143 research outputs found
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
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
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
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
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
Spin dynamics of the S=1/2 antiferromagnetic zig-zag ladder with anisotropy
We use exact diagonalization and the modified Lanczos method to study the
finite energy and finite momentum spectral weight of the longitudinal and
transverse spin excitations of the anisotropic zig-zag ladder. We find that the
spin excitations form continua of gapless or gapped spinons in the different
regions of the phase diagram. The results obtained are consistent with a
picture previously proposed that in the anisotropic case there is a transition
from a gapped regime to a gapless regime, for small interchain coupling. In
this regime we find a sharp low-energy peak in the structure function for the
transverse spin excitations, consistent with a finite stiffness.Comment: 17 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
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
Non-Fermi Liquid Behavior in Dilute Quadrupolar System PrLaPb with 0.05
We have studied the low-temperature properties of PrLaPb
with non-Kramers quadrupolar moments of the crystal-electric-field
ground state, for a wide concentration range of Pr ions. For 0.05, the
specific heat increases monotonically below =1.5 K, which can be
scaled with a characteristic temperature defined at each concentration
. The electrical resistivity in the corresponding temperature
region shows a marked decrease deviating from a Fermi-liquid behavior
. The Kondo effect arising from the correlation
between the dilute moments and the conduction electrons may give
rise to such anomalous behavior
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