Modern society has been sustained through the development of semiconductor devices based on Moore’s law. However, as the era of Moore’s law is drawing to a close, semiconductor research should shift toward next-generation technologies, namely beyond-CMOS technologies. In this context, electronic devices utilising strongly correlated electron materials have attracted attention as promising candidates. In particular, Mott memory that utilises an electric-field-induced Mott transition has been attracting attention as a promising device capable of operating at high speeds with low power consumption.

The 4d-electron Mott insulator Ca₂RuO₄ (CRO) has garnered significant attention as a promising candidate material for Mott memory because it shows an insulator-metal transition, namely switching (SW), induced by an electric field as low as 40 V/cm at room temperature (~295 K). However, in previous SW experiments on CRO, an electric field was applied using electrodes directly formed on the sample. Therefore, a large bias current inevitably flowed through the sample simultaneously with its metallisation. Consequently, whether an insulating CRO can be switched by an electric field alone, irrespective of the influence of Joule heating, remains unclear. Metallisation experiments using electrostatic methods are desired to eliminate the influence of heat. As a suitable technique, we adopted an electric double-layer transistor (EDLT), which is a type of field-effect transistor (FET) that uses ionic liquids or electrolytes, because it can accumulate nearly 100 times higher carrier density than MOSFET-type transistors on a material surface.

The EDLT device was fabricated on a cleaved single-crystalline CRO. The resistance was reduced at positive gate voltage V_G≥+3 V, corresponding to electron doping. When V_G returned from +4 to 0 V, the reduced resistance recovered by more than 90%. The reversibility of the gating process indicated that this resistance drop was an electrostatic phenomenon rather than an electrochemical reaction.

During the challenging COVID-19 lockdown, we accidentally discovered the most surprising feature of ion-gated metallisation of CRO: the long-term progression of resistance reduction up to 97%, by maintaining the gating experiments during the lockdown. This aging effect is difficult to explain solely in terms of electrostatic carrier doping on the surface. Indeed, the estimated thickness of the metallised layer (~ 100 nm) far exceeds that expected from surface doping alone. Due to the strong electron-lattice coupling, the structure of CRO is highly sensitive to metallisation. The enormous decrease in the resistance observed in this study may be explained by a percolation-like mechanism. That is, the initial surface metallisation triggered by gating induces structural changes, leading to a cascading progression of metallisation into the bulk interior. Our research deepens the understanding of nonlinear and nonequilibrium phenomena in correlated electron systems and opens up exciting possibilities for novel electronic devices, such as Mott memory.　

(Written by Fumihiko Nakamura and Tsutomu Nojima on behalf of all the authors)

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