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.     

Studies of superconducting A-15 vanadium-based alloys

Superconductivity of binary alloys spanning the A-15 compounds V₃Si, V₃Ge, V₃Ga, and V₃Al and the pseudobinary derivatives of these stoichiometric compounds was surveyed by studying samples prepared by rf-sputtering from alloy cathodes. The possible formation of the hypothetical A-15 binaries V₃P, V₃B, and V₃C and their pseudobinary formation with V₃Si was also explored. Efforts to form these hypothetical alloys were not successful. The T _c's were measured resistively and the structure and lattice constants were determined by x-ray analysis. A maximum T _cof 11.7 K was obtained for A-15 V₃Al, and the importance of a suitable deposition temperature and high sputtering pressures was examined. It is proposed that large variations of T _camong pseudobinary alloys imply a rapid variation of the density of states at the Fermi level.     

Irradiation damage of A15-Nb3Si and Nb7Si

The behavior of the superconducting temperatureT _c and the lattice parametera ₀ of polycrystalline Nb₃Si and Nb₇Si thin films as a function of 300-kV -particle fluence was investigated. It was found that theT _c depression of A15 Nb₃Si is of the same size and nature as for other A15 materials. In contrast, the initialT _c degradation of Nb₇Si is a factor of three larger. The observed minimum in theT _c vs fluence dependence of Nb₃Si is discussed and a simple qualitative model is presented.On leave of absence from the Institute of Physics, Czechoslovak Academy of Sciences, Prague, Czechoslovakia.     

Nb3Si prepared by high-rate sputtering followed by quenching

Nb₃Si films with the A-15 structure have been prepared by sputtering onto a substrate whose temperature is about 1000 C, followed by quenching to liquid nitrogen temperature. All processes were carried out by using magnetron sputtering in a container full of liquid nitrogen. The quenching just after the deposition was achieved by thermal conduction using a sample holder in contact with the liquid nitrogen container. The highest onset temperature T _on and critical temperature (midpoint of the transition) T _c of the samples are 17.3 and 16.4 K, respectively, with a sharp transition. X-ray diffraction patterns show fine-grained A-15 structure and give a lattice constant of 5.09 Å.Work supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education.     

Changes ofT c,J c,B c2 and the lattice parameter of the Nb3Sn phase formed at the initial stage of growth in a multifilamentary superconductive wire

Investigations were made of the superconducting transition temperature,T _c, the upper critical flux density,B _c2, and the critical current density,J _c, of Nb₃Sn layers in filamentary wire in a bronze matrix. The lattice parameter,a ₀, andT _c of Nb₃Sn layers in 259-filament wire were determined after removal of the bronze matrix. The microstructure and layer thickness were studied using scanning electron microscopy. The diffusion formation of Nb₃Sn phase at 1023 K was studied until the complete reaction of the niobium filaments. It was found that the Nb₃Sn layer begins to form in the manufacturing process during the intermediate annealing at 793 K, and that there is a considerable degradation of critical parameters due to the non-stoichiometry of the Nb₃Sn phase in layers thinner than 1m.     

Is Nb3Si a 25 K superconductor?

Nb₃Si is predicted to have the highest superconducting critical temperature of all known A15 compounds, but it can only be prepared by non-equilibrium methods and resulting T_c values are disappointing. Experimental results on Nb₃Si prepared by a variety of methods are reviewed. By analogy with Nb₃Ge it is argued that T_c for stoichiometric fully ordered A15 Nb₃Si should be 25 K.     

Superconductivity in new A-15 niobium alloys

A number of previously unreported A-15 Nb alloys have been studied, using samples prepared by dual target sputtering.T _c was measured resistively and structure determined by x-ray diffraction. Samples were “chemically” analyzed by anodization. New alloys include A-15 Nb₃Si and pseudobinary derivatives, as well as some derivatives of “Nb₃P.”T _c and the A-15 lattice parameter were surveyed for these, and for previously studied alloys related to Nb₃Al, Nb₃Ga, and Nb₃Ge. The “chemical” variation ofT _c was slight, except near pure Nb₃Si and for alloys based upon “Nb₃P,” where the samples show considerable disorder. Maxima inT _c occur for the alloys Nb₃Al_0.7Ge_0.3 and Nb₃Al_0.5Ga_0.5. The first is well known; trends measured here indicate that the second may be equally high. It is proposed that the dominant factor forT _c in this group of alloys is the state of order achieved in the samples. No evidence was seen to associate enhancedT _c with A-15 phase boundaries.     

The volume dependence ofT c of ternary A-15 phases

Results are presented of measurements of the superconducting transition temperatureT _c , lattice constanta, magnetic susceptibility , and critical fieldH _c2 for many Nb₃Al and V₃Si-based ternary phases and Nb₃SnH _x . For V₃Si-based ternary systems and Nb₃SnH_x the density of states at the Fermi levelN(_F) sharply decreases with the concentration of the alloying element. The variation ofN(_F) in these ternary systems cannot be explained by the variation ofa. In ternary phases Nb₃(Al_1–x Y _x ), where Y can be Ge, Ga, Sb, or Se, a quite clear correlation is revealed betweenT _c anda. The dependence ofT _c onV in these systems is due to the variation of the matrix element of the electron-phonon interaction I ².     

