Possible multi-orbital ground state in CeCu$_2$Si$_2$

TL;DR

This study investigates the crystal-field ground state of CeCu$_2$Si$_2$ using x-ray absorption spectroscopy, revealing a stable $ ext{Gamma}_7^{(1)}$ ground state with multi-orbital character and no orbital switching across temperatures.

Contribution

It provides the first detailed temperature-dependent analysis of the crystal-field ground state in CeCu$_2$Si$_2$, highlighting its multi-orbital nature and stability.

Findings

The ground state is predominantly $ ext{Gamma}_7^{(1)}$ with thermal population of excited states.

Presence of Ce 4$f^0$ configuration suggests multi-orbital ground state.

Crystal-field scheme remains unchanged across temperature range.

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.