Superconductivity in the Ti-D system under pressure

The pressure dependence of the superconducting transition temperature in TiD_0.74 has been measured up to 30 GPa in a diamond high-pressure chamber. It is found that the deuteride TiD_0.74 becomes a superconductor at pressures corresponding to the transition to the high-pressure ζ phase, with a transition temperature that increases from 4.17 to 4.43 K in the interval P=14–30 GPa. The value extrapolated to atmospheric pressure T _c (0)=4.0 K is significantly lower than the superconducting transition temperature (T _c =5.0 K) measured earlier in the metastable state obtained by quenching TiD_0.74 under pressure. It is assumed that the significant difference of the extrapolated value from the superconducting transition temperature in the metastable state after quenching under pressure is caused by a phase transition on the path from the stability region of the ζ phase under pressure to the region of the metastable state at atmospheric pressure. Fiz. Tverd. Tela (St. Petersburg) 40, 2153–2155 (December 1998)     

Superconducting transition temperature in mercury HTSC-cuprates under hydrostatic pressure

A study is made of the properties of the homologous series of mercury HTSC-cuprates HgBa₂Ca_n−1Cu_nO_2n +2+δ with n=1–8. Experiments are conducted under pressure for samples with n=1–5. The Hg-1223 and Hg-1234 phases were synthesized using a controlled high pressure chamber. The oxygen content of an initial mixture corresponding to the Hg-1234 phase was varied by changing the composition of the initial BaO/BaO₂ oxides. The dependence of the superconducting transition temperature T _c on the lattice constant a (and, therefore, on the oxygen content) and of T _c ^max and dT _c ^max /dp on n are convex upward up to n=4, 5. The maximum values always correspond to the Hg-1223 phase. Experimental T _c ^max (n) curves for the phases with n=1–6 and dT _c ^max /dp curves for n=1–5 are compared with Anderson’s theory (the so-called RVB model). A general analysis of these results indicates that the mercury cuprates have an ideal structure for HTSC. The Hg-1223 phase is the “champion” in this ideal structure and the critical temperature corresponding to this phase (T _c =135 K) is the highest at atmospheric pressure. Zh. éksp. Teor. Fiz. 113, 1474–1483 (April 1998)     

Negative Poisson coefficient of fractal structures

On the basis of a fractal model the macroscopic elastic properties of an inhomogeneous medium with random structure have been determined. It is shown that if the ratio of the bulk moduli of the phases K ₂/K ₁→0, then the percolation threshold p _c the Poisson coefficient is equal to 0.2. A study of the behavior of a two-phase medium with negative Poisson coefficient is carried out. Fiz. Tverd. Tela (St. Petersburg) 41, 2147–2153 (December 1999)     

Ferroelectric properties of SrTiO3-PbTiO3 solid solutions

Dielectric properties of ceramic samples of Sr_1−x Pb_xTiO₃ solid solutions for x varying from 0 to 0.3 have been studied. The ferroelectric phase transition in this system has been established to persist down to x=0.005. Within the x values of 0.002 to 0.05, the ferroelectric transition temperature is shown to follow the relation T _c=A(x−x _c)^1/2, with A=440 K and x _c=0.002. Fiz. Tverd. Tela (St. Petersburg) 39, 714–717 (April 1997)     

Some comparative thermodynamic characteristics of fullerite and various covalent elements

Measurements of the specific heat and elastic wave velocities for a C₆₀ fullerene sample treated at high pressure and temperature are used to estimate the Debye temperature and the function ΔC=C _p−C _v, and also to calculate the thermal expansion work in the ideal approximation. Similar calculations were made for graphite, diamond, silicon, germanium, and various refractory metals. The results were used to draw qualitative conclusions on the structural stability of a new material obtained from fullerene C₆₀ which possesses extremely high hardness. Fiz. Tverd. Tela (St. Petersburg) 40, 1387–1389 (July 1998)     

Superconductivity in covalent semiconductors

This review covers recent advances in superconductivity of diamond, Si, SiC, group III–V and II–IV semiconductors, metal-intercalated graphite and fullerites. The results are critically analyzed and prospects are given for future research directions. In particular, it is argued that the highest transition temperatures of ∼9 K in diamond and 11.5 K in CaC₆ can further be enhanced and that no reliable evidence exists yet for superconductivity in III-V semiconductors.      

