Effect of nitrogen on some properties of vapor-deposited Nb3Sn

Measurements of critical currentsI _c, transition temperaturesT _c, as well as transmission electron microscopy, scanning electron microscopy and X-ray analysis have established a correlation between critical currentI _c, the tape heating temperature during deposition, and the presence of a fine dispersion of precipitates. As the tape heating current (temperature) increased from 3 to 5 Amps,I _c increased from 20 to 430 Amps/3 mm tape width at 50 kG. A fine precipitate of the Nb₂N phase was observed for highest tape heating temperatures. It is concluded that the precipitate particles may act as flux-pinning centers in the Nb₃Sn layer.     

The density of states of b.c.c. Nb3Au: A comparison with A-15 Nb3Au

When Nb₃Au is converted from the A-15 structure to the b.c.c. allotrope, a sharp decrease in the superconducting transition temperature, T_c, occurs (11.1 to 1.5 K). To study possible causes for this sharp decrease, specific heat measurements have been made on the b.c.c. form of Nb₃Au yielding values of γ=19.2 mJ/mole-K², β=0.274 mJ/mole-K⁴ and θ_D=305 K. These parameters compared with published values for the A-15 Nb₃Au indicate that a sharp drop in the density of states at the Fermi level, N(0), occurs when the A-15 structure is converted to the b.c.c. form.     

Optical properties of cubic and tetragonal KTa0.5Nb0.5O3 by density functional theory

Using density functional methods, optical properties of KTa_0.5Nb_0.5O₃ (KTN) are investigated. The imaginary part of dielectric function ε₂(ω), the optical absorption coefficient I(ω) and the reflectivity R(ω) of KTN in the cubic (paraelectric) and tetragonal (ferroelectric) phases are calculated. The origin of the different behavior of the optical spectra between the two phases is discussed.     

Thermodynamics of superconducting Nb3Sn

Shen's experimental data on α²(ω)F(ω) and μ¹ in superconducting Nb₃Sn is used to calculate its thermodynamic properties. It is found to exhibit the same size strong coupling corrections to BSC as does Pb. Comparison with experimental data shows satisfactory agreement.     

Optical spectroscopy of electronic surface states

In this paper a number of optical spectroscopic methods for investigating surface electronic structure are discussed, including reflectance techniques, ellipsometry, surface photoconductivity and surface photovoltage spectroscopy. In addition to electron scattering techniques and UV-photoemission, optical spectroscopic methods have contributed much in recent times to the understanding of electronic surface states on solids. A discussion and comparison is given of the nature and significance of information obtained by these methods and exemplary experimental results are presented to illustrate the contribution of the optical techniques to the present knowledge about surface states. The relation between information obtained from optical measurements and electron spectroscopy is considered.     

Site-specific phonon density of states discerned using electronic states

We have performed the measurement of the site-specific phonon densities of states (PDOS) discerned using electronic states. As far as we know, no general method could give the site-specific PDOS, although oscillating properties of the individual atoms in nonequivalent positions are not necessarily equivalent. However, the combination of the energy and time domain measurements of the nuclear resonant scattering of synchrotron radiation allows the identification of site-specific PDOS. We measured the site-specific PDOS of iron atoms in magnetite, which is a mixed valent compound, and the difference between partial phonon densities of the iron sites was clearly observed.     

On mossbauer dynamics in Nb3Sn

We compare the anharmonic Lamb Mossbauer factor and the q-Lamb Mossbauer factor by studying the anharmonicity observed in the f-factor data of Nb₃Sn. We also show that this anharmonicity does not arise due to the presence of potential.     

