The existence of polyexcitons, the N -body complexes of excitons for N > 2 in 3D bulk systems, has been controversial for more than 40 years since its first theoretical suggestion. We investigated the stability of fundamental excitonic complexes in diamond numerically with the stochastic variational method (SVM) and an explicitly correlated Gaussian (ECG) basis. The electron-hole many-body system is described by an effective mass Hamiltonian. Our model includes the effective mass anisotropy and multiple valley and band degrees of freedom. We show that the excitons, trions, biexcitons, charged biexcitons, and triexcitons are stable in diamond. Numerical calculations reproduce from 81% to 86% of the experimentally reported binding energies for neutral bound states.
Recent developments of ultrafast laser pulse techniques enable us to study the subpicosecond scale dynamics out of thermal equilibrium. Multiple temperature models (MTMs) are frequently used to describe such dynamics where the total system is divided into subsystems each of which is in local thermal equilibrium. Typical examples include the electron-lattice two temperature model and electron-spin-phonon three temperature model. We present the exact analytical solutions of linear MTM, based on the Fourier series expansion and the Green’s function method. We then discuss their properties for the case of the two and three temperature models. We show that the general solution of MTM is expressed as a linear combinations of a spatially uniform, single-temperature stationary mode and the other non-oscillatory, decaying “eigenmodes” characterized by different wave vectors and well-defined mode lifetimes. The eigenmode picture enables us to explore the hierarchical structure of models with respect to space, time and the coupling parameter. Excitation by source term is included by the Green’s function method. As an example, we derive an analytical solution for a Gaussian type source term. We report a phenomenon “temperature inversion” where the lattice temperature exceeds electron’s temperature for ns time scale. Furthermore, we show how physical requirements such as energy conservation and equilibration are realized in the general linear MTM in terms of the eigenmode picture.
Recent advances in ultrashort laser pulse techniques have opened up a wide variety of applications in both fundamental physics and industrial fields. In this work, ab initio molecular dynamics simulations based on time-dependent density functional theory revealed a steady deceleration of lattice distortion propagation in an aluminum slab with increasing laser pulse intensity. Analysis of the interatomic force revealed a significant reduction in the harmonic terms and non-monotonic growth of anharmonicity. This behavior was characterized by spatially non-uniform force screening by plasmons, which is missing from Born-Oppenheimer molecular dynamics, and is consistent with the current interpretation of laser-induced periodic structure patterning. This work provides a semi-quantitative criterion for modifying the phonon properties of non-equilibrium systems.
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