Theoretical and experimental evidence for a nodal energy gap in MgB2

摘要

We present a phenomenological model that strongly suggests 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 conventional s-wave. The model is an extension of the BCS gap equation 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 sound velocities and masses, 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 the literature that is relevant to the energy-gap symmetry. We find that the evidence from the literature for s-wave is inconclusive. Our finding is that the pi gap has six nodal lines.