Effect of phase fluctuations upon penetration depth

The temperature dependence of the static penetration (T) has been used as a guide to the nature of the superconducting state in high-T _c materials. It has been argued that an algebraic temperature dependence in the ratio (T)/(0) [(T) — (0)]/(0) at low temperature is evidence for d-wave pairing. This paper examines the effect of superconducting phase fluctuations upon (T) and finds an algebraic dependence over a broad range of temperature.     

Pair and Quasiparticle States of YBa2Cu3O7?x Films Deduced from the Surface Impedance and a Comparison with Nb3Sn

Surface impedance data at 19 and 87 GHz of high-quality epitaxial YBCO films on different substrates are compared with data for Nb₃Sn films on sapphire in terms of pair and quasiparticle (qp) transport. Surface resistance R _s and penetration depth of YBCO are strongly affected by temperature dependent qp scattering, which is depressed in films with enhanced lattice strain. All films showed a comparable residual resistance R _res(19 GHz)90 constituting a qp reservoir which is likely to be caused by the electronic configuration and by impurities. Subtracting R _res from R _s (T) revealed activated behavior with a reduced energy gap ₀/k _B T _c0.9 for a film on sapphire, but power-law behavior for the other films. The penetration depth did not reveal power-law dependences at T0.5 ·T _c, but was consistent with a reduced energy gap of 0.45 for a film on MgO. The increase of (T) at T0.5 · T _c was related to qp scattering, which also caused an extremal conductivity ₁(T). A shoulder in (T) at T=(0.6–0.7) · T _c confirmed evidence for the existence of two superconducting bands. The magnetic-field induced recovery of (B) of various YBCO films hinted for an important role of magnetic scattering. The results are in contradiction to a d-wave symmetry of the order parameter, at least for the chain band.     

Critical Behavior of 2 − Ions in Argon Gas

We measured the drift mobility of ₂ ^– ions in argon gas close to the critical point for (1.005 < T/T _c < 1.04) above T _c 150.7 K in the density range (0.025 <N/N _c< 1.733) around the bulk critical density N _c = 8.08 atoms . nm^–3. The density-normalized zero-field mobility ₀ N of the ions shows a deep minimum as a function of the gas density N as T T _c ⁺ This anomalous reduction of ₀ N occurs at a density N _m 0.76N _c. We believe that this behavior is due to the strong electrostriction exerted by the ion on the highly compressible gas. By introducing suitable contributions to the effective ion radius R due to the large gas compressibility and taking into account short-range local density and viscosity augmentation due to electrostriction, the hydrodynamic Stokes formula ₀ = e/6R, where is the gas viscosity, is in good agreement with the experimental data.     

Viscosity of steam in the critical region

The shear viscosity of fluids exhibits an anomalous enhancement in the close vicinity of the critical point. A detailed experimental study of the viscosity of steam in the critical region has been reported by Rivkin and collaborators. A reanalysis of the experimental data indicates that the behavior of the viscosity of steam near the critical point is similar to that observed for other fluids near the critical point. An interpolating equation for the viscosity of water and steam is presented that incorporates the critical viscosity enhancement.Nomenclature a critical region equation of state parameter - a _k coefficients in equation for ₀ - a _ij coefficients in equation for ¯ - b critical region equation of state parameter - c _p specific heat at constant pressure - c _v specific heat at constant volume - k critical region equation of state parameter - k _B Boltzmann constant - P pressure - P _r 22.115 MPa - P ^* P/P _r - P _c critical pressure - P _i coefficients in critical region equation of state - R~P (P-P _c )/P _c - q parameter in equation for critical viscosity enhancement - r parametric variable in critical region equation of state - T temperature in K (IPTS-48) - T _r 647.27 K - T ^* T/T _r - T _c critical temperature - T (T–T _c )/T _c - V volume - critical exponent of specific heat - critical exponent of coexistence curve - critical exponent of compressibility - critical exponent of chemical potential at T=T _c - dynamic viscosity - ₀ lim ₀ - ¯ normal viscosity - critical viscosity enhancement - ¯ thermal conductivity - normal thermal conductivity - critical thermal conductivity enhancement - parametric variable in critical region equation of state - correlation length - ₀ correlation length amplitude above T _c at = _c - critical exponent of correlation length - density - _r 317.763 kg/m³ - ^* / _r - _c critical density - (– _c )/ _c - _p estimated error of pressure - _T estimated error of temperature - estimated error of viscosity - exponent of critical viscosity enhancement - _t (/P) _T symmetrized compressibility - _T ^* _T P _r / _r ² - _{t t} P _c / _c ²     

