Electronic specific heat of La2−xSrxCu1−yMyO4(M=Ga,Zn): Impurity effects on a D-wave superconductor

The electronic specific heat C_el was studied in Ga- and Zn-doped La_2–xSr_xCuO₄ (0.16x0.22) at T10K. Partial substitution of Ga or Zn for Cu suppresses T_c and revives the T-linear electronic specific heat, T, markedly. The (y)/n vs T_c/T_c0 relation for Zn-doped samples with x0.2 is in good agreement with the theoretical one for resonant impurity scattering in a d-wave superconductor, while those for Ga-doped samples and for Zn-doped samples with x 0.2 deviate slightly from the theoretical curve. The deviation will be discussed in relation to changes in the magnetic properties of 3d electrons.     

Enhanced Phonon Scattering Below T c Caused by Zn and Ni Substitution in La1.85Sr0.15CuO4

The thermal conductivity has been measured for La _2-X Sr _X CuO ₄ and La _1.85 Sr _0.15 Cu _0.98 M _0.02 O ₄ (M; Zn, Ni). The characteristic enhancement in below T _c was confirmed for La _1.85 Sr _0.15 CuO ₄ . The phonon scattering mechanisms were analyzed in detail and the electron-phonon coupling constant was estimated to be 0.05. The enhancement becomes indetectable on Zn or Ni substitution for Cu, which indicated survival of the phonon scattering by charge carriers down to lower temperatures as a result of reduced T _c .     

Effects of Substituting Ni and Zn for Cu in the Electron Doped Sr0.9La0.1CuO2 Superconductor

We used X-ray diffraction, energy dispersive spectroscopy and low field magnetization to characterize Sr_0.9La_0.1Cu_1–x R _x O₂ samples for R=Zn and Ni. We found that both Ni and Zn are substituted at the Cu-site. The value of T _c was almost independent of composition until x0.03 for R=Zn while it was reduced to 30 K at x0.02 for R=Ni. This trend of T _c reduction is very similar to the pair breaking-effect in conventional superconductors and electron-doped (Nd, Ce)₂CuO_4– but the rate of reduction of T _c for R=Ni is larger in Sr_0.9La_0.1CuO₂ than in (Nd, Ce)₂CuO_4–. These results are discussed along with the recent experimental observation of gap symmetry in electron-doped superconductors.     

Temperature dependence of surface impedance Z(T, ω) and mean free paths I(T) of YBCO-superconductors

The surface impedance Z(T,) at 10 and 145 GHz and between 4 and 300 K is obtained experimentally. Z(T_aTT_c) is quantitatively fitted by the BCS theory with a mean free path I(T) increasing rapidely below T_c. This I-increase in the frame of the BCS-theory is limited at T_a by inelastic surface scattering at weak or strong links, e.g., by twin boundaries in distances a_TW which dominates scattering for a_Tw2I(T_a). Below T_a the enforced energy transfer from YBCO-crystallites to weak links may enhance Z(Ta) until at T T* the weak link surface impedance dominates Z_res(T相似文献   

Surface impedance of epitaxial YBCO thin films — Comparison of experimental data with a d-wave theory of superconductivity

The surface impedance of two 350nm thick YBCO films was measured for temperatures between 4.2K and 150K in a copper cavity at 87 GHz. Both films, one grown by electron beam evaporation on MgO, the other one by high oxygen-pressure dc sputtering from a stoichiometric target on LaAlO₃, provide critical temperatures of about 91K, low residual surface resistances of R_s(4.2K) < 1m and low specific resistivities in the normal state of (100K) < 85cm. The experimental data obtained on these two films are compared to a d-wave model of superconductivity which incorporates elastic and inelastic scattering. Good agreement between theory and the experimental results for both the surface resistance and the penetration depth in the whole temperature range is achieved for scattering phase shifts near 0.4 and order parameter amplitudes in the range of 2₀(0)/k_BT_c = 6.0 – 7.5 without subtracting an extrinsic residual surface resistance.     

Shear viscosity of liquid4He and3He-4He mixtures,especially near the superfluid transition

