55,773 research outputs found
Chaotic temperature dependence in a model of spin glasses
We address the problem of chaotic temperature dependence in disordered glassy
systems at equilibrium by following states of a random-energy random-entropy
model in temperature; of particular interest are the crossings of the
free-energies of these states. We find that this model exhibits strong, weak or
no temperature chaos depending on the value of an exponent. This allows us to
write a general criterion for temperature chaos in disordered systems,
predicting the presence of temperature chaos in the Sherrington-Kirkpatrick and
Edwards-Anderson spin glass models, albeit when the number of spins is large
enough. The absence of chaos for smaller systems may justify why it is
difficult to observe chaos with current simulations. We also illustrate our
findings by studying temperature chaos in the naive mean field equations for
the Edwards-Anderson spin glass.Comment: 10 pages, 5 figures; To be published in European Physics Journal
Large-scale low-energy excitations in 3-d spin glasses
We numerically extract large-scale excitations above the ground state in the
3-dimensional Edwards-Anderson spin glass with Gaussian couplings. We find that
associated energies are O(1), in agreement with the mean field picture. Of
further interest are the position-space properties of these excitations. First,
our study of their topological properties show that the majority of the
large-scale excitations are sponge-like. Second, when probing their geometrical
properties, we find that the excitations coarsen when the system size is
increased. We conclude that either finite size effects are very large even when
the spin overlap q is close to zero, or the mean field picture of homogeneous
excitations has to be modified.Comment: 11 pages, typos corrected, added reference
Energy exponents and corrections to scaling in Ising spin glasses
We study the probability distribution P(E) of the ground state energy E in
various Ising spin glasses. In most models, P(E) seems to become Gaussian with
a variance growing as the system's volume V. Exceptions include the
Sherrington-Kirkpatrick model (where the variance grows more slowly, perhaps as
the square root of the volume), and mean field diluted spin glasses having +/-J
couplings. We also find that the corrections to the extensive part of the
disorder averaged energy grow as a power of the system size; for finite
dimensional lattices, this exponent is equal, within numerical precision, to
the domain-wall exponent theta_DW. We also show how a systematic expansion of
theta_DW in powers of exp(-d) can be obtained for Migdal-Kadanoff lattices.
Some physical arguments are given to rationalize our findings.Comment: 12 pages, RevTex, 9 figure
Multifractality and intermediate statistics in quantum maps
We study multifractal properties of wave functions for a one-parameter family
of quantum maps displaying the whole range of spectral statistics intermediate
between integrable and chaotic statistics. We perform extensive numerical
computations and provide analytical arguments showing that the generalized
fractal dimensions are directly related to the parameter of the underlying
classical map, and thus to other properties such as spectral statistics. Our
results could be relevant for Anderson and quantum Hall transitions, where wave
functions also show multifractality.Comment: 4 pages, 4 figure
Anticoherence of spin states with point group symmetries
We investigate multiqubit permutation-symmetric states with maximal entropy
of entanglement. Such states can be viewed as particular spin states, namely
anticoherent spin states. Using the Majorana representation of spin states in
terms of points on the unit sphere, we analyze the consequences of a
point-group symmetry in their arrangement on the quantum properties of the
corresponding state. We focus on the identification of anticoherent states (for
which all reduced density matrices in the symmetric subspace are maximally
mixed) associated with point-group symmetric sets of points. We provide three
different characterizations of anticoherence, and establish a link between
point symmetries, anticoherence and classes of states equivalent through
stochastic local operations with classical communication (SLOCC). We then
investigate in detail the case of small numbers of qubits, and construct
infinite families of anticoherent states with point-group symmetry of their
Majorana points, showing that anticoherent states do exist to arbitrary order.Comment: 15 pages, 5 figure
The silicate absorption profile in the ISM towards the heavily obscured nucleus of NGC 4418
The 9.7-micron silicate absorption profile in the interstellar medium
provides important information on the physical and chemical composition of
interstellar dust grains. Measurements in the Milky Way have shown that the
profile in the diffuse interstellar medium is very similar to the amorphous
silicate profiles found in circumstellar dust shells around late M stars, and
narrower than the silicate profile in denser star-forming regions. Here, we
investigate the silicate absorption profile towards the very heavily obscured
nucleus of NGC 4418, the galaxy with the deepest known silicate absorption
feature, and compare it to the profiles seen in the Milky Way. Comparison
between the 8-13 micron spectrum obtained with TReCS on Gemini and the larger
aperture spectrum obtained from the Spitzer archive indicates that the former
isolates the nuclear emission, while Spitzer detects low surface brightness
circumnuclear diffuse emission in addition. The silicate absorption profile
towards the nucleus is very similar to that in the diffuse ISM in the Milky Way
with no evidence of spectral structure from crystalline silicates or silicon
carbide grains.Comment: 7 Pages, 3 figures. MNRAS in pres
A comparison of two magnetic ultra-cold neutron trapping concepts using a Halbach-octupole array
This paper describes a new magnetic trap for ultra-cold neutrons (UCNs) made
from a 1.2 m long Halbach-octupole array of permanent magnets with an inner
bore radius of 47 mm combined with an assembly of superconducting end coils and
bias field solenoid. The use of the trap in a vertical, magneto-gravitational
and a horizontal setup are compared in terms of the effective volume and
ability to control key systematic effects that need to be addressed in high
precision neutron lifetime measurements
Magnetic exponents of two-dimensional Ising spin glasses
The magnetic critical properties of two-dimensional Ising spin glasses are
controversial. Using exact ground state determination, we extract the
properties of clusters flipped when increasing continuously a uniform field. We
show that these clusters have many holes but otherwise have statistical
properties similar to those of zero-field droplets. A detailed analysis gives
for the magnetization exponent delta = 1.30 +/- 0.02 using lattice sizes up to
80x80; this is compatible with the droplet model prediction delta = 1.282. The
reason for previous disagreements stems from the need to analyze both singular
and analytic contributions in the low-field regime.Comment: 4 pages, 4 figures, title now includes "Ising
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