4,458 research outputs found
Long-range and short-range magnetic correlations, and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14
Spin-spin correlations and microscopic origin of net magnetization in the
spin-1 trimer chain compound CaNi3P4O14 have been investigated by powder
neutron diffraction. The present study reveals a 3D long-range magnetic
ordering below 16 K where the magnetic structure consists of ferromagnetic
trimers that are coupled ferromagnetically along the spin-chain. The moment
components along the a and c axes arrange antiferromagnetically. Our study
establishes that the uncompensated moment components along the b axis result in
a net magnetization per unit cell. The magnetic structure, determined in the
present study, is in agreement with the results of recent first principles
calculation; however, it is in contrast to a fascinating experimental
prediction of ferrimagnetic ordering based on the periodicity of the exchange
interactions in CaNi3P4O14. Our study also confirms the presence of broad
diffuse magnetic scattering, due to 1D short-range spin-spin correlations, over
a wide temperature range below ~50 K down to a temperature well below the Tc.
Total neutron scattering analysis by the RMC method reveals that the dominating
spin-spin correlation above Tc is ferromagnetic and along the b axis. The
nearest neighbour spin-spin correlations along the a and c axes are found to be
weakly antiferromagnetic. The nature of the trimer spin structure of the
short-range state is similar to that of the 3D long-range ordered state. The
present investigation of microscopic nature of the magnetic ground state also
explains the condition required for the 1/3 magnetization plateau to be
observed in the trimer spin-chains. In spite of the S=1 trimer chain system,
the present compound CaNi3P4O14 is found to be a good realization of 3D magnet
below the Tc=16 K with full ordered moment values of ~2 mu_B/Ni2+ (1.98 and
1.96 mu_B/Ni2+ for two Ni sites, respectively) at 1.5 K.Comment: 10 pages, 8 figure
The replica symmetric behavior of the analogical neural network
In this paper we continue our investigation of the analogical neural network,
paying interest to its replica symmetric behavior in the absence of external
fields of any type. Bridging the neural network to a bipartite spin-glass, we
introduce and apply a new interpolation scheme to its free energy that
naturally extends the interpolation via cavity fields or stochastic
perturbations to these models. As a result we obtain the free energy of the
system as a sum rule, which, at least at the replica symmetric level, can be
solved exactly. As a next step we study its related self-consistent equations
for the order parameters and their rescaled fluctuations, found to diverge on
the same critical line of the standard Amit-Gutfreund-Sompolinsky theory.Comment: 17 page
Graphene via large N I: Renormalization
We analyze the competing effects of moderate to strong Coulomb
electron-electron interactions and weak quenched disorder in graphene. Using a
one-loop renormalization group calculation controlled within the large-N
approximation, we demonstrate that, at successively lower energy (temperature
or chemical potential) scales, a type of non-Abelian vector potential disorder
always asserts itself as the dominant elastic scattering mechanism for generic
short-ranged microscopic defect distributions. Vector potential disorder is
tied to both elastic lattice deformations ("ripples") and topological lattice
defects. We identify several well-defined scaling regimes, for which we provide
scaling predictions for the electrical conductivity and thermopower, valid when
the inelastic lifetime due to interactions exceeds the elastic lifetime due to
disorder. Coulomb interaction effects should figure strongly into the physics
of suspended graphene films, where rs > 1; we expect vector potential disorder
to play an important role in the description of transport in such films.Comment: 25 pages, 21 figure
Statistical mechanics of temporal association in neural networks with transmission delays
We study the representation of static patterns and temporal sequences in neural networks with signal delays and a stochastic parallel dynamics. For a wide class of delay distributions, the asymptotic network behavior can be described by a generalized Gibbs distribution, generated by a novel Lyapunov functional for the determination dynamics. We extend techniques of equilibrium statistical mechanics so as to deal with time-dependent phenomena, derive analytic results for both retrieval quality and storage capacity, and compare them with numerical simulations
Statistical properties of an ensemble of vortices interacting with a turbulent field
We develop an analytical formalism to determine the statistical properties of
