Current driven insulator-to-metal transition without Mott breakdown in Ca$_2$RuO$_4$

TL;DR

This study reveals that the insulator-to-metal transition in Ca$_2$RuO$_4$ driven by current does not involve Mott breakdown but results from structural domain coexistence reducing the electronic gap, challenging previous assumptions.

Contribution

The paper demonstrates that the current-induced IMT in Ca$_2$RuO$_4$ occurs without Mott breakdown, highlighting the role of structural inhomogeneity and domain boundaries in conductivity increase.

Findings

The IMT occurs with minimal reorganization of the Mott state.

Structural domains coexist and facilitate gap reduction.

The transition does not require metallic and insulating phase coexistence.

Abstract

The electrical control of a material's conductivity is at the heart of modern electronics. Conventionally, this control is achieved by tuning the density of mobile charge carriers. A completely different approach is possible in Mott insulators such as Ca$_2$RuO$_4$, where an insulator-to-metal transition (IMT) can be induced by a weak electric field or current. This phenomenon has numerous potential applications in, e.g., neuromorphic computing. While the driving force of the IMT is poorly understood, it has been thought to be a breakdown of the Mott state. Using in operando angle-resolved photoemission spectroscopy, we show that this is not the case: The current-driven conductive phase arises with only a minor reorganisation of the Mott state. This can be explained by the co-existence of structurally different domains that emerge during the IMT. Electronic structure calculations show that the boundaries between domains of slightly different structure lead to a drastic reduction of the overall gap. This permits an increased conductivity, despite the persistent presence of the Mott state. This mechanism represents a paradigm shift in the understanding of IMTs, because it does not rely on the simultaneous presence of a metallic and an insulating phase, but rather on the combined effect of structurally inhomogeneous Mott phases.