Two-dimensional ground-state mapping of a Mott-Hubbard system in a flexible field-effect device

Abstract

A Mott insulator sometimes induces unconventional superconductivity in its neighbors when doped and/or pressurized. Because the phase diagram should be strongly related to the microscopic mechanism of the superconductivity, it is important to obtain the global phase diagram surrounding the Mott insulating state. However, the parameter available for controlling the ground state of most Mott insulating materials is one-dimensional owing to technical limitations. Here, we present a two-dimensional ground-state mapping for a Mott insulator using an organic field-effect device by simultaneously tuning the bandwidth and bandfilling. The observed phase diagram showed many unexpected features such as an abrupt first-order superconducting transition under electron doping, a recurrent insulating phase in the heavily electron-doped region, and a nearly constant superconducting transition temperature in a wide parameter range. These results are expected to contribute toward elucidating one of the standard solutions for the Mott-Hubbard model.

Control of Organic Superconducting Field-Effect Transistor by Cooling Rate

“…A novel finding is the particular enhancement of superconductivity at half filling observed at cooling rate of 0.3 K min −1 , which is indicated by reddish violet region. Taking into account that the carrier doping here is very small (~a few % of band-filling) and takes place only at the first monolayer at FET interface [15], the current superconductivity enhancement should be difficult to understand in terms of the electron-phonon scattering [33] or the peculiar charge ordered state observed in cuprates [34]. At this stage, the mechanism behind this strange SC transition is an open question owing to the lack of appropriate experimental methods or theories.…”

“…Recently, it has also become possible to induce band-filling controlled Mott transition [5][6][7][8][9] or superconducting (SC) transition [10][11][12][13][14][15] in κ-BEDT-TTF salts, since our research group has developed field-effect transistors (FETs) utilizing the thin (~100 nm) single crystals [16,17]. Furthermore, flexible FETs [11] and electric-double-layer transistors (EDLTs) [15] enabled simultaneous control of band-filling and bandwidth in κ-BEDT-TTF salts. Based on the experimental results, we constructed a ground state phase diagram of strongly correlated system against band-filling and electron correlation [10,15].…”

“…Furthermore, flexible FETs [11] and electric-double-layer transistors (EDLTs) [15] enabled simultaneous control of band-filling and bandwidth in κ-BEDT-TTF salts. Based on the experimental results, we constructed a ground state phase diagram of strongly correlated system against band-filling and electron correlation [10,15]. However, an unexplored room still remains, particularly in the vicinity of bandwidth-driven Mott transition, where SC and Mott-insulating phases coexist [18].…”

Electrolyte‐Gating‐Induced Metal‐Like Conduction in Nonstoichiometric Organic Crystalline Semiconductors under Simultaneous Bandwidth Control

“…The four‐probe resistance of the sample was measured down to 1.9 K in a chamber of the Physical Property Measurement System (Quantum Design). The effect of the electrolyte gating could be limited within few nanometers from the sample surface; however, the total resistance of bulk samples could be tuned by electrolyte gating as reported for other organic salt of ET …”

“…A metal‐like conduction is achieved by positive charge injection into the β″‐layer by the electrolyte gating under simultaneous stress control. The present study opens up new approaches to heavy doping of organic semiconductors for novel electronic functions using metal–insulator transitions and superconductivity of strongly correlated electron systems …”