Steps and facets on annealed LaAlO3{100} and {110} surfaces

Lanthanum-aluminate (LaAlO₃) is one of the optimum substrates for epitaxic growth of thin oxide films. In this paper, the structures of the {100} and {110} surfaces of annealed LaAlO₃ are studied using reflection electron microscopy (REM). 010 steps have been observed on {100}, these are the lowest energy steps. The {100} surface is atomically flat, but the {110} surfaces exhibit high-density fine structures distributed on large surface terraces. These fine structures correspond to the formation of small width (100) and (010) facets on the (110) surface. A growth model is given to interpret the formation of large steps and large terraces on the {110} surfaces.     

X-ray diffraction studies of epitaxy in orthorhombic- DyMnO3/Er1Ba2Cu3O7-δ thin film heterostructures on LaAlO3 (100) substrates

The heteroepitaxy in DyMnO₃/Er₁Ba₂Cu₃O_7-δ bilayer thin films on LaAlO₃ (100) substates was characterized by four-circle X-ray diffractometry. The Er₁Ba₂Cu₃O_7-δ thin films on LaAlO₃ (100) substrates were prepared by molecular-beam deposition (MBD) and post-growth annealing in wet and dry O₂ at 880°C, whereas the DyMnO₃ thin films on the Er₁Ba₂Cu₃O_7-δ/LaAlO₃ (100) heterostructure were deposited by MBD and post-growth annealing in dry O₂ at 750°C. The conventional X-ray diffraction (XRD) patterns as well as pole figures (φ-scans) for specific (hkl) reflections were acquired. The Er₁Ba₂Cu₃O_7-δ thin film in the DyMnO₃/Er₁Ba₂Cu₃O_7-δ/LaAlO₃ (100) heterostructure showed [001] oriented epitaxial growth, as expected. The DyMnO₃ thin film on the Er₁Ba₂Cu₃O_7-δ epilayer in the heterostructure grew with (110) epitaxy in its metastable orthorhombic phase (lattice constants: a_o=5.272 Å, b_o=5.795 Å and c_o=7.38 Å). The heteroepitaxial relationships at the orthorhombic-DyMnO O₃ (110) /Er₁Ba₂Cu₃O_7-δ (001) interface was determined as the following: DyMnO₃ (110) Er₁Ba₂Cu₃O_7-δ (001), DyMnO₃ [1 0] ¶r; Er₁Ba₂Cu₃O_7-δ[100] or Er₁Ba₂Cu₃O_7-δ[010], and DyMnO₃ [001] ¶r; Er₁Ba₂Cu₃O _7-δ[010] or Er₁Ba₂Cu₃O_7-δ [100].     

Magneto-elastic effects in single-crystal USb0.8Te0.2

A high resolution X-ray diffraction study has been performed on a single crystal of USb_0.8Te_0.2. At room temperature this material has the cubic FCC (NaCl) structure. On cooling through the Néel point of 205 K USb_0.8Te_0.2 is known to enter a triple-q antiferromagnetic phase. Below T_C 175 K USb_0.8Te0.2 is a ferromagnet with net magnetisation along the 1 1 1 body diagonal. Four-circle X-ray scattering data from high-symmetry {h O h} and {h h h} reflections are presented as a function of temperature. We confirm that this magnetisation directly leads to a magneto-elastic distortion of the crystal lattice with a rhombohedral extension along the 1 1 1 body diagonals. At 130 K, this distortion is found to correspond to a rhombohedral bond angle of = 89.856° ± 0.004° consistent with previous results. In this single-crystal study we also present evidence for additional associated effects which we attribute to inter-grain stresses and sample mosaic structure.     

Structure of the Si(011)-(16 × 2) surface and hydrogen desorption kinetics investigated using temperature-programmed desorption

D₂ temperature-programmed desorption (TPD) was used to probe the structure of the Si(011)-(16 × 2) surface. Deuterium was adsorbed at 200°C to coverages θ_D ranging up to complete saturation (approximately 1.1 ML) and the sample heated at 5°C s⁻¹. TPD spectra exhibited three second-order desorption peaks labelled β₂, β*₁ and β₁ centered at 430, 520 and 550°C. Of the proposed models for the Si(011)-(16 × 2) reconstruction, the present TPD results as a function of θ_D provide support for the adatom/dimer model with the β₂ peak assigned to D₂ desorption from the dihydride phase, while the β*₁ and β₁ peaks arise from adatom and surface-atom monohydride phases.     

