Spin transport and optically-probed coherence in magnetic semiconductor heterostructures

Molecular beam epitaxy is used to "spin engineer" an environment wherein quantum-confined electronic states in a wide band gap II-VI semiconductor quantum well (Zn_1-xCd_x Se) are strongly exchange-coupled to systematic 2D distributions of localized spins (Mn²⁺ ions). Magneto-optical spectroscopy of undoped structures demonstrates that such a scheme successfully produces well-confined excitonic states whose Zeeman splitting in modest magnetic fields greatly exceeds the inhomogeneous line widths. In modulation-doped structures, a combination of magneto-transport and magneto-optical measurements shows the formation of a "magnetic" two-dimensional electron gas characterized by spin gaps which are much larger than Landau level gaps. This results in a novel quantum Hall system which can be highly spin polarized even at large filling factors. Time-resolved Faraday/Kerr effect measurements in the Voigt geometry probe the electronic spin dynamics of the exciton/electron gas, revealing terahertz and gigahertz oscillations that originate from the coherent spin precession of electrons and local moments, respectively.