Determination of Gap Symmetry from Angle-DependentHc2Measurements onCeCu2Si2

The tetragonal heavy-fermion compound ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ exhibits a superconducting ground state ($S$ type, $T_c=0.67\mathrm{K}$) close to a magnetic instability. Here, we present angle-resolved resistivity measurements of the upper critical field $H_{c2}$. In-plane rotation of $S$-type ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ single crystals reveals a fourfold oscillation of $H_{c2}$. An extended weak-coupling BCS model for a $d$-wave symmetry including strong Pauli-limiting effects confirms the aforementioned angular dependence and points towards $d_{xy}$ symmetry of the order parameter.