Magnetic-field-induced formation of exciton magnetic polarons in ZnSe/Zn1−xMnxSe quantum-well structures

Abstract

Cw and time-resolved photoluminescence spectroscopy is used to study ZnSe/${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Mn}}_{\mathit{x}}$Se quantum wells with semimagnetic barriers in an external magnetic field. The data demonstrate a change of the dominant energy relaxation mechanism from disorder localization of light-hole excitons at zero-field to heavy-hole exciton interface magnetic polaron formation at intermediate fields and, again, disorder localization of heavy-hole excitons at large magnetic fields. The formation of the interface magnetic polaron is promoted by a magnetic-field-induced type-I–type-II transition for heavy-hole excitons. Despite the transition, neither the exciton lifetime nor its optical oscillator strength is dramatically altered. This is, as we confirm by numerical solution of the two-particle Schrödinger equation, a result of the electron-hole Coulomb interaction. The polaron formation time is initially 110 ps, but decreases with growing magnetic field down to 70 ps (B=5 T). A theoretical investigation of the polaron formation dynamics shows that the associated change of the exciton wave function is smaller, the closer the ${\mathrm{Mn}}^{2+}$ spin system is driven into saturation by the external field. As a consequence, the polaron formation time approaches the characteristic ${\mathrm{Mn}}^{2+}$ spin response time. Our measurement uncovers a fast primary localization prior to the polaron process—but also of magnetic origin—that we believe to be necessary to start the polaron formation. © 1996 The American Physical Society.