Superparamagnetic iron oxide particles and positive enhancement for myocardial perfusion studies assessed by subsecond T1-weighted MRI

Superparamagnetic iron oxide particles (SPIOs) are usually referred to as T₂ MR contrast agents, reducing signal intensity (SI) on T₂-weighted MR images (negative enhancement). This study reports the original use of SPIOs as T₁-enhancing contrast agents, primarily assessed in vitro, and then applied to an in vivo investigation of a myocardial perfusion defect. Using a strongly T₁-weighted subsecond MR sequence with SPIOs intravenous (IV) bolus injection, MR imaging of myocardial vascularization after reperfusion was performed, on a dog model of coronary occlusion followed by reperfusion. Immediately after the intravenous bolus injection of 20 μmol/kg of SPIOs, a positive signal intensity enhancement was observed respectively, in the right and left ventricular cavity and in the nonischemic left myocardium. Moreover, compared to normal myocardium, the remaining ischemic myocardial region (anterior wall of the left ventricle) appeared as a lower and delayed SI enhancing area (cold spot). Mean peak SIE in the nonischemic myocardium (posterior wall) was significantly higher than in the ischemic myocardium (anterior wall) (110 ± 23% vs. 74 ± 22%, Mann-Whitney test < 1%, n₁ = 6, n₂ − n₁ = 0, U > 2). In conclusion, the T₁ effect of SPIOs at low dose, during their first intravascular distribution, suggests their potential use as positive markers to investigate the regional myocardial blood flow and some perfusion defects such as the “no-reflow phenomenon”.     

Surface tension and density measurements for indium and uranium using a sessile-drop apparatus with glow discharge cleaning

The sessile-drop method is used to measure the surface tension and density of liquid indium and uranium under high vacuum. Measurements are made over the temperature range 156–500°C for In and at the melting point for U. Surface oxides are efficiently removed with a glow discharge system. Drop profiles are captured by photograph and processed using nonlinear regression to yield the surface tension and density. In this regression procedure, normal distances from calculated profiles to data points are minimized. For indium, the density and surface tension measurements yield _mp = 7.05 × 10³kg/m³, d/dT = −0.776 kg/m³·°C, and γ_mp = 0.568 N/m, dγ/dT = −9.45 × 10⁻⁵ N/m·°C. The results for uranium at the melting point are _mp = 17.47 × 10³ kg/m³ and γ_mp = 1.653 N/m.     

Electrons and holes in undoped Nd2CuO4

Nd₂CuO_4±δ is the non-superconducting prototype of the Re_2−xM_xCuO₄t−y family (Re=Pr, Nd, Sm and M=Ceor Th) of n-type oxide superconductors. Four-probe DC conductivity, EMF in P(O₂) gradient, and thermopower measurements have been used to characterise its electric transport and defect structure between 300 and 900°C and between 5×10⁻⁴ and 1 atm oxygen partial pressure.

The results show that Nd₂CuO_4±δ can be oxygen under-stoichiometric (with n-type conductivity), near-stoichiometric, and over-stoichiometric (with p-type conductivity) in different T, P(O₂) ranges.     

On the pp → ppη(η′) reactions near threshold

The production rate for η′ in pp → ppη′ at rest is calculated in a covariant one boson exchange model, previously applied to study π⁰ and η production in NN collisions. The transition amplitudes for the elementary BN → η′N processes with B being the meson exchanged (B = π, σ, η, , ω and a₀) are taken to be the sum of s- and u-channels with a nucleon in the intermediate states, and an a₀ meson pole in a t-channel. The couplings of the η′ to hadrons are a factor 0.4 weaker than the respective η-hadron couplings, as suggested by a quark model and a singlet-octet mixing angle θ = −23°. The model reproduces near threshold cross sections for the quasielastic processes π⁻p → nη(η′) and pp → ppη(η′) reactions.     

Multilayer relaxation of Rh(311)

The multilayer relaxation of the Rh(311) surface was investigated by means of LEED structure determination both for vertical and surface parallel (registry) relaxations. Excellent agreement between experimental and calculated spectra could be achieved mirrored by a minimum Pendry R-factor R = 0.174. The first three layer spacings are oscillatorily relaxed by Δd₁₂/d₀ = −14.5 ± 1.8%, Δd₂₃/d₀ = +4.9 ±2.0% and Δd₃₄/d₀ = −1.0 ±2.0%. There seems to be a coherent registry shift of the fir Δs = 0.03 ± 0.07 Å which, however, is within the error limits of the structure determination. Moreover, an energy dependent inner potential is detected. The results are discussed in comparison to equivalent surfaces of other materials as well as for the less open surfaces of rhodium.     

