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The Devereaux Group uses a variety of computational methods in various research projects. The methods include different flavors of quantum Monte Carlo, exact diagonalization and sparse matrix techniques, dynamical mean-field theory and cluster extensions, density matrix renormalization group methods, nonequilibrium Keldysh techniques, and density functional theory and other materials specific computational methods. Simulations and calculations are performed on different platforms including an in-house SIMES cluster, SHERLOCK housed in the Stanford Research Computing Facility (SRCF), and the resources of the National Energy Research Scientific Computing Center (NERSC).
Photoemission spectroscopy (PES) provides valuable information about single-particle dynamics in condensed matter systems and the technique recently has been extended to the time-domain (tr-PES) using pump-probe, femto-second techniques. These experiments have been able to provide new information on electron dynamics directly in the time domain. New information also has become available from time domain experiments conducted using different x-ray scattering techniques.
X-ray scattering techniques provide important experimental probes for studying strongly correlated materials, giving access to dynamics in the charge and spin channels with materials specificity. In inelastic techniques like RIXS or NIXS, photon-in/photon-out processes, the frequency shift and polarization change of the outgoing photon compared to the incoming photon provides information about the physics of charge transfer processes and dynamics associated with the electronic structure with momentum- and energy-dependence. Other methods, like x-ray absorption (XAS), provide complimentary information about the unoccupied electronic states that can give a more complete materials specific picture when combined with ARPES or x-ray emission spectroscopy.
ARPES is a photon-in/electron-out spectroscopy based on the photoelectric effect. An incoming photon, generated by an X-ray, VUV, or laser system, directed at a sample and absorbed, supplies sufficient energy to an electron inside a material to overcome the work function. This technique provides valuable information about the single-particle dynamics of materials and can shed light on the coupling between various degrees of freedom and reveal signatures of strong correlations. It provides access to the occupied electron states in a material that can compliment x-ray scattering techniques.
Scanning tunneling microscopy (STM) probes the surface electronic structure of materials. In STM experiments, a metallic tip is held a distance above the sample in high vacuum and a bias voltage is applied across the tip-vacuum-sample interface. This results in a tunnel current proportional to the convolution of the tip and sample density-of-states (DOS).