Nanoscale orbital excitations and the infrared spectrum of a molecular Mott insulator: A15-Cs$_{3}$C$_{60}$

TL;DR

This paper reveals that nanoscale orbital excitations in the Mott insulator Cs₃C₆₀ explain its infrared spectral features without static distortions, emphasizing the importance of quantum treatment of nuclear and orbital dynamics.

Contribution

It introduces a quantum model for orbital excitations in a molecular Mott insulator, explaining IR spectra without static distortions and highlighting dynamic nuclear and orbital effects.

Findings

IR spectral features explained by quantum orbital excitations

Absence of static Jahn-Teller distortions in spectra

Temperature-dependent spectral changes due to low-lying multiplet states

Abstract

The quantum physics of ions and electrons behind low-energy spectra of strongly correlated {\it molecular} conductors, superconductors and Mott insulators is poorly known, yet fascinating especially in orbitally degenerate cases. The fulleride insulator Cs$_{3}$C$_{60}$ (A15), one such system, exhibits infrared (IR) spectra with low temperature peak features and splittings suggestive of static Jahn-Teller distortions with breakdown of orbital symmetry in the molecular site. That is puzzling, for there is no detectable static distortion, and because the features and splittings disappear upon modest heating, which they should not. Taking advantage of the Mott-induced collapse of electronic wavefunctions from lattice-extended to nanoscale localized inside a caged molecular site, we show that unbroken spin and orbital symmetry of the ion multiplets explains the IR spectrum without adjustable parameters. This demonstrates the importance of a fully quantum treatment of nuclear positions and orbital momenta in the Mott insulator sites, dynamically but not statically distorted. The observed demise of these features with temperature is explained by the thermal population of a multiplet term whose nuclear positions are essentially undistorted, but whose energy is very low-lying. That term is in facts a scaled-down orbital excitation analogous to that of other Mott insulators, with the same spin $1/2$ as the ground state, but with a larger orbital moment of two instead of one.