2,480 research outputs found
A large-N analysis of the local quantum critical point and the spin-liquid phase
We study analytically the Kondo lattice model with an additional
nearest-neighbor antiferromagnetic interaction in the framework of large-N
theory. We find that there is a local quantum critical point between two
phases, a normal Fermi-liquid and a spin-liquid in which the spins are
decoupled from the conduction electrons. The local spin susceptibility displays
a power-law divergence throughout the spin liquid phase. We check the
reliability of the large-N results by solving by quantum Monte Carlo simulation
the N=2 spin-liquid problem with no conduction electrons and find qualitative
agreement. We show that the spin-liquid phase is unstable at low temperatures,
suggestive of a first-order transition to an ordered phase.Comment: 4 pages and 1 figur
Theory of the spin galvanic effect at oxide interfaces
The spin galvanic effect (SGE) describes the conversion of a non-equilibrium
spin polarization into a transverse charge current. Recent experiments have
demonstrated a large conversion efficiency for the two-dimensional electron gas
formed at the interface between two insulating oxides, LaAlO and SrTiO.
Here we analyze the SGE for oxide interfaces within a three-band model for the
Ti t orbitals which displays an interesting variety of effective
spin-orbit couplings in the individual bands that contribute differently to the
spin-charge conversion. Our analytical approach is supplemented by a numerical
treatment where we also investigate the influence of disorder and temperature,
which turns out to be crucial to provide an appropriate description of the
experimental data.Comment: 5 pages, 3 figure
Self-consistent Modeling of the of HTS Devices: How Accurate do Models Really Need to Be?
Numerical models for computing the effective critical current of devices made
of HTS tapes require the knowledge of the Jc(B,theta) dependence, i.e. of the
way the critical current density Jc depends on the magnetic flux density B and
its orientation theta with respect to the tape. In this paper we present a
numerical model based on the critical state with angular field dependence of Jc
to extract the Jc(B,theta) relation from experimental data. The model takes
into account the self-field created by the tape, which gives an important
contribution when the field applied in the experiments is low. The same model
can also be used to compute the effective critical current of devices composed
of electromagnetically interacting tapes. Three examples are considered here:
two differently current rated Roebel cables composed of REBCO coated conductors
and a power cable prototype composed of Bi-2223 tapes. The critical currents
computed with the numerical model show good agreement with the measured ones.
The simulations reveal also that several parameter sets in the Jc(B,theta) give
an equally good representation of the experimental characterization of the
tapes and that the measured Ic values of cables are subjected to the influence
of experimental conditions, such as Ic degradation due to the manufacturing and
assembling process and non-uniformity of the tape properties. These two aspects
make the determination of a very precise Jc(B,theta) expression probably
unnecessary, as long as that expression is able to reproduce the main features
of the angular dependence. The easiness of use of this model, which can be
straightforwardly implemented in finite-element programs able to solve static
electromagnetic problems, is very attractive both for researchers and devices
manufactures who want to characterize superconducting tapes and calculate the
effective critical current of superconducting devices
Fluorescent thermal imaging of a non-insulated pancake coil wound from high temperature superconductor tape
We have wound a 157-turn, non-insulated pancake coil with an outer diameter
of 85 mm and we cooled it down to 77 K with a combination of conduction and gas
cooling. Using high-speed fluorescent thermal imaging in combination with
electrical measurements we have investigated the coil under load, including
various ramping tests and over-current experiments. We have found found that
the coil does not heat up measurably when being ramped to below its critical
current. Two over-current experiments are presented, where in one case the coil
recovered by itself and in another case a thermal runaway occurred. We have
recorded heating in the bulk of the windings due to local defects, however the
coil remained cryostable even during some over-critical conditions and heated
only to about 82-85 K at certain positions. A thermal runaway was observed at
the center, where the highest magnetic field and a resistive joint create a
natural defect. The maximum temperature, ~100 K, was reached only in the few
innermost windings around the coil former
Density inhomogeneities and Rashba spin-orbit coupling interplay in oxide interfaces
There is steadily increasing evidence that the two-dimensional electron gas
(2DEG) formed at the interface of some insulating oxides like LaAlO3/SrTiO3 and
LaTiO3/SrTiO3 is strongly inhomogeneous. The inhomogeneous distribution of
electron density is accompanied by an inhomogeneous distribution of the
(self-consistent) electric field confining the electrons at the interface. In
turn this inhomogeneous transverse electric field induces an inhomogeneous
Rashba spin-orbit coupling (RSOC). After an introductory summary on two
mechanisms possibly giving rise to an electronic phase separation accounting
for the above inhomogeneity,we introduce a phenomenological model to describe
the density-dependent RSOC and its consequences. Besides being itself a
possible source of inhomogeneity or charge-density waves, the density-dependent
RSOC gives rise to interesting physical effects like the occurrence of
inhomogeneous spin-current distributions and inhomogeneous quantum-Hall states
with chiral "edge" states taking place in the bulk of the 2DEG. The
inhomogeneous RSOC can also be exploited for spintronic devices since it can be
used to produce a disorder-robust spin Hall effect.Comment: 13 pages, 15 figure
- …