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.     

T c enhancement in La2−x Sr x CuO4 under anisotropic pressure

We have measured _c (T) for La _2–x Sr _x CuO ₄ (LSCO) with x=0.15 under anisotropic pressures. The c-axis compression, which decreases _c (T), reduces T _c drastically, whereas the c-axis stretch, which increases _c (T), enhances T _c from 38K at the ambient pressure to 51.6K at 8GPa. The T _c = 51.6K is the highest record so far reported for LSCO. The variation of T _c is well scaled with variations of _c . We deduce that the enhancement of T _c in LSCO is mainly caused by the suppression of interlayer coupling, though two dimensionality is generally known as a destructive factor for conventional superconductivity.     

Thermal conductivity of methane

This paper reports thermal conductivity data for methane measured in the temperature range 120–400 K and pressure range 25–700 bar with a maximum uncertainty of ± 1%. A simple correlation of these data accurate to within about 3% is obtained and used to prepare a table of recommended values.Nomenclature a _k ,b _ij ,b _k Parameters of the regression model, k= 0 to n; i =0 to m; j =0 to n - P Pressure (MPa or bar) - Q _kl Heat flux per unit length (mW · m^–1) - t time (s) - T Temperature (K) - T _cr Critical temperature (K) - T _r reduced temperature (= T/T _cr) - T _w Temperature rise of wire between times t ₁ and t ₂ (deg K) - T ^* Reduced temperature difference (TT _cr)/T _cr - Thermal conductivity (mW · m^–1 · K^–1) - ₁ Thermal conductivity at 1 bar (mW · m^–1 · K^–1) - _bg Background thermal conductivity (mW · m^–1 · K^–1) - _cr Anomalous thermal conductivity (mW · m^–1 · K^–1) - _e Excess thermal conductivity (mW · m^–1 · K^–1) - Density (g · cm^–3) - _cr Critical density (g · cm^–3) - _r Reduced density (= / _cr) - ^* Reduced density difference (– _cr )/ _cr      

Application of Neural Networks to a Predictive Extended Corresponding States Model for Pure Halocarbon Thermodynamics

The extended corresponding states (ECS) model has been extensively studied for representing the thermodynamic surface of pure fluids and mixtures in the a ^R (, T) form, and the most advanced version is currently the one for hydrofluorocarbons, but the shape factors (, T) and (, T) have yet to be determined as analytical functions for the whole PT surface of a pure fluid. For a sample of pure halocarbons, this work aims to solve the fundamental problem of determining the individual shape functions over the entire PT domain through an innovative predictive procedure using a density model requiring only a single saturated liquid density input. An original algorithm using artificial neural networks enables the determination of the (, T) and (, T) functions from a priori knowledge of their functional forms. The proposed algorithm focuses on the determination of the residual Helmholtz energy a ^R (, T) for each fluid, subsequently allowing any other thermodynamic residual function to be calculated through the first and second derivatives of temperature and density. For each fluid studied, the model has been validated for residual functions against the same functions coming from highly accurate dedicated equations of state. The prediction accuracies reach average absolute deviation values ranging from 0.3 to 7.8%, spanning from vapor and liquid regions to supercritical conditions, while the corresponding results of the conventional ECS method range from 0.54 to 20%.     

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.     

Anisotropy in the resistive superconducting transition under magnetic fields in single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8

Anisotropie properties of the single crystal Pb₂Sr₂Ho_0.5Ca_0.5Cu₃O₈ have been investigated by measuring the electrical resistivity in theab-plane _ab (H, ,T), which depends on the angle between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values of _ab (H, ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter (m _c ^* /m _ab ^* )^1/2 is estimated as 12–13, which is larger than that of YBa₂Cu₃O₇ and much smaller than that of Bi₂Sr₂CaCu₂O₈. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence of _ab (H,,T).     

