Critical Behavior in Doping-Driven Metal-Insulator Transition on Single-Crystalline Organic Mott-FET

Link between T-Linear Resistivity and Quantum Criticality in Ambipolar Black Phosphorus

The interplay between strong Coulomb interactions and kinetic energy leads to intricate many-body competing ground states owing to quantum fluctuations in 2D electron and hole gases. However, the simultaneous observation of quantum critical phenomena in both electron and hole regimes remains elusive. Here, we utilize anisotropic black phosphorus (BP) to show density-driven metal-insulator transition with a critical conductance ∼e2/h which highlights the significant role of quantum fluctuations in both hole and electron regimes. We observe a T-linear resistivity from the deep metallic phase to the metal-insulator boundary at moderate temperatures, while it turns to Fermi liquid behavior in the deep metallic phase at low temperatures in both regimes. An analysis of the resistivity suggests that disorder-dominated transport leads to T-linear behavior in the hole regime, while in the electron regime, the T-linear resistivity results from strong Coulomb interactions, suggestive of strange-metal behavior. Successful scaling collapse of the resistivity in the T-linear region demonstrates the link between quantum criticality and the T-linear resistivity in both regimes. Our study provides compelling evidence that ambipolar BP could serve as an exciting testbed for investigating exotic states and quantum critical phenomena in hole and electron regimes of 2D semiconductors.

Percolation-Based Metal-Insulator Transition in Black Phosphorus Field Effect Transistors

The existence of a novel phenomenon, such as the metal-insulator transition (MIT) in two-dimensional (2D) systems, affords emerging functional properties that provide new aspects for future electronics and optoelectronics. Here, we report the observation of the MIT in black phosphorus field effect transistors by tuning the carrier density (n) controlled by back-gate bias. We find that the conductivity follows an n dependence as σ(n) ∝ n^α with α ∼ 1, which indicates the presence of screened Coulomb impurity scattering at high carrier densities in the temperature range of 10-300 K. As n decreases, the screened Coulomb impurity scattering breaks down, developing strong charge density inhomogeneity leading to a percolation-based transition at the critical carrier density (n_C). At low carrier densities (n < n_C), the system is in the insulating regime, which is expressed by Mott variable range hopping that demonstrates the role of disorder in the system. In addition, the extracted average values of critical exponent δ are ∼1.29 ± 0.01 and ∼1.14 ± 0.01 for devices A and B, respectively, consistent with the 2D percolation exponent of ⁴/₃, confirming the 2D percolation-based MIT in BP devices. Our findings strongly suggest that the 2D MIT observed in BP is a classical percolation-based transition caused by charge inhomogeneity induced by screened Coulomb charge impurity scattering around a transition point controlled by n through back-gate bias.

Two-dimensional hole gas in organic semiconductors

A highly conductive metallic gas that is quantum mechanically confined at a solid-state interface is an ideal platform to explore non-trivial electronic states that are otherwise inaccessible in bulk materials. Although two-dimensional electron gases have been realized in conventional semiconductor interfaces, examples of two-dimensional hole gases, the counterpart to the two-dimensional electron gas, are still limited. Here we report the observation of a two-dimensional hole gas in solution-processed organic semiconductors in conjunction with an electric double layer using ionic liquids. A molecularly flat single crystal of high-mobility organic semiconductors serves as a defect-free interface that facilitates two-dimensional confinement of high-density holes. A remarkably low sheet resistance of 6 kΩ and high hole-gas density of 10¹⁴ cm^-2 result in a metal-insulator transition at ambient pressure. The measured degenerate holes in the organic semiconductors provide an opportunity to tailor low-dimensional electronic states using molecularly engineered heterointerfaces.

Electric Double Layer Doping of Charge-Ordered Insulators α-(BEDT-TTF)2I3 and α-(BETS)2I3

Field-effect transistors based on strongly correlated insulators are an excellent platform for studying the electronic phase transition and simultaneously developing phase transition transistors. Molecular conductors are suitable for phase transition transistors owing to the high tunability of the electronic states. Molecular Mott transistors show field-induced phase transitions including superconducting transitions. However, their application to charge-ordered insulators is limited. In this study, we fabricated electric double layer transistors based on quarter-filled charge-ordered insulators α-(BEDT-TTF)2I3 and α-(BETS)2I3. We observed ambipolar field effects in both compounds where both electron and hole doping (up to the order of 1013 cm−2) reduces the resistance by the band filling shift from the commensurate value. The maximum field-effect mobilities are approximately 10 and 55 cm2/Vs, and the gate-induced conductivities are 0.96 and 3.6 e2/h in α-(BEDT-TTF)2I3 and α-(BETS)2I3, respectively. However, gate-induced metallic conduction does not emerge. The gate voltage dependence of the activation energy in α-(BEDT-TTF)2I3 and the Hall resistance in α-(BETS)2I3 imply that the electric double layer doping in the present experimental setup induces hopping transport rather than band-like two-dimensional transport.

