470 research outputs found
Coherent Charge Transport in Metallic Proximity Structures
We develop a detailed microscopic analysis of electron transport in normal
diffusive conductors in the presence of proximity induced superconducting
correlation. We calculated the linear conductance of the system, the profile of
the electric field and the densities of states. In the case of transparent
metallic boundaries the temperature dependent conductance has a non-monotoneous
``reentrant'' structure. We argue that this behavior is due to nonequilibrium
effects occuring in the normal metal in the presence of both superconducting
correlations and the electric field there. Low transparent tunnel barriers
suppress the nonequilibrium effects and destroy the reentrant behavior of the
conductance. If the wire contains a loop, the conductance shows Aharonov-Bohm
oscillations with the period as a function of the magnetic flux
inside the loop. The amplitude of these oscillations also demonstrates
the reentrant behavior vanishing at and decaying as at relatively
large temperatures. The latter behavior is due to low energy correlated
electrons which penetrate deep into the normal metal and ``feel'' the effect of
the magnetic flux . We point out that the density of states and thus the
``strengh'' of the proximity effect can be tuned by the value of the flux
inside the loop. Our results are fully consistent with recent experimental
findings.Comment: 16 pages RevTeX, 23 Postscript figures, submitted to Phys. Rev.
Stochastic Theory in the Strong Coupling Limit
The stochastic -theory in dimensions dynamically develops domain
wall structures within which the order parameter is not continuous. We develop
a statistical theory for the -theory driven with a random forcing which
is white in time and Gaussian-correlated in space. A master equation is derived
for the probability density function (PDF) of the order parameter, when the
forcing correlation length is much smaller than the system size, but much
larger than the typical width of the domain walls. Moreover, exact expressions
for the one-point PDF and all the moments are given. We then
investigate the intermittency issue in the strong coupling limit, and derive
the tail of the PDF of the increments . The scaling laws
for the structure functions of the increments are obtained through numerical
simulations. It is shown that the moments of field increments defined by,
, behave as , where
for , and for Comment: 22 pages, 6 figures. to appear in Nuclear. Phys.
Critical fluctuation conductivity in layered superconductors in strong electric field
The paraconductivity, originating from critical superconducting
order-parameter fluctuations in the vicinity of the critical temperature in a
layered superconductor is calculated in the frame of the self-consistent
Hartree approximation, for an arbitrarily strong electric field and zero
magnetic field. The paraconductivity diverges less steep towards the critical
temperature in the Hartree approximation than in the Gaussian one and it shows
a distinctly enhanced variation with the electric field. Our results indicate
that high electric fields can be effectively used to suppress order-parameter
fluctuations in high-temperature superconductors.Comment: 11 pages, 2 figures, to be published in Phys. Rev.
Three-dimensional stability of Burgers vortices
Burgers vortices are explicit stationary solutions of the Navier-Stokes
equations which are often used to describe the vortex tubes observed in
numerical simulations of three-dimensional turbulence. In this model, the
velocity field is a two-dimensional perturbation of a linear straining flow
with axial symmetry. The only free parameter is the Reynolds number , where is the total circulation of the vortex and is
the kinematic viscosity. The purpose of this paper is to show that Burgers
vortex is asymptotically stable with respect to general three-dimensional
perturbations, for all values of the Reynolds number. This definitive result
subsumes earlier studies by various authors, which were either restricted to
small Reynolds numbers or to two-dimensional perturbations. Our proof relies on
the crucial observation that the linearized operator at Burgers vortex has a
simple and very specific dependence upon the axial variable. This allows to
reduce the full linearized equations to a vectorial two-dimensional problem,
which can be treated using an extension of the techniques developped in earlier
works. Although Burgers vortices are found to be stable for all Reynolds
numbers, the proof indicates that perturbations may undergo an important
transient amplification if is large, a phenomenon that was indeed observed
in numerical simulations.Comment: 31 pages, no figur
The Impact of the Physical Activity Policy Research Network
