Scanning tunneling microscopy observation of a square Abrikosov lattice in LuNi2B2

We present scanning tunneling microscopy measurements of the (001) surface of a LuNi(2)B2C borocarbide single crystal at 4.2 K. In zero field, the conductance versus voltage characteristics recorded at various locations on the sample reproducibly provide a gap value of 2.2 meV. In a magnetic field of 1.5 and 0.375 T, the recordings of the conductance as a function of position reveal a regular square vortex lattice tilted by 45 degrees with respect to the crystalline a axis. This unusual result is correlated with an in-plane anisotropy of the upper critical field H(c2)parallel to(45 degrees)/H(p)arallel to(c2)(0) = 0.92 at 4.2 K and is analyzed within the framework of Ginzburg-Landau theory.