Germanium–vacancy color center in isotopically enriched diamonds synthesized at high pressures

We report on the high-pressure synthesis of novel nano- and microcrystalline diamonds with germanium–vacancy (Ge–V) color centers emitting at 602 nm. The synthesis was carried out in non-metallic growth systems C–H–Ge and C–H–O–Ge enriched with germanium and carbon isotopes. We demonstrate germanium and carbon isotope shifts in the fine structure of the luminescence, which allows us to unambiguously associate the center with the germanium impurity entering into the diamond lattice. We show that there are two ground-state energy levels with the separation of 0.7 meV and two excited-state levels separated by 4.6 meV in the electronic structure of the center and suggest a split-vacancy structure of this center. High-intensity and narrow-line emission of high-pressure synthesized small diamonds with Ge–V centers makes them promising candidates for single-photon emitters.     

Bulk and surface electron transport in topological insulator candidate YbB6–δ

We report the study of transport and magnetic properties of the YbB_6–δsingle crystals grown by inductive zone melting. A strong disparity in the low temperature resistivity, Seebeck and Hall coefficients is established for the samples with the different level of boron deficiency. The effective parameters of the charge transport in YbB_6–δ are shown to depend on the concentration of intrinsic defects, which is estimated to range from 0.09% to 0.6%. The pronounced variation of Hall mobility μ_H found for bulk holes is induced by the decrease of transport relaxation time from τ ≈ 7.7 fs for YbB_5.994 to τ ≈ 2.2 fs for YbB_5.96. An extra contribution to conductivity from electrons with μ_H≈ –1000 cm² V^–1 s^–1 and the very low concentration n /n_Yb≈ 10^–6 discovered below 20 K for all the single crystals under investigation is suggested to arise from the surface electron states appeared in the inversion layer due to the band bending. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The influence of dimerization on the orientational phase transition in the C<Subscript>60</Subscript> fullerite

The influence of dimerization of a C₆₀ fullerite subjected to thermobaric treatment on the change in linear dimensions upon the orientational phase transition is investigated. It is demonstrated that the effects associated with the dimerization of fullerites substantially affect the thermal expansion coefficient only for samples synthesized under the conditions P_syn≥8 GPa and T_syn≥70°C (where P_syn and T_syn are the pressure and temperature of the synthesis, respectively). These effects bring about a smearing of the phase transition, a shift of the transition toward low temperatures, and a decrease in the volume jump.     

High-pressure phases in the GaSb-Mn system

The effect of high pressure (6 GPa) on the formation of new phases in a polycrystalline mixture GaSb: Mn = 1: 1 upon heating was studied. Sphalerite-type solid solutions with a small amount of Mn form at temperatures below 520–600 K. At higher temperatures, new crystalline GaSbMn phases are synthesized: a phase with a simple cubic structure with a lattice parameter a = 2.946 ± 0.001 Å (at 620–670 K) and a phase with a tetragonal CuAl₂-type structure (space group I4/mcm) with lattice parameters a = 6.426 ± 0.004 Å and c = 5.349 ± 0.004 Å (at 690–870 K). These new phases are metastable under normal conditions and have magnetic properties. The structure, conductivity, and thermal stability of the synthesized phases are investigated, and the products of decomposition of these new phases upon annealing are analyzed.     

Pressure-induced structural transformations and the anomalous behavior of the viscosity in network chalcogenide and oxide melts

It is known that a number of compressed melts undergo structural phase transitions. Data on the structural changes at high pressures in chalcogenides (AsS, As₂S₃) and oxide (B₂O₃) melts with a network structure have been reviewed. Viscosity is one of the fundamental physical properties of a liquid. For various melts, it varies in a very wide range. Structural transformations in melts induce the corresponding changes in all physical properties, in particular viscosity. The measurements of the viscosity of a number of melts at high pressures and temperatures by the radiographic method have been reported. Changes in the viscosity by several orders of magnitude have been detected when the pressure is varied by several gigapascals. The diffusion mechanism in network-structure melts at various pressures has been analyzed. The prediction of the behavior of the viscosity of various melts at superhigh pressures is of high importance for the physics of glass transition, geophysics, and materials science.     

