38 research outputs found
Magnetic interference pattern in planar SNS Josephson junctions
We study the Josephson current through a ballistic normal metal layer of
thickness on which two superconducting electrodes are deposited within a
distance of each other. In the presence of an ({\it in-layer}) magnetic
field we find that the oscillations of the critical current with
the magnetic flux are significantly different from an ordinary magnetic
interference pattern. Depending on the ratio and temperature,
-oscillations can have a period smaller than flux quantum ,
nonzero minima and damping rate much smaller than . Similar anomalous
magnetic interference pattern was recently observed experimentally.Comment: 6 pages, 4 figures, Accepted by Phys. Rev.
Thermodynamics of low dimensional spin-1/2 Heisenberg ferromagnets in an external magnetic field within Green function formalism
The thermodynamics of low dimensional spin-1/2 Heisenberg ferromagnets (HFM)
in an external magnetic field is investigated within a second-order two-time
Green function formalism in the wide temperature and field range. A crucial
point of the proposed scheme is a proper account of the analytical properties
for the approximate transverse commutator Green function obtained as a result
of the decoupling procedure. A good quantitative description of the correlation
functions, magnetization, susceptibility, and heat capacity of the HFM on a
chain, square and triangular lattices is found for both infinite and
finite-sized systems. The dependences of the thermodynamic functions of 2D HFM
on the cluster size are studied. The obtained results agree well with the
corresponding data found by Bethe ansatz, exact diagonalization, high
temperature series expansions, and quantum Monte Carlo simulations.Comment: 11 pages, 14 figure
Magnetic interference patterns in superconducting junctions: Effects of anharmonic current-phase relations
A microscopic theory of the magnetic-field modulation of critical currents is
developed for plane Josephson junctions with anharmonic current-phase
relations. The results obtained allow examining temperature-dependent
deviations of the modulation from the conventional interference pattern. For
tunneling through localized states in symmetric short junctions with a
pronounced anharmonic behavior, the deviations are obtained and shown to depend
on distribution of channel transparencies. For constant transparency the
deviations vanish not only near Tc, but also at T=0. If Dorokhov bimodal
distribution for transparency eigenvalues holds, the averaged deviation
increases with decreasing temperature and takes its maximum at T=0.Comment: 6 pages, 6 figure
Anderson localization of a weakly interacting one dimensional Bose gas
We consider the phase coherent transport of a quasi one-dimensional beam of
Bose-Einstein condensed particles through a disordered potential of length L.
Among the possible different types of flow identified in [T. Paul et al., Phys.
Rev. Lett. 98, 210602 (2007)], we focus here on the supersonic stationary
regime where Anderson localization exists. We generalize the diffusion
formalism of Dorokhov-Mello-Pereyra-Kumar to include interaction effects. It is
shown that interactions modify the localization length and also introduce a
length scale L* for the disordered region, above which most of the realizations
of the random potential lead to time dependent flows. A Fokker-Planck equation
for the probability density of the transmission coefficient that takes this new
effect into account is introduced and solved. The theoretical predictions are
verified numerically for different types of disordered potentials. Experimental
scenarios for observing our predictions are discussed.Comment: 20 pages, 13 figure
Phonon scattering in ortho-para hydrogen solid solutions (role of configurational relaxation)
The experimental data on the thermal conductivity of ortho-parahydrogen
solutions are analyzed on the basis of a relaxation-time model taking account
of configurational relaxation of the ortho subsystem. The influence of
configurational relaxation on the thermal conductivity is analyzed using
resonance scattering of phonons by pair clusters of orthomolecules taking
account of their rotational spectrum.Comment: 7 pages, 4 figure
On the theory of Josephson effect in a diffusive tunnel junction
Specific features of the equilibrium current-carrying state of a Josephson
tunnel junction between diffusive superconductors are studied theoretically in
the 1D geometry. It is found that the Josephson current induces localized
states of electron excitations in the vicinity of the tunnel barrier, which are
a continuous analog of Andreev levels in a ballistic junction. The depth of the
corresponding ``potential well'' is much greater than the separation between an
Andreev level and the continuous energy spectrum boundary for the same
transmissivity of the barrier. In contrast to a ballistic junction in which the
Josephson current is transported completely by localized excitations, the
contribution to current in a diffusive junction comes from whole spectral
region near the energy gap boundary, where the density of states differs
considerably from its unperturbed value. The correction to the Josephson
current in the second order of the barrier transmissivity, which contains the
second harmonic of the phase jump, is calculated and it is found that the true
expansion parameter of the perturbation theory for a diffusive junction is not
the tunneling probability itself, but a much larger parameter .Comment: 8 pages, 5 Postscript figures, submitted to Low Temp. Phy
Andreev reflection and cyclotron motion at superconductor -- normal-metal interfaces
We investigate Andreev reflection at the interface between a superconductor
and a two--dimensional electron system (2DES) in an external magnetic field
such that cyclotron motion is important in the latter. A finite Zeeman
splitting in the 2DES and the presence of diamagnetic screening currents in the
superconductor are incorporated into a microscopic theory of Andreev edge
states, which is based on the Bogoliubov--de Gennes formalism. The
Andreev--reflection contribution to the interface conductance is calculated.
The effect of Zeeman splitting is most visible as a double--step feature in the
conductance through clean interfaces. Due to a screening current, conductance
steps are shifted to larger filling factors and the formation of Andreev edge
states is suppressed below a critical filling factor.Comment: 8 pages, 6 figure
One-dimensional classical diffusion in a random force field with weakly concentrated absorbers
A one-dimensional model of classical diffusion in a random force field with a
weak concentration of absorbers is studied. The force field is taken as
a Gaussian white noise with \mean{\phi(x)}=0 and \mean{\phi(x)\phi(x')}=g
\delta(x-x'). Our analysis relies on the relation between the Fokker-Planck
operator and a quantum Hamiltonian in which absorption leads to breaking of
supersymmetry. Using a Lifshits argument, it is shown that the average return
probability is a power law \smean{P(x,t|x,0)}\sim{}t^{-\sqrt{2\rho/g}} (to be
compared with the usual Lifshits exponential decay in
the absence of the random force field). The localisation properties of the
underlying quantum Hamiltonian are discussed as well.Comment: 6 pages, LaTeX, 5 eps figure