<Superscript>10</Superscript>B–<Superscript>11</Superscript>B isotope substitution and superconductivity in ZrB<Subscript>12</Subscript>

The specific heat of the ZrB₁₂ compound in the normal and superconducting states (T _C ≈ 6 K) has been studied in the 1.9–7 K temperature range for high-quality single crystals with different relative contents of boron isotopes. For Zr¹⁰B₁₂, Zr^natB₁₂, and Zr¹¹B₁₂ dodecaborides, the electron density of states and the electronphonon coupling constant, λ_e-ph ∼ 0.4, are found. The dependence of the thermodynamic and upper critical fields, as well as of the Ginzburg-Landau parameter (κ = 0.8–1.14) on temperature and isotope composition is determined. The results suggest the existence of the magnetic field induced phase transition at T* = 4–5 K, which is not related to the transition from type-I to type-II superconductivity. The possibilities of the existence of two-gap superconductivity and a structural phase transition at T* in zirconium dodecaboride are discussed.     

Comment on magnetism and superconductivity in rutheno cuprates: RuSr2GdCu2O8 and RuSr2Gd1.5Ce0.5Cu2O10

Both RuSr₂GdCu₂O_8-δ (Ru-1212) and RuSr₂Gd_1.5Ce_0.5Cu₂O_10-δ (Ru-1222) exhibits magnetism and superconductivity, as seen by magnetization vs. temperature behavior measured in 5 Oe field. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization data show branching at around 140 K and 100 K with a cusp at 135 K and 80 K and a diamagnetic transition around 20 K and 30 K in the ZFC part, for Ru-1212 and Ru-1222, respectively. The isothermal magnetization possesses a non-linear contribution due to a ferromagnetic component at low temperatures below 50 K for both samples. The resistance vs. temperature behavior of the samples in applied fields of 0, 3 and 7 T confirmed superconductivity, with a different type of broadening of the superconductivity transition under magnetic fields for Ru-1212 from that known for conventional high-T _c superconductors. The magnetoresistance (MR) is negative above the Ru magnetic ordering temperature at 135 K. Below the Ru magnetic ordering temperature, MR displays a positive peak at low fields and becomes negative at higher fields for Ru-1212. For Ru-1222, MR remains negative both above and below the ordering temperature. A maximum of 2% is observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction pattern obtained for the Ru-1212 sample shows two types of superstructure: one has a weak spot at the centre of the a–b rectangle, and the other only along the b direction. Interestingly, Ru-1222 shows only clean a–b and a–c planes, without any superstructures.     

Superconductivity in Bi2.2Ca2.8−x Sr x Cu2O y system and effect of rare earth substitution

The oxide system Bi_2.2Ca_2.8−x Sr _x Cu₂O _y has been investigated for superconductivity as a function of Ca/Sr ratio withx=0.7, 1.0, 1.4 and 1.8. All these compositions are found to be superconducting with onset temperature in the range of 80–90 K implying a large homogeneity range between Sr and Ca for the superconductivity to occur in this system. Effect of partial replacement of Bi by rare earths has been studied and it is observed that 20% replacement of Bi by Nd and Eu does not significantly affect the superconducting transition temperature.     

Influence of magnesium oxide nanoparticles embedded in Bi-2212/Ag tapes on their superconducting properties

A MgO-doped Bi₂Sr₂CaCu₂O _y (Bi-2212) high-temperature superconductor is investigated experimentally. The influence of the MgO particle concentration and size on the high-temperature superconductivity parameters is studied. It is shown that, when incorporated into the superconductor, MgO particles do not change their structure. The optimal parameters of the particles exerting the strongest effect on the material are found. The optimal size of the particle is in the range 40–50 nm for a cubic structure. Measures should be taken against clustering. Ways of particle preparation and incorporation into the superconductor are considered. The superconductivity of the material with embedded MgO particles is investigated. At temperatures close to 20 K, the magnetic field of doped tapes exceeds that of undoped ones by a factor of 2.5–10.0 depending on the MgO concentration. The temperature dependences of the superconducting properties of the doped tapes at different magnetic fields are presented.     

