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

Abstract

We present the carrier transport properties in the vicinity of a doping-driven Mott transition observed at a field-effect transistor (FET) channel using a single crystal of the typical two-dimensional organic Mott insulator κ-(BEDT-TTF)CuN(CN)Cl (κ-Cl). The FET shows a continuous metal-insulator transition (MIT) as electrostatic doping proceeds. The phase transition appears to involve two-step crossovers, one in Hall measurement and the other in conductivity measurement. The crossover in conductivity occurs around the conductance quantum e/h, and hence is not associated with "bad metal" behavior, which is in stark contrast to the MIT in half-filled organic Mott insulators or that in doped inorganic Mott insulators. Through in-depth scaling analysis of the conductivity, it is found that the above carrier transport properties in the vicinity of the MIT can be described by a high-temperature Mott quantum critical crossover, which is theoretically argued to be a ubiquitous feature of various types of Mott transitions.

Fabrication and Operation of Monolayer Mott FET at Room Temperature

“…For Mott-FET fabricated with κ-type single crystal κ-(BEDT-TTF) 2 Cu-[N(CN) 2 ]Cl, mobility over 200 cm 2 V ¹1 s ¹1 was achieved, while metallic conduction was observed only at low temperature because of "the contribution from the undoped bulk channel". 5,6 In this case, the Mott gap is even smaller than the previous cases, and thus Mott insulator crystal with hundreds of nanometers thickness could not offer a high resistance like a normal organic band insulator with large HOMO-LUMO gap. Indeed, the transport carriers are accumulated only in the first few layers in FET operation, 7 while most of the layers without carrier accumulation still remain in Mott insulating state.…”

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

“…Slight modification of parameters may alter the stability of the two. Now in κ-Cl, both electron and hole doping are realized using the EDLT 46,47 . The filling-control MI transition and the emergence of SC have been confirmed with significant electron-hole doping asymmetry.…”

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

“…Remarkably, SC transition could be driven by the small carrier modulation d M ≈ 6.3 × 10 12 cm −2 corresponding to ≈0.035 electron or hole per BEDT‐TTF dimer. However, the field‐induced resistance change in device 1 is not so large as those in previous FETs based on κ‐type salts . It is probably because the upper layers of the κ‐NCS channel far from interface between channel and gate insulator have non‐negligible conductivity.…”

“…We previously reported that the transport properties of FETs based on κ‐Br or κ‐Cl are strongly influenced by cooling‐induced 2D strain that is caused by mismatch of thermal expansion coefficients between organic channel and the substrate . The κ‐Br device on SiO 2 /Si substrate exhibited insulating nature, reminiscent of bulk κ‐Cl, suggesting a negative pressure effect of about −50 MPa (Figure e).…”

“…This system has small carrier density (≈1.8 × 10 14 cm −2 ) corresponding to approximately one third of that of cuprates owing to molecule‐based structure with large crystal lattice. An important character of the devices utilizing κ‐BEDT‐TTF salts is susceptible nature toward strain effect, with which the low‐temperature electronic state and FET property can be controlled . However, we have been utilizing only κ‐(BEDT‐TTF) 2 Cu[N(CN) 2 ]Br (κ‐Br) and κ‐(BEDT‐TTF) 2 Cu[N(CN) 2 ]Cl (κ‐Cl) for FETs in previous reports, and the chemical variety is still limited.…”