X iv : c on d - m at / 9 50 60 10 v 1 2 J un 1 99 5 Microwave - induced Resonant Reflection and Localization of Ballistic Electrons in Quantum Microchannels

We show that electron transport in a ballistic microchannel supporting both propagating and reflected modes can be completely blocked by applying a microwave electromagnetic field. The effect is due to resonant reflection caused by multiple coherent electron-photon scattering involving at least two spatially localized scattering centers in the channel. With many such scattering centers present the conductance is shown to have an irregular dependence on bias voltage, gate voltage and frequency with irregu-larily spaced dips corresponding to resonant reflection. When averaged over bias, gate voltage or frequency the conductance will decay exponentially with channel length in full analogy with the localization of 1D electrons caused by impurity scattering. The dynamics of a mesoscopic system subject to a time dependent field depends cruically on the relation between the phase breaking time τ φ and the time t 0 needed for electrons to pass through the system. In the absence of phase breaking processes, τ φ ≫ t 0 , one could expect phase coherent dynamic phenomena to occur in mesoscopic systems in close analogy with the well known static phenomena. They should be highly tunable by for instance a magnetic field or by electric fields due to applied bias-or gate voltages. The photoconductance of a ballistic microchannel created in a gated AlGaAs heterostructure is an example. In a channel with a cross section that varies so slowly that the adiabatic approximation applies, coherent electron-photon scattering corresponding to transitions between different transverse modes has been shown to be localized in space and to lead to transitions between propagating and reflected modes [1]. This type of indirect backscattering has many features in common with impurity scattering and has been suggested [2] to give rise to quantum interference effects in the electron transport properties. In particular the photoconductance caused by single photon scattering (absorption) was shown to be an oscillating function of an applied gate voltage in a quasi-one dimensional channel containing a microwidening [2]. At large enough electromagnetic fields, multiple coherent electron-photon scattering results in a coherent resonant coupling of two different transverse modes in the channel. Because the electron-photon interaction is localized in space, this resonant coupling can be expressed in terms of a Landau-Zener breakdown [3]. In this paper we pursue this line of argument and show that resonant electron-photon scattering strongly modifies the electron transport and that it may lead to resonant (total) reflection of electrons and hence block the …