Low-temperature heat capacity of small Nb3Sn polycrystals by ac calorimetry

It is shown by an ac calorimetry technique that the multiple heat capacity anomalies which occur below the superconducting transition temperature for small polycrystalline Nb₃Sn samples are intrinsic to these samples. The recent suggestions that shear stresses can account for these results are analyzed for their validity. The dependence of the occurrence of these multiple anomalies upon the thermal history of the samples was investigated.Supported in part by U.S. Atomic Energy Commission under the Contract No. USAEC-AT-(04-3)-34 at the Department of APIS, UCSD, La Jolla, California. Present work supported under the auspices of ERDA.Supported by Air Force Office of Scientific Research, Air Force Systems Command USAF, under AFOSR Contract No. AFOSR/F-44620-72-C-0017.     

Vortex behavior in Nb3Ge microbridges

We have fabricated Nb₃Ge microbridges, including those with variablethickness geometry, with submicrometer lengths, by means of plasma etching combined with lithographic techniques. The measuredI-V characteristic and the temperature variation of the critical current are consistent with that expected from the vortex concept. However, a deviation is observed in the microwave steps: the magnitude of the critical current oscillates with microwave power in a manner which agrees with an analog simulation based on the RSJ model. We have found a strong correlation between the bridge behavior and the film parametersT _c and ρ _n (normal resistivity). The critical current is reduced rapidly with a decrease inT _c and an increase in ρ _n , while in high-T _c and low-ρ _n bridges a linearI-V characteristic appears at low voltages. Taking into account the influence of pinning, we discuss the results in terms of vortex motion.     

Superconductivity and phase stability of Nb3Ge

A method is described for calculating the surface impedance of thin superconducting films using the theory of Mattis and Bardeen of the surface impedance of superconductors. An approximate extension of the theory to the case where the superconducting order parameter inside the film varies slowly in space is presented. Calculations using the theory are compared with experimental measurements of the surface impedance at 3 GHz of thin tin films on bulk lead substrates. Agreement between theory and experiment is good.     

Low-pressure sputtering of high-T c Nb3Ge

A modified technique for the sputtering of high-T _c Nb₃Ge using low Ar pressure is reported. The experimental arrangement involved only conventional equipment and ordinary vacuum conditions. Nb₃Si with the A-15 structure was also prepared in a similar manner.The earlier stage of this work was supported by the U.S. Atomic Energy Commission under contract AEC-AT-(04-3)-34.     

The synthesis of Nb3Ge by RF sputtering

The conditions necessary for the formation of Nb ₃ Ge by low-pressure rf sputtering with a superconducting transition temperatureT _c >21 K have been investigated. Samples have been deposited onto cooled substrates so that the film is first amorphous and then is crystallized by a subsequent annealing, and onto hot substrates, in which case the film is crystalline upon deposition. The highestT _c samples were obtained for a substrate temperature of 735±25° C. The optimum substrate temperature is the same as the optimum annealing temperature for crystallizing films which were first deposited onto cooled substrates. Special conditions are necessary for the formation of single-phase A15 samples of Nb ₃ Ge with an optimumT _c . We have utilized the collisions that the sputtered atoms undergo with the sputtering gas molecules to thermalize the sputtered atoms. We report here on sputtering in both krypton and in argon.     

Surface structure of high-T c Nb3Ge films

We have studied the surface structure of rf-sputtered Nb₃Ge films by means of reflection electron diffraction and Auger electron spectroscopy. It is found that a tetragonal Nb₅Ge₃ phase exists, being partly disordered, at the surface of high-T _c films, whereas A15 phase is predominant in the body of the film as proved by x-ray diffraction. The tetragonal surface phase is replaced by an amorphous phase in thin films of <1000 Å, where reducedT _c 's as compared with those of thick films are found. Single A15 phase occurs at the surface of Ge-deficient films which have lowT _c 's. Auger analysis has shown a pronounced Ge-rich layer with a depth of 60–100 Å, depending on film composition and thickness. At the top of the surface layer, Ge is enriched beyond a composition corresponding to Nb₅Ge₃. It is inferred that the Ge-rich layer is responsible for formation of the tetragonal phase.Work supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.     

ac losses in Nb3Ge composite tape conductors

ac losses of Nb₃Ge tapes prepared by chemical vapour deposition have been measured between 4 and 16 K. Several tapes were observed to exhibit complete shielding of the normal substrate by the Nb₃Ge coating, which is continuous around the tape circumference. These tapes had measured losses that were acceptably low for power transmission up to 14 K. Another class of tapes showed high losses characteristic of ohmic currents flowing through the normal substrates. Microscopic examination revealed continuous longitudinal edge cracks for these samples. A correlation observed between ‘edge cracking’ and Nb³Ge coat thickness is discussed in terms of the differential contraction from coating temperature between the Nb₃Ge and the substrate in a completely enclosed geometry.     

