高压下富铌化合物超导电性的理论研究

Since the discovery of superconductivity at 40K in MgB2, it is found that although the conventional superconductor usually have low superconducting critical temperatures (Tc), however, due to their simple structure, low cost and other advantages, they have a wide range of application perspective, including geological prospecting, medical instrument, military, environment and so on. Except for MgB2, the Niobium-rich compounds also have been known as the superconductors with high Tc among those simple compound superconductors . As we have known, pressure can bring great change of Tc in the conventional superconductors, thus the Niobium-rich compounds with higher Tc may be found probably under pressure. In this project, we investigate the new structure, phase transition mechanism, superconducting properties and so on in the Niobium-rich compunds（Nb3Ge, Nb3Sn,Nb3Ga,NbN）under pressure by the computer simulations. The main two contents are as follows. 1、Determination of the high-pressure phases (including thermodynamics, lattice dynamics and mechanical stability ). 2、superconducting properties (including Fermi surface nesting function, phonon and electron-phonon coupling) under pressure, the finish of our project can provide the knowledge reserve for revealing the general laws of superconducting mechanism in solid materials under high pressure, show clearly direction to design the superconductors with high Tc.

自从具有40 K 超导转变温度的MgB2超导体被发现以来，人们发现即使是超导转变温度较低的传统超导体，由于其结构简单、成本低廉等优点，在大地探矿、医疗仪器、环境和军事方面具有广泛应用前景。其中富铌化合物是目前除了MgB2之外已知的具有较高超导转变温度的简单化合物超导体。总所周知，压力能极大的改变传统超导体的转变温度，因此在高压下可能找到具有更高超导转变温度的富铌化合物。本项目通过计算机模拟研究高压下富铌化合物（Nb3Ge、Nb3Sn、Nb3Ga、NbN）的新结构、相变机制、超导电性等关键的基本物理学特征。主要内容包括两方面：1、高压结构的确定（热力学、晶格动力学和机械稳定性）。2、高压下的超导电性（费米面嵌套函数、声子、电-声子耦合）。为揭示高压下材料超导机制的一般性规律提供知识储备，并为设计具有较高超导转变温度的超导材料指明方向。

项目以VN为例研究了高压下VN、NbN化合物的结构相变。采用PSO蚁群结构搜索方法预测了压力下VN的高压相，找到了比实验发现的NaCl结构能量更低的三个新相，并对其硬度进行了计算，发现它们是具有较高硬度的过渡金属超硬材料。对典型富铌超导化合物NbxSiy在高压下可能结构进行了变组分预测，发现其在高压下主要以NbSi2、NbSi以及Nb2Si3存在。在超导研究上，采用BCS理论对高压下简单金属Ba，Se和Te不同高压相的超导电性进行了研究，发现BCS理论可以很好地描述其压力下的超导电性，并对它们新高压相的超导温度进行了预测，发现Ba在压力下可能具有7.8-9.5K的超导转变温度，由此Ba也是具有较高超导温度的元素之一；而Se和Te的fcc相在压力下则有5.7K和4.6K的超导温度，我们同时从理论上阐述了上述元素超导温度随压力变化的内在机制。