1,517 research outputs found
Spin-glass phase transition and behavior of nonlinear susceptibility in the Sherrington-Kirkpatrick model with random fields
The behavior of the nonlinear susceptibility and its relation to the
spin-glass transition temperature , in the presence of random fields, are
investigated. To accomplish this task, the Sherrington-Kirkpatrick model is
studied through the replica formalism, within a one-step
replica-symmetry-breaking procedure. In addition, the dependence of the
Almeida-Thouless eigenvalue (replicon) on the random fields
is analyzed. Particularly, in absence of random fields, the temperature
can be traced by a divergence in the spin-glass susceptibility ,
which presents a term inversely proportional to the replicon . As a result of a relation between and , the
latter also presents a divergence at , which comes as a direct consequence
of at . However, our results show that, in the
presence of random fields, presents a rounded maximum at a temperature
, which does not coincide with the spin-glass transition temperature
(i.e., for a given applied random field). Thus, the maximum
value of at reflects the effects of the random fields in the
paramagnetic phase, instead of the non-trivial ergodicity breaking associated
with the spin-glass phase transition. It is also shown that still
maintains a dependence on the replicon , although in a more
complicated way, as compared with the case without random fields. These results
are discussed in view of recent observations in the LiHoYF
compound.Comment: accepted for publication in PR
Gestão e políticas públicas em Minas Gerais: cultura e patrimônio cultural
Aborda-se o papel do Estado e dos seus gestores em relação à adoção de políticas de preservação da cultura e do patrimônio cultural em Minas Gerais. O objetivo central é convidar o leitor a refletir sobre as políticas públicas de preservação do patrimônio cultural e o papel do gestor em relação às ações desenvolvidas no Estado de Minas Gerais, nos diferentes municípios que compõe esse estado
Incorporação de novos acessos de microrganismos solubilizadores de fosfato à coleção de microrganismos multifuncionais da Embrapa Milho e Sorgo.
Quantum Critical Point in the Spin Glass-Kondo Transition in Heavy Fermion Systems
The Kondo-Spin Glass competition is studied in a theoretical model of a Kondo
lattice with an intra-site Kondo type exchange interaction treated within the
mean field approximation, an inter-site quantum Ising exchange interaction with
random couplings among localized spins and an additional transverse field in
the x direction, which represents a simple quantum mechanism of spin flipping.
We obtain two second order transition lines from the spin-glass state to the
paramagnetic one and then to the Kondo state. For a reasonable set of the
different parameters, the two second order transition lines do not intersect
and end in two distinct QCP.Comment: 20 pages; 1 figure; to appear in Physical Review
Ultrastable lasers based on vibration insensitive cavities
We present two ultra-stable lasers based on two vibration insensitive cavity
designs, one with vertical optical axis geometry, the other horizontal.
Ultra-stable cavities are constructed with fused silica mirror substrates,
shown to decrease the thermal noise limit, in order to improve the frequency
stability over previous designs. Vibration sensitivity components measured are
equal to or better than 1.5e-11 per m.s^-2 for each spatial direction, which
shows significant improvement over previous studies. We have tested the very
low dependence on the position of the cavity support points, in order to
establish that our designs eliminate the need for fine tuning to achieve
extremely low vibration sensitivity. Relative frequency measurements show that
at least one of the stabilized lasers has a stability better than 5.6e-16 at 1
second, which is the best result obtained for this length of cavity.Comment: 8 pages 12 figure
Measurement of interaction energy near a Feshbach resonance in a 6Li Fermi gas
We investigate the strongly interacting regime in an optically trapped Li
Fermi mixture near a Feshbach resonance. The resonance is found at G
in good agreement with theory. Anisotropic expansion of the gas is interpreted
by collisional hydrodynamics. We observe an unexpected and large shift (G)
between the resonance peak and both the maximum of atom loss and the change of
sign of the interaction energy.Comment: 4 pages, 4 figure
Route to turbulence in a trapped Bose-Einstein condensate
We have studied a Bose-Einstein condensate of atoms under an
oscillatory excitation. For a fixed frequency of excitation, we have explored
how the values of amplitude and time of excitation must be combined in order to
produce quantum turbulence in the condensate. Depending on the combination of
these parameters different behaviors are observed in the sample. For the lowest
values of time and amplitude of excitation, we observe a bending of the main
axis of the cloud. Increasing the amplitude of excitation we observe an
increasing number of vortices. The vortex state can evolve into the turbulent
regime if the parameters of excitation are driven up to a certain set of
combinations. If the value of the parameters of these combinations is exceeded,
all vorticity disappears and the condensate enters into a different regime
which we have identified as the granular phase. Our results are summarized in a
diagram of amplitude versus time of excitation in which the different
structures can be identified. We also present numerical simulations of the
Gross-Pitaevskii equation which support our observations.Comment: 6 pages, 3 figure
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