Anharmonic theory of superconductivity in the high-pressure materials

Electron-phonon superconductors at high pressures have displayed the highest values of critical superconducting temperature $T_c$ on record, now rapidly approaching room temperature. Despite the importance of high-$P$ superconductivity in the quest for room-temperature superconductors, a mechanistic understanding of the effect of pressure and its complex interplay with phonon anharmonicity and superconductivity is missing, as numerical simulations can only bring system-specific details clouding out key players controlling the physics. Here we develop a minimal model of electron-phonon superconductivity under an applied pressure which takes into account the anharmonic decoherence of the optical phonons. We find that $T_c$ behaves non-monotonically as a function of the ratio $\Gamma/\omega_0$, where $\Gamma$ is the optical phonon damping and $\omega_0$ the optical phonon energy at zero pressure and momentum. Optimal pairing occurs for a critical ratio $\Gamma/\omega_0$ when the phonons are on the verge of decoherence ("diffuson-like" limit). Our framework gives insights into recent experimental observations of $T_c$ as a function of pressure in the complex BCS material TlInTe$_2$

A Unified Understanding of minimum lattice thermal conductivity

We propose a first-principles model of minimum lattice thermal conductivity based on a unified theoretical treatment of thermal transport in crystals and glasses

The Effect of Na Atom on TlInSe2 and TlInTe2 Compounds

Thermoelectric materials have widely used applications in technological areas such as electronic devices and data storage. TlInSe2 and TlInTe2 compounds are among these thermoelectric materials. In this study, while the structural, electronic, and optical properties of TlInSe2, TlInTe2, Tl0.75Na0.25InSe2 and Tl0.75Na0.25InTe2 compounds have been examined with the WIEN2k program based on DFT, their thermoelectric properties have been calculated with another program BoltzTrap. The electronic calculations show that, all compounds exhibit indirect band gap properties. In addition, the band gap energy of Tl0.75Na0.25InSe2 is shifted in the electromagnetic spectrum. The optical properties are found to change depending on the direction for all compounds. Finally, the thermoelectric parameters have been calculated depending on temperature. It is thought that especially the results for Na-doped compounds will be a leading reference for experimental studies

Ab initio study of NaSrSb and NaBaSb as potential thermoelectric prospects

Zintl phases are excellent thermoelectric prospects to put the waste heat to good use. In the quest of the same, using first-principles methods combined with Boltzmann transport theory, we explored two recent phases NaSrSb and NaBaSb. We found low lattice thermal conductivity of 1.9 and 1.3 W m$^{-1}$ K$^{-1}$ at 300~K for NaSrSb and NaBaSb, respectively, which are of the same order as other potential Zintl phases such as Sr$_3$AlSb$_3$ and BaCuSb. We account for such low values to short phonon lifetimes, small phonon group velocities, and lattice anharmonicity in the crystal structure. The calculated electrical transport parameters based on acoustic deformation potential, ionized impurity, and polar optical phonon scattering mechanisms reveal large Seebeck coefficients for both materials. Further, we obtain a high figure of merit of ZT$\sim$2.0 at 900~K for \textit{n}-type NaSrSb. On the other hand, the figure of merit of \textit{n}-type NaBaSb surpasses the unity. We are optimistic about our findings and believe our work would set a basis for future experimental investigations.Comment: 10 figures, 1 tabl

Current-voltage characteristic of InGaTe?

