Niobium oxides and niobates physical properties: Review and prospects

LiTi 2
 O
 4
 is one end member of the homogeneity range of the spinel phase Li
 1+x
 Ti
 2−x
 O
 4
 (0≤x≤1/3) and is superconducting at temperatures up to 13.7 K. Various measurements were carried out in order to characterize the superconducting and normal state properties of LiTi 2
 O
 4
 and of other compositions within the homogeneity range of the spinel phase. These measurements establish LiTi 2
 O
 4
 as ad-band superconductor and show thatT

 c
 decreases to <1.5 K forx≳0.1. This disappearance of superconductivity with increasingx was found to be correlated with anomalous changes in the lattice parameter with composition, and, from electrical resistivity measurements, is tentatively attributed to the occurrence of a composition-induced metal-semiconductor transition atx≈0.1. The metallic character of LiTi 2
 O
 4
 and the composition dependence of the observed electrical properties are shown to follow from crystallographic considerations.

Superconducting and normal state properties of Li 1+x Ti 2−x O 4 spinel compounds. I. Preparation, crystallography, superconducting properties, electrical resistivity, dielectric behavior, and magnetic susceptibility

The O−Ti (Oxygen-Titanium) system

We have made a detailed study of the fabrication process of Nb/Al‐AlOx/Nb Josephson junctions with the aim of introducing guiding principles to obtain high‐quality junctions. Junctions were fabricated by a novel process named the self‐aligned contact process. We investigated the deposition conditions of Nb and Al films and optimized them. Oxidation conditions for obtaining target critical current were also investigated. Then composition of the tunneling barrier material and the coverage of the thin Al layer on the base electrode were analyzed by x‐ray photoelectron spectroscopy, Auger electron spectroscopy, etc. We investigated aging and annealing effects on the junctions. We confirmed that the current‐voltage (I‐V) characteristics of the junctions did not change after storage for 450 days at room temperature or when subjected to temperatures of up to 150 °C for shorter periods. Applying the McMillan theory to our experimental results, we explained the characteristics of the junctions. As a result, we hav...

Experimental investigations and analysis for high-quality Nb/Al-AlOx/Nb Josephson junctions

The initial stages in the oxidation of Nb have been studied by the uv‐photoemission technique for photon energies below 12 eV. Changes of structure in the electron distribution curves for different oxygen exposures and temperatures are correlated with formation of different types of Nb oxides. Electronic structure changes due to formation of at least three different oxides or mixtures of oxide have been observed. NbO2 and the NbO oxides seem to form first as a thin protective layer. The pentoxide, Nb2O5, is obtained after heavy oxidation. Practical implications of the formation of NbO in the interface between Nb2O5 and Nb metal are discussed.

Oxidation of Nb as studied by the uv-photoemission technique

This paper presents electrical resistivity and superconductivity studies for the B1 (NaCl) structure oxides of titanium, vanadium, and niobium. In samples of nominal composition TiO
 x
 , VO
 x
 , and NbO
 x
 ,x was varied from 0.8 to 1.2. It was found that all three of these oxides exhibit room-temperature electrical resistivities characteristic of metallic behavior. With decreasing temperature, the resistivity drops steeply in the case of NbO
 x
 , but remains steady or rises somewhat in the case of TiO
 x
 and VO
 x
 , depending on the exact value ofx. It is suggested that in TiO
 x
 and VO
 x
 there is a large resistivity component due to scattering of carriers by disordered vacancies. Superconductivity was observed in NbO
 x
 (T
c=1.38 K,x=1.0) and TiO
 x
 (T
c=1.0 K,x=1.07). In the latter case the material showed a well-defined maximum ofT
c as a function of composition, withT
c 1.20. Several VO
 x
 samples remained normal to 0.07 K.

C. K. Jones

Papers

Superconductivity in the TiO and NbO systems