46,434 research outputs found
Kosterlitz-Thouless Phase Transition of the ANNNI model in Two Dimensions
The spin structure of an axial next-nearest-neighbor Ising (ANNNI) model in
two dimensions (2D) is a renewed problem because different Monte Carlo (MC)
simulation methods predicted different spin orderings. The usual equilibrium
simulation predicts the occurrence of a floating incommensurate (IC)
Kosterlitz-Thouless (KT) type phase, which never emerges in non-equilibrium
relaxation (NER) simulations. In this paper, we first examine previously
published results of both methods, and then investigate a higher transition
temperature, , between the IC and paramagnetic phases. In the usual
equilibrium simulation, we calculate the layer magnetization on larger lattices
(up to sites) and estimate with
frustration ratio . We examine the nature of
the phase transition in terms of the Binder ratio of spin overlap
functions and the correlation-length ratio . In the NER simulation, we
observe the spin dynamics in equilibrium states by means of an autocorrelation
function, and also observe the layer magnetization relaxations from the ground
and disordered states. These quantities exhibit an algebraic decay at . We conclude that the two-dimensional ANNNI model actually
admits an IC phase transition of the KT type.Comment: 20 pages, 16 figure
Structure of the breakpoint region in CVC of the intrinsic Josephson junctions
A fine structure of the breakpoint region in the current-voltage
characteristics of the coupled intrinsic Josephson junctions in the layered
superconductors is found. We establish a correspondence between the features in
the current-voltage characteristics and the character of the charge
oscillations in superconducting layers in the stack and explain the origin of
the breakpoint region structure.Comment: 5 pages, 5 figures. Accepted for Phys.Rev.
Supersolid of Hardcore Bosons on the Face Centered Cubic Lattice
We investigate a supersolid state in hardcore boson models on the
face-centered-cubic (FCC) lattice. The supersolid state is characterized by a
coexistence of crystalline order and superfluidity. Using a quantum Monte Carlo
method based on the directed-loop algorithm, we calculate static structure
factors and superfluid density at finite temperature, from which we obtain the
phase diagram. The supersolid phase exists at intermediate fillings between a
three-quarter-filled solid phase and a half-filled solid phase. We also discuss
the mechanism of the supersolid state on the FCC lattice.Comment: 5pages, 6figure
Quantum phase transitions in the sub-ohmic spin-boson model: Failure of the quantum-classical mapping
The effective theories for many quantum phase transitions can be mapped onto
those of classical transitions. Here we show that such a mapping fails for the
sub-ohmic spin-boson model which describes a two-level system coupled to a
bosonic bath with power-law spectral density, J(omega) ~ omega^s. Using an
epsilon expansion we prove that this model has a quantum transition controlled
by an interacting fixed point at small s, and support this by numerical
calculations. In contrast, the corresponding classical long-range Ising model
is known to have an upper-critical dimension at s = 1/2, with mean-field
transition behavior controlled by a non-interacting fixed point for 0 < s <
1/2. The failure of the quantum-classical mapping is argued to arise from the
long-ranged interaction in imaginary time in the quantum model.Comment: 4 pages, 3 figs; (v2) discussion extended; (v3) marginal changes,
final version as published; (v4) added erratum pointing out that main
conclusions were incorrect due to subtle failures of the NR
Linkages in thermal copolymers of lysine
The thermal copolymerization of lysine with other alpha-amino acids was studied. The identity of the second amino acid influences various properties of the polymer obtained, including the proportion of alpha and epsilon linkages of lysine. A review of linkages in proteinoids indicates alpha and beta linkages for aspartic acid, alpha and gamma linkages for glutamic acid, alpha and epsilon linkages for lysine, and alpha linkages for other amino acids. Thermal proteinoids are thus more complex in types of linkage than are proteins
Sum Rules of the Multiple Giant Dipole States
Various sum rules for multiple giant dipole resonance states are derived. For
the triple giant dipole resonance states, the energy-weighted sum of the
transition strengths requires a model to be related to those of the single and
double giant dipole resonance states. It is also shown that the non-diagonal
matrix elements of the double commutator between the dipole operator and the
nuclear Hamiltonian give useful identities for the excitation energy and
transition strength of each excited state. Using those identities, the
relationship between width of the single dipole state and those of the multiple
ones is qualitatively discussed.Comment: 8 pages, 1 figure, using PTPTeX styl
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