How a dc Electric Field Drives Mott Insulators Out of Equilibrium.

Out of equilibrium phenomena are a major issue of modern physics. In particular, correlated materials such as Mott insulators experience fascinating long-lived exotic states under a strong electric field. Yet, the origin of their destabilization by the electric field is not elucidated. Here we present a comprehensive study of the electrical response of canonical Mott insulators GaM_{4}Q_{8} (M=V, Nb, Ta, Mo; Q=S, Se) in the context of a microscopic theory of electrical breakdown where in-gap states allow for a description in terms of a two-temperature model. Our results show how the nonlinearities and the resistive transition originate from a massive creation of hot electrons under an electric field. These results give new insights for the control of the long-lived states reached under an electric field in these systems which has recently open the way to new functionalities used in neuromorphic applications.

Structural, optical, and dielectric properties of Bi(1.5-x)Zn(0.92-y)Nb(1.5)O(6.92-δ) thin films grown by PLD on R-plane sapphire and LaAlO3 substrates.

Bi(1.5-x)Zn(0.92-y)Nb(1.5)O(6.92-δ) thin films have the potential to be implemented in microwave devices. This work aims to establish the effect of the substrate and of the grain size on the optical and dielectric properties. Bi(1.5-x)Zn(0.92-y)Nb(1.5)O(6.92-δ) thin films were grown at 700 °C via pulsed-laser deposition on R-plane sapphire and (100)(pc) LaAlO(3) substrates at various oxygen pressures (30, 50, and 70 Pa). The structure, morphology, dielectric and optical properties were investigated. Despite bismuth and zinc deficiencies, with respect to the Bi(1.5)Zn(0.92)Nb(1.5)O(6.92) stoichiometry, the films show the expected cubic pyrochlore structure with a (100) epitaxial-like growth. Different morphologies and related optical and dielectric properties were achieved, depending on the substrate and the oxygen pressure. In contrast to thin films grown on (100)(pc) LaAlO(3), the films deposited on R-plane sapphire are characterized by a graded refractive index along the layer thickness. The refractive index (n) at 630 nm and the relative permittivity (ε(r)) measured at 10 GHz increase with the grain size: on sapphire, n varies from 2.29 to 2.39 and ε(r) varies from 85 to 135, when the grain size increases from 37 nm to 77 nm. On the basis of this trend, visible ellipsometry can be used to probe the characteristics in the microwave range quickly, nondestructively, and at a low cost.

Indirect reduction of aryldiazonium salts onto cathodically activated platinum surfaces: formation of metal-organic structures.

Platinum phases of general formula [Pt(n-), M+, MX] can be electrogenerated from cathodic polarization in dry dimethylformamide containing a supporting electrolyte, MX. The reaction of these electrogenerated Pt phases as reducing agent with aryldiazonium salts was investigated for preparing controlled metal-organic interfaces and characterizing the reactivity of the "reduced platinum phases". In a two-step process, the "reduced platinum phase" locally reacts with aryldiazonium salts, leading to the attachment of aryl groups onto the metal surface in the previously modified areas. Detailed experiments using cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and in situ electrochemical atomic force microscopy (EC-AFM) were carried out to follow the reaction in solution with the example of NaI as supporting electrolyte (MX = NaI). These studies demonstrate the irreversible attachment of aryl groups onto the platinum electrode. Comparison between the direct electroreduction of aryldiazonium compounds (4-nitrophenyl- and 4-bromophenyldiazonium) on a platinum electrode and their reaction with [Pt2-, Na+, NaI] suggests that a similar general mechanism is responsible for the grafting. However in the second case, no applied potential is required to stimulate the binding thanks to the reductive properties of [Pt2-, Na+, NaI]. Competitive reduction of the organic layer and growth of the layer were observed and analyzed as a function of the injected charge used to initially produce [Pt2-, Na+, NaI]. Similar reactions are highly probable with other MX salts owing to the redox properties observed for this type of platinum phase ([Pt(n-), M+, MX]).

Cathodic modifications of platinum surfaces in organic solvent: reversibility and cation type effects.

Cathodic modification of platinum surfaces leads to the formation of iono-platinic phases ([Pt(n-), M+, MX]), which involves the insertion of cations and salts into the platinum electrode. This process was investigated at the local scale by in situ observation of surface electrochemical processes by atomic force microscopy (EC-AFM) techniques as a function of the salt and the injected charge, with special attention about the process reversibility. AFM images recorded in solution after the cathodic modifications of well-defined platinum surfaces [epitaxial platinum deposit on (100) MgO substrate] show drastic modification on the morphology of the surface, confirming previous ex situ studies. The amplitude of the modifications directly depends on both the nature of supporting electrolyte and the quantity of charge injected into the platinum. As long as the injected charge remains small enough to maintain the adhesion of the Pt deposit onto the MgO substrate, the process was found to be fully reversible. Indeed, impressive morphology changes occur under the cathodic treatment (formation of [Pt(n-), M+, MX]) but the initial geometry is totally recovered after reoxidation of the iono-platinic phase. This cycle of reduction-reoxidation can be performed several times without any significant alteration of the recovered surface and of its structural characteristics. It is suggested that the modification starts at the interface solution platinum surface and then its insertion into the platinum surface.

Microstructure of Cu(x)Mo6S8 Chevrel phase thin films on R-plane sapphire

Cu(x)Mo6S8 Chevrel phase thin films epitaxially grown on R-plane sapphire substrates have been studied using field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HREM). For samples grown in optimal conditions, the SEM images evidence oriented grains of about 100 nm average size; the HREM images of cross-section samples allow to deduce the local epitaxial relations between the substrates and films. HREM also evidences two grain families, previously deduced from X-ray and reflection high-energy electron diffraction studies. No defects have been observed in the films; a higher molybdenum content in the films results in Mo-precipitation at the film-substrate interface.