Pressure and volume dependence of thelo-to phonons in InAs

The pressure dependence of thelo-to phonons in InAs has been investigated by Raman scattering using the diamond anvil cell. Indium arsenide transforms, presumably to the rock-salt structure at 70±1 kbar. The mode Grüneisen parameters for thelo-to phonons are γ _lo =0.99±0.03, γ _to =1.2±0.03 respectively. The effective charge,e* _T , for InAs decreases slightly with pressure and this trend is in accordance with the behaviour of other III–V zinc blende structured semiconductors: The structural phase transition is discussed in the light of theoretical calculations for phase stability of III–V compounds, as well as recent high pressure x-ray diffraction studies.     

Mobility–diffusivity relationship for heavily doped organic semiconductors

The relationship between the diffusivity D _n and the mobility μ _n of chemically doped organic n-type semiconductors exhibiting a disordered band structure is presented. These semiconductors have a Gaussian-type density of states. So, calculations have been performed to elucidate the dependence of D _n /μ _n on the various parameters of this Gaussian density of states. Y. Roichman and N. Tessler (Appl. Phys. Lett. 80:1948, 2002), and subsequently Peng et al. (Appl. Phys. A 86:225, 2007), conducted numerical simulations to study this diffusivity–mobility relationship in organic semiconductors. However, almost all other previous studies of the diffusivity–mobility relationship for inorganic semiconductors are based on Fermi–Dirac integrals. An analytical formulation has therefore been developed for the diffusivity/mobility relationship for organic semiconductors based on Fermi–Dirac integrals. The D _n /μ _n relationship is general enough to be applicable to both non-degenerate and degenerate organic semiconductors. It may be an important tool to study electrical transport in these semiconductors.     

Magnetic susceptibility of ternary compounds I–III–VI2

Summary The temperature dependence of the magnetic susceptibility ((300÷78) K) of some polycrystalline I–III–VI₂ semiconductors has been determined. Some of the materials show a temperature-independent diagmagnetic behaviour. CuInS₂ and CuInSe₂ show changes in the diamagnetic susceptibility assumed to be due to the presence of a temperature-dependent paramagnetic term from noninteracting trapped carriers. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Effect of a pulsed magnetic field on the transverse resistance of a layered superconductor

The field dependences of the transverse resistance of a single crystal of the layered superconductor Bi₂Sr₂CaCu₂O_y (BSCCO-2212) with T _c 0≃92 K are studied in magnetic fields up to 50 T in the perpendicular orientation H⊥(ab). It is established that in the resistive region the resistance is a power-law function of the field, and the temperature dependence of the barrier height for flux creep is obtained. It is found that in a wide temperature range, 50–125 K, the transverse magnetoresistance of the crystal in the normal state and under conditions of superconductivity suppression by a strong magnetic field is negative and can be approximated by a linear law with a temperature-dependent slope. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 420–425 (10 September 1998)     

Electronic structure and stability relations of CaF2 and NaCl phases in a Ti-H system

Calculations are made of the electronic structures of TiH_0.73 and TiH hydrides having a NaCl crystal structure and TiH_1.5, TiH_0.73, and TiH having a CaF₂ structure. Estimates of their superconductivity temperature indicate that this should be significantly higher for the stoichiometric NaCl phase compared to nonstoichiometric phases. The results are compared with the characteristics of the χ phase of titanium hydride TiH_0.73 which undergoes a superconducting transition at 4.3 K. Fiz. Tverd. Tela (St. Petersburg) 40, 195–197 (February 1998)     

Muon/Muonium in an Isotopically Pure 13C Diamond

A preliminary study of the diamagnetic (μ_d) and the paramagnetic (Mu _T ) states in a synthetic ¹³C diamond has been performed using the Transverse Field Muon Spin Rotation method. This system could be used to verify the quantum diffusion behaviour observed before, however, with a more reliable extraction of the hopping rate. The results were obtained in an applied magnetic field of 7.5 mT and at sample temperatures of 10 K, 100 K and 200 K. The prompt fraction, f, of the μ_d state remains constant at 22(5)% in the range 10–200 K; that of the Mu _T state increases from 53(10)% at 10 K to 78(10)% at 200 K. The fractions of the two states add to 100% at 200 K, suggesting non-population of the bond-centred state, Mu_BC, which is often observed in other diamond samples. The μ_d state has a spin relaxation rate of 0.20(5) μs⁻¹, in contrast to the zero value obtained in type II diamond samples. This indicates appreciable interaction of the μ_d state with the ¹³C atoms. The Mu _T state has a large spin relaxation rate ranging from 3.0(5) μs⁻¹ at 10 K to 7.0(5) μs⁻¹ at 200 K, consistent with values obtained in diamond samples with defects. This work is part of ongoing studies of muon/muonium-defect interactions in diamonds. This revised version was published online in September 2006 with corrections to the Cover Date.     