Superconductivity and possible overdoping in Lu1−x Ca x Ba2Cu3O7−δ

Structural and superconducting properties of the system Lu_1−x Ca _x Ba₂ Cu₃O_7−δ, both in bulk as well as thin film form, have been investigated. Presence of large Ca-ions at the Lu-sites is responsible for phase stability (in 1–2–3 phase) in bulk. In argon annealed tetragonal samples (δ:1) superconductivity is reinstated as in these samples, presence of Ca-ions lead to the generation of holes in the Cu−O planes. In oxygen annealed bulk samples and thin films, there is a large depression ofT _c due to divalent Ca-ions present in concentrationx<0.2. Our data indicate that this depression ofT _c is likely due to overdoping effect.     

Physical properties of superhard and ultrahard fullerites created from solid C60 by high-pressure-high-temperature treatment

60 in the pressure range and the temperature range were studied. These properties are the specific gravity, specific heat in the range , sound velocities, elastic moduli, electrical properties, resistance to uniaxial stress and stability against oxidation. These properties are different from those of diamond and other carbon forms. The hardness of ultrahard fullerites exceed the hardness of diamond. The structures of the samples are described on the basis of X-ray data. Received: 27 September 1996/Accepted: 19 December 1996     

The structural, elastic and thermodynamic properties of intermetallic compound CeGa2

The structural, elastic and thermodynamic characteristics of CeGa₂ compound in the AlB₂ (space group: P6/mmm) and the omega trigonal (space group: P-3m1) type structures are investigated using the methods of density functional theory within the generalized gradient approximation (GGA). The thermodynamic properties of the considered structures are obtained through the quasi-harmonic Debye model. The results on the basic physical parameters, such as the lattice constant, the bulk modulus, the pressure derivative of bulk modulus, the phase-transition pressure (P _t ) from P6/mmm to P-3m1 structure, the second-order elastic constants, Zener anisotropy factor, Poisson’s ratio, Young’s modulus, and the isotropic shear modulus are presented. In order to gain further information, the pressure and temperature-dependent behavior of the volume, the bulk modulus, the thermal expansion coefficient, the heat capacity, the entropy, Debye temperature and Grüneisen parameter are also evaluated over a pressure range of 0–6 GPa and a wide temperature range of 0–1800 K. The obtained results are in agreement with the available experimental and the other theoretical values.     

Inhomogeneous magnetostriction states in uniaxial ferromagnetic films

Surface magnetoelastic Love waves and nonuniform distributions of the magnetization and elastic strains are investigated in a uniaxial ferromagnetic film on a massive nonmagnetic substrate in a tangential external magnetic field. A new inhomogeneous phase is predicted having spatial modulation of the order parameter, arising from magnetostrictive coupling of the magnetization with lattice strains near the interface of the magnetoelastic and elastic media. It is shown that, at some critical magnetic field H _c, different from the orientational transition field in an isolated sample, a magnetoelastic Love wave propagating parallel to the magnetization vector in the film plane becomes unstable. The frequency and group velocity of the wave vanish at wave number k=k _c≠0 and the wave freezes, forming a domain structure localized in the film and adjoining substrate. Fiz. Tverd. Tela (St. Petersburg) 41, 665–671 (April 1999)     

The influence of a magnetic field on the inelastic properties of LiF crystals

The effect of weak magnetic fields (0.1–0.8 T) on the internal friction and Young’s-modulus defect of LiF crystals is investigated over a range of relative strain amplitudes ɛ ₀ from 10⁻⁶ to 10⁻⁴ at frequencies of 40 and 80 kHz. Experiments with these fields show that the internal friction increases and the effective elastic modulus decreases, indicating an increase in the plasticity of the samples. Plots are obtained of the internal friction versus the magnitude of the magnetic field at various values of the strain amplitude ɛ ₀. Fiz. Tverd. Tela (St. Petersburg) 41, 1035–1040 (June 1999)     

Young’s modulus and internal friction in porous biocarbon white pine wood precursors

This paper reports on a study performed in the temperature range 100–293 K, in air and in vacuum, for the amplitude and time dependences of the Young’s modulus and the internal friction (ultrasound damping) of biocarbon precursors prepared from white pine wood at two pyrolysis (carbonization) temperatures of 1000 and 2400°C. The measurements have been conducted by the resonance technique with a composite vibrator on samples cut along and across the tree growth direction. The desorption of molecules of the external medium at low amplitudes of ultrasonic vibrations has been found to produce the pronounced influence on the effective elastic modulus and elastic vibration decrement. The data obtained from acoustic measurements of the amplitude dependences of the elastic modulus have been used to estimate the microplastic properties of the samples. It has been shown that increasing the carbonization temperature gives rise to noticeable changes in the Young’s modulus and internal friction, as well as to reduction of the microplastic stress σ _y of the biomaterial studied. The stress σ _y of the samples cut across the growth direction has been found to be substantially smaller than that of the “longitudinal” samples. The elastic and microplastic properties of precursors prepared from white pine wood have been compared with those of the white eucalyptus wood.     

