619 research outputs found
Renormalization-group approach to the stochastic Navier--Stokes equation: Two-loop approximation
The field theoretic renormalization group is applied to the stochastic
Navier--Stokes equation that describes fully developed fluid turbulence. The
complete two-loop calculation of the renormalization constant, the
function, the fixed point and the ultraviolet correction exponent is performed.
The Kolmogorov constant and the inertial-range skewness factor, derived to
second order of the \eps expansion, are in a good agreement with the
experiment. The possibility of the extrapolation of the \eps expansion beyond
the threshold where the sweeping effects become important is demonstrated on
the example of a Galilean-invariant quantity, the equal-time pair correlation
function of the velocity field. The extension to the -dimensional case is
briefly discussed.Comment: 20 pages, 3 figure
Premartensitic Transition in Ni2+xMn1-xGa Heusler Alloys
The temperature dependencies of the resistivity and magnetization of a series
of Ni2+XMn1-XGa (X = 0 - 0.09) alloys were investigated. Along with the
anomalies associated with ferromagnetic and martensitic transitions,
well-defined anomalies were observed at the temperature of premartensitic
transformation. The premartensitic phase existing in a temperature range 200 -
260 K in the stoichiometric Ni2MnGa is suppressed by the martensitic phase with
increasing Ni content and vanishes in Ni2.09Mn0.91Ga composition
O(1/N_f) Corrections to the Thirring Model in 2<d<4
The Thirring model, that is, a relativistic field theory of fermions with a
contact interaction between vector currents, is studied for dimensionalities
2<d<4 using the 1/N_f expansion, where N_f is the number of fermion species.
The model is found to have no ultraviolet divergences at leading order provided
a regularization respecting current conservation is used. Explicit O(1/N_f)
corrections are computed, and the model shown to be renormalizable at this
order in the massless limit; renormalizability appears to hold to all orders
due to a special case of Weinberg's theorem. This implies there is a universal
amplitude for four particle scattering in the asymptotic regime. Comparisons
are made with both the Gross-Neveu model and QED.Comment: 22 pages in plain TeX, with 7 figs included using psfig.tex (Minor
conceptual changes - algebra unaffected
Anomalous thermal conductivity of NaV2O5 as compared to conventional spin-Peierls system CuGeO3
A huge increase of thermal conductivity k is observed at the phase transition
in stoichiometric NaV2O5. This anomaly decreases and gradually disappears with
deviation from stoichiometry in Na(1-x}V2O5 (x = 0.01, 0.02, 0.03, and 0.04).
This behavior is compared with that of pure and Zn-doped CuGeO3 where only
modest kinks in the k(T) curves are observed at the spin-Peierls transition.
The change of k at critical temperature Tc could be partially attributed to the
opening of an energy gap in the magnetic excitation spectrum excluding the
scattering of thermal phonons on spin fluctuations. However, the reason for
such a strong anomaly in the k(T) may lie not only in the different energy
scales of CuGeO3 and NaV2O5, but also in the different character of the phase
transition in NaV2O5 which can have largely a structural origin, e.g. connected
with the charge ordering.Comment: PostScript 4 pages, 4 PostScript pictures. Submitted to Physical
Review Letter
Finite groups isospectral to simple groups
The spectrum of a finite group is the set of element orders of this group.
The main goal of this paper is to survey results concerning recognition of
finite simple groups by spectrum, in particular, to list all finite simple
groups for which the recognition problem is solved
Integral Transforms for Conformal Field Theories with a Boundary
A new method is developed for solving the conformally invariant integrals
that arise in conformal field theories with a boundary. The presence of a
boundary makes previous techniques for theories without a boundary less
suitable. The method makes essential use of an invertible integral transform,
related to the radon transform, involving integration over planes parallel to
the boundary. For successful application of this method several nontrivial
hypergeometric function relations are also derived.Comment: 20 pagess, LateX fil
A Model Study of the Low-Energy Charge Dynamics of NaV_2O_5
An exact-diagonalization technique on small clusters is used to calculate the
dynamical density correlation functions of the dimerized t-J chain and coupled
anisotropic t-J ladders (trellis lattice) at quarter filling, i.e., the systems
regarded as a network of pairs (dimers or rungs) of sites coupled weakly via
the hopping and exchange interactions. We thereby demonstrate that the
intersite Coulomb repulsions between the pairs induce a low-energy collective
mode in the charge excitations of the systems where the internal charge degrees
of freedom of the pairs play an essential role. Implications to the electronic
states of NaV_2O_5, i.e., fluctuations of the valence state of V ions and phase
transition as a charge ordering, are discussed.Comment: 4 pages, 4 gif figures. Hardcopies of figures (or the entire
manuscript) can be obtained by e-mail request to [email protected]
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