ABSTRACT
Perhaps the most important aspect of contemporary condensed matter physics involves understanding strong Coulomb interactions among the large numbers of electrons in a solid. Electronic correlations lead to the emergence of new system properties, such as metal-insulator transitions, superconductivity, magnetoresistance, Bose-Einstein condensation, the formation of excitonic gases, or the integer and fractional quantum Hall effects. The discovery of high-TC superconductivity in particular was a watershed event, leading to dramatic experimental and theoretical advances in the eld of correlated-electron systems.1-10 Such materials often exhibit competition among the charge, lattice, spin, and orbital degrees of freedom, whose cause-effect relationships are difcult to ascertain. Experimental insight into the properties of solids is traditionally obtained by time-averaged probes, which measure, for example, linear optical spectra, electrical conduction properties, or the occupied band structure in thermal equilibrium. Many novel physical properties arise from excitations out of the ground state into energetically higher states by thermal, optical, or electrical means. This leads to fundamental interactions between the system’s constituents, such as electron-phonon and electron-electron interactions, which occur on ultrafast timescales. Although these interactions underlie the physical properties of solids, they are often only indirectly inferred from time-averaged measurements.