Stokes parameters measurement of light over a wide wavelength range by judicious choice of azimuthal settings of quarter-wave plate and linear polarizer

The Stokes parameters (S_p, S₁, S₂, S₃) of light are measured at an arbitrary wavelength over a wide wavelength range. The Stokes parameters S₁ and S₂ are easily obtained. But the Stokes parameter S₃ at the arbitrary wavelength is affected by the phase difference error Δ₁ of the quarter-wave plate mismatch and the Stokes parameters S₁ or S₂. Therefore, in this paper, S₃ is obtained from both the intensity measurements by the circular polarizer rotating the quarter-wave plate by 0° and 90°. Then, S₃ is obtained by considering only Δ₁, but is not affected by S₁ and S₂. Also, though Δ₁ is not accounted for, S₃ is measured more accurately.     

Modification of UO2 crystal morphologies through hydroxylation

Atomic scale computer simulation is used to predict the surface energies of UO₂, subject to different hydroxide coverages. It was found that the {1 1 1} surface dominates dry UO₂, resulting in an octahedral morphology. However, the {1 0 0} surfaces were strongly stabilized by hydroxylation relative to the {1 1 1} surfaces. Consequently, even a modest hydroxylation of 30% substantially truncates the octahedron crystal morphology, and a fully cubic morphology is predicted at 80% hydroxide coverage.     

Spin-lattice relaxation of Fe nuclei in yttrium iron garnet

Pulse measurements of T₁ for ⁵⁷Fe nuclei in very pure, single crystals of YIG are reported. The temperature was varied from 2° to 292°K, and the externally applied field ranged from 0 to 6000 Oe. The temperature variation of T₁ is quite strong, being three orders of magnitude in the range 2°–40°K. At constant temperature, T₁ changed approximately one order of magnitude between saturation field and 6000 Oe. The data are compared with the results of a calculation by Beeman and Pincus, in which a second-order Raman process and the three-magnon process are assumed to predominate below 50°K. Agreement is only qualitative, the experimental values of T₁ being larger than predicted. At 4.2°K in zero field, it is found that a polycrystalline sample containing particles of ≈ 5 × 10⁻⁴ cm dia. has a value of 1/T₁ which is some two orders of magnitude larger than for a macroscopic crystal. The presence of a relaxation mechanism associated with surface effects is suggested.     

Entropy of planar random surfaces on the lattice

We study planar random surfaces on a hypercubic lattice in two and three dimensions by Monte Carlo techniques. Our data are consistent with the formula n₀(A;C) A^b₀A, where n₀(A;C) is the number of planar random surfaces with area A and boundary C. We find b₀ = −1.4 ± 0.2, = 5.31 ± 0.03 (for d = 2) and b₀ = −1.5 ± 0.2, = 7.13 ± 0.05 (for d = 3). The values of b₀ disagree with those obtained from the Polyakov string model.     

The structure of the Newtonian limit

We consider a smooth one-parameter family of four-dimensional manifolds X,≥0, each one endowed with a covariant metric g. It is assumed that g is a Lorentz metric for each >0, i.e., the signature of g is (+,−,−,−) for >0, while the limit metric g₀ on X₀ is assumed to be degenerated of rank 1, i.e., the signature of g₀ is (+,0,0,0). We characterize when the limit manifold X₀ inherits the geometric structure of a Newtonian gravitation. The limit manifold X₀ is a Newtonian gravitation if and only if there exist the limits of the Levi-Civita connection , the curvature operator and the contravariant Einstein tensor G² as →0. Moreover, the existence of these limits is characterized in terms of the Taylor expansion of the family {g} with respect to the parameter .     

Longitudinal nuclear relaxation measurements in hcp H2

Measurements of T₁ in the hep phase of H₂, over the temperature range 2°–12°K and the ortho concentration range between 0.5 and 0.97 are presented. At temperatures below 10°K, the thermally activated self-diffusion is negligible and the mechanism for nuclear relaxation is that attributed by Moryia and Motizuki and by Harris to intramolecular dipolar interaction, modulated by intennolecular electric quadrupole-quadrupole (EQQ) interaction. The gaussian approximation for the correlation function was used by these authors to predict T₁. From the comparison between experiment and theory, we determine the EQQ parameter Γ/k_B to be 0.67°K. Above 10°K the effect of diffusion influences T₁, and the experimental results for an 88 per cent ortho H₂ sample up to the melting point suggest that the relaxation mechanisms resulting from EQQ interaction and diffusion are not independent of one another.     

