Impurity Resonant States and Electroconductance Oscillations in Quantum Ballistic Nanostructures

The properties of electronic transport have been studied in quantum ballistic nanostructures, such as GaAs -AlGaAs semiconductor heterostructures. The well-known quantization of conductance as a function of the Fermi energy or the channel width (typically modulated through a gate voltage) is strongly affected in the presence of impurities within the narrow constrictions. We present a tight-binding recursive Green's function method for coupled transverse modes to study dramatic impurity effects on the otherwise quantized conductance. Noticeable conductance features arise due to resonant tunneling through a quasi -bound state in strongly attractive impurities. The conductance curve also exhibits downward dips just before the onset of each plateau, and associated with quasi-bound states in transverse energy levels, which originate from the shifted transverse quantum states due to the attractive impurity. A variety of quantum waveguide structures, which have both geometrical and impurity effects and yield quantum transport anomalies, has been studied in singly- and multiply-connected nanoconstrictions. A quantum interference phenomenon in ballistic nanostructures, analogous to the electrostatic Aharonov-Bohm effect is also presented. Electroconductance oscillations are shown to exist and to arise as a quantum interference effect between phase-shifted branches of the wavefunction, by modulating an applied transverse potential along one of the branches of a multiply-connected system. Studies of temperature dependence of these aperiodic oscillations, the expected phase changes, and electric depopulation of subbands are presented.