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 ...

An innovative strategy in dislocation analysis, based on comparison between continuous and tessellated film, demonstrates that vertical dislocations, extending straight up to the surface, easily dominate in thick Ge layers on Si(001) substrates. The complete elimination of dislocations is achieved by growing self-aligned and self-limited Ge microcrystals with fully faceted growth fronts, as demonstrated by AFM extensive etch-pit counts.

The fabrication of advanced devices increasingly requires materials with different properties to be combined in the form of monolithic heterostructures. In practice this means growing epitaxial semiconductor layers on substrates often greatly differing in lattice parameters and thermal expansion coefficients. With increasing layer thickness the relaxation of misfit and thermal strains may cause dislocations, substrate bowing and even layer cracking. Minimizing these drawbacks is therefore essential for heterostructures based on thick layers to be of any use for device fabrication. Here we prove by scanning X-ray nanodiffraction that mismatched Ge crystals epitaxially grown on deeply patterned Si substrates evolve into perfect structures away from the heavily dislocated interface. We show that relaxing thermal and misfit strains result just in lattice bending and tiny crystal tilts. We may thus expect a new concept in which continuous layers are replaced by quasi-continuous crystal arrays to lead to dramatically improved physical properties.

Laser emission using photonic crystal microcavities (PCM) [1] has opened new ways towards very low threshold and highly efficient solid state lasers with also very small size [2,3]. Recently, the term "thresholdless" has been used in the literature [4] to identify lasers presenting two main features: a spontaneous emission coupling factor (β ) close to 1 and low non radiative losses. Non radiative losses are reduced by several orders of magnitude at cryogenic temperatures, although they can never be completely suppressed. When the spontaneous emission factor β is equal to 1 every photon emitted by the device is emitted in the lasing mode. Such "thresholdless" lasers were proposed by Noda [4] to be realized by combining QDs as light emitters and PCMs as high quality resonators. Using that recipe, ultra-low threshold lasing has been achieved at cryogenic temperatures using an ever-decreasing number of QDs within PCMs. [2,[5][6][7] That strategy was adopted by Strauf et al. to demonstrate near thresholdless lasing at low temperature (4.5 K) by using few QDs (between 2 to 4) as active emitters and a high β =0.85 with power theshold values of 124 nW.[5] Khajavikhan et al. recently demonstrated thresholdless operation at low temperature (4 K) using metallic microcavities instead PCMs. [8] In this work we report a RT continuous wave (c.w.) laser with emission characteristics close to those of an deal thresholdless laser. [6]

In this work we report the stacking of 50 InAs/GaAs quantum dot layers with a GaAs spacer thickness of 18 nm using GaP monolayers for strain compensation. We find a good structural and optical quality of the fabricated samples including a planar growth front across the whole structure, a reduction in the quantum dot size inhomogeneity, and an enhanced thermal stability of the emission. The optimized quantum dot stack has been embedded in a solar cell structure and we discuss the benefits and disadvantages of this approach for high efficiency photovoltaic applications.

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The authors have studied the use of antimony for the optimization of the InAs∕GaAs(001) self-assembled quantum dot (QD) luminescence characteristics in the 1.3μm spectral region. The best results have been obtained by capping InAs QDs with 2 ML of GaSb grown on top of a 3 ML GaAs barrier separating the InAs and the GaSb layers. This results in an order of magnitude enhancement of the room temperature luminescence intensity at 1.3μm emission wavelength.

Long distance ͑1.4 m͒ interaction of two different InAs/GaAs quantum dots in a photonic crystal microcavity is observed. Simultaneous coupling of both quantum dots to the cavity is demonstrated by Purcell effect measurements. Resonant optical excitation in the p state of any of the quantum dots, results in an increase in the s-state emission of the other one. The cavity-mediated coupling can be controlled by varying the excitation intensity. These results represent an experimental step toward the realization of quantum logic operations using distant solid-state qubits. DOI: 10.1103/PhysRevB.81.193301 PACS number͑s͒: 78.67.Hc, 42.50.Ex, 78.67.De Efficient quantum information applications require qubits with low decoherence rates, fast manipulation times, and easy scalability.1 These requirements are met by qubits based on electron spins or excitons in semiconductor quantum dots ͑QDs͒. Coupling of single semiconductor QD excitons to a microcavity confined electromagnetic mode has different advantages depending on the coupling strength. Weak coupling allows enhanced optical efficiency associated to the exciton decay time reduction by the Purcell effect.2 In the strong-coupling regime, the system presents entangled lightmatter states that can be used as building blocks for transmission of quantum information, 3 qubit readout, 4 production of entangled pairs by compensation of the natural exciton fine structure splitting, 5 and lasing. 6 Single QD-cavity coupling has been demonstrated in the past years, 7-13 showing interesting cavity-quantum electrodynamics effects. The possibility of using two or more qubits coupled by a single optical microcavity is appealing for it can provide techniques for long distance, fast interactions between qubits.14-16 New dynamical phenomena are expected in these systems, which are dependent on the relative energy scales of the coupling between qubits and between qubits and the cavity mode ͑CM͒. In randomly distributed QDs samples it is statistically difficult to have two or more QDs both spatially and spectrally coupled to a microcavity mode. Some approaches have been proposed to obtain this type of coupled system, [17][18][19] which rely on the deterministic location of the QD in the cavity.12,20 Coupling of several QDs to a single cavity mode has been reported as the origin of lasing at very low threshold. 21In this Brief Report, we show that exciton states of two semiconductor quantum dots with large lateral separation interact through a microcavity confined optical mode. Individual and simultaneous coupling of the QDs to the CM is demonstrated by changes in photoluminescence ͑PL͒ emission intensity and spontaneous emission rate ͑Purcell effect͒ when the QD excitons are brought into resonance with the CM. Cavity-mediated inter-QD interaction is demonstrated by PL excitation ͑PLE͒ measurements, in which resonant excitation at the p state of any of the QDs increases the s-state emission of the other one. The microcavity-mediated interaction of the two QDs can be controlled by vary...