The paleoinformation value of natural remanent magnetization of several traps in Yakutia

Research was carried out to estimate the possibility of determining the direction and strength of the ancient geomagnetic field (H _an ) by natural remanent magnetization (I _n ) of nine oriented samples from traps of the Minor Botuoba Region (Yakutia) aged 260 Ma. Five samples (Pi-10, K-4, K-6, 315–13, and Ki-2) are characterized by negative polarity of In, while four samples (nos. 334-5, 331–2, 315–11, 299–2) have positive polarity as does the recent geomagnetic field in this region. The ferrimagnetic constituent of the samples with reverse I _n polarity appears to be quite variable: samples K-4 and K-5 are characterized by low Curie points (T _c ≈ 200°C) of the ferrimagnetic phase, sample Ki-2 contains single-phase oxidized titanomagnetite with T _c ≈ 310°C, and the T _c of the Pi-10 ferrimagnetic phase is 540°C. Hence, it may be concluded that the primary remanent magnetization of the first two samples was formed in a reverse polarity field. These samples also may be used to determine the paleostrength of the geomagnetic field. The properties of traps containing single-phase oxidized (sample Ki-2) and disintegrated (sample Pi-10) titanomagnetite require additional investigation. Samples with positive In polarities are characterized by the self-reversal phenomenon upon thermal demagnetization of the natural remanent magnetization, which was most likely caused by the occurrence of titanomagnetite exsolution textures in ferrimagnetic grains. The paleoinformation value of the I _n of these samples is doubtful.     

Correlation between the electric-field effect and weak-link type in YBa2Cu3−x Oy and YBa2Cu3−x Oy /Agx high-T c superconducting ceramics

An experimental study is reported of the effect of an electric field E⩽120 MV/m and of temperature T on the critical current I _c and I-V characteristics of yttrium-based high-T _c superconducting ceramics. The materials studied were copper-deficient ceramics, YBa₂Cu_3−x O_y (D samples), and YBa₂Cu_3−x O_y/Ag_x ceramics [S samples with silver present in amounts equal to the copper deficiency (0⩽x⩽0.4)]. It has been established that in D samples at 77 K, the electric field increases I _c and reduces substantially R for I>I _c, whereas in S samples no field effect is observed. Measurements of the I _c(T) dependence near the critical temperature showed that they can be described for all samples by a relation of the type I _c =const(1−T/T _c )^α, where α≈1 for the D samples, and α≈2 for the S samples. The results obtained suggest that the electric-field effect correlates with the existence in the ceramic of SIS-type weak links at grain boundaries. Fiz. Tverd. Tela (St. Petersburg) 40, 1195–1198 (July 1998)     

Lattice mechanical properties of noble and transition metals

A model pseudopotential depending on an effective core radius but otherwise parameter free is used to study the interatomic interactions, phonon dispersion curves (inq and r-space analysis), phonon density of states, mode Grüneisen parameters, dynamical elastic constants (C ₁₁,C ₁₂ andC ₄₄), bulk modulus (B), shear modulus (C′), deviation from Cauchy relation (C ₁₂–C ₄₄), Poisson’s ratio (σ), Young’s modulus (Y), behavior of phonon frequencies in the elastic limit independent of the direction (Y ₁), limiting value in the [110] direction (Y ₂), degree of elastic anisotropy (A), maximum frequencyω _max, mean frequency 〈ω〉, 〈ω ²〉^1/2=(〈ω〉/〈ω ⁻¹〉)^1/2, fundamental frequency 〈ω ²〉, and propagation velocities of the elastic constants in Cu, Ag, Au, Ni, Pd, and Pt. The contribution of s-like electrons is calculated in the second-order perturbation theory for the model potential while that of d-like electrons is taken into account by introducing repulsive short-range Born-Mayer like term. Very recently proposed screening function due to Sarkar et al. has been used to obtain the screened form factor. The theoretical results are compared with experimental findings wherever possible. A good agreement between theoretical investigations and experimental findings has proved the ability of our model potential for predicting a large number of physical properties of transition metals.     

