21,202 research outputs found
Direct sampling of complex landscapes at low temperatures: the three-dimensional +/-J Ising spin glass
A method is presented, which allows to sample directly low-temperature
configurations of glassy systems, like spin glasses. The basic idea is to
generate ground states and low lying excited configurations using a heuristic
algorithm. Then, with the help of microcanonical Monte Carlo simulations, more
configurations are found, clusters of configurations are determined and
entropies evaluated. Finally equilibrium configuration are randomly sampled
with proper Gibbs-Boltzmann weights.
The method is applied to three-dimensional Ising spin glasses with +- J
interactions and temperatures T<=0.5. The low-temperature behavior of this
model is characterized by evaluating different overlap quantities, exhibiting a
complex low-energy landscape for T>0, while the T=0 behavior appears to be less
complex.Comment: 9 pages, 7 figures, revtex (one sentence changed compared to v2
Geometric Phases and Critical Phenomena in a Chain of Interacting Spins
The geometric phase can act as a signature for critical regions of
interacting spin chains in the limit where the corresponding circuit in
parameter space is shrunk to a point and the number of spins is extended to
infinity; for finite circuit radii or finite spin chain lengths, the geometric
phase is always trivial (a multiple of 2pi). In this work, by contrast, two
related signatures of criticality are proposed which obey finite-size scaling
and which circumvent the need for assuming any unphysical limits. They are
based on the notion of the Bargmann invariant whose phase may be regarded as a
discretized version of Berry's phase. As circuits are considered which are
composed of a discrete, finite set of vertices in parameter space, they are
able to pass directly through a critical point, rather than having to
circumnavigate it. The proposed mechanism is shown to provide a diagnostic tool
for criticality in the case of a given non-solvable one-dimensional spin chain
with nearest-neighbour interactions in the presence of an external magnetic
field.Comment: 7 Figure
Non-resonant kaon pair production and medium effects in proton-nucleus collisions
We study the non-resonant (non-) production of pairs by
protons of 2.83 GeV kinetic energy on C, Cu, Ag, and Au targets within the
collision model, based on the nuclear spectral function, for incoherent primary
proton--nucleon and secondary pion--nucleon creation processes. The model takes
into account the initial proton and final kaon absorption, target nucleon
binding and Fermi motion as well as nuclear mean-field potential effects on
these processes. We calculate the antikaon momentum dependences of the
exclusive absolute and relative pair yields in the acceptance window
of the ANKE magnetic spectrometer, used in a recent experiment performed at
COSY, within the different scenarios for the antikaon-nucleus optical
potential. We demonstrate that the above observables are strongly sensitive to
this potential. Therefore, they can be useful to help determine the
optical potential from the direct comparison of the results of our calculations
with the data from the respective ANKE-at-COSY experiment. We also show that
the pion--nucleon production channels dominate in the low-momentum ,
production in the considered kinematics and, hence, they have to be accounted
for in the analysis of these data.Comment: 19 page
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