Light-absorption and luminescence processes in nano-sized materials can be modelled either by using computational approaches developed for quantum chemical calculations or by applying computational methods in the effective mass approximation (EMA) originally intended for solid-state theory studies. An overview of the theory and implementation of an ab initio correlation EMA method for studies of luminescence properties of embedded semiconductor quantum dots is presented. The applicability of the method and the importance of correlation effects are demonstrated by calculations on InGaAs/GaAs quantum-dot and quantum-ring samples. Ab initio and density functional theory (DFT) quantum chemical studies of optical transitions in freestanding silicon nanoclusters are also discussed. The accuracy of the optical gaps and oscillator strengths for silicon nanoclusters obtained using different computational methods is addressed. Changes in the cluster structures, excitation energies and band strengths upon excitation are reported. The role of the surface termination and functional groups on the silicon nanocluster surfaces is discussed.
The local nonequilibrium quasiparticle distribution function in a normal-metal wire depends on the applied voltage over the wire and the type and strength of different scattering mechanisms. We show that in a setup with superconducting reservoirs, in which the supercurrent and the dissipative current flow ͑anti͒ parallel, the distribution function can also be tuned by applying a supercurrent between the contacts. Unlike the usual control by voltage or temperature, this leads to a Peltier-like effect: the supercurrent converts an externally applied voltage into a difference in the effective temperature between two parts of the system maintained at the same potential. We suggest an experimental setup for probing this phenomenon and mapping out the controlled distribution function.
PACS 71.35.Pq, 72.20.Jv, 73.21.La Powerful computational methods are presented for studies of energy levels, photon-recombination rates, and phonon-relaxation rates of neutral and charged multiexciton complexes at correlated levels of theory. The electron -hole system is described by a two-band effective-mass Hamiltonian. The one-particle functions are expanded in a basis set consisting of anisotropic Gaussian functions. The many-body Hamiltonian constructed in the space of the antisymmetric products of one-particle functions is diagonalized using general coupled-cluster and configuration-interaction methods. The expansion coefficients of the coupledcluster and configuration-interaction wave functions are obtained by solving the corresponding equations using direct iterative algorithms. We demonstrate the potential of the computational approaches by calculating total energies of multiexciton complexes at coupled-cluster and configuration-interaction levels.Computational methods for studies of radiative recombination and phonon-relaxation rates have also been developed and results are reported for radiative recombination rates and recombination energies of the exciton, biexciton, and of the positive and the negative trions confined in a InGaAs/GaAs quantum-dot sample. Phonon-relaxation rates have been calculated for a few low-lying g ∆ states of the exciton complex of the same quantum-dot sample.
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