690 research outputs found
Two-dimensional magnetoexcitons in the presence of spin-orbit coupling
We study theoretically the effect of spin-orbit coupling on quantum well
excitons in a strong magnetic field. We show that, in the presence of an
in-plane field component, the excitonic absorption spectrum develops a
double-peak structure due to hybridization of bright and dark magnetoexcitons.
If the Rashba and Dresselhaus spin-orbit constants are comparable, the
magnitude of splitting can be tuned in a wide interval by varying the azimuthal
angle of the in-plane field. We also show that the interplay between spin-orbit
and Coulomb interactions leads to an anisotropy of exciton energy dispersion in
the momentum plane. The results suggest a way for direct optical measurements
of spin-orbit parameters.Comment: 9 pages, 6 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.
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
Non-Linear Algebra and Bogolubov's Recursion
Numerous examples are given of application of Bogolubov's forest formula to
iterative solutions of various non-linear equations: one and the same formula
describes everything, from ordinary quadratic equation to renormalization in
quantum field theory.Comment: LaTex, 21 page
Magnetic properties of Ni2.18Mn0.82Ga Heusler alloys with a coupled magnetostructural transition
Polycrystalline Ni2.18Mn0.82Ga Heusler alloys with a coupled
magnetostructural transition are studied by differential scanning calorimetry,
magnetic and resistivity measurements. Coupling of the magnetic and structural
subsystems results in unusual magnetic features of the alloy. These uncommon
magnetic properties of Ni2.18Mn0.82Ga are attributed to the first-order
structural transition from a tetragonal ferromagnetic to a cubic paramagnetic
phase.Comment: 4 pages, 4 figures, revtex
Anomalous scaling of a passive scalar advected by the turbulent velocity field with finite correlation time and uniaxial small-scale anisotropy
The influence of uniaxial small-scale anisotropy on the stability of the
scaling regimes and on the anomalous scaling of the structure functions of a
passive scalar advected by a Gaussian solenoidal velocity field with finite
correlation time is investigated by the field theoretic renormalization group
and operator product expansion within one-loop approximation. Possible scaling
regimes are found and classified in the plane of exponents ,
where characterizes the energy spectrum of the velocity field in the
inertial range , and is related to the
correlation time of the velocity field at the wave number which is scaled
as . It is shown that the presence of anisotropy does not disturb
the stability of the infrared fixed points of the renormalization group
equations which are directly related to the corresponding scaling regimes. The
influence of anisotropy on the anomalous scaling of the structure functions of
the passive scalar field is studied as a function of the fixed point value of
the parameter which represents the ratio of turnover time of scalar field
and velocity correlation time. It is shown that the corresponding one-loop
anomalous dimensions, which are the same (universal) for all particular models
with concrete value of in the isotropic case, are different (nonuniversal)
in the case with the presence of small-scale anisotropy and they are continuous
functions of the anisotropy parameters, as well as the parameter . The
dependence of the anomalous dimensions on the anisotropy parameters of two
special limits of the general model, namely, the rapid-change model and the
frozen velocity field model, are found when and ,
respectively.Comment: revtex, 25 pages, 37 figure
A simple analytical model for dark matter halo structure and adiabatic contraction
A simple analytical model for describing inner parts of dark matter halo is
considered. It is assumed that dark matter density is power-law. The model
deals with dark matter distribution function in phase space of adiabatic
invariants (radial action and angular momentum). Two variants are considered
for the angular part of the distribution function: narrow and broad
distribution. The model allows to describe explicitly the process of adiabatic
contraction of halo due to change of gravitational potential caused by
condensation of baryonic matter in the centre. The modification of dark matter
density in the centre is calculated, and is it shown that the standard
algorithm of adiabatic contraction calculation overestimates the compressed
halo density, especially in the case of strong radial anisotropy.Comment: 5 pages, 3 figures. v3 - major improvements, another halo model
introduced, discussion extende
Studying a LW-VCSEL-Based Resonant Cavity Enhanced Photodetector and Its Application in Microwave Photonics Circuits
A detailed comparative experimental study was carried out to pursue advanced performances corresponding to the key parameters of two photodetectors based on vertical cavity surface emitting laser (VCSEL) operating in free-running or optically injection locked mode, as well as an inherent pin-photodetector. During the preliminary study, the key static and dynamic parameters were quantitatively determined and the optimal operating modes were derived for the both versions of VCSEL-based photodetectors as separate microwave-photonics circuit elements. Based on them, a final experiment was conducted to evaluate the processing quality, when one of the versions of VCSEL-based photodetectors or a inherent pin-photodetector is implemented as an optical-to-electrical converter for a typical microwave-photonics circuit that processes 120-Mbps 16-position quadrature amplitude modulated signal on the radio frequency carrier of 1–6 GHz. As a result, it was confirmed that better processing quality, i.e. Error Vector Magnitude value of less than 4%, could be obtained by using the free-running VCSEL-based photodetector version
Energy representation for out-of-equilibrium Brownian-like systems: steady states and fluctuation relations
Stochastic dynamics in the energy representation is employed as a method to
study non-equilibrium Brownian-like systems. It is shown that the equation of
motion for the energy of such systems can be taken in the form of the Langevin
equation with multiplicative noise. Properties of the steady states are
examined by solving the Fokker-Planck equation for the energy distribution
functions. The generalized integral fluctuation theorem is deduced for the
systems characterized by the shifted probability flux operator. There are a
number of entropy and fluctuation relations such as the Hatano-Sasa identity
and the Jarzynski's equality that follow from this theorem.Comment: revtex4-1, 18 pages, extended discussion, references adde
Features of Magneto-Optical Resonances in an Elliptically Polarized Traveling Light Wave
The parameters of nonlinear absorption magneto-optical resonances in the
Hanle configuration have been studied as functions of the ellipticity of a
traveling light wave. It has been found that these parameters (amplitude,
width, and amplitude-to-width ratio) depend strongly on the polarization of the
light wave. In particular, the resonance amplitude can increase by more than an
order of magnitude when the polarization changes from linear to optimal
elliptic. It has been shown that this effect is associated with the Doppler
frequency shift for atoms in a gas. The theoretical results have been
corroborated in experiments in Rb vapor.Comment: 5 page
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