Introduction Lack of physical activity is one of the greatest challenges of the 21st century. The Physical Activity Policy Research Network (PAPRN) is a thematic network established in 2004 to identify determinants, implementation, and outcomes of policies that are effective in increasing physical activity. The purpose of this study is to describe the products of PAPRN and make recommendations for future research and best practices. Methods A mixed methods approach was used to obtain both quantitative and qualitative data on the network. First, in 2014, PAPRN's dissemination products from 2004 to 2014 were extracted and reviewed, including 57 publications and 56 presentations. Next, semi-structured qualitative interviews were conducted with 25 key network participants from 17 locations around the U.S. The transcripts were transcribed and coded. Results The results of the interviews indicated that the research network addressed several components of its mission, including the identification of physical activity policies, determinants of these policies, and the process of policy implementation. However, research focusing on physical activity policy outcomes was limited. Best practices included collaboration between researchers and practitioners and involvement of practitioners in research design, data collection, and dissemination of results. Conclusions PAPRN is an example of a productive research network and has contributed to both the process and content of physical activity policy research over the past decade. Future research should emphasize physical activity policy outcomes. Additionally, increased partnerships with practitioners for collaborative, cross-sectoral physical activity policy research should be developed
Exactly solvable model of quantum diffusion
We study the transport property of diffusion in a finite translationally
invariant quantum subsystem described by a tight-binding Hamiltonian with a
single energy band and interacting with its environment by a coupling in terms
of correlation functions which are delta-correlated in space and time. For weak
coupling, the time evolution of the subsystem density matrix is ruled by a
quantum master equation of Lindblad type. Thanks to the invariance under
spatial translations, we can apply the Bloch theorem to the subsystem density
matrix and exactly diagonalize the time evolution superoperator to obtain the
complete spectrum of its eigenvalues, which fully describe the relaxation to
equilibrium. Above a critical coupling which is inversely proportional to the
size of the subsystem, the spectrum at given wavenumber contains an isolated
eigenvalue describing diffusion. The other eigenvalues rule the decay of the
populations and quantum coherences with decay rates which are proportional to
the intensity of the environmental noise. On the other hand, an analytical
expression is obtained for the dispersion relation of diffusion. The diffusion
coefficient is proportional to the square of the width of the energy band and
inversely proportional to the intensity of the environmental noise because
diffusion results from the perturbation of quantum tunneling by the
environmental fluctuations in this model. Diffusion disappears below the
critical coupling.Comment: Submitted to J. Stat. Phy
Phase Diagram of a Spin Ladder with Cyclic Four Spin Exchange
We present the phase diagram of the Heisenberg model on the two leg
ladder with cyclic four spin exchange, determined by a combination of Exact
Diagonalization and Density Matrix Renormalization Group techniques. We find
six different phases and regimes: the rung singlet phase, a ferromagnetic
phase, two symmetry broken phases with staggered dimers and staggered scalar
chiralities, and a gapped region with dominant vector chirality or collinear
spin correlations. We localize the phase transitions and investigate their
nature.Comment: 4 pages, 6 figures, REVTeX 4, published versio
An Assessment of the Current Data Affecting Tritium Retention and its Use to Project Towards T Retention in ITER
Evidence of two-electron tunneling interference in Nb/InAs junctions
The impact of junction transparency in driving phase-coherent charge transfer
across diffusive semiconductor-superconductor junctions is demonstrated. We
present conductivity data for a set of Nb-InAs junctions differing only in
interface transparency. Our experimental findings are analyzed within the
quasi-classical Green-function approach and unambiguously show the physical
processes giving rise to the observed excess zero-bias conductivity.Comment: 10 pages (RevTex), 4 figures (PostScript), accepted for pubblication
in Physical Review
Effective action approach and Carlson-Goldman mode in d-wave superconductors
We theoretically investigate the Carlson-Goldman (CG) mode in two-dimensional
clean d-wave superconductors using the effective ``phase only'' action
formalism. In conventional s-wave superconductors, it is known that the CG mode
is observed as a peak in the structure factor of the pair susceptibility
only just below the transition temperature T_c and only
in dirty systems. On the other hand, our analytical results support the
statement by Y.Ohashi and S.Takada, Phys.Rev.B {\bf 62}, 5971 (2000) that in
d-wave superconductors the CG mode can exist in clean systems down to the much
lower temperatures, . We also consider the manifestations of
the CG mode in the density-density and current-current correlators and discuss
the gauge independence of the obtained results.Comment: 23 pages, RevTeX4, 12 EPS figures; final version to appear in PR
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