Optical study of strained double Ge/Si quantum dot layers

status: publishe

Транспортные свойства ориентированной и изотропной бумаги из одностенных углеродных нанотрубок

Buckypapers (BP) with carbon nanotubes (CNT) are very promising for a lot of applications, in which their high conductance, strength and small weight are required. In this work, isotropic BP were prepared using the solution-based deposition that includes the single walled carbon nanotubes (SWCNT) dispersion and the dispersion filtration from a solvent. To increase the BP conductivity, the orientation of the SWCNT bundles composing BP and a following iodine doping were applied. The method of extrusion through the narrow (300 µm) gap was used for the SWCNT orientation. The temperature dependences of conductance for isotropic, oriented and doped BP were studied to understand the effect of CNT alignment and the mechanism of transport through SWCNT BP. It was shown that bundle orientation increases the BP conductivity from ~103 S × cm-1 to ~104 S × cm-1, and iodine doping of oriented samples additionally increase the conductivity by an order. The fluctuation – assisted tunneling between CNT bundles was used to describe the mechanism of low temperature conductivity.Бумага (buckypaper – BP) из одностенных углеродных нанотрубок (ОУНТ) является перспективным материалом для использования в качестве компонентов микро- и наноэлектроники, в которых требуется высокая удельная электро- и теплопроводность, а также высокая удельная прочность. Изотропные образцы BP из ОУНТ сформированы фильтрацией дисперсии из ОУНТ для удаления растворителя. Для увеличения проводимости ВР проводилась ориентация ОУНТ вдоль выделенного направления, а также дополнительное легирование ОУНТ в парах йода. Ориентация ОУНТ осуществлялась с помощью экструзии через щель раствора из ОУНТ. Проведено сравнение температурных зависимостей электропроводности изотропных, ориентированных и легированных образцов ВР для выявления механизма проводимости и роли ориентации ОУНТ. Показано, что ориентирование пучков ОУНТ вдоль выделенного направления позволяет увеличить проводимость ВР с ~103 См/cм до ~104 См/cм, а легирование ориентированных образцов в парах йода увеличивает электропроводность еще на порядок. Механизм низкотемпературной проводимости по ВР описан флуктуационно-индуцированным туннелированием носителей заряда между пучками ОУНТ

Structural, Optical and Electronic Properties of the Wide Bandgap Topological Insulator Bi1.1Sb0.9Te2S

Successful applications of a topological insulator (TI) in spintronics require its bandgap to be wider then in a typical TI and the energy position of the Dirac point in the dispersion relations to be away from the valence and conduction bands. In this study we grew Bi1.1Sb0.9Te2S crystals and examined their elemental composition, structural, optical and electronic properties as well as the electronic band structure. The high structural quality of the grown crystals was established by X-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy demonstrated a near parabolic character of the valence and conduction bands and a direct bandgap of 0.36 eV. The dispersion relations also revealed a Dirac cone, confirming the topological insulator nature of this material, with the position of the Dirac point being 100 meV above the valence band maximum. Far infrared reflectivity spectra revealed a plasma edge and two phonon dips. Fitting these spectra with theoretical functions based on the Drude-Lorentz model allows determination of the high frequency dielectric constant (41.3), plasma frequency (936 cm−1) and the frequencies of two infrared phonons (177.7 cm−1 and 77.4 cm−1). © 2021 Elsevier B.V.The reported study was funded by RFBR, project number 19-29-12061 . The part of optical research was carried out within the state assignment of Ministry of Science and Higher Education of the Russian Federation (theme "Spin" No AAAA-A18-118020290104-2 and No AAAA-A19-119081990020-8 and theme "Electron" No AAAAA18-118020190098-5 ). The study was also supported by the Russian Science Foundation (Project No. 17-12-01047 ) in the part of the crystal growth and state assignment of ISP SB RAS ( 0306–2019-0007 ) and IGM SB RAS. The Raman measurements were partially supported by the grant of the Russian Foundation for Basic Research (Project No. 19-52-18008 ). This work is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through project-ID 258499086 – SFB 1170 (A01), the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter–ct.qmat Project-ID 390858490 – EXC 2147

Private Sector Union Density and the Wage Premium: Past, Present, and Future

The rise and decline of private sector unionization were among the more important features of the U.S. labor market during the twentieth century. Following a dramatic spurt in unionization after passage of the depression-era National Labor Relations Act (NLRA) of 1935, union density peaked in the mid-1950s, and then began a continuous decline. At the end of the century, the percentage of private wage and salary workers who were union members was less than 10 percent, not greatly different from union density prior to the NLRA

Raman scattering of Ge dot superlattices

Self-organised Ge dot superlattices grown by molecular beam epitaxy of Ge and Si layers utilizing Stranski-Krastanov growth mode were investigated by Raman spectroscopy. An average size of Ge quantum dots was obtained from transmission electron microscopy measurements. The strain and interdiffusion of Ge and Si atoms in Ge quantum dots were estimated from the analysis of frequency positions of optical phonons observed in the Raman spectra. Raman scattering by folded longitudinal acoustic phonons in the Ge dot superlattices was observed and explained using of elastic continuum theory