Resistivity of Lightly Doped Ferromagnetic Semiconductors

In metallic magnets with a low carrier density, scattering from magnetic fluctuations above and near the transition temperature T _c provides a large contribution to the electrical resistance. Because the fluctuations can be suppressed by a magnetic field, a large negative magnetoresistance ensues. In a simple model, we find the low field magnetoresistance scales with the ratio of field induced magnetization m(H) to the saturation magnetization m _sat: /=((T, 0)–(T, H))/(T, 0)C(m/m _sat)². At very low carrier densities magnetic polarons should form in a range of temperatures above T _c. The CMR perovskite manganites cannot be explained without strong coupling of the magnetic order to lattice distortions (of the Jahn–Teller type) above T _c.     

Transport properties of oxygen-deficient YBa2Cu3O7?x thin films

We report on measurements of the in-plane resistivity and Hall coefficientR _H (Bc) of various oxygen-deficient epitaxial films of YBa₂Cu₃O_7–x in the normal state. The superconducting transition temperaturesT _c of the samples vary from 14 to 90 K. Both the resistivity and the Hall coefficient exhibit a strong dependence on the oxygen content and the temperature. Asx increases,T _c decreases continuously, while andR _H gradually increase in magnitude. Furthermore, also the characteristic linear dependences ofT andR _H T ^–1 of the highly doped compounds changes to a nonlinear behavior for the samples withT _c lower than 60 K. The unusual doping and temperature dependence ofR _H will be compared to the predictions of our calculations, based on a two-dimensional tight-binding model using the relaxation-time approximation. The model considers also the next-nearest-neighbor hopping, which strongly influences the predicted Hall coefficient. Additionally, the cotangent of the Hall angle cot( _H ) is discussed in the framework of the two-dimensional Luttinger liquid theory.We gratefully acknowledge financial support from the Bundesministerium fiir Forschung und Technologic.     

Anomalous Behaviour of PrNi2B2C Borocarbide

Electrical resistivity (T), specific heat C _p(T), magnetic susceptibility (T), and magnetization M(H) have been measured for polycrystalline PrNi ₂ B ₂ C. Maxima in C _p(T) and |d(T)/dT| dependencies were found at T _N 4.3 K connected with AFM ordering observed earlier by neutron diffraction. The Sommerfeld coefficient is at least 250 mJ/mol K ² . The values of T _N for PrNi ₂ B ₂ C and |dT _c /dx| for Y _1–x Pr _x Ni ₂ B ₂ C were found to be much larger than estimated by the de Gennes scaling from the Gd-based borocar-bides. These facts as well as anomalous decrease of (T) found below 20K are connected with moderate heavy fermion behaviour of antiferromagnetic PrNi ₂ B ₂ C.     

Investigation of UBe13 and other beryllium and uranium compounds in normal and superconducting states

The Hall voltageU _xy (H), magnetoresistance (H), magnetization in the superconducting stateM(H), and the second superconducting critical magnetic field of UBe₁₃ have been investigated. It is found thatU _xy (H) is not linear, but has a maximum nearH=70 kOe atT=1.9 K. When the temperature rises from 1.9 to 100 K, the Hall coefficientR(H0) decreases about 20 times from 2.5×10^–10 to 1.35×10^–11 -cm/Oe. The rise of the magnetic field causes a strong decrease of the electrical resistance: atT=1.7 K, (0)/(210kOe)5.8. From theH _c2 investigation for single crystals it can be seen that near T_c the anisotropy ofH _c2 is absent (it becomes visible only upon loweringT to 0.9T _c ). TheM is similar to the typical dependence for type II superconductors. The thermodynamic critical fieldH _c (0)1 kG, the Ginzburg-Landau parameter x 60, and the penetration depth 4.10^–5 cm. The effects of alloying a small quantity of iron and of neutron irradiation are investigated. The effects of alloying with other compounds of U and Be are also discussed. It is possible to suppose that UBe₁₃ has two groups of carriers, heavy and light, with the anomalous properties of UBe₁₃ caused by the specificity of the interaction between these two groups of carriers.     

