Coexistence of the topological state and a two-dimensional electron gas on the surface of Bi2Se3

Topological insulators are a recently discovered class of materials with fascinating properties: While the inside of the solid is insulating, fundamental symmetry considerations require the surfaces to be metallic. The metallic surface states show an unconventional spin texture, electron dynamics and stability. Recently, surfaces with only a single Dirac cone dispersion have received particular attention. These are predicted to play host to a number of novel physical phenomena such as Majorana fermions, magnetic monopoles and unconventional superconductivity. Such effects will mostly occur when the topological surface state lies in close proximity to a magnetic or electric field, a (superconducting) metal, or if the material is in a confined geometry. Here we show that a band bending near to the surface of the topological insulator Bi$_2$Se$_3$ gives rise to the formation of a two-dimensional electron gas (2DEG). The 2DEG, renowned from semiconductor surfaces and interfaces where it forms the basis of the integer and fractional quantum Hall effects, two-dimensional superconductivity, and a plethora of practical applications, coexists with the topological surface state in Bi$_2$Se$_3$. This leads to the unique situation where a topological and a non-topological, easily tunable and potentially superconducting, metallic state are confined to the same region of space.Comment: 12 pages, 3 figure

Emergent Phenomena Induced by Spin-Orbit Coupling at Surfaces and Interfaces

Spin-orbit coupling (SOC) describes the relativistic interaction between the spin and momentum degrees of freedom of electrons, and is central to the rich phenomena observed in condensed matter systems. In recent years, new phases of matter have emerged from the interplay between SOC and low dimensionality, such as chiral spin textures and spin-polarized surface and interface states. These low-dimensional SOC-based realizations are typically robust and can be exploited at room temperature. Here we discuss SOC as a means of producing such fundamentally new physical phenomena in thin films and heterostructures. We put into context the technological promise of these material classes for developing spin-based device applications at room temperature

Rational Surface Design of Upconversion Nanoparticles with Polyethylenimine Coating for Biomedical Applications: Better Safe than Brighter?

Upconversion nanoparticles (UCNPs) coated with polyethylenimine (PEI) are popular background-free optical contrast probes and efficient drug and gene delivery agents attracting attention in science, industry, and medicine. Their unique optical properties are especially useful for subsurface nanotheranostics applications, in particular, in skin. However, high cytotoxicity of PEI limits safe use of UCNP@PEI, and this represents a major barrier for clinical translation of UCNP@PEI-based technologies. Our study aims to address this problem by exploring additional surface modifications to UCNP@PEI to create less toxic and functional nanotheranostic materials. We designed and synthesized six types of layered polymer coatings that envelop the original UCNP@PEI surface, five of which reduced the cytotoxicity to human skin keratinocytes under acute (24 h) and subacute (120 h) exposure. In parallel, we examined the photoluminescence spectra and lifetime of the surface-modified UCNP@PEI. To quantify their brightness, we developed original methodology to precisely measure the colloidal concentration to normalize the photoluminescence signal using a nondigesting mass spectrometry protocol. Our results, specified for the individual coatings, show that, despite decreasing the cytotoxicity, the external polymer coatings of UCNP@PEI quench the upconversion photoluminescence in biologically relevant aqueous environments. This trade-off between cytotoxicity and brightness for surface-coated UCNPs emphasizes the need for the combined assessment of the viability of normal cells exposed to the nanoparticles and the photophysical properties of postmodification UCNPs. We present an optimized methodology for rational surface design of UCNP@PEI in biologically relevant conditions, which is essential to facilitate the translation of such nanoparticles to the clinical applications

Method for Supplying Glass-Lubricating Material to Outer Surface of Workpiece Before Pressing

Изобретение относится к области обработки металлов давлением и может быть использовано при нанесении стеклосмазочного материала на наружную поверхность заготовки перед горячим прессованием прутков или труб. Способ включает подачу стеклосмазочного материала на наклонный стол стеклосмазки, нагрев заготовки 1 до требуемой температуры, выгрузку заготовки и прокатывание по наклонному столу стеклосмазки 2 при помощи толкателя и последующую загрузку обработанной заготовки в контейнер пресса. Стеклосмазочный материал подают на стол стеклосмазки 2 снизу через сетку или перфорированный лист 4 путем подъема платформы 6 со стеклосмазочным материалом 5, расположенной в бункере 7. Контролируют толщину слоя стеклосмазочного материала 5 путем контроля величины подъема платформы 6 со стеклосмазочным материалом 5. Равномерность подаваемого стеклосмазочного материала 5 обеспечивают выравниванием стеклосмазочного материала 5 на платформе 6 автоматически за счет вибрации, встряхивания или перемешивания. Обеспечивается качественный слой стеклосмазочного материала на наружной поверхности заготовки за счет возможности регулирования толщины и равномерности наносимого слоя стеклосмазочного материала, а также исключается потеря стеклосмазочного материала, что снижает его расход. 3 ил., 1 табл.FIELD: metalworking. SUBSTANCE: invention relates to the field of metalworking by pressure; it can be used when applying glass-lubricating material to the outer surface of a workpiece before hot pressing rods or pipes. The method includes supplying glass-lubricating material to an inclined glass-lubricating table, heating workpiece 1 to the required temperature, unloading the workpiece and rolling along inclined glass-lubricating table 2 using a pusher, and then, loading the processed workpiece into a press container. Glass-lubricating material is supplied to glass-lubricating table 2 from below through a grid or perforated sheet 4 by lifting platform 6 with glass-lubricating material 5 located in hopper 7. The thickness of a layer of glass-lubricating material 5 is controlled by controlling the amount of lifting of platform 6 with glass-lubricating material 5. The uniformity of supplied glass-lubricating material 5 is provided by leveling glass-lubricating material 5 on platform 6 automatically due to vibration, shaking or mixing. EFFECT: high-quality layer of glass-lubricating material is provided on the outer surface of the workpiece due to the possibility of regulating the thickness and uniformity of the applied layer of glass-lubricating material, and also the loss of glass-lubricating material is eliminated, which reduces its consumption. 1 cl, 3 dwg, 1 tbl

Pressure-induced topological phases of KNa2Bi

This work is licensed under a Creative Commons Attribution 4.0 International License.We report an ab initio study of the effect of hydrostatic pressure and uniaxial strain on electronic properties of KNa 2 Bi, a cubic bialkali bismuthide. It is found that this zero-gap semimetal with an inverted band structure at the Brillouin zone center can be driven into various topological phases under proper external pressure. We show that upon hydrostatic compression KNa 2 Bi turns into a trivial semiconductor with a conical Dirac-type dispersion of electronic bands at the point of the topological transition while the breaking of cubic symmetry by applying a uniaxial strain converts the compound into a topological insulator or into a three-dimensional Dirac semimetal with nontrivial surface Fermi arcs depending on the sign of strain. The calculated phonon dispersions show that KNa 2 Bi is dynamically stable both in the cubic structure (at any considered pressures) and in the tetragonal phase (under uniaxial strain).We acknowledge financial support of the University of the Basque Country UPV/EHU (grant No. GIC13-IT-756-13), the Departamento de Educación del Gobierno Vasco, the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-01), the Spanish Ministry of Economy and Competitiveness MINECO Project FIS2013-48286-C2-1-P, and the Saint Petersburg State University (project No. 15.61.202.2015).Peer Reviewe