23,450 research outputs found
Nontrivial critical crossover between directed percolation models: Effect of infinitely many absorbing states
At non-equilibrium phase transitions into absorbing (trapped) states, it is
well known that the directed percolation (DP) critical scaling is shared by two
classes of models with a single (S) absorbing state and with infinitely many
(IM) absorbing states. We study the crossover behavior in one dimension,
arising from a considerable reduction of the number of absorbing states
(typically from the IM-type to the S-type DP models), by following two
different (excitatory or inhibitory) routes which make the auxiliary field
density abruptly jump at the crossover. Along the excitatory route, the system
becomes overly activated even for an infinitesimal perturbation and its
crossover becomes discontinuous. Along the inhibitory route, we find continuous
crossover with the universal crossover exponent , which is
argued to be equal to , the relaxation time exponent of the DP
universality class on a general footing. This conjecture is also confirmed in
the case of the directed Ising (parity-conserving) class. Finally, we discuss
the effect of diffusion to the IM-type models and suggest an argument why
diffusive models with some hybrid-type reactions should belong to the DP class.Comment: 8 pages, 9 figure
Fulde-Ferrel-Larkin-Ovchinnikov Inhomogeneous Superconducting State and Phase Transitions Induced by Spin Accumulation in a Ferromagnet--Wave Superconductor-Ferromagnet Tunnel Junction
Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting (SC)
state, first- and second-order phase transitions, and quantum criticality
induced by spin accumulation in a ferromagnet--wave
superconductor-ferromagnet tunnel junction are theoretically predicted. A
complex phase diagram in the temperature-bias voltage plane is determined. It
is found that the phase transitions from the homogeneous BCS state to the
inhomogeneous FFLO state, and from the FFLO state with the momentum 's azimuthal angle to that with , are of the first-order; while the transitions from all SC
states to the normal state at critical voltages are of the second-order. A
Lifshitz point, a bicritical point and a quantum critical point are identified.Comment: 5 pages, 5 figure
Dithered GMD Transform Coding
The geometric mean decomposition (GMD) transform coder (TC) was recently introduced and was shown to achieve the optimal coding gain without bit loading under the high bit rate assumption. However, the performance of the GMD transform coder is degraded in the low rate case. There are mainly two reasons for this degradation. First, the high bit rate quantizer model becomes invalid. Second, the quantization error is no longer negligible in the prediction process when the bit rate is low. In this letter, we introduce dithered quantization to tackle the first difficulty, and then redesign the precoders and predictors in the GMD transform coders to tackle the second. We propose two dithered GMD transform coders: the GMD subtractive dithered transform coder (GMD-SD) where the decoder has access to the dither information and the GMD nonsubtractive dithered transform coder (GMD-NSD) where the decoder has no knowledge about the dither. Under the uniform bit loading scheme in scalar quantizers, it is shown that the proposed dithered GMD transform coders perform significantly better than the original GMD coder in the low rate case
Topological Gauge Structure and Phase Diagram for Weakly Doped Antiferromagnets
We show that the topological gauge structure in the phase string theory of
the {\rm t-J} model gives rise to a global phase diagram of antiferromagnetic
(AF) and superconducting (SC) phases in a weakly doped regime. Dual confinement
and deconfinement of holons and spinons play essential roles here, with a
quantum critical point at a doping concentration . The complex
experimental phase diagram at low doping is well described within such a
framework.Comment: 4 pages, 2 figures, modified version, to appear in Phys. Rev. Let
Debris Disks around Solar-Type Stars: Observations of the Pleiades with Spitzer Space Telescope
We present Spitzer MIPS observations at 24 um of 37 solar-type stars in the
Pleiades and combine them with previous observations to obtain a sample of 71
stars. We report that 23 stars, or 32 +/- 6.8%, have excesses at 24 um at least
10% above their photospheric emission. We compare our results with studies of
debris disks in other open clusters and with a study of A stars to show that
debris disks around solar-type stars at 115 Myr occur at nearly the same rate
as around A-type stars. We analyze the effects of binarity and X-ray activity
on the excess flux. Stars with warm excesses tend not to be in equal-mass
binary systems, possibly due to clearing of planetesimals by binary companions
in similar orbits. We find that the apparent anti-correlations in the incidence
of excess and both the rate of stellar rotation and also the level of activity
as judged by X-ray emission are statistically weak.Comment: 34 pages; accepted for publication in ApJ; new version included
corrections of typos, etc to match published versio
The Haldane Energy Gap of A Doped Linear-Chain Heisenberg Antiferromagnet
Using the valence-bond-solid (VBS) approach and the Schwinger boson mean
field approximation, we study the dependence of the Haldane gap of a spin-1
linear chain Heisenberg antiferromagnet on impurity doping with different
spins. The impurity spins affect the singlet pairing order parameter
and the constraint factor . As a result, the Haldane gap is reduced by
a factor , with as the impurity concentration, and
eventually collapses at with as the VBS correlation
length. This theoretical prediction can be verified by neutron scattering
experiments.Comment: REVTEX, 12 pages, no figure
Relativistic precession and spin dynamics of an elliptic Rydberg wave packet
Time evolution of wave packets built from the eigenstates of the Dirac
equation for a hydrogenic system is considered. We investigate the space and
spin motion of wave packets which, in the non-relativistic limit, are
stationary states with a probability density distributed uniformly along the
classical, elliptical orbit (elliptic WP). We show that the precession of such
a WP, due to relativistic corrections to the energy eigenvalues, is strongly
correlated with the spin motion. We show also that the motion is universal for
all hydrogenic systems with an arbitrary value of the atomic number Z.Comment: Latex2e, uses IOP style files (included), 10 pages, 5 jpg figures, 1
postscript figure. Relation between precession time and radiative liftime
added (eq.(12)). Accepted for publication in J. Phys.
Midgap States in Antiferromagnetic Heisenberg Chains with A Staggered Field
We study low-energy excitations in antiferromagnetic Heisenberg chains with a
staggered field which splits the spectrum into a longitudinal and a transverse
branch. Bound states are found to exist inside the field induced gap in both
branches. They originate from the edge effects and are inherent to spin-chain
materials. The sine-Gordon scaling (: the
staggered field) provides an accurate description for the gap and midgap
energies in the transverse branch for and the midgap energies in both
branches for over a wide range of magnetic field; however, it can fit
other low-energy excitations only at much lower field. Moreover, the
integer-spin S=1 chain displays scaling behavior that does not fit this scaling
law. These results reveal intriguing features of magnetic excitations in
spin-chain materials that deserve further investigation.Comment: 4 pages, 4figure
Mott-Peierls Transition in the extended Peierls-Hubbard model
The one-dimensional extended Peierls-Hubbard model is studied at several band
fillings using the density matrix renormalization group method. Results show
that the ground state evolves from a Mott-Peierls insulator with a correlation
gap at half-filling to a soliton lattice with a small band gap away from
half-filling. It is also confirmed that the ground state of the Peierls-Hubbard
model undergoes a transition to a metallic state at finite doping. These
results show that electronic correlations effects should be taken into account
in theoretical studies of doped polyacetylene. They also show that a
Mott-Peierls theory could explain the insulator-metal transition observed in
this material.Comment: 4 pages with 3 embedded eps figure
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