Effect of Nuclear Field on Magnetotransport Quantum Oscillations in InSb1

In combining magneto-transport investigations with NMR (nuclear magnetic resonance) we measured the effect of the nuclear hyperfine field B_HF on quantum oscillations in the transverse magneto-resistivity _xx and in the Hall resistivity _xy of metallically doped n-InSb. Quantitative analysis of the B_HF-induced change in _xx demonstrates that this experiment allows to separate spin splitting phenomena in magneto-transport from effects due to the external magnetic field B. This is used to show that an oscillatory structure in R_H=_xy/B is directly related to a redistribution in the occupation of the two spin states in the lowest Landau level.     

Temperature dependence of electrical resistivity of deformed and undeformed V-rich V3Si single crystals

The electrical resistivity (T) of V-rich V₃Si single crystals (T _c-11.4 K) was measured from 4.2 to 300 K along the directions of [1 0 0] and [1 1 1] before and after plastic deformation at 1573 K. Anisotropy of (T) was observed although V₃Si has the cubic A15 structure. Plastic deformation does not affect the normal-state (T) behaviour but changes the normal-superconducting transition width T_c. At low temperatures (T _c<T 40 K), (T) varies approximately as T ⁿ where n-2.5 and this behaviour does not contradict the (0)- phase-diagram plot proposed by Gurvitch, where is the electron-phonon coupling constant and (0) is the residual resistivity.     

Velocity of second sound and superfluid density near the superfluid transition in3He-4He mixtures

Using superleak condenser transducers, the velocity of second soundU ₂ has been measured near the superfluid transition temperature T in³He-⁴He mixtures with molar concentrationsX of³He of 0.0, 0.038, 0.122, 0.297, and 0.440. We have obtained the superfluid density _s/ fromU ₂ on the basis of linearized two-fluid hydrodynamics. The results for _s/ are consistent with those obtained from the oscillating disk method, as expected from two-fluid hydrodynamics. The value of _s/ at eachX could be expressed by a single power law, _s/=k, where =1-T/R, with the experimental uncertainty. It is found that the exponent is independent of concentration forX0.44 within the experimental uncertainty. This concentration independence of is in agreement with the universality concept. From the conclusion that the values of are universal forX0.44, the concentration dependence of the superfluid component _s is expressed by an empirical equation _s(X, )=²_s(0, ). It is found that corresponds to the volume fraction of⁴He in the superfluid³He-⁴He mixture. The value of is in agreement with that obtained from the measurement of the molar volume by others.This paper is based on a thesis submitted to Tokyo University of Education in partial fulfillment of the requirements for the Ph.D. degree.     

Equation of State and Thermodynamic Properties of Pure D2O and D2O + H2O Mixtures in and Beyond the Critical Region

A parametric crossover model is adapted to represent the thermodynamic properties of pure D₂O in the extended critical region. The crossover equation of state for D₂O incorporates scaling laws asymptotically close to the critical point and is transformed into a regular classical expansion far from the critical point. An isomorphic generalization of the law of corresponding states is applied to the prediction of thermodynamic properties and the phase behavior of D₂O + H₂O mixtures over a wide region around the locus of vapor-liquid critical points. A comparison is made with experimental data for pure D₂O and for the D₂O + H₂O mixture. The equation of state yields a good representation of thermodynamic property data in the range of temperatures 0.8T _c(x)T1.5T _c(x) and densities 0.35_c(x)1.65_c(x).     

Energy Scales in the High-Tc Superconductor YBa2Cu3O6+x

The optical conductivity sum rule is used to examine the evolution of the spectral weight N() in both the normal and superconducting states of optimally and underdoped YBa₂Cu₃O_6+x along the a axis. Differences in N() above and below T _c allow the strength of the superconducting condensate _s to be determined. In the optimally-doped material, _s is fully formed at energies comparable to the full superconducting gap maximum (0.1 eV), while in the underdoped material the energy scale for convergence is considerably higher (0.6 eV). This difference is discussed in terms of normal-state properties.     