Nb3Al AND Nb3(Al,Ge) SUPERCONDUCTING TAPES BY LASER BEAM ANNEALING

Nb₃Al AND Nb₃(Al,Ge) tapes with excellent superconducting properties have been fabricated using high energy density laser beam irradiation, irradiations were carried out continuously on Nb-25at%AI and Nb-20at%Al -5at%Ge tapes prepared by powder metallurgical processing. With the high power density and short irradiation time, the tape could be heated and cooled much faster than a tape heat treated by conventional methods. The results were fine structure and large critical current density J_c values at high magnetic fields. J_c at 23 Tesla was over 10⁴ A/cm². This value was much greater than that obtained from a commercial Nb₃Sn superconductor. New continuous irradiation systems have been developed for scale-up development. Nb₃Al tapes 10 m in length were successfully fabricated. There was some scatter in J_c along the tape length. Small coil tests in magnetic fields indicated that the tape had excellent stability against a flux jump and mechanical strain.     

Measurement of isobaric heat capacity of R125

The specific isobaric heat capacity (c_p) was measured for R125 (pentafluoroethane) in the gas phase by using a flow calorimeter. Twelve measurements for R125 were obtained at temperatures from 313 to 333 K and at pressures from 0.8 to 2.4 MPa. Some of them are close to the saturation curve. The expanded uncertainty (k = 2) of the temperature measurements is estimated to be less than 23 mK, and that of the pressure measurements is less than 14 kPa. The expanded uncertainty for c_p is estimated to range from 12 to 22 J kg⁻¹ K⁻¹. Also, the experimental data were compared with available equations of state. From the results, it became clear that these data will be essential to improve available models so as to represent more reliable thermodynamic properties of R125 and refrigerant mixtures with R125 that are used for refrigeration and air conditioning systems.     

Superconducting properties of Nb3Ge thin films prepared by coevaporation

Nb₃Ge films with thicknesses between 60 and 800 nm were deposited onto heated sapphire substrates by thermal coevaporation. Stoichiometric Nb₃Ge samples crystallize in a single-phase A15 structure with a lattice parameter of 5.14 ». Critical current densities and upper critical magnetic inductions were measured as a function of temperature, film thickness, and magnetic field orientation. The critical currents and fields show maxima for parallel field orientation. This is due to the random orientation of the crystallites. With decreasing thickness an increasing second maximum for normal field orientation is observed. This behavior can be explained by additional precipitates caused by internal stress.Dedicated to Professor Dr. B. T. Matthias on the occasion of his 60th birthday.Research supported by Deutsche Forschungsgemeinschaft.     

Superconducting properties of Nb3Ge thin films prepared by cosputtering

Nb₃Ge films with thicknesses between 0.5 and 1.5 m have been deposited on heated sapphire substrates by dc sputtering in a pure argon atmosphere. With a maximum superconducting onset temperature T _c= 22.7 K, these high-T _csamples crystallize in a single-phase A15 structure with lattice parameters down to 5.14 ». Critical current densities and upper critical magnetic inductions have been measured as a function of magnetic field orientation, temperature, and film thickness. The anisotropies of the critical currents and fields depend on the microstructure of the samples and show maxima for the field orientation normal to the sample surface in those films that exhibit columnar growth of the crystallites normal to the substrate. The temperature, field, and angular dependences of the pinning forces in these films are compared to existing pinning theories.Dedicated to Prof. Dr. B. T. Matthias on the occasion of his 60th birthday.Supported by Deutsche Forschungsgemeinschaft.     

Superconductivity in irradiated A-15 compounds at low fluences II. Alpha-particle-irradiated Nb3Sn and Nb3Ge

The T _c behavior of vapor-deposited Nb₃Ge and Nb₃Sn is examined as a function of low-fluence alpha-particle irradiation. It is found that for Nb₃Sn with 15 -cm the T _c is insensitive to low doses of radiation, whereas Nb₃Ge with ₀ 50 -cm has its T _c depressed immediately with irradiation. It is suggested that the T _c behavior of A-15 superconductors in the regime of small dose is strongly influenced by the initial state of the sample. Furthermore, it is argued that the behavior of the T _c with dose can be qualitatively explained by considering a sharp structure in the density of states N(E), the smearing of which by defects leads to a depression in T _c .Work performed under the auspices of the U.S. Energy Research and Development Administration.Also at Materials Science Department, SUNY at Stony Brook.Also at Physics Department, SUNY at Stony Brook.     

