RESUMO
We numerically solve the Bloch-Torrey equation by discretizing the differential operators in real space using finite differences. The differential equation is either solved directly in time domain as initial-value problem or in frequency domain as boundary-value problem. Especially the solution in time domain is highly efficient and suitable for arbitrary domains and dimensions. As examples, we calculate the average magnetization and the frequency distribution for capillaries and cells which are idealized as cylinders and spheres, respectively. The solution is compared with the commonly used Gaussian approximation and the strong-collision approximation. While these approximations become exact in limiting cases (small or large diffusion coefficient), they strongly deviate from the numerical solution for intermediate values of the diffusion coefficient.
Assuntos
Algoritmos , Capilares/citologia , Células Cultivadas/citologia , Interpretação de Imagem Assistida por Computador/métodos , Imageamento por Ressonância Magnética/métodos , Modelos Biológicos , Simulação por Computador , Campos EletromagnéticosRESUMO
A theory of time-resolved luminescence from photoexcited semiconductors is presented. It combines quantum kinetics of hot-carrier relaxation and quantum theory of spontaneous emission. Model calculations show the "transfer" of photoluminescence from the initial signal at the pump frequency via subsequent phonon replicas until the buildup of luminescence at the excitonic resonance. Time-resolved photoluminescence is predicted to be a sensitive measure of electron-LO-phonon quantum kinetics and bottleneck effects.
RESUMO
We investigate the energy spectrum and the electron dynamics of a band in a semiconductor superlattice as a function of the electric field. Linear optical spectroscopy shows that, for high fields, the well-known localization of the Bloch states is followed by a field-induced delocalization, associated with Zener breakdown. Using time-resolved measurements, we observe Bloch oscillations in a regime where they are damped by Zener breakdown.
