NbX(X=C,N,O)的能带结构研究

The Investigation on the Band Structures of NbX(X=C, N, O)

本文采用EHT近似下的紧束缚能带方法,计算了NbX(X=C,N,O)的能带结构。结果表明,它们的能带结构相近,Nb—X间存在较强的成键作用,传导电子主要具有Nb的4d特征,Nb—Nb键与超导电性相关,从C到N,Nb—Nb键共价性削弱,Tc提高。尽管计算所得NbO中Nb—Nb键强度介于NbC和NbN之间,但其实际Tc却比NbC和NbN的都低,这是NbO的空位效应所致,这一结果可从我们对有序缺陷的Nb_(0.75)O_(0.75)晶体能带结构的计算得到验证。

In this paper, the band structures of NbX crystals have been studied by using the tight-binding energy band method within the Extend Huckel scheme. It is found that the band structures of NbX crystals are roughly akin to each other. The bonding between the atom Nb and the atom X is relatively strong. .The electric conductions are demonstrated to be dominated by 4d character of niobium. The Nb - Nb bonding is found to be connected with the superconducting transition temperatures of NbX crystals. As a result, the weakness of the Nb-Nb bonds from NbC to NbN has lead to the increases of their Tc accordingly. Although the calculated strength of the Nb-Nb bonds in the ideal NbO crystal lies between the strengths of the Nb-Nb bonds in NbC and NbN crystals, the superconducting transition temperature measured on the real crystal (i. e. Nb0.75O0.75) is lower than those of NbN and NbC crystals. The effect of vacancies in the real NbO crystal is viewed to be responsible for the low superconducting transition temperature, which is supported by the band structure calculation of the ordered defect crystal Nb0.75O0.75.