Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities (~1010 cm-3). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength

Nanostructure and strain properties of core-shell GaAs/AlGaAs nanowires

GaAs/AlGaAs core–shell nanowires (NWs) were grown on Si(111) by Ga-assisted molecular beam epitaxy via the vapor–liquid–solid mechanism. High-resolution and scanning transmission electron microscopy observations showed that NWs were predominantly zinc-blende single crystals of hexagonal shape, grown along the [111] direction. GaAs core NWs emerged from the Si surface and subsequently, the NW growth front advanced by a continuous sequence of (111) rotational twins, while the AlGaAs shell lattice was perfectly aligned with the core lattice. Occasionally, single or multiple stacking faults induced wurtzite structure NW pockets. The AlGaAs shell occupied at least half of the NW's projected diameter, while the average Al content of the shell, estimated by energy dispersive x-ray analysis, was x = 0.35. Furthermore, molecular dynamics simulations of hexagonal cross-section NW slices, under a new parametrization of the Tersoff interatomic potential for AlAs, showed increased atom relaxation at the hexagon vertices of the shell. This, in conjunction with the compressively strained Al0.35Ga0.65As shell close to the GaAs core, can trigger a kinetic surface mechanism that could drive Al adatoms to accumulate at the relaxed sites of the shell, namely along the diagonals of the shell's hexagon. Moreover, the absence of long-range stresses in the GaAs/Al0.35Ga0.65As core–shell system may account for a highly stable heterostructure. The latter was consolidated by temperature-dependent photoluminescence spectroscopy

THz emission from Fe/Pt spintronic emitters with L10_{0}-FePt alloyed interface

Recent developments in nanomagnetism and spintronics have enabled the use of ultrafast spin physics for terahertz (THz) emission. Spintronic THz emitters, consisting of ferromagnetic FM / non-magnetic (NM) thin film heterostructures, have demonstrated impressive properties for the use in THz spectroscopy and have great potential in scientific and industrial applications. In this work, we focus on the impact of the FM/NM interface on the THz emission by investigating Fe/Pt bilayers with engineered interfaces. In particular, we intentionally modify the Fe/Pt interface by inserting an ordered L1$_{0}$-FePt alloy interlayer. Subsequently, we establish that a Fe/L1$_{0}$-FePt (2\,nm)/Pt configuration is significantly superior to a Fe/Pt bilayer structure, regarding THz emission amplitude. The latter depends on the extent of alloying on either side of the interface. The unique trilayer structure opens new perspectives in terms of material choices for the next generation of spintronic THz emitters

GRAIN BOUNDARY STRUCTURE AND TEXTURE IN A Ni-BASE ODS SUPERALLOY

Les caractéristiques microscopiques du superalliage MA6000, renforcé par dispersion d'oxydes, ont été étudiées à l'aide de la microscopie électronique en transmission. Les joints de grains entre les grains allongés sont des joints de faible désorientation, avec des angles compris entre 1° et 6°. Des zones sans particules d'oxyde sont observées dans le matériau de départ, alors qu'aucune indication de leur présence n'est trouvée dans l'échantillon déformé en fluage. Après fluage, nous observons essentiellement une forte déformation (rafting) des particules γ'. L'existence d'une texture a été mise en évidence. Les grains ont en commun une axe <110> ± 5° parallèle à la direction d'extrusion.The microstructural features of the oxide dispersion strengthened superalloy MA6000 have been investigated, by means of Transmission Electron Microscopy (TEM). The grain boundaries between the elongated grains are of the low-angle type, with rotation angles between 1° and 6°. Oxide depleted zones are observed in the as-received material, whereas no indication of their presence is found in a creep tested specimen. The occurrence of γ' rafting, perpendicular to the stress axis, was the only effect observed after creep deformation. The presence of a texture has been examined. The grains shared a <110> ± 5° axis parallel to the extrusion direction

A Game of Cops and Robbers on Graphs with Periodic Edge-Connectivity.

This paper considers a game in which a single cop and a single robber take turns moving along the edges of a given graph G. If there exists a strategy for the cop which enables it to be positioned at the same vertex as the robber eventually, then G is called cop-win, and robber-win otherwise. In contrast to previous work, we study this classical combinatorial game on edge-periodic graphs. These are graphs with an infinite lifetime comprised of discrete time steps such that each edge is assigned a bit pattern of length le, with a 1 in the i-th position of the pattern indicating the presence of edge in the i-th step of each consecutive block of le steps. Utilising the known framework of reach-ability games, we obtain an O(LCM(L)·n3) time algorithm to decide if a given n-vertex edge-periodic graph Gτ is cop-win or robber-win as well as compute a strategy for the winning player (here, L is the set of all edge pattern lengths le, and LCM(L) denotes the least common multiple of the set L). For the special case of edge-periodic cycles, we prove an upper bound of 2·l·LCM(L) on the minimum length required of any edge-periodic cycle to ensure that it is robber-win, where l= 1 if LCM(L)≥2·maxL, and l= 2 otherwise. Furthermore, we provide constructions of edge-periodic cycles that are cop-win and have length1.5·LCM(L) in the l= 1 case and length 3·LCM(L) in the l= 2 case.</p