The electronic structure of EuPd$_2$Si$_2$ in the vicinity of the critical endpoint

TL;DR

This study investigates the electronic structure of EuPd$_2$Si$_2$ near a critical endpoint, revealing valence crossover, phase transitions, and correlated band structure changes using spectroscopy and ab initio theory.

Contribution

It provides new insights into the valence crossover and phase transition phenomena in EuPd$_2$Si$_2$, especially near the critical end point, combining experimental spectroscopy with theoretical calculations.

Findings

Valence band states change significantly at temperature T_V.

Au and Ge doping alter T_V and induce antiferromagnetic order.

Near the critical end point, conduction bands split and flat bands emerge.

Abstract

Hard X-ray angle-resolved photoemission spectroscopy reveals significant alterations in the valence band states of EuPd$_2$Si$_2$ at a temperature $T_V$, where the Eu ions undergo a temperature-induced valence crossover from a magnetic Eu$^{2+}$ state to a low-temperature valence-fluctuating state. The introduction of small amounts of Au on Pd lattice sites and Ge on Si sites, respectively, results in a decrease in $T_V$ and the emergence of an antiferromagnetic state at low temperatures without valence fluctuations. It has been proposed that the boundary between AFM order and valence crossover represents a first-order phase transition associated with a specific type of second-order critical end point. In this scenario, strong coupling effects between fluctuating charge, spin, and lattice degrees of freedom are to be expected. In the case of EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ with x=0.13, which is situated close to the critical end point, a splitting of conduction band states and the emergence of flat bands with one-dimensional character have been observed. A comparison with ab initio theory demonstrates a high degree of correlation with experimental findings, particularly in regard to the bands situated in proximity to the critical end point.