We report a comprehensive discussion of quantum interference effects due to the finite structure of excitons in quantum rings and their first experimental corroboration observed in the optical recombinations. Anomalous features that appear in the experiments are analyzed according to theoretical models that describe the modulation of the interference pattern by temperature and built-in electric fields.PACS numbers: 71.35.Ji, 73.21.La, 78.20.Ls, 78.67.Hc The nanoscale ring structures, or quantum rings (QRs), have attracted the interest of the scientific community due to their unique rotational symmetry and the possibility to verify quantum mechanical phenomena.[1, 2, 3] Among these, the study of Aharonov-Bohm (AB)-like effects has gained a significant impetus, [4,5,6] and these efforts have gone beyond the original discussion of the AB interpretation on the nature of electromagnetic potentials and their role in quantum mechanics. [7] It is reasonable to say that the study of coherent interference occurring in transport properties of nanoscopic QRs, as proposed in Ref. 7 encounters, at the moment, serious scale limitations which has encouraged the search for optical implications associated to AB-effects.These endeavors applied to nanoscopic QRs do not strictly meet the original conditions for the ABconfiguration since the carriers are confined within regions with finite values of magnetic field. However, we still consider an observed effect as of AB-type if it can be explained assuming that the magnetic field is ideally concentrated in the middle of the QRs, i. e., when such effect comes essentially from potential vector-mediated quantum interference. As also considered in Ref. 8, in stationary systems this interference is generally reflected in a boundary condition and it is not as explicit as in the famous picture of an AB scattering situation.In this work we consider AB-interference in excitonic states as proposed theoretically in Refs. 9, 10, 12. Instead of looking only at the oscillatory dependence on magnetic flux of the electron-hole (e − h) recombination energy during photo-luminescence (PL), we also consider the excitonic oscillator strength whose oscillatory behavior reflects directly the changes in the exciton wavefunction as the magnetic flux increases. A similar experimental work was reported in Ref. 6 for type-II QRs, however, here we study type-I systems where both electron and hole move in the ring so that the correlation between them is crucial to the oscillatory behavior found in the PL integrated intensity. The samples studied here were grown using a RIBER 32P solid-source molecular beam epitaxy chamber and the QRs were grown using the following procedure. A 0.5 µm GaAs buffer layer was grown on semi-insulating (100) GaAs substrates at 580• C, after oxide desorption. Then, it was followed by 2.2 ML of InAs and the formation of quantum dots (QDs) at 520• C. The dots were obtained using the Stranski-Krastanov growth mode. Cycles of 0.14 ML of InAs plus a 2 s interruption under As 2 flux were r...

We have studied the luminescence of narrow quantum wires at photoexcitation densities of up to ϳ3 3 10 6 cm 21 . We show that even at these densities, which are well above the expected Mott density of 8 3 10 5 cm 21 , excitonic recombination dominates over other recombination channels in stark contrast with the behavior of quantum wells and bulk structures at equivalent densities. As we observe no significant shift in the peak energy with density, an upper limit to the band gap renormalization can be set. [S0031-9007(97)03044-5] 71.27. + a, 78.47. + p Excitonic effects in a one dimensional (1D) system are expected to be even more significant than they are in two and three dimensional systems [1][2][3]. An enhancement of excitonic correlations [4] and an increased oscillator strength [5] have been observed experimentally. The question that then beckons is, up to what densities will excitons remain stable in quantum wires (QWRs); put otherwise one may wonder at what density the Mott transition occurs in optically excited QWRs. A fascinating aspect of high density studies, the Mott transition has been an active field of research over the past three decades [6], and there remain a number of open questions regarding this transition in three, two, and one dimensions [7]. In QWRs the transition is expected to occur when the insulating excitonic phase transforms into a conducting free electron-hole (e-h) plasma at a carrier density of about 8 3 10 5 cm 21 [3]. This transition has so far not been observed in optical studies, although it would be most interesting to probe it and associated effects such as a possible hysteresis in the Mott density [8].Semiconductor QWRs have also been actively studied, as it is expected that the singularity in the density of states in a quasi-1D system may lower threshold current densities, thus improving the performance of semiconductor lasers [9][10][11]. The common procedure of computing the optical gain using a 1͞ p E density of states is, however, incomplete as a number of effects such as inhomogeneities, Coulomb interactions, and many body effects are neglected. A better understanding of the recombination of a dense e-h plasma in QWRs is thus called for, and the subject has attracted considerable attention of late [4,[12][13][14][15][16].Recent progress in the growth of semiconductor nanostructures has made available wires of good quality which should open the way for rigorous studies that probe the different interactions that occur in quasi-1D systems. It is then unfortunate that the picture that emerges from the published literature on high density phenomena in QWRs is confused and contradictory. A prime example of such ambiguity is the issue of band gap renormalization (BGR). Experimental reports range from evidence for a very large band gap energy shift [12,15], to no shift at all [4,13,14], with some authors observing shifts only due to the carriers in other subbands [16]. The theoretical results are equally ambiguous. Some models predict a very large BGR [17 -19], even large...

