Evidences for local non-centrosymmetricity and strong phonon anomaly in EuCu2As2: A Raman spectroscopy and lattice dynamics study

TL;DR

This study uses Raman spectroscopy and lattice dynamics calculations to reveal local non-centrosymmetricity and phonon anomalies in EuCu2As2, indicating possible electronic density wave instability below 165 K.

Contribution

It provides the first evidence of local non-centrosymmetricity in EuCu2As2 and links phonon anomalies to electronic instabilities, challenging the assumption of a symmetric crystal structure.

Findings

Presence of seven Raman peaks consistent with non-centrosymmetric structure

Anomalous phonon behavior observed around 165 K

Possible electronic density wave instability below 165 K

Abstract

Phonon modes and their association with the electronic states have been investigated for the metallic EuCu$_{2}$As$_{2}$ system. In this work, we present the Raman spectra of this pnictide system which clearly shows the presence of seven well defined peaks above $100$~cm$^{-1}$ that is consistent with the locally non-centrosymmetric {\it P4/nmm} crystal structure, contrary to that what is expected from the accepted symmorphic {\it I4/mmm} structure. Lattice dynamics calculations using the {\it P4/nmm} symmetry attest that there is a commendable agreement between the calculated phonon spectra at the $\Gamma$ point and the observed Raman mode frequencies, with the most intense peak at $\sim 232$~cm$^{-1}$ being ascribed to the A$_{1g}$ mode. Temperature dependent Raman measurements show that there is a significant deviation from the expected anharmonic behaviour around $165$~K for the A$_{1g}$ mode, with anomalies being observed for several other modes as well, although to a lesser extent. Attempts are made to rationalize the observed anomalous behavior related to the hardening of the phonon modes, with parallels being drawn from metal dichalcogenide and allied systems. Similarities in the evolution of the Raman peak frequencies with temperature seem to suggest a strong signature of a subtle electronic density wave instability below $165$~K in this compound.