High-resolution measurements of are reported for liquid⁴He and³He-⁴He mixtures at saturated vapor pressures between 1.2 and 4.2 K with particular emphasis on the superfluid transition. Here is the mass density, the shear viscosity, and in the superfluid phase both and are the contributions from the normal component of the fluid ( _n and _n ). The experiments were performed with a torsional oscillator operating at 151 Hz. The mole fraction X of³He in the mixtures ranged from 0.03 to 0.65. New data for the total density and data for _n by various authors led to the calculation of . For⁴He, the results for are compared with published ones, both in the normal and superfluid phases, and also with predictions in the normal phase both over a broad range and close to T. The behavior of and of in mixtures if presented. The sloped/dT near T and its change at the superfluid transition are found to decrease with increasing³He concentration. Measurements at one temperature of versus pressure indicate a decreasing dependence of on molar volume asX(³He) increases. Comparison of at T, the minimum of _n in the superfluid phase and the temperature of this minimum is made with previous measurements. Thermal conductivity measurements in the mixtures, carried out simultaneously with those of , revealed no difference in the recorded superfluid transition, contrary to earlier work. In the appendices, we present data from new measurements of the total density for the same mixtures used in viscosity experiments. Furthermore, we discuss the data for _n determined for⁴He and for³He-⁴He mixtures, and which are used in the analysis of the data.     

Functional derivative ofT c with α2(ω)F(ω) for weak coupling anisotropic superconductors

Approximate analytic calculations of the functional derivative ofT _c with respect to ²()F() for anisotropic superconductors are presented, with the primary purpose of identifying the determining material parameters. The square-well model for the phonon-mediated electron-electron interaction, the weak coupling limit ( _c /2T _c 1), and separable anisotropy are used. The general behavior of T _c /²()F() is the same as that found in numerical calculations for the case of small anisotropy, a ²/(–*)1; the regime of * is also discussed.     

A dilatometric method to measure the thermal diffusivity of nonmetallic liquids

A new method to measure the thermal diffusivity of liquids is presented. It requires determination of the time dependence of the thermal expansion of the liquid when it is subjected to a heat source at the top of the cell containing the liquid. The high accuracy of the method (about 3%) is due to an essential reduction of convective currents and also to the absence of temperature detectors, which generally introduce unwanted perturbations on the thermal Field.Nomenclature Thermal conductivity - c Specific heat - Density - c = specific heat x density - h Newton coefficient - Thermal diffusivity - T, 0 Temperature - tV Electric signal - Calibration coefficient - _exp, _th Volume change of the liquid     

The role of Y/Ba on the formation of high-Tc superconducting phase in Y-based system

The high-T _c phase in the Y-Ba-Cu-O system was prepared and studied using specimens with various Y/Ba ratios. The d.c. conductivity was measured, and a theoretical relation between the normalized resistance and temperature at different polaron stabilization energies was taken into account to study the behavior of superconductor materials in the normal state. The 0.5 [Y/Ba] 1 and 3 [Y+Ba] 4 were found to be the most preferential ranges for the formation of a single superconducting phase.     

Superconductivity in bulk A15 Nb3Si under pressure

We have determined the effect of hydrostatic pressureP on the superconducting transition temperatureT _c of bulk, A15 Nb₃Si. For 0P20 kbar (2 GPa),T _c decrease linearly with increasing pressure at a rate T _c/P=–2.67×10^–5 K/bar. From an estimate of T _c/P obtained using recent band structure calculations for the density-of-electronic-states change as a function of lattice parameter in Nb₃Si, we conclude that the pressure dependence of the electron-phonon interaction primarily determines T _c/P.Work performed under the auspices of the U.S. DOE.     

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.     

Pressure effect on anomalous suppression ofT c around x=1/8 in La2−xBaxCuO4 and La1.8−xx Nd0.2BaxCuO4

In La_2–xBa_xCuO₄ (LBCO) and La_1.8–xNd_0.2Ba_xCuO₄ (LNBCO), the low-temperature structural change at T₁ (=60–110K), resulting from tilt of the CuO₆ octahedra, is easily suppressed at high pressures. However, the structural change of LNBCO (x=0.125) remains up to 2GPa though it jumps from T₁(110K) to T* (60K), where the electric resistivity begins to upturn, at 0-1.5GPa. The upturn of is also suppressed at high pressures, and the superconducting critical temperature T_c is fully restored except in a very narrow x-range around x=0.125. The T_c for x 0.125 is appreciably lower at 2GPa than those for other Ba-contents.     

Experimental study of the boundary between single-phase convection and boiling in underheated cryogenic liquids

The effect of pressure and underheating on the position of the boundary between heat-transfer regimes in liquid helium and hydrogen is investigated.Notation q heat flux - p pressure - =T_s–T underheating - T_s saturation temperature - T temperature of liquid - T=T_wa – T T_s=T_wa – T_s - T_wa temperature of heat-emitting surface - A,a, B, b, C constants - m, n indices - Nu Nusselt number - Ra Rayleigh number - thermal conductivity - coefficient of cubical expansion - kinematic viscosity - g acceleration - standard deviation Indices 01 conditions of convection-boiling transition - 02 conditions of boiling-convection transition Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 42, No. 1, pp. 5–11, January, 1982.     

