Theory of Finite-temperature and Non-equilibrium X-ray Absorption

Abstract

There has been considerable interest in non-equilibrium phenomena in recent years. For example, the development of femtosecond lasers has led to many experiments involving non-equilibrium systems. In these experiments, the electrons and lattice are in an out of-equilibrium state due to the significantly longer electron-lattice relaxation time than the energy absorption of the electrons. Understanding the interaction between the electrons and atomic structures in these short live non-equilibrium state of matter can provide new insight for next-generation spintronic devices and high energy density physics. Time-resolved x-ray absorption spectroscopy (TR-XAS) has an advantage over diffraction spectroscopy by probing the local atomic structure and local electronic properties. Analyzing the spectra requires accurate modeling of the excitations across a wide range of conditions but such analysis codes are not widely available. Our goal is to develop a first-principle theoretical framework which quantitatively agrees with absorption near-edge structure (XANES). This thesis aims to develop a finite-temperature electronic structure theory that accounts for the temperature dependence of the exchange-correlation and the phonon interactions. We first describe the theory of x-ray absorption at finite temperature with the inclusion of temperature dependent exchange-correlation using the multiple-scattering formalism. Dynamic effects from the lattice can be included in the theory using molecular dynamics or the computation of dynamical matrix. This method has been implemented in the real-space multiple scattering code, FEFF10. Finally, we present the finite temperature generalization of the COHSEX approximation to the quasiparticle electron self-energy. This contribution highlights the importance of the dynamic corrections to the COH approximation especially for high energy unoccupied states. These developments make possible simulations of a wide variety of systems and experiments with temperatures up to the warm dense matter regime.