Discovery of superconductivity in KTaO3 by electrostatic carrier doping

We present in this paper an overview of the work that has been performed on high-temperature superconductor superlattices, with a special emphasis on the superconducting properties of multilayers. The critical temperature and superconductivity in ultra-thin films, the vortex dynamics, and the field-temperature vortex phase diagram in multilayers are studied and compared to bulk behaviour. In particular, the roles of the individual layer thicknesses and the conductivity of the spacer material are studied, as well as the coupling between superconducting layers and the effect of finite multilayer thickness. Comparisons with other systems are made, and an interpretation is proposed for the understanding of superconductivity in ultra-thin oxide films. The field-temperature vortex phase diagram is determined in coupled multilayers and compared to single crystal results. Finally, we discuss the development of novel oxide systems and heterostructures, and the possibilities brought out by new material combinations.

The emergence of metallic conduction in layered dichalcogenide semiconductor materials by chemical doping is one of key issues for two-dimensional (2D) materials engineering. At present, doping methods for layered dichalcogenide materials have been limited to an ion intercalation between layer units or electrostatic carrier doping by electrical bias owing to the absence of appropriate substitutional dopant for increasing the carrier concentration. Here, we report the occurrence of metallic conduction in the layered dichalcogenide of SnSe2 by the direct Se-site doping with Cl as a shallow electron donor. The total carrier concentration up to ~1020 cm−3 is achieved by Cl substitutional doping, resulting in the improved conductivity value of ~170 S·cm−1 from ~1.7 S·cm−1 for non-doped SnSe2. When the carrier concentration exceeds ~1019 cm−3, the conduction mechanism is changed from hopping to degenerate conduction, exhibiting metal-insulator transition behavior. Detailed band structure calculation reveals that the hybridized s-p orbital from Sn 5s and Se 4p states is responsible for the degenerate metallic conduction in electron-doped SnSe2.

Density functional theory for differential capacitance of planar electric double layers in ionic liquids

Electro-static carrier doping was attempted in a layered transition metal disulphide MoS2 by constructing an electric double-layer transistor with an ionic liquid. With the application of gate voltage VG higher than 3 V, a metallic behavior was observed in the MoS2 channel. We found an onset of electric field-induced superconductivity in the field induced metallic phase. A maximum TC ∼ 9.4 K was observed, which could be higher than those in chemically doped bulk materials.

We report the modulation of electrical properties of charge-ordered YbFe2O4 films using electrostatic carrier doping. By utilizing the current switching effect under a high DC electric field as a probe, we detected the charge-ordered phase in the c-axis-oriented YbFe2O4 films grown by pulsed-laser deposition. A three-terminal electric double-layer device with a thin channel of YbFe2O4 and an ionic liquid (IL) gate dielectric showed reversible modulation of channel conductance under a positive gate voltage. Furthermore, we demonstrated that conductance modulation could be enhanced by customizing the molecular structure of the IL.

Room-temperature superconductivity in carbons – a mini review

High transition temperature (${T}_{\mathrm{c}}$) superconductivity in FeSe/$\mathrm{SrTi}{\mathrm{O}}_{3}$ has been widely discussed on the possible mechanisms in conjunction with the various effects of interface between FeSe and $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrate. By employing an electric-double-layer transistor configuration, which enables both the electrostatic carrier doping and electrochemical thickness tuning, we investigated the interfacial effect on the high-${T}_{\mathrm{c}}$ phase at around 40 K in FeSe films deposited on $\mathrm{SrTi}{\mathrm{O}}_{3}$, MgO, and $\mathrm{KTa}{\mathrm{O}}_{3}$ substrates. The systematic study on thickness dependence of transport properties under a certain gate voltage reveals the universal trend of the onset ${T}_{\mathrm{c}}$ against the Hall coefficient in all the FeSe films, irrespective of the substrate materials in which the different contribution of interfacial effect is expected. The independence of the highest ${T}_{\mathrm{c}}$ on substrate materials evidences that the high-${T}_{\mathrm{c}}$ superconductivity at around 40 K does not primarily originate from a specific interface combination but from a charge carrier filling at specific electronic band structure.

Electrochemical impedance spectroscopy on the capacitance of ionic liquid–acetonitrile electrolytes

Classical density functional theory (cDFT) is used to investigate electrosorption of ionic liquids in porous electrodes within the framework of a coarse-grained model. The purpose of this study is to clarify the influence of the side alkyl chains of imidazolium cations on the electric double layer (EDL) capacitance that was studied in a number of recent investigations but with contradictory trends. For an ionic liquid near a planar electrode, cDFT predicts that the capacitance falls by extending the alkyl chain length of cations because neutral segments reduce the packing density of counterions thus the charge density. The side-chain effect is more complicated for ionic liquids in micropores owing to space-charge competition. Adding neutral segments to imidazolium cations always reduces the capacitance in cases where the surface electrical potential of micropores is sufficiently large. However, the capacitance shows a nonmonotonic dependence on the alkyl chain length at intermediate surface potentials. Surprisingly, addition of neutral segments to the cations has the most pronounced effect on the EDL capacitance in cases when the surface potential is positively charged. These findings challenge the conventional assumption that the alkyl side chains of imidazolium ions only negatively impact ionic liquid performance in charge storage.

