284 research outputs found
Electric-field switchable magnetization via the Dzyaloshinskii-Moriya interaction: FeTiO_3 versus BiFeO_3
In this article we review and discuss a mechanism for coupling between
electric polarization and magnetization that can ultimately lead to
electric-field switchable magnetization. The basic idea is that a ferroelectric
distortion in an antiferromagnetic material can "switch on" the
Dzyaloshinskii-Moriya interaction which leads to a canting of the
antiferromagnetic sublattice magnetizations, and thus to a net magnetization.
This magnetization M is coupled to the polarization P via a trilinear free
energy contribution of the form P(M x L), where L is the antiferromagnetic
order parameter. In particular, we discuss why such an invariant is present in
R3c FeTiO_3 but not in the isostructural multiferroic BiFeO_3. Finally, we
construct symmetry groups that in general allow for this kind of
ferroelectrically-induced weak ferromagnetism.Comment: 15 pages, 3 images, to appear in J. Phys: Condens. Matter Focus Issue
on Multiferroic
First-principles study of spontaneous polarization in multiferroic BiFeO
The ground-state structural and electronic properties of ferroelectric
BiFeO are calculated using density functional theory within the local
spin-density approximation and the LSDA+U method. The crystal structure is
computed to be rhombohedral with space group , and the electronic
structure is found to be insulating and antiferromagnetic, both in excellent
agreement with available experiments. A large ferroelectric polarization of
90-100 C/cm is predicted, consistent with the large atomic
displacements in the ferroelectric phase and with recent experimental reports,
but differing by an order of magnitude from early experiments. One possible
explanation is that the latter may have suffered from large leakage currents.
However both past and contemporary measurements are shown to be consistent with
the modern theory of polarization, suggesting that the range of reported
polarizations may instead correspond to distinct switching paths in structural
space. Modern measurements on well-characterized bulk samples are required to
confirm this interpretation.Comment: (9 pages, 5 figures, 5 tables
STUDY OF THE STRUCTURE AND PROPERTIES OF THE COATINGS OBTAINED BY THE METHOD OF ELECTRON BEAM SURFACING OF POWDER MIXTURES (NB+C, TI+NB+C)
В данной исследовательской работе были сопоставлены возможности различных приемов когнитивной графики для контроля процесса и продукта в черной металлургии с целью управления качеством продукции без изменения технологии по существу. Использование предложенных подходов показало высокую эффективность для поиска существенных закономерностей в системе «управляющие параметры – свойства».In this research work have been compared the possibilities of different methods of cognitive graphics for process control and product in the steel industry for the purpose of quality control of production without substantive changes of technology. Using the proposed approach has shown high efficiency to find significant of regularities in the system of "control parameters – property"
Origin of magnetoelectric behavior in BiFeO
The magnetoelectric behavior of BiFeO has been explored on the basis of
accurate density functional calculations. The structural, electronic, magnetic,
and ferroelectric properties of BiFeO are predicted correctly without
including strong correlation effect in the calculation. Moreover, the
experimentally-observed elongation of cubic perovskite-like lattice along the
[111] direction is correctly reproduced. At high pressure we predicted a
pressure-induced structural transition and the total energy calculations at
expanded lattice show two lower energy ferroelectric phases, closer in energy
to the ground state phase. Band-structure calculations show that BiFeO will
be an insulator in A- and G-type antiferromagnetic phases and a metal in other
magnetic configurations. Chemical bonding in BiFeO has been analyzed using
various tools and electron localization function analysis shows that
stereochemically active lone-pair electrons at the Bi sites are responsible for
displacements of the Bi atoms from the centro-symmetric to the
noncentrosymmetric structure and hence the ferroelectricity. A large
ferroelectric polarization (88.7 C/cm) is predicted in accordance
with recent experimental findings. The net polarization is found to mainly (
98%) originate from Bi atoms. Moreover the large scatter in experimentally
reported polarization values is due to the large anisotropy in the spontaneous
polarization.Comment: 19 pages, 12 figures, 4 table
Determining the nature of faint X-ray sources from the ASCA Galactic center survey
© 2015, Pleiades Publishing, Inc. We present the results of the the identification of six objects from the ASCA Galactic center and Galactic plane surveys: AX J173548-3207, AX J173628-3141, AX J1739.5-2910, AX J1740.4-2856, AX J1740.5-2937, and AX J1743.9-2846. Chandra, XMM-Newton, and XRT/Swift X-ray data have been used to improve the positions of the optical counterparts to these sources. Thereafter, we have carried out a series of spectroscopic observations of the established optical counterparts at the RTT-150 telescope. Analysis of X-ray and optical spectra as well as photometric measurements in a wide wavelength range based on optical and infrared catalogs has allowed the nature of the program sources to be determined. Two X-ray objects have been detected in the error circle of AX J173628-3141: one is a coronally active G star and the other may be a symbiotic star, a red giant with an accreting white dwarf. Three sources (AX J1739.5-2910, AX J1740.5-2937, AX J1743.9-2846) have turned out to be active G-K stars, presumably RS CVn objects, one (AX J1740.4-2856) is an M dwarf, and another one (AX J173548-3207) most likely a low-mass X-ray binary in its low state. The distances and corresponding luminosities of the sources in the soft X-ray band (0.5–10 keV) have been estimated; analysis of deep INTEGRAL Galactic center observations has not revealed a statistically significant flux at energies >20 keV from any of them
Impurity breakdown and terahertz luminescence in n-GaN epilayers under external electric field
