Electric-field-induced superconductivity in an insulator

Controlling electron-phonon interactions in graphene at ultrahigh carrier densities

We report on the temperature dependent electron transport in graphene at different carrier densities n. Employing an electrolytic gate, we demonstrate that n can be adjusted up to 4 × 10(14)  cm(-2) for both electrons and holes. The measured sample resistivity ρ increases linearly with temperature T in the high temperature limit, indicating that a quasiclassical phonon distribution is responsible for the electron scattering. As T decreases, the resistivity decreases more rapidly following ρ(T) ∼ T(4). This low temperature behavior can be described by a Bloch-Grüneisen model taking into account the quantum distribution of the two-dimensional acoustic phonons in graphene. We map out the density dependence of the characteristic temperature Θ(BG) defining the crossover between the two distinct regimes, and show that, for all n, ρ(T) scales as a universal function of the normalized temperature T/Θ(BG).

Electrostatic and electrochemical nature of liquid-gated electric-double-layer transistors based on oxide semiconductors

The electric-double-layer (EDL) formed at liquid/solid interfaces provides a broad and interdisciplinary attraction in terms of electrochemistry, photochemistry, catalysts, energy storage, and electronics because of the large interfacial capacitance coupling and its ability for high-density charge accumulation. Much effort has recently been devoted to the fundamental understanding and practical applications of such highly charged EDL interfaces. However, the intrinsic nature of the EDL charging, whether it is electrostatics or electrochemistry, and how to distinguish them are far from clear. Here, by combining electrical transport measurements with electrochemical impedance spectroscopy (EIS), we studied the charging mechanisms of highly charged EDL interfaces between an ionic liquid and oxide semiconductor, ZnO. The direct measure for mobile carriers from the Hall effect agreed well with that from the capacitance-voltage integration at 1 Hz, implying that the pseudocapacitance does not contribute to carrier transport at EDL interfaces. The temperature-frequency mapping of EIS was further demonstrated as a "phase diagram" to distinguish the electrostatic or electrochemical nature of such highly charged EDL interfaces with densities of up to 8 × 10(14) cm(-2), providing a guideline for electric-field-induced electronic phenomena and a simple method for distinguishing electrostatic and electrochemical charging in EDLTs not only based on a specific oxide semiconductor, ZnO, but also commonly applicable to all types of EDL interfaces with extremely high-density carrier accumulation.

Field-induced water electrolysis switches an oxide semiconductor from an insulator to a metal

Water is composed of two strong electrochemically active agents, H(+) and OH(-) ions, but has not been used as an active electronic material in oxide semiconductors. In this study, we demonstrate that water-infiltrated nanoporous glass electrically switches an oxide semiconductor from insulator to metal. We fabricated a field-effect transistor structure on an oxide semiconductor, SrTiO(3), using water-infiltrated nanoporous glass-amorphous 12CaO·7Al(2)O(3)-as the gate insulator. Positive gate voltage, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO(3) surface at room temperature. This leads to the formation of a thin (~3 nm) metal layer with an extremely high electron concentration (10(15)-10(16) cm(-2)), which exhibits exotic thermoelectric behaviour. The electron activity of water as it infiltrates nanoporous glass may find many useful applications in electronics or in energy storage.

Electrically defined ferromagnetic nanodots

While ferromagnetic nanodots are being widely studied from fundamental as well as application points of views, so far all the dots have been physically defined; once made, one cannot change their dimension or size. We show that ferromagnetic nanodots can be electrically defined. To realize this, we utilize an electric field to modulate the in-plane distribution of carriers in a ferromagnetic semiconductor (Ga,Mn)As film with a meshed gate structure having a large number of nanoscaled windows.

Dimensionality-driven insulator-metal transition in A-site excess non-stoichiometric perovskites

Coaxing correlated materials to the proximity of the insulator–metal transition region, where electronic wavefunctions transform from localized to itinerant, is currently the subject of intensive research because of the hopes it raises for technological applications and also for its fundamental scientific significance. In general, this tuning is achieved by either chemical doping to introduce charge carriers, or external stimuli to lower the ratio of Coulomb repulsion to bandwidth. In this study, we combine experiment and theory to show that the transition from well-localized insulating states to metallicity in a Ruddlesden-Popper series, La_0.5Sr_n+1−0.5Ti_nO_3n+1, is driven by intercalating an intrinsically insulating SrTiO3 unit, in structural terms, by dimensionality n. This unconventional strategy, which can be understood upon a complex interplay between electron–phonon coupling and electron correlations, opens up a new avenue to obtain metallicity or even superconductivity in oxide superlattices that are normally expected to be insulators.

