X-ray Studies of Charge-Order Dynamics in Complex Materials

Laboratory Directed Research and Development

Robert W. Schoenlein, Zahid Hussain (ALS)

The purpose of this project is to develop novel ultrafast resonant x-ray scattering techniques, and apply them to study charge and orbital ordering dynamics in complex materials where strong interplay between charge, spin, and subtle lattice distortions give rise to rich phase diagrams. This project will focus on two classes of transition-metal oxides, and their dynamic response to tailored vibrational and electronic excitations: (i) CMR manganites exhibiting charge/orbital ordering, and (ii) blue-bronze (K0.3MoO3) compounds exhibiting charge-density waves.

Resonant x-ray scattering (RXS) is a powerful quantitative probe of charge and orbital ordering in complex materials. However, studies to date have focused on ordering phenomena in the ‘quasi-static’ regime, as a function of adiabatic changes in doping, pressure, temperature, and applied field. Understanding the dynamics of such ordering phenomena, their time scales, correlation lengths, and coupling to different degrees of freedom, is essential for revealing the driving force behind the intriguing physics and novel properties of correlated materials. Resonant x-ray diffraction measurements are made on Bragg reflections that are nominally forbidden by the lattice unit-cell symmetry. Anisotropy of the electron density (e.g. from charge/orbital ordering) results in non-zero Bragg reflections. These reflections are particularly enhanced at absorption resonances (e.g. L-edge or K-edge) since they directly probe the valence states most affected by the symmetry-breaking. Time-resolved resonant x-ray diffraction experiments will be conducted at ALS BL6.0.1 and BL6.0.2, the only beamlines in the U.S. providing the required tunable femtosecond x-ray pulses in the 0.2-10 keV range, accompanied by a laser system covering the visible and mid-IR range (for tailored excitation). For these studies, a dedicated vacuum chamber and diffractometer will be constructed that will incorporate a cryostat for low-temperature measurements