Vibrating reed studies on high-T c superconductors

We present vibrating reed (VR) measurements on single crystal and ceramic “1-2-3” and melt-processed polycrystalline Bi-based compounds in a wide range of temperature (4·2–100 K) and magnetic field (B=0–4 T). The “depinning line” (DL) determined by the VR technique is equivalent to the “irreversibility line” determined by magnetization and susceptibility measurements. A comparison of the results on single crystal and polycrystalline 1-2-3 compounds indicates that the VR technique is sensitive to the intragranular properties of the polycrystalline reed. It is found that the DL for 1-2-3 compounds is much steeper than that for Bi-based compounds, reflecting an intrinsically different pinning in both the materials, in agreement with the measured elastic coupling (Labusch constant α(B, T)).     

Self-diffusion at the melting point: From H2 and N2 to liquid metals

A nominal lower bound to the mean free diffusion time at the melting point T _m was obtained earlier which provided a factor-two type estimate for self-diffusion coefficients of the alkali halides, alkali metals, eight other metals, and Ar. The argument was based on the classical Uncertainty Principle applied to the solid crystal, whereby maximum-frequency phonons lose validity as collective excitations and degenerate into aperiodic, single-particle diffusive motion at the melting point. Because of the short time scale of this motion, the perfect-gas diffusion equation and true mass can be used to obtain the self-diffusion coefficient in the Debye approximation to the phonon spectrum. This result for the self-diffusion coefficient also yields the scale factor that determines the order of magnitude of liquid self-diffusion coefficients, which has long been an open question. The earlier theory is summarized and clarified, and the results extended to the more complex molecular liquids H₂ and N₂. It is also demonstrated that combining Lindemann's melting law with the perfect-gas diffusion equation estimate yields a well-known empirical expression for liquid-metal self-diffusion at T _m. Validity of the self-diffusion estimate over a melting temperature range from 14 to more than 1300 K and over a wide variety of crystals provides strong confirmation for the existence of the specialized diffusive motion at the melting point, as well as confirmation of a relation between the phonon spectrum of the solid crystal and diffusive motion in the melt.     

Mathematical Model for Transitional Processes in Josephson Cryotrons Based on Tunnel Junctions

In this work, we consider controlling of logical states of Josephson memory cells (cryotrons) based on superconductor-insulator-superconductor (SIS) Josephson tunnel junctions by external current impulses. A mathematical model for the transitional processes that take place during direct logical transitions “0” → “1” and inverse logical transitions “1” → “0” is proposed. By means of mathematical modeling, we investigate transitional processes in cryotrons during the change of their logical state and obtain their transitional characteristics for operational temperatures T ₁=11.6 K and T ₂=81.2 K, close to the boiling temperatures of helium and nitrogen, respectively. It is shown that such memory cells can efficiently operate under the temperature T ₂=81.2 K. The behavior of the Josephson cryotrons as well as their operational stability is explored.     

Laser-induced surface decomposition of Al2O3 excited in the V-center absorption band

The primary products of desorption from Al₂O₃ surface excited by laser pulses (pulse duration τ∼15 ns; wavelength λ=354 nm, radiant power density P/S<10⁸ W/cm²) in the V-center absorption band were studied by the time-of-flight (TOF) spectroscopy. The TOF spectra show evidence of the desorption of one “ cold” (T ₁=300 K) and two “hot” (T ₂=1000 K, T ₃=4300 K) groups of oxygen molecules with the Maxwell velocity distributions, as well as of the hot Al and O atoms with nonequilibrium energy distributions (E ₁=0.37 eV, E ₂=0.38 eV). A model describing the oxygen desorption as initiated by the electron transitions is suggested, in which escape of the cold O₂ molecules from the surface is related to discharge of the O ₂ ⁻ anions adsorbed on the V-centers, desorption of the hot atoms is attributed to discharge of the surface O⁻ anions, and the appearance of the hot O² molecules is related to the associative desorption of two O⁻ anions localized at the same V-center discharged by a pair of excitons.     

Low-Temperature (T=250–400 K) oxygen adsorption on YBa2Cu3O6.9 ceramics

Oxygen adsorption on YBa₂Cu₃O_6.9 ceramics at T=250–400 K was studied by thermal desorption (TD) mass spectrometry. It was established that, depending on the adsorption temperature (T _a), oxygen occurs on the substrate surface either in two forms (γ1 and γ2, for T _a < 350 K) or in a single form (γ2, for T _a ≥350 K). In the TD spectrum, γ1 adsorbed oxygen species are manifested by a peak with a maximum at T _max=330–340 K. When the adsorption temperature increases from 290 to 400 K, the TD peak of γ2 oxygen species shifts from T _max≈380 K to 440 K, which is accompanied by a decrease in the temperature “boundary” for the desorption of structural oxygen. In addition, γ2 oxygen species are capable of participating in isotope exchange with the oxygen of the ceramics. For both forms of adsorbed oxygen, TD most probably proceeds by the associative mechanism with an activation energy of 0.63±0.08 eV (γ1) and 1.13±0.02 eV (γ2). A model qualitatively explaining oxygen sorption in ceramics is proposed, according to which γ2 species are formed in the initial adsorption stage.     

