B.D. McCombe

Aharonov-Bohm excitons at elevated temperatures in type-II ZnTe/ZnSe quantum dots.

Optical emission from type-II ZnTe/ZnSe quantum dots demonstrates large and persistent oscillations in both the peak energy and intensity indicating the formation of coherently rotating states. Furthermore, these Aharonov-Bohm oscillations are shown to be remarkably robust and persist until 180 K. This is at least one order of magnitude greater than the typical temperatures in lithographically defined rings. To our knowledge, this is the highest temperature at which the AB effect has been observed in solid-state and molecular nanostructures.

Robust magnetic polarons in type-II (Zn,Mn)Te/ZnSe magnetic quantum dots

We present a magneto-optical study of magnetic polarons in type-II Zn,Mn Te quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. In our photoluminescence studies, the magnetic polarons are detected at temperatures up to 150 K, with a formation energy of 40 meV. The emission from these dots exhibits an unusually small Zeeman shift with applied magnetic field 2 meV at 8 T and at the same time a very large circular polarization. We attribute this apparently contradictory behavior by a low and weakly temperaturedependent magnetic susceptibility due to antiferromagnetic coupling of the Mn spins.

Coherent Aharonov-Bohm oscillations in type-II (Zn,Mn)Te/ZnSe quantum dots

The magneto-photoluminescence of type-II (Zn,Mn)Te quantum dots is presented. As a result of the type-II band alignment, Aharonov-Bohm (AB) oscillations in the photoluminescence intensity are evident. In addition, an interesting interplay between the AB effect and the spin polarization in these diluted magnetic semiconductor quantum dots is observed. The intensity of the AB oscillations increases with both magnetic field and the degree of optical polarization, indicating that the suppression of spin fluctuations improves the coherence of the system.

Charged Magnetoexcitons in Two Dimensions: Isolated X— and Many‐Electron Effects

The states of charged magnetoexcitons in two-dimensional systems are considered. Exact optical selection rules for intra- and inter-band processes are discussed. The effect of excess electrons on internal transitions of negatively charged excitons X in quantum wells is studied experimentally and theoretically. An experimentally observed blue-shift with excess electron density is explained in terms of collective excitations, magnetoplasmons bound to a valence band hole.

Internal transitions of two-dimensional charged magneto-excitons X−: theory and experiment

Abstract Internal spin-singlet and spin-triplet transitions of charged excitons X− in magnetic fields in quantum wells have been studied experimentally and theoretically. The allowed X− transitions are photoionizing and exhibit a characteristic double-peak structure, which reflects the rich structure of the magnetoexciton continua in higher Landau levels (LLs). We discuss a novel exact selection rule, a hidden manifestation of translational invariance, that governs transitions of charged mobile complexes in a magnetic field.

Pressure Tuning of Many-Electron Impurity Interactions in Confined Semiconductor Structures

We report studies of the free carrier and donor-bound FIR excitations of a confined electron gas in modulation doped GaAs/AlGaAs quantum wells (QW) as a function of the QW electron density. Applied pressure is used to tune the electron density via the Γ–X well–barrier crossover. As electrons are removed from the QWs, we observe successively the quenching of cyclotron resonance, the evolution of the D— singlet-like magnetoplasmon resonance into the D— singlet transition of isolated donors, and the emergence of the neutral donor D0 1s–2p+ line. Calculations predict a sharp drop in the QW electron density for 2.3 to 3.1 GPa, in accordance with experiment. A rapid decrease with B-field in the blue shift of the magnetoplasmon resonance at 2.2 GPa in one sample shows that pressure has shifted the ν < 1 filling-factor regime to a factor-of-two lower field.

Barrier impurity states in modulation doped AlGaAs/GaAs multi-quantum wells

Abstract MQW samples doped with donors in the barrier have been studied by far infrared magneto-spectroscopy to point out the effect of modulation doping in confined structures. Additional absorption features appear in the spectra; the lowest energy feature is shown to be due to electrons in the QWs bound to their parent donor ions in the barrier. Investigation of edge doped samples reveal that other features are not related to edge donors.

Metal–insulator transition of spatially separated electrons and holes in mixed type I–type II GaAs/AlAs quantum wells

Low temperature photo-induced far-infrared and microwave absorption studies of mixed type I–type II GaAs/AlAs multiple quantum wells have revealed features related to a metal–insulator transition. The far infrared experiments show an absorption feature at fields below electron cyclotron resonance (eCR) that shifts up with increasing electron–hole density and eventually pins to the eCR field position. In microwave experiments a broad, photoinduced absorption line is observed at fields above that of eCR; the position and strength of this feature depend on excitation intensity, temperature and microwave power. This disparate behavior can be understood qualitatively in terms of internal transitions of spatially separated electrons bound to holes localized laterally in short-range well-width fluctuations and clustered in long-range potential maxima in the narrow wells. These transitions evolve with increasing excitation intensity via a metal-insulator transition into dimensional magnetoplasma resonances of “puddles” of an electron liquid confined laterally by the potential of the clustered holes.

Grating-coupler-induced intersubband transitions in semiconductor multiple quantum wells

Abstract Intersubband transitions induced in a simple Faraday transmission geometry by metallic grating couplers have been studied in several GaAs/Al 0.3 Ga 0.7 As MQW samples lightly doped with donors and with well widths between 210 A and 320 A. A sensitive pumping and probing technique was employed in which chopped visible pump light and spatially modulated infrared radiation are transmitted simultaneously through the MQW. The excess free electron density in the well created by 2 of red light is estimated to be ≅ 10 9 cm −2 per well. The measured E 1 – E 0 energies are in good agreement with a simple ID model calculatio found that the grating coupling efficiency drops from 15% to 5% when the ratio of the transition wavelength to the grating period is increased from 1.5 to 3.6.

Time-resolved magnetophotoluminescence studies of magnetic polaron dynamics in type-II quantum dots

We used continuous wave photoluminescence (cw-PL) and time resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe/(Zn,Mn)Se system the holes are confined in the non-magnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn,Mn)Se matrix. On the other hand, in the (Zn,Mn)Te/ZnSe system, the holes are confined in the magnetic (Zn,Mn)Te QDs, while the electrons remain in the surrounding non-magnetic ZnSe matrix. The magnetic polaron formation energies in both systems were measured from the temporal red-shift of the band-edge emission. The magnetic polaron exhibits distinct characteristics depending on the location of the Mn ions. In the ZnTe/(Zn,Mn)Se system the magnetic polaron shows conventional behavior with decreasing with increasing temperature T and increasing magnetic field B. In contrast, in the (Zn,Mn)Te/ZnSe system has unconventional dependence on temperature T and magnetic field B; is weakly dependent on T as well as on B. We discuss a possible origin for such a striking difference in the MP properties in two closely related QD systems.