November 2010 : Electronic properties of materials: the metal-insulator transition delivers part of its secrets under the light of the synchrotron
The behavior of electrons in materials induces two generally distinct states, either metallic or insulating depending on whether they are conducting an electrical current. Some materials however can change from one state to the other under the influence of external parameters. Understanding this metal-insulator transition is essential to masterize the electronic properties of materials. We have studied this transition in Cr doped V2O3, a model material of strongly correlated electrons systems using spectroscopy and diffraction technique. Nat. Comm. 1 (2010), 105. Here is the abstract :
V2O3 is the prototype system for the mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal-to-insulator transition (mIT) between a paramagnetic metal (Pm) and a paramagnetic insulator. This or related mITs have a high technological potential, among others, for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the mIT in Cr-doped V2o3 with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated Pm phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure, that promotes a genuine mott transition to an eventually homogeneous metallic state.
March 2010: RIXS at high pressure
Inelastic x-ray scattering (IXS) in the hard x-ray range has emerged as a powerful probe for the electornic properties of materials under high pressure conditions. In a recent review article "Inelastic x-ray scattering by electronic excitations under high pressure" published in Rev. Mod. Phys. 82, 847–896 (2010), we provide a complete and timely information about this research activity that will be possible on the GALAXIES beamline in a near future. Here is the abstract :
Investigating electronic structure and excitations under extreme conditions gives access to a rich variety of phenomena. High pressure typically induces behavior such as magnetic collapse and the insulator-metal transition in 3d transition-metal compounds, valence fluctuations or Kondo-like characteristics in f-electron systems, and coordination and bonding changes in molecular solids and glasses. This article reviews research concerning electronic excitations in materials under extreme conditions using inelastic x-ray scattering (IXS). IXS is a spectroscopic probe of choice for this study because of its chemical and orbital selectivity and the richness of information it provides. Being an all-photon technique, IXS has a penetration depth compatible with high-pressure requirements. Electronic transitions under pressure in 3d transition-metal compounds and f-electron systems, most of them strongly correlated, are reviewed. Implications for geophysics are mentioned. Since the incident x-ray energy can easily be tuned to absorption edges, resonant IXS, often employed, is discussed at length. Finally studies involving local structure changes and electronic transitions under pressure in materials containing light elements are reviewed.
The Workshop HAXPES has been held at SOLEIL from 18 to 19 January 2010.