468 research outputs found
Temperature-driven single-valley Dirac fermions in HgTe quantum wells
We report on temperature-dependent magnetospectroscopy of two HgTe/CdHgTe
quantum wells below and above the critical well thickness . Our results,
obtained in magnetic fields up to 16 T and temperature range from 2 K to 150 K,
clearly indicate a change of the band-gap energy with temperature. The quantum
well wider than evidences a temperature-driven transition from
topological insulator to semiconductor phases. At the critical temperature of
90 K, the merging of inter- and intra-band transitions in weak magnetic fields
clearly specifies the formation of gapless state, revealing the appearance of
single-valley massless Dirac fermions with velocity of
ms. For both quantum wells, the energies extracted from
experimental data are in good agreement with calculations on the basis of the
8-band Kane Hamiltonian with temperature-dependent parameters.Comment: 5 pages, 3 figures and Supplemental Materials (4 pages
Structural Information in Two-Dimensional Patterns: Entropy Convergence and Excess Entropy
We develop information-theoretic measures of spatial structure and pattern in
more than one dimension. As is well known, the entropy density of a
two-dimensional configuration can be efficiently and accurately estimated via a
converging sequence of conditional entropies. We show that the manner in which
these conditional entropies converge to their asymptotic value serves as a
measure of global correlation and structure for spatial systems in any
dimension. We compare and contrast entropy-convergence with mutual-information
and structure-factor techniques for quantifying and detecting spatial
structure.Comment: 11 pages, 5 figures,
http://www.santafe.edu/projects/CompMech/papers/2dnnn.htm
Particles Sliding on a Fluctuating Surface: Phase Separation and Power Laws
We study a system of hard-core particles sliding downwards on a fluctuating
one-dimensional surface which is characterized by a dynamical exponent . In
numerical simulations, an initially random particle density is found to coarsen
and obey scaling with a growing length scale . The structure
factor deviates from the Porod law in some cases. The steady state is unusual
in that the density-segregation order parameter shows strong fluctuations. The
two-point correlation function has a scaling form with a cusp at small argument
which we relate to a power law distribution of particle cluster sizes. Exact
results on a related model of surface depths provides insight into the origin
of this behaviour.Comment: 5 pages, 5 Postscript figure
Dynamics of Social Balance on Networks
We study the evolution of social networks that contain both friendly and
unfriendly pairwise links between individual nodes. The network is endowed with
dynamics in which the sense of a link in an imbalanced triad--a triangular loop
with 1 or 3 unfriendly links--is reversed to make the triad balanced. With this
dynamics, an infinite network undergoes a dynamic phase transition from a
steady state to "paradise"--all links are friendly--as the propensity p for
friendly links in an update event passes through 1/2. A finite network always
falls into a socially-balanced absorbing state where no imbalanced triads
remain. If the additional constraint that the number of imbalanced triads in
the network does not increase in an update is imposed, then the network quickly
reaches a balanced final state.Comment: 10 pages, 7 figures, 2-column revtex4 forma
Coarsening in a Driven Ising Chain with Conserved Dynamics
We study the low-temperature coarsening of an Ising chain subject to
spin-exchange dynamics and a small driving force. This dynamical system reduces
to a domain diffusion process, in which entire domains undergo nearest-neighbor
hopping, except for the shortest domains -- dimers -- which undergo long-range
hopping. This system is characterized by two independent length scales: the
average domain length L(t)~t^{1/2} and the average dimer hopping distance l(t)~
t^{1/4}. As a consequence of these two scales, the density C_k(t) of domains of
length k does not obey scaling. This breakdown of scaling also leads to the
density of short domains decaying as t^{-5/4}, instead of the t^{-3/2} decay
that would arise from pure domain diffusion.Comment: 7 pages, 9 figures, revtex 2-column forma
Massless Dirac fermions in III-V semiconductor quantum wells
We report on the clear evidence of massless Dirac fermions in two-dimensional
system based on III-V semiconductors. Using a gated Hall bar made on a