A self-consistent analytical solution of the problem of the superconductivity of ultrathin metal films is found within the tight-binding model for normal-metal electrons with a simple example of a film of three atomic layers. Superconductivity is not destroyed in atomically thin films if the energies of the electron subsystem lie near the Fermi surface at least for some values of the quasimomentum component along the film. A substantial increase in the critical temperature of an ultrathin metal film as compared to its bulk value is possible if the electron excitation spectrum contains low-energy modes with an anomalously weak dispersion.

The evolution of superconductivity in ultrathin films of Sn, Pb, Ga, Al, and In has been examined as a function of thickness and temperature. The films were grown in increments by condensation from the vapor onto substrates held at temperatures below 18 K. For each metal, global superconductivity or zero electrical resistance was found when the normal-state sheet resistance ${R}_{N}$ fell below a value close to h/4${e}^{2}$, or 6.45 k\ensuremath{\Omega}/\ensuremath{\square}, an observation uncorrelated with either structural or material parameters such as thickness or transition temperature. Prior evidence of superconductivity with nonzero resistance, local superconductivity, was found at earlier stages of film growth. All evidences of superconducting behavior were observed at temperatures close to the bulk transition temperature beginning in the range of thicknesses for which normal-state resistivities were greater than 200 \ensuremath{\mu}\ensuremath{\Omega}-cm and were rapidly changing with thickness. This implies that the films consisted of fully superconducting grains connected by tunneling junctions. The strong disorder represented by a broad distribution of junction parameters can be renormalized into weak disorder. Thus theoretical calculations based on regular arrays of superconducting sites coupled by (Josephson) junctions appear to be relevant. The extreme thinness of the films implies very small junction capacitances leading to large quantum fluctuations of the phase differences of their superconducting order parameters. Two classes of theories explaining a nearly universal resistance threshold for superconductivity have emerged. Both classes involve the quenching of these quantum fluctuations. In the limit of very small junction capacitances the threshold occurs at resistance values near h/4${e}^{2}$, and is essentially independent of the capacitance and the energy gap, in good agreement with the experimental data. Not contained in any of the models is an explanation of the observed regular variation of the low-temperature resistances of the films with the normal-state sheet resistance for values just above the resistance threshold.

Controlling the electronic properties of functional oxide materials via external electric fields has attracted increasing attention as a key technology for next-generation electronics. For transition-metal oxides with metallic carrier densities, the electric-field effect with ionic liquid electrolytes has been widely used because of the enormous carrier doping capabilities. The gate-induced redox reactions revealed by recent investigations have, however, highlighted the complex nature of the electric-field effect. Here, we use the gate-induced conductance modulation of spinel ZnxFe3−xO4 to demonstrate the dual contributions of volatile and non-volatile field effects arising from electronic carrier doping and redox reactions. These two contributions are found to change in opposite senses depending on the Zn content x; virtual electronic and chemical field effects are observed at appropriate Zn compositions. The tuning of field-effect characteristics via composition engineering should be extremely useful for fabricating high-performance oxide field-effect devices.

The oxygen-tolerant operation of ionic-gating has been achieved by all-solid-state ionic devices with a nano-grained Yttria-stabilized ZrO2 (YSZ) proton conductor/SrTiO3 (STO) single crystal. The drain current of the transistor was well tuned, even under atmospheric conditions (including approximately 200 mbar of oxygen gas), in contrast to STO-based ionic-gating transistors composed of ionic liquids, in which carrier doping is completely suppressed by even a small amount of oxygen gas (0.1 mbar). This oxygen-tolerance is attributed to the gas-tight nature of all-solid-state devices. Furthermore, complete suppression of metallization in the presence of oxygen gas, which was previously seen as evidence of electrochemical carrier doping (ECCD), was found to be unrelated to the type of carrier doping, i.e. electrostatic carrier doping or ECCD. The oxygen-tolerant operation achieved by all-solid-state ionic-gating devices is a great advantage for practical applications.

Electrostatic carrier doping using a field-effect-transistor structure is an intriguing approach to explore electronic phases by critical control of carrier concentration. We demonstrate the reversible control of the insulator–metal transition (IMT) in a quasi-two-dimensional (2D) electron gas at the interface of insulating SrTiO 3 single crystals. Superconductivity was observed in one device doped far beyond the IMT, which may imply the presence of 2D metal–superconductor transition. This realization of a quasi-two-dimensional metallic state on the most widely-used perovskite oxide is the best manifestation of the potential of oxide electronics.

The two-dimensional superconductor-insulator transition