Multi-orbital physics in lithium-molybdenum purple-bronze: going beyond paradigm

TL;DR

This paper explores the complex multi-orbital interactions in lithium molybdenum purple bronze, providing a new excitonic model that explains various experimental phenomena including spectral broadening, resistivity behavior, and anomalous thermal and magnetic properties.

Contribution

It introduces a novel excitonic model to describe multi-orbital effects in purple bronze, advancing understanding beyond traditional single-orbital approaches.

Findings

Spectral function broadening with Gaussian profile matches experiments.

Resistivity upturn at 28K explained by multi-orbital excitations.

Suppression of resistivity component under magnetic field aligns with model predictions.

Abstract

We investigate the role of inter-orbital fluctuations in the low energy physics of a quasi-1D material - lithium molybdenum purple bronze (LMO). It is an exceptional material that may provide us a long sought realization of a Tomonaga-Luttinger liquid (TLL) physics, but its behaviour at temperatures of the order of $T^*\approx 30$K remains puzzling despite numerous efforts. Here we make a conjecture that the physics around $T^*$ is dominated by multi-orbital excitations. Their properties can be captured using an excitonic picture. Using this relatively simple model we compute fermionic Green's function in the presence of excitons. We find that the spectral function is broadened with a Gaussian and its temperature dependence acquires an extra $T^1$ factor. Both effects are in perfect agreement with experimental findings. We also compute the resistivity for temperatures above and below critical temperature $T_o$. We explain an upturn of the resistivity at 28K and interpret the suppression of this extra component of resistivity when a magnetic field is applied along the conducting axis. Furthermore, in the framework of our model, we qualitatively discuss and consistently explain other experimentally detected peculiarities of purple bronze: the breaking of Wiedmann-Franz law and the magnetochromatic behaviour.