High-temperature superconductivity in copper oxides arises when a parent insulator compound is doped beyond some critical concentration; what exactly happens at this superconductor-insulator transition is a key open question. The cleanest approach is to tune the carrier density using the electric field effect; for example, it was learned in this way that weak electron localization transforms superconducting SrTiO(3) into a Fermi-glass insulator. But in the copper oxides this has been a long-standing technical challenge, because perfect ultrathin films and huge local fields (>10(9) V m(-1)) are needed. Recently, such fields have been obtained using electrolytes or ionic liquids in the electric double-layer transistor configuration. Here we report synthesis of epitaxial films of La(2- x)Sr(x)CuO(4) that are one unit cell thick, and fabrication of double-layer transistors. Very large fields and induced changes in surface carrier density enable shifts in the critical temperature by up to 30 K. Hundreds of resistance versus temperature and carrier density curves were recorded and shown to collapse onto a single function, as predicted for a two-dimensional superconductor-insulator transition. The observed critical resistance is precisely the quantum resistance for pairs, R(Q) = h/(2e) = 6.45 kΩ, suggestive of a phase transition driven by quantum phase fluctuations, and Cooper pair (de)localization.

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The hybrid halide perovskites, the very performant compounds in photovoltaic applications, possess large Seebeck coefficient and low thermal conductivity making them potentially interesting high figure of merit (ZT) materials. For this purpose one needs to tune the electrical conductivity of these semiconductors to higher values. We have studied the CH3NH3MI3 (M=Pb,Sn) samples in pristine form showing very low ZT values for both materials; however, photoinduced doping (in M=Pb) and chemical doping (in M=Sn) indicate that, by further doping optimization, ZT can be enhanced toward unity and reach the performance level of the presently most efficient thermoelectric materials.X. Mettan et al. 04.26.2015 1

Tungsten oxide and its associated bronzes (compounds of tungsten oxide and an alkali metal) are well known for their interesting optical and electrical characteristics. We have modified the transport properties of thin WO3 films by electrolyte gating using both ionic liquids and polymer electrolytes. We are able to tune the resistivity of the gated film by more than five orders of magnitude, and a clear insulator-to-metal transition is observed. To clarify the doping mechanism, we have performed a series of incisive operando experiments, ruling out both a purely electronic effect (charge accumulation near the interface) and oxygen-related mechanisms. We propose instead that hydrogen intercalation is responsible for doping WO3 into a highly conductive ground state and provide evidence that it can be described as a dense polaronic gas.

We report on in-plane paraconductivity measurements in thin YBa2Cu306 9 films. Our analysis of the data shows that the temperature dependence of paraconductivity is affected by lattice disorder and deviates at all temperatures from the universal power laws predicted by both scaling and mean-field theories.This gives evidence for the absence of critical fluctuations and for the failure of the Aslamazov-Larkin universal relation between critical exponent and dimensionality of the spectrum of Gaussian fluctuations. We account quantitatively for the data within the experimental error by introducing a shortwavelength cutoff into this spectrum. This implies that three-dimensional short-wavelength Gaussian fluctuations dominate in YBa2Cu306 9 and suggests a rapid attenuation of these fluctuations with decreasing wavelength in short-coherence-length systems as compared to the case of the conventional Ginzburg-Landau theory.