Collective modes, Yb-valence instability and metal-insulator transition in the cage-cluster borides RB12 (R- Ho, Er, Tm, Yb, Lu, Zr)

TL;DR

This study investigates the dynamic conductivity and collective modes in RB12 borides, revealing how Yb-valence instability influences metal-insulator transition and charge transport through Jahn-Teller dynamics and localized modes.

Contribution

It provides new insights into the role of Yb-valence instability and Jahn-Teller effects in the metal-insulator transition of RB12 borides, supported by comprehensive spectral analysis.

Findings

Yb-valence instability correlates with the metal-insulator transition.

Charge transport is dominated by non-equilibrium electrons and collective Jahn-Teller modes.

The MIT involves a decrease in Drude carriers and redistribution to localized modes.

Abstract

A thorough study of the wide-range (40-35000 cm-1) dynamic conductivity spectra of the rare-earth (RE) dodecaborides RB12 (R- Ho, Er and Tm) and Tm1-xYbxB12 substitutional solid solutions was carried out at room temperature. Both the Drude-type components and overdamped excitations have been separated and analyzed. An additional absorption band observed above 200 cm-1 in these RB12 with magnetic RE ions is attributed to the cooperative Jahn-Teller dynamics of the B12 complexes, which depends crucially on the RE-ion cage space and is compared with the same effect found in the non-magnetic LuB12 and ZrB12. It was shown that non-equilibrium (hot) electrons participating in the formation of the collective JT modes dominate in charge transport, and portion of Drude-type carriers changes by 20-40% in these compounds with unstable boron lattice. Strong renormalization of the infrared response is observed in Tm1-xYbxB12 solid solutions with metal-insulator transition (MIT) and is discussed in terms of localized collective modes caused by Yb-ion valence instability. We demonstrate that even at room temperature the MIT is accompanied with simultaneous decrease in concentration of Drude-type electrons and redistribution of carriers to localized JT collective modes.