Thermodynamic Properties of Sulfur Hexafluoride

We present new vapor phase speed-of-sound data u(P, T), new Burnett density–pressure–temperature data (P, T), and a few vapor pressure measurements for sulfur hexafluoride (SF₆). The speed-of-sound data spanned the temperature range 230 KT460 K and reached maximum pressures that were the lesser of 1.5 MPa or 80% of the vapor pressure of SF₆. The Burnett (P, T) data were obtained on isochores spanning the density range 137 mol·m^–34380 mol·m^–3 and the temperature range 283 KT393 K. (The corresponding pressure range is 0.3 MPaP9.0 MPa.) The u(P, T) data below 1.5 MPa were correlated using a model hard-core, Lennard–Jones intermolecular potential for the second and third virial coefficients and a polynomial for the perfect gas heat capacity. The resulting equation of state has very high accuracy at low densities; it is useful for calibrating mass flow controllers and may be extrapolated to 1000 K. The new u(P, T) data and the new (P, T) data were simultaneously correlated with a virial equation of state containing four terms with the temperature dependences of model square-well potentials. This correlation extends nearly to the critical density and may help resolve contradictions among data sets from the literature.     

Equation of state of a3He-4He mixture near its liquid-vapor critical point

Measurements of the pressure coefficient (P/T)_,x are reported for a ³ He- ⁴ He mixture with a mole fractionX=0.805 of ³ He in the neighborhood of the liquid-vapor critical point. These include data on 16 isochores taken over the density interval–0.50.5 and over the temperature range–0.1 t0.1, where =(– _c )/ _c andt=(T-T _c )/T _c ,with _c andT _c ,respectively, the critical density and temperature of the mixture. From the discontinuity of (P/T)_,x at the boundary between the two-phase and the one-phase regions we determine the dew-bubble curve nearT _c with better precision than was done in recentPVT experiments. From the extrapolation of data not approachingT _c closer than1 mK, (P/T)_,x along the critical isochore appears to be discontinuous atT _c ,while for the isochore / _c 0.92, (P/T)_,x is continuous across the dew curve. It is found that this latter isochore cuts the dew curve at its highest temperature. These observations are discussed in terms of general thermodynamic arguments and theoretical predictions of the asymptotic behavior. We calculate (P/T)_,x from the scaling equation of state proposed by Leung and Griffiths for ³ He- ⁴ He mixtures, using their numerical parameters. In spite of some systematic deviations, especially in the two-phase region, there is in general good agreement with experimental results. In particular, the shape of the measured dew-bubble curve and the apparent discontinuity of (P/T)_,x along the critical isochore show excellent agreement with theory.Work supported by a grant from the National Science Foundation. A report of this work has been presented at the Washington Meeting of the APS [Bull. Am. Phys. Soc. 20, 618 (1975)].     

Systematic Evolution of the Magnetotransport Properties of Bi2Sr2−x La x CuO6 with Carrier Concentration

We report that it is possible to obtain a series of high-quality crystals of Bi ₂ Sr _2–x La _x CuO ₆ , of which the transport properties have been believed to be dirtier than those of other cuprates. In our crystals, the normal-state transport properties display behaviors which are in good accord with other cuprates; for example, in the underdoped region the in-plane resistivity _ab shows the pseudogap feature and in the overdoped region the T dependence of _ab changes to T ⁿ with n>1. The characteristic temperatures of the pseudo-gap deduced from the resistivity and the Hall coefficient data are presented.     

Flux flow resistivity and upper critical field in ideal type II amorphous superconductors

Flux flow resistivity _f and upper critical fieldH _c2 of ideal type II amorphous bulk supercbnductors Zr₃Ni and Zr₃Rh on both as-quenched and thermally relaxed states have been studied. It is found that thermal annealing does not change the temperature dependence ofH _c2 in homogeneous superconductors. The temperature and field dependence of _f in all samples studied exhibits a universal scaling relation of the form _f / _n =f(h, t), where _n is the normal state resistivity, andh andt are the reduced field and reduced temperature, respectively. The results are compared with predictions of the time-dependent microscopic theories for bulk superconductors in the dirty limit. In the low-field region (HH _c2 ) the viscosity coefficient contains both the ordinary (Bardeen-Stephen, Tinkham) and anomalous (Gor'kov-Kopnin) terms. ForHH _c2 the results agree qualitatively with the theory of Imai with pair-breaking in the anomalous term. Implications of the present results are discussed.This work is sponsored by the National Science Foundation under Grant DMR-82-02624.