The dynamics of spin-lattice relaxation of the Mn-ions in (Zn,Mn)Se-based diluted-magnetic-semiconductor quantum wells is studied by time-resolved photoluminescence.The spin-lattice relaxation time varies by five orders of magnitude from 10 -3 down to 10 -8 s, when the Mn content increases from 0.4 up to 11%. Free carriers play an important role in this dynamics. Hot carriers with excess kinetic energy contribute to heating of the Mn system, while cooling of the Mn system occurs in the presence of cold background carriers provided by modulation doping. In a Zn 0.89 Mn 0.11 Se quantum well structure, where the spin-lattice relaxation process is considerably shorter than the characteristic lifetime of nonequilibrium phonons, also the phonon dynamics and its contribution to heating of the Mn system are investigated.PACS: 75.50. Pp, 78.55.Et, 78.20.Ls, 2

Heating of the spin system of magnetic Mn ions by means of photoexcited carriers has been studied in undoped ͑Zn, Mn͒Se/͑Zn, Be͒Se multiple quantum well structures. Elevated spin temperature of the magnetic ions has been documented by a suppression of the giant Zeeman splitting of excitonic states measured in photoluminescence and reflectivity spectra. Low densities of photoexcitation ͑about 1 W/cm 2 ͒ induce strong heating of the Mn spin system. The heating shows a strong dependence on the Mn content varying from 0.004 to 0.06. It decreases with increasing Mn content due to the shortening of the spin-lattice relaxation time.

For a system of coplanar electrons and holes the photoluminescence spectrum in the fractional quantum Hall regime does not exhibit anomalies associated with the fractional quantum Hall effect. However, when electron and hole layers are separated a new peak in the photoluminescence spectrum is introduced when the filling factor exceeds a fraction vo at which an incompressible state occurs. The new peak is separated from the main spectral feature by the quasiparticle-quasihole gap and corresponds to a process in which a fractionally charged quasiparticle recombines with a hole leaving behind fractionally charged quasiholes.PACS numbers: 78.55.Cr, 73.20.DxIn a magnetic field the kinetic energy of two-dimensional electrons is quantized into extensively degenerate Landau levels. In the strong-magnetic-field limit where all electrons can be accommodated within a single partially filled Landau level electron-electron interactions cannot be treated perturbatively. At certain fractional Landau-level fillings (v) the electrons condense into incompressible fluid states leading to the fractional quantum Hall (FQH) eff*ect. It has proved to be diflficult to probe the properties of the unique many-body states which occur for v at or near filling factor at which an incompressibility occurs.Recently photoluminescence (PL) experiments [l] have opened the possibility of obtaining new spectroscopic information on FQH states. Considerable uncertainty and confusion remains, however, in the interpretation of features in the PL spectra. In this Letter we establish the essential role of geometry in determining the PL spectra. We show that for an ideal electron-hole system PL data provide no spectroscopic information on the state of the many-electron system. On the other hand, if the electrons and holes are widely separated the PL spectra are related to the one-particle Green's function of the electron system which contains important information about the many-electron states.We consider ideal two-dimensional electrons and holes confined to a quantum well in the strong-magnetic-field limit where all electrons and holes are in the lowest spinpolarized Landau level and Landau-level mixing can be ignored. We restrict our attention to the experimentally relevant limit of dilute holes so that the PL spectra are given by Pico) Zh l,cxp(-E^VkBT)\{^M'i^n')\^S{hco-(E^^-E^)),where ^n] and ^^', respectively, denote exact many-body eigenstates of the pure electronic system and the excitonic system where one hole is present. (Z is the partition function of the excitonic system.) In Eq. (1) ^ is [2] a constant proportional to atomic dipole transition rates and to the overlap between electron and hole envelope functions and L is the luminescence operator which in second quantization takes the form L=lLemh, (dT(t>%{T)(l>f(T)=J,emhm .(2)Here 0^, and ^Z' are electron and hole single-particle wave functions in the lowest Landau level and e^ and hm are electron and hole annihilation operators. To obtain the second form for Eq. (2) we have noted that for eac...

The optical properties of two-dimensional electron gases in ZnSe/(Zn,Be)Se and ZnSe/(Zn,Be,Mg)Se modulation-doped quantum wells with electron densities up to 1.4 × 10 12 cm −2were studied by photoluminescence, photoluminescence excitation and reflectivity in a temperature range between 1.6 and 70 K and in external magnetic fields up to 48 T. In these structures, the Fermi energy of the two-dimensional electron gas falls in the range between the trion binding energy and the exciton binding energy. Optical spectra in this regime are shown to be strongly influenced by the Coulomb interaction between electrons and photoexcited holes. In high magnetic fields, when the filling factor of the two-dimensional electron gas becomes smaller than two, a change from Landau-level-like spectra to exciton-like spectra occurs. We attempt to provide a phenomenological description of the evolution of optical spectra for quantum wells with strong Coulomb interactions.

The dynamical response of magnetic ion system to pulsed laser excitation is studied in ͑Zn, Mn͒Se/ ͑Zn, Be͒Se and ͑Cd, Mn͒Te/ ͑Cd, Mg͒Te quantum well structures. Contributions of a direct heating of the Mn system by photocarriers and an indirect heating via nonequilibrium phonons are distinguished. Their relative efficiency is measured at different excitation densities and over a wide range of Mn concentrations from 0.4 to 11%. For all studied regimes the direct energy transfer from carriers dominates in ͑Zn,Mn͒Se structures. In ͑Cd,Mn͒Te structures a regime where the direct heating is still prominent but weaker than the phonon contribution is found.