A generalized scaled equation of state for n-alkanes (methane to n-nonane) in the critical region

A generalized scaled equation of state has been developed to calculate thermodynamic properties of n-alkanes from methane (CH₄) to n-nonane (C₉H₂₀) in the critical region. The equation is valid in the reduced density range 0.7 _c1.3 at T=T _c and up to 1.2T _c at = _c.     

Efficiency of heating methods for polymer fibers with formation of adhesion compounds

Efficiency of contact and contactless methods of heating of a fiber polymer material upon formation of adhesion compounds between its layers is analyzed. The advisability of contact heating in the preliminary stage and contactless heating in the final stage of formation of compounds is shown.Notation e spectral density of radiation - T temperature - a spectral absorption coefficient - radiation density within the spectral region ₂–₁, and 0–, respectively - wavelength - _max wavelength of the maximum of black-body radiation - E _0– total density of black-body radiation - the Boltzmann constant - E _ef effective radiation density - E radiation density absorbed by the material - X dimensionless coordinate - r radius of the filament of the lamp - h distance from the lamp to the surface of the material - S area - P power absorbed by the portion of the material - T _m temperature of the onset of variations in the appearance, shrinking, and melting of the material - Q heat - T ₀ initial temperature of heating - c specific heat capacity of the polymer - m mass of the material being heated - time of heating toT _m - y coordinate - R thickness - T _s stabilized temperature - Fo Fourier number - thermal conductivity of the material Institute of Mechanics of Metal-Polymer Systems, Academy of Sciences of Belarus, Gomel', Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 3, May–June, 1995.     

Anisotropic Electrical Transport in MgB2 Single Crystals

We report on study of transport properties of MgB₂ single crystals. The normal state resistivity has been found to be anisotropic with resistivity ratio _c / _ab =3.5. In agreement with the results of band structure calculations the normal state Hall effect measurements with H//ab-planes and H//c-axis show two type carrier behavior. Below T _c, the in-plane as well as the out-of-plane Hall resistivity, _xy and _zx , display no sign change anomaly. Furthermore, both _xy and _zx have been found to scale with corresponding longitudinal resistivity with the same exponent =1.5.     

Preparation and analysis of high-T c Nb-Ge films

The dependence of T_c on sputtering conditions, chemical composition, and phase structure for Nb-Ge films has been studied. It was found that T_c varied with composition, but was not dependent on exact stoichiometry of the film; the Nb/Ge ratio was <3 for very high T_c 23 K films and 4 for films with T_c 20 K. X-ray results showed that the films with T_c 23 K contained a certain amount of tetragonal Nb₅Ge₃ with a lattice parameter of A15 phase 5.14 Å. The depth profile of very high T_c films showed no increase of oxygen concentration at the film-substrate interface; no correlation of high T_c and impurities was found.     

Magnetic penetration depth measurements with the microstrip resonator technique

The microstrip resonator technique is a convenient way to sensitively measure the temperature dependence of the magnetic penetration depth (T) in superconducting thin films. Because the method relies on measuring the resonant frequency of a high-Q transmission line resonator at microwave frequencies, one can very precisely measure small changes in (T). This technique is applied to studying the low-temperature dependence of (T), since that is in principle a measure of the low-lying pair-breaking excitations of the superconductor. We find that the penetration depth in niobium films is consistent with the predictions of weak coupled BCS theory. The low-temperature dependence of (T) inc-axis YBa₂Cu₃O_7– films can be interpreted as either a weak exponential or as a power law. In addition, the measured value of (0) is found to be strongly dependent on the form of the temperature dependence for (T) used in fitting the data. Best fits over the entire temperature range are obtained with a BCS temperature dependence having values for 2(0)/k _BT_c strictly less than 3.5, consistent with our measurements of the temperature dependence of (T) at low temperatures in YBa₂Cu₃O_7– .     

