102 research outputs found
Ultra--fast carriers relaxation in bulk silicon following photo--excitation with a short and polarized laser pulse
A novel approach based on the merging of the out--of--equilibrium Green's
function method with the ab-initio, Density--Functional--Theory is used to
describe the ultra--fast carriers relaxation in Silicon. The results are
compared with recent two photon photo--emission measurements. We show that the
interpretation of the carrier relaxation in terms of L -> X inter--valley
scattering is not correct. The ultra--fast dynamics measured experimentally is,
instead, due to the scattering between degenerate states that is activated
by the non symmetric population of the conduction bands induced by the laser
field. This ultra--fast relaxation is, then, entirely due to the specific
experimental setup and it can be interpreted by introducing a novel definition
of the quasi--particle lifetimes in an out--of--equilibrium context.Comment: 4 page, 2 figure
An ab-initio approach to describe coherent and non-coherent exciton dynamics
The use of ultra-short laser pulses to pump and probe materials activates a
wealth of processes which involve the coherent and non coherent dynamics of
interacting electrons out of equilibrium. Non equilibrium (NEQ) many body
perturbation theory (MBPT) offers an equation of motion for the density-matrix
of the system which well describes both coherent and non coherent processes. In
the non correlated case there is a clear relation between these two regimes and
the matrix elements of the density-matrix. The same is not true for the
correlated case, where the potential binding of electrons and holes in
excitonic states need to be considered. In the present work we discuss how
NEQ-MBPT can be used to describe the dynamics of both coherent and non-coherent
excitons in the low density regime. The approach presented is well suited for
an ab initio implementation
Non equilibrium optical properties in semiconductors from first--principles: a combined theoretical and experimental study of bulk silicon
The calculation of the equilibrium optical properties of bulk silicon by
using the Bethe--Salpeter equation solved in the Kohn--Sham basis represents a
cornerstone in the development of an ab--initio approach to the optical and
electronic properties of materials. Nevertheless calculations of the {\em
transient} optical spectrum using the same efficient and successful scheme are
scarce. We report, here, a joint theoretical and experimental study of the
transient reflectivity spectrum of bulk silicon. Femtosecond transient
reflectivity is compared to a parameter--free calculation based on the
non--equilibrium Bethe--Salpeter equation. By providing an accurate description
of the experimental results we disclose the different phenomena that determine
the transient optical response of a semiconductor. We give a parameter--free
interpretation of concepts like bleaching, photo--induced absorption and
stimulated emission, beyond the Fermi golden rule. We also introduce the
concept of optical gap renormalization, as a generalization of the known
mechanism of band gap renormalization. The present scheme successfully
describes the case of bulk silicon, showing its universality and accuracy.Comment: 14 pages, 13 figure
Ab initio circular dichroism with the Yambo code: applications to dipeptides
Circular dichroism (CD) spectroscopy is a useful technique for characterizing chiral molecules. It is more sensitive than total absorption to molecule conformation, and it is routinely used to identify enantiomers. We present here a first principles implementation of CD with application to three cyclo-dipeptides. Our CD approach for molecules has been integrated in the 5.0 release of the Yambo code, distributed under GPL
Exciton-Phonon Interaction and Relaxation Times from First Principles
Electron-phonon interactions are key to understanding the dynamics of electrons in materials and can be modeled accurately from first principles. However, when electrons and holes form Coulomb-bound states (excitons), quantifying their interactions and scattering processes with phonons remains an open challenge. Here we show a rigorous approach for computing exciton-phonon (ex-ph) interactions and the associated exciton dynamical processes from first principles. Starting from the ab initio Bethe-Salpeter equation, we derive expressions for the ex-ph matrix elements and relaxation times. We apply our method to bulk hexagonal boron nitride, for which we map the ex-ph relaxation times as a function of exciton momentum and energy, analyze the temperature and phonon-mode dependence of the ex-ph scattering processes, and accurately predict the phonon-assisted photoluminescence. The approach introduced in this work is general and provides a framework for investigating exciton dynamics in a wide range of materials
- …