Estimate of density-of-states changes with strain in A15 Nb3Sn superconductors

Abstract

The experimental datasets are analyzed which show that the bare density of states N(EF) changes dramatically, as does the superconducting transition temperature Tc, in Nb3Sn samples which are strained in different states and levels. By taking into account the strain induced change in the electron–phonon coupling strength, the density of states as function of strain is estimated via a formula deduced from the strong-coupling modifications to the theory of type-II superconductivity. The results of the analysis indicate that (i) as the Nb3Sn material undergoes external axial strain ɛ, the value of N(EF) decreases 15% as Tc varies from ∼17.4 to ∼16.6K; (ii) the N(EF)–ɛ curve exhibits a changing asymmetry of shape, in qualitative agreement with a recent first principle calculations; (iii) the relationship between the density of states and the superconducting transition temperature in strained A15 Nb3Sn strands shows significant difference between tensile and compression loads, while for the trend of the strain-induced drop in electron–phonon coupling strength versus Tc of distorted Nb3Sn sample under different stress conditions, the curves show consistency in a wide strain range. A general model for characterizing the effect of strain states on the N(EF) in A15 Nb3Sn superconductors is suggested, and the density of states behavior in different modes of deformation can be well described with the modeling formalism. The present results are useful in order to understand the origin of the strain sensitivity of the superconducting properties of A15 Nb3Sn superconductor, and develop a comprehensive theory describing the strain tensor-dependent superconducting behavior of A15 Nb3Sn strands.