Accession Number : AD1032198

Title :   Theoretical and Experimental Evidence for a Nodal Energy Gap in MgB2

Descriptive Note : Journal Article - Open Access

Corporate Author : MIT Lincoln Laboratory Lexington United States

Personal Author(s) : Oates,Daniel E ; Agassi,Y D

Full Text :

Report Date : 17 Feb 2017

Pagination or Media Count : 32

Abstract : We present a phenomenological model that indicates with high probability that the smaller of the two energy gaps in MgB2, the so-called pi gap, contains nodal lines with a six-fold symmetry (i-wave). The model also indicates that the larger gap, the so-called sigma gap, is a conventional s wave. The model is an extension of the BCS theory that accounts for the elastic anisotropy in MgB2 and the Coulomb repulsion. It is based on a phononic pairing mechanism and assumes no coupling between the two energy gaps in MgB2 at zero temperature. All of the parameters of the model, such as the sound velocities, are independently determined material constants. The results agree with a previous ad-hoc hypothesis that the pi energy gap has six nodal lines. That hypothesis was motivated by low-temperature measurements of the surface impedance and intermodulation distortion in high-quality thin films. We briefly review experimental evidence in support of our hypothesis and review evidence in the literature that has led to the conclusion that both gaps are s wave. We find that the evidence from the literature for s wave is inconclusive. Our finding is that the pi gap has six nodal lines.

Descriptors :   thin films , energy gaps , RESONATORS , MICROWAVE FREQUENCY , transition temperature , crystal structure

Subject Categories : Solid State Physics

Distribution Statement : APPROVED FOR PUBLIC RELEASE