Control of Organic Superconducting Field-Effect Transistor by Cooling Rate

A new superconducting field-effect transistor (FET) in the vicinity of bandwidth-controlled Mott transition was developed using molecular strongly correlated system κ-(BEDT-TTF)2Cu[N(CN)2]Br [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] laminated on CaF2 substrate. This device exhibited significant cooling-rate dependence of resistance below about 80 K, associated with glass transition of terminal ethylene group of BEDT-TTF molecule, where more rapid cooling through glass transition temperature leads to the decrease in bandwidth. We demonstrated that the FET properties such as ON/OFF ratio and polarity can be controlled by utilizing cooling rate. Our result may give a novel insight into the design of molecule-based functional devices.

Mechanism of superconductivity and electron-hole doping asymmetry in κ-type molecular conductors

Unconventional superconductivity in molecular conductors is observed at the border of metal-insulator transitions in correlated electrons under the influence of geometrical frustration. The symmetry as well as the mechanism of the superconductivity (SC) is highly controversial. To address this issue, we theoretically explore the electronic properties of carrier-doped molecular Mott system κ-(BEDT-TTF)₂X. We find significant electron-hole doping asymmetry in the phase diagram where antiferromagnetic (AF) spin order, different patterns of charge order, and SC compete with each other. Hole-doping stabilizes AF phase and promotes SC with d_xy-wave symmetry, which has similarities with high-T_c cuprates. In contrast, in the electron-doped side, geometrical frustration destabilizes the AF phase and the enhanced charge correlation induces another SC with extended-s + \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$d_{x^2 - y^2}$$\end{document}dx2-y2wave symmetry. Our results disclose the mechanism of each phase appearing in filling-control molecular Mott systems, and elucidate how physics of different strongly-correlated electrons are connected, namely, molecular conductors and high-T_c cuprates.

The mechanism of unconventional superconductivity in molecular conductors remains controversial. Here, Watanabe et al. theoretically study and report electron-hole doping asymmetry and competing orders with superconductivity in a doped molecular Mott system.

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

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.

An Ambipolar Superconducting Field-Effect Transistor Operating above Liquid Helium Temperature

Superconducting (SC) devices are attracting renewed attention as the demands for quantum-information processing, meteorology, and sensing become advanced. The SC field-effect transistor (FET) is one of the elements that can control the SC state, but its variety is still limited. Superconductors at the strong-coupling limit tend to require a higher carrier density when the critical temperature (T_C ) becomes higher. Therefore, field-effect control of superconductivity by a solid gate dielectric has been limited only to low temperatures. However, recent efforts have resulted in achieving n-type and p-type SC FETs based on organic superconductors whose T_C exceed liquid He temperature (4.2 K). Here, a novel "ambipolar" SC FET operating at normally OFF mode with T_C of around 6 K is reported. Although this is the second example of an SC FET with such an operation mode, the operation temperature exceeds that of the first example, or magic-angle twisted-bilayer graphene that operates at around 1 K. Because the superconductivity in this SC FET is of unconventional type, the performance of the present device will contribute not only to fabricating SC circuits, but also to elucidating phase transitions of strongly correlated electron systems.

Field-, strain- and light-induced superconductivity in organic strongly correlated electron systems

Stimulated by the discovery of high-temperature superconductivity in 1986, band-filling control of strongly correlated electron systems has been a persistent challenge over the past three decades in condensed matter science. In particular, recent efforts have been focused on electrostatic carrier doping of these materials, utilising field-effect transistor (FET) structures to find novel superconductivity. Our group found the first field-induced superconductivity in an organic-based material in 2013 and has been developing various types of superconducting organic FETs. In this perspective, we summarise our recent results on the development of novel superconducting organic FETs. In addition, this perspective describes novel functionality of superconducting FETs, such as strain- and light-responsivity. We believe that the techniques and knowledge described here will contribute to advances in future superconducting electronics as well as the understanding of superconductivity in strongly correlated electron systems.