three-layer InAs/GaSb/InAs quantum well, we restore the Landau levels fan chart
by magnetotransport and unequivocally demonstrate a gapless state in our
sample. Measurements of cyclotron resonance at different electron
concentrations directly indicate a linear band crossing at the point
of Brillouin zone. Analysis of experimental data within analytical Dirac-like
Hamiltonian allows us not only determing velocity m/s of
massless Dirac fermions but also demonstrating significant non-linear
dispersion at high energies.Comment: Main text and Supplemental Materials, 14 pages, 9 figure
Equation of State for Macromolecules of Variable Flexibility in Good Solvents: A Comparison of Techniques for Monte Carlo Simulations of Lattice Models
The osmotic equation of state for the athermal bond fluctuation model on the
simple cubic lattice is obtained from extensive Monte Carlo simulations. For
short macromolecules (chain length N=20) we study the influence of various
choices for the chain stiffness on the equation of state. Three techniques are
applied and compared in order to critically assess their efficiency and
accuracy: the repulsive wall method, the thermodynamic integration method
(which rests on the feasibility of simulations in the grand canonical
ensemble), and the recently advocated sedimentation equilibrium method, which
records the density profile in an external (e.g. gravitation-like) field and
infers, via a local density approximation, the equation of state from the
hydrostatic equilibrium condition. We confirm the conclusion that the latter
technique is far more efficient than the repulsive wall method, but we find
that the thermodynamic integration method is similarly efficient as the
sedimentation equilibrium method. For very stiff chains the onset of nematic
order enforces the formation of isotropic-nematic interface in the
sedimentation equilibrium method leading to strong rounding effects and
deviations from the true equation of state in the transition regime.Comment: 32 pages, 18 figures, submitted to Phys.Rev.E; one paragraph added to
conclusions sectio
Persistence in Cluster--Cluster Aggregation
Persistence is considered in diffusion--limited cluster--cluster aggregation,
in one dimension and when the diffusion coefficient of a cluster depends on its
size as . The empty and filled site persistences are
defined as the probabilities, that a site has been either empty or covered by a
cluster all the time whereas the cluster persistence gives the probability of a
cluster to remain intact. The filled site one is nonuniversal. The empty site
and cluster persistences are found to be universal, as supported by analytical
arguments and simulations. The empty site case decays algebraically with the
exponent . The cluster persistence is related to the
small behavior of the cluster size distribution and behaves also
algebraically for while for the behavior is
stretched exponential. In the scaling limit and with fixed the distribution of intervals of size between
persistent regions scales as , where is the average interval size and . For finite the
scaling is poor for , due to the insufficient separation of the two
length scales: the distances between clusters, , and that between
persistent regions, . For the size distribution of persistent regions
the time and size dependences separate, the latter being independent of the
diffusion exponent but depending on the initial cluster size
distribution.Comment: 14 pages, 12 figures, RevTeX, submitted to Phys. Rev.
Anomalous self-diffusion in the ferromagnetic Ising chain with Kawasaki dynamics
We investigate the motion of a tagged spin in a ferromagnetic Ising chain
evolving under Kawasaki dynamics. At equilibrium, the displacement is Gaussian,
with a variance growing as . The temperature dependence of the
prefactor is derived exactly. At low temperature, where the static
correlation length is large, the mean square displacement grows as
in the coarsening regime, i.e., as a finite fraction of the
mean square domain length. The case of totally asymmetric dynamics, where
(resp. ) spins move only to the right (resp. to the left), is also
considered. In the steady state, the displacement variance grows as . The temperature dependence of the prefactor is derived exactly,
using the Kardar-Parisi-Zhang theory. At low temperature, the displacement
variance grows as in the coarsening regime, again proportionally to
the mean square domain length.Comment: 22 pages, 8 figures. A few minor changes and update
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