The Application Of First Principles And Flat/Steep Band Theory In A15Superconductor

Nb3Sn and other A15members have been widely applied in nuclear power, nuclear magnetic resonance (NMR), high-energy particle accelerators for their high critical current density (Jc), and upper critical field (Bc2). There have been comprehensive and intensive studies on the applications, the fundamental lattice dynamic and electronic properties etc. of A15superconductors.A15type compound Nb3X (X=Al, Ga, Ge, Pb, Sb, Si, In, Sn) are selected in this work, Based on the density functional theory (DFT) and the "Flat/steep" band model"Flat/steep" band model, The electronic structure of15type compound is calculated in this work, the main conclusions are as follows:1. Based on the density functional theory (DFT) in the generalized gradient approximation (GGA), the exchange-correlation potential has been calculated by using the Perdew, Burke and Ernzerhof (PBE) formalism. The electronic structure of Nb3Al/Nb3Sn has been studied. The obtained results agree well with those from earlier studies and show clearly flat bands around the Fermi level. The steep bands characterized in this work locate around the M point in the first Brillouin zone (BZ). The obtained results reveal that Nb3Al/Nb3Sn fits more to the "Flat/steep" band model than to the van-Hove singularity scenario.2. The electron structure of Nb3Al indicates that the states around the Fermi level are essentially contributed from the Nb d-states. It is thus obvious that the superconducting properties are crucially decided by the Nb atoms. In this work, several hypothetical structure Nb3*, Nb3Nb, Nb3Al0.5are selected to invest the role of Al in Nb3Al. Their electron structure indicates the Al play an important role in the stability of Nb3Al; the s states electrons of Al are major contributions in lowest energy of Nb3Al. The p states of Al atoms interact strongly with the d+of Nb complex. The complex interaction has greater influence on its performance.3. The elastic performance and electronic properties under pressure is studied, and the elastic constants, young’s modulus and other important parameters of Nb3Al is calculated.