6,487 research outputs found
Interplay between disorder, quantum and thermal fluctuations in ferromagnetic alloys: The case of UCu2Si(2-x)Ge(x)
We consider, theoretically and experimentally, the effects of structural
disorder, quantum and thermal fluctuations in the magnetic and transport
properties of certain ferromagnetic alloys.We study the particular case of
UCu2Si(2-x)Ge(x). The low temperature resistivity, rho(T,x), exhibits Fermi
liquid (FL) behavior as a function of temperature T for all values of x, which
can be interpreted as a result of the magnetic scattering of the conduction
electrons from the localized U spins. The residual resistivity, rho(0,x),
follows the behavior of a disordered binary alloy. The observed non-monotonic
dependence of the Curie temperature, Tc(x), with x can be explained within a
model of localized spins interacting with an electronic bath whose transport
properties cross-over from ballistic to diffusive regimes. Our results clearly
show that the Curie temperature of certain alloys can be enhanced due to the
interplay between quantum and thermal fluctuations with disorder.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Negative Hopping Magnetoresistance and Dimensional Crossover in Lightly Doped Cuprate Superconductors
We show that, due to the weak ferromagnetism of LaSrCuO, an
external magnetic field leads to a dimensional crossover 2D 3D for the
in-plane transport. The crossover results in an increase of the hole's
localization length and hence in a dramatic negative magnetoresistance in the
variable range hopping regime. This mechanism quantitatively explains puzzling
experimental data on the negative magnetoresistance in the N\'eel phase of
LaSrCuO.Comment: 6 pages, 3 figures; published versio
Vortex liquid crystals in anisotropic type II superconductors
In a type II superconductor in a moderate magnetic field, the superconductor
to normal state transition may be described as a phase transition in which the
vortex lattice melts into a liquid. In a biaxial superconductor, or even a
uniaxial superconductor with magnetic field oriented perpendicular to the
symmetry axis, the vortices acquire elongated cross sections and interactions.
Systems of anisotropic, interacting constituents generally exhibit liquid
crystalline phases. We examine the possibility of a two step melting in
homogeneous type II superconductors with anisotropic superfluid stiffness from
a vortex lattice into first a vortex smectic and then a vortex nematic at high
temperature and magnetic field. We find that fluctuations of the ordered phase
favor an instability to an intermediate smectic-A in the absence of intrinsic
pinning
Dirac fermions in a power-law-correlated random vector potential
We study localization properties of two-dimensional Dirac fermions subject to
a power-law-correlated random vector potential describing, e.g., the effect of
"ripples" in graphene. By using a variety of techniques (low-order perturbation
theory, self-consistent Born approximation, replicas, and supersymmetry) we
make a case for a possible complete localization of all the electronic states
and compute the density of states.Comment: Latex, 4+ page
Hamiltonian symplectic embedding of the massive noncommutative U(1) Theory
We show that the massive noncommutative U(1) theory is embedded in a gauge
theory using an alternative systematic way, which is based on the symplectic
framework. The embedded Hamiltonian density is obtained after a finite number
of steps in the iterative symplectic process, oppositely to the result proposed
using the BFFT formalism. This alternative formalism of embedding shows how to
get a set of dynamically equivalent embedded Hamiltonian densities.Comment: 16 pages, no figures, revtex4, corrected version, references
additione
Quantum Cosmology in Scalar-Tensor Theories With Non Minimal Coupling
Quantization in the minisuperspace of non minimal scalar-tensor theories
leads to a partial differential equation which is non separable. Through a
conformal transformation we can recast the Wheeler-DeWitt equation in an
integrable form, which corresponds to the minimal coupling case, whose general
solution is known. Performing the inverse conformal transformation in the
solution so found, we can construct the corresponding one in the original
frame. This procedure can also be employed with the bohmian trajectories. In
this way, we can study the classical limit of some solutions of this quantum
model. While the classical limit of these solutions occurs for small scale
factors in the Einstein's frame, it happens for small values of the scalar
field non minimally coupled to gravity in the Jordan's frame, which includes
large scale factors.Comment: latex, 18 page
Noncommutative Geometry and Cosmology
We study some consequences of noncommutativity to homogeneous cosmologies by
introducing a deformation of the commutation relation between the
minisuperspace variables. The investigation is carried out for the
Kantowski-Sachs model by means of a comparative study of the universe evolution
in four different scenarios: the classical commutative, classical
noncommutative, quantum commutative, and quantum noncommutative. The comparison
is rendered transparent by the use of the Bohmian formalism of quantum
trajectories. As a result of our analysis, we found that noncommutativity can
modify significantly the universe evolution, but cannot alter its singular
behavior in the classical context. Quantum effects, on the other hand, can
originate non-singular periodic universes in both commutative and
noncommutative cases. The quantum noncommutative model is shown to present
interesting properties, as the capability to give rise to non-trivial dynamics
in situations where its commutative counterpart is necessarily static.Comment: 22 pages, 5 figures, substantial changes in the presentation, results
are the same, to appear in Physical Review
Weakly Coupled Antiferromagnetic Quantum Spin Chains
Quasi-one-dimensional quantum antiferromagnets formed by a d-dimensional
hypercubic lattice of weakly coupled spin-1/2 antiferromagnetic Heisenberg
chains are studied by combining exact results in one-dimension and
renormalization group analyses of the interchain correlations. It is shown that
d-dimensional magnetic long-range order develops at zero-temperature for
infinitesimal antiferromagnetic or ferromagnetic interchain couplings. In the
presence of weak bond alternations, the order-disorder transition occurs at a
finite interchain coupling. Relevances to the lightly doped quantum
antiferromagnets and multi-layer quantum Hall systems are discussed.Comment: 12 revtex pages, no figures, revised final version to appear in PR
Scoping the potencial for outdoor microalgae production in the Azores
Microalgae cultivation has received much attention due to some desirable characteristics
such as fast growth rates, high photosynthetic efficiencies and the production of valuable
biochemicals. Mass cultivation of microalgae for commercial purposes is already a reality
in some locations being an activity with approximately 45 years. The Azores archipelago,
located in the North Atlantic Ocean off the coast of Portugal, features a temperate climate.
To evaluate the feasibility of local outdoor microalgae production, certain fundamental
factors must be analysed. In this study water, light, temperature, carbon sources, nutrients,
international outlook and regional context were analyzed from the perspective of a local
outdoor commercial microalgae production. A SWOT analysis was applied to the analysed
factors to evaluate the strengths, weaknesses, opportunities and threats. The results
nominate the Azores as a promising location to implement an outdoor production of valueadded
products from microalgae.Oma – Observatório do Mar dos Açores ; MARE – Centro de Ciências do Mar e do Ambiente ; IMAR – Instituto do Mar ; DOP – Departamento de Oceanografia e Pescas ; Universidade dos Açoresinfo:eu-repo/semantics/publishedVersio
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