Superconductivity in W-containing diamond-like nanocomposite films

Abstract

Superconductivity in a tungsten-containing carbon-oxide film was reported. The film with 500 nm thickness was deposited onto polycrystalline silicon oxides using chemical vapor deposition and the co-sputtering of a tungsten metal target. The bonding state of the carbon atoms and the macroscopic and microscopic crystal structure of the film were investigated by Raman spectroscopy, X-ray diffraction and transmission electron microscopy measurements. From the experimental results, we determined that this film essentially had an amorphous structure. The temperature dependence on resistivity was measured in the temperature range of 2–300 K. Resistive superconducting transition was observed at 3.8 K. The dc magnetizations were measured in the temperature range of 1.8–6.5 K. The diamagnetism resulting from a superconductive state was observed below 3.75 K, which is consistent with a resistive superconducting transition. It is thought that the finite sized clusters of the different superconductive transition temperatures cooperatively produce a macroscopic superconducting phenomenon.

Introduction

Diamond-like nanocomposites and metals containing diamond-like nanocomposite films consist mainly of carbon, hydrogen, silicon, and oxygen. These composite materials can overcome the weaknesses of diamond-like carbon (DLC), such as the abruption and generation of micro-cracks. One approach for further improving this property is the addition of other elements. In the past, amorphous carbon films mixed with boron and nitrogen [1], [2], or metal [3] have been studied for this purpose.

As a technique for improving not only the mechanical characteristics, but also the electrical characteristics of DLC, the method of adding metals has attracted much attention in recent years. In the deposition process, metal is added by the physical vapor deposition process; for example, by sputtering. In many cases, the added metal forms granular structures within the thin film [4], where the electrical conductivity is greatly affected by the size of the metal species, grain sizes, concentration, and the degree of metal dispersion [5], [6]. The contained metal is dispersed in a carbon base matrix in the shape of nanoclusters.

On the other hand, a hot topic associated with these boron-containing carbon materials is the discovery of superconducting properties due to the high boron doping in the diamond bulk [7] and film [8]. While diamond is essentially an insulator, the electrical properties of this material are derived from the concentration of the doped boron. The superconductive transition temperature of samples prepared at high pressure and high temperature is 4.0 K [7], while the sample temperature prepared using microwave plasma CVD (MWCVD) is 7.4 K [8]. Thus, it appears that the superconductive transition temperature depends on the preparation conditions.

However, there has been little research reported on the superconducting properties of carbon materials that contain relatively heavy elements. We have already reported on the crystal structure and conductivity of films doped with Cr, W, Nb and Si [9], [10], [11], [12]. The superconductive transition temperature of amorphous carbon films doped with tungsten and silicon is 2 K < T_c < 4 K [13], and it is reported that the transition temperature depends on the composition of tungsten and carbon.

In this report, we discussed the bonding state of the carbon atoms and the macroscopic crystal structure of tungsten-containing carbon-oxide films, and considered the superconducting properties of these composite thin films, prepared through a combination of chemical vapor deposition and magnetron sputtering.