We report on the observation and experimental studies of impurity breakdown and terahertz luminescence in n-GaN epilayers under external electric field. The terahertz electroluminescence is observed in a wide range of doping levels (at noncompensated donor density from 4.5×10[sup 16] to 3.4×10[sup 18] cm[sup −3]). Spectra of terahertz luminescence and photoconductivity are studied by means of Fourier transform spectrometry. Distinctive features of the spectra can be assigned to intracenter electron transitions between excited and ground states of silicon and oxygen donors and to hot electron transitions to the donor states.Peer reviewe
Quantum Hall effect in a high-mobility two-dimensional electron gas on the surface of a cylinder
The quantum Hall effect is investigated in a high-mobility two-dimensional
electron gas on the surface of a cylinder. The novel topology leads to a
spatially varying filling factor along the current path. The resulting
inhomogeneous current-density distribution gives rise to additional features in
the magneto-transport, such as resistance asymmetry and modified longitudinal
resistances. We experimentally demonstrate that the asymmetry relations
satisfied in the integer filling factor regime are valid also in the transition
regime to non-integer filling factors, thereby suggesting a more general form
of these asymmetry relations. A model is developed based on the screening
theory of the integer quantum Hall effect that allows the self-consistent
calculation of the local electron density and thereby the local current density
including the current along incompressible stripes. The model, which also
includes the so-called `static skin effect' to account for the current density
distribution in the compressible regions, is capable of explaining the main
experimental observations. Due to the existence of an
incompressible-compressible transition in the bulk, the system behaves always
metal-like in contrast to the conventional Landauer-Buettiker description, in
which the bulk remains completely insulating throughout the quantized Hall
plateau regime
Influence of Anomalous Dispersion on Optical Characteristics of Quantum Wells
Frequency dependencies of optical characteristics (reflection, transmission
and absorption of light) of a quantum well are investigated in a vicinity of
interband resonant transitions in a case of two closely located excited energy
levels. A wide quantum well in a quantizing magnetic field directed normally to
the quantum-well plane, and monochromatic stimulating light are considered.
Distinctions between refraction coefficients of barriers and quantum well, and
a spatial dispersion of the light wave are taken into account. It is shown that
at large radiative lifetimes of excited states in comparison with nonradiative
lifetimes, the frequency dependence of the light reflection coefficient in the
vicinity of resonant interband transitions is defined basically by a curve,
similar to the curve of the anomalous dispersion of the refraction coefficient.
The contribution of this curve weakens at alignment of radiative and
nonradiative times, it is practically imperceptible at opposite ratio of
lifetimes . It is shown also that the frequency dependencies similar to the
anomalous dispersion do not arise in transmission and absorption coefficients.Comment: 10 pages, 6 figure
Principals of the theory of light reflection and absorption by low-dimensional semiconductor objects in quantizing magnetic fields at monochromatic and pulse excitations
The bases of the theory of light reflection and absorption by low-dimensional
semiconductor objects (quantum wells, wires and dots) at both monochromatic and
pulse irradiations and at any form of light pulses are developed. The
semiconductor object may be placed in a stationary quantizing magnetic field.
As an example the case of normal light incidence on a quantum well surface is
considered. The width of the quantum well may be comparable to the light wave
length and number of energy levels of electronic excitations is arbitrary. For
Fourier-components of electric fields the integral equation (similar to the
Dyson-equation) and solutions of this equation for some individual cases are
obtained.Comment: 14 page
Methods for the diagnosis and treatment of oligometastases in patients with prostate cancer and progressive disease after radical treatment
Prostate cancer (PC) is one of the most urgent problems of modern urologic oncology in Russia because of a steady rise in morbidity and mortality rates associated with this disease. Radical prostatectomy (RP) and external beam radiotherapy (EBRT) or brachytherapy are mainstay treatments in patients with localized and locally advanced PC. Just the same, the rate of lymphogenic disease progression after radical treatment is as high as 15–40 %. At the same time, a search for a tumor focus leading to an increase in the marker is extremely important. If a local recurrence is detected after previous RP, there may be salvage EBRT. After verifying the relapse after salvage EBRT or brachytherapy, RP may be recommended to patients in the specialized centers having appropriate experience with this type of surgery. In patients with distant metastases, palliative hormonal therapy has been the only conventional method so far. Nonetheless, in a considerable number of patients, a marker recurrence may be associated with the so-called oligometastatic progression of the disease, in which the number of detected metastatic foci is minimal. The clinical introduction of promising and precise diagnostic methods having high sensitivity and specificity even with the minimal levels of prostate-specific antigen, such as magnetic resonance imaging and positron tomography, could substantially increase the number of detectable patients with oligometastases and propose new therapeutic conceptions for the treatment of this contingent of patients. Recently, in the literature there have been increasingly more works suggesting the possibility of using local treatments (surgery or radiotherapy) in patients with oligometastases when the disease progresses after radical treatment. The results of investigations show that a personalized approach and surgical or radiation treatments in selected patients make it possible to increase survival prior to the use of hormonal treatment and, in a number of cases, to completely abandon the latter. The paper gives a review of the literature data dealing with this problem
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