Time-resolved reflectivity and Raman studies of the interplay of enigmatic orders in Mo$_8$O$_{23}$

Abstract

Monoclinic semi-metallic Mo$_8$O$_{23}$ belongs to a multifunctional series of compounds showing multiple ordering phenomena that have not achieved much attention till now. Previous X-rays studies of this compound have revealed an incommensurate ordering transition at $T_{\mathrm{IC}}\sim350$ K, followed by a structural transition to commensurate order at $T_{\mathrm{IC-C}}=285$ K. In addition, an enigmatic resistance maximum is observed at $T_{\mathrm{el}}\sim150$ K, whose origin has so far proved elusive. Aiming to disentangle these multiple orders we use the polarized transient optical spectroscopy supplemented by Raman spectroscopy to study the electronic relaxation dynamics and lattice vibrational modes in Mo$_8$O$_{23}$ single crystals. Remarkably, both the coherent vibrational mode response and single particle response display extrema of damping/relaxation times close to $T_{\mathrm{el}}$ with the concurrent appearance of new coherent vibrational modes and a characteristic polarization asymmetry which saturates below $T_{\mathrm{el}}$. The single-particle relaxation data analysis shows the appearance of a temperature-independent gap in the electronic excitation spectrum below $T_{\mathrm{IC}}$ and additional temperature-dependent gap opening near $T_{\mathrm{el}}$. Concurrently, a low frequency vibrational mode shows anomalous softening around $T_{\mathrm{m}}\sim200$ K, far below $T_{\mathrm{IC-C}}$ and $T_{\mathrm{IC}}$. The observations are interpreted in terms of the appearance of a hidden gapped state below $T_{\mathrm{el}}$ that has so far eluded detection by structural analyses.