Growth and superconductivity of niobium titanium alloy thin films on strontium titanate (001) single-crystal substrates for superconducting joints

First high-pressure XAFS results at the bending-magnet-based energy-dispersive XAFS beamline BL-8 at the Indus-2 synchrotron facility

The static focusing optics of the existing energy-dispersive XAFS beamline BL-8 have been advantageously exploited to initiate diamond anvil cell based high-pressure XANES experiments at the Indus-2 synchrotron facility, India. In the framework of the limited photon statistics with the 2.5 GeV bending-magnet source, limited focusing optics and 4 mm-thick diamond windows of the sample cell, a (non-trivial) beamline alignment method for maximizing photon statistics at the sample position has been designed. Key strategies include the selection of a high X-ray energy edge, the truncation of the smallest achievable focal spot size to target size with a slit and optimization of the horizontal slit position for transmission of the desired energy band. A motor-scanning program for precise sample centering has been developed. These details are presented with rationalization for every step. With these strategies, Nb K-edge XANES spectra for Nb₂O₅ under high pressure (0-16.9 GPa) have been generated, reproducing the reported spectra for Nb₂O₅ under ambient conditions and high pressure. These first HPXANES results are reported in this paper. The scope of extending good data quality to the EXAFS range in the future is addressed. This work should inspire and guide future high-pressure XAFS experiments with comparable infrastructure.

Correlation between Crystal Structure, Surface/Interface Microstructure, and Electrical Properties of Nanocrystalline Niobium Thin Films

Niobium (Nb) thin films, which are potentially useful for integration into electronics and optoelectronics, were made by radio-frequency magnetron sputtering by varying the substrate temperature. The deposition temperature (Ts) effect was systematically studied using a wide range, 25-700 °C, using Si(100) substrates for Nb deposition. The direct correlation between deposition temperature (Ts) and electrical properties, surface/interface microstructure, crystal structure, and morphology of Nb films is reported. The Nb films deposited at higher temperature exhibit a higher degree of crystallinity and electrical conductivity. The Nb films' crystallite size varied from 5 to 9 (±1) nm and tensile strain occurs in Nb films as Ts increases. The surface/interface morphology of the deposited Nb films indicate the grain growth and dense, vertical columnar structure at elevated Ts. The surface roughness derived from measurements taken using atomic force microscopy reveal that all the Nb films are characteristically smooth with an average roughness <2 nm. The lowest electrical resistivity obtained was 48 µΩ cm. The correlations found here between growth conditions electrical properties as well as crystal structure, surface/interface morphology, and microstructure, could provide useful information for optimum conditions to produce Nb thin films for utilization in electronics and optoelectronics.

Relation between Crystal Structure and Transition Temperature of Superconducting Metals and Alloys

Using the Roeser–Huber equation, which was originally developed for high temperature superconductors (HTSc) (H. Roeser et al., Acta Astronautica 62 (2008) 733), we present a calculation of the superconducting transition temperatures, T c , of some elements with fcc unit cells (Pb, Al), some elements with bcc unit cells (Nb, V), Sn with a tetragonal unit cell and several simple metallic alloys (NbN, NbTi, the A15 compounds and MgB 2 ). All calculations used only the crystallographic information and available data of the electronic configuration of the constituents. The model itself is based on viewing superconductivity as a resonance effect, and the superconducting charge carriers moving through the crystal interact with a typical crystal distance, x. It is found that all calculated T c -data fall within a narrow error margin on a straight line when plotting ( 2 x ) 2 vs. 1 / T c like in the case for HTSc. Furthermore, we discuss the problems when obtaining data for T c from the literature or from experiments, which are needed for comparison with the calculated data. The T c -data presented here agree reasonably well with the literature data.