Revising the 4f symmetry in CeCu2Ge2: Soft x-ray absorption and hard x-ray photoemission spectroscopy

We present a detailed study on the $4f$ ground-state symmetry of the pressure-induced superconductor ${\mathrm{CeCu}}_{2}{\mathrm{Ge}}_{2}$ probed by soft x-ray absorption and hard x-ray photoemission spectroscopy. The revised Ce $4f$ ground states are determined as $|{\mathrm{\ensuremath{\Gamma}}}_{7}\ensuremath{\rangle}=\sqrt{0.45}|{J}_{z}=\ifmmode\pm\else\textpm\fi{}\frac{5}{2}\ensuremath{\rangle}\ensuremath{-}\sqrt{0.55}|\ensuremath{\mp}\frac{3}{2}\ensuremath{\rangle}$ with $\mathrm{\ensuremath{\Sigma}}$-type in-plane rotational symmetry. This gives an in-plane magnetic moment consistent with the antiferromagnetic moment as reported in neutron measurements. Since the in-plane symmetry is the same as that for the superconductor ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$, we propose that the charge distribution along the $c$ axis plays an essential role in driving the system into a superconducting phase.