The optical properties of a GaTa(4)Se(8) single crystal are investigated under high pressure. At ambient pressure, the optical conductivity exhibits a charge gap of ≈0.12 eV and a broad midinfrared band at ≈0.55 eV. As pressure is increased, the low energy spectral weight is strongly enhanced and the optical gap is rapidly filled, pointing to an insulator to metal transition around 6 GPa. The overall evolution of the optical conductivity demonstrates that GaTa(4)Se(8) is a Mott insulator which undergoes a bandwidth-controlled Mott metal-insulator transition under pressure, in remarkably good agreement with theory. With the use of our optical data and ab initio band structure calculations, our results were successfully compared to the (U/D, T/D) phase diagram predicted by dynamical mean field theory for strongly correlated systems.

We have performed electron spin resonance (ESR) measurements on a large assembly of graphene oxide (GO) and reduced graphene oxide (RGO) flakes. In GO samples the Curie tail is coming from 1.4 x 10(18) cm(-3) of localized spins. Although reduction of GO was expected to reestablish the pristine properties of graphene, no Pauli-like contribution was detected and only a low concentration of 1.2 x 10(16) cm(-3) spin carrying defects were measured. Our study, completed by resistivity measurements, shows that the carrier transport in RGO samples is dominated by hopping. The incomplete reduction of GO leaves behind a large number of defects, presumably the majority of which are ESR silent, causing the Anderson localization of the electronic states. Slight doping with potassium indicates the appearance of a Pauli contribution in the spin susceptibility.

We have measured the pressure and temperature dependences of the resistivity and the thermoelectric power of anatase, TiO2. The resistivity varies with T-3 at high temperatures, and its absolute value is in the 1 Omega cm range. Below 60 K, the resistivity is activated. Most surprisingly, the activation energy shows non-monotonic pressure dependence. The thermoelectric power has a very high value and its temperature dependence resembles that of polaronic materials. We suggest a large polaronic model to describe the temperature and pressure dependence of the two transport coefficients. Copyright (c) EPLA, 2012

Cubic ZnMnO3 powder in the form of well-crystalline nanoflakes have been synthesized at low temperatures from a nitrate precursor. The electrical properties of cubic ZuMnO(3) samples have been established by DC resistivity (rho) and thermo-electric power (Seebeck coefficient) measurements on a pressed pellet. The material exhibits insulator behavior with 0.7 eV acceptor ionization energy in the measured temperature range of 170-300 K. The thermo-electric power indicates a positive sign of the charge carriers. The obtained material exhibits a superparamagnetic signature with a blocking temperature of 9 K and the ZFC-FC splitting temperature of 15 K. (C) 2010 Elsevier Ltd. All rights reserved.

We report here on a new type of non volatile resistive switching that we discovered in the whole class of Mott Insulator compounds AM4X8 (A = Ga, Ge; M= V, Nb, Ta; X = S, Se). The mechanism of this resistive switching differs from the thermochemical or electrochemical effects reported so far to explain the resistive switching in other materials. We found that this resistive switching is related to an electric field effect which induces an electronic phase change in the AM4X8 compounds from the Mott insulating state to a metallic-like state. This new type of resistive switching is observed both on crystals and on polycrystalline thin films with fast writing/erasing times (50 ns to 10 μs) and resistance ratios (ΔR/Rlow) higher than 33% at room temperature. These results appear very promising at developing a new class of Resistive Random Access Memory (RRAM).

We report the first experimental evidence for a strong electromechanical coupling in the Mott insulator GaTa4Se8 allowing a highly reproducible nano-writing with a Scanning Tunneling Microscope (STM). The local electric field across the STM junction is observed to have a threshold value above which the clean (100) surface of GaTa4Se8 becomes mechanically instable: At voltage biases V > 1.1V the surface suddenly inflates and comes in contact with the STM tip, resulting in nanometer size craters. The formed pattern can be indestructibly "read" by STM at lower voltage bias, thus allowing a 5 Tdots/inch2 dense writing/reading at room temperature. The discovery of the electromechanical coupling in GaTa4Se8 might give new clues in the understanding of the Electric Pulse Induced Resistive Switching recently observed in this stoechiometric Mott insulator.

There is accumulated evidence today that an electric pulse can drastically modify the physical properties of correlated materials. An electric pulse was shown for example to induce an insulator-to-metal transition in manganites or in organic Mott insulators. We report here the first experimental evidence of a non-volatile electric pulse-induced insulator-to-metal transition and possible superconductivity in the Mott insulator GaTa4Se8. This resistive switching is concomitant to an electronic phase separation induced by the pulse. This phenomena most probably differs from the thermal, electronic injection or ionic diffusion processes explaining the resistive switching in materials foreseen for non-volatile memory (RRAM) applications.

We have synthesized and characterized the first Ti member of the AM(4)X(8) (A = Ga, Ge; M = V, Nb, Ta, Mo, X = S, Se) family of compounds that exhibit a deficient spinel structure with tetrahedral transition metal clusters. Single-crystal X-ray structure determination of Ga0.87Ti4S8 (space group F-43m, a = 9.9083(l 1) A) revealed the presence of Ga vacancies and a partial declustering of the transition metal atoms compared to other members of the family. This declustering is likely associated to the decrease of the electronic filling of the clusters from 7 to I I electrons in known AM4X8 compounds to similar to 3 electrons in Gao.87Ti4S8- XPS measurements show that the valence band of Ga0.87Ti4S8 is more dispersed than that of the Mott insulator GaV4S8, which suggests a strengthening of the transfer integrals induced by the slackening of the transition metal clusters. A Fermi step consistent with a metallic character is observed in Ga-0.87Ti4S8 while a gap appears (0.2 eV) below the Fermi level in the Mott insulator GaV4S8. The resistivity measured on crystals of Ga0.87Ti4S8 showed a metallic behavior with a small residual resistivity ratio. Ga0.87Ti4S8 exhibits large electronic specific heat (gamma = 58 mJ K-2 mol(-1)) and Pauli susceptibility. All these measurements as well as the Wilson and Kadowaki-Woods ratios demonstrate that Ga0.87Ti4S8 exhibits a correlated metal type behavior and is no longer a paramagnetic Mott insulator like other members of the family.