a system consisting of an ensemble of vortices with random position in plane
interacting with a turbulent field. We calculate the generating functional by
path-integral methods. The function space is the statistical ensemble composed
of two parts, the first one representing the vortices influenced by the
turbulence and the second one the turbulent field scattered by the randomly
placed vortices.Comment: Third version; Important corrections in the normalization for the gas
of vortices, et
Quark Number Fluctuations in a Chiral Model at Finite Baryon Chemical Potential
We discuss the net quark and isovector fluctuations as well as off-diagonal
quark flavor susceptibilities along the chiral phase transition line in the
Nambu--Jona-Lasinio (NJL) model. The model is formulated at non-zero quark and
isospin chemical potentials with non-vanishing vector couplings in the
iso-scalar and iso-vector channels. We study the influence of the quark
chemical potential on the quark flavour susceptibilities in detail and the
dependence of the results on model parameters as well as on the quark mass. The
NJL model findings are compared with recent lattice results obtained in
two--flavor QCD at finite chemical potential. On a qualitative level, the NJL
model provides a consistent description of the dependence of quark number
fluctuations on temperature and baryon chemical potential. The phase diagram
and the position of the tricritical point in the NJL model are also discussed
for different parameter sets.Comment: 33 pages, 11 figures; final version accepted for publication in Phys.
Rev.
Dimensional crossover in dipolar magnetic layers
We investigate the static critical behaviour of a uniaxial magnetic layer,
with finite thickness L in one direction, yet infinitely extended in the
remaining d dimensions. The magnetic dipole-dipole interaction is taken into
account. We apply a variant of Wilson's momentum shell renormalisation group
approach to describe the crossover between the critical behaviour of the 3-D
Ising, 2-d Ising, 3-D uniaxial dipolar, and the 2-d uniaxial dipolar
universality classes. The corresponding renormalisation group fixed points are
in addition to different effective dimensionalities characterised by distinct
analytic structures of the propagator, and are consequently associated with
varying upper critical dimensions. While the limiting cases can be discussed by
means of dimensional epsilon expansions with respect to the appropriate upper
critical dimensions, respectively, the crossover features must be addressed in
terms of the renormalisation group flow trajectories at fixed dimensionality d.Comment: 25 pages, Latex, 12 figures (.eps files) and IOP style files include
Microcanonical finite-size scaling in specific heat diverging 2nd order phase transitions
A Microcanonical Finite Site Ansatz in terms of quantities measurable in a
Finite Lattice allows to extend phenomenological renormalization (the so called
quotients method) to the microcanonical ensemble. The Ansatz is tested
numerically in two models where the canonical specific-heat diverges at
criticality, thus implying Fisher-renormalization of the critical exponents:
the 3D ferromagnetic Ising model and the 2D four-states Potts model (where
large logarithmic corrections are known to occur in the canonical ensemble). A
recently proposed microcanonical cluster method allows to simulate systems as
large as L=1024 (Potts) or L=128 (Ising). The quotients method provides
extremely accurate determinations of the anomalous dimension and of the
(Fisher-renormalized) thermal exponent. While in the Ising model the
numerical agreement with our theoretical expectations is impressive, in the
Potts case we need to carefully incorporate logarithmic corrections to the
microcanonical Ansatz in order to rationalize our data.Comment: 13 pages, 8 figure
Defect fugacity, Spinwave Stiffness and T_c of the 2-d Planar Rotor Model
We obtain precise values for the fugacities of vortices in the 2-d planar
rotor model from Monte Carlo simulations in the sector with {\em no} vortices.
The bare spinwave stiffness is also calculated and shown to have significant
anharmonicity. Using these as inputs in the KT recursion relations, we predict
the temperature T_c = 0.925, using linearised equations, and using next higher order corrections, at which vortex unbinding commences
in the unconstrained system. The latter value, being in excellent agreement
with all recent determinations of T_c, demonstrates that our method 1)
constitutes a stringent measure of the relevance of higher order terms in KT
theory and 2) can be used to obtain transition temperatures in similar systems
with modest computational effort.Comment: 7 pages, 4 figure
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