Phonons in a highly-correlated electron system: the heavy-fermion superconductor CeCu₂Si₂

Full text: CeCu₂Si₂ crystallizes in the tetragonal ThCr₂Si₂-type structure with 5 atoms in the primitive unit cell. It exhibits non-conventional superconductivity driven by low-energy magnetic excitations. The Ce³⁺ Hund's rule J=5/2 ground state is split by the action of the crystalline electric field into 3 doublets, with two excited doublets forming a quasi-quartet at around 30 meV. Except for Raman data no information about the lattice dynamics was available up to date. We therefore performed inelastic neutron scattering at low temperatures (3 K and 10 K) on a large single crystal on the thermal triple-axis spectrometers PUMA (FRM II) and TAIPAN (OPAL) to determine the phonon dispersion relations in the [001/110] plane. The measured dispersion curves will be compared with ab-initio DFT calculations. In addition we could refine the crystal field level scheme resulting in the observation that the excited quasiquartet actually consists of two considerably broadened doublets situated at around 28 meV and 35 meV, respectively. (author)