Discovery of superconductivity in KTaO3 by electrostatic carrier doping.

Superconductivity at interfaces has been investigated since the first demonstration of electric-field-tunable superconductivity in ultrathin films in 1960(1). So far, research on interface superconductivity has focused on materials that are known to be superconductors in bulk. Here, we show that electrostatic carrier doping can induce superconductivity in KTaO(3), a material in which superconductivity has not been observed before. Taking advantage of the large capacitance of the self-organized electric double layer that forms at the interface between an ionic liquid and KTaO(3) (ref. 12), we achieve a charge carrier density that is an order of magnitude larger than the density that can be achieved with conventional chemical doping. Superconductivity emerges in KTaO(3) at 50 mK for two-dimensional carrier densities in the range 2.3 × 10(14) to 3.7 × 10(14) cm(-2). The present result clearly shows that electrostatic carrier doping can lead to new states of matter at nanoscale interfaces.

Electrically induced ferromagnetism at room temperature in cobalt-doped titanium dioxide.

The electric field effect in ferromagnetic semiconductors enables switching of the magnetization, which is a key technology for spintronic applications. We demonstrated electric field-induced ferromagnetism at room temperature in a magnetic oxide semiconductor, (Ti,Co)O(2), by means of electric double-layer gating with high-density electron accumulation (>10(14) per square centimeter). By applying a gate voltage of a few volts, a low-carrier paramagnetic state was transformed into a high-carrier ferromagnetic state, thereby revealing the considerable role of electron carriers in high-temperature ferromagnetism and demonstrating a route to room-temperature semiconductor spintronics.

Liquid-gated interface superconductivity on an atomically flat film.

Liquid/solid interfaces are attracting growing interest not only for applications in catalytic activities and energy storage, but also for their new electronic functions in electric double-layer transistors (EDLTs) exemplified by high-performance organic electronics, field-induced electronic phase transitions, as well as superconductivity in SrTiO(3) (ref. 12). Broadening EDLTs to induce superconductivity within other materials is highly demanded for enriching the materials science of superconductors. However, it is severely hampered by inadequate choice of materials and processing techniques. Here we introduce an easy method using ionic liquids as gate dielectrics, mechanical micro-cleavage techniques for surface preparation, and report the observation of field-induced superconductivity showing a transition temperature T(c)=15.2 K on an atomically flat film of layered nitride compound, ZrNCl. The present result reveals that the EDLT is an extremely versatile tool to induce electronic phase transitions by electrostatic charge accumulation and provides new routes in the search for superconductors beyond those synthesized by traditional chemical methods.

Electric-field-induced superconductivity in an insulator.

Electric field control of charge carrier density has long been a key technology to tune the physical properties of condensed matter, exploring the modern semiconductor industry. One of the big challenges is to increase the maximum attainable carrier density so that we can induce superconductivity in field-effect-transistor geometry. However, such experiments have so far been limited to modulation of the critical temperature in originally conducting samples because of dielectric breakdown. Here we report electric-field-induced superconductivity in an insulator by using an electric-double-layer gating in an organic electrolyte. Sheet carrier density was enhanced from zero to 10(14) cm(-2) by applying a gate voltage of up to 3.5 V to a pristine SrTiO(3) single-crystal channel. A two-dimensional superconducting state emerged below a critical temperature of 0.4 K, comparable to the maximum value for chemically doped bulk crystals, indicating this method as promising for searching for unprecedented superconducting states.

Superconductivity of doped Ar@C60.

The synthesis of a mg amount of pure argon containing fullerene allowed the synthesis of the first endohedral superconductors with critical temperatures lower than expected, an indication of the strong influence of the argon atom on the C60 cage.

Control of carrier density by self-assembled monolayers in organic field-effect transistors.

Organic thin-film transistors are attracting a great deal of attention due to the relatively high field-effect mobility in several organic materials. In these organic semiconductors, however, researchers have not established a reliable method of doping at a very low density level, although this has been crucial for the technological development of inorganic semiconductors. In the field-effect device structures, the conduction channel exists at the interface between organic thin films and SiO(2) gate insulators. Here, we discuss a new technique that enables us to control the charge density in the channel by using organosilane self-assembled monolayers (SAMs) on SiO(2) gate insulators. SAMs with fluorine and amino groups have been shown to accumulate holes and electrons, respectively, in the transistor channel: these properties are understood in terms of the effects of electric dipoles of the SAMs molecules, and weak charge transfer between organic films and SAMs.

Electronic interactions in a new fullerene dimer: C(122)H(4), with two methylene bridges

The isolation of a new fullerene dimer, C(122)H(4), and its structural characterization by (13)C NMR and (1)H NMR spectroscopy and by UV/vis and IR spectroscopy are reported. The structure of this dimer consists of two fullerene cages, which are directly connected through two C-C bonds and two methylene bridges. Consequently, adjacent hexagonal faces of the two fullerene cages are arranged in a face to face manner. Molecular orbital calculations indicate that the proximity of the fullerene cages results in significant through space overlap in both the HOMO and LUMO. As a consequence of this overlap, the electrochemistry of the dimer shows electronic communication with stepwise reduction of each cage.