Mechanism of the formation of a diamond nanocomposite during transformations of C60 fullerite at high pressure

The results of an investigation of the transformation of C₆₀ fullerite to diamond under pressure through intermediate three-dimensionally polymerized and amorphous phases are reported. It is found that treatment of fullerite C₆₀ at pressures 12–14 GPa and temperatures ∼1400°C produces a nanocrystalline graphite-diamond composite with a concentration of the diamond component exceeding 50%. At lower temperatures (700–1200°C) nanocomposites consisting of diamondlike (sp ³) and graphitic (sp ²) amorphous phases are formed. The nanocomposites obtained have extremely high mechanical characteristics: hardness comparable to that of best diamond single crystals and fracture resistance two times greater than that of diamond. Mechanisms leading to the transformation of C₆₀ fullerite into diamond-based nanocomposites and the reasons for the high mechanical characteristics of these nanocomposites are discussed. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 11, 822–827 (10 June 1999)     

Recent advances in superconductivity of covalent superconductors

A summary and critical analysis are given of recent results on superconductivity in diamond, Si, SiC, III–V and IV–VI semiconductors, as well as graphite and fullerites intercalated with alkali metals. Superconductivity in III–V semiconductors is questioned, and proposals are outlined for further enhancement of the transition temperature in diamond and intercalated graphite.     

Intraband luminescence spectra of insulators and semiconductors excited by pulsed electron beams or electric fields

The features of the intraband luminescence spectrum of wide-gap insulators (KI, KBr, CsCl, etc.) and semiconductors (GaP, CdS, α-SiC, and ZnS) are studied in the temperature interval 80–760 K. The spectra of the intraband luminescence are compared with the spectra of the pre-breakdown electroluminescence of GaP and α-SiC surface-barrier diode structures and of a ZnS thin-film electroluminescence indicator. In alkali halide crystals the short-wavelength boundary hν _m of the intraband luminescence is less than the band gap E _g and is governed by complex excitonic processes. In semiconductors with indirect transitions hν _m>E _g. The differences in the spectra of the intraband luminescence and the intraband pre-breakdown electroluminescence can be explained by differences in the distributions of the hot charge carriers over levels of the allowed bands and in the maximum energies of the carriers involved in the formation of the spectra. Fiz. Tverd. Tela (St. Petersburg) 39, 613–617 (April 1997)     

Comparison of optical properties and mechanical/electrical degradation of some low density polyethylene-and polypropylene-based polymer blends

In this study, the optical properties as well as mechanical and electrical degradation of low-density polyethylene (LDPE)/polypropylene fiber (PP fiber) (10–50% PP fiber), polypropylene (PP)/PP fiber (10–50% PP fiber), and LDPE/diamond (0.1–3% diamond) blends, which are prepared by hot pressing method, with changing thicknesses ranging from 30 to 225 μm, are compared. The spectra, in the wavelength range 200–2500 nm, are examined. Based on optical absorption spectra obtained, Tauc graphs are plotted. Determined values of the direct optical energy gap (E _d ^opt ), the indirect optical energy gap (E _i ^opt ), the width of the band (ΔE), and ultraviolet transmittance (T_UV) are listed. The direct E _d ^opt and indirect E _i ^opt values for organic blends are in the range of 3.10–3.17 eV and 1.52–2.99 eV; for inorganic blends they are 1.80–4.13 eV and 1.55–4.7 eV respectively. The electrical strength (ε) and the mechanical tension (σ) have been investigated, and graphs (the dependence of the electrical life time log τ_ε on ε) are given. The experimental results are analyzed from the viewpoint of the validity of the thermofluctuation theory. LDPE and LDPE/0.5% diamond composite parameters consecutively changed: σ from 68 to 82 MPa, ε from 60·10⁶ to 85·10⁶ V/m, mechanical lifetime τ_σ from 10 to 1.5·10⁵ sec, electrical lifetime τ_ε from 2· 10³ to 2·10⁵ sec, and structure-sensitive parameters γ and χ — from 1.48 to 1.18 (J)MPa/mole and from 0.97 to 0.70 (J)Vm⁻¹/mole respectively. The values of mechanical and electrical durability were observed to increase by 20 and 41%, respectively, for LDPE/0.5% diamond composite. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 677–683, September–October, 2007.     

