First-principles study of spontaneous polarization in multiferroic BiFeO3_3

The ground-state structural and electronic properties of ferroelectric BiFeO$_3$ 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 $R3c$, 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 $\mu$C/cm$^2$ 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 BiFeO3_3

The magnetoelectric behavior of BiFeO$_3$ has been explored on the basis of accurate density functional calculations. The structural, electronic, magnetic, and ferroelectric properties of BiFeO$_3$ 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$_3$ will be an insulator in A- and G-type antiferromagnetic phases and a metal in other magnetic configurations. Chemical bonding in BiFeO$_3$ 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 $\mu$C/cm$^{2}$) 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

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 $\alpha$, $\beta$, $\gamma$, $\kappa$ and $\mu$ 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

Меланома полового члена и уретры: обзор литературы и наблюдения из практики

One of clinical and anatomical types of melanoblastoma is melanoma of the glans penis appearing on the skin or mucosa of the penis (external opening of the urethra or ureter, internal or external layer of the foreskin). It’s a rare disease, and most of specialists are unfamiliar with it, including oncologists and urologists. Thus, the conducted research has practical importance as it may improve diagnostics and treatment of patients with this oncological pathology. Одной из клинико-анатомических разновидностей меланобластомы является меланома головки полового члена, возникающая на коже полового члена или на слизистой оболочке (наружном отверстии мочеиспускательного канала или уретры, внутреннем или наружном листке крайней плоти). Это редкое заболевание, малоизвестное широкому кругу специалистов, в том числе онкологам и урологам, поэтому выполненная работа имеет большое практическое значение, так как позволит улучшить диагностику и лечение больных данной онкопатологией

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

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