Many technological applications would benefit from new ways of bringing complex materials near the insulator–metal transition region. The authors induce, in a Lantanum Strontium Titanate, a transition from insulating to metallic behaviour by structural intercalation of intrinsically insulating units, opening new avenues to engineer these materials.

The fermi level dependent electronic properties of the smallest (2,2) carbon nanotube

A metal-semiconductor transition in the smallest (2,2) single-wall carbon nanotube (SWNT) is predicted theoretically as a function of gate voltage. By hole-doping (or heavy electron-doping), the energy gap of a neutral (2,2) SWNT vanishes with structural change, and the (2,2) SWNT becomes metallic. The (2,2) tube assumes a doubly degenerate ground state around the charge neutral condition with an energy barrier, while this tube has only one nondegenerate metallic ground state over an energy window of -0.12 to +0.40 eV. Because of a high density of states at the Fermi energy for hole-doped (2,2) SWNTs, a possible superconducting transition is expected.

Hydrogenation-induced surface polarity recognition and proton memory behavior at protic-ionic-liquid/oxide electric-double-layer interfaces

The electric-double-layer (EDL) formed at liquid/solid interfaces provides a broad and interdisciplinary attraction in terms of electrochemistry, photochemistry, catalysts, energy storage, and electronics. Especially in recent years, much effort has been devoted to the fundamental understanding and practical applications of transistor configurations with EDLs because of their ability for high-density charge accumulation. However, to exploit additional new functionalities of such an emerging interface is not only of great importance but also a huge challenge. Here, we demonstrate that, by introducing protic ionic liquid (PIL) as the gate dielectric for ZnO EDL transistors (EDLTs), small and chemically active ions, such as protons and hydroxyls, can serve as an adsorption medium to extend the interfacial functionalities of EDLTs. By selectively driving the H(+) or OH(-) groups onto ZnO channel surfaces with an electric field, the charged adsorbates interact with surface atoms in different adsorption mechanisms, showing remarkable variations in electron transport and providing a possibility for the recognition of surface polarity. Most significantly, the large hysteresis in the transfer characteristics of PIL-EDLTs makes the device available and promising for nonvolatile proton memory devices via surface hydrogenation and dehydrogenation processes. Such a finding provides us with new opportunities to understand liquid/solid heterogeneous interface phenomena and to extend the practical functions of EDLs through controllable interfacial interaction.

Carrier localization on surfaces of organic semiconductors gated with electrolytes

Organic semiconductor single crystals gated with electrolytes exhibit a pronounced maximum in channel conductance at hole densities >10(13)   cm(-2). The cause is a strong decrease in the hole mobility with increasing charge density, which is explained in terms of a percolation model that incorporates trapping of holes by ions at the semiconductor-electrolyte interface. In the case of rubrene crystals, the peak channel conductance occurs at hole densities near 3 × 10(13)  cm(-2). The magnitude of the effect will be large for semiconductors with low dielectric constants and narrow bandwidths, and thus is likely to be a general phenomenon in organic semiconductors gated with electrolytes.

Epitaxial SrTiO3 films with electron mobilities exceeding 30,000 cm2 V(-1) s(-1)

The study of quantum phenomena in semiconductors requires epitaxial structures with exceptionally high charge-carrier mobilities. Furthermore, low-temperature mobilities are highly sensitive probes of the quality of epitaxial layers, because they are limited by impurity and defect scattering. Unlike many other complex oxides, electron-doped SrTiO(3) single crystals show high (approximately 10(4) cm(2) V(-1) s(-1)) electron mobilities at low temperatures. High-mobility, epitaxial heterostructures with SrTiO(3) have recently attracted attention for thermoelectric applications, field-induced superconductivity and two-dimensional (2D) interface conductivity. Epitaxial SrTiO(3) thin films are often deposited by energetic techniques, such as pulsed laser deposition. Electron mobilities in such films are lower than those of single crystals. In semiconductor physics, molecular beam epitaxy (MBE) is widely established as the deposition method that produces the highest mobility structures. It is a low-energetic, high-purity technique that allows for low defect densities and precise control over doping concentrations and location. Here, we demonstrate controlled doping of epitaxial SrTiO(3) layers grown by MBE. Electron mobilities in these films exceed those of single crystals. At low temperatures, the films show Shubnikov-de Haas oscillations. These high-mobility SrTiO(3) films allow for the study of the intrinsic physics of SrTiO(3) and can serve as building blocks for high-mobility oxide heterostructures.

