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

Although the local resistivity of semiconducting silicon in its standard crystalline form can be changed by many orders of magnitude by doping with elements, superconductivity has so far never been achieved. Hybrid devices combining silicon's semiconducting properties and superconductivity have therefore remained largely underdeveloped. Here we report that superconductivity can be induced when boron is locally introduced into silicon at concentrations above its equilibrium solubility. For sufficiently high boron doping (typically 100 p.p.m.) silicon becomes metallic(1). We find that at a higher boron concentration of several per cent, achieved by gas immersion laser doping, silicon becomes superconducting. Electrical resistivity and magnetic susceptibility measurements show that boron-doped silicon (Si:B) made in this way is a superconductor below a transition temperature T-c approximate to 0.35 K, with a critical field of about 0.4 T. Ab initio calculations, corroborated by Raman measurements, strongly suggest that doping is substitutional. The calculated electron-phonon coupling strength is found to be consistent with a conventional phonon-mediated coupling mechanism(2). Our findings will facilitate the fabrication of new silicon-based superconducting nano-structures and mesoscopic devices with high-quality interfaces.

Superconductivity in doped cubic silicon.

Despite the amount of experimental and theoretical work on doping-induced superconductivity in covalent semiconductors based on group IV elements over the past four years, many open questions and puzzling results remain to be clarified. The nature of the coupling (whether mediated by electronic correlation, phonons or both), the relationship between the doping concentration and the critical temperature (T(c)), which affects the prospects for higher transition temperatures, and the influence of disorder and dopant homogeneity are debated issues that will determine the future of the field. Here, we present recent achievements and predictions, with a focus on boron-doped diamond and silicon. We also suggest that innovative superconducting devices, combining specific properties of diamond or silicon with the maturity of semiconductor-based technologies, will soon be developed.

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Superconducting group-IV semiconductors.

Changes in susceptibility of GeTe with temperature and sample composition are shown in transition curves for four pressed and sintered samples. Measurements show that GeTe becomes superconductivity in the predicted tempernture range. A plot of the transition temperatures as a function of the carrier concentration is also shown. (L.B.S.)

Superconductivity in Germanium Telluride

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.

Superconductivity in the TiO and NbO systems

Superconductivity of Tungsten

Very pure Ir is found to be superconducting. The critical field varies from 16 gauss at 0.04 deg K to 0 gauss at 0.14 deg K. In the purification process, special care is taken to remove as much Fe as possible. (T.F.H.)

Superconductivity of iridium

The theoretical and experimental results for the superconducting properties of Nb-Mo alloys are correlated. (T.F.H.)

J. W. Gibson

Papers

Superconductivity in Germanium Telluride

Superconductivity in the TiO and NbO systems

Superconductivity of Tungsten

Superconductivity of iridium

Superconductivity in the NbMo System