Channelingeffekte bei der Lichtausbeute von Anthrazen bei Beschuß mit α-Teilchen

The scintillation response of anthracene crystals to α-particles has been investigated as a function of angle of incidence with respect to the crystal axes. Small local minima of the scintillation response occured, when the α-particles were incident along the directionsc, [102], [111], [1¯11], [1¯11], [¯1¯11], [121], [1¯21] and along theb c and (¯201) plane. In naphthalene and terphenyl similar minima could not be traced. The observed effects are interpreted as channeling effects.     

Dissociation of non-screw dislocations in B.C.C. lattice

A non-planar dissociation of non-screw 1/2[¯111] (110) dislocations is proposed. The dissociation is possible for dislocations of [1¯11] and [1¯10] directions and the dislocations of these directions become sessile. The [1¯11] and [1¯10] directions are in agreement with experimentally observed shape of dislocations in the (110) glide plane.     

Quantitative HRTEM Investigations of a Symmetrical Tilt Σ11 Grain Boundary with Two Different Grain Boundary Planes in α-Al2O3

High resolution transmission electron microscopy was used for the determination of the Σ11 grain boundary structure in α-Al₂O₃. Two orientations of the Σ11 boundary were investigated, the (101¯1) and the (2¯116) interface planes, respectively. The atomistic structures of the grain boundary core were quantitatively determined by comparing the experimental micrographs with simulated images. Each GB was imaged along two perpendicular directions. 3D models are proposed for each of the grain boundary orientations. For the Σ11 (101¯1) grain boundary an empirical shell model was used as the starting model for the structure refinement, whereas a CSL type model was used for the Σ11 (2¯116) grain boundary. After the quantitative evaluation both Σ11 grain boundaries revealed that the best matching structures were non CSL model structures. Non-mirror symmetry was observed for both configurations for at least one investigated zone axes. For the Σ11 (101¯1) interface a relative shift of one crystal halve with respect to the other was observed perpendicular to the grain boundary plane, where the (101¯1) lattice spacing is reduced by Δy = 0.04 ± 0.01 nm. For the Σ11 (2¯116) interface the resulting atomic structure is shifted by Δx = 0.322 ± 0.035 nm in the [21¯1¯1] direction from the CSL model structure. This corresponds to half of the repeat length of the [21¯1¯1] direction. No relaxation perpendicular to the interface plane was detected.     

Comparison of the calculated electronic structures of [111] and [001] GaAs/AℓxGa1-xAs semiconductor superlattices

A theoretical comparison of the electronic structures of long-period (about 300 Å) N_GaAs×N_AℓGaAs GaAs/Aℓ_0.3Ga_0.7As superlattices grown along the [111] direction and superlattices grown along the [001] direction is presented. Almost all qualitative features of the theoretical results are in good agreement with experiments by Hayakawa et al. The observed optical transition enhancement in the [111]-oriented quantum-well structures is caused by the topological difference in the superlattices, and is only partly due to the fact that the heavy holes in the [111] superlattices have larger transverse effective masses, and therefore have larger two-dimensional valence band densities of states.     

Magnetic anisotropy of multilayer [Fe/Pt]n structures

The possibility of controlling the magnetic anisotropy of multilayer [Fe/Pt]n structures grown by magnetron sputtering of Fe and Pt plates by varying number n of layers is studied. Mössbauer spectroscopy data and measured magnetic hysteresis loops demonstrate that the multilayer [Fe/Pt]n structures at n = 16 have a predominantly perpendicular magnetic anisotropy. The results of X-ray photoelectron spectroscopy and micromagnetic simulation point to the presence of intermediate layers enriched in iron ions in the structures. The magnetic anisotropy perpendicular to the surface of the [Fe/Pt]n films at n = 16 is found to be caused by the anisotropy of the intermediate layers.     

Growth and characterization of ferroelectric SrBi2Ta2O9 single crystals via high-temperature self-flux solution method

SrBi₂Ta₂O₉ (SBT) single crystals were produced by the high-temperature self-flux solution method using a Bi₂O₃ flux modified with B₂O₃. The processing conditions were optimized to obtain large and translucent SBT crystals with a layered habit and typical dimensions of approximately 7 × 5 × 0.2 mm. X-ray diffraction and x-ray topography measurements revealed that the major faces of the crystals with natural rectangular platelet morphology are perfectly (001)-oriented with edges directed along the [110] directions. The high quality of the crystals was confirmed by rocking curves (half-width of 0.04° for the (0018) reflection) and by ferroelectric measurements. The anisotropy in the dielectric and ferroelectric properties was investigated both along the [110] (ab plane) and the [001] (c axis) directions. The growth mechanism, morphology, and dielectric anisotropy of the SBT crystal platelets are discussed based on its crystallographic structure. This article was submitted by the authors in English     

