Scanning tunnelling microscopy and spectroscopy of MgB₂

Experiments of the scanning tunnelling microscopy and spectroscopy (STM/STS) have been carried out on the layered superconductor MgB₂ with T_c 39 K. The measurements were done at 5 K using a Pt-Ir tip and the single-crystal grain. The STM images show characteristic hexagonal patterns with the distances of nearest neighbour atoms to be 0.3 - 0.35 nm and 0.15 - 0.2 nm, respectively, which are consistent with the Mg and B lattice sizes. The STS measurements clarify the superconducting gap 2Δ 20 - 24 meV, which leads to the strong-coupling ratio 2Δ/k_BT_c 5 - 6. The tunnelling spectra can be described by the correlated 2-gap model. These features are consistent with our previous break-junction measurements. The gap structure is homogeneous at least within the range of 200 nm × 200 nm.