Enlarged magnetic focusing radius of photoinduced currents in mesoscopic circuits

Abstract

We exploit GaAs-based quantum point contacts as mesoscopic detectors to analyze the ballistic flow of photogenerated electrons in a two-dimensional electron gas at a perpendicular magnetic field. Whereas charge transport experiments always measure the classical cyclotron radius, we show that this changes dramatically when detecting the photoinduced non-equilibrium current in magnetic fields. The experimentally determined radius of the trajectories surprisingly exceeds the classical cyclotron value by far. Monte Carlo simulations suggest electron-electron scattering as the underlying reason. PACS: 73.23.Ad, 42.82.Fv, 81.07.St, 85.35.Be, 72.20.Dp KEYWORDS. Ballistic optoelectronic quantum transport, nanoscale optoelectronics, nonequilibrium transport, electron-electron scattering 2In the presence of a magnetic field B, propagating charge carriers obey the Lorenz force and move on curved trajectories in the plane perpendicular to the field. The corresponding classical cyclotron radius r cyclo (| |) = |v| | |(1) for a charge q is a measure of its momentum and thus contains both mass m and velocity v of the carrier. Based on Eq. 1, cyclotron resonance experiments on twodimensional electron systems have been used to verify the 2D nature, to determine the components of the effective mass tensor in solids and to obtain information on Fermi contours [1]. However, following Kohn's theorem, the mass measured in cyclotron resonance experiments is expected to be the bare mass, not affected by electron-electron interactions [2]. In magnetic focusing experiments, quantum point contacts (QPCs) have been used both as injectors and detectors to indirectly obtain information on cyclotron trajectories of ballistic currents in two-dimensional electron gases (2DEGs) [3,4,5,6]. At 2DEGs' boundaries, even skipping cyclotron orbits have been visualized using a scanning gate technique [7,8,9,10], and different electron trajectories have been shown to interfere with each other [8,11,12]. The trajectories extracted were in accordance with Eq. (1) and the expected effective mass GaAs * of the GaAs-based circuits.Here, we exploit an optical beam induced current (OBIC) technique to directly analyze the flowpatterns of photogenerated electrons in a 2DEG embedded in an AlGaAs/GaAs quantum well at a perpendicular magnetic field. In contrast to the magnetic focusing results gained from transport experiments, we uncover the magnetic focusing dynamics of photogenerated non-equilibrium excess charge carriers in a 2DEG. A laser locally creates charge carriers at a certain position in the 2DEG, and the ballistic photocurrent of the photogenerated electrons across an adjacent QPC is measured as a function of the laser position [13,14]. This OBIC-technique allows adjusting the excitation position independent of the applied |B|. Thus, it is possible to directly map the magnetic flow patterns of photogenerated electrons in the 2DEG. We observe curved trajectories with a radius being inversely proportional to |B| as expected from Eq. (1). However...