The radiative recombination rates of interacting electron-hole pairs in a quantum dot are strongly affected by quantum correlations among electrons and holes in the dot. Recent measurements of the biexciton recombination rate in single self-assembled quantum dots have found values spanning from two times the single exciton recombination rate to values well below the exciton decay rate. In this paper, a Feynman path-integral formulation is developed to calculate recombination rates including thermal and many-body effects. Using real-space Monte Carlo integration, the path-integral expressions for realistic three-dimensional models of InGaAs/GaAs, CdSe/ZnSe, and InP/InGaP dots are evaluated, including anisotropic effective masses. Depending on size, radiative rates of typical dots lie in the regime between strong and intermediate confinement. The results compare favorably to recent experiments and calculations on related dot systems. Configuration interaction calculations using uncorrelated basis sets are found to be severely limited in calculating decay rates.

Near-field photoluminescence imaging spectroscopy of naturally occurring GaAs quantum dots (QDs) is presented. We successfully mapped out center-of -mass wave functions of an exciton confined in a GaAs QD in real space due to the enhancement of spatial resolution up to 30 nm. As a consequence, we discovered that the spatial profile of the exciton emission, which reflects the shape of a monolayer-high island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. This novel technique can be extensively applied to wave function engineering in the design and the fabrication of quantum devices.

Strong optical anisotropy is observed in the photoluminescence ͑PL͒ bands of both the InP self-assembled quantum dots and the Ga 0.5 In 0.5 P matrix. From the linearly polarized PL spectra measured under weak excitation, we found that large size quantum dots show strong anisotropy. The luminescence from a single quantum dot observed by the micro-PL technique revealed a doublet fine structure of the exciton levels that obey the linear polarization selection rule. The observed fine structure is shown to arise from an interplay of the electron-hole exchange interaction and the asymmetric crystal structure of the InP/Ga 0.5 In 0.5 P system. ͓S0163-1829͑99͒50608-5͔ RAPID COMMUNICATIONS

We observe magnetic-field-induced level mixing and quantum superposition phenomena between three approaching single-particle states in a quantum dot probed via the ground state of an adjacent quantum dot by single-electron resonant tunneling. The mixing is attributed to anisotropy and anharmonicity in realistic dot confining potentials. The pronounced anticrossing and transfer of strengths (both enhancement and suppression) between resonances can be understood with a simple coherent level mixing model. Superposition can lead to the formation of a dark state by complete cancellation of an otherwise strong resonance, an effect resembling coherent population trapping in a three-level-system of quantum and atom optics.

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We have observed ferromagnetism in dilute (∼1–4at.%) Mn-doped crystalline ZnO nanowires at temperatures up to 400K. Arrays of freestanding single crystal ZnO:Mn nanowires were fabricated by Au-catalyzed vapor-liquid-solid growth. Structure and compositional analyses revealed that Mn was incorporated into the ZnO lattice. From the observed saturation magnetization, the magnetic moment per Mn atom is estimated to be between 0.3μB and 1.2μB. Photoluminescence measurements show a strong suppression of defect related midgap emission, indicative of an interplay between Mn doping and native point defects.

The size dependence of the lowest electron-hole state in semiconductor microcrystals is calculated using the variational principle with a three-parameter Hylleraas-type wave function. For very small particles the Coulomb interaction may be treated as a perturbation.For larger particles the size dependence of the energy is much sharper than that expected in previous work.Wannier excitons in semiconductor rnicrocrystals provide an interesting spectroscopic system for the study of size quantization effects. Recently, several groups' have studied this phenomenon in colloidal spherical particles of CdS and spherical microcrystals of CuC1, CdS, CdSe, and CdS Se& ", etc. , in a glass matrix. Brus' has given a variational calculation for the size dependence of the electron-hole pair state while Efros and Efros' have calculated the spectra in some limiting cases. For a particle radius smaller than the Bohr radius ao of the exciton, the size quantization of the electron and hole band states dominates and the effect of the Coulomb attraction between the electron and hole can be treated as a perturbation. For very large particles, Efros and Efros conclude that the lowest energy of the electron-hole pair in the microcrystal is larger than that in the bulk due to size quantization of the center-of-mass motion. The size dependence of the measured energy levels can thus yield a good estimate of the total mass I &+m2 of the electron-hole pair. From a semiconductor-physics point of view this would be especially interesting because the total mass is sensitive to the heavier-particle mass, usually the hole, while the reduced mass involved in the bulk exciton binding energy is sensitive to the lighter-particle mass, usually the electron. Experimental results for CuCl particles give a total mass much smaller than that generally accepted.In this Brief Report we report an accurate variational calculation for the lowest electron-hole pair state. The dependence of the energy levels as well as the wave functions on the particle size are critically evaluated. Our calculations clearly indicate that the determination of the to-tal mass by this method is not satisfactory.In the effective-mass approximation the essential problem is to solve the Schrodinger equation for the envelope function f: AV' AV 2m ) 2m2 2 47TEOE7 )2 + &0 P(ri, r~) = EP(r&, r2), (1) where m~, mz and r&, r2 are the electron and hole masses and position vectors, respectively, and r~2 --~r , -rT he confining potential Vo is 0 inside the spherical crystallite of radius R and infinite outside this sphere, with the corresponding boundary condition Q(rl, r2)=0 for r, or r2)R .(2) For small particles the energy levels were calculated by treating the Coulomb interaction between the electron and hole as a perturbation.From single-particle wave functions linear combinations corresponding to zero angular momentum were formed. Using a finite set of these as the basis set, the matrix representation for the Hamiltonian in Eq. (1) was obtained and diagonalized. While the unperturbed level...