On the microwave absorption of superconducting Nb3Sn

Measurements ofdc-resistance, surface resistance at 9.6 GHz and x-ray diffraction were made on Nb₃Sn layers having thicknesses between 4 and 7 μ. Samples were prepared by condensing tin on niobium foil which was then annealed at high temperature. This process was repeated, in some cases as many as twenty times. The best samples showed steep transition curves close to 18.2 K. X-ray measurements indicated a high degree of stoechiometry. The surface resistance decreased from 5 · 10^?2Ω atT _c to a residual value of 3 · 10^?4 Ω. This relatively high value can be due to minor deviations from stoechiometry or possibly to similar mechanisms as proposed for pure elements.     

On the electronic density of states of the transition metals

XIS measurements of the elements Ir, Pt and Au are reported. High precision and reproducibility is obtained by the application of a “π/2-method” which approximates a Bragg angle of 90° at the dispersing crystal. Theoretical densities of states exist for Pt and Au. The agreement with the measured isochromats is good. A rigid-band model for Ir, Pt, and Au is ruled out by the measurements. Rather they suggest (combined with results of photoemission experiments) a narrowing of thed-bands from Ir through Pt to Au. Moreover, thed-band of Ir lies relative to thesp-band at a lower energy than thed-bands of Pt and Au. A fitting parameter concerning the experimental resolution is explained and considered as a possible indication of localization for XIS.     

On the electronic density of states of the transition metals

A superposition model is suggested to describe the electronic density of states curves of binary alloys with the aid of those of the component metals. It is based on a charge transfer between the different atomic cells. The application of this model to different spectroscopic methods studying the density of states brings up the question of the localizing and averaging properties of the corresponding experiments.     

Critical parameters of thin diffusion Nb3Sn layers

Measurements of the critical temperatureT _c and the upper critical magnetic field H_c2 nearT _c of thin diffusion Nb₃Sn layers formed on a niobium core are presented. These parameters are dependent on the Nb₃Sn layer thickness, if it decreases to values comparable with the London penetration depth. With decreasing layer thickness the critical temperature decreases and both the width of the transition curve and the upper critical field increase.Hc ₂ for the thinnest investigated layers ( 1950 Å) is about 50% higher than that of the bulk materials.The presented work has been made possible and supported by the Alexander von Humboldt — Foundation and for the most part performed at the Institute of Applied Physics, University of Giessen.The author is indebted to Prof. E. Saur for encouraging this work, Dr. G. Otto for help in the sample preparation, and the Deutsche Forschungsgemeinschaft for financial support of this work.He also wishes to thank Prof. W. Buckel for his interest in this work and many valuable discussions. His thanks go to his co-worker, S. Tak~fcs, for valuable remarks.     

The density of unoccupied electronic states in copper

The density of unoccupied electronic states in Cu has been measured employing soft X-ray appearance-potential spectroscopy (SXAPS). The L_III le     

High T c,structural instabilities and electronic properties of Nb3Ge

Abstract

A number of Nb₃Ge samples with rather high T _c,on (T _c,on ? 22.5 K) were prepared by CVD and characterized by the electrical resistance, X-ray diffraction and scanning electron microscope patterns. It was found that the crystal growth patterns are roughly divided into two categories, (i) a particles-pattern in a nearly single phase of Nb₃Ge and (ii) a network pattern with a fairly large amount of the Nb₅Ge₃ phase but with rather a large resistance ratio and a small resistivity at T ? T _c,on. The latter type of samples shows generally higher T _c,on. It is deduced that the origin of high T _c,on comes mainly from a deformation of the atomic potential at the Nb-site into an anharmonic and rather unstable shape due to the coexistence of the Nb₅Ge₃ phase. The mechanism of the formation of different patterns is discussed on the basis of the theory of non-equilibrium thermodynamics of Prigogine.     

Spin-polarized electronic density of states in amorphous iron

The spin-polarized electronicd-band density of states in amorphous iron are reported using recursion method. The magnetic moment of amorphous iron is found to be almost as high as BCC iron. This lends support to the view expressed by Kemény et al. that the extrapolation of experimental results obtained on samples containing less than 2% impurities to deduce the magnetic moment of pure amorphous iron may not be justified.