Anomalous State Sandwiched between Fermi Liquid and Charge Ordered Mott-Insulating Phases of Ti4O7

TL;DR

This study investigates the electronic structure of Ti4O7 across its phase transitions, revealing an anomalous state between a Fermi liquid and a Mott insulator, characterized by pseudogap behavior and coherent screening.

Contribution

It provides detailed spectroscopic evidence of an intermediate anomalous phase in Ti4O7, highlighting its unique electronic properties between known phases.

Findings

Presence of a metallic Fermi-edge above Tc2

Insulating gap of 100 meV below Tc1

Pseudogap and coherent screening in the intermediate phase

Abstract

The Magneli phase Ti4O7 exhibits two sharp jumps in resistivity with coupled structural transitions as a function of temperature at Tc1=142 K and Tc2=154 K. We have studied electronic structure changes across the two transitions using 7 eV laser, soft x-ray and hard x-ray (HX) photoemission spectroscopy (PES). Ti 2p-3d resonant PES and HX-PES show a clear metallic Fermi-edge and mixed valency above Tc2. The low temperature phase below Tc1 shows a clear insulating gap of 100 meV. The intermediate phase between Tc1 and Tc2 indicates a pseudogap coexisting with remnant coherent states. HX-PES and complementary calculations have confirmed the coherent screening in the strongly correlated intermediate phase. The results suggest existence of a highly anomalous state sandwiched between the mixed-valent Fermi liquid and charge ordered Mott-insulating phase in Ti4O7.