Цель работы: Получение и исследование вольтамперной характеристики монокристаллов и тонких пленок соединения InGaTe2 в статическом и динамическом режимах, при различных температурах и при различных площадях контактов. Методы исследования. Монокристаллы InGaTe? были выращены методами Бриджмена-Стокбаргера, а тонкие пленки получены с использованием метода конденсации из паровой фазы. Рентгенограммы InGaTe? получены на дифрактометре ДРОН-2 в CuKa излучении (l=1,54178C). Параметры элементарной ячейки определили с точностью 0,005C. Вольтамперная характеристика исследована на образцах прямоугольной формы размерами 7?1?1 ммі . Контактами служили I. n и Cu. Ток, снабжающий концы прямоугольных образцов, ориентирован так, что ток через образец протекает вдоль по оси c6 монокристалла InGaTe?. Вольтамперная характеристика исследовалась на постоянном токе в статическом и динамическом режимах. Результаты: Рентгенофазовым анализом выявлено, что соединение InGaTe2 кристаллизуется в тетрагональной сингонии с параметрами решетки а=8,463C; с=6,981C. Исследовались статическая и динамическая вольтамперные характеристики InGaTe2 при различных температурах, изменение температуры образцов в области отрицательного дифференциального сопротивления, зависимость порогового напряжения от температуры, вольтамперная характеристика тонких пленок InGaTe2 при различных площадях контактов. Выявлено, что данная фаза обладает переключающими свойствами с памятью и с уменьшением температуры величина порогового напряжения увеличивается, в результате S-образная характеристика становится ярко выраженной. Проанализировано изменение порогового напряжения с изменением температуры. Исследовались ВАХ тонких пленок в зависимости от площади и от размера самой пленки. Выявлено, что пленки также обладают переключающими свойствами и с уменьшением площади контактов величина порогового напряжения, при котором происходит переключение, уменьшается. А также изменением температуры и площади контактов управляется величина порогового напряжения. А это означает, что данная фаза может успешно применяться при создании быстродействующих и высокочувствительных приборов.The main aim of the research is to prepare and study the current-voltage characteristics of single crystals and thin films of InGaTe2 compound in static and dynamic modes at different temperatures and for different contact area. Methods: InGaTe? single crystals were grown by the Bridgman- Stockbarger, and thin films were prepared by the method of physical vapor deposition. InGaTe2 radiographs were obtained on a DRON-2 CuKa radiation (l=1,54178C). The unit cell parameters were determined with an accuracy 0,005C. The current-voltage characteristics was investigated on rectangular samples with dimensions 7?1?1 mmі . In and Cu served as contacts. Current supplying the ends of rectangular samples is oriented in the way that current flows through the sample along a single crystal axis c6 of InGaTe?monocrystal. The current-voltage characteristics were examined for constant current in static and dynamic modes. Results. X-ray analysis revealed that InGaTe2 compound crystallizes in tetragonal syngony with lattice parameters a=8,463C; a=6,981C. The authors have investigated the static and dynamic current-voltage characteristics of InGaTe4 at different temperatures, change in the samples temperature in the region of negative differential resistance, dependence of the threshold voltage on the temperature, current-voltage characteristics of InGaTe2 thin films at different contact area. It was ascertained that this phase has switching properties with memory. The threshold voltage increases at temperature decreas. The S-pattern becomes clearly expressed. The authors analyzed the change in the threshold voltage at temperature change. The CVC of thin films were investigated depending on the area and the size of the film itself. It was revealed that the films also exhibit switching properties and decreasing contact area with the magnitude of the threshold voltage at which the switching is reduced. The threshold voltage value is controlled by the changes in temperature and contact area. This means that this phase can be successfully used in the design of fast and highly sensitive instrumentation

Electronic Structure of a Chain-like Compound: TlSe

An ab-initio pseudopotential calculation using density functional theory within the local density approximation has been performed to investigate the electronic properties of TlSe which is of chain-like crystal geometry. The energy bands and effective masses along high symmetry directions, the density of states and valence charge density distributions cut through various planes are presented. The results have been discussed in terms of previously existing experimental and theoretical data, and comparisons with similar compounds have been made.Comment: 7 page

Upgrade of the MARI spectrometer at ISIS

The MARI direct geometry time-of-flight neutron spectrometer at ISIS has been upgraded with an $m=3$ supermirror guide and new detector electronics. This has resulted in a flux gain of ${\approx}6{\times}$ at $\lambda=1.8$ {\AA}, and improvements on discriminating electrical noise, allowing MARI to continue to deliver a high quality science program well into its fourth decade of life

Upgrade of the MARI spectrometer at ISIS

The MARI direct geometry time-of-flight neutron spectrometer at ISIS has been upgraded with an m = 3 supermirror guide and new detector electronics. This has resulted in a flux gain of approximate to 6x at lambda = 1.8 angstrom, and improvements on discriminating electrical noise, allowing MARI to continue to deliver a high quality science program well into its fourth decade of life