Angle-resolved photoemission spectroscopy (ARPES)
ARPES is a powerful experimental technique for measuring the bulk and surface electronic structure of crystalline solids in momentum space. Coupled with theory, this technique can elucidate Fermi surfaces and band dispersions of new materials, quantify the magnitude and momentum dependence of spectral gaps which are related to the order parameter of many types of emergent phases, and reveal effects of strong coupling of electrons to other electrons and to bosonic excitations.
ARPES is based on the, for which Einstein received his Nobel prize. UV light shines on a sample, and electrons are photoemitted from the sample surface. When these electrons are photoemitted, they still carry information about how they were moving back inside the sample, except they received a (known) energy kick from the photons. The momentum imparted by the photons is negligible at the wavelengths used in most experiments. By precisely measuring the kinetic energy and emission angle of photoelectrons, one can construct their energy vs momentum relationship (dispersion relation or band structure) of electrons inside the sample.
Ultrafast spectroscopies use sub-picosecond laser pulses to manipulate and investigate quantum materials on timescales comparable to characteristic processes such as Cooper pair formation. These experimental techniques have made important contributions to chemistry and semiconductor physics, but using them to investigate complex strongly correlated materials, some of which are still poorly understood by equilibrium probes, is relatively new. These experiments hold promise for exciting/observing collective modes relevant to emergent phases, distinguishing coexisting phases via their distinct electron dynamics, and isolating inelastic electron scattering processes relevant to the formation of a given electronic phase. Time-domain experiments usually take the form of a pump-probe experiment: first, a pump pulse of light excites (usually) electrons out of equilibrium, and then, after a short time delay, a probe pulse studies how the system has responded. Ultrafast spectroscopies encompass a suite of different experiments with varying pump photon energies which perturb the system in different ways and varying probe schemes which study different aspects of the electronic response. The experiments I will initially set up—studying pump-induced transient changes in reflectivity—are perhaps the most versatile: they can be applied to almost any sample without stringent requirements for size, thickness, or surface quality.
New materials or improved control over existing materials are the basis of new discoveries in hard condensed matter physics. In addition to collaborating with other faculty at Davis and elsewhere who synthesize bulk crystals and thin films of quantum materials of contemporary interest, I plan on implementing optical floating-zone synthesis, as a means of growing large, clean single-crystals of oxide materials.