975 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
Enhanced longitudinal mode spacing in blue-violet InGaN semiconductor laser
A novel explanation of observed enhanced longitudinal mode spacing in InGaN
semiconductor lasers has been proposed. It has been demonstrated that e-h
plasma oscillations, which can exist in the laser active layer at certain
driving conditions, are responsible for mode clustering effect. The resonant
excitation of the plasma oscillations occurs due to longitudinal mode beating.
The separation of mode clusters is typically by an order of magnitude larger
that the individual mode spacing.Comment: 3 pages, 2 figure
A possibility for precise Weinberg angle measurement in centrosymmetric crystals with axis
We demonstrate that parity nonconserving interaction due to the nuclear weak
charge Q_W leads to nonlinear magnetoelectric effect in centrosymmetric
paramagnetic crystals. It is shown that the effect exists only in crystals with
special symmetry axis k. Kinematically, the correlation (correction to energy)
has the form H_PNC ~ Q_W (E,[B,k])(B,k), where B and E are the external
magnetic and electric fields. This gives rise to magnetic induction M_PNC ~ Q_W
{k(B,[k,E]) + [k,E](B,k)}. To be specific we consider rare-earth trifluorides
and, in particular, dysprosium trifluoride which looks the most suitable for
experiment. We estimate the optimal temperature for the experiment to be of a
few kelvin. For the magnetic field B = 1 T and the electric field E = 10 kV/cm,
the expected magnetic induction is 4 \pi M_PNC = 0.5 * 10^-11 G, six orders of
magnitude larger than the best sensitivity currently under discussion.
Dysprosium has several stable isotopes, and so, comparison of the effects for
different isotopes provides possibility for precise measurement of the Weinberg
angle.Comment: 7 pages, 1 figure, 2 tables; version 2 - added discussion of neutron
distribution uncertaint
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
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
Leading infrared logarithms for sigma-model with fields on arbitrary Riemann manifold
We derive non-linear recursion equation for the leading infrared logarithms
(LL) in four dimensional sigma-model with fields on an arbitrary Riemann
manifold. The derived equation allows one to compute leading infrared
logarithms to essentially unlimited loop order in terms of geometric
characteristics of the Riemann manifold.
We reduce the solution of the SU(oo) principal chiral field in arbitrary
number of dimensions in the LL approximation to the solution of very simple
recursive equation. This result paves a way to the solution of the model in
arbitrary number of dimensions at N-->ooComment: Talk given by MVP at the conference devoted to memory of A.N.
Vasilie
Description of paramagnetic--spin glass transition in Edwards-Anderson model in terms of critical dynamics
Possibility of description of the glass transition in terms of critical
dynamics considering a hierarchy of the intermodal relaxation time is shown.
The generalized Vogel-Fulcher law for the system relaxation time is derived in
terms of this approach. It is shown that the system satisfies the
fluctuating--dissipative theorem in case of the absence of the intermodal
relaxation time hierarchy.Comment: 10 pages, 6 figure
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
Origin of magnetic interactions and their influence on the structural properties of Ni2MnGa and related compounds
In this work, we perform first principles DFT calculations to investigate the
interplay between magnetic and structural properties in Ni2MnGa. We demonstrate
that the relative stability of austenite (cubic) and non-modulated martensite
(tetragonal) phases depends critically on the magnetic interactions between Mn
atoms. While standard approximate DFT functionals stabilize the latter phase, a
more accurate treatment of electronic localization and magnetism, obtained with
DFT+U, suppresses the non-modulated tetragonal structure for the stoichiometric
compound, in better agreement with the experiments. We show that the Anderson
impurity model, with Mn atoms treated as magnetic impurities, can explain this
observation and that the fine balance between super-exchange RKKY type
interactions mediated by Ni d and Ga p orbitals determines the equilibrium
structure of the crystal. The Anderson model is also demonstrated to capture
the effect of the number of valence electrons per unit cell on the structural
properties, often used as an empirical parameter to tune the behavior of
Ni2MnGa based alloys. Finally, we show that off-stoichiometric compositions
with excess Mn promote transitions to a non-modulated tetragonal structure, in
agreement with experiments.Comment: 16 pages, 25 figure
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.
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