Carrier diffusion in InGaAs/GaAs quantum wells grown on vicinal GaAs(0 0 1) substrates

We have investigated the influence of vicinal GaAs substrates on the optical and electronic properties of InGaAs/GaAs quantum wells (QWs). A single In_0.10Ga_0.90As QW was grown by molecular-beam epitaxy on a vicinal GaAs(0 0 1) substrate with a miscut angle of 0° (nominal), 2°, 4° and 6° towards [1 1 0]. The carrier diffusion was obtained by a micro-photoluminescence scan technique that permits to observe the effective diffusion length characterized by the lateral spread of carriers in the QW followed by radiative recombination. The carrier diffusion length was obtained parallel (L_||) and perpendicular (L) to the atomic steps. The diffusion length decreases as the temperature increases up to 100 K. Above this temperature we found different behaviours that depend on the sample miscut angle.     

Theoretical analysis of 2D laser angle sensor and several design parameters

Having adopted several sets of new technology and techniques, such as cube-corner prism of higher refractive index, orthogonal holographic grating, non-linear compensation, and fringe fractionization, etc., we have developed a new type of laser angle sensor that can be used to measure two-dimensional angle of moving target. The sensor has high sensitivity, high precision and longer measuring range compared with the original sensor, and its measuring range will achieve ±35°, while its minimum resolution angle is 0.004°. The experimental results show that the measuring error is not greater than ±0.01°. In this paper, several design parameters and measuring results for 2D laser angle sensor are given.     

Facile method to prepare stable superhydrophobic Co3O4 surface

A facile and novel method was developed to fabricate rough Co₃O₄ surface with hierarchical micro- and nanostructures by the combination of simple solid state reactions and coating process. After modification with stearic acid, a superhydrophobic surface with water contact angle of 155 ± 1.8° and sliding angle of 2° was obtained. The superhydrophobic Co₃O₄ surface remained superhydrophobic property in a wide pH range from 3 to 14. The superhydrophobic Co₃O₄ surface also showed excellent self-cleaning property and high stability in ambient environments.     

新颖的微流控电调谐空间光开关

提出了一种微流控电调谐非机械空间光开关器件,该器件的基本形式为“光输入阵列+光交换空间+光输出阵列”的结构,采用“水/油/水”液体棱镜作为偏光控制单元.在特定电压范围（30~110 V）内,通过电润湿效应作用的液体棱镜光束偏转角可在约－15°~15°之间连续可调.由此可构造多种平面甚至立体光开关阵列.     

One- and two-dimensional C–H/N–H dipolar correlation experiments under fast magic-angle spinning for determining the peptide dihedral angle φ

A recently proposed ¹³C–¹H recoupling sequence operative under fast magic-angle spinning (MAS) [K. Takegoshi, T. Terao, Solid State Nucl. Magn. Reson. 13 (1999) 203–212.] is applied to observe ¹³C–¹H and ¹⁵N–¹H dipolar powder patterns in the ¹H–¹⁵N–¹³C–¹H system of a peptide bond. Both patterns are correlated by ¹⁵N-to-¹³C cross polarization to observe one- or two-dimensional (1D or 2D) correlation spectra, which can be simulated by using a simple analytical expression to determine the H–N–C–H dihedral angle. The 1D and 2D experiments were applied to N-acetyl[1,2-¹³C,¹⁵N] -valine, and the peptide φ angle was determined with high precision by the 2D experiment to be ±155.0°±1.2°. The positive one is in good agreement with the X-ray value of 154°±5°. The 1D experiment provided the value of φ=±156.0°±0.8°.     

Rapid-Pulsing Artifact-Free Double-Quantum-Filtered Homonuclear Spectroscopy: The 2D-INADEQUATE Experiment Revisited

Rapid pulsing artifacts are observed in the conventional phase-cycled carbon-13 2D INADEQUATE experiment. By using the product operator formalism, it is shown that they result from the effects of imperfect 90° and 180° excitation pulses on the most abundant molecules containing only one isolated carbon-13 nucleus. The labeled longitudinal magnetization remaining at the end of one scan is recycled by the subsequent acquisition, giving rise to multiple-quantum (p= 0, ±1, ±2, …) artifacts in theF₁dimension. By considering pairs of scans instead of single scans, a new phase cycle is proposed. It is based on a scheme for compensating for imperfections in the excitation cluster by a proper combination of the pulse phases in two consecutive scans. Because the artifacts are 90° out of phase compared to the desired signal, a concomitant rearrangement of the receiver phase achieves suppression of all unwanted signals. Experiments are presented on menthol dissolved in CDCl₃as a test compound. Improvements in spectrum quality as well as increased sensitivity are discussed.     