Kinetics of calcium molybdate crystallization

Kinetics of crystallization of calcium molybdate from unstirred molten solutions of lithium chloride of low to medium supersaturation in platinum crucibles by the process of continuous cooling at 5° C hr⁻¹ from temperaturesT ₀=700 and 750°C are investigated. The crystal size measured by optical microscopy for different crystallization periods reveals that both crystal length and width generally increase with cooling period. The degree of crystallizationα _t,also increases with cooling period, attaining a maximum of 0·90. The diffusion rate constants,K _Dlat 700 and 750°C are 0·0776 and 0·1138 respectively. The effect of variation of the crystallization temperature on the crystal size and their number is also studied.     

Low-field magnetization curves of YBa2Cu3O7−x and BiSrCaCu2O5.5: Effects of Josephson medium

We present the measurement of dc magnetization curve of three samples of high-T _c superconductors in relatively low magnetic field of ±50 mT and at temperaturesT=4.2 K andT=77 K by a SQUID susceptometer. The samples were polycrystalline YBa₂Cu₃O_7–x, a single crystal of YBa₂Cu₃O_7–x, and polycrystalline BiCaSrCu₂O_5.5. We have especially studied the influence of the Josephson medium inside our samples on their low-field magnetization. We have found closed hysteretic loops in the field ±4 mT with virgin and granular slopes in the sample of polycrystalline YBaCuO and clear flux jumps corresponding to the penetration of low-field (Josephson) vortices. The pinning of these vortices in our case is rather low. In polycrystalline sample of BiCaSrCuO, we have found only reversible magnetization curves corresponding to very low pinning and/or to relatively low number of weak links.     

SEM investigations of iron surface ion erosion as a function of specimen temperature and incidence angle

Ion bombardment of iron surfaces produced an ion eroded surface microtopography strongly dependent on the incidence angle and specimen temperature. For bombardment temperatures exceeding the self-diffusion temperature, T _d, a microstructural smoothing process due to the thermally activated diffusion effects, assisted by ion bombardment, can be observed by SEM. The role of migrational processes which should be taken into account for a correct interpretation of the final surface topography formed by hot sputtering, was also considered.     

Estimation of the critical constants of long-chain normal alkanes

Correlations between the critical constants of normal alkanes and the number of carbon atoms in a molecule have been considered. In an approximation of a self-consistent field for a polymeric fluid, an equation of state of the van der Waals type has been written, and the dependences of the critical constants of chain molecules on the number of mers have been obtained. It has been found that for an infinitely long alkyl chain, the limiting values of the critical temperature, the critical pressure, and the critical density are equal to, respectively, 1135 K, 0 M Pa, and 0 kg · m^–3. A method of pulse heating of a wire probe immersed in the substance under investigation has been used to measure the dependence of the temperature of the attainable superheatT ^* of low-density polyethylene on the pressure p and the duration of heating pulset ^*. Extrapolation has been used to obtain an estimation of the attainable-superheat temperature of polyethyleneT ^*(p=0,t ^*=0)= 1175 K, which can be treated as the critical temperature of polyethylene.     

Electrothermal instability of a polar polymer dielectric above the glass transition temperature

A stationary temperature state of a polar polymer dielectric exposed to an electric field is studied. A special feature of the dielectrics of this type is the presence of a maximum in the loss factor ε″(T) above the glass transition temperature. Using an approach based on the Kirkwood formula for ε″(T), exact solutions to a nonlinear model problem are obtained. These solutions show the multiplicity of the stationary temperature states and the surface of these states in the “load-ambient temperature” space of parameters exhibits a topological singularity of the “fold” type.     

Two-gap structure in Yb(Y)Ba2Cu3O7?x single crystals

Tunneling effects in YbBa₂Cu₃O_7–x (YbBCO) and YbBa₂Cu₃O_7–x (YBCO) single crystals have been investigated using an improved technique of break junction preparation. A two-gap structure with BCS-like density of states has been observed. The value of the large gap is in the range of 24–28 meV for YBCO (T _c =85–90 K) and 29–30 meV for YbBCO (T _c =87–90 K). A variation of the value of the small gap between 0.5 and 11 meV has been detected, even for the same crystal. As a possible explanation, two-band superconductivity, surface properties, or strong gap anisotropy will be discussed.     

