LANS Informal Seminar: John-Paul Castellan

Seminar Title: Challenges of using Diffuse Scattering to Measure Short Range Correlations in Crystalline Materials
Speaker: John-Paul Castellan, MSD

Date/Time: 2009-07-30 15:00
Location: Building 221, room A-261

Description:
Many recent phenomena of high interest in condensed matter physics are linked to the existence of short-ranged correlations.  The most prominent examples are provided by transition metal oxides, where complex disorder results in a balance of the spin, orbital, charge and strain degrees of freedom and give rise to competing ground states with incompatible order and exotic phenomena such as colossal magnetoresistance, negative thermal expansion, quantum spin liquids, and high temperature superconductivity.  Traditional crystallography provides well developed and sophisticated tools for obtaining the long-range order of crystalline solids, but only indirect and insufficient evidence of disorder.  While there are several techniques that are very sensitive to the existence of local disorder, none provide detailed information on the correlations between defects or the length scales of short-range ordering processes.  Single crystal diffuse scattering probes both the local distortions around point defects as well as the defect to defect correlations on length scales of 1 to 10nm and provides the most powerful tool for studying complex short range correlations, such as the formation of stripes, checkerboards, ladders, or phase separation.  However, there remain formidable difficulties in obtaining and analyzing large enough volumes of data with sufficient momentum and energy resolution required for accurate modeling.  I will discuss several systems that we are currently studying at MSD as well as some new techniques being developed to collect and analyze diffuse scattering.