Spontaneous nuclear ferromagnetic ordering of in nuclei in AuIn2 Part I. Nuclear specific heat and nuclear susceptibility

We have measured the nuclear specific heat C_n and nuclear susceptibility _n of In nuclei (I=9/2, =5.5 _n) in the cubic intermetallic compound AuIn₂ (Korringa constant =0.11 Ksec) in the normal conducting state at 30K10mK and 2mTB115 mT. Our data show a positive nuclear Weiss temperature =+ 43 K and that the In nuclei undergo a nuclear ferromagnetic transition at T_c=35 K. The In nuclei experience an internal field of about 10 mT (obtained from C_n at T>T_c ). The nuclear ordering temperature T_c and the internal field increase with applied magnetic field. From the data we deduce exchange constants for the investigated system. The critical entropy reduction S(T_c)/S_max=8.6% and critical enthalpy E=0.28 RT_c are in reasonable agreement with the measured ordering temperature T_c,applying the Heisenberg model for a simple cubic I=9/2 spin system. The nuclear spin relaxation time calculated from the real and imaginary parts of _n is 10 msec at T>50 K, but drops to <1msec at T_c.This is the first observation of a spontaneous nuclear magnetic ordering transition in a not-hyperfine-enhanced metal at thermal equilibrium, i.e. at T _nuclear =T _electron .     

Transport properties and determination of the basic parameters of superconductors with overlapping bands

The thermoconductivity and ultrasonic attenuation in non-single-band super-conductors are investigated. It is proved that the ratio ₂/₁ atT T _c is equal to that atT=0. It is also shown that ultrasonic measurements and data on the phonon thermoconductivity can be used very effectively for definition of the basic parameters of the superconductors with overlapping bands.     

Dynamically enhanced supercurrents in granular superconducting Y-Ba-Cu-O thin films

Thin superconducting films of Y-Ba-Cu-O withT _c089 K have been prepared by the aerosol deposition method. The voltage current characteristic of microbridges (typically 10×10µm²) made from the finely granular films, which behave like a network of boundary Josephson junctions, is highly nonlinear and shows besides thermal effects evidence for dynamic enhancement of the supercurrent. Our experimental data are in qualitative agreement with a theory developed for conventional superconductors by Aslamazov and Larkin and also by Tinkham. A value for the electron phonon scattering time has been derived: _E 15 ps.     

Intensity of critical opalescence of helium-4

We present measurements of the critical opalescence of helium-4. The results are analyzed by the Einstein and Ornstein-Zernike theory and the power laws. We obtain ==1.17±0.02, ==0.62±0.1,/=4.5±0.3,P _c =1706.008 mm Hg, andT _c =5,189.863 mK (T ₅₈ ). The critical behavior of helium-4 is almost the same as that of classical fluids and the influence of the quantum nature of helium-4 is not as evident as has been claimed.     

The Impact of Nuclear Magnetism on Superconductivity in a Metal with Nuclear Electric Quadrupole Splitting: Indium

We report the first investigation of the impact of nuclear magnetism on superconductivity in the tetragonal metal indium. We have measured the superconducting critical field B_c(T) and in its vicinity the nuclear magnetic heat capacity at ultralow temperatures, 170 KT200 mK. We compare the measured quantities with calculations which consider the nuclear magnetic Zeeman and the dominating nuclear electric quadrupole interaction in indium. The heat capacity data support the occurence of a positive sign of the electrical field gradient at nuclear sites and in consequence the existence of a nuclear low spin ground state. Surprisingly, at lowest investigated temperatures, 170 KT1 mK, the reduction of the critical field B_c(T) clearly exceeds the size of the calculated magnetization ₀ M(B_c, T) which is limited by the nuclear low spin ground state. In all other materials the interplay of nuclear magnetism and superconductivity has been studied so far (Al, AuAl ₂ , AuIn ₂ , Rh, and Sn), the bare nuclear magnetization appeared as an upper limit of the reduction of the critical field.     