Variation of the coefficient of self-diffusion in a crystal caused by changes in the isotope composition

Variation of the parameters of self-diffusion in a crystal of simple substance caused by changes in the isotope composition have been theoretically studied in the regions of high and low temperatures. The results of numerical calculations performed for diamond and bcc lithium crystals are presented. It is shown that the isotope dependence of the self-diffusion coefficient D is weak at high temperatures but becomes substantial at low temperatures: at T = 0, the D value increases by an order of magnitude on the passage from ⁷Li to ⁶Li and decreases by three orders of magnitude on the passage from ¹²C to ¹³C. This isotope effect is related to the phenomena of quantum tunneling and zero-point oscillations of atoms in crystals at T = 0. The isotope dependence of the interatomic interaction potential significantly influences the self-diffusion coefficient only at high temperatures.     

Electrical conductivity,self-diffusion,and volume expansion of alkali halides at the melting point

In an earlier paper, Heisenberg's uncertainty principle was invoked at the melting point T _m of crystalline solids to provide fundamental justification for Lindemann's melting law and to compute diffusion coefficients of several alkali halides. The uncertainty principle defines breakdown of Debye zone boundary (ZB) phonons as valid collective excitations when phonon energies and line widths due to anharmonicity become comparable at T _m. Upon breakdown, random, high-frequency single-particle motion or partial decoupling of crystal ions sets in. Lifetimes of these single-particle ZB motions are determined from the minimum-uncertainty product inequality by assuming that it becomes an equality at T _m for ZB phonons. The present paper addresses improved formulation of that work and extended application to ionic electrical conductivities of 18 molten alkali halides at T _m. It is shown that use of the Debye model produces an approximate lower bound to the mean free time, not the unconstrained direct estimate previouslu implied. This feature is generally reflected in results for ionic conductivities and alkali halide diffusion coefficients for which comparison experimental data were found. However, in spite of this lower-bound formulation and the simple nature of the computation, the results compare favorably with experiment. A model of random single-particle harmonic motion superimposed on the lower-frequency collective motion is proposed to account for volume expansion accompanying the partial decoupling for hard-sphere ions. Experimental comparisons for 15 alkali halides show the decoupling volume change to account largely for the total volume change of melting (in the hard-sphere approximation), yielding a closer agreement with experiment than recent calculations aimed explicitly at the total volume change.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Nitrogen diffusion in a subsurface region of ion-implanted molybdenum

The dynamics of variation of the surface concentration of nitrogen in an ion-doped (100)-oriented single crystal Mo foil was studied by Auger electron spectroscopy and secondary-ion mass spectrometry techniques. The samples were implanted with nitrogen ions at an energy of 50 or 100 keV to a dose of 5×10¹⁷cm₋₂ and then annealed at 800–900°C. The diffusion coefficient of nitrogen, estimated by the time of appearance of the maximum nitrogen concentration on the sample surface, is 7–9 orders of magnitude lower than the value for a solid solution of nitrogen in molybdenum. On the other hand, this estimate is 3–5 orders of magnitude higher than the self-diffusion coefficient of molybdenum. It is suggested that a “supersaturated” solid solution of nitrogen is formed in ion-implanted molybdenum, in which the excess (over the solubility limit) nitrogen is deposited at the radiation defects and migrates with these defects in the course of the diffusion annealing.     

Spin-lattice relaxation in gadolinium-doped calcium tungstate

The spin-lattice relaxation of the S-state ion Gd³⁺ in a calcium tungstate host lattice has been examined at 37.5 GHz over the temperature range 1.5 to 30 K. The gadolinium concentrations in the doped single crystals used were about 50 ppm. Single exponential recovery was observed and the spin-lattice relaxation time (T ₁) varied from about 14 msec at 1.5 K to 0.03 msec at 30 K, measured with =90° and =8°. It was found that T ₁ varied with temperature (T) as T ₁ T ^–1 below 8 K and as T ₁ T ^–3 between 8 and 30 K. The experimental data was fitted by the expressions T ₁ ^–1 =35 T+0.5 T ³ and T ₁ ^–1 =35T+0.1 T ^3.6 for crystals of nominal gadolinium concentrations 0.005 wt % and 0.05 wt % respectively. The difference between the observed dependence and the T ^–5 variation predicted in the Raman region for an S-state ion in a perfect lattice is attributed to defects. Measurements in the -plane at 4.2 K showed that T ₁ was anisotropic with a maximum value at =25° about three times greater than the minimum value obtained at =55°. The angular positions at which these features occur show a remarkable coincidence with the acoustic axes of symmetry of the crystal, which have recently been determined by ultrasonic methods.     

