Critical field and specific heat of superconducting Tl-Pb-Bi alloys

The Eliashberg kernels ² ()F() obtained from tunneling experiments for a series of weak to strong coupling Tl-Pb-Bi alloys are used to calculate numerically on the imaginary axis the thermodynamic properties of these alloys in the superconducting state. The functional derivatives ofT _c andH _c with respect to changes in the kernel are also calculated.Research supported by the National Research Council of Canada.Partially supported by Conacyt, Mexico.     

Angular dependence of the critical magnetic field in granular superconducting films

The critical magnetic fieldH _c () of granular Al films has been measured as a function of the angle between the field direction and the plane of the film at temperatures nearT _c0 .The film thicknessd is smaller than the temperature-dependent coherence length (T), the bulk electron mean free path1 is smaller than the BCS coherence length ₀, and 1 d. The experimental data onH _c () are well fitted by the Tinkham formula. However, the observed values ofH _c/H _care not always consistent with and increase with1/d. This fact suggests that the boundary scattering of electrons at the film surface enhancesH _c () and that the enhancement ofH _cis larger than that ofH _c.On leave from Department of Physics, Faculty of Science, Kyushu University, Fukuoka, Japan.     

The effect of neutron irradiation on the upper critical magnetic field of superconducting Mo3Os

The effect of irradiation with high-energy (E>1 MeV) reactor neutrons on the upper critical magnetic fieldH _c2 (T) and the superconducting transition temperatureT _c has been determined for the A15 compound Mo₃Os. An increase ofH _c2(0) by 37% relative to its unirradiated value was observed for a neutron fluence of1.7×10¹⁹ n/cm². For the same fluence,T _c was found to decrease by 12%. The results are discussed in terms of defects in the A15 structure and their influence onH _c2(0).     

Superconductors with crystalline-field split impurities: Upper critical field and specific heat

We continue the investigation of crystalline-field effects on the properties of superconductors containing rare-earth impurities. Attention is focused on impurities with a nonmagnetic ground state. We calculate the upper critical fieldH _{c 2} and the jump of the specific heat at the phase transition.     

Low-temperature specific heat of a semiconductor in a strong magnetic field

We calculate the electronic specific heat of a semiconductor (GaAs) at very low temperatures when subjected to a magnetic field strong enough to make the electron distribution nondegenerate. The transition to nondegeneracy is characterized by a large and rapid increase in the specific heat. For large fields the value approaches that of a one-dimensional nondegenerate gas, after first exceeding this value. Zeeman splitting, even with a very smallg factor (0.32), almost doubles the maximum in the specific heat as a function of field.On leave from Centre de Recherches sur les Très Basses Températures, Grenoble, France.     

Comparative critical field study of superconducting ternary borides

The upper critical field (H _c2) and electrical transport properties have been determined for ternary rare-earth transition-metal borides with a variety of crystal structures; namely, ScRu₄B₄ (I4₁/acd), LuRh₄B₄ (I4₁/acd), ThRh₄B₄ (P4₂/nmc), LuOs₃B₂ (P6/mmm), LuRuB₂ (Pnma), and YRuB₂ (Pnma). For the majority of the compounds the critical field measurements are analyzed in terms of the dirty limit [l_tr/_BCS(0)1] theory of Werthamer, Helfand, and Hohenberg (WHH). For the superconductor YRuB₂, which is in the clean limit, critical field data are analyzed in terms of the Ginzburg-Landau theory. From the experimental data for ,T _c, and residual resistivity _res, we also have determined the heat capacity coefficient and BCS coherence length _BCS(0). The values indicate that these compounds are all strong type II superconductors.     

The upper critical field of thin superconducting films with large resistance

The influence of the electron-electron interaction on the upper critical fieldH _c2 of thin superconducting films with large resistance and of layered superconductors is investigated theoretically. The orbital effect only is taken into account. It is shown that the electron-electron interaction in the diffusion and Cooper channels influencesH _c2 in different manners. The interaction in the diffusion channel (dynamically screened Coulomb interaction) leads to essential deviations from the standard BCS dirty limitH _c2 (T) curve at low temperatures and as a result upward curvature inH _c2 (T) is possible. The interaction in the Cooper channel is significant only at temperaturesT _c–TT_c and enlarges the slopedH _c2 (T)/dt atT=T _c.     

Critical magnetic field curve of superconducting tungsten

The critical magnetic field of superconducting tungsten has been measured as a function of temperature down to 3 mK. In common with other weak-coupling superconductors the agreement with the theoretical BCS curve is very good. The critical temperatureT _cof our specimen is found to be 15.4±0.2 mK. Analysis of the low temperature data gives the values 1.15±0.01 G forH ₀and 0.90±0.06 mJ/mole K₂ for the electronic specific heat parameter . Very large supercooling and superheating effects are observed for an electropolished specimen. The Ginzburg-Landau parameter estimated from the supercooling field is =(2.6 ± 1.6) × 10^–3. The problem of providing thermal contact at very low temperatures to the sample is discussed with emphasis on our use of liquid He³.Supported in part by the U.S. Atomic Energy Commission under Contract No. AT(11-1)-1198. This work was submitted by W. C. Black in partial fulfillment of the requirements for the Ph.D. degree at the University of Illinois.     

