Theoretical description of nonequilibrium behavior in charge density wave
Freericks, Jim K
The fast development of time resolved photoemission (TRPES) techniques allow us to discover the rich features of nonequilibrium phenomena which may not appear in equilibrium. One of the most explored topics is the nonequilibrium behavior of a charge density wave (CDW) material. Being an ordered phase at low temperature, the CDW state provides a fertile ground to study electron-electron and electron-ion interactions. By driving this material out of equilibrium and taking ultrafast time resolution snapshots of its behavior, TRPES helps us understand these interactions and sheds light on the mechanisms behind these and other complex material properties, such as metal-insulator transitions, high temperature superconductivity, and magnetic phenomena. Recent experiments on TRPES in CDW materials show an ultrafast CDW gap closure in systems such as 1T-TaS2 and TbTe3 and the subsequent separation of time scales for the electron-electron interaction and the electron-lattice interaction. But it is still not clear what happens during the ultrashort period (in first 100 femtosecond). In this dissertation, we solve a two band model describing this ultrafast process in a CDW system. By fixing the lattice distortion effect in the CDW, we studied the nonequilibrium excitations of the electrons under a strong electric field. This research is performed by calculating nonequilibrium Green's functions (NGF) along the Kadanoff-Baym-Keldysh contour. We solved this nonequilibrium problem exactly. We show non-perturbative results and explore the nonlinear electronic behavior under an ultrashort light pulses. In addition to the TRPES problem, we also examined the behavior of Bloch oscillations under a large DC field, the response to an AC electric field, high harmonic generation from solids, and the crossover between frequency-driven excitation and amplitude-driven excitation.
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