7,719 research outputs found
Phase diagrams of XXZ model on depleted square lattice
Using quantum Monte Carlo (QMC) simulations and a mean field (MF) theory, we
investigate the spin-1/2 XXZ model with nearest neighbor interactions on a
periodic depleted square lattice. In particular, we present results for 1/4
depleted lattice in an applied magnetic field and investigate the effect of
depletion on the ground state. The ground state phase diagram is found to
include an antiferromagnetic (AF) phase of magnetization and an
in-plane ferromagnetic (FM) phase with finite spin stiffness. The agreement
between the QMC simulations and the mean field theory based on resonating
trimers suggests the AF phase and in-plane FM phase can be interpreted as a
Mott insulator and superfluid of trimer states respectively. While the thermal
transitions of the in-plane FM phase are well described by the
Kosterlitz-Thouless transition, the quantum phase transition from the AF phase
to in-plane FM phase undergo a direct second order insulator-superfluid
transition upon increasing magnetic field.Comment: 7 pages, 8 figures. Revised version, accepted by PRB
Theory of the tunneling spectroscopy of ferromagnetic superconductors
We study tunneling conductance in normal metal / insulator / ferromagnetic
superconductor junctions. The tunneling spectra show a clear difference between
spin-singlet s-wave pairing, spin-triplet opposite spin pairing and
spin-triplet equal spin pairing: These pairings exhibit, respectively, gap
struture, double peak structure and zero bias peak in the spectra. The obtained
result may serve as a tool for determining the pairing symmetry of
ferromagnetic superconductors.Comment: 4 pages, 2 figure
Andreev reflection at half-metal-superconductor interfaces with non-uniform magnetization
Andreev reflection at the interface between a half-metallic ferromagnet and a
spin-singlet superconductor is possible only if it is accompanied by a spin
flip. Here we calculate the Andreev reflection amplitudes for the case that the
spin flip originates from a spatially non-uniform magnetization direction in
the half metal. We calculate both the microscopic Andreev reflection amplitude
for a single reflection event and an effective Andreev reflection amplitude
describing the effect of multiple Andreev reflections in a ballistic thin film
geometry. It is shown that the angle and energy dependence of the Andreev
reflection amplitude strongly depends on the orientation of the gradient of the
magnetization with respect to the interface. Establishing a connection between
the scattering approach employed here and earlier work that employs the
quasiclassical formalism, we connect the symmetry properties of the Andreev
reflection amplitudes to the symmetry properties of the anomalous Green
function in the half metal.Comment: 13 pages, 4 figure
A few comments on the high-energy behavior of string scattering amplitudes in warped spacetimes
The high-energy behavior of string scattering in warped spacetimes is studied
to all orders in perturbation theory. If one assumes that the theory is finite,
the amplitudes exactly fall as powers of momentum.Comment: 6 page
Non-divergent pseudo-potential treatment of spin-polarized fermions under 1D and 3D harmonic confinement
Atom-atom scattering of bosonic one-dimensional (1D) atoms has been modeled
successfully using a zero-range delta-function potential, while that of bosonic
3D atoms has been modeled successfully using Fermi-Huang's regularized s-wave
pseudo-potential. Here, we derive the eigenenergies of two spin-polarized 1D
fermions under external harmonic confinement interacting through a zero-range
potential, which only acts on odd-parity wave functions, analytically. We also
present a divergent-free zero-range potential treatment of two spin-polarized
3D fermions under harmonic confinement. Our pseudo-potential treatments are
verified through numerical calculations for short-range model potentials.Comment: 9 pages, 4 figures (subm. to PRA on 03/15/2004
Possible role of 3He impurities in solid 4He
We use a quantum lattice gas model to describe essential aspects of the
motion of 4He atoms and of 3He impurities in solid 4He. This study suggests
that 3He impurities bind to defects and promote 4He atoms to interstitial sites
which can turn the bosonic quantum disordered crystal into a metastable
supersolid. It is suggested that defects and interstitial atoms are produced
during the solid 4He nucleation process where the role of 3He impurities (in
addition to the cooling rate) is known to be important even at very small (1
ppm) impurity concentration. It is also proposed that such defects can form a
glass phase during the 4He solid growth by rapid cooling.Comment: 4 two-column Revtex pages, 4 figures. Europhysics Letters (in Press
Evolution of a bosonic mode across the superconducting dome in the high-Tc cuprate Pr(2-x)Ce(x)CuO(4-{\delta})
We report a detailed spectroscopic study of the electron doped cuprate
superconductor Pr(2-x)Ce(x)CuO(4-{\delta}) using point contact junctions for
x=0.125(underdoped), x=0.15(optimally doped) and x=0.17(overdoped). From our
conductance measurements we are able to identify bosonic resonances for each
doping. These excitations disappear above the critical temperature, and above
the critical magnetic field. We find that the energy of the bosonic excitations
decreases with doping, which excludes lattice vibrations as the paring glue. We
conclude that the bosonic mediator for these cuprates is more likely to be spin
excitations.Comment: 4 page
Density of states in d-wave superconductors of finite size
We consider the effect of the finite size in the ab-plane on the surface
density of states (DoS) in clean d-wave superconductors. In the bulk, the DoS
is gapless along the nodal directions, while the presence of a surface leads to
formation of another type of the low-energy states, the midgap states with zero
energy. We demonstrate that finiteness of the superconductor in one of
dimensions provides the energy gap for all directions of quasiparticle motion
except for \theta=45 degrees (\theta is the angle between the trajectory and
the surface normal); then the angle-averaged DoS behaves linearly at small
energies. This result is valid unless the crystal is 0- or 45-oriented (\alpha
\ne 0 or 45 degrees, where \alpha is the angle between the a-axis and the
surface normal). In the special case of \alpha=0, the spectrum is gapped for
all trajectories \theta; the angle-averaged DoS is also gapped. In the special
case of \alpha=45, the spectrum is gapless for all trajectories \theta; the
angle-averaged DoS is then large at low energies. In all the cases, the
angle-resolved DoS consists of energy bands that are formed similarly to the
Kronig-Penney model. The analytical results are confirmed by a self-consistent
numerical calculation.Comment: 9 pages (including 5 EPS figures), REVTeX
Re-entrant localization of single particle transport in disordered Andreev wires
We study effects of disorder on the low energy single particle transport in a
normal wire surrounded by a superconductor. We show that the heat conductance
includes the Andreev diffusion decreasing with increase in the mean free path
and the diffusive drift produced by a small particle-hole asymmetry,
which increases with increasing . The conductance thus has a minimum as a
function of which leads to a peculiar re-entrant localization as a
function of the mean free path.Comment: 4 pages, 2 figure
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