The synthesis of the Nb3(Al-Ge) system by sputtering

The variation of the superconducting transition temperatureT _chas been studied in the Nb₃Al _1–x Ge _x system for0x1 for samples produced by rf sputtering onto heated substrates. The effect that different sputtering gas pressures have upon the rate of energy loss of the sputtered atoms due to collisions with neutral sputtering gas atoms is considered. Also considered is how thermalization can be achieved in the fewest number of collisions by matching the mass of the sputtering gas atoms to that of the sputtered atoms. For the case of Nb ₃ (Al-Ge) we show that it is advantageous to use a mixture of sputtering gases so that the light Al atoms can be thermalized as well as the heavier Nb and Ge atoms. It is also thus shown that the same sputtering conditions that are optimal for forming high-T _cNb₃Ge onto heated substrates are not optimal for forming high-T _cNb ₃ Al.Supported by CUNY FRAP and National Science Foundation DMR 74-18138.     

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.     

Upper critical field measurements on A15 NbGe superconducting films

Measurements of the upper critical fieldB _c2 (T) have been made on a range of high-transition-temperature A15 niobium germanium samples prepared by dc sputtering with varying additions of oxygen and differing sputtering conditions. An overview is presented on the formation of Nb₃Ge with particular reference to lattice parameter, composition, and normal state residual resistivity. The problem of sample inhomogeneity and its effect onB _c2 (T) measurements is discussed. Correlations of are made withT _cand resistivity, and the results compared with previous studies of neutron-irradiated and coevaporated material. Existing theories of are summarized and the problem of comparison of theory and experiment is discussed. An analysis is presented that does not depend directly on assumptions about the Fermi surface structure. The data indicate thatB _c2 has a fundamental dependence onT _cand resistivity that is similar to that found in Nb₃Sn. There is evidence that the mean Fermi velocity for clean Nb₃Ge is about twice the value for Nb₃Sn; the mean Fermi surface area is also about twice the value for Nb₃Sn. This suggests that Nb₃Ge is more strongly coupled than Nb₃Sn.     

Transition metal superconductivity in a generalized Nb3Al system

The variation of the superconducting transition temperatureT _c in (Nb_1–y M _y )_3– Al_1–x Ge _x composite systems, where M is one of the IVB transition elements Ti, Zr, or Hf, was studied. It was found that the transition metal group dominates in determining theT _c of the system. The variation ofT _c is shown to be similar for all three IVB elements reflecting the similarity of the behavior ofT _c for Nb_1–yM _y binary alloys. The behavior of the binary alloys is analyzed in terms of the McMillan equation to test the importance of the electronic density of states N(O) in determining the electron-phonon coupling constant . TheT _c variation for M equal to Zr was investigated for the entire range ofx andy from 0 to I. The transition temperature is qualitatively independent ofx suggesting that only physical quantities relevant to the transition metal alone influence the transition temperature.Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Chicago.     

Effect of cold rolling on the superconducting and electronic properties of two amorphous alloys; Nb50Zr35Si15 and Nb70Zr15Si15

The effect of cold rolling on the superconducting properties was examined for amorphous Nb₅₀Zr₃₅Si₁₅ and Nb₇₀Zr₁₅Si₁₅ superconductors. Cold rolling to 10 to 20% reduction in thickness results in a rise of superconducting transition temperature (T _c) and a decrease in transition width (T _c), upper critical field gradient near , critical current density [J _c(H)] and normal electrical resistivity (_n). Changes of about 7% forT _c 33% for T _c, 12% for and 70% forJ _c(H) are found. The rise ofT _c upon cold rolling was considered to originate from the increase in the electron-phonon coupling constant () due to an increase in the electronic density of states at the Fermi level [N(E _f)] and a decrease in the phonon frequency (), while the decreases in T_c,J _c(H) and _n were attributed to the decrease in fluxoid pinning force due to an increase in homogeneity in the amorphous structure. From the results described above, the following two conclusions were derived: (a) cold rolling causes changes in electronic and phonon-states in the quenched amorphous phase, and (b) deformation upon cold rolling occurs not only in the coarse deformation bands observable by optical microscopy, but also on a much finer scale comparable to the coherence length (7.7 nm).     

Superconducting properties of A15 V3Ga and Nb3Al by precipitation from supersaturated bcc phase

Structurally multiconnected filamentary A15 V₃Ga and Nb₃Al in their respective bcc phases are formed by heat treating as quenched and deformed supersaturated bcc phases. It was found that T_c of these compounds are ～14 and ～12 K V₃Ga and Nb₃Al, respectively, after appropriate heat treatments. The kinetics and morphology of precipitation of these compounds as well as the critical current density are presented.     