Computation of streamwise vorticity in a compressible flow of a winglet nozzle-based COIL device

Chemical oxygen iodine laser (COIL) is a high-power laser with potential applications in both military as well as in the industry. COIL is the only chemical laser based on electronic transition with a wavelength of 1.315 μm, which falls in the near-infrared (IR) range. Thus, COIL beam can also be transported via optical fibers for remote applications such as dismantling of nuclear reactors. The efficiency of a supersonic COIL is essentially a function of mixing specially in systems employing cross-stream injection of the secondary lasing (I₂) flow in supersonic regime into the primary pumping (O₂¹Δ_g) flow. Streamwise vorticity has been proven to be among the most effective manner of enhancing mixing and has been utilized in jet engines for thrust augmentation, noise reduction, supersonic combustion, etc. Therefore, a computational study of the generation of streamwise vorticity in the supersonic flow field of a COIL device employing a winglet nozzle with various delta wing angles of 5°, 10°, and 22.5° has been carried out. The study predicts a typical Mach number of approximately 1.75 for all the winglet geometries. The analysis also confirms that the winglet geometry doubles up both as a nozzle and as a vortex generator. The region of maximum turbulence and fully developed streamwise vortices is observed to occur close to the exit, at x/λ of 0.5, of the winglets making it the most suitable region for secondary flow injection for achieving efficient mixing. The predicted length scale of the scalloped mixer formed by the winglet nozzle is 4λ. Also, the winglet nozzle with 10° lobe angle is most suitable from the point of view of mixing developing cross-stream velocity of 120 m/s with acceptable pressure drop of 0.7 Torr. The winglet geometry with 5° lobe angle is associated with a low cross-stream velocity of 60 m/s, whereas the one with 22.5° lobe angle is associated with a large static and total pressure drop of 1.87 and 9.37 Torr, respectively, making both the geometries unsuitable for COIL systems. The experimental validation shows a close agreement with the computationally predicted values. The studies for the most suitable 10° lobe angle geometry show an observed Mach number of 1.72 with an improved mixing efficiency of 74% due to the occurrence of predicted streamwise vortices in the flow.     

Microwave spectrum, structure, dipole moment, and internal rotation of fluoromethyl methyl ether

Microwave spectra of fluoromethyl methyl ether and its 10 isotopically substituted species were measured. The r_s structure of this molecule was determined from the observed moments of inertia. Structural parameters obtained for this molecule, which was in the gauche form, were compared with those of the analogous molecules. Dipole moments of the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment is 1.744 ± 0.029 D making an angle of 100°54′ with the O---CH₂ bond toward the C---F direction and lies in the plane whose dihedral angles with the FCO and COC planes are 114°9′ and 44°56′, respectively. The barrier to internal rotation of the methyl group was calculated taking into account the coupling effect with the skeletal torsion using the observed splitting data of the spectra in the ground, first excited methyl torsional, and skeletal torsional states. The barrier, skeletal torsional frequency, and coupling term were determined to be V₃ = 1538 ± 40 cal/mole, ω_t = 158 ± 4 cm⁻¹, and V_s = 490 ± 500 cal/mole, respectively.     

Lorentz-mikroskopie von Bloch- und Néelwänden in Ni-Fe-kristallen

We prepared thin single crystal foils of 50 Ni 50 Fe for observation in a 100 kV electron microscope. Due to its high saturation induction and low anisotropy, magnetic domain walls can be easily observed in this material. The effective anisotropy which governs the wall angles consists of crystal anisotropy and superimposed random stress anistropies so that wall angles of any magnitude are found.In accordance with theory three distinct wall types are identified for film thickness between 500 Å and 2000 Å as a function of wall angle θ: (i) the asymmetric Bloch wall for 180° θ #62; 140°, (ii) the asymmetric Néel wall for 140° #62; θ #62; θ_s, and (iii) the symmetric Néel wall for θ_s #62; θ, where θ_s is an angle which decreases with increasing film thickness.     

In situ low-angle cross sectioning: Bevel slope flattening due to self-alignment effects

Low-angle cross sections are produced inside an Auger microprobe using the equipped depth profile ion sputter gun. Simply the sample is partly covered by a mask. Utilizing the edge of this mask the sample is sputtered with ions. Due to the shading of the mask a cross section is produced in the sample. The slope of this cross section is considerably shallower than given by the geometrical setup. This is attributed to self-alignment effects, which are due to missing sputter cascades in the transition area between sputtered and shaded sample regions and a chamfering of the mask edge.These self-alignment effects are studied here using a 104.6 nm thick SiO₂ layer thermally grown on a Si substrate. In this study on one hand for a fixed ion impact angle of 15.8° as function of the sputter time several in situ low-angle cross sections were produced. This way slope angles between an ultimate low slope angle of 0.014° and 0.085° were achieved. On the other hand for a fixed sputter time the ion impact angle was varied between 14.8° and 70.8°. For these samples cross section slope angles between 0.031° and 0.32° are observed. These results demonstrate the distinct slope flatting of in situ cross sectioning.     