Piezoelectric properties of tetragonal single-domain Mn-doped NBT-6 %BT single crystals

We report a study of properties of Mn-doped NBT-6 %BT single crystals. We show that tetragonal single-domain states can be stabilized by poling along a [001] direction. For carefully prepared crystals, the piezoelectric coefficient \(d_{33}\) can reach 570 pC/N. When poled along non-polar directions, the crystals exhibit ferroelectric domain structures consistent with tetragonal micron-sized domains, as revealed by optical observation and Raman spectroscopy. The multidomain crystals have lower \(d_{33}\) values, 225 and 130 pC/N for [011] and [111]-oriented crystals, respectively. This trend is commented on from a domain-engineering perspective.     

Die Kantenstreifung im elektronenmikroskopischen Bild würfelförmiger MgO-Kristalle bei Durchstrahlung in Richtung der Raumdiagonale

In electron microscope images of MgO crystals orientated with their {111} lattice planes normal to the direction of the incident electron beam a sine-forme periodicity of fringe structure parallel to the crystal edges is mostly observed. This simple fringe structure cannot generally be expected since six strong diffractions (¯202), (0¯22) etc. are simultaneously excited. — In such micrographs spacing and contrast of the fringes vary strongly while from the projection images of the crystals there are no remarkable differences in crystal orientations. For this case, the region near the [111]-direction of incidence, the fringe structure was theoretically investigated by Eigenvalue method and Bethe approximation. In exact [111] direction, only two wave fields exist which produce sinusoidal fringes. Small deviations from the [111] direction keep the two wave fields prevailing, the fringe structure remaining nearly sinusoidal, but induce strong variation of spacing and contrast. The theoretical results are in qualitative agreement with the observed fringes of micrographs, but are not yet suitable for quantitative comparison because of the considerable aperture of illumination (10^?3), used in the observation.     

Atom-probe fim studies of β-SiC whiskers

The 〈111〉-oriented whiskers of β-SiC, which is a semiconductor of the zinc blende structure, were studied by an atom-probe field ion microscope. Spectra of the number of field-evaporated ions versus their mass to charge ratio for A [111]-and B [1̄1̄1̄]-oriented tips were different from each other. Both the atomic ions and the molecular ions of SiC were observed for the two oriented tips. The mechanism of the field-evaporation of β-SiC has been discussed from the spectra in connection with the fact that the distinct difference between the two orientations was found in the field emission patterns of the field evaporated tips.     

SrTiO3(110) thin films grown directly on different oriented silicon substrates

We have grown (110)-oriented SrTiO₃ (STO) thin films on silicon without any buffer layer, by means of pulsed laser deposition technique. The crystal structures of the grown films were examined by X-ray diffraction analysis including θ–2θ scan and rocking curve as well as Laue diffraction methods. STO films with single (110) out-of-plane orientation were formed on all (100), (110) and (111)-oriented Si substrates. The in-plane alignments for the epitaxial STO films grown directly on Si (100) were found as STO[001]//Si[001] and STO[11̄0]//Si[010]. The results should be of interest for better understanding of the growth of perovskite oxide thin films on silicon wafers. PACS 77.55.+f; 68.55.JK; 81.15Fg     

Magnetization reversal and domains structures in (Co/Ni/Co/Pt) multilayers

Multilayers of [Co/Ni(t_Ni)/Co/Pt]_×4 are investigated for different Ni insertion layer thicknesses. The resulting magnetic properties and magnetic domain structures are compared with [Co/Ni]_×8 multilayers. As determined by magneto-optical Kerr effect microscopy and a vibrating sample magnetometer measurements, all multilayers exhibited a perpendicular magnetic anisotropy. It is found that the nucleation field and magnetic coercivity of [Co/Ni(t)/Co/Pt]_×4 multilayers are lower than (Co/Ni)_×8 and decreased with Ni thickness. Magnetization decay measurements reveal that these multilayers did not show an exponential decay behavior as was observed in rare earth transition metal alloys. Very narrow wires will remain stables for several hours even with an applied magnetic field closer to the coercivity. Insertion of very thin Ni in (Co/Pt) multilayers offers a good way to optimize the magnetic properties of the material and adjust the domain size for nanowire-based devices.     

