Spectroscopic evidence for anisotropics-wave pairing symmetry inMgB2

Scanning tunneling spectroscopy of superconducting ${\mathrm{MgB}}_2$ ${(T}_c=39\mathrm{}\hspace{0.5em}\mathrm{K})$ were studied on high-density pellets and c-axis oriented films. The sample surfaces were chemically etched to remove surface carbonates and hydroxides, and the data were compared with calculated spectra for all symmetry-allowed pairing channels. The pairing potential $\left({\Delta }_k\right)$ is best described by an anisotropic s-wave pairing model, with ${\Delta }_k={\Delta }_{\mathrm{xy}}{\sin }^2{\theta }_k+{\Delta }_z{\cos }^2{\theta }_k,$ where ${\theta }_k$ is the angle relative to the crystalline c-axis, ${\Delta }_z\sim 8.0\hspace{0.5em}\mathrm{meV},$ and ${\Delta }_{\mathrm{xy}}\sim 5.0\hspace{0.5em}\mathrm{meV}.$