Molecular Orbital Degeneracy Lifting in a Tetrahedral Cluster System NbSeI

TL;DR

This study uncovers two mechanisms that lift orbital degeneracy in NbSeI's Nb4 clusters, revealing complex electronic states and phase transitions driven by local distortions and molecular orbital interactions.

Contribution

It demonstrates the existence of two distinct degeneracy-lifting mechanisms in a high-symmetry cluster compound, advancing understanding of electronic order in molecular cluster systems.

Findings

NbSeI is a nonmagnetic insulator below 106 K.

Local distortions persist above 106 K, indicating a molecular orbital-liquid or frozen state.

The material exhibits a noncooperative Jahn-Teller distortion stabilizing the insulating phase.

Abstract

The lifting of degenerate electronic states, in which multiple electronic states share the same energy, is a fundamental issue in the physics of crystalline solids. In real materials, this problem has been extensively studied in transition metal compounds, where various quantum phenomena arise from the spin and orbital degeneracy of the d electrons on individual transition-metal atoms. In contrast, materials containing high-symmetry clusters composed of multiple transition-metal atoms are expected to exhibit more emergent phenomena due to the entanglement of the electronic degrees of freedom across multiple atoms. Here, we report the discovery of two distinct mechanisms of orbital-degeneracy lifting in NbSeI, which comprises Nb4 tetrahedral clusters with molecular orbital degrees of freedom and whose average crystal structure is predicted to host a flat-band metal. Below 106 K, NbSeI is found to be a nonmagnetic molecular orbital-ordered insulator. Above this temperature, the average structure becomes face-centered cubic without any superlattice, while the orbital degeneracy remains lifted by significant local distortions of Nb4 tetrahedra, which may be associated with a molecular orbital-liquid or orbital-frozen state. This noncooperative Jahn-Teller distortion stabilizes a nonmagnetic insulating state above 106 K, in stark contrast to the flat-band metal predicted from the average structure.