Ab initio Calculation of Impurity–Vacancy Complexes in Diamond at High Pressure

Journal of Experimental and Theoretical Physics - Optical centers in diamond are possible candidates for single-photon emitters for applications in quantum communication, biology, and medicine. The...     

Thermopower in quasi-two-dimensional (BEDT-TTF) m X n organic conductors

Thermopower of (BEDT-TTF) _m X _n organic conductors has been studied using a dedicated measurement technique in the temperature range of 4.2 to 300 K. It turned out that some features of the thermopower in quasi-two-dimensional metals, namely the presence of a peak in the thermopower of α-(BEDT-TTF)₂MHg(SCN)₄ and a plateau in κ-(BEDT-TTF)₂Cu(NCS)₂ in the temperature interval between 10 and 50 K, are probably due to the phonon drag effect. Similar temperature dependences of the Seebeck coefficient can be satisfactorily interpreted in terms of a simple model taking into account the real experimental curve of the phonon heat capacity versus temperature, C ∝ T ², which is not described by the Debye formula. One feature distinguishing organic superconductors from magnetically ordered metals is a stronger temperature dependence of the characteristic electron-phonon scattering time τ _e-ph(T). Phonon drag effects also determine the behavior of the thermopower in the (BEDT-TTF)₃Cl₂·2H₂O organic conductor, which is characterized by a metal-insulator transition at T∼150 K. An analysis of measurements of the conductivity and thermopower vs. temperature taken together indicates that the transition in this compound has a complex nature: first (at T∼150 K) a metal-insulator transition occurs, which produces an energy gap in the band spectrum, then at a lower temperature (T∼20 K) a transition to a charge-density wave state takes place. Zh. éksp. Teor. Fiz. 113, 323–338 (January 1998)     

Ground state formation in a strong hubbard correlation regime in iron monosilicide

Low-temperature anomalies in the physical properties of iron monosilicide are analyzed based on the results of thorough measurements of the conductivity, Hall coefficients, thermo emf, and magnetic characteristics of high-quality single-crystal FeSi samples at liquid helium (LHe) and intermediate temperatures. It is demonstrated that the most adequate and consistent interpretation of the experimental magnetic, transport, and optical characteristics can be given within the framework of the Hubbard model. The model parameters are determined and the arguments are presented which provide evidence of the spin polaron formation and the density of state (DOS) renormalization taking place in FeSi in the vicinity of the Fermi energy at intermediate temperatures. It was found that a decrease in the sample temperature in the region of T < T _c ≈ 15 K is accompanied by a transition to a coherent regime of the spin density fluctuations. As a result, the ferro-magnetic character of the interaction leads to the formation of magnetic microdomains with a characteristic size ～10 Å. The exchange-induced magnetization enhancement in the vicinity of charge carriers in these microdomains probably accounts for the anomalous components in the Hall coefficient and the magnetization hysteresis observed in FeSi at LHe temperatures. The nature of the low-temperature transition at T _m ≈ 7 K in the system of interacting magnetic microparticles in iron monosilicide is discussed.     

Hopping conductivity of carbynes modified under high pressures and temperatures: Galvanomagnetic and thermoelectric properties

The conductivity, thermopower, and magnetoresistance of carbynes structurally modified by heating under a high pressure are investigated in the temperature range 1.8–300 K in a magnetic field up to 70 kOe. It is shown that an increase in the synthesis temperature under pressure leads to a transition from 1D hopping conductivity to 2D and then to 3D hopping conductivity. An analysis of transport data at T ≤ 40 K makes it possible to determine the localization radius a ～ (56?140) Å of the wave function and to estimate the density of localized states _g(E _F) for various dimensions d of space: _g(E _F) ≈ 5.8 × 10⁷ eV^?1 cm^?1 (d=1), _g(E _F) ≈5×10¹⁴ eV^?1 cm ^?2 (d=2), and _g(E _F)≈1.1×10²¹ eV^?1 cm_?3 (d=3). A model for hopping conductivity and structure of carbynes is proposed on the basis of clusterization of sp ² bonds in the carbyne matrix on the nanometer scale.