Electron Transport and Optical Properties in Mesoscopic Systems

Doctoral thesis, 1994

The main subject of this Thesis is optical properties of quantum point contact systems and their influence on the electron transport. A quantum point contact is a narrow constriction in a two-dimensional electron gas, where the width of the constriction can be controlled by means of a gate voltage. The size of the system is comparable to the Fermi wavelength and the electron transport is in the ballistic regime, i.e. the electrons do not suffer any collisions during their motion across the structure. In papers I-III we consider a quantum point contact system subject to a high frequency electromagnetic field. First, the optical absorption is calculated, paper I. We show that optical point contact spectroscopy is possible, since there is a clear frequency separation between absorption in the center of the microconstriction and in the wide contact regions. Further, we calculate the photoconductance of the system, paper II. We find very pronounced step-like oscillations of the photoconductance as a function of gate voltage. The oscillations are cut off when the number of propagating modes becomes too large. In paper III we demonstrate that interference effects, due to inelastic electron-photon scattering, are possible in a ballistic microstructure. The reason is that the electron-photon scattering plays the same role as impurity scattering does in a "dirty" system. The interference effects can be controlled by the gate voltage or by the frequency of the electromagnetic field. The second part of the Thesis deals with persistent currents in mesoscopic rings, paper IV. Here, we investigate the role of statistics on the temperature behaviour of persistent currents in a 1D ballistic ring. We find that the cross-over temperature, separating the low- and high-temperature regimes, is approximately a factor of two larger for a system with a fixed number of particles (canonical ensemble) than for a system with a fixed chemical potential (grand canonical ensemble). The third and last part of the Thesis concerns double barrier resonant tunneling, paper V. We present a phenomenological model describing the effect of inelastic scattering on resonant and sequential tunneling in a double barrier heterostructure. We show that the peak current is independent of whether the tunneling mechanism is coherent or sequential.