Anomalous expansion of the copper-apical-oxygen distance in superconducting cuprate bilayers

We have introduced an improved x-ray phase-retrieval method with unprecedented speed of convergence and precision, and used it to determine with sub-Angstrom resolution the complete atomic structure of epitaxial La(2-x)Sr(x)CuO(4) ultrathin films. We focus on superconducting heterostructures built from constituent materials that are not superconducting in bulk samples. Single-phase metallic or superconducting films are also studied for comparison. The results show that this phase-retrieval diffraction method enables accurate measurement of structural modifications in near-surface layers, which may be critically important for elucidation of surface-sensitive experiments. Specifically we find that, while the copper-apical-oxygen distance remains approximately constant in single-phase films, it shows a dramatic increase from the metallic-insulating interface of the bilayer towards the surface by as much as 0.45 A. The apical-oxygen displacement is known to have a profound effect on the superconducting transition temperature.

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.

Percolative theories of strongly disordered ceramic high-temperature superconductors

Optimally doped ceramic superconductors (cuprates, pnictides, etc.) exhibit transition temperatures T(c) much larger than strongly coupled metallic superconductors like Pb (T(c) = 7.2 K, E(g)/kT(c) = 4.5) and exhibit many universal features that appear to contradict the Bardeen, Cooper, and Schrieffer theory of superconductivity based on attractive electron-phonon pairing interactions. These complex materials are strongly disordered and contain several competing nanophases that cannot be described effectively by parameterized Hamiltonian models, yet their phase diagrams also exhibit many universal features in both the normal and superconductive states. Here we review the rapidly growing body of experimental results that suggest that these anomalously universal features are the result of marginal stabilities of the ceramic electronic and lattice structures. These dual marginal stabilities favor both electronic percolation of a dopant network and rigidity percolation of the deformed lattice network. This "double percolation" model has previously explained many features of the normal-state transport properties of these materials and is the only theory that has successfully predicted strict lowest upper bounds for T(c) in the cuprate and pnictide families. Here it is extended to include Coulomb correlations and percolative band narrowing, as well as an angular energy gap equation, which rationalizes angularly averaged gap/T(c) ratios, and shows that these are similar to those of conventional strongly coupled superconductors.

Dominant mobility modulation by the electric field effect at the LaAlO3/SrTiO3 interface

Caviglia et al. [Nature (London) 456, 624 (2008)] have found that the superconducting LaAlO3/SrTiO3 interface can be gate modulated. A central issue is to determine the principal effect of the applied electric field. Using magnetotransport studies of a gated structure, we find that the mobility variation is almost 5 times that of the sheet carrier density. Furthermore, superconductivity can be suppressed at both positive and negative gate bias. These results indicate that the relative disorder strength strongly increases across the superconductor-insulator transition.

Polaronic hole trapping in doped BaBiO3

The present ab initio study shows that in BaBiO3, Bi3+ sites can trap two holes from the valence band to form Bi5+ cations. The trapping is accompanied by large local lattice distortions; therefore the composite particle consisting of the electronic hole and the local lattice phonon field forms a polaron. Our study clearly shows that even sp elements can trap carriers at lattice sites, if local lattice relaxations are sufficiently large to screen the localized hole. The derived model describes all relevant experimental results, and settles the issue of why hole-doped BaBiO3 remains semiconducting upon moderate hole doping.

Large electric field effect in electrolyte-gated manganites

We have studied electrostatic field-induced doping in La0.8Ca0.2MnO3 transistors using electrolyte as a gate dielectric. For positive gate bias, electron doping drives a transition from a ferromagnetic metal to an insulating ground state. The thickness of the electrostatically doped layer depends on bias voltage but can extend to 5 nm requiring a field doping of 2x10;{15} charges per cm;{2} equivalent to 2.5 electrons per unit cell area. In contrast, negative gate voltages enhance the metallic conductivity by 30%.