20 research outputs found
Interlayer Exchange Coupling Mediated by Valence Band Electrons
The interlayer exchange coupling mediated by valence band electrons in
all-semiconductor IV-VI magnetic/nonmagnetic superlattices is studied
theoretically. A 3D tight-binding model, accounting for the band and magnetic
structure of the constituent superlattice components is used to calculate the
spin-dependent part of the total electronic energy. The antiferromagnetic
coupling between ferromagnetic layers in EuS/PbS superlattices is obtained, in
agreement with the experimental evidences. The results obtained for the
coupling between antiferromagnetic layers in EuTe/PbTe superlattices are also
presented.Comment: 8 pages, 6 figures, to be submitted to Phys.Rev.
Zitterbewegung in External Magnetic Field: Classic versus Quantum Approach
We investigate variations of the Zitterbewegung frequency of electron due to
an external static and uniform magnetic field employing the expectation value
quantum approach, and compare our results with the classical model of spinning
particles. We demonstrate that these two so far compatible approaches are not
in agreement in the presence of an external uniform static magnetic field, in
which the classical approach breaks the usual symmetry of free particles and
antiparticles states, i.e. it leads to CP violation. Hence, regarding the
Zitterbewegung frequency of electron, the classical approach in the presence of
an external magnetic field is unlikely to correctly describe the spin of
electron, while the quantum approach does, as expected. We also show that the
results obtained via the expectation value are in close agreement with the
quantum approach of the Heisenberg picture derived in the literature. However,
the method we use is capable of being compared with the classical approach
regarding the spin aspects. The classical interpretation of spin produced by
the altered Zitterbewegung frequency, in the presence of an external magnetic
field, are discussed.Comment: 16 pages, no figure
Magnetic interactions in EuTe epitaxial layers and EuTe/PbTe superlattices
The magnetic properties of antiferromagnetic (AFM) EuTe epitaxial layers and
short period EuTe/PbTe superlattices (SLs), grown by molecular beam epitaxy on
(111) BaF substrates, were studied by magnetization and neutron diffraction
measurements. Considerable changes of the N\'eel temperature as a function of
the EuTe layer thickness as well as of the strain state were found. A mean
field model, taking into account the variation of the exchange constants with
the strain-induced lattice distortions, and the nearest neighbor environment of
a Eu atoms, was developed to explain the observed changes in wide
range of samples. Pronounced interlayer magnetic correlations have been
revealed by neutron diffraction in EuTe/PbTe SLs with PbTe spacer thickness up
to 60 \AA. The observed diffraction spectra were analyzed, in a kinematical
approximation, assuming partial interlayer correlations characterized by an
appropriate correlation parameter. The formation of interlayer correlations
between the AFM EuTe layers across the nonmagnetic PbTe spacer was explained
within a framework of a tight-binding model. In this model, the interlayer
coupling stems from the dependence of the total electronic energy of the
EuTe/PbTe SL on the spin configurations in adjacent EuTe layers. The influence
of the EuTe and PbTe layer thickness fluctuations, inherent in the epitaxial
growth process, on magnetic properties and interlayer coupling is discussed.Comment: 17 pages, 19 figures, accepted to PR
Constraints on cosmological models from strong gravitational lensing systems
Strong lensing has developed into an important astrophysical tool for probing
both cosmology and galaxies (their structure, formation, and evolution). Using
the gravitational lensing theory and cluster mass distribution model, we try to
collect a relatively complete observational data concerning the Hubble constant
independent ratio between two angular diameter distances from
various large systematic gravitational lens surveys and lensing by galaxy
clusters combined with X-ray observations, and check the possibility to use it
in the future as complementary to other cosmological probes. On one hand,
strongly gravitationally lensed quasar-galaxy systems create such a new
opportunity by combining stellar kinematics (central velocity dispersion
measurements) with lensing geometry (Einstein radius determination from
position of images). We apply such a method to a combined gravitational lens
data set including 70 data points from Sloan Lens ACS (SLACS) and Lens
Structure and Dynamics survey (LSD). On the other hand, a new sample of 10
lensing galaxy clusters with redshifts ranging from 0.1 to 0.6 carefully
selected from strong gravitational lensing systems with both X-ray satellite
observations and optical giant luminous arcs, is also used to constrain three
dark energy models (CDM, constant and CPL) under a flat universe
assumption. For the full sample () and the restricted sample ()
including 36 two-image lenses and 10 strong lensing arcs, we obtain relatively
good fitting values of basic cosmological parameters, which generally agree
with the results already known in the literature. This results encourages
further development of this method and its use on larger samples obtained in
the future.Comment: 22 pages, 5 figures, 2 tables; accepted by JCA
Classical Simulation of Relativistic Quantum Mechanics in Periodic Optical Structures
Spatial and/or temporal propagation of light waves in periodic optical
structures offers a rather unique possibility to realize in a purely classical
setting the optical analogues of a wide variety of quantum phenomena rooted in
relativistic wave equations. In this work a brief overview of a few optical
analogues of relativistic quantum phenomena, based on either spatial light
transport in engineered photonic lattices or on temporal pulse propagation in
Bragg grating structures, is presented. Examples include spatial and temporal
photonic analogues of the Zitterbewegung of a relativistic electron, Klein
tunneling, vacuum decay and pair-production, the Dirac oscillator, the
relativistic Kronig-Penney model, and optical realizations of non-Hermitian
extensions of relativistic wave equations.Comment: review article (invited), 14 pages, 7 figures, 105 reference