Evidences for local non-centrosymmetricity andstrong phonon anomaly in EuCu 2 As 2 :A Raman spectroscopy and lattice dynamics study.

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 P4/nmm
crystal structure, contrary to that what is expected from the accepted symmorphic
I4/mmm structure. Lattice dynamics calculations using the P4/nmm symmetry attest
that there is a commendable agreement between the calculated phonon spectra at
the Γ point and the observed Raman mode frequencies, with the most intense peak
at ∼ 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.