Simultaneous Measurements of Thermal,Electrical, and Acoustic Properties of BaTiO<Subscript>3</Subscript> – New Feature of 403 K Phase Transition

In order to obtain useful information on the transient process of phase transitions in ferroelectrics by various methods including calorimetry, the “mK-stabilized cell” of a small size has been developed. It is based on the heat flux differential scanning calorimeter (DSC) and has a temperature stability better than 1 mK. The “cell” can be used to change the temperature under nearly quasi-static conditions by an infinitesimally small rate not only on heating but also on cooling while passing through the transition points. It enables simultaneous measurements of endothermic heat and exothermic heat along with dielectric constants, displacement currents, etc. with a high degree of temperature resolution. X-ray diffraction measurements for sensing thermal anomalies are also possible by a minor modification of the “cell.” Precise and simultaneous measurements of thermal, electrical, and acoustic properties were carried out at the 403 K phase transition in BaTiO₃ single crystal grown by the top-seeded solution growth method. It has been clarified that the exothermic heat at the transition on cooling has more useful information than the endothermic heat on heating; in the cooling process two thermal anomalies occur separately at T₁ and T₂ although the transition is in a narrow temperature range. It is recognized from other methods that the nature of the transition on cooling is not of a single but of multiple steps. Resonant ultrasonic measurements using the “cell” were carried out, after developing a new excitation method. The sample does not have a simple softening approaching the transition point on cooling but has different elastic moduli for the two thermal anomalies at T₁ and T₂. The dielectric constant also has an intermediate constant value between T₁ and T₂. The crystal structure in the room temperature phase below the transition point has been determined by X-ray diffraction. In this region, tetragonal and monoclinic structures coordinating with each other exist. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Influence of a Weak Magnetic Field on Photoconductivity Spectrum of C60 Single Crystal

Abstract

Comparative research of the excitation photoconductivity spectra (quantum light energy 2–5 eV) of C₆₀ single crystal in and out of magnetic field at the temperature T = 250–350 K has been carried. The spectral evolution at this temperature range is described. It is shown that the spectra changes abrupt at temperature T ₁ ～ 260 K and T ₂ ～ 315 K. An increase in the photoconductivity up to 15% was observed in the magnetic field (B = 0.4 T) within the photon energy range 2.5–4.5 eV. Local photoconductivity peak's appearances in the magnetic field have been proven that the charge transfer excitons take part in a photoconductivity.     

Synthesis and transition temperatures of the superconducting Chevrel-phase system CuxMo6S6I2 with x=0 to 1.2

We report the synthesis, lattice parameters, and superconducting transition temperatures of the Chevrel-phase system Cu_xMo₆S₆I₂, (x=0–1.2). The diffusion of copper into Mo₆S₆I₂ is carried out at 470°C and leads to a depression of the superconducting transition temperature from T_c=13.5 K for x=0 to T_c=4.8 K for x=1.2.     

The T-P-H effects on the magnetoresistance of La0.7Ca0.3Mn1.0O3−δ ceramics and films

Effects of the temperature (T=77–325 K), high hydrostatic pressures (P=0–2.1 GPa), and magnetic fields (H=0–8 kOe) on the electric resistance (R) and magnetoresistance (ΔR/R ₀) were studied in La_0.7Ca_0.3Mn_1.0O_3−δ based ceramics and single crystal films. A significant difference between the magnetoresistance peak temperatures (T _P) observed in the ceramic and film samples is explained by their different deviations from the stoichiometry with respect to oxygen, that is, by a greater concentration of anion vacancies in the film perovskite structure. An increase in the magnetic field strength H and the pressure P leads to a decrease in the electric resistance R. The magnetoresistance grows with increasing field strength H and decreasing pressure P. A growth in the hydrostatic pressure leads to an increase in the T _P value by 12 K for the ceramics and by 40 K for the films.     

Photoinduced Quasiparticle Relaxation Dynamics in Near-optimally Doped SmFeAsO0.8F0.2 Single Crystals

We investigate the quasiparticle relaxation and low-energy electronic structure in a near-optimally doped pnictide superconductor with T _c=49.5 K by means of femtosecond spectroscopy. Multiple relaxation processes are evident, with distinct superconducting state dynamics and a clear “pseudogap”-like feature with an onset above 180 K indicating the existence of a temperature-independent gap of magnitude Δ _PG=61±9 meV above and below T _c. The fluence and temperature dependence of the superconducting state dynamics shows similar behaviour to the cuprate superconductors with the superconducting-condensate vaporization energy of E _va/k _B≈1.5 K per Fe ion.