Abstract
The crystal-field ground state wave function of CeCu$_2$Si$_2$ has been investigated with linear polarized $M$-edge x-ray absorption spectroscopy from 250mK to 250K, thus covering the superconducting ($T_{\text{c}}$=0.6K), the Kondo ($T_{\text{K}}$$\approx$20K) as well as the Curie-Weiss regime. The comparison with full-multiplet calculations shows that the temperature dependence of the experimental linear dichroism is well explained with a $\Gamma_7^{(1)}$ crystal-field ground-state and the thermal population of excited states at around 30meV. The crystal-field scheme does not change throughout the entire temperature range thus making the scenario of orbital switching unlikely. Spectroscopic evidence for the presence of the Ce 4$f^0$ configuration in the ground state is consistent with the possibility for a multi-orbital character of the ground state. We estimate from the Kondo temperature and crystal-field splitting energies that several percents of the higher lying $\Gamma_6$ state and $\Gamma_7^{(2)}$ crystal-field states are mixed into the primarily $\Gamma_7^{(1)}$ ground state. This estimate is also supported by re-normalized band-structure calculations that uses the experimentally determined crystal-field scheme.
Highlights
Heavy fermion compounds are f electron systems where, at low temperatures, the hybridization of localized 4 f or 5 f and conduction electrons (c f hybridization) forms an entangled ground state with quasiparticles that can have effective masses up to three orders of magnitude larger than the free electron mass [1,2]
The data analysis was performed with a full multiplet calculation, the analysis becomes more intuitive when expressing the linear dichroism of each crystal-field state in terms of incoherent sums of the individual DJz. This is only possible when the rotational symmetry is higher than twofold and as long as the crystal-field splitting is small with respect to the spin orbit splitting [29]. Both are fulfilled for CeCu2Si2 so that we can write for the linear dichroism of the
The X-ray absorption spectroscopy (XAS) spectra and the linear dichroism therein of In CeCu2Si2 have been measured in the wide temperature range from 250 mK to 250 K
Summary
Heavy fermion compounds are f electron systems where, at low temperatures, the hybridization of localized 4 f or 5 f and conduction electrons (c f hybridization) forms an entangled ground state with quasiparticles that can have effective masses up to three orders of magnitude larger than the free electron mass [1,2]. This is only possible when the rotational symmetry is higher than twofold and as long as the crystal-field splitting is small with respect to the spin orbit splitting [29] Both are fulfilled for CeCu2Si2 so that we can write for the linear dichroism of the (1) 7 state:. The satellite indicating the presence of 4 f 0 in the ground state shows up at the high-energy tail of the 4 f 1 absorption lines; here we are always referring to the configurations of the initial state [29] These satellites can be used for measuring relative changes of the 4 f shell occupation with temperature [31].
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