218 research outputs found
Vorticity and magnetic shielding in a type-II superconductor
We study in detail, solving the Bogoliubov-de Gennes equations, the magnetic
field, supercurrent and order parameter profiles originated by a solenoid or
magnetic whisker inserted in a type-II superconductor. We consider solutions of
different vorticities, n, in the various cases. The results confirm the
connection between the vorticity, the internal currents and the boundstates in
a self-consistent way. The number of boundstates is given by the vorticity of
the phase of the gap function as in the case with no external solenoid. In the
limiting case of an infinitely thin solenoid, like a Dirac string, the solution
is qualitatively different. The quasiparticle spectrum and wave functions are a
function of n-n_ext, where n_ext is the vorticity of the solenoid. The flux is
in all cases determined by the vorticity of the gap function.Comment: revised version, 25 pages, LaTex, 10 figure
Overscreened Single Channel Kondo Problem
We consider the single channel Kondo problem with the Kondo coupling between
a spin impurity and conduction electrons with spin . These problems
arise as multicritical points in the parameter spaces of two- and higher-level
tunneling systems, and some impurity models of heavy fermion compounds. In
contrast to the previous Bethe-anstaz conjectures, it turns out that the
dynamics of the spin sector is the same as that of a spin impurity coupled
to channels of spin electrons with . As a
result, for , the system shows non-Fermi liquid behavior with the
same exponents for the thermodynamic quantities as those of channel
Kondo problem. However, both the finite-size spectrum and the operator content
are different due to the presence of the other sectors and can be obtained by
conformal field theory techniques.Comment: 4 pages, revtex, no figures. Revised Versio
Low-cost multipurpose sensor network integrated with iot and webgis for fire safety concerns
Fire emergencies cause severe damage to Brazilian federal universities. An appropriate and efficient tool to prevent or detect such events early is multisensory networks from the Internet of Things (IoT). In this study, we present the stages of development of a WebGIS system which integrates the IoT that allows the detection and helps manage such incidents. The approach consists of a network of multipurpose sensors that can identify different sources of fire hazards. If a potential source is registered, information about environmental conditions is transmitted in real-time to the system. Depending on the severity level, an alert is issued to WebGIS. Location is represented on a map. The entire system consists of single-board devices. Software components are based on open-source tools. The whole network only needs little power and, therefore, theoretically, could be carried out as an autonomous system powered by batteries. The entire system has been tested with flame, temperature, gas, smoke, and humidity sensors. The experiments allowed us to show its potential, formulate recommendations and indications for future studies
Charge dynamics in half-filled Hubbard chains with finite on-site interaction
We study the charge dynamic structure factor of the one-dimensional Hubbard
model with finite on-site repulsion U at half filling. Numerical results from
the time-dependent density matrix renormalization group are analyzed by
comparison with the exact spectrum of the model. The evolution of the line
shape as a function of U is explained in terms of a relative transfer of
spectral weight between the two-holon continuum that dominates in the limit
U\to \infty and a subset of the two-holon-two-spinon continuum that
reconstructs the electron-hole continuum in the limit U\to 0. Power-law
singularities along boundary lines of the spectrum are described by effective
impurity models that are explicitly invariant under spin and \eta-spin SU(2)
rotations. The Mott-Hubbard metal-insulator transition is reflected in a
discontinuous change of the exponents of edge singularities at U=0. The sharp
feature observed in the spectrum for momenta near the zone boundary is
attributed to a Van Hove singularity that persists as a consequence of
integrability.Comment: 22 pages, 13 figure
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
Magnetic impurities in a superconductor: Effect of domain walls and interference
We consider the effect of magnetic impurities, modeled by classical spins, in
a conventional superconductor. We study their effect on the quasiparticles,
specifically on the spin density and local density of states (LDOS). As
previously emphasized, the impurities induce multiple scatterings of the
quasiparticle wave functions leading to complex interference phenomena. Also,
the impurities induce quantum phase transitions in the many-body system.
Previous authors studied the effect of either a small number of impurities
(from one to three) or a finite concentration of impurities, typically in a
disordered distribution. In this work we assume a regular set of spins
distributed inside the superconductor in such a way that the spins are
oriented, forming different types of domain walls, assumed stable. This
situation may be particularly interesting in the context of spin transfer due
to polarized currents traversing the material.Comment: 26 pages, 26 figures (72 in total
Superconductivity Driven by Chain Coupling and Electronic Correlations
We present an analysis of a system of weakly coupled Hubbard chains based on
combining an exact study of spectral functions of the uncoupled chain system
with a renormalization group method for the coupled chains. For low values of
the onsite repulsion and of the doping , the leading instability is
towards a superconducting state. The process includes excited states above a
small correlation pseudogap. Similar features appear in extended Hubbard models
in the vicinity of commensurate fillings. Our theoretical predictions are
consistent with the phase diagram observed in the (TMTTF)X and (TMTSF)X
series of organic compounds.Comment: 7 pages, 2 figure
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
Numerical Renormalization Group Study of non-Fermi-liquid State on Dilute Uranium Systems
We investigate the non-Fermi-liquid (NFL) behavior of the impurity Anderson
model (IAM) with non-Kramers doublet ground state of the f configuration
under the tetragonal crystalline electric field (CEF). The low energy spectrum
is explained by a combination of the NFL and the local-Fermi-liquid parts which
are independent with each other. The NFL part of the spectrum has the same form
to that of two-channel-Kondo model (TCKM). We have a parameter range that the
IAM shows the divergence of the magnetic susceptibility together with
the positive magneto resistance. We point out a possibility that the anomalous
properties of UThRuSi including the decreasing resistivity
with decreasing temperature can be explained by the NFL scenario of the TCKM
type. We also investigate an effect of the lowering of the crystal symmetry. It
breaks the NFL behavior at around the temperature, , where
is the orthorhombic CEF splitting. The NFL behavior is still expected above the
temperature, .Comment: 25 pages, 12 figure
Interplay of disorder and magnetic field in the superconducting vortex state
We calculate the density of states of an inhomogeneous superconductor in a
magnetic field where the positions of vortices are distributed completely at
random. We consider both the cases of s-wave and d-wave pairing. For both
pairing symmetries either the presence of disorder or increasing the density of
vortices enhances the low energy density of states. In the s-wave case the gap
is filled and the density of states is a power law at low energies. In the
d-wave case the density of states is finite at zero energy and it rises
linearly at very low energies in the Dirac isotropic case
(\alpha_D=t/\Delta_0=1, where t is the hopping integral and \Delta_0 is the
amplitude of the order parameter). For slightly higher energies the density of
states crosses over to a quadratic behavior. As the Dirac anisotropy increases
(as \Delta_0 decreases with respect to the hopping term) the linear region
decreases in width. Neglecting this small region the density of states
interpolates between quadratic and back to linear as \alpha_D increases. The
low energy states are strongly peaked near the vortex cores.Comment: 12 REVTeX pages, 15 figure
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