Scanning tunneling microscopy observation of a square abrikosov lattice in LuNi2B2C

We present scanning tunneling microscopy measurements of the (001) surface of a LuNi{sub 2}B {sub 2}C 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 H{sup {parallel}}{sub c2}(45{degree})/H{sup {parallel}}{sub c2}(0)=0.92 at 4.2K and is analyzed within the framework of Ginzburg-Landau theory. {copyright} {ital 1997} {ital The American Physical Society}