Enhancement of superconductivity in Pb-Bi-Sn and Pb-Bi-In alloy filaments produced by glass-coated melt spinning

The enhancement ofT _c in Pb-Bi-Sn and Pb-Bi-In system alloy filaments produced by glass-coated melt spinning was investigated as a means of producing a new type of superconducting filament with highT _c. Long filaments of Pb-Bi-Sn alloy withT _c higher than 10 K and Pb-Bi-In alloy withT _c higher than 9 K were obtained from the molten state at a temperature of 1500 K with a winding speed of 2.63 m sec^–1. For example, a Pb₄₅Bi₃₅Sn₂₀ filament withT _c of 10.1 K was 15 m in diameter and polycrystalline with a grain size of 100 nm. The structure of the filament was a mixture of , tin and bismuth phases and a metastable phase of mixed structure of bismuth and supersaturated solid solution of tin in -phase was detected. TheT _c of the filament decreased on heat treatment. A metastable phase of mixed structure of bismuth and -phase was also detected for a Pb₄₅Bi₄₅In₁₀ filament withT _c of 9.3 K. As the metastable phase for the Pb-Bi-In filament was more unstable than that for the Pb-Bi-Sn filament, theT _c of the filament was drastically decreased by heat treatment. The metastable phase was considered to play an important role in the enhancement ofT _c for Pb-Bi-Sn and Pb-Bi-In alloy filaments.     

Incommensurate Dynamical Properties of Optimally Doped YBa2Cu3O6.9

The resonance and incommensurate peaks in optimally doped high-T _c superconductor YBa₂Cu₃O_6.9 are observed by means of TOF neutron scattering technique. Obtained overall dynamical properties in a wide Q-E space show distinct incommensurate structure around antiferromagnetic (AF) zone center (, ). The incommensurability of =0.1 at 36 meV corresponds to the previous triple-axis neutron measurement. Variation of temperature dependence suggests the spectral weight of antiferromagnetic fluctuation observed around 25 meV at (, ) shifts to resonance peak position below T _c. Moreover obtained results with E _i=80 meV neutron energy indicate new feature of dynamical incommensurate structure at higher frequency region up to 53 meV.     

Phonon anomalies in La2CuO4 + δ

The phonon dispersion of oxygen doped La₂CuO_{4 +} has been investigated by inelastic neutron scattering on a large single crystal with 0.02 (T_c = 31 K). Most of the phonons were found to be very similar to those of La_1.9Sr_0.1CuO₄. However, in measurements of a phonon branch which exhibits a pronounced anomaly in (La, Sr)₂CuO₄ a two-peak structure was observed: one series of peaks corresponds to the phonon frequencies of undoped La₂CuO₄, the other one to that of La_2–xSr_xCuO₄ with x 0.1. This finding is presumably associated with a phase separation into domains with low and high , respectively.     

Electron-phonon coupling as a mechanism for high-temperature superconductivity

We have solved the Eliashberg gap equations which are valid for arbitrary phonon frequency, (_ph), electron-phonon coupling constant, (), and screened Coulomb interaction, ( ^*). We have used values of (_ph),, and ( ^*) appropriate to the cuprate superconductors, and calculated the density of states, the pair potential., and the value of the gap at T=0 K. Using the linearized Eliashberg equations in the matrix representation, we have calculatedT _c and 2/k T _c . We have found that we can account for the highT _c 's in the cuprates with reasonable values for, ^*, and _ph.     

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.     

Transport properties of Fermi fluids at finite temperatures

A method based on Landau's Fermi-fluid theory is presented for calculating the temperature-dependent viscosity and thermal conductivity of a Fermi fluid. It is argued that both O(T) and O(T² ln T) terms appear in the temperature series expansions for T ² and T. Exact expressions for the zero-temperature limit of T ² and T are derived for the dilute hard-sphere Fermi gas (DHSFG). The leading finite-temperature corrections to these quantities due to the s dependence of the quasiparticle scattering amplitude are also derived for the DHFSG (s is a dimensionless ratio of energy transfer to momentum transfer). The results are compared with previous expressions of Emery and of Dy and Pethick; some discrepancies are noted and discussed.Based in part on a Ph.D. thesis by J.C.R. submitted to the University of Colorado.     

Saturation curve equations for normal and heavy water

Compact and precise equations are obtained for the saturation curves of normal and heavy water.Notation p saturated vapor pressure - p_c critical pressure - T absolute temperature - T_c critical temperature - =T/T_c dimensionless temperature - =1 – ; r heat of vaporization - c _p ⁰ isobaric heat capacity of vapor in ideal gas state - c_s liquid heat capacity along saturation curve - v specific volume of vapor - a _i coefficients of Eq. (1) Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 5, pp. 894–897, May, 1981.