1,570 research outputs found
Cocliques of maximal size in the prime graph of a finite simple group
In this paper we continue our investgation of the prime graph of a finite
simple group started in http://arxiv.org/abs/math/0506294 (the printed version
appeared in [1]). We describe all cocliques of maximal size for all finite
simple groups and also we correct mistakes and misprints from our previous
paper. The list of correction is given in Appendix of the present paper.Comment: published version with correction
Spatial effects in superradiant Rayleigh scattering from Bose-Einstein condensates
We present a detailed theoretical analysis of superradiant Rayleigh
scattering from atomic Bose-Einstein condensates. A thorough investigation of
the spatially resolved time-evolution of optical and matter-wave fields is
performed in the framework of the semiclassical Maxwell-Schroedinger equations.
Our theory is not only able to explain many of the known experimental
observations, e.g., the behavior of the atomic side-mode distributions, but
also provides further detailed insights into the coupled dynamics of optical
and matter-wave fields. To work out the significance of propagation effects, we
compare our results to other theoretical models in which these effects are
neglected.Comment: 14 pages, 13 figure
Theory of interacting electrons on the honeycomb lattice
The low-energy theory of electrons interacting via repulsive short-range
interactions on graphene's honeycomb lattice at half filling is presented. The
exact symmetry of the Lagrangian with local quartic terms for the Dirac field
dictated by the lattice is D_2 x U_c(1) x (time reversal), where D_2 is the
dihedral group, and U_c(1) is a subgroup of the SU_c(2) "chiral" group of the
non-interacting Lagrangian, that represents translations in Dirac language. The
Lagrangian describing spinless particles respecting this symmetry is
parameterized by six independent coupling constants. We show how first imposing
the rotational, then Lorentz, and finally chiral symmetry to the quartic terms,
in conjunction with the Fierz transformations, eventually reduces the set of
couplings to just two, in the "maximally symmetric" local interacting theory.
We identify the two critical points in such a Lorentz and chirally symmetric
theory as describing metal-insulator transitions into the states with either
time-reversal or chiral symmetry being broken. In the site-localized limit of
the interacting Hamiltonian the low-energy theory describes the continuous
transitions into the insulator with either a finite Haldane's (circulating
currents) or Semenoff's (staggered density) masses, both in the universality
class of the Gross-Neveu model. The picture of the metal-insulator transition
on a honeycomb lattice emerges at which the residue of the quasiparticle pole
at the metallic and the mass-gap in the insulating phase both vanish
continuously as the critical point is approached. We argue that the Fermi
velocity is non-critical as a consequence of the dynamical exponent being fixed
to unity by the emergent Lorentz invariance. Effects of long-range interaction
and the critical behavior of specific heat and conductivity are discussed.Comment: 16 revtex pages, 4 figures; typos corrected, new and updated
references; published versio
High frequency dielectric and magnetic anomaly at the phase transition in NaV2O5
We found anomalies in the temperature dependence of the dielectric and the
magnetic susceptibiliy of NaV_2O_5 in the microwave and far infrared frequency
ranges. The anomalies occur at the phase transition temperature T_c, at which
the spin gap opens. The real parts of the dielectric constants epsilon_a and
epsilon_c decrease below T_c. The decrease of epsilon_a (except for the narrow
region close to T_c) is proportional to the intensity of the x-ray reflection
appearing at T_c. The dielectric constant anomaly can be explained by the
zigzag charge ordering in the ab-plane appearing below T_c. The anomaly of the
microwave magnetic losses is probably related to the coupling between the spin
and charge degrees of freedom in vanadium ladders.Comment: 3 PS-figures, LATEX-text, new experimental data added, typos
correcte
Coherent interaction of laser pulses in a resonant optically dense extended medium under the regime of strong field-matter coupling
Nonstationary pump-probe interaction between short laser pulses propagating
in a resonant optically dense coherent medium is considered. A special
attention is paid to the case, where the density of two-level particles is high
enough that a considerable part of the energy of relatively weak external
laser-fields can be coherently absorbed and reemitted by the medium. Thus, the
field of medium reaction plays a key role in the interaction processes, which
leads to the collective behavior of an atomic ensemble in the strongly coupled
light-matter system. Such behavior results in the fast excitation interchanges
between the field and a medium in the form of the optical ringing, which is
analogous to polariton beating in the solid-state optics. This collective
oscillating response, which can be treated as successive beats between light
wave-packets of different group velocities, is shown to significantly affect
propagation and amplification of the probe field under its nonlinear
interaction with a nearly copropagating pump pulse. Depending on the probe-pump
time delay, the probe transmission spectra show the appearance of either
specific doublet or coherent dip. The widths of these features are determined
by the density-dependent field-matter coupling coefficient and increase during
the propagation. Besides that, the widths of the coherent features, which
appear close to the resonance in the broadband probe-spectrum, exceed the
absorption-line width, since, under the strong-coupling regime, the frequency
of the optical ringing exceeds the rate of incoherent relaxation. Contrary to
the stationary strong-field effects, the density- and coordinate-dependent
transmission spectra of the probe manifest the importance of the collective
oscillations and cannot be obtained in the framework of the single-atom model.Comment: 10 pages, 8 figures, to be published in Phys. Rev.
Ground state order and spin-lattice coupling in tetrahedral spin systems Cu2Te2O5X2
High-resolution ac susceptibility and thermal conductivity measurement on
Cu2Te2O5X2(X=Br,Cl) single crystals are reported. For Br-sample, sample
dependence prevents to distinguish between possibilities of magnetically
ordered and spin-singlet ground states. In Cl-sample a three-dimensional
transition at 18.5 K is accompanied by almost isotropic behavior of
susceptibility and almost switching behavior of thermal conductivity. Thermal
conductivity studies suggest the presence of a tremendous spin-lattice coupling
characterizing Cl- but not Br-sample. Below the transition Cl-sample is in a
complex magnetic state involving AF order but also the elements consistent with
the presence of a gap in the excitation spectrum.Comment: version accepted for publication in Phys.Rev.B-Rapid Communicatio
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