J. A. Gaj

Light and Electric Field Control of Ferromagnetism in Magnetic Quantum Structures

A strong influence of illumination and electric bias on the Curie temperature and saturation value of the magnetization is demonstrated for semiconductor structures containing a modulation-doped p-type Cd(0.96)Mn(0.04)Te quantum well placed in various built-in electric fields. It is shown that both light beam and bias voltage generate an isothermal and reversible crossover between the paramagnetic and ferromagnetic phases, in the way that is predetermined by the structure design. The observed behavior is in quantitative agreement with the expectations for systems, in which ferromagnetic interactions are mediated by the weakly disordered two-dimensional hole liquid.

Influence of an in-plane electric field on exciton fine structure in InAs-GaAs self-assembled quantum dots

The influence of an in-plane electric field on the optical properties of single quantum dots is investigated. On a sample containing a plane of InAs∕GaAs dots, micrometer-size electro-optical structures were produced in order to apply an external electric field in the dot plane. A large decrease of the anisotropic exchange splitting, correlated with the in-plane Stark shift, is observed.

Optical manipulation of a single Mn spin in a CdTe-based quantum dot.

Two coupled CdTe quantum dots, selected from a self-assembled system, one of them containing a single Mn ion, were studied by continuous wave and modulated photoluminescence, photoluminescence excitation, and photon correlation experiments. Optical writing of information on the spin state of the Mn ion has been demonstrated, using the orientation of the Mn spin by spin-polarized carriers transferred from the neighboring quantum dot. Mn spin orientation time values from 20 to 100 ns were measured, depending on the excitation power. Storage time of the information on the Mn spin was found to be enhanced by application of a static magnetic field of 1 T, reaching hundreds of microseconds in the dark. Simple rate equation models were found to describe correctly the static and dynamical properties of the system.

Effect of the s , p − d exchange interaction on the excitons in Zn 1 − x Co x O epilayers

We present a spectroscopic study of ZnCoO layers grown by molecular beam epitaxy on sapphire substrates. ZnCoO is commonly considered as a promising candidate for being a Diluted Magnetic Semiconductor ferromagnetic at room temperature. We performed magneto-optical spectroscopy in the Faraday configuration, by applying a magnetic field up to 11T, at temperatures down to 1.5K. For very dilute samples (x<0.5%), the giant Zeeman splitting of the A and B excitons is observed at low temperature. It is proportional to the magnetization of isolated Co ions, as calculated using the anisotropy and g-factor deduced from the spectroscopy of the d-d transitions. This demonstrates the existence of spin-carrier coupling. Electron-hole exchange within the exciton has a strong effect on the giant Zeeman splitting observed on the excitons. From the effective spin-exciton coupling, =0.4eV, we estimate the difference of the exchange integrals for free carriers, N0 |alpha - beta|=0.8=eV. The magnetic circular dichroism observed near the energy gap was found to be proportional to the paramagnetic magnetization of anisotropic Co ions even for higher Co contents.

Magnetic polarons in exciton luminescence of Cd1-xMnxTe

Systematic investigations of the photoluminescence spectra of Cd1-xMnxTe mixed crystals were performed for 0<or=x<or=0.4 as a function of temperature and magnetic field. Two luminescence lines are distinguished in the excitonic region. The first line is caused by recombination of excitons bound to neutral acceptors. A detailed theoretical interpretation of this line is given in terms of the bound magnetic polaron. For the second line a structure on the curve of temperature dependence is observed in the vicinity of the spin-glass phase transition. That line is attributed to a magnetic localisation of excitons.

Excitation mechanisms of individual Cd Te ∕ Zn Te quantum dots studied by photon correlation spectroscopy

Systematic measurements of auto- and cross-correlations of photons emitted from individual

Optically induced energy and spin transfer in nonresonantly coupled pairs of self-assembled CdTe/ZnTe quantum dots

\mathrm{Cd}\mathrm{Te}∕\mathrm{Zn}\mathrm{Te}

Introduction to the physics of diluted magnetic semiconductors

quantum dots under pulsed excitation were used to elucidate nonresonant excitation mechanisms in this self-assembled system. Memory effects extending over a few excitation pulses have been detected in agreement with previous reports and quantitatively described by a rate equation model, fitting a complete set of correlation and PL intensity results. The important role of single carrier trapping in the quantum dot was established. An explanation was suggested for the unusually wide antibunching dip observed previously in

Picosecond charge variation of quantum dots under pulsed excitation

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p-type doping of II–VI heterostructures from surface states: Application to ferromagnetic Cd1−xMnxTe quantum wells

autocorrelation experiments on quantum dots under cw excitation.