Tc measurement of sputter-deposited Nb3Ge films by a balance-bridge method

T_c values of sputter-deposited Nb₃Ge films were measured using both the balancebridge method and the four-probe resistance method.     

Resistivity of high-T c, 1 at % Ti-substituted Nb3Ge

Measurements are presented of the resistivity in samples of (Nb_0.99Ti_0.01)_1–x Ge_x, 0.17 x 0.24, from the superconducting transition temperature T _cto 300 K. The T _cattains its maximum value of above 22 K and the residual resistivity its minimum value at the same off-stoichiometric composition when x = 0.205. At this same value of x the transition width is narrowest and the residual resistivity extrapolates to a finite value, indicating a comparatively low resistance ratio is to be expected. The resistance ratio achieved in these samples with Ti substitution is comparable to that achieved in the best samples of pure Nb₃Ge and is indicative of the short mean free path in this system.Supported by NSF DMR 78-03217 and PSC-BHE Research Award Program.     

Effect of doping with Nb and Dy on the heat capacity of Si

The heat capacity of undoped Si and Si doped with Nb (10¹⁸ cm^-3) and Dy (10¹⁷ cm^-3) was measured by differential scanning calorimetry (DSM-2M and Mettler instruments). The phase transition from diamond Si to the high-pressure phase Si II was found to occur in undoped Si between 550 and 650 K. No phase transition was observed in Si doped with Nb or Dy, indicating that the dopants strengthen the Si lattice, thereby impeding structural changes.     

Measurement of low temperature heat capacity of metals under pressure

An alternating heat flow method is used to measure the heat capacity of metals at low temperatures under hydrostatic compression. The phase shift method is developed for taking account of the losses in the medium. The method was tested by measuring the heat capacity of Sn at pressures up to 10 k bar. The errors of measurements are ～3%.     

The film growth process in the bias sputtering of superconducting Nb3Ge

Metastable A15 Nb₃Ge films were sputter deposited onto sapphire substrates under thermalizing conditions. Simultaneously the configuration of the electric potential was varied by the application of a negative bias to the tantalum substrate holder or to both the holder and the growing Nb-Ge film. Films deposited with no bias showed an initial nucleation with disordered crystallites of low super-conducting transition temperature T_c and a subsequent “homoepitaxial” growth process towards a well-ordered state of high T_c. In contrast, bias sputtering led to well-ordered A15 crystallites at an earlier growth stage, which was then succeeded by a non-homoepitaxial growth process. These results will be discussed in the context of ion bombardment of the growing film surface and of the tantalum holder.     

Surface acoustic wave determination of the superconducting fraction of a Nb3Ge film

Surface acoustic waves at 1200 MHz were used to investigate the properties of a 0.5-µm-thick film of Nb₃Ge. This film was rf-sputtered onto a substrate consisting of a piezoelectrically active 3.5-µm AlN layer which was chemically vapor-deposited over a sapphire substrate. The attenuation coefficient of the surface acoustic waves was measured from 0.8 to 30 K. The raw data are analyzed to eliminate interference effects due to splitting of the wave into two components. One of these is at the surface of the film and the other may be a surface skimming bulk mode which is at the film-substrate interface. The resultant curve of attenuation versus temperature in the superconducting region is then used to determine the distribution function of the superconducting transition temperature of the film. Although the film starts to become superconducting at 21 K and the majority of the film appears to become superconducting at 18 K, it is also found that a significant amount of the film does not become superconducting until 10 K. The difference between the attenuation measured in the normal state and the superconducting state is used to obtain the electron mean free path in the film. This is compared to values obtained from electrical measurements.Research supported by the U.S. Air Force Office of Scientific Research under Grant No. AFOSR 81-0002.Research supported by the U.S. Air Force Office of Scientific Research under Contract No. AFOSR F49620-78-C-0031.     

Magnetic field properties of 1 at.% transition metal substituted Nb3Ge

Measurements of the critical magnetic field slope near the superconducting transition temperature T _care reported for samples of Nb_1–x Ge _x , (Nb_0.99Ti_0.01)_1–x Ge _x , and (Nb_0.99Zr_0.01)_1–x Ge _x for 0.15x0.25. The samples investigated were produced by trisputtering onto heated polycrystalline alumina substrates. The critical magnetic field slope near T _cfor substituted samples is comparable to that of Nb₃Ge alone. The highest T _cvalues, the highest resistance ratios, and the narrowest transitions, however, occur for samples with excess transition metal component.Supported by PSC-BHE Research Award Program.