324 research outputs found
Quantum spherical model with competing interactions
We analyze the phase diagram of a quantum mean spherical model in terms of
the temperature , a quantum parameter , and the ratio ,
where refers to ferromagnetic interactions between first-neighbor
sites along the directions of a hypercubic lattice, and is
associated with competing antiferromagnetic interactions between second
neighbors along directions. We regain a number of known results for
the classical version of this model, including the topology of the critical
line in the space, with a Lifshitz point at , for , and
closed-form expressions for the decay of the pair correlations in one
dimension. In the T=0 phase diagram, there is a critical border,
for , with a singularity at the Lifshitz point if
. We also establish upper and lower critical dimensions, and analyze
the quantum critical behavior in the neighborhood of .Comment: 18 pages, 3 figures, refs added, minor modifications to match
published versio
Exact renormalization group equation for the Lifshitz critical point
An exact renormalization equation (ERGE) accounting for an anisotropic
scaling is derived. The critical and tricritical Lifshitz points are then
studied at leading order of the derivative expansion which is shown to involve
two differential equations. The resulting estimates of the Lifshitz critical
exponents compare well with the calculations. In the case
of the Lifshitz tricritical point, it is shown that a marginally relevant
coupling defies the perturbative approach since it actually makes the fixed
point referred to in the previous perturbative calculations
finally unstable.Comment: Final versio
Susceptibility Amplitude Ratios Near a Lifshitz Point
The susceptibility amplitude ratio in the neighborhood of a uniaxial Lifshitz
point is calculated at one-loop level using field-theoretic and
-expansion methods. We use the Schwinger parametrization of the
propagator in order to split the quadratic and quartic part of the momenta, as
well as a new special symmetry point suitable for renormalization purposes. For
a cubic lattice (d = 3), we find the result .Comment: 7 pages, late
First-principles prediction of coexistence of magnetism and ferroelectricity in rhombohedral Bi2FeTiO6
First principles calculations based on the density functional theory within
the local spin density approximation plus U(LSDA+U)scheme, show rhombohedral
BiFeTiO is a potential multiferroic in which the magnetism and
ferroelectricity coexist . A ferromagnetic configuration with magnetic moment
of 4 per formula unit have been reported with respect to the minimum
total energy. Spontaneous polarization of 27.3 C/cm, caused mainly by
the ferroelectric distortions of Ti, was evaluated using the berry phase
approach in the modern theory of polarization. The Bi-6s stereochemical
activity of long-pair and the `d-ness' criterion in off-centring of Ti were
coexisting in the predicted new system. In view of the oxidation state of
Bi,Fe,Ti, and O from the orbital-resolved density
of states of the Bi-6p, Fe-3d,Ti-3d, and O-2p states,the valence state of
BiFeTiO in the rhombohedral phase was found to be
BiFeTiO.Comment: 22 pages, 9 figures. submitted to Physics Letters
Quantum Criticality and Yang-Mills Gauge Theory
We present a family of nonrelativistic Yang-Mills gauge theories in D+1
dimensions whose free-field limit exhibits quantum critical behavior with
gapless excitations and dynamical critical exponent z=2. The ground state
wavefunction is intimately related to the partition function of relativistic
Yang-Mills in D dimensions. The gauge couplings exhibit logarithmic scaling and
asymptotic freedom in the upper critical spacetime dimension, equal to 4+1. The
theories can be deformed in the infrared by a relevant operator that restores
Poincare invariance as an accidental symmetry. In the large-N limit, our
nonrelativistic gauge theories can be expected to have weakly curved gravity
duals.Comment: 10 page
Reply to "Comment on Renormalization group picture of the Lifshitz critical behaviors"
We reply to a recent comment by Diehl and Shpot (cond-mat/0305131)
criticizing a new approach to the Lifshitz critical behavior just presented (M.
M. Leite Phys. Rev. B 67, 104415(2003)). We show that this approach is free of
inconsistencies in the ultraviolet regime. We recall that the orthogonal
approximation employed to solve arbitrary loop diagrams worked out at the
criticized paper even at three-loop level is consistent with homogeneity for
arbitrary loop momenta. We show that the criticism is incorrect.Comment: RevTex, 6 page
The antiferromagnetic phi4 Model, II. The one-loop renormalization
It is shown that the four dimensional antiferromagnetic lattice phi4 model
has the usual non-asymptotically free scaling law in the UV regime around the
chiral symmetrical critical point. The theory describes a scalar and a
pseudoscalar particle. A continuum effective theory is derived for low
energies. A possibility of constructing a model with a single chiral boson is
mentioned.Comment: To appear in Phys. Rev.
Possible Existence of an Extraordinary Phase in the Driven Lattice Gas
We report recent simulation results which might indicate the existence of a
new low-temperature "phase" in an Ising lattice gas, driven into a
non-equilibrium steady state by an external field. It appears that this
"phase", characterized by multiple-strip configurations, is selected when
square systems are used to approach the thermodynamic limit. We propose a
quantitative criterion for the existence of such a "phase". If confirmed, its
observation may resolve a long-standing controversy over the critical
properties of the driven Ising lattice gas.Comment: 10 pages; 4 figure
Surface states in nearly modulated systems
A Landau model is used to study the phase behavior of the surface layer for
magnetic and cholesteric liquid crystal systems that are at or near a Lifshitz
point marking the boundary between modulated and homogeneous bulk phases. The
model incorporates surface and bulk fields and includes a term in the free
energy proportional to the square of the second derivative of the order
parameter in addition to the usual term involving the square of the first
derivative. In the limit of vanishing bulk field, three distinct types of
surface ordering are possible: a wetting layer, a non-wet layer having a small
deviation from bulk order, and a different non-wet layer with a large deviation
from bulk order which decays non-monotonically as distance from the wall
increases. In particular the large deviation non-wet layer is a feature of
systems at the Lifshitz point and also those having only homogeneous bulk
phases.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Bulk and Boundary Critical Behavior at Lifshitz Points
Lifshitz points are multicritical points at which a disordered phase, a
homogeneous ordered phase, and a modulated ordered phase meet. Their bulk
universality classes are described by natural generalizations of the standard
model. Analyzing these models systematically via modern
field-theoretic renormalization group methods has been a long-standing
challenge ever since their introduction in the middle of the 1970s. We survey
the recent progress made in this direction, discussing results obtained via
dimensionality expansions, how they compare with Monte Carlo results, and open
problems. These advances opened the way towards systematic studies of boundary
critical behavior at -axial Lifshitz points. The possible boundary critical
behavior depends on whether the surface plane is perpendicular to one of the
modulation axes or parallel to all of them. We show that the semi-infinite
field theories representing the corresponding surface universality classes in
these two cases of perpendicular and parallel surface orientation differ
crucially in their Hamiltonian's boundary terms and the implied boundary
conditions, and explain recent results along with our current understanding of
this matter.Comment: Invited contribution to STATPHYS 22, to be published in the
Proceedings of the 22nd International Conference on Statistical Physics
(STATPHYS 22) of the International Union of Pure and Applied Physics (IUPAP),
4--9 July 2004, Bangalore, Indi
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