A new mechanism is proposed for the energy relaxation of hot carriers in single-wall carbon nanotubes: scattering with the emission of surface optical phonons into the semiconductor substrate. The theory involves intrasubband and intersubband forward and backward scattering. The analytical result and numerical data indicate that intrasubband forward scattering is the main process: the corresponding lifetime comprises several femtoseconds for a quartz substrate, which allows this mechanism of energy relaxation to be considered dominating for a nanotube on the surface of a polar semiconductor or a dielectric.

We report on efficient terahertz emission in the region of 1.5-2 THz from high electric field biased 6H-SiC structures with a natural superlattice (NSL) at liquid helium temperatures. The shape of the THz emission spectrum, the linear dependence of its maximum on the bias, the characteristic field strength required to achieve the emission, and the linear polarization of the emission along the natural superlattice axis allow it to be attributed to steady-state Bloch oscillations of electrons in the 6H-SiC natural superlattice.

The theory of the remote Coulomb impurity scattering in single-wall carbon nanotubes is developed within a one-electron approximation. The Boltzmann equation is solved within the drift-diffusion model to obtain the tube conductivity. The conductivity depends on the type of the nanotube band structure (metal or semiconductor) and on the electron Fermi energy. We found that the exponential dependence of the conductivity on the Fermi energy is due to the Coulomb scattering rate having a strong dependence on the momentum transfer. We calculate intrasubband and intersubband scattering rates and present general expressions for the conductivity. Numerical results, as well as obtained analytical expressions, show that the degenerately doped semiconductor tubes may have very high mobility unless the doping level becomes too high and the intersubband transitions impede the electron transport.

A general expression for optical matrix elements describing interlevel absorption in size-quantized systems in the presence of electric and magnetic fields is derived. For rectangular and triangular confining potentials, this approach does not require detailed knowledge of the electron wave function and considerably simplifies the problem. As a result, selection rules and analytical expressions for the matrix elements are obtained for many cases where the standard procedure is much more complicated. As examples, optical matrix elements and selection rules are calculated for some particular shapes of quantum wells: triangular, single, and double asymmetric rectangular. The problem is generalized to the case of heterostructures with effective-mass mismatch. A similar approach to quantum wires and dots is also considered. Interlevel absorption spectra for quantum wells in a parallel magnetic field are calculated.