We report photoluminescence measurements on stacked self-assembled InP quantum dots in magnetic fields up to 50 T. For triply stacked layers the dots become strongly coupled when the layer separation is 4 nm or less. In contrast, doubly stacked layers show no sign of coupling. We explain this puzzling difference in coupling by proposing a model in which the holes are weakly confined in the Ga x In 1Ϫx P layers separating the layers of dots, and are responsible for the coupling. Since only one such intervening layer exists in the doubly stacked dots coupling is excluded. Our model is strongly supported by the exciton masses and radii derived from our experimental results, and is consistent with available theory.

We report magnetophotoluminescence measurements of stacked layers of self-assembled InP quantum dots. With a magnetic field applied in the growth direction we have determined the exciton reduced mass from the field dependence of the photoluminescence energy. By applying a magnetic field perpendicular to the growth direction, we have analyzed the spatial confinement of the dots in the growth direction. We observe a large increase in the shift of the exciton energy between 0 and 50 T when the thickness of the GaInP spacer layer between the dots is reduced from 8 to 4 nm. This indicates a decrease in spatial confinement in the growth direction which we attribute to strong electronic coupling between vertically stacked dots.During the last few years much effort has been put into the study of self-assembled semiconductor quantum dots ͑QDs͒, due to their promising optical and electrical properties for applications such as lasers. 1 Photoluminescence ͑PL͒ measurements on these QDs reveal an intense emission of visible ͑InP͒ 2-4 or infrared ͓In͑Ga͒As, 1,5-9 InSb, 10 and InAlAs͔ 11 light, with linewidths of the order of 40 meV, from the confined excitons in the dots. These rather broad lines are caused by the luminescence of large amounts of QDs, ϳ10 10 cm Ϫ2 , with a typical 10% size nonuniformity. On the other hand, by repeating layers of dots separated by a few nanometers a vertical alignment occurs giving a reduction in the linewidth due to a more homogeneous size distribution and electronic coupling. 3,5,6 Here we present PL measurements of stacked InP QDs in magnetic fields up to 50 T. Our data show that reducing the thickness of the GaInP barrier between layers of dots results in strong electronic coupling between vertically stacked InP dots. The control of the electronic coupling of QDs offers the potential for wavefunction engineering, which is interesting, not only for fundamental physics, 12 but also for laser applications. Indeed, the first injection laser based on stacked InP quantum dots has recently been reported. 13 The InP quantum dots were grown by solid-source molecular beam epitaxy on a GaAs substrate with a GaAs buffer layer, and embedded in a Ga 0.52 In 0.48 P waveguide layer. The lattice mismatch between the InP and the Ga 0.52 In 0.48 P is 3.7%. For sample A, only one layer of dots was grown by depositing nominally 3 ML of InP. For samples B, C, and D three layers of InP dots were grown, separated by GaInP spacer layers with nominal thicknesses of 8, 4, and 2 nm, respectively. A detailed description of these samples can be found elsewhere. 3 Cross-sectional transmission electron microscopy ͑TEM͒ pictures show that for samples B, C and D the InP QDs are nicely stacked above each other. 3 The dots have typical dimensions of 16 nm diameter and about 2 nm height, with a density of 5ϫ10 10 cm Ϫ2 per layer. The PL experiments were performed at 4.2 K in pulsed magnetic fields up to 50 T, applied parallel and perpendicular to the growth direction, z. The excitation was provided by a continuous wave frequen...

The phonon dynamics of LiV 2 O 5 single crystals is studied using infrared and Raman spectroscopy techniques. The infrared-active phonon frequencies and dielectric constants are obtained by oscillator fitting procedure of the reflectivity data measured at room temperature. The Raman scattering spectra are measured at room temperature and at T=10 K in all nonequivalent polarized configurations. The assignment of the phonons is done by comparing the infrared and Raman spectra of LiV 2 O 5 and NaV 2 O 5 . The factor-group-analysis of the LiV 2 O 5 crystal symmetry and of its constituent layers is performed to explain the symmetry properties of the observed modes. We concluded that layer symmetry dominates in the vibrational properties of this compound.

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We have studied the interface region between the superconducting YBa2Cu3O7−x (123) matrix and the embedded nonsuperconducting Y2BaCuO5 (211) inclusions in fast melt processed samples, by recording micro-Raman spectra along lines crossing these inclusions. The observed relative shifts of the 123 Raman modes indicate the accumulation of compressive stress in the bulk of the 123 material and a release of this stress around the 211 inclusions. These results show that for practical applications of the fast melt processed materials, beside strong pinning, proper mechanical properties around the pinning centers are important.

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