Microwave spectrum, structure, and dipole moment of the trans-trans isomer of methylpropylether

Microwave spectra of the trans-trans (TT) isomer of methylpropylether and its 12 isotopically substituted species were measured. The r_s structure of this isomer was determined from the observed moments of inertia. Structural parameters of this isomer were roughly equal to those of the reported r_s structures of trans-ethylmethylether and propane. Dipole moments of the TT isomer for the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment was μ_a = 0.082 ± 0.010, μ_b = 1.104 ± 0.013, and μ_total = 1.107 ± 0.013 D making angles of 4°17′ with the b-inertial axis, of 6°7′ with the bisector of the COC angle. The barrier to internal rotation of the CH₃C group was calculated to be 3300 ± 60 cal/mole from A-A splittings of the spectra in the CH₃C excited torsional state.     

Improved SNR in linear reordered 2D bSSFP imaging using variable flip angles

This article presents a variable flip-angle approach for balanced steady-state free precession (bSSFP) imaging, which allows increases in signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) while keeping specific absorption rate (SAR) constant or reduces SAR for given CNR and SNR. The gain in SNR is achieved by utilizing the higher signal in the transient phase. Flip-angle variation during the echo train is realized using a trigonometric function with M steps (ramp length). Variation is combined with a linear k-space reordering such that outer parts of k-space are sampled using a lower flip angle α_min, while the central part of k-space is acquired with a higher flip angle α_max. No additional preparation or dummy cycles are applied prior to data acquisition. Several variation schemes with different starting flip angles α_min and ramp length M are considered. For example, using α_min=1° and M=96, α_max can be set to 47° without exceeding SAR limits at 3 T and gaining up to 50% in SNR, while, conventionally, α=34° is the maximal possible flip angle. Resolution seems unaffected in volunteer imaging. In all cases, no transient artifacts due to flip-angle variation were observed. This article demonstrates the use of flip-angle variations in bSSFP to increase SNR and CNR while keeping SAR constant, which is especially important at higher field strengths. Flip-angle variation can also be combined with other methods such as parallel imaging techniques for further SAR reduction.     

Optical resonator for high-power transverse flow CO2 lasers

A new design of the U-type resonator is described. In this way, a laser beam with symmetrical intensity profile (regarding to a symmetry plane) can be extracted from an active medium that exhibits gain asymmetry along one of the transverse directions. The whole area of the active medium cross-section can be used, and consequently the laser efficiency will be increased. This resonator structure was applied for efficiency power extraction (as a low order TEM modes laser beam) from a DC excited transverse flow CO₂ laser with cylindrical geometry. Although the cross-section area of the discharge was entirely used (including the cathode fall region), a symmetrical intensity profile of the laser beam (regarding to the two orthogonal symmetry planes) was obtained in the near field as well as in the far field; the gain asymmetry along the flow direction was compensated by the gas circulation fluidodynamical circuit with two counterflowing discharge channels. A double-U optical resonator was introduced in order to provide a laser beam with axial symmetry.For the practical construction of these two types of optical resonators we have developed two new types of 90° deflection elements: the first one, which does not reverse the image (and which has the properties of the pentaprism), and the second one, which rotates the image with 90° angle. Both elements exhibit good focusability if they are equipped with two concave mirrors.     

Experimental research on a novel fiber-optic cantilever-type inclinometer

Experimental setup for tilt measurement and a novel signal detection method for demodulation of fiber Bragg grating (FBG) wavelength shift are proposed. Being attached, respectively, on the upper and lower surface of only one single pendulum-type cantilever element, only a couple of matched FBGs are used for both tilt angle sensing and wavelength shift signal demodulating. So the received light power will change due to the split of the two reflected spectra of FBGs, which corresponds to the tilt angle. In addition, the cross-sensitivity effect of FBG-based sensors will be solved automatically due to a differential signal process method. Experimental results indicate the measurement accuracy is about ± 0.03°, and measurement resolution is estimated to be about 0.002°.     

Magnetische streufelder über den bereichen sehr genau orientierter Ni-einkristalloberflächen zwischen 5.5–500 K

The previously described electron optical measurements of the magnetic stray fields of Ni single crystals above the domain structure of plane ( 10)-surfaces oriented within 0.01° at room temperature are now extended to the temperature range 5.5–500 K. Their dependence on the angle ₀, 0° ₀ 5.41°, between exact 10 direction and the normal to the surface is investigated along the crystal axis 110. The results are discussed and compared with calculations of the μ^*-correction.     

