Metal-insulator transition in PrRu$_4$P$_{12}$ and SmRu$_4$P$_{12}$ investigated by optical spectroscopy

TL;DR

This study uses optical spectroscopy to investigate the electronic structures of PrRu$_4$P$_{12}$ and SmRu$_4$P$_{12}$, revealing energy gaps and phonon anomalies associated with their metal-insulator transitions, and discusses possible density wave or orbital ordering mechanisms.

Contribution

First optical spectroscopy analysis of PrRu$_4$P$_{12}$ and SmRu$_4$P$_{12}$ elucidating their energy gap formation and phonon anomalies during MIT.

Findings

Energy gap of ~10 meV develops below $T_{MI}$

Optical phonon peaks show broadening, shifts, and new peaks below $T_{MI}$

Characteristics differ from Kondo semiconductors

Abstract

Electronic structures of the filled-skutterudite compounds PrRu$_4$P$_{12}$ and SmRu$_4$P$_{12}$, which undergo a metal-insulator transition (MIT) at $T_{\rm MI}$ = 60 K and 16 K, respectively, have been studied by means of optical spectroscopy. Their optical conductivity spectra develop an energy gap of $\sim$ 10 meV below $T_{\rm MI}$. The observed characteristics of the energy gap are qualitatively different from those of the Kondo semiconductors. In addition, optical phonon peaks in the spectra show anomalies upon the MIT, including broadening and shifts at $T_{\rm MI}$ and an appearance of new peaks below $T_{\rm MI}$. These results are discussed in terms of density waves or orbital ordering previously predicted for these compounds.