We report the direct measurement of the persistent current carried by a single electron by means of magnetization experiments on self-assembled InAs=GaAs quantum rings. We measured the first Aharonov-Bohm oscillation at a field of 14 T, in perfect agreement with our model based on the structural properties determined by cross-sectional scanning tunneling microscopy measurements. The observed oscillation magnitude of the magnetic moment per electron is remarkably large for the topology of our nanostructures, which are singly connected and exhibit a pronounced shape asymmetry. DOI: 10.1103/PhysRevLett.99.146808 PACS numbers: 73.21.La, 73.23.Ra, 78.67.Hc In quantum mechanics, particular attention is paid to phenomena occurring due to the phase coherence of charge carriers in doubly connected (ring) topologies. Electrons confined to a submicron ring manifest a topologically determined quantum-interference phenomenon, known as the Aharonov-Bohm (AB) effect [1], as a result of the oscillatory behavior of their energy levels as a function of an applied magnetic field. This behavior is usually associated with the occurrence of oscillatory persistent currents in the ring [2 -4]. Experimental evidence for AB oscillations has been detected in the mesoscopic regime in metallic [5,6] and semiconducting [7,8] rings, containing many electrons. We address the occurrence of the AB effect in defect-free self-assembled semiconductor nanostructures [9][10][11][12][13]. The ability to fill nanostructures with only a few (1-2) electrons offers the unique possibility to detect magnetic field induced oscillations in the persistent current carried by single electron states. We report the first direct measurement by means of ultrasensitive magnetization experiments of the oscillatory persistent current carried by a single electron in self-assembled InAs/GaAs ''volcanolike'' nanostructures. Remarkably, this single electron current occurs even in the absence of an opening [14] in our nanostructures, which is required for the AB effect in the standard treatment [1]. The magnetic field at which the first oscillation in the magnetic moment arises is much higher than expected from the diameter of the quantum rings as determined by atomic force microscopy [13]. However, the experiments are in good agreement with a model based on the structural parameters as determined with cross-sectional scanning tunneling microscopy (XSTM) measurements.The persistent current was determined via the magnetic moment of electrons in a highly homogeneous ensemble of InAs self-assembled nanostructures. The sample was grown by molecular beam epitaxy and contains 29 mutually decoupled periods [ Fig. 1(a)] [15]. Each period consists of a nanostructured InAs layer, between two 24 nm GaAs layers, and a 2 nm doped (7 10 16 cm ÿ3 Si) GaAs layer that provides electrons to the InAs nanostructures. We used a one-dimensional Poisson solver [16] to estimate the average number of electrons per nanostructure to be about 1.5. Considering the two possible spin orientations we ...

The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2431758͔There is currently great interest in producing entangled photons on demand for applications in quantum information processing. 1 One proposal which spurred much research is using radiative biexciton-exciton cascade in semiconductor quantum dots ͑QDs͒ to produce pairs of polarization entangled photons. 2 In an idealized QD, the bright exciton states ͑M = ±1͒ are degenerate. In this case the two decay paths from the biexciton to the vacuum state via the intermediate single exciton are indistinguishable in energy; thus photons emitted in the radiative cascade are polarization entangled. However, in practice, the rotational symmetry of a self-assembled QD is broken and the electron-hole exchange interaction mixes the bright exciton states into a nondegenerate doublet ͓referred to as a fine-structure splitting ͑FSS͔͒. This leads to an energetically distinguishable recombination path for the biexciton-exciton cascade. Polarization correlations are observed in the linear basis but polarization entanglement is destroyed due to the FSS. 3,4 For photons to be polarization entangled using this scheme, the requirement that the FSS be less than the homogeneous linewidth must be met. The FSS is typically 10-100 eV, while the homogeneous linewidth of self-assembled InGaAs QDs is ϳ1 eV. 5 Techniques used to actively tune the FSS include an inplane electric 6 or magnetic 7 field and an in situ uniaxial stress. 8 Also, QDs which are smaller due to the growth process 9 or subsequent annealing 10 have a smaller FSS. Unfortunately, such QDs are higher in energy and the QD photons become difficult to distinguish from those produced in the wetting layer. Recently, polarization entangled photons have been reported from specific QDs with energetically overlapping bright exciton states 11 and initially nondegenerate states tuned via a magnetic field. 12 However, a robust approach that would allow one to actively tune the FSS from a large value to zero for each QD is still necessary to realize an event ready entangled photon pair source. To this end we further explore the effect of a lateral electric field on the FSS.There are three basic characteristics of an exciton in a lateral electric field attributed to the quantum confined Stark effect, as has been investigated for quantum wells 13 and QDs: 14 a redshift in recombination energy, decreased oscillator strength, and an increase in nonradiative carrier tunneling probability. Additionally, electric fi...

We have measured the vertical Stark effect of excitons confined to individual self-assembled ring-shaped quantum dots. We find that the excitons have very large permanent dipole moments corresponding to electronhole separations up to 2.5 nm, comparable to the nanostructures' physical height. We find a trend of both permanent dipole moment and polarizability on the emission energy, but a very strong correlation between the permanent dipole moment and the polarizability.

Offermans, P.; Koenraad, P.M.; Wolter, J.H.; Granados, D.; Garcia, J.M.; Fomin, V.; Gladilin, V.N.; Devreese, J.T.

We report the formation of nanosize ferromagnetic MnAs crystallites imbedded in low-temperature grown GaAs using Mn+ ion implantation and subsequent annealing. The structural and magnetic properties of the crystallites have been characterized by transmission electron microscopy, electron beam induced x-ray fluorescence, and superconducting quantum interference device magnetometry. After an optimized thermal annealing at 750 °C, MnAs crystallites of 50 nm in size are formed. These nanomagnets show room temperature ferromagnetism.