Can Photo- and Cathodoluminescence be Regarded as Complementary Techniques?

Photoluminescence (PL) usually provides macroscopic, high quality spectroscopic data. Cathodoluminescence (CL), on the other hand, offers the same information with microscopic imaging. However, replicating PL signatures in a CL system is not straightforward since matching experimental conditions, such as temperature and excitation density, is difficult. The matter is further exacerbated by inherent differences in the nature of excitation: electrons versus photons. Our work with high purity semiconductors suggests that CL is generally more sensitive to excitation circumstance than PL. For example, electrons can cause sample charging and contamination-related phenomena that dramatically affect CL. Changes in surface attributes (e.g., by chemical passivation) also affect PL and CL signals differently. Here, we extend previous work on GaAs by exploring the role of surface topography (by atomic force microscopy) and temperature (1.8K-100K) on excitonic lineshapes. We find that topographic subtleties strongly influence the character of exciton-polariton luminescence. We interpret these changes in terms of non-classical scattering phenomena derived from microscopic roughness. These microscopic changes also influence the temperature behaviour of excitons in crystals. Specifically, we find that passivated samples are brighter partly because there is a corresponding reduction in the (Arrhenius) activation energy for excitonic processes. In summary, the changes in surface topography and corresponding recombination physics seem well correlated

Quasiperiodic functions theory and the superlattice potentials for a two-dimensional electron gas

We consider Novikov problem of the classification of level curves of quasiperiodic functions on the plane and its connection with the conductivity of two-dimensional electron gas in the presence of both orthogonal magnetic field and the superlattice potentials of special type. We show that the modulation techniques used in the recent papers on the 2D heterostructures permit to obtain the general quasiperiodic potentials for 2D electron gas and consider the asymptotic limit of conductivity when $\tau \to \infty$. Using the theory of quasiperiodic functions we introduce here the topological characteristics of such potentials observable in the conductivity. The corresponding characteristics are the direct analog of the "topological numbers" introduced previously in the conductivity of normal metals.Comment: Revtex, 16 pages, 12 figure