124 research outputs found

### Field theory of bi- and tetracritical points: Relaxational dynamics

We calculate the relaxational dynamical critical behavior of systems of
$O(n_\|)\oplus O(n_\perp)$ symmetry by renormalization group method within the
minimal subtraction scheme in two loop order. The three different bicritical
static universality classes previously found for such systems correspond to
three different dynamical universality classes within the static borderlines.
The Heisenberg and the biconical fixed point lead to strong dynamic scaling
whereas in the region of stability of the decoupled fixed point weak dynamic
scaling holds. Due to the neighborhood of the stability border between the
strong and the weak scaling dynamic fixed point corresponding to the static
biconical and the decoupled fixed point a very small dynamic transient
exponent, of $\omega_v^{{\cal B}}=0.0044$, is present in the dynamics for the
physically important case $n_\|=1$ and $n_\perp=2$ in $d=3$.Comment: 8 figure

### Scaling of the Hysteresis Loop in Two-dimensional Solidification

The first order phase transitions between a two-dimensional (2d) gas and the
2d solid of the first monolayer have been studied for the noble gases Ar, Kr
and Xe on a NaCl(100) surface in quasi-equilibrium with the three-dimensional
gas phase. Using linear temperature ramps, we show that the widths of the
hysteresis loops of these transitions as a function of the heating rate, r,
scales with a power law r^alpha with alpha between 0.4 and 0.5 depending on the
system. The hysteresis loops for different heating rates are similar. The
island area of the condensed layer was found to grow initially with a t^4 time
dependence. These results are in agreement with theory, which predicts alpha =
0.5 and hysteresis loop similarity.Comment: 4 pages, 5 figures, Revte

### Topological classification of vortex-core structures of spin-1 Bose-Einstein condensates

We classify vortex-core structures according to the topology of the order
parameter space. By developing a method to characterize how the order parameter
changes inside the vortex core. We apply this method to the spin-1
Bose-Einstein condensates and show that the vortex-core structures are
classified by winding numbers that are locally defined in the core region. We
also show that a vortex-core structure with a nontrivial winding number can be
stabilized under a negative quadratic Zeeman effect.Comment: 16 pages, 6 figure

### A proposal to detect vortices above the superconducting transition temperature

We propose a simple experiment to determine whether vortices persist above
the superconducting transition temperature Tc in the pseudogap phase of high
temperature cuprate superconductors. This involves using a magnetic dot to
stabilize a vortex in a thin cuprate film beneath the dot. We calculate the
magnetic field profile as a function of distance from the dot if a vortex is
present, and discuss possible measurements that could be done to detect this.
Finally, we comment on the temperature range where a stable vortex should be
observable.Comment: 3 pages, 2 figure

### Effects of Strain coupling and Marginal dimensionality in the nature of phase transition in Quantum paraelectrics

Here a recently observed weak first order transition in doped SrTiO3 is
argued to be a consequence of the coupling between strain and order parameter
fluctuations. Starting with a semi-microscopic action, and using
renormalization group equations for vertices, we write the free energy of such
a system. This fluctuation renormalized free energy is then used to discuss the
possibility of first order transition at zero temperature as well as at finite
temperature. An asymptotic analysis predicts small but a finite discontinuity
in the order parameter near a mean field quantum critical point at zero
temperature. In case of finite temperature transition, near quantum critical
point such a possibility is found to be extremely weak. Results are in accord
with some experimental findings on quantum paraelectrics such as SrTiO3 and
KTaO3.Comment: Revised versio

### Mesoscopic field and current compensator based on a hybrid superconductor-ferromagnet structure

A rather general enhancement of superconductivity is demonstrated in a hybrid
structure consisting of submicron superconducting (SC) sample combined with an
in-plane ferromagnet (FM). The superconducting state resists much higher
applied magnetic fields for both perpendicular polarities, as applied field is
screened by the FM. In addition, FM induces (in the perpendicular direction to
its moment) two opposite current-flows in the SC plane, under and aside the
magnet, respectively. Due to the compensation effects, superconductivity
persists up to higher applied currents. With increasing current, the sample
undergoes SC-"resistive"-normal state transitions through a mixture of
vortex-antivortex and phase-slip phenomena.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let

### Theory of Josephson effect in chiral p-wave superconductor / diffusive normal metal / chiral p-wave superconductor junctions

We study the Josephson effect between chiral p-wave superconductor /
diffusive normal metal (DN) / chiral p-wave superconductor (CP/DN/CP) junctions
using quasiclassical Green's function formalism with proper boundary
conditions. The px+ipy-wave symmetry of superconducting order parameter is
chosen which is believed to be a pairing state in Sr2RuO4. It is shown that the
Cooper pairs induced in DN have an odd-frequency spin-triplet s-wave symmetry,
where pair amplitude is an odd function of Matsubara frequency. Despite the
peculiar symmetry properties of the Cooper pairs, the behavior of the Josephson
current is rather conventional. We have found that the current phase relation
is almost sinusoidal and the Josephson current is proportional to exp(-L/xi),
where xi is the coherence length of the Cooper pair in DN and L is the length
of DN. The Josephson current between CP / diffusive ferromagnet metal (DF) / CP
junctions is also calculated. It is shown that the 0-pi transition can be
realized by varying temperature or junction length L similar to the case of
conventional s-wave junctions. These results may serve as a guide to study
superconducting state of Sr2RuO4.Comment: 9 pages, 9 figure

### Vortex Plasma in a Superconducting Film with Magnetic Dots

We consider a superconducting film, placed upon a magnetic dot array.
Magnetic moments of the dots are normal to the film and randomly oriented. We
determine how the concentration of the vortices in the film depends on the
magnetic moment of a dot at low temperatures. The concentration of the
vortices, bound to the dots, is proportional to the density of the dots and
depends on the magnetization of a dot in a step-like way. The concentration of
the unbound vortices oscillates about a value, proportional to the magnetic
moment of the dots. The period of the oscillations is equal to the width of a
step in the concentration of the bound vortices.Comment: RevTeX, 4 page

### Crossed Andreev reflection-induced magnetoresistance

We show that very large negative magnetoresistance can be obtained in
magnetic trilayers in a current-in-plane geometry owing to the existence of
crossed Andreev reflection. This spin-valve consists of a thin superconducting
film sandwiched between two ferromagnetic layers whose magnetization is allowed
to be either parallelly or antiparallelly aligned. For a suitable choice of
structure parameters and nearly fully spin-polarized ferromagnets the
magnetoresistance can exceed -80%. Our results are relevant for the design and
implementation of spintronic devices exploiting ferromagnet-superconductor
structures.Comment: 5 pages, 4 figures, final published versio

- â€¦