COMMUNICATION 1805715 (1 of 6) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de An Ambipolar Superconducting Field-Effect Transistor Operating above Liquid Helium Temperature Genta Kawaguchi,* Andrey A. Bardin, Masayuki Suda, Mikio Uruichi, and Hiroshi M. Yamamoto* Dr. G. Kawaguchi, Dr. M. Suda, Dr. M. Uruichi, Prof. H. M. Yamamoto Research Center of Integrative Molecular Systems (CIMoS) Institute for Molecular Science Okazaki, Aichi 444-8585, Japan E-mail: gkawaguchi@ims.ac.jp; yhiroshi@ims.ac.jp Dr. A. A. Bardin Institute of Problems of Chemical Physics Russian Academy of Sciences Chernogolovka, Moscow Region 142432, Russia Dr. M. Suda, Prof. H. M. Yamamoto SOKENDAI (The Graduate University for Advanced Studies) Okazaki, Aichi 444-8585, Japan Dr. M. Suda, Prof. H. M. Yamamoto RIKEN Wako, Saitama 351-0198, Japan The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201805715. DOI: 10.1002/adma.201805715 parametric amplifiers, [5] or quantum computing. [6] Various SC FETs have been developed using high critical temperature (T C ) superconductors, [7] transition metal dichalcogenides, [8] graphenes, [9] SrTiO 3 / LaAlO 3 interface, [10] and so on. However, SC devices have inevitable constraints of operation conditions. For example, high T C SC device can raise operation tem- perature, but at the same time, minimum carrier density (d M ) required for supercon- ductivity switching increases. In general, the trade-off relation between T C and d M is known for strong-coupling limit. [9] High carrier density control by electric-double- layer-transistor (EDLT) is one of the solu- tions to this problem, but the ionic liquid becomes frozen at low temperature to pre- vent real-time switching. We are focusing on organic supercon- ductors κ-(BEDT-TTF) 2 X, where BEDT- TTF and X represent bis(ethylenedithio) tetrathiafulvalene and anion species, respectively. These materials are considered to be unconven- tional superconductors similar to high T C cuprates, showing relatively high T C > 10 K for organics. [11] This system has small carrier density (≈1.8 × 10 14 cm -2 ) corresponding to approxi- mately 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. [12–17] 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. Here, we report on an ambipolar SC FET operating with con- ventional SiO 2 dielectric above liquid He temperature (4.2 K). This is the first active FET utilizing κ-(BEDT-TTF) 2 Cu(NCS) 2 (κ-NCS) channel, designed by careful consideration of chem- ical pressure effect and strain from a SiO 2 /Si substrate. Recently, an ambipolar SC FET is also reported in magic-angle twisted bilayer graphene on SiO 2 /Si substrate, but its T C is below 1.7 K. [9] Therefore, to our knowledge, our κ-NCS device is the first example of normally OFF ambipolar SC FET that operates above 4.2 K, which might lead to novel and innova- tive applications. We also observed the resistance hysteresis at field-induce phase transition, which might not only be useful for switching functionality but also give a fresh insight into 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. Superconducting Devices Field-effect transistors (FETs) are essential for modern electronic devices, where conductance can be controlled by modulating carrier density via gate electric field. [1] Superconducting (SC) FETs [2] or Josephson FETs [3] that can switch between SC and normal states have attracted much attention, because they can not only replace conventional semiconductor-based FETs but also offer novel applications such as single photon detection, [4] Adv. Mater. 2018, 1805715