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, ${T}_{c}=0.67\text{ }\text{ }\mathrm{K}$) 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.