26,762 research outputs found
Dimensionality effects in the LDOS of ferromagnetic hosts probed via STM: spin-polarized quantum beats and spin filtering
We theoretically investigate the local density of states (LDOS) probed by a
STM tip of ferromagnetic metals hosting a single adatom and a subsurface
impurity. We model the system via the two-impurity Anderson Hamiltonian. By
using the equation of motion with the relevant Green functions, we derive
analytical expressions for the LDOS of two host types: a surface and a quantum
wire. The LDOS reveals Friedel-like oscillations and Fano interference as a
function of the STM tip position. These oscillations strongly depend on the
host dimension. Interestingly, we find that the spin-dependent Fermi wave
numbers of the hosts give rise to spin-polarized quantum beats in the LDOS.
While the LDOS for the metallic surface shows a damped beating pattern, it
exhibits an opposite behavior in the quantum wire. Due to this absence of
damping, the wire operates as a spatially resolved spin filter with a high
efficiency.Comment: revised tex
Finite size analysis of a two-dimensional Ising model within a nonextensive approach
In this work we present a thorough analysis of the phase transitions that
occur in a ferromagnetic 2D Ising model, with only nearest-neighbors
interactions, in the framework of the Tsallis nonextensive statistics. We
performed Monte Carlo simulations on square lattices with linear sizes L
ranging from 32 up to 512. The statistical weight of the Metropolis algorithm
was changed according to the nonextensive statistics. Discontinuities in the
m(T) curve are observed for . However, we have verified only one
peak on the energy histograms at the critical temperatures, indicating the
occurrence of continuous phase transitions. For the regime, we
have found continuous phase transitions between the ordered and the disordered
phases, and determined the critical exponents via finite-size scaling. We
verified that the critical exponents , and depend
on the entropic index in the range in the form , and . On the other hand, the critical exponent does not
depend on . This suggests a violation of the scaling relations and and a nonuniversality of the
critical exponents along the ferro-paramagnetic frontier.Comment: accepted for publication in Phys. Rev.
Experimental analysis of lateral impact on planar brittle material: spatial properties of the cracks
The breakup of glass and alumina plates due to planar impacts on one of their
lateral sides is studied. Particular attention is given to investigating the
spatial location of the cracks within the plates. Analysis based on a
phenomenological model suggests that bifurcations along the cracks' paths are
more likely to take place closer to the impact region than far away from it, i.
e., the bifurcation probability seems to lower as the perpendicular distance
from the impacted lateral in- creases. It is also found that many observables
are not sensitive to the plate material used in this work, as long as the
fragment multiplicities corresponding to the fragmentation of the plates are
similar. This gives support to the universal properties of the fragmentation
process reported in for- mer experiments. However, even under the just
mentioned circumstances, some spatial observables are capable of distinguishing
the material of which the plates are made and, therefore, it suggests that this
universality should be carefully investigated
Experimental investigation of linear-optics-based quantum target detection
The development of new techniques to improve measurements is crucial for all
sciences. By employing quantum systems as sensors to probe some physical
property of interest allows the application of quantum resources, such as
coherent superpositions and quantum correlations, to increase measurement
precision. Here we experimentally investigate a scheme for quantum target
detection based on linear optical measurment devices, when the object is
immersed in unpolarized background light. By comparing the quantum
(polarization-entangled photon pairs) and the classical (separable polarization
states), we found that the quantum strategy provides us an improvement over the
classical one in our experiment when the signal to noise ratio is greater than
1/40, or about 16dB of noise. This is in constrast to quantum target detection
considering non-linear optical detection schemes, which have shown resilience
to extreme amounts of noise. A theoretical model is developed which shows that,
in this linear-optics context, the quantum strategy suffers from the
contribution of multiple background photons. This effect does not appear in our
classical scheme. By improving the two-photon detection electronics, it should
be possible to achieve a polarization-based quantum advantage for a signal to
noise ratio that is close to 1/400 for current technology.Comment: comments are welcome, submitted to PR
Quantum phase transition triggering magnetic BICs in graphene
Graphene hosting a pair of collinear adatoms in the phantom atom
configuration has pseudogap with cubic scaling on energy,
which leads to the appearance of
spin-degenerate bound states in the continuum (BICs) [Phys. Rev. B 92, 045409
(2015)]. In the case when adatoms are locally coupled to a single carbon atom
the pseudogap scales linearly with energy, which prevents the formation of
BICs. In this Letter, we explore the effects of non-local coupling
characterized by the Fano factor of interference tunable by changing
the slope of the Dirac cones in the graphene band-structure. We demonstrate
that three distinct regimes can be identified: i) for (critical
point) a mixed pseudogap appears
yielding a phase with spin-degenerate BICs; ii) near when
the system undergoes a quantum phase
transition in which the new phase is characterized by magnetic BICs and iii) at
a second critical value the cubic scaling of the pseudogap with
energy characteristic to the phantom atom
configuration is restored and the phase with non-magnetic BICs is recovered.
The phase with magnetic BICs can be described in terms of an effective
intrinsic exchange field of ferromagnetic nature between the adatoms mediated
by graphene monolayer. We thus propose a new type of quantum phase transition
resulting from the competition between the states characterized by
spin-degenerate and magnetic BICs
Skeletopy of the intumescentia lumbalis and conus medullaris applied to epidural anaesthesia in Leopardus geoffroyi
Background: Leopardus geoffroyi is a Neotropical wild felid with wide distribution in the south of the South American continent. The objective was to investigate the skeletopy of the intumescentia lumbalis (IL) and conus medullaris (CM) from 11 specimens of L. geoffroyi collected dead on highways.
Materials and methods: The cadavers were fixed in formaldehyde solution and dissected to allow the dorsal exposure of IL and CM. The cranial and caudal limits were marked with radiopaque pins and radiographic projections were used to determine the skeletopy. The lengths of IL and CM were measured with a pachymeter.
Results: In most specimens, the IL was located at the level of L4 and L5 vertebrae, although in 4 (1 male and 3 females) individuals its cranial limit was L3 and in 3 specimens (2 male and 1 female) the caudal limit was L6. The length of IL was 35.6 ± 6.7 mm. The CM had its base predominantly at the level of the L5 vertebra, although in some specimens the base was in L4 and in others in L6. The apex of the CM can be found since the lumbosacral junction until the level of the Cd2 vertebra. The CM measured 74.4 ± 14.3 mm.
Conclusions: Based on the skeletopy, it can be suggested that epidural anaesthesia procedures in L. geoffroyi are safer with the introduction of the catheter through the sacrocaudal interarcual space, as recommended by some anaesthetists for the domestic cat
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