Transmission spectra of thin CdS and CdSe crystals near higher-order isotropic points

Summary For photon energies below the absorption edge in CdS, CdSe and other II–VI crystals the polariton dispersion curves forE⊥c andE‖c (c-axis in a wurtzite crystal) corss at some points called isotropic points (IP). The occurrence of isotropic points provides the possibility of mode coupling between ordinary and extraordinary waves. Since the consequences of mode coupling on the optical properties for photon energies near the lowest IP lying much below the first excitonic state were widely discussed in recent years, more attention is now paid to isotropic points lying near the band gap and related to then=2,3, … excitonic states (?higher isotropic points?). Making use of Stahl's real density matrix approach we derive the polariton dispersion relationsk _⊥ (ω), andk _‖ (ω), for CdS and CdSe bulk crystals and determine the positions of IP's due to the crossing of theB-polariton with higherA-excitonic resonances. By the method of multiple internal reflection we calculate the transmission spectra for various crystal thicknesses (between 3 μm and 0.5 mm) and coupling mechanisms. The calculated transmission shows sharply peaked structures centred at the isotropic points.     

Low-temperature resistivity of YBa2Cu3O6+x single crystals in the normal state

A scan of the superconductor-nonsuperconductor transformation in single crystals of YBa₂Cu₃O_6+x (x≈0.37) is done in two alternative ways, namely, by applying a magnetic field and by reducing the hole concentration through oxygen rearrangement. The in-plane normal-state resistivity ρ_ab obtained in the two cases is quite similar; its temperature dependence can be fitted by a logarithmic law in a temperature range of almost two decades. However, an alternative representation of the temperature dependence of σ_ab=1/ ρ _ab by a power law, typical for a 3D material near a metal-insulator transition, is also plausible. The vertical conductivity σ_c=1/ρ_c followed a power law, and neither σ_c(T), nor ρ_c(T) could be fitted by log T. It follows from the ρ_c measurements that the transformation at T=0 is split into two transitions: superconductor-normal-metal and normal-metal-insulator. In our samples, they are separated in oxygen content by Δx≈0.025. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 834–839 (10 June 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Relation between the reduced and unreduced hardness in nanomicroindentation tests

Hooke’s law is generalized to the case of arbitrary elastic or plastic indentation , where ɛ=q/E _r is the elastic strain, q is the average pressure over the contact area, E _r is the reduced elastic (Young’s) modulus, A is the projected area of the contact, w ₁ is the deformation in elastic indentation by a flat punch. On this basis a relation is obtained between the reduced hardness H and unreduced hardness H _h, which depends on the ratio w _s/w₁=m _s; w _s is the elastic deformation along the perimeter of the indent, and m _s≅0.78. It is shown that the correction ΔE _r to the elastic modulus E _r determined from the condition of linearity of the initial part of the unloading diagram, is ΔE _r=0.27(ΔP/P _m), where ΔP is the value used in the calculation of E _r for the length of the linear part of the diagram, reckoned from the maximum load P _m. It is shown that for metallic construction materials of medium hardness one has q=HM, where HM is the Meyer hardness. With increasing HM and increasing angle ϕ at the tip of the indenter, the ratio HM/q grows by an exponential law. Zh. Tekh. Fiz. 69, 42–48 (July 1999)     

Elastic moduli of single-crystal C60

The matrix of elastic constants of the fcc phase of solid C₆₀ has been determined experimentally from measurements of the the velocity of 5 MHz ultrasound in single-crystal samples with different crystallographic orientations. The following values were obtained for the elastic moduli: C ₁₁=14.9±0.9 GPa, C ₁₂=8.8±1.0 GPa, and C ₄₄=6.6±0.18 GPa. The results are compared with theoretical estimates of the elastic moduli and data obtained in previous measurements of the elastic characteristics of solid C₆₀. Fiz. Tverd. Tela (St. Petersburg) 40, 173–175 (January 1998)     

Magnetostriction and thermal expansion anomalies near the Curie point of the compound La0.7Sr0.3MnO3 with the perovskite structure

It is found that the bulk part ω of the magnetostriction near the Curie temperature T _c in a La_0.7Sr_0.3MnO₃ single crystal with the perovskite structure is negative and that the temperature dependence of |ω| has a maximum near T _c . The quantity |ω| at the maximum increases rapidly with increasing magnetic field. The thermal expansion coefficient near T c increases with temperature much faster than linearly. The paramagnetic Curie temperature determined from the Curie-Weiss law, which the paramagnetic susceptibility of this crystal satifies, was found to be lower than T _c . These anomalies and also the near-T _c metal-insulator transition which is characteristic for this material are explained by the existence of a magnetically two-phase state consisting of a conducting ferromagnetic matrix containing antiferromagnetic insulating microregions occupying not more than 5% of the sample volume. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 449–453 (25 March 1997)