Unveiling the electronic transformations in the semi-metallic correlated-electron transitional oxide Mo8O23

V Nasretdinova,A Meden,V Kabanov,Ya A Gerasimenko,G Gatti,A Yu Kuntsevich,J Mravlje,D Mihailovic,M Grioni,P Sutar,D Svetin

doi:10.1038/s41598-019-52231-4

Abstract

Mo8O23 is a low-dimensional chemically robust transition metal oxide coming from a prospective family of functional materials, MoO3−x, ranging from a wide gap insulator (x = 0) to a metal (x = 1). The large number of stoichometric compounds with intermediate x have widely different properties. In Mo8O23, an unusual charge density wave transition has been suggested to occur above room temperature, but its low temperature behaviour is particularly enigmatic. We present a comprehensive experimental study of the electronic structure associated with various ordering phenomena in this compound, complemented by theory. Density-functional theory (DFT) calculations reveal a cross-over from a semi-metal with vanishing band overlap to narrow-gap semiconductor behaviour with decreasing temperature. A buried Dirac crossing at the zone boundary is confirmed by angle-resolved photoemission spectroscopy (ARPES). Tunnelling spectroscopy (STS) reveals a gradual gap opening corresponding to a metal-to-insulator transition at 343 K in resistivity, consistent with CDW formation and DFT results, but with large non-thermal smearing of the spectra implying strong carrier scattering. At low temperatures, the CDW picture is negated by the observation of a metallic Hall contribution, a non-trivial gap structure in STS below ∼170 K and ARPES spectra, that together represent evidence for the onset of the correlated state at 70 K and the rapid increase of gap size below ∼30 K. The intricate interplay between electronic correlations and the presence of multiple narrow bands near the Fermi level set the stage for metastability and suggest suitability for memristor applications.

Highlights

Mo8O23 is a low-dimensional chemically robust transition metal oxide coming from a prospective family of functional materials, MoO3−x, ranging from a wide gap insulator (x = 0) to a metal (x = 1)
No noticeable softening around the structural transitions in either of the visible phonon modes were observed in the recent combined transient reflectivity spectroscopy and Raman studies[11], questioning this scenario too
To unveil the nature of the ground state and the sequence of transformations leading to it, here we explore the evolution of the electronic structure in the phase diagram of Mo8O23 using spectroscopic (STS, angle-resolved photoemission spectroscopy (ARPES)) and magnetotransport measurements on high-quality single crystals, and compare them with the Density-functional theory (DFT) predictions

Summary

Introduction

We find that despite quantitative agreement between DFT calculations and spectroscopic data on the scale of ∼1 eV, the low-temperature low-energy behaviour of Mo8O23 electronic spectra is that of a strongly correlated multiband material, in contrast with a narrow band semi-metal to semiconductor CDW-like transition model anticipated from DFT. This discrepancy is driven by the previously unresolved purely electronic transition at 70 K with no structural involvement. The present data allows building of the unifying picture behind the phase diagram of Mo8O23

Methods

Results

Conclusion