Pressure-induced phase transition of Bi2Te3 into the bcc structure

The pressure-induced phase transition of bismuth telluride, Bi2Te3, has been studied by synchrotron x-ray diffraction measurements at room temperature using a diamond-anvil cell (DAC) with loading pressures up to 29.8 GPa. We found a high-pressure body-centered cubic (bcc) phase in Bi2Te3 at 25.2 GPa, which is denoted as phase IV, and this phase apperars above 14.5 GPa. Upon releasing the pressure from 29.8 GPa, the diffraction pattern changes with pressure hysteresis. The original rhombohedral phase is recovered at 2.43 GPa. The bcc structure can explain the phase IV peaks. We assumed that the structural model of phase IV is analogous to a substitutional binary alloy; the Bi and Te atoms are distributed in the bcc-lattice sites with space group Im-3m. The results of Rietveld analysis based on this model agree well with both the experimental data and calculated results. Therefore, the structure of phase IV in Bi2Te3 can be explained by a solid solution with a bcc lattice in the Bi-Te (60 atomic% tellurium) binary system.Comment: 12 pages, 5 figure

High pressure transport properties of the topological insulator Bi2Se3

We report x-ray diffraction, electrical resistivity, and magnetoresistance measurements on Bi2Se3 under high pressure and low temperature conditions. Pressure induces profound changes in both the room temperature value of the electrical resistivity as well as the temperature dependence of the resistivity. Initially, pressure drives Bi2Se3 towards increasingly insulating behavior and then, at higher pressures, the sample appears to enter a fully metallic state coincident with a change in the crystal structure. Within the low pressure phase, Bi2Se3 exhibits an unusual field dependence of the transverse magnetoresistance that is positive at low fields and becomes negative at higher fields. Our results demonstrate that pressures below 8 GPa provide a non-chemical means to controllably reduce the bulk conductivity of Bi2Se3

Phase Diagram of Pressure-induced Superconductivity and its Relation to Hall Coefficient in Bi2Te3 Single Crystal

Pressure-induced superconductivity and its relation to corresponding Hall coefficient (RH) have been reported for Bi2Te3, one of known topological insulators. A full phase diagram is presented which shows a complex dependence of the superconducting transition temperature as a function of pressure over an extensive range. High-pressure RH measurements reveal a close relation of these complex behaviors, particularly, a dramatic change of dRH/dP before structural phase transition and a pressure-induced crossover on RH in the high pressure phase were observed.Comment: 14 pages and 3 figure

Диагностическая эффективность перфузионной компьютерной томографии миокарда с чреспищеводной электрокардиостимуляцией у больных с исходным диагнозом «острый коронарный синдром»

Introduction. Computed tomography angiography (CTA) is widely used to detect atherosclerotic changes in coronary arteries (CA). However, the method is limited by the impossibility to perform functional assessment of detected stenoses. Perfusion computed tomography of the myocardium (PCT) can be used for this purpose.Aim of the study. To assess diagnostic accuracy of PCT of the myocardium with transesophageal electrocardiostimulation (TEES) and stress-echocardiography (stress-echoCG) with veloergometry (VEM) in detection of transient ischemia in patients with initial diagnosis of “acute coronary syndrome” (ACS) with borderline stenoses (50–75%) in CA in relation to measurements of fractional flow reserve (FFR).Materials and Methods. The study included 30 patients with the initial diagnosis of ACS with borderline (50–75%) stenoses in CA according to CTA or coronary angiography (CAG). Subsequently, they underwent myocardial PCT with 320-row detector with TEES, as well as stress-echoCG with VEM. Invasive FFR measurement was used as a reference method. FFR value < 0.8 indicated hemodynamic significance of stenosis. Myocardial perfusion was assessed visually. Regional myocardial contractility of the left ventricle was assessed by stress-echoCG.Results. All patients were examined according to the stated protocol. PCT with TEES revealed FFR-significant stenoses with sensitivity, specificity, prognostic value of positive result and prognostic value of negative result 56, 93, 90, 65% respectively, stress-echoCG with VEM 62, 93, 91, 68% respectively.Conclusion. Myocardial PCT with TEES enables to detect perfusion defects associated with transient ischemia, and the diagnostic accuracy of the method in comparison with FFR is comparable with the already well-proven stress-echoCG. The use of PCT with TEES in combination with CTA can be considered as a promising diagnostic tool in patients without known coronary anatomy and with suspected ACS.Введение. Компьютерная томографическая ангиография (КТА) широко используется для выявления атеросклеротических изменений в коронарных артериях (КА). Однако метод ограничен невозможностью проведения функциональной оценки выявленных стенозов. С этой целью может использоваться перфузионная компьютерная томография миокарда (ПКТ).Цель исследования. Оценить диагностическую точность методов ПКТ миокарда с чреспищеводной электрокардиостимуляцией (ЧПЭС) и стресс-эхокардиографии (стресс-ЭхоКГ) с велоэргометрией (ВЭМ) в выявлении преходящей ишемии у больных с исходным диагнозом «острый коронарный синдром» (ОКС) с пограничными стенозами (50–75%) в КА по отношению к измерениям фракционного резерва кровотока (ФРК).Материалы и методы. В исследование были включены 30 больных с исходным диагнозом ОКС с пограничными (50–75%) стенозами в КА по данным КТА или коронарной ангиографии (КАГ). Впоследствии им были выполнены ПКТ миокарда на томографе с 320-рядным детектором с ЧПЭС, а также стресс-ЭхоКГ с ВЭМ. В качестве референтного метода использовалось инвазивное измерение ФРК. Значение показателя ФРК < 0,8 указывало на гемодинамическую значимость стеноза. Перфузия миокарда оценивалась визуально. При проведении стресс-ЭхоКГ оценивалась региональная сократимость миокарда левого желудочка.Результаты. Все больные обследованы согласно заявленному протоколу. ПКТ с ЧПЭС выявила ФРК-значимые стенозы с чувствительностью, специфичностью, прогностической ценностью положительного результата и прогностической ценностью отрицательного результата 56, 93, 90, 65% соответственно, стресс-ЭхоКГ с ВЭМ 62, 93, 91, 68% соответственно.Выводы. ПКТ миокарда с ЧПЭС позволяет выявить дефекты перфузии, ассоциированные с преходящей ишемией, а диагностическая точность метода при сравнении с ФРК сопоставима с уже хорошо зарекомендовавшей себя стресс-ЭхоКГ. Использование ПКТ с ЧПЭС в комбинации с КТА можно рассматривать как перспективный диагностический инструмент у больных без известной коронарной анатомии и с подозрением на ОКС