Photoelectric characteristics of nanostructured hydrochemically deposited films of the Pb<Subscript>0.975</Subscript>Sn<Subscript>0.025</Subscript>Se substitutional solid solution

The photoelectric characteristics of Pb_0.975Sn_0.025Se solid solution films prepared by the hydrochemical codeposition of PbSe and SnSe with the subsequent heat treatment in air at 573–700 K have been investigated. The thermal and optical band gaps, the temperature coefficient of the optical band gap, the dark resistance, the volt-watt sensitivity, and the range of spectral sensitivity have been determined in the temperature range of 220–300 K. It has been found that, after heat treatment below 573 K, the films of the Pb_0.975Sn_0.025Se solid solutions possess metallic conductivity, while being heat treated at elevated temperatures, they become semiconductors with p-type conductivity. The composition of the solid solution is independent of the heat treatment temperature; it is formed during deposition.     

Contribution of nonequilibrium optical phonons to the Peltier and Seebeck effects in polar semiconductors

Additive contributions to the Seebeck and Peltier coefficients made by nonequilibrium longitudinal optical phonons have been calculated. The results obtained are valid for any temperature and applicable to polar nondegenerate semiconductors with low carrier concentrations. The calculated components of the thermoelectric coefficients are exponentially small in the low-temperature domain and reach a maximum at k _BT∼ħω ₀. In materials with a large carrier mass and strong electron-phonon coupling the contribution of optical phonons to the Seebeck coefficient can exceed 1 mV/K. Fiz. Tverd. Tela (St. Petersburg) 40, 1209–1215 (July 1998)     

Anomalous behavior of the specific heat and upper critical magnetic field in the superconducting single crystal La1.85Sr0.15CuO4

The specific heat and resistive upper critical magnetic field of the single crystal La_1.85Sr_0.15CuO₄ are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated. Zh. éksp. Teor. Fiz. 112, 1386–1395 (October 1997)     

Long-lived excited state of Te donors in GaP

The kinetics of the photoresponses in constant and microwave electric fields and the variation of the absorption of background radiation in GaP doped with Te (2×10¹⁷ cm⁻³) upon impurity excitation at 5–50 K are investigated. The lifetime of the excited state of the Te donors is determined (∼10⁻² s). It is shown that the results presented are consistent with the model of carrier accumulation in long-lived impurity excited states in semiconductors. These results are compared with the results previously obtained for diamond-structure semiconductors. Zh. éksp. Teor. Fiz. 114, 2204–2210 (December 1998)     

A quantitative study of diffracted X-ray intensities from type I natural diamond crystals by high resolution X-ray diffractometry and comparison with nearly perfect silicon single crystals

Results of accurate measurements of peak and integrated intensities of , 111 and 333 reflections of natural diamonds of type I and nearly perfect silicon single crystals are reported. Highly monochromated and collimated MoK _{α 1} exploring beam was used. A quadrupole crystal X-ray diffractometer was employed in (+, −, +) and (+, −, +, −) settings. (111) platelets of diamond and silicon crystals with thicknesses of about 1 mm were selected. High resolution diffraction curves, stationary and traverse topographs were recorded. Diffraction curve half widths of diamond and silicon crystals were in the range: 45–200 arc sec and a few arc sec respectively. The experimental values of integrated intensitiesρ for diamond crystals were found to lie between the theoretical values for ideally perfect and ideally imperfect crystals. Experimental values ofρ for silicon were closer to the “perfect crystal” values. This is consistent with the results of diffractometric and topographic evaluation. The peak intensities of all reflections were higher for diamond crystals in comparison to the silicon crystals. The ratioI _C/I _Si lies in the range 1.3 (111 reflection) to 10.5 and (333) reflections. This is anomalous and cannot be accounted for by considering the degree of perfection, structure factor and difference in absorption coefficient.     

Superconductivity and magnetism in HoNi2B2C

The effect of magnetic ordering on superconductivity in HoNi₂B₂C is studied. It is concluded that the abrupt suppression of superconductivity in the region of incommensurate antiferromagnetic ordering is due to a modification of the wave functions of the conduction electrons by the long-range magnetic order. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 9, 702–707 (10 May 1996)