Quantum Materials Phenomena Out of Equilibrium
Studying the out of equilibrium dynamics of quantum many-particle systems is an emerging field that has been enabled by experimental advances on different fronts. Ultrasmall systems like quantum dots and single molecule devices probe the quantum limits of electronic devices. The small size means that such devices are easily driven far from equilibrium by an external voltage. The energy gained by electrons traversing the device can be transferred to quantized molecular vibrations or other quantum degrees of freedom. Ultracold experiments on atoms and molecules isolate quantum matter from its environment. This allows experimentalists to study how statistical properties of a complex system emerge from its dynamics. An alternative way to avoid the effects of coupling to the environment is to probe a material on very short time scales, as in ultrafast optical pump-probe spectroscopy. Rice researchers are at the forefront, studying phenomena ranging from solitons in Bose-Einstein condensates to conduction in single molecule transistors, and revealing superfluorescent bursts from quantum-degenerate electron-hole gases and coherent quantum dynamics in two-dimensional electron gases and carbon nanotubes.
A weakly interacting system can be described by a distribution function, which assigns particular particles to particular states. This idea is not helpful when the system is strongly interacting, however, for the same reason that it is not helpful to characterize individual H2O molecules in describing the flow of liquid water. What is the analogous framework for non-equilibrium quantum dynamics, and what are the intrinsic departures from classical hydrodynamics? Theoretical results by Rice researchers suggest that ultrafast and ultracold experiments can realize new quantum states of matter that only exist out of equilibrium.