Effect of a magnetic field on the specific heat jump of a superconducting alloy containing paramagnetic impurities with local states within the gap

The effect of a uniform magnetic field on the specific heat jump \({\Delta C}\) of a superconductor containing paramagnetic impurities has been investigated. The impurities are described by the Shiba-Rusinov model. Taking the normalized position of the bound state within the BCS gap ε₀=0.8 and 0.0 and the parameter α(=〈S _z ² 〉/S²)=1.0 and 1/3 as typical values, we find the detailed dependence of \({\Delta C}\) on the impurity concentration \(\bar c\) and the transition temperature T_c. In some cases \({\Delta C}\) diverges at certain values of \(\bar c\) or T_c, signaling a change of the normal-superconductor phase transition from second order to first order.     

The influence of an external magnetic field on the magnetic part of the specific heat of a ferromagnetic substance

An equation is derived for the magnetic part of the specific heat of iron using the Weiss Molecular Field Theory. Numerical values are obtained, using a computer, for the magnetic part of the specific heat with an external magnetic field. The results show that an external magnetic field has a considerable influence on the magnetic specific heat at the Curie temperature.     

Free energy and specific heat of critical films

Based on the field-theoretical renormalization group theory forO(N) symmetric systems in 4 — dimensions the free energy of critical films confined between two parallel plates at distanceL is analyzed. For temperatures aboveT _c,bulk the universal scaling functions are presented and discussed. At bulk criticality the scaling functions reduce to the universal Casimir amplitudes. Using these field-theoretical results quantitative predictions on the specific heat of⁴He films and films of³He-⁴He mixtures close to the onset of superfluidity are made. Wetting of a substrate by⁴He or³He-⁴He mixtures close to the — transition is studied in detail and thus the full scaling function is probed.     

NiTi合金热容的测定及传热的研究

使用调制式差示扫描量热器（MDSC）精确测定了Ni-Ti合金的比热容。研究了以不同的速度加热和冷却合金时相变点的变化,并用试验测定的热容值,计算了电流加热的NiTi合金丝的温度变化历程,这将为科学合理地计算加热时合金的温度场提供参考信息。     

The temperature and magnetic field dependence of superconducting critical current densities of multifilamentary Nb3Sn and NbTi composite wires

Commercial NbTi and Nb3Sn multifilamentary superconducting wire is becoming increasing important for use in research and commercial magnet systems. In both materials the temperature dependence of Jcplays a major role in the determination of magnet system operating parameters and design stability margins. We report here critical current density measurements as a function of temperature from 4.2 to 19 K and of applied magnetic field upto 8 T for multifilamentary Nb3Sn wire and for 2 alloys of NbTi superconducting wire. From this data [partial J_{c}(H_{a})/partialT] and[partialH_{c2}/partialT]T=T_{c}can be obtained and stability criteria and other superconducting parameters of the wires may be extracted.     

Low-temperature quadrupole and crystalline field effects in the specific heat of solid D2

Measurements have been made of the specific heat of solid D ₂ with concentrations of para-D ₂ between approximately0.9 and6%. The measurements are an extension of previously reported results to 150 mK and to various para-D ₂ concentrations. An anomalous specific heat is observed below 0.6 K which cannot be accounted for by the nearest-neighbor electric quadruple-quadrupole (EQQ) interactions betweenJ=1 molecules. This anomaly is explained in terms of next- and next-next-nearest-neighbor interactions between pairs and nearest-neighbor interactions between triple configurations ofJ=1 molecules. In addition, estimates are presented of the crystalline field splitting parameter (/k) in solid deuterium with lowJ=1 concentration.This work was partially supported by a grant from the National Science Foundation and by a contract with the Office of Naval Research.     

A loss-less superconducting kilo-oersted ac magnetic field system

A simple superconducting system which allows almost loss-less ac magnetic field to be generated in the kilo-oested region and in the frequency range from dc up to more than 1 kHz is described. The behaviour of commercially available superconducting wires and the influence of winding of the coils are also considered.     

Upper critical field and related properties of superconducting amorphous alloys Zr-Si

The upper critical fieldH _c2 has been measured for the superconducting amorphous alloys Zr_100–x Si _x withx=13, 15, and 17 over a wide temperature range down to 50 mK. Measurements for related properties, such as the depinning current densityJ _p , pinning force densityF _p , and flux flow resistivity _f , reveal that these systems are well characterized as typically soft and dirty type II superconductors with an extremely lowJ _p of 1 A/cm² andF _p of 1.5×10⁴ N/m³ ath (H/H _c2)=0.5 andt (T/T _c )=0.5. The temperature dependence ofH _c2 is well fitted by a curve of the Werthamer-Helfand-Hohenberg theory with Maki parameter =1.7–1.5 and spin-orbit interaction parameter _so=1–2.4. These results indicate that the Pauli spin-paramagnetic effect plays an important role in the low-temperature behavior ofH _c2 in these low-T _c and high-H _c2 amorphous superconductors. Corrections of the electron-phonon coupling (_ep=0.6) to the Pauli spin-paramagnetic limiting field lead to a reduced importance of the spin-orbit interaction effect, with _so less than 1.0. A discussion is given of the structural inhomogeneities in an amorphous phase, which are sensitively reflected in the strength of the fluxoid line pinning interaction (J _p ,F _p ) and are intimately related to the nonintrinsic anomaly, observed in many amorphous alloys, thatH _c2 depends linearly on temperature even at low temperatures, exceeding the theoretically allowedH _c2 values.