 research outputs found
Thermal conductance of pressed aluminum and stainless steel contacts at liquid helium temperatures
The thermal conductance of aluminum and stainless steel 304 sample pairs with surface finishes ranging from 0.1 to 1.6 microns rms roughness was investigated over a temperature range from 1.6 to 6.0 k. The thermal conductance follows a simple power law function of temperature, with the exponent ranging from 0.5 to 2.25, increases asymptotically with increasing applied force, and exhibits an anomaly for surface finishes in the 0.4 micron region
Hydrodynamic description of transport in strongly correlated electron systems
We develop a hydrodynamic description of the resistivity and
magnetoresistance of an electron liquid in a smooth disorder potential. This
approach is valid when the electron-electron scattering length is sufficiently
short. In a broad range of temperatures, the dissipation is dominated by heat
fluxes in the electron fluid, and the resistivity is inversely proportional to
the thermal conductivity, . This is in striking contrast with the
Stokes flow, in which the resistance is independent of and
proportional to the fluid viscosity. We also identify a new hydrodynamic
mechanism of spin magnetoresistance
Density of States in Superconductor - Normal Metal - Superconductor Junctions
We consider the chi_0 dependence of the density of states inside the normal
metal of a superconductor - normal metal - superconductor (SNS) junction.Here
chi_0 is the phase difference of two superconductors of the junction. It is
shown that in the absence of electron-electron interaction the energy
dependence of the density of states has a gap which decreases as chi_0
increases and closes at chi_0= pi. Both the analytical expressions for the
chi_0 dependence of the density of states and the results of numerical
simulations are presented.Comment: 7 pages with 4 included epsf figures, published version with small
change
Characterisation and representation of non-dissipative electromagnetic medium with a double light cone
We study Maxwell's equations on a 4-manifold N with a medium that is
non-dissipative and has a linear and pointwise response. In this setting, the
medium can be represented by a suitable (2,2)-tensor on the 4-manifold N.
Moreover, in each cotangent space on N, the medium defines a Fresnel surface.
Essentially, the Fresnel surface is a tensorial analogue of the dispersion
equation that describes the response of the medium for signals in the geometric
optics limit. For example, in isotropic medium the Fresnel surface is at each
point a Lorentz light cone. In a recent paper, I. Lindell, A. Favaro and L.
Bergamin introduced a condition that constrains the polarisation for plane
waves. In this paper we show (under suitable assumptions) that a slight
strengthening of this condition gives a pointwise characterisation of all
medium tensors for which the Fresnel surface is the union of two distinct
Lorentz null cones. This is for example the behaviour of uniaxial medium like
calcite. Moreover, using the representation formulas from Lindell et al. we
obtain a closed form representation formula that pointwise parameterises all
medium tensors for which the Fresnel surface is the union of two distinct
Lorentz null cones. Both the characterisation and the representation formula
are tensorial and do not depend on local coordinates
Controlling the Sign of Magnetoconductance in Andreev Quantum Dots
We construct a theory of coherent transport through a ballistic quantum dot
coupled to a superconductor. We show that the leading-order quantum correction
to the two-terminal conductance of these Andreev quantum dots may change sign
depending on (i) the number of channels carried by the normal leads or (ii) the
magnetic flux threading the dot. In contrast, spin-orbit interaction may affect
the magnitude of the correction, but not always its sign. Experimental
signatures of the effect include a non-monotonic magnetoconductance curve and a
transition from an insulator-like to a metal-like temperature dependence of the
conductance. Our results are applicable to ballistic or disordered dots.Comment: Final version (4pages 3figs)- improved presentation and fig 3, and
updated reference
Andreev Probe of Persistent Current States in Superconducting Quantum Circuits
Using the extraordinary sensitivity of Andreev interferometers to the
superconducting phase difference associated with currents, we measure the
persistent current quantum states in superconducting loops interrupted by
Josephson junctions. Straightforward electrical resistance measurements of the
interferometers give continuous read-out of the states, allowing us to
construct the energy spectrum of the quantum circuit. The probe is estimated to
be more precise and faster than previous methods, and can measure the local
phase difference in a wide range of superconducting circuits.Comment: Changes made in light of referees comments; to appear in PR
Mesoscopic mechanism of adiabatic charge transport
We consider adiabatic charge transport through mesoscopic metallic samples
caused by a periodically changing external potential. We find that both the
amplitude and the sign of the charge transferred through a sample per period
are random sample specific quantities. The characteristic magnitude of the
charge is determined by the quantum interference.Comment: 4 pages, 2 figure
A theory of \pi/2 superconducting Josephson junctions
We consider theoretically a Josephson junction with a superconducting
critical current density which has a random sign along the junction's surface.
We show that the ground state of the junction corresponds to the phase
difference equal to \pi/2. Such a situation can take place in superconductor-
ferromagnet junction
Covariant coarse-graining of inhomogeneous dust flow in General Relativity
A new definition of coarse-grained quantities describing the dust flow in
General Relativity is proposed. It assigns the coarse--grained expansion, shear
and vorticity to finite-size comoving domains of fluid in a covariant,
coordinate-independent manner. The coarse--grained quantities are all
quasi-local functionals, depending only on the geometry of the boundary of the
considered domain. They can be thought of as relativistic generalizations of
simple volume averages of local quantities in a flat space. The procedure is
based on the isometric embedding theorem for S^2 surfaces and thus requires the
boundary of the domain in question to have spherical topology and positive
scalar curvature. We prove that in the limit of infinitesimally small volume
the proposed quantities reproduce the local expansion, shear and vorticity. In
case of irrotational flow we derive the time evolution for the coarse-grained
quantities and show that its structure is very similar to the evolution
equation for their local counterparts. Additional terms appearing in it may
serve as a measure of the backreacton of small-scale inhomogeneities of the
flow on the large-scale motion of the fluid inside the domain and therefore the
result may be interesting in the context of the cosmological backreaction
problem. We also consider the application of the proposed coarse-graining
procedure to a number of known exact solutions of Einstein equations with dust
and show that it yields reasonable results.Comment: 17 pages, 5 figures. Version accepted in Classical and Quantum
Gravity
Transport and triplet superconducting condensate in mesoscopic ferromagnet-superconductor structures
We calculate the conductance of a superconductor/ferromagnet (S/F) mesoscopic
structure in the dirty limit. First we assume that the ferromagnet exhibits a
homogeneous magnetization and consider the case that the penetration of the
condensate into the F wire is negligible and the case in which the proximity
effect is taken into account. It is shown that if the exchange field is large
enough, the conductance below the critical temperature , is always smaller
than the conductance in the normal state. At last, we calculate the conductance
for a F/S structure with a local inhomogeneity of the magnetization in the
ferromagnet. We demonstrate that a triplet component of the condensate is
induced in the F wire.This leads to a increase of the conductance below .Comment: 31 pages, 6 figures. to be published in International Journal of
Modern Physics B; references adde
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