Phonon thermal transport in InAs nanowires with different size and growth directions

We investigate the phonon thermal transport properties in InAs nanowires with different size and growth directions by using nonequilibrium molecular dynamics methods. The results show a remarkable anisotropy for the thermal conductivity in InAs nanowire. It is found that the thermal conductivity along [110] growth direction is about three times larger than that along [100] or [111] direction. With the increase of temperature, the thermal conductivity along [110] direction decreases significantly. However, the thermal conductivity along other two directions is not sensitive to temperature. Moreover, we find a crossover from ballistic to ballistic-diffusive thermal transport for a certain length of InAs nanowire. A brief physical analysis of these results is given. It is suggested that the anisotropy of thermal conductivity is common for nanowires with zinc blende structures.     

Vibration amplification, damping, and self-oscillations in micromechanical resonators induced by optomechanical coupling through carrier excitation

Carrier-induced dynamic backaction in micromechanical resonators is demonstrated. Thermal vibration of an n-GaAs/i-GaAs bilayer cantilever is amplified by optical band-gap excitation, and for the excitation power above a critical value, self-oscillations are induced. These phenomena are found in the [1[over ˉ]10]-oriented cantilever, whereas the damping (deamplification) is observed in the [1[over ˉ]10] orientation. This optomechanical coupling does not require any optical cavities but is instead based on the piezoelectric effect that is generated by photoinduced carriers.     

Optical distortion parameter of KCl laser windows at 10.6 μm

Thermal lensing in [111]-oriented KCl laser windows for CO₂ lasers is re-examined in the light of new measurements of the temperature coefficient of refractive index; it is concluded that the effective optical distortion parameter should be close to 8.5 × 10⁻⁶ K⁻¹.     

Site-specific reactivity of oxygen at Cu(110) step defects: an STM study of ammonia dehydrogenation

Defects on surfaces, such as steps, are known to influence catalytic activity considerably, but few microscopic observations have been made of such site-specific reactivity. Using atomic-resolution scanning tunneling microscopy (STM), we have observed the reactivity of oxygen adsorbed in the (2 × 1) structure at step sites on the Cu(110) surface with ammonia. It is found that the reactivity is high at both the top and bottom of a [11¯0] step and at the bottom of a [001] step, whereas almost no reaction occurs at the sites bordering the top of a [001] step. Reaction at the bottom of [001] steps appears to lead to deposition of copper atoms at the step edge.     

Growth of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) on Cu(110) studied by STM

. The structure of thin 3,4,9,10-perylenetetracarb-oxylic-dianhydride (PTCDA) films on Cu(110) was studied by scanning tunnelling microscopy (STM) from submonolayer to monolayer coverage. While no long-range ordering was found after deposition at room temperature, the formation of a well-defined superstructure is observed after thermal annealing. It appears that the formation of the superstructure is driven by the interaction between the oxygen atoms of the PTCDA and the copper atoms of the substrate. While the distance between the molecules fits well to the atomic lattice of the Cu(110) surface along the [1[`(^[¯])]1^[¯]\bar\Box1\Box]]0] direction, the mismatch along the [001] direction leads to a periodic buckling normal to the surface accompanied by a restructuring of the substrate.     

Channelingeffekte bei der Lichtausbeute von Anthrazen-, Naphthalin- undp-Terphenylkristallen bei Anregung mit α-Teilchen

The scintillation light yield of anthracene atT=300 °K andT=80 °K and of naphthalene andp-terphenyl atT=80 °K was investigated for impact directions in the (a, c)-plane. The light yield was measured for welldefined integration times. The relative depth of the channeling-minimum observed for impact directions parallel to thec-axis of anthracene is found to increase with increasing integration times. For the first time channeling-minima of the light yield were observed at impact directions parallel to thec- and the [102]-axis of naphthalene and thec- and [¯102]-axis ofp-terphenyl. The [¯102]-axis ofp-terphenyl shows a more pronounced effect than thec-axis.     

Transmission electron microscopy of dislocations in cementite deformed at high pressure and high temperature

Polycrystalline aggregates of cementite (Fe₃C) and (Fe,Ni)₃C have been synthesised at 10 GPa and 1250 °C in the multianvil apparatus. Further, deformation of the carbides by stress relaxation has been carried out at temperature of 1250 °C and for 8 h at the same pressure. Dislocations have been characterised by transmission electron microscopy. They are of the [1?0?0] and [0?0?1] type, [1?0?0] being the most frequent. [1?0?0] dislocations are dissociated and glide in the (0?1?0) plane. [0?0?1] dislocations glide in (1?0?0) and (0?1?0). Given the plastic anisotropy of cementite, the morphology of the lamellae in pearlitic steels appears to have a major role in the strengthening role played by this phase, since activation of easy slip systems is geometrically inhibited in most cases.