9,870 research outputs found
High Dimensional Apollonian Networks
We propose a simple algorithm which produces high dimensional Apollonian
networks with both small-world and scale-free characteristics. We derive
analytical expressions for the degree distribution, the clustering coefficient
and the diameter of the networks, which are determined by their dimension
Ricci Flat Black Holes and Hawking-Page Phase Transition in Gauss-Bonnet Gravity and Dilaton Gravity
It is well-known that there exists a Hawking-Page phase transition between a
spherical AdS black hole and a thermal AdS space. The phase transition does not
happen between a Ricci flat AdS black hole whose horizon is a Ricci flat space
and a thermal AdS space in the Poincare coordinates. However, the Hawking-Page
phase transition occurs between a Ricci flat AdS black hole and an AdS soliton
if at least one of horizon coordinates for the Ricci flat black hole is
compact. We show a similar phase transition betwen the Ricci flat black holes
and deformed AdS solitons in the Gauss-Bonnet gravity and the dilaton gravity
with a Liouville-type potential including the gauged supergravity coming from
the spherical reduction of Dp-branes in type II supergravity. In contrast to
Einstein gravity, we find that the high temperature phase can be dominated
either by black holes or deformed AdS solitons depending on parameters.Comment: Latex, 17 pages without figure
Ferromagnetic tendency at the surface of CE charge-ordered manganites
Most previous investigations have shown that the surface of a ferromagnetic
material may have antiferromagnetic tendencies. However, experimentally the
opposite effect has been recently observed: ferromagnetism appears in some
nano-sized manganites with a composition such that the antiferromagnetic
charge-ordered CE state is observed in the bulk. A possible origin is the
development of ferromagnetic correlations at the surface of these small
systems. To clarify these puzzling experimental observations, we have studied
the two-orbital double-exchange model near half-doping n=0.5, using open
boundary conditions to simulate the surface of either bulk or nano-sized
manganites. Considering the enhancement of surface charge density due to a
possible AO termination (A = trivalent/divalent ion composite, O = oxygen), an
unexpected surface phase-separated state emerges when the model is studied
using Monte Carlo techniques on small clusters. This tendency suppresses the CE
charge ordering and produces a weak ferromagnetic signal that could explain the
experimental observations.Comment: 8 pages, 5 figure
Macroscopic quantum coherence in spinor condensates confined in an anisotropic potential
We investigate the macroscopic quantum coherence of a spin-1 Rb condensate
confined in an anisotropic potential. Under the single-mode approximation, we
show that the system can be modeled as a biaxial quantum magnet due to the
interplay between the magnetic dipole-dipole interaction and the anisotropic
potential. By applying a magnetic field along the hard-axis, we show that the
tunneling splitting oscillates as a function of the field strength. We also
propose an experimental scheme to detect the oscillatory behavior of the
tunneling splitting by employing the Landau-Zener tunneling.Comment: 5 pages, 4 figure
Exchange interactions and temperature dependence of the magnetization in half--metallic Heusler alloys
We study the exchange interactions in half-metallic Heusler alloys using
first-principles calculations in conjunction with the frozen-magnon
approximation. The Curie temperature is estimated within both mean-field (MF)
and random-phase-approximation (RPA) approaches. For the half-Heusler alloys
NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In
this case the MF and RPA estimations differ strongly. The RPA approach provides
better agreement with experiment. The exchange interactions are more complex in
the case of full-Heusler alloys CoMnSi and CoCrAl where the dominant
effects are the inter-sublattice interactions between the Mn(Cr) and Co atoms
and between Co atoms at different sublattices. For these compounds we find that
both MF and RPA give very close values of the Curie temperature slightly
underestimating experimental quantities. We study the influence of the lattice
compression on the magnetic properties. The temperature dependence of the
magnetization is calculated using the RPA method within both quantum mechanical
and classical approaches.Comment: New figures and discussio
Discrete chaotic states of a Bose-Einstein condensate
We find the different spatial chaos in a one-dimensional attractive
Bose-Einstein condensate interacting with a Gaussian-like laser barrier and
perturbed by a weak optical lattice. For the low laser barrier the chaotic
regions of parameters are demonstrated and the chaotic and regular states are
illustrated numerically. In the high barrier case, the bounded perturbed
solutions which describe a set of discrete chaotic states are constructed for
the discrete barrier heights and magic numbers of condensed atoms. The chaotic
density profiles are exhibited numerically for the lowest quantum number, and
the analytically bounded but numerically unbounded Gaussian-like configurations
are confirmed. It is shown that the chaotic wave packets can be controlled
experimentally by adjusting the laser barrier potential.Comment: 7 pages, 5 figure
Band Narrowing and Mott Localization in Iron Oxychalcogenides La2O2Fe2O(Se,S)2
Bad metal properties have motivated a description of the parent iron
pnictides as correlated metals on the verge of Mott localization. What has been
unclear is whether interactions can push these and related compounds to the
Mott insulating side of the phase diagram. Here we consider the iron
oxychalcogenides La2O2Fe2O(Se,S)2, which contain an Fe square lattice with an
expanded unit cell. We show theoretically that they contain enhanced
correlation effects through band narrowing compared to LaOFeAs, and we provide
experimental evidence that they are Mott insulators with moderate charge gaps.
We also discuss the magnetic properties in terms of a Heisenberg model with
frustrating J1-J2-J2' exchange interactions on a "doubled" checkerboard
lattice.Comment: 4 pages, 5 eps figures. Version to appear in Phys. Rev. Let
Incompressible Navier-Stokes Equations from Einstein Gravity with Chern-Simons Term
In (2+1)-dimensional hydrodynamic systems with broken parity, the shear and
bulk viscosity is joined by the Hall viscosity and curl viscosity. The dual
holographic model has been constructed by coupling a pseudo scalar to the
gravitational Chern-Simons term in (3+1)-dimensional bulk gravity. In this
paper, we investigate the non-relativistic fluid with Hall viscosity and curl
viscosity living on a finite radial cutoff surface in the bulk. Employing the
non-relativistic hydrodynamic expansion method, we obtain the incompressible
Navier-Stokes equations with Hall viscosity and curl viscosity. Unlike the
shear viscosity, the ratio of the Hall viscosity over entropy density is found
to be cutoff scale dependent, and it tends to zero when the cutoff surface
approaches to the horizon of the background spacetime.Comment: 22 pages, published versio
CVM studies on the atomic ordering in complex perovskite alloys
The atomic ordering in complex perovskite alloys is investigated by the
cluster variation method (CVM). For the 1/3\{111\}-type ordered structure, the
order-disorder phase transition is the first order, and the order parameter of
the 1:2 complex perovskite reaches its maximum near x=0.25. For the
1/2\{111\}-type ordered structure, the ordering transition is the second order.
Phase diagrams for both ordered structures are obtained. The order-disorder
line obeys the linear law.Comment: 10 pages, 6 figure
Oscillatory behavior of the in-medium interparticle potential in hot gauge system with scalar bound states
We investigate the in-medium interparticle potential of hot gauge system with
bound states by employing the QED and scalar QED coupling. At finite
temperature an oscillatory behavior of the potential has been found as well as
its variation in terms of different free parameters. We expect the competition
among the parameters will lead to an appropriate interparticle potential which
could be extended to discuss the fluid properties of QGP with scalar bound
states
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