Superconductivity and disorder-driven metal-insulator transition in quench-condensed tin films

We have studied the effects of disorder in a series of Sn films quench-condensed onto cryogenically cooled substrates. We find a simple scaling of the conductivity with film thickness ~ (d - d _c ), with 0.85, and a metal-insulator transition near d _c 40–50 ». The superconducting transition temperatures depend linearly on 1/d, provided d is not too small, and the films exhibit strong annealing effects. These features are explained using a model in which grain boundary scattering dominates intragrain scattering.     

Nonequilibrium contributions to the electric response of dirty superconductors

Comparison of the measured microwave response in the presence of a dc current in narrow films of tin nearT _c with the results from microscopic theory shows good agreement. Nonequilibrium effects become important when the frequency becomes of the order of the inverse inelastic relaxation time _E ^–1 (k _B T _c ³ / _D ² ) (_D is the Debye temperature) even if remains well below the gap frequency ₀(T)/.     

Influence of interstitial oxygen on the superconducting transition temperature of vanadium

The critical temperatureT _c and residual resistivity ₀ were measured in homogeneously oxidized thin vanadium foils. The per at % oxygen induced variations are respectively T _c =–1.2K and ₀=53 n-m. TheT _c data are used to calculate the electron-phonon coupling constant, which decreases 5% per at % of oxygen.     

Flow of a heated ferrofluid over a stretching sheet in the presence of a magnetic dipole

Summary The flow of a viscous ferrofluid over a stretching sheet in the presence of a magnetic dipole is considered. The fluid momentum and thermal energy equations are fomulated as a five-parameter problem, and the influence of the magneto-thermomechanical coupling is explored numerically. It is concluded that the primary effect of the magnetic field is to decelerate the fluid motion as compared to the hydrodynamic case, thereby increasing the skin friction and reducing the heat transfer rate at the sheet.Nomenclature a distance - c constant - c _p specific heat at constant pressure - C _f wall friction coefficient - e 2.71828 ... - f dimensionless stream function - H magnetic field - k thermal conductivity - K constant - M magnetization - Nu _x local Nusselt number - p pressure - P dimensionless pressure - Pr Prandtl number, c _p/k - Re _x local Reynolds number, cx ²/ - T temperature - u velocity component along the sheet - v velocity component normal to the sheet - x coordinate along the sheet - y coordinate normal to the sheet - dimensionless distance - ferrohydrodynamic interaction parameter - constant - dimensionless Curie temperature - dimensionless coordinate - dimensionless temperature - viscous dissipation parameter - dynamic viscosity - ₀ permeability - dimensionless coordinate - density - shear stress - magnetic potential - stream function     

Nonmonotonic Temperature Dependence of the Thermal Hall Angle of a YBa2Cu3O6.95 Single Crystal

We have performed high-resolution measurements of the magnetic field (0 TB9 T) and temperature (10 KT<140 K) dependence of the longitudinal and transverse Hall thermal conductivity of a twinned YBa₂Cu₃O_6.95 single crystal. We have used and compared two recently published methods to extract the thermal Hall angle _H(T, B). Our results indicate that cot(_H) varies quite accurately as T⁴ in the intermediate temperature range 0.3c. It shows a well defined minimum at T_m20 K which resembles that observed in the c-axis microwave conductivity. The electronic part of the longitudinal and the transverse thermal conductivity show the scaling behavior for transport properties predicted for d-wave superconductors in the temperature range 18 KT30 K.     

Temperature dependence of resistivity for thermally diffused V3Si multilayer films

Results concerning V₃Si films produced by a simple annealed multilayer technique are reported together with X-ray diffraction patterns, Auger spectroscopy, and Rutherford backscattering analysis. Low-temperature electrical resistivity measurements are discussed. It is found that the V₃Si films exhibit aT ² dependence in the temperature rangeT _c T23 K and aT ^2.6 dependence in the rangeT _c T40 K. The normal-state resistivity in the whole temperature range (T _c T600 K) is analyzed in the framework of Cote-Meisel theory. Consistent values of the saturation resistivity _m and of the Debye temperature are obtained by fitting the experimental data with the Cote-Meisel expression for (T).