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""We measured the absorption coefficient and the non-radiative lifetime of intersubband transitions in Germanium quantum wells by steady-state and pump-probe terahertz. spectroscopy.""

We study the low-energy electronic structure of massless and massive zigzag graphene nanoribbons and the effect due to edge and bulk model perturbations. We consider also the presence of a uniform magnetic field threading the ribbons and the induced energy splitting of the two Dirac valleys. The emergence of energy gaps and valley asymmetry enriches the perspectives of novel devices based on relativistic-like carriers.

We present a tight-binding continued-fraction description of the energy spectrum of zigzag and armchair graphene ribbons. The effect of a uniform magnetic field threading biased ribbons and the presence of top gates are also discussed. Stationary currents distribution is obtained by the nonequilibrium Keldysh-Green's function method. A pictorial description of conductive channels in the quantum Hall regime is provided. The analysis of the currents under applied gates shows that the Dirac-like nature of the states at the corners of the graphene Brillouin zone can be exploited to realize current manipulation.

The study of current profiles in the two-dimensional electron gas in strong magnetic fields is central for a microscopic interpretation of the integer quantum Hall effect. Within the framework of the nonequilibrium Keldysh-Green's function formalism, we consider the general expressions of energy-resolved and space-resolved current distributions in Hall samples. Formal elaboration, corroborated also by numerical calculations, shows that chirality of current flow, thermodynamic equilibration of conductive channels, as well as conductance quantization, are all a straight consequence of the semilocal nature of retarded (or advanced) Green's functions in strong magnetic fields.

The conductive channels and the current distributions through a narrow quantum point contact (QPC) in a two-dimensional electron gas threaded by a uniform magnetic field are studied using the Keldysh nonequilibrium Green's function formalism and a site representation of the electron Hamiltonian. We show that when the conditions of chiral transport regime are met in the two regions separated by the QPC, exact quantization in integer multiples of 2e/sup 2//h is maintained both for the conductance of the incident current and the backscattered plus transmitted currents.