Effect of structural defects on InSb/AlxIn1−x Sb quantum wells grown on GaAs (0 0 1) substrates

We investigate oriented abrupt steps (OASs), a type of surface defect in InSb/Al_xIn_1−xSb quantum-well (QW) samples grown on GaAs (0 0 1) substrates. Previous atomic force microscopy studies have reported that the OASs are oriented along the [1 1 0] and directions and have an inclination angle of 5°–15° with respect to the sample surface. Our plan-view and cross-sectional transmission electron microscopy analyses reveal that the OASs are the terminal edges of threading micro-twins at the sample surface. Hall effect measurements indicate that the density of OASs correlates with the electron mobility in the InSb QWs.     

N Chemical Shift Tensor Magnitude and Orientation in the Molecular Frame of Uracil Determined via MAS NMR

The potential of heteronuclear MAS NMR spectroscopy for the characterization of ¹⁵N chemical shift (CS) tensors in multiply labeled systems has been illustrated, in one of the first studies of this type, by a measurement of the chemical shift tensor magnitude and orientation in the molecular frame for the two ¹⁵N sites of uracil. Employing polycrystalline samples of ¹⁵N₂ and 2-¹³C,¹⁵N₂-labeled uracil, we have measured, via ¹⁵N–¹³C REDOR and ¹⁵N–¹H dipolar-shift experiments, the polar and azimuthal angles (θ, ψ) of orientation of the ¹⁵N–¹³C and ¹⁵N–¹H dipolar vectors in the ¹⁵N CS tensor frame. The (θ_NC, ψ_NC) angles are determined to be (92 ± 10°, 100 ± 5°) and (132 ± 3°, 88 ± 10°) for the N1 and N3 sites, respectively. Similarly, (θ_NH, ψ_NH) are found to be (15 ± 5°, −80 ± 10°) and (15 ± 5°, 90 ± 10°) for the N1 and N3 sites, respectively. These results obtained based only on MAS NMR measurements have been compared with the data reported in the literature.     

Microwave spectrum and conformation of 6-thiabicyclo[3.1.0]hexane

The microwave spectrum of 6-thiabicyclo[3.1.0]hexane (cyclopentene sulfide) has been measured in the region 26,500-40,000 MHz. The experimental data are consistent with a single stable conformation. Furthermore, these data can only be satisfactorily explained by assuming that this conformation is the boat form. Rotational constants were obtained, both for the ground state and two excited vibrational states, while centrifugal distortion coefficients were obtained for the ground state and one excited vibrational state. The ground state rotational constants found were A₀ = 5026.243 ± 0.003 MHz, B₀ = 2833.813 ± 0.003 MHz, and C₀ = 2411.679 ± 0.03 MHz. For the ground state of the molecule, the electric dipole moment components were found to be μ_a = 1.800 ± 0.012 D and μ_c = 1.155 ± 0.024 D, yielding a total dipole moment μ = 2.139 ± 0.027 D.     

A High-Resolution C 3D CSA-CSA-CSA Correlation Experiment by Means of Magic Angle Turning

It is shown in this paper that a previously reported 90° sample flipping ¹³C 2D CSA-CSA correlation experiment may be carried out alternatively by employing constant slow sample rotation about the magic angle axis and by synchronizing the read pulse to of the rotor cycle. A high-resolution 3D CSA-CSA-CSA correlation experiment based on the magic angle turning technique is reported in which the conventional 90° 2D CSA-CSA powder pattern for each carbon in a system containing a number of inequivalent carbons may be separated according to the isotropic chemical shift value. The technique is demonstrated on 1,2,3-trimethoxybenzene in which all of the overlapping powder patterns that cannot be segregated by the 2D CSA-CSA experiment are resolved successfully by the 3D CSA-CSA-CSA experiment, including even the two methoxy groups (M₁ and M₃) whose isotropic shifts, confirmed by high-speed MAS, are separated by only 1 ppm. A difference of 4 ppm in the principal value component (δ₃₃) between M₁ and M₃ is readily obtained.     

Tagging of the Magnetization with the Transition Zones of 360° Rotations Generated by a Tandem of Two Adiabatic DANTE Inversion Sequences

A new technique is presented for generating myocardial tagging using the signal intensity minima of the transition zones between the bands of 0° and 360° rotations, induced by a tandem of two adiabatic delays alternating with nutations for tailored excitation (DANTE) inversion sequences. With this approach, the underlying matrix corresponds to magnetization that has experienced 0° or 360° rotations. The DANTE sequences were implemented from adiabatic parent pulses for insensitivity of the underlying matrix to B₁ inhomogeneity. The performance of the proposed tagging technique is demonstrated theoretically with computer simulations and experimentally on phantom and on the canine heart, using a surface coil for both RF transmission and signal reception. The simulations and the experimental data demonstrated uniform grid contrast and sharp tagging profiles over a twofold variation of the B₁ field magnitude.