Scanning Tunneling Microscopy Observation of a Square Abrikosov Lattice in LuNi2B2C

We present scanning tunneling microscopy measurements of the (001) surface of a LuNi2B 2C borocarbide single crystal at 4.2K. In zero field, the conductance versus voltage characteristics recorded at various locations on the sample reproducibly provide a gap value of 2.2meV. In a magnetic field of 1.5 and 0.375T, the recordings of the conductance as a function of position reveal a regular square vortex lattice tilted by 45 degree with respect to the crystalline a axis. This unusual result is correlated with an in-plane anisotropy of the upper critical field Hparallelc2(45 degree)/Hparallelc2(0)=0.92 at 4.2K and is analyzed within the framework of Ginzburg-Landau theory. copyright 1997 The American Physical Society