A-15 phases: Occurrence,Geller radii,and electronic structure

Reported A-15 phases are tabulated using a comparison between measured and predicted lattice parameter as a criterion. An enlarged set of Geller radii is calculated, and trends in these are used to discuss electronic features of these compounds. The small variation of superconductingT _c among some compounds related to Nb₃Sn is explained in terms of constant electronic feature deduced from the Geller radii.     

Effect of annealing on the superconducting properties of two amorphous alloys: Nb70Zr15Si15 and Zr85Si15

The changes in the superconducting and electronic properties of amorphous Nb₇₀Zr₁₅Si₁₅ and Zr₈₅Si₁₅ alloys with annealing were examined with an aim to evaluate the effect of structural relaxation on the superconductivity of metal-metalloid type amorphous alloys.T _c rises once from 3.99 to 4.42 K on annealing at temperatures below about 473 K for the Nb-Zr-Si alloy and from 2.71 to 2.75 K at temperatures below about 373 K for the Zr-Si alloy, and with further rising annealing temperature,t _d, lowers monotonically to a final relaxed value (3.15 K for Nb₇₀Zr₁₅Si₁₅ and 2.49 K for Zr₈₅Si₁₅), which is independent of the previous thermal cycling. These results indicate that the thermal relaxation of an amorphous phase occurs through at least two stages. The lowering ofT _c occurs exponentially witht _d, and an activation energy for the relaxation process and the frequency of jump over the barrier were estimated to be about 2.03 eV and 2.4×10¹⁴ sec^–1 for Nb₇₀Zr₁₅Si₁₅ and about 1.28 eV and 1.2×10¹¹ sec^–1 for Zr₈₅Si₁₅, respectively. The high frequencies indicate that the relaxations occur more or less independently of each other in a non-co-operative manner. The dressed density of electronic states at the Fermi level,N(E _f) (1+), which was calculated from the measured values of _n and (dH _c2dT)T_c, exhibited a similar annealing temperature dependence to that ofT _c. From this the change inT _c on thermal relaxation was interpreted as due to the changes in and/orN(E _f). From the depressions ofJ _c(H) and fluxoid pinning force on annealing in a temperature range of 473 to 873 K, it was concluded that the structural relaxation from a less homogeneous quenched-in state to a homogeneous stable state occurred on the scale of coherence length (7.5 nm) during the annealing.     

Magnetic and nonmagnetic impurities in some A-15 structure systems. I. Superconducting properties

We present measurements of the critical temperature T _c and the critical field H _c2 of the A-15 structure systems Nb _y ,Pt_100–y–x M _x where M = Fe and Ru (y = 73–78). T _c is influenced by two effects: one due to the magnetic Fe atoms, which causes a strong decrease, and the other due to the nonmagnetic effect of the Fe atoms, which causes a smaller but non-negligible effect. For the study of the nonmagnetic effect we introduced Ru impurities instead of Fe. Anomalous high critical field values are found as T 0, and the curves H _c2 versus T do not follow the WHH theory generalized to include magnetic impurities. The introduction of an empirical parameter permits us to fit our experimental results. This abnormal behavior of H _c2 is discussed in terms of strong-coupling and anisotropy effects.     

Superconductivity,structural disorder,and phase transitions in ion-irradiated Nb3Al films

Thin films of single-phase A15 Nb₃Al were irradiated with protons and nitrogen ions. X-ray intensity analysis revealed a decrease of the Bragg-Williams long-range order parameterS and an increase of the average displacement amplitude of the atomsU in the fluence region where a decrease of the superconducting transition temperatureT _c was observed. The functional dependence ofT _c onS andU was found to depend on the ion species used for irradiation. After irradiation with 10¹⁶ N²⁺ / cm² the A15 phase transformed into an A2 phase. Furnace annealing caused a retransformation into the A15 phase with improved superconducting properties.     

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.     

Superconductivity and Mössbauer effect of Nb3Sn produced by Sn implantation

We have implanted Sn, Ge, and Si into Nb films. The resulting Nb-Sn compounds and their annealing behavior have been analyzed by the Mössbauer effect and compared to samples obtained by diffusion of Sn into Nb foils. Mössbauer spectra show that Nb₃Sn is obtained just by implantation, but with a T _cof only 5 K. The 925°C annealing temperature necessary to form the A15 structure with long-range order of Nb chains and T _cvalues up to 17.8 K is at least 100 °C higher in implanted samples than in samples prepared by diffusion of Sn into Nb. This is explained in terms of implantation-induced lattice defects. The metastable A15 phases of Nb₃Ge and Nb₃Si could not be formed by Ge or Si implantation, regardless of target or annealing temperature. It is suggested that the high-energy ions only form phases stable at high temperatures and with low T _cvalues.On leave from North Dakota State University, Fargo, North Dakota.