233 research outputs found
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
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
Complete solution of a constrained tropical optimization problem with application to location analysis
We present a multidimensional optimization problem that is formulated and
solved in the tropical mathematics setting. The problem consists of minimizing
a nonlinear objective function defined on vectors over an idempotent semifield
by means of a conjugate transposition operator, subject to constraints in the
form of linear vector inequalities. A complete direct solution to the problem
under fairly general assumptions is given in a compact vector form suitable for
both further analysis and practical implementation. We apply the result to
solve a multidimensional minimax single facility location problem with
Chebyshev distance and with inequality constraints imposed on the feasible
location area.Comment: 20 pages, 3 figure
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
Symmetries of a class of nonlinear fourth order partial differential equations
In this paper we study symmetry reductions of a class of nonlinear fourth
order partial differential equations \be u_{tt} = \left(\kappa u + \gamma
u^2\right)_{xx} + u u_{xxxx} +\mu u_{xxtt}+\alpha u_x u_{xxx} + \beta u_{xx}^2,
\ee where , , , and are constants. This
equation may be thought of as a fourth order analogue of a generalization of
the Camassa-Holm equation, about which there has been considerable recent
interest. Further equation (1) is a ``Boussinesq-type'' equation which arises
as a model of vibrations of an anharmonic mass-spring chain and admits both
``compacton'' and conventional solitons. A catalogue of symmetry reductions for
equation (1) is obtained using the classical Lie method and the nonclassical
method due to Bluman and Cole. In particular we obtain several reductions using
the nonclassical method which are no} obtainable through the classical method
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
BAs and boride III-V alloys
Boron arsenide, the typically-ignored member of the III-V arsenide series
BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma
conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an
X_1c-like indirect band gap, and its bond charge is distributed almost equally
on the two atoms in the unit cell, exhibiting nearly perfect covalency. The
reasons for these are tracked down to the anomalously low atomic p orbital
energy in the boron and to the unusually strong s-s repulsion in BAs relative
to most other III-V compounds. We find unexpected valence band offsets of BAs
with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is
significantly higher than that of AlAs, despite the much smaller bond length of
BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects
result from the unusually strong mixing of the cation and anion states at the
VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and
composition-independent band gap bowing. This means that while addition of
small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of
boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the
conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing
enthalpies which are smaller than in GaN-GaAs alloys. The unique features of
boride III-V alloys offer new opportunities in band gap engineering.Comment: 18 pages, 14 figures, 6 tables, 61 references. Accepted for
publication in Phys. Rev. B. Scheduled to appear Oct. 15 200
Size Quantization in Planar Graphene-Based Heterostructures: Pseudospin Splitting, Interface States, and Excitons
A planar quantum-well device made of a gapless graphene nanoribbon with edges
in contact with gapped graphene sheets is examined. The size-quantization
spectrum of charge carriers in an asymmetric quantum well is shown to exhibit a
pseudospin splitting. Interface states of a new type arise from the crossing of
dispersion curves of gapless and gapped graphene materials. The exciton
spectrum is calculated for a planar graphene quantum well. The effect of an
external electric field on the exciton spectrum is analyzed.Comment: 15 pages, 14 figure
Методы диагностики и лечения олигометастазов у больных раком предстательной железы с прогрессированием заболевания после проведенного радикального лечения
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.Рак предстательной железы (РПЖ) является одной из наиболее актуальных проблем современной онкоурологии в России по причине неуклонного роста показателей заболеваемости и смертности от данной патологии. Основными методами лечения больных локализованным и местно-распространенным РПЖ являются радикальная простатэктомия (РПЭ) и дистанционная лучевая терапия (ДЛТ) или брахитерапия. Тем не менее частота лимфогенного прогрессирования заболевания после проведения радикального лечения достигает 15–40 %. При этом крайне важным является поиск опухолевого очага, приводящего к повышению маркера. В случае выявления местного рецидива после выполненной ранее РПЭ может быть проведена спасительная ДЛТ. При верификации рецидива после спасительной ДЛТ или брахитерапии больным может быть рекомендована РПЭ в специализированных центрах, обладающих должным опытом выполнения данного вида хирургического вмешательства. У пациентов с появлением отдаленных метастазов единственным общепринятым методом до настоящего времени являлась паллиативная гормональная терапия. Тем не менее у значительного числа больных маркерный рецидив может быть ассоциирован с так называемым олигометастатическим прогрессированием заболевания, при котором количество выявленных метастатических очагов минимально. Внедрение в клиническую практику перспективных и высокоточных методов диагностики, обладающих высокой чувствительностью и специфичностью даже при минимальных уровнях простатического специфического антигена, таких как магнитно-резонансная и позитронно-эмиссионная томография, позволило существенно увеличить число выявляемых пациентов с олигометастазами и предложить новые терапевтические концепции для лечения данного контингента больных. В последнее время в литературе появляется все больше работ, свидетельствующих о возможности применения локальных методов терапии (хирургии или лучевой терапии) у пациентов с наличием олигометастазов при прогрессировании заболевания после радикального лечения. Как показывают результаты исследований, персонализированный подход и использование хирургических или лучевых методов терапии у отобранных пациентов позволяет существенно увеличить продолжительность времени до назначения гормонального лечения, а в ряде случаев – полностью отказаться от его проведения. В статье приведен обзор данных литературы, освещающих данную проблему
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