141 research outputs found
Microwave response of an NS ring coupled to a superconducting resonator
A long phase coherent normal (N) wire between superconductors (S) is
characterized by a dense phase dependent Andreev spectrum . We probe this
spectrum in a high frequency phase biased configuration, by coupling an NS ring
to a multimode superconducting resonator. We detect a dc flux and frequency
dependent response whose dissipative and non dissipative components are related
by a simple Debye relaxation law with a characteristic time of the order of the
diffusion time through the N part of the ring. The flux dependence exhibits
periodic oscillations with a large harmonics content at temperatures
where the Josephson current is purely sinusoidal. This is explained considering
that the populations of the Andreev levels are frozen on the time-scale of the
experiments.Comment: 5 pages,4 figure
Enhancement of Superconductivity in Disordered Films by Parallel Magnetic Field
We show that the superconducting transition temperature T_c(H) of a very thin
highly disordered film with strong spin-orbital scattering can be increased by
parallel magnetic field H. This effect is due to polarization of magnetic
impurity spins which reduces the full exchange scattering rate of electrons;
the largest effect is predicted for spin-1/2 impurities. Moreover, for some
range of magnetic impurity concentrations the phenomenon of {\it
superconductivity induced by magnetic field} is predicted: superconducting
transition temperature T_c(H) is found to be nonzero in the range of magnetic
fields .Comment: 4 pages, 2 figure
Universal temperature dependence of the conductivity of a strongly disordered granular metal
A disordered array of metal grains with large and random intergrain
conductances is studied within the one-loop accuracy renormalization group
approach. While at low level of disorder the dependence of conductivity on log
T is nonuniversal (it depends on details of the array's geometry), for strong
disorder this dependence is described by a universal nonlinear function, which
depends only on the array's dimensionality. In two dimensions this function is
found numerically. The dimensional crossover in granular films is discussed.Comment: 6 pages, 6 figures, submitted to JETP Letter
Fractional power-law susceptibility and specific heat in low temperature insulating state of o-TaS_{3}
Measurements of the magnetic susceptibility and its anisotropy in the
quasi-one-dimensional system o-TaS_{3} in its low-T charge density wave (CDW)
ground state are reported. Both sets of data reveal below 40 K an extra
paramagnetic contribution obeying a power-law temperature dependence
\chi(T)=AT^{-0.7}. The fact that the extra term measured previously in specific
heat in zero field, ascribed to low-energy CDW excitations, also follows a
power law C_{LEE}(0,T)=CT^{0.3}, strongly revives the case of random exchange
spin chains. Introduced impurities (0.5% Nb) only increase the amplitude C, but
do not change essentially the exponent. Within the two-level system (TLS)
model, we estimate from the amplitudes A and C that there is one TLS with a
spin s=1/2 localized on the chain at the lattice site per cca 900 Ta atoms. We
discuss the possibility that it is the charge frozen within a soliton-network
below the glass transition T_{g}~40 K determined recently in this system.Comment: 7 pages, 3 figures, submitted to Europhysics Letter
Broad relaxation spectrum and the field theory of glassy dynamics for pinned elastic systems
We study thermally activated, low temperature equilibrium dynamics of elastic
systems pinned by disorder using one loop functional renormalization group
(FRG). Through a series of increasingly complete approximations, we investigate
how the field theory reveals the glassy nature of the dynamics, in particular
divergent barriers and barrier distributions controling the spectrum of
relaxation times. A naive single relaxation time approximation for each
wavevector is found to be unsatisfactory. A second approximation based on a
random friction model, yields a size (L) dependent log-normal distribution of
relaxation times (mean barriers ~L^\theta and variance ~ L^{\theta/2}) and a
procedure to estimate dynamical scaling functions. Finally, we study the full
structure of the running dynamical effective action within the field theory. We
find that relaxation time distributions are non-trivial (broad but not
log-normal) and encoded in a closed hierarchy of FRG equations. A thermal
boundary layer ansatz (TBLA) appears as a consistent solution. It extends the
one discovered in the statics which was shown to embody droplet thermal
fluctuations. Although perturbative control remains a challenge, the structure
of the dynamical TBLA which encodes barrier distributions opens the way for
deeper understanding of the field theory approach to glasses
Monte-Carlo calculation of longitudinal and transverse resistivities in a model Type-II superconductor
We study the effect of a transport current on the vortex-line lattice in
isotropic type-II superconductors in the presence of strong thermal
fluctuations by means of 'driven-diffusion' Monte Carlo simulations of a
discretized London theory with finite magnetic penetration depth. We calculate
the current-voltage (I-V) characteristics for various temperatures, for
transverse as well as longitudinal currents I. From these characteristics, we
estimate the linear resistivities R_xx=R_yy and R_zz and compare these with
equilibrium results for the vortex-lattice structure factor and the helicity
moduli. From this comparison a consistent picture arises, in which the melting
of the flux-line lattice occurs in two stages for the system size considered.
In the first stage of the melting, at a temperature T_m, the structure factor
drops to zero and R_xx becomes finite. For a higher temperature T_z, the second
stage takes place, in which the longitudinal superconducting coherence is lost,
and R_zz becomes finite as well. We compare our results with related recent
numerical work and experiments on cuprate superconductors.Comment: 4 pages, with eps figure
Quantum superconductor-metal transition
We consider a system of superconducting grains embedded in a normal metal. At
zero temperature this system exhibits a quantum superconductor-normal metal
phase transition. This transition can take place at arbitrarily large
conductance of the normal metal.Comment: 13 pages, 1 figure include
Degenerate Bose liquid in a fluctuating gauge field
We study the effect of a strongly fluctuating gauge field on a degenerate
Bose liquid, relevant to the charge degrees of freedom in doped Mott
insulators. We find that the superfluidity is destroyed. The resulting metallic
phase is studied using quantum Monte Carlo methods. Gauge fluctuations cause
the boson world lines to retrace themselves. We examine how this world-line
geometry affects the physical properties of the system. In particular, we find
a transport relaxation rate of the order of 2kT, consistent with the normal
state of the cuprate superconductors. We also find that the density excitations
of this model resemble that of the full tJ model.Comment: 4 pages. Uses RevTeX, epsf, multicols macros. 5 postscript figure
Low-frequency dynamics of disordered XY spin chains and pinned density waves: from localized spin waves to soliton tunneling
A long-standing problem of the low-energy dynamics of a disordered XY spin
chain is re-examined. The case of a rigid chain is studied where the quantum
effects can be treated quasiclassically. It is shown that as the frequency
decreases, the relevant excitations change from localized spin waves to
two-level systems to soliton-antisoliton pairs. The linear-response correlation
functions are calculated. The results apply to other periodic glassy systems
such as pinned density waves, planar vortex lattices, stripes, and disordered
Luttinger liquids.Comment: (v2) Major improvements in presentation style. One figure added (v3)
Another minor chang
Hysteretic dynamics of domain walls at finite temperatures
Theory of domain wall motion in a random medium is extended to the case when
the driving field is below the zero-temperature depinning threshold and the
creep of the domain wall is induced by thermal fluctuations. Subject to an ac
drive, the domain wall starts to move when the driving force exceeds an
effective threshold which is temperature and frequency-dependent. Similarly to
the case of zero-temperature, the hysteresis loop displays three dynamical
phase transitions at increasing ac field amplitude . The phase diagram in
the 3-d space of temperature, driving force amplitude and frequency is
investigated.Comment: 4 pages, 2 figure
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