SEMICONDUCTOR NANOSTRUCTURES IN HIGH MAGNETIC FIELDS: RECENT RESULTS AT HFML NIJMEGEN

Abstract

A short overview of the current experimental facilities at the HFML Nijmegen, including an update on the planned development of a 45 T Hybrid magnet in conjunction with a Free Electron Laser, was presented. That was followed by a brief highlight of recent results on the high field physical properties of semiconductor nanostructures, such as quantum rings1 and dots,2 graphene3,4 and organic nanostructures.5 The majority of the talk was devoted to the investigation of negatively charged excitons (negative trions, two electrons bound to one hole) in semiconductor quantum wells in high magnetic fields. A comprehensive overview of photoluminescence (PL) experiments on GaAs and CdTe quantum wells (QWs) of variable well widths, containing a low density electron gas (2DEG) was presented. Particular interest was given to the magnetic field range where the ground state of negative trions is changing from a singlet (both electrons having opposite spin) to a triplet (parallel electron spins) state. Using polarized magneto-PL (excitation) and reflectivity spectroscopy we have identified (for both GaAs and CdTe QWs) the four typical excitonic peaks, usually denoted as the singlet, dark triplet and bright triplet trion peaks, as well as the neutral exciton peak. All peaks exhibit a pronounced dependence on polarization, 2DEG density and temperature. CdTe QWs exhibit all four peaks at the entire field range used (< 44 T), whereas the dark triplet emission disappears at high fields for GaAs QWs. We have determined the field strength at which the singlet-triplet crossover occurs for all QWs, which allows us to critically compare our experimental results with theoretical predictions. Remarkably, the actual singlet-triplet transition is hidden in GaAs QWs and a narrow (12 nm) CdTe QW. Since the PL emission energy equals the energy difference between the initial (trion) and final (electron) states, the PL lines themselves do not cross at the singlet-triplet crossover as a result of the different Zeeman energies of the final electron states.6 In contrast the cross-over is directly visible in the PL lines of a wide (20 nm) CdTe QW. This behaviour can be readily understood by the dependence of the electron- and hole g -factors on well width and host material. Note from Publisher: This article contains the abstract only.