2,007 research outputs found
Controlling antiferromagnetic domains in patterned La0.7Sr0.3FeO3 thin films
Transition metal oxide thin films and heterostructures are promising platforms to achieve full control of the antiferromagnetic (AFM) domain structure in patterned features as needed for AFM spintronic devices. In this work, soft x-ray photoemission electron microscopy was utilized to image AFM domains in micromagnets patterned into La0.7Sr0.3FeO3 (LSFO) thin films and La0.7Sr0.3MnO3 (LSMO)/LSFO superlattices. A delicate balance exists between magnetocrystalline anisotropy, shape anisotropy, and exchange interactions such that the AFM domain structure can be controlled using parameters such as LSFO and LSMO layer thickness, micromagnet shape, and temperature. In LSFO thin films, shape anisotropy gains importance only in micromagnets where at least one extended edge is aligned parallel to an AFM easy axis. In contrast, in the limit of ultrathin LSFO layers in the LSMO/LSFO superlattice, shape anisotropy effects dominate such that the AFM spin axes at micromagnet edges can be aligned along any in-plane crystallographic direction
Triaxiality and the determination of the cubic shape parameter K3 from five observables
The absolute and the relative quadrupole shape invariants q3 and K3 provide a
model independent measure of triaxiality for beta-rigid nuclei. We will show
that one can obtain q3 and K3 from a small number of observables. The
approximations which are made will be shown to hold within a few percent both
in the rigid triaxial rotor model and the interacting boson model. The shape
parameter K3 is given for an exemplary set of nuclei and is translated into
effective values of the geometrical deformation parameters beta and gamma.Comment: 16 pages, 4 figure
Periodic chiral magnetic domains in single-crystal nickel nanowires
We report on experimental and computational investigations of the domain
structure of ~0.2 x 0.2 x 8 {\mu}m single-crystal Ni nanowires (NWs). The Ni
NWs were grown by a thermal chemical vapor deposition technique that results in
highly-oriented single-crystal structures on amorphous SiOx coated Si
substrates. Magnetoresistance measurements of the Ni NWs suggest the average
magnetization points largely off the NW long axis at zero field. X-ray
photoemission electron microscopy images show a well-defined periodic
magnetization pattern along the surface of the nanowires with a period of
{\lambda} = 250 nm. Finite element micromagnetic simulations reveal that an
oscillatory magnetization configuration with a period closely matching
experimental observation ({\lambda} = 240 nm) is obtainable at remanence. This
magnetization configuration involves a periodic array of alternating chirality
vortex domains distributed along the length of the NW. Vortex formation is
attributable to the cubic anisotropy of the single crystal Ni NW system and its
reduced structural dimensions. The periodic alternating chirality vortex state
is a topologically protected metastable state, analogous to an array of
360{\deg} domain walls in a thin strip. Simulations show that other remanent
states are also possible, depending on the field history. Effects of material
properties and strain on the vortex pattern are investigated. It is shown that
at reduced cubic anisotropy vortices are no longer stable, while negative
uniaxial anisotropy and magnetoelastic effects in the presence of compressive
biaxial strain contribute to vortex formation.Comment: 15 pages, 11 figure
Transient magnetoconductivity of photoexcited electrons
Transient magnetotransport of two-dimensional electrons with
partially-inverted distribution excited by an ultrashort optical pulse is
studied theoretically. The time-dependent photoconductivity is calculated for
GaAs-based quantum wells by taking into account the relaxation of electron
distribution caused by non-elastic electron-phonon interaction and the
retardation of the response due to momentum relaxation and due to a finite
capacitance of the sample. We predict large-amplitude transient oscillations of
the current density and Hall field (Hall oscillations) with frequencies
corresponding to magnetoplasmon range, which are initiated by the instability
owing to the absolute negative conductivity effect.Comment: 21 pages, 6 fig
Weights for relative motives; relation with mixed complexes of sheaves
The main goal of this paper is to define the so-called Chow weight structure
for the category of Beilinson motives over any 'reasonable' base scheme
(this is the version of Voevodsky's motives over defined by Cisinski and
Deglise). We also study the functoriality properties of the Chow weight
structure (they are very similar to the well-known functoriality of weights for
mixed complexes of sheaves). As shown in a preceding paper, the Chow weight
structure automatically yields an exact conservative weight complex functor
(with values in ). Here is the heart of the Chow weight
structure; it is 'generated' by motives of regular schemes that are projective
over . Besides, Grothendiek's group of -motives is isomorphic to
; we also define a certain 'motivic Euler characteristic' for
-schemes. We obtain (Chow)-weight spectral sequences and filtrations for any
cohomology of motives; we discuss their relation to Beilinson's 'integral part'
of motivic cohomology and to weights of mixed complexes of sheaves. For the
study of the latter we introduce a new formalism of relative weight structures.Comment: a few minor corrections mad
Alternative Interpretation of Sharply Rising E0 Strengths in Transitional Regions
It is shown that strong 0+2 -> 0+1 E0 transitions provide a clear signature
