18,197 research outputs found
Analytical study of tunneling times in flat histogram Monte Carlo
We present a model for the dynamics in energy space of multicanonical
simulation methods that lends itself to a rather complete analytic
characterization. The dynamics is completely determined by the density of
states. In the \pm J 2D spin glass the transitions between the ground state
level and the first excited one control the long time dynamics. We are able to
calculate the distribution of tunneling times and relate it to the
equilibration time of a starting probability distribution. In this model, and
possibly in any model in which entering and exiting regions with low density of
states are the slowest processes in the simulations, tunneling time can be much
larger (by a factor of O(N)) than the equilibration time of the probability
distribution. We find that these features also hold for the energy projection
of single spin flip dynamics.Comment: 7 pages, 4 figures, published in Europhysics Letters (2005
Gravitational waves in the generalized Chaplygin gas model
The consequences of taking the generalized Chaplygin gas as the dark energy
constituent of the Universe on the gravitational waves are studied and the
spectrum obtained from this model, for the flat case, is analyzed. Besides its
importance for the study of the primordial Universe, the gravitational waves
represent an additional perspective (besides the CMB temperature and
polarization anisotropies) to evaluate the consistence of the different dark
energy models and establish better constraints to their parameters. The
analysis presented here takes this fact into consideration to open one more
perspective of verification of the generalized Chapligin gas model
applicability. Nine particular cases are compared: one where no dark energy is
present; two that simulate the -CDM model; two where the gas acts like
the traditional Chaplygin gas; and four where the dark energy is the
generalized Chaplygin gas. The different spectra permit to distinguish the
-CDM and the Chaplygin gas scenarios.Comment: Latex file, 9 pages, 11 figures eps forma
Fluctuating local moments, itinerant electrons and the magnetocaloric effect: the compositional hypersensitivity of FeRh
We describe an ab-initio Disordered Local Moment Theory for materials with
quenched static compositional disorder traversing first order magnetic phase
transitions. It accounts quantitatively for metamagnetic changes and the
magnetocaloric effect. For perfect stoichiometric B2-ordered FeRh, we calculate
the transition temperature of the ferromagnetic-antiferromagnetic transition to
be 495K and a maximum isothermal entropy change in 2 Tesla of J~K~kg. A large (40\%) component of is
electronic. The transition results from a fine balance of competing electronic
effects which is disturbed by small compositional changes - e.g. swapping just
2\% Fe of `defects' onto the Rh sublattice makes drop by 290K. This
hypersensitivity explains the narrow compositional range of the transition and
impurity doping effects.Comment: 11 pages, 4 figure
Tuning paramagnetic spin-excitations of single adatoms
Around 50 years ago, Doniach [Proc. Phys. Soc. 91, 86 (1967)] predicted the
existence of paramagnons in nearly ferromagnetic materials, recently measured
in bulk Pd [Phys. Rev. Lett. 105, 027207 (2010)]. Here we predict the analogous
effect for single adatoms, namely paramagnetic spin-excitations (PSE). Based on
time-dependent density functional theory, we demonstrate that these overdamped
excitations acquire a well-defined peak structure in the meV energy region when
the adatom's Stoner criterion for magnetism is close to the critical point. In
addition, our calculations reveal a subtle tunability and enhancement of PSE by
external magnetic fields, exceeding by far the response of bulk paramagnons and
even featuring the atomic version of a quantum phase transition. We further
demonstrate how PSE can be detected as moving steps in the
signal of state-of-the-art inelastic scanning tunneling spectroscopy, opening a
potential route for experimentally accessing fundamental electronic properties
of non-magnetic adatoms, such as the Stoner parameter.Comment: 6 pages, 3 figure
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