We investigated experimentally and theoretically the valence-band structure of wurtzite InP nanowires. The wurtzite phase, which usually is not stable for III-V phosphide compounds, has been observed in InP nanowires. We present results on the electronic properties of these nanowires using the photoluminescence excitation technique. Spectra from an ensemble of nanowires show three clear absorption edges separated by 44 meV and 143 meV, respectively. The band edges are attributed to excitonic absorptions involving three distinct valence-bands labeled: A, B, and C. Theoretical results based on "ab initio" calculation gives corresponding valence-band energy separations of 50 meV and 200 meV, respectively, which are in good agreement with the experimental results.

We present a systematic experimental and theoretical study of the first-order phase transition of epitaxially grown MnAs thin films under biaxial tensile stress. Our results give direct information on the dependence of the phase-transition temperature of MnAs films on the lattice parameters. We demonstrate that an increase of the lattice constant in the hexagonal plane raises the phase-transition temperature (T p ), while an increase of the perpendicular lattice constant lowers T p . The results of calculations based on density functional theory are in good agreement with the experimental ones. Our findings open exciting prospects for magneto-mechanical devices, where the critical temperature for ferromagnetism can be engineered by external stress. DOI: 10.1103/PhysRevLett.95.077203 PACS numbers: 75.70.Ak, 61.50.Ks, 68.60.2p MnAs presents a first-order phase transition at 40 C, changing from ferromagnetic/hexagonal ( -phase NiAs structure) to paramagnetic/orthorhombic ( -phase MnP structure) [1]. This magneto-structural phase transition has important implications for technological applications. The magneto-elastic effects are useful for transducers [2], while their magneto-caloric properties are interesting for developing refrigeration devices [3]. In recent years, the attention given to MnAs has been strongly amplified by the possibility of epitaxial growth on GaAs substrates [4]. The integration of ferromagnetic materials with semiconductors is a subject of great interest for spintronics, and MnAs grown on GaAs is a strong candidate for spin injection devices [5].From the theoretical point of view, the treatment of the first-order phase transition of materials with magnetoelastic properties is a rather complex issue. Early simple phenomenological thermodynamic treatments based on the localized Heisenberg model [1] have been used to explain the properties of MnAs under an external hydrostatic pressure and magnetic field. Sophisticated band structure calculations are required for a precise quantitative analysis, although in this case it is difficult to introduce a statistical treatment to describe a first-order phase transition [6].We present an experimental and theoretical investigation of the magneto-structural phase transition of MnAs films grown on GaAs. Those films present a nonabrupt phase transition with the coexistence [7] of the two phases in form of periodically alternating stripes [8,9] for a large temperature range ( 20 C) [7][8][9][10]. As a result of this phase coexistence a considerable fraction of the volume of the MnAs epitaxial films is usually in the paramagnetic phase at 30 C, which is a strong limitation for room temperature spintronic devices. The growth of MnAs films on different crystal orientations has been suggested as an alternative that can provide higher phase-transition temperatures [10]. The detailed mechanism that associates the crystal distortion (lattice parameter variation) with the phase-transition temperature is, however, still unclear, This issue was addressed in the ear...

We report a systematic study of the optoelectronic properties of ZnSe1−xTex alloys grown by molecular beam epitaxy over the entire range of compositions. The band-gap energy as a function of the composition presents a minimum at x≂0.65. The main luminescence emission observed at 5 K becomes narrower and closer to the band-gap energy as we increase the Te content. The linewidth and the difference between the emission peak and band-gap energy decrease significantly with increasing x and present a break in the slope at x≂0.65.