Vortex lattice and quasiparticle density of states in CeRu2 studied by scanning tunneling spectroscopy

Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements hal e been performed on an intermetallic compound CeRu2 down to 2.2 K. Vacuum t unneling spectra have been measured on the surface prepared by cracking the sample in helium gas at 4.2 K. The shape and temperature dependence of the quasiparticle density of stales, N-s(E, r), have been well interpreted in terms of s-wave BCS superconductors. In magnetic fields, the hexagonal vort ex lattice has been imaged by measurements of N-s(E = 0, r) up to 2 T. This is the first observation of the vortex lattice by STS in heavy-fermion sup erconductors.