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 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 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 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.     

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.     

Dependence of Tc on electronic density of states

From the Eliashberg gap equations generalized to include approximately an energy dependence in the electron density of states N(ε) we compute the functional derivative of the critical temperature T_c with N(ε). It is found to be a rapidly decreasing function of the increasing energy away from the Fermi energy. In an appropriate limit, a comparison with the predictions of the McMillan equation shows that it is inadequate in a study of δT_c/δN(ε).     

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.     

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     

Pseudogap in the electronic density of states of superconducting glasses

Two loop order results for the electronic density of states (DOS) of both Ising- and XY-type superconducting glass phases are presented. Maximal depression of the DOS by critical fluctuation corrections to its finite mean field value is found to occur at the Fermi energyE _F . The theory is renormalizable and exponentiation of the loop expansion yields (E _F)=0 for the DOS in two dimensions and for arbitrarily small disorder. It is also concluded that the DOS assumes nonzero values, when either |E–E _F|, or magnetic field H, or temperature T become nonzero. A pseudogap (atT=0,H=0) or a smeared gap (forH0 orT0) is thus obtained with a DOS-minimum at the Fermi level. A rough estimate of the effective width is given. In three dimensions, the DOS atE _F vanishes like (n _c ^I –n)^1/2 for the Ising-type glass and like (n _c ^XY –n)¹ for the XY-type glass as the impurity concentrationn exceeds characteristic values. At second loop order, the calculation contains a nontrivial indication of a lower critical dimension two.