116 research outputs found
Theory of Andreev reflection in a two-orbital model of iron-pnictide superconductors
A recently developed theory for the problem of Andreev reflection between a
normal metal (N) and a multiband superconductor (MBS) assumes that the incident
wave from the normal metal is coherently transmitted through several bands
inside the superconductor. Such splitting of the probability amplitude into
several channels is the analogue of a quantum waveguide. Thus, the appropriate
matching conditions for the wave function at the N/MBS interface are derived
from an extension of quantum waveguide theory. Interference effects between the
transmitted waves inside the superconductor manifest themselves in the
conductance. We provide results for a FeAs superconductor, in the framework of
a recently proposed effective two-band model and two recently proposed gap
symmetries: in the sign-reversed s-wave () scenario
resonant transmission through surface Andreev bound states (ABS) at nonzero
energy is found as well as destructive interference effects that produce zeros
in the conductance; in the extended s-wave ()
scenario no ABS at finite energy are found.Comment: 4 pages, 5 figure
Estudo do polimorfismo do gene do hormônio de crescimento em rebanhos da raça Nelore do estado da Bahia.
Título em inglês: Study of the polymorphism of the gene of the hormone of growth in a Nelore breed of the state of the Bahia
Mixed-valent regime of the two-channel Anderson impurity as a model for UBe_13
We investigate the mixed-valent regime of a two-configuration Anderson
impurity model for uranium ions, with separate quadrupolar and magnetic
doublets. With a new Monte Carlo approach and the non-crossing approximation we
find: (i) A non-Fermi-liquid fixed point with two-channel Kondo model critical
behavior; (ii) Distinct energy scales for screening the low-lying and excited
doublets; (iii) A semi-quantitative explanation of magnetic-susceptibility data
for UThBe assuming 60-70% quadrupolar doublet ground-state
weight, supporting the quadrupolar-Kondo interpretation.Comment: 4 Pages, 3 eps figures; submitted to Phys. Rev. Let
Spin-density Wave in Ising-coupled Antiferromagnetic Chains
The effect of anisotropy in the nearest-neighbor spin interactions that
couple consecutive spin-1/2 antiferromagnetic chains is studied
theoretically by considering the limit where the coupling is purely of the
Ising type. An analysis based on the equivalent Luttinger model reveals that
the groundstate is an Ising antiferromagnet in general.Comment: 11 pgs. of plain TeX, one postscript fig., to appear in Phys. Rev.
Supercurrent induced domain wall motion
We study the dynamics of a magnetic domain wall, inserted in, or juxtaposed
to, a conventional superconductor, via the passage of a spin polarized current
through a FSF junction. Solving the Landau-Lifshitz-Gilbert equation of motion
for the magnetic moments we calculate the velocity of the domain wall and
compare it with the case of a FNF junction. We find that in several regimes the
domain wall velocity is larger when it is driven by a supercurrent.Comment: 10 pages, 8 figure
Dephasing in Metals by Two-Level Systems in the 2-Channel-Kondo Regime
We point out a novel, non-universal contribution to the dephasing rate
1/\tau_\phi \equiv \gamma_\phi of conduction electrons in metallic systems:
scattering off non-magnetic two-level systems (TLSs) having almost degenerate
Kondo ground states. In the regime \Delta_{ren} < T < T_K (\Delta_{ren} =
renormalized level splitting, T_K = Kondo temperature), such TLSs exhibit
non-Fermi-liquid physics that can cause \gamma_\phi, which generally decreases
with decreasing T, to seemingly saturate in a limited temperature range before
vanishing for T \to 0. This could explain the saturation of dephasing recently
observed in gold wires [Mohanty et al. Phys. Rev. Lett. 78, 3366 (1997)].Comment: Final published version, including minor improvements suggested by
referees. 4 pages, Revtex, 1 figur
Gapless spectrum in a class of S=1 exchange models with long-range interactions
We present evidence for the absence of a gap in a class of S=1
antiferromagnetic exchange models. The spin exchange is long-ranged of the type
where . We have shown previously that
without the alternating factor the model for (S=1 Haldane-Shastry
model) has a gap, exponentially decaying correlation functions and
exponentially small susceptibility at very low temperatures. In the case of the
alternated interaction the stabilizing next nearest neighbor ferromagnetic
interaction changes qualitatively the behavior of the system. We have studied
the groundstate and first excited state using a modified Lanczos algorithm for
system sizes up to 16 sites. Also, we performed exact diagonalization for
systems up to 8 sites and obtained the thermodynamics. The correlation
functions decay with distance like a power law. These models define a new class
of integer spin chains that do not show a Haldane gap. The results may be
relevant to describe impurity spins coupled by a RKKY-interaction through a
half-filled conduction-electron band.Comment: 19 pages, 13 figures, RevTex, improved version, to appear in Journal
of Physics: Condensed Matte
Single vortex structure in two models of iron pnictide superconductivity
The structure of a single vortex in a FeAs superconductor is studied in the
framework of two formulations of superconductivity for the recently proposed
sign-reversed wave () scenario: {\it (i)} a continuum model taking
into account the existence of an electron and a hole band with a repulsive
local interaction between the two; {\it (ii)} a lattice tight-binding model
with two orbitals per unit cell and a next-nearest-neighbour attractive
interaction. In the first model, the local density of states (LDOS) at the
vortex centre, as a function of energy, exhibits a peak at the Fermi level,
while in the second model such LDOS peak is deviated from the Fermi level and
its energy depends on band filling. An impurity located outside the vortex core
has little effect on the LDOS peak, but an impurity close to the vortex core
can almost suppress it and modify its position.Comment: 17 pages, 15 figures. Accepted for publication in New Journal of
Physic
Replacement of soybean meal with soybean cake reduces methane emissions in dairy cows and an assessment of a face-mask technique for methane measurement.
Kondo Effect in Systems with Spin Disorder
We consider the role of static disorder in the spin sector of the one- and
two-channel Kondo models. The distribution functions of the disorder-induced
effective energy splitting between the two levels of the Kondo impurity are
derived to the lowest order in the concentration of static scatterers. It is
demonstrated that the distribution functions are strongly asymmetric, with the
typical splitting being parametrically smaller than the average rms value. We
employ the derived distribution function of splittings to study the temperature
dependence of the low-temperature conductance of a sample containing an
ensemble of two-channel Kondo impurities. The results are used to analyze the
consistency of the two-channel Kondo interpretation of the zero-bias anomalies
observed in Cu/(Si:N)/Cu nanoconstrictions.Comment: 16 pages, 5 figures, REVTe
- …