The optical properties of the wurtzite (WZ) GaAs crystal phase found in nanowires (NWs) are a highly controversial topic. Here, we study high-quality pure WZ GaAs/AlGaAs core-shell NWs grown by Au-assisted molecular beam epitaxy (MBE) with microphotoluminescence spectroscopy (μ-PL) and (scanning) transmission electron microscopy on the very same single wire. We determine the room temperature (294 K) WZ GaAs bandgap to be 1.444 eV, which is ∼20 meV larger than in zinc blende (ZB) GaAs, and show that the free exciton emission at 15 K is at 1.516 eV. On the basis of time- and temperature-resolved μ-PL results, we propose a Γ(8) conduction band symmetry in WZ GaAs. We suggest a method for quantifying the optical quality of NWs, taking into consideration the difference between the room and low temperature integrated PL intensity, and demonstrate that Au-assisted GaAs/AlGaAs core-shell NWs can have high PL brightness up to room temperature.

A. WANIC (a), and W. WOLSKI (d) Neutron spectrometric investigations are made of goethite (a-FeOOH) of mineralogical and synthetic origin (a-FeOOH of natural isotopic composition) and also of a-FeOOD using a monochromatic neutron beam of I = 1.12 A. Some powder diffraction patterns in the temperature range from -190 to +120 "C are measured. The obtained coordinates for all atoms (including those for hydrogen) in the unit cell are given. The existence of antiferromagnetic spin ordering a t temperatures below +SO "C is observed. A model is proposed in which the spins are aligned parallel to the b-axis. EbIn EICCJIeHOBaH reTIIT (cc-FeOOH) MuHepanora4ecKoro IIPOMCXOmAeHHR EI CEIH-TeTElseCHIlfi (eCTeCTBeHHOr0 E130TOnWleCKOrO COCTaBa) a TaKme AefiTepEI3OBaHHbIfi W T H T a-FeOOD. kICCJIeAOBaHHfI BeJIEICb B B m s a (~rOCJIaBEIH) IIpH peaKTope PA. ,&JIH M3MepeHHn HCIZOJI3OBPHO KPaKOBCKllfi HefiTpOHHblfi KpEICTNIJlEI~eCKEl~ CIIeK-TpOMeTp EI IIyuOK HefiTpOHOB C AJIUHOfi BOJlHbI A = 1,12 A. TeMnepaTypa o6paaua U3MeHIIJIaCb B IIpeAeJlaX OT -190 HO +I20 "c. YTOYHeHO HOOPAEIHaTbI aTOMOB BbInO 06HapymeHO CyrrleCTBOBaHIIe aHTEl-meneaa II wicnopona B s n e~e~~a p~o t m e h e , a TaKme B nepwfi pa3 6b1no onpe-#eppoMarHmHoro ynopHAoseHm CIIEIHOB EIOHOB meneaa B TeMnepaTypax HHme +80 "C. ~P M B O A E I T C R MoAen C~E~H O B O~~ peluema B KOTOPOP C~E I H~I napanenmb1 OCM b.HeJIeHO IIOJIOmeHEIe aTOMOB BOAOpOAa.

NiMnGe is investigated by meam of X-ray diffraction, neutron diffraction, and magnetometric methods. The orthorhombic distorsion of hexagonal lattice is observed below 470 K. The compound NiMnGe is antiferromagnetic with a NBel temperature T N = 346 K.The jump observed a t temperature Tt = 186 K on the temperature dependence of magnetic susceptibility points a t the change of a magnetic structure. From the analysis of the neutron diffraction patterns .the following models of spiral magnetic structure are obtained: a t T < Tt the spiral axis is in the bc-plane. The angle between the spiral axis and the b-axis is 45", magnetic moment a t 80 K is 2 . 7 5~~; at T, < T < T N the spiral axis is along the a-axis. !The magnetic moment a t 295 K is 2 . 2 ,~~.

No abstract

Magnetic and crystallographic nature of the compounds NiMnGe1−nSin (0 ≦ n ≦ 1) are studied in the temperature range from 80 to 1000 K by X‐ray and neutron diffraction, magnetometric and DTA measurements. On the basis of these experimental data, crystallographic and magnetic phase diagram are constructed. The compounds have NiTiSi type structure at low temperatures and they transform into the Ni2In type structure at high temperatures. Magnetic and neutron diffraction measurement revealed the existence of three regions of different types of magnetic orderings: helicoidal (0 ≦ n ≦ 0.25), noncollinear (0.3 ≦ n ≦ 0.55), and ferromagnetic (0.6 ≦ n ≦ 1.0). The transition from antiferromagnetic to ferromagnetic phase implies the decrease of the MnMn distances.

Les résultats de travaux sur la structure magnétique de MnCoSi sont présentés. Les mesures ont été effectuées sur deux échantillons obtenus par des méthodes différentes. Une transition de phase ferro‐antiferromagnétique a été observée sur les deux échantillons. La température de transition montre une forte dépendance vis à vis de la méthode de préparation. A basse température, ces échantillons possédent une structure en hélice du type MnP. On observe aussi une forte dépendance du vecteur d'onde k avec la température.

h i g h l i g h t s 28 Belgian cases of persistent masonry efflorescence were investigated. Gypsum is the major persistent efflorescence component. The formation mechanism is related to moisture transport and not to air pollution. Both brick and mortar can be the source of gypsum efflorescence. Many aspects remain hypothetical, rising important research questions. a b s t r a c tBelgian masonry facades are being increasingly affected by unsightly persistent efflorescence. This results in disappointed customers and consequently creates a threat for the brick industry. Our paper presents a field survey and literature review on the topic. An investigation of Belgian cases reveals gypsum abundance in the deposit. The specific characteristics and literature review indicate masonry as the source and moisture transfer as the transport mechanism. However, there is currently no sound explanation for the crystallisation of gypsum on the surface and its only recent occurrence. One hypothesis points at mortar additives, which may affect the transport and crystallisation of gypsum.

The objective of primary cementing is to protect the casing and to ensure zonal isolation. It can be difficult to obtain a good cement job along the full length of a well, and casing centralization is one of the main factors that influence this. Even if the dependence of cement placement on casing centralization is well-known, little information is available on how the degree of casing centralization affects the well during its production phase. Well temperatures cycle up and down as a part of normal production operations – and well barrier materials, in particular steel, cement and rock, will consequently repeatedly expand and contract their volumes. Over time, this is likely to induce debonding and radial cracking of the cement sheath which threatens well integrity. This paper reports the results of an experimental study mapping how, where and when the annular cement loses its sealing ability upon temperature variations, and how this is dependent on casing centralization. The studied samples consisted of rock, cement and casing, and the temperature was cycled in a controlled and programmable manner. In-situ monitoring by Acoustic Emission (AE) sensors detected the development of cracking and debonding in the samples during thermal cycling. Initial and post-experiment computed tomography (CT) scans provided complementary three-dimensional (3D) information on the geometry and location of the induced cracks and debonding. Our study compared the thermal cycling resistance of two samples, one with centralized casing and one with a 50% casing stand-off. The AE monitoring results indicated that most of the cracking/debonding occurred during the actual heating and cooling, and not in between cycles when the temperature was held constant. The CT analyses showed that the thermal cycling caused considerable enlargement of cracks and voids initially present in the cement sheath, and this enlargement was significantly more severe when the casing was not centralized. The paper presents, for the first time, a 3D visualization of cracks and debonded volumes in the cement sheath, and it underlines the importance of obtaining a good initial cement job. Also, it is shown that it is important to obtain a good casing centralization during well construction – not only for optimal cement placement, but also for maintaining well integrity during production.