of phase transitional behavior in finite nuclei. Calculations using the IBA
show that these transition strengths exhibit a dramatic and robust increase in
spherical-deformed shape transition regions, that this rise matches well the
existing data, that the predictions of these E0 transitions remain large in
deformed nuclei, and that these properties are intrinsic to the way that
collectivity and deformation develop through the phase transitional region in
the model, arising from the specific d-boson coherence in the wave functions,
and that they do not necessarily require the explicit mixing of normal and
intruder configurations from different IBA spaces.Comment: 6 pages, 3 figure
Hanle effect in the solar Ba II D2 line: a diagnostic tool for chromospheric weak magnetic fields
The physics of the solar chromosphere depends in a crucial way on its
magnetic structure. However there are presently very few direct magnetic field
diagnostics available for this region. Here we investigate the diagnostic
potential of the Hanle effect on the Ba II D2 line resonance polarization for
the determination of weak chromospheric turbulent magnetic fields......Comment: In press in astronomy and astrophysic
Thermoelectric properties of the bismuth telluride nanowires in the constant-relaxation-time approximation
Electronic structure of bismuth telluride nanowires with the growth
directions [110] and [015] is studied in the framework of anisotropic effective
mass method using the parabolic band approximation. The components of the
electron and hole effective mass tensor for six valleys are calculated for both
growth directions. For a square nanowire, in the temperature range from 77 K to
500 K, the dependence of the Seebeck coefficient, the electron thermal and
electrical conductivity as well as the figure of merit ZT on the nanowire
thickness and on the excess hole concentration are investigated in the
constant-relaxation-time approximation. The carrier confinement is shown to
play essential role for square nanowires with thickness less than 30 nm. The
confinement decreases both the carrier concentration and the thermal
conductivity but increases the maximum value of Seebeck coefficient in contrast
to the excess holes (impurities). The confinement effect is stronger for the
direction [015] than for the direction [110] due to the carrier mass difference
for these directions. The carrier confinement increases maximum value of ZT and
shifts it towards high temperatures. For the p-type bismuth telluride nanowires
with growth direction [110], the maximum value of the figure of merit is equal
to 1.3, 1.6, and 2.8, correspondingly, at temperatures 310 K, 390 K, 480 K and
the nanowire thicknesses 30 nm, 15 nm, and 7 nm. At the room temperature, the
figure of merit equals 1.2, 1.3, and 1.7, respectively.Comment: 13 pages, 7 figures, 2 tables, typos added, added references for
sections 2-
The Hanle Effect in 1D, 2D and 3D
This paper addresses the problem of scattering line polarization and the
Hanle effect in one-dimensional (1D), two-dimensional (2D) and
three-dimensional (3D) media for the case of a two-level model atom without
lower-level polarization and assuming complete frequency redistribution. The
theoretical framework chosen for its formulation is the QED theory of Landi
Degl'Innocenti (1983), which specifies the excitation state of the atoms in
terms of the irreducible tensor components of the atomic density matrix. The
self-consistent values of these density-matrix elements is to be determined by
solving jointly the kinetic and radiative transfer equations for the Stokes
parameters. We show how to achieve this by generalizing to Non-LTE polarization
transfer the Jacobi-based ALI method of Olson et al. (1986) and the iterative
schemes based on Gauss-Seidel iteration of Trujillo Bueno and Fabiani Bendicho
(1995). These methods essentially maintain the simplicity of the
Lambda-iteration method, but their convergence rate is extremely high. Finally,
some 1D and 2D model calculations are presented that illustrate the effect of
horizontal atmospheric inhomogeneities on magnetic and non-magnetic resonance
line polarization signals.Comment: 14 pages and 5 figure
Hanle effect in the CN violet system with LTE modeling
Weak entangled magnetic fields with mixed polarity occupy the main part of
the quiet Sun. The Zeeman effect diagnostics fails to measure such fields
because of cancellation in circular polarization. However, the Hanle effect
diagnostics, accessible through the second solar spectrum, provides us with a
very sensitive tool for studying the distribution of weak magnetic fields on
the Sun. Molecular lines are very strong and even dominate in some regions of
the second solar spectrum. The CN system is
one of the richest and most promising systems for molecular diagnostics and
well suited for the application of the differential Hanle effect method. The
aim is to interpret observations of the CN
system using the Hanle effect and to obtain an estimation of the magnetic field
strength. We assume that the CN molecular layer is situated above the region
where the continuum radiation is formed and employ the single-scattering
approximation. Together with the Hanle effect theory this provides us with a
model that can diagnose turbulent magnetic fields. We have succeeded in fitting
modeled CN lines in several regions of the second solar spectrum to
observations and obtained a magnetic field strength in the range from 10--30 G
in the upper solar photosphere depending on the considered lines.Comment: Accepted for publication in Astronomy and Astrophysic
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