Synchrotron Radiation Studies Research

Overview

This program develops new capabilities using the nation's synchrotron radiation facilities and applies them to cutting-edge problems in materials science. In particular, we aim to play a leading scientific role at the Advanced Photon Source (APS).

  • X-ray scattering studies take advantage of the high brilliance APS x-ray source for in-situ and time-resolved studies of surface and thin film structure. These include investigations of synthesis processes such as vapor-phase epitaxy and electrochemical deposition, and studies of electric-field-driven ferroelectric domain dynamics.
  • High-resolution angle-resolved photoemission is used to understand the nature of superconductivity in the hi-Tc materials.

New thrusts focus on exploring science enabled by future facilities such as an x-ray nanoprobe, a high-energy photoemission microscope, and a coherent x-ray source.

Research Areas

X-ray Scattering

Our approach is to take advantage of the unique capabilities of the high-brilliance APS x-ray source, e.g. for brilliance-limited time-resolved experiments. For example, to date there have been few direct measurements of the atomic-scale processes occurring during vapor-phase processes such as MOCVD growth because the reactive vapor-phase environment is not suitable for many probes such as electron diffraction and STM. We have pioneered the use of x-ray techniques that can penetrate this environment for in-situ, real-time measurements of surface structure and morphology during growth. Likewise, we are studying liquid/solid interfaces, which are the most prevalent interfaces in our daily living and industrial environment due to the presence of water even in our air. Synchrotron x-ray techniques are uniquely suited for atomic-scale structural studies of such buried interfaces. Another project has been to develop techniques using coherent x-rays, such as time correlation spectroscopy, and apply them to problems in statistical physics.

Photoemission

The problem of high temperature superconductivity has not been solved. Although much progress has been made in understanding the electronic structure, and angle-resolved photoemission spectroscopy (ARPES) has played a major role in this progress, much remains to be done. We need to understand the relationship of the boson observed to be interacting with the electrons to superconductivity. Is it a consequence, or the cause of superconductivity? And when the system undergoes the superconducting transition, where is the condensation energy coming from? Is it from the potential energy, the kinetic energy, or perhaps a more exotic source, such as the superexchange energy? ARPES can make significant contributions to these fundamental questions.

Related Facilities

Research

  • Vapor-Phase Epitaxy. We have used in situ x-ray scattering to understand the atomic-scale growth mechanisms and surface structures occurring during MOCVD growth of GaN and PbTiO3. Homoepitaxial growth mode transitions and surface reconstructions were mapped as a function of process conditions for the first time.
  • Electrocatalytic Surfaces. In situ x-ray scattering during electrodeposition and electrocatalysis was used to understand the structures and catalytic mechanisms in systems such as Rh/Au and RuO2 under electrolytes. We observed surface structures responsible for hydrogen and oxygen evolution, surface redox processes, and an unexpected reversible activation of RuO2 (110) surfaces that involves conversion of Ru between +3 and +4 valance states.
  • Surface Resonance X-ray Scattering. We have demonstrated a new technique sensitive to monolayer-level changes of chemical states at buried interfaces. The Pt (111) surface was studied in situ in the electrochemical environment and found to have a large (9 eV) increase in the x-ray resonance energy due to incipient anodic oxidation.
  • Quasiparticles in High-Tc Superconductors. The nature of the carriers in high-Tc superconductors has been elucidated using angle-resolved photoemission. We have identified a particular point on the Fermi surface where the superconducting energy gap vanishes below Tc, and determined the nature of the excitations that dominate the properties of the system.
  • New Methods in Angle-Resolved Photoemission. The unconventional nature of the electronic structure of the cuprate superconductors, coupled with the small size of their Brillouin zones, makes accurate determination of the Fermi surface by angle-resolved photoemission rather difficult. We have developed several new methods for the determination of the Fermi surface, making possible detailed measurements of new complex materials.
  • Switching of Ferroelectric Domains. A new technique to observe domain polarity based on interference between scattering from the film and substrate has been used to shed light on the mechanism of imprint and fatigue in ferroelectric films. These studies are being extended using stroboscopic techniques to allow observation of domain dynamics in nanoscale structures on nanosecond time scales.
  • Time-Correlation Spectroscopy with Coherent X-rays. We have developed this new technique for observing atomic- and nano-scale dynamics and used it observe non-linear diffusion in concentrated colloids and fluctuations during domain coarsening. These studies with coherent x-rays are providing part of the scientific basis for the next generation of synchrotron x-ray sources.