Time evolution of interface roughness during thermal oxidation on Si(0 0 1)

The surface morphological change at an initial stage of thermal oxidation on Si(0 0 1) surface with O₂ was investigated as a function of oxide coverage by a real-time monitoring method of Auger electron spectroscopy (AES) combined with reflection high energy electron diffraction (RHEED). At 653 °C where oxide islands grow laterally, protrusions were observed to develop under the oxide islands as a consequence of concurrent etching of the surface. The rate of etching was measured from a periodic oscillation of RHEED half-order spot intensity I_(1/2,0) and I_(0,1/2). At 549 °C where Langmuir-type adsorption proceeds, it was observed that both I_(1/2,0) and I_(0,1/2) decrease more rapidly in comparison with an increase of oxide coverage and the intensity ratio between them decreases gradually with O₂ exposure time. These suggest that Langmuir-type adsorption occurs at sites where O₂ adsorbs randomly, leading to subdivision of the 2×1 and 1×2 domains by oxidized regions, and that Si atoms are ejected due to volume expansion in oxidation to change the ratio between 2×1 and 1×2 domains.     

Phase transitions in the superionic protonic conductor CsHSeO4 investigated by Brillouin spectroscopy

A Brillouin investigation in CsHSeO₄ has been performed over the temperature range 20–165 °C which includes two phase transitions, in particular the transition to the superionic phase near T_s = 129 °C. We observed strong discontinuities for elastic constants C₁₁, C₂₂ and C₃₃ at T_s and a broadening of the Brillouin lines above T_s. The results are discussed on the basis of a linear coupling between strains and mobile protons.     

Compton scattering by the proton through θcms = 75° and 90° in the Δ-resonance region

Differential cross sections for Compton scattering by the proton have been measured in the energy interval between 200 and 500 MeV at scattering angles of θcms = 75° and θcms = 90° using the CATS, the CATS/TRAJAN, and the COPP setups with the Glasgow Tagger at MAMI (Mainz). The data are compared with predictions from dispersion theory using photo-meson amplitudes from the recent VPI solution SM95. The experiment and the theoretical procedure are described in detail. It is found that the experiment and predictions are in agreement as far as the energy dependence of the differential cross sections in the Δ-range is concerned. However, there is evidence that a scaling down of the resonance part of the M1+3/2 photo-meson amplitude by (2.8 ± 0.9)% is required in comparison with the VPI analysis. The deduced value of the M1+3/2-photoproduction amplitude at the resonance energy of 320 MeV is: |M1+3/2| = (39.6 ± 0.4) × 10−3 mπ+−1.     

Low-temperature phase and magnetic interactions in FCC Fe-Cr-Ni alloys

The low-temperature (5 K < T < 300 K) magnetic properties of a set of nine isostructural fcc Fe-Cr-Ni (Fe ≈ 68 at %, Cr ≈ 20 at%, Ni ≈ 9 at%) alloys were studied by SQUID magnetometry, neutron diffraction and ultrasonic techniques. Type-1 antiferromagnetic (AF) ordering was observed below the Néel temperature, T_N. The dc susceptibility, χ(T), did not exhibit a simple Curie-Weiss dependence. Above T_N, atemperature independent component χ₀ was observed, i.e., χ(T) = χ₀ + C / (itT + θ. T_N was systematically influenced by the lattice parameter, a, decreasing from (47.9 ± 0.5) K to (35.0 ± 0.5) K as a increased by only 0.25%. The average magnetic moment of ≈ 0.6μ_B obtained from neutron scattering was lower than the ≈ 1 μ_B obtained from the SQUID data. Mean field estimates of antiferromagnetic nearest-neighbors exchange interaction (J₁) and ferromagnetic second-nearest-neighbors interaction (J₂) indicate that |J₂/J₁|≈ 1.5. We believe that this is evidence of the RKKY interaction, and self-consistently argue that only the external d electrons are responsible for the localized average moment. This may mean that s-d hybridization of the external electrons is weak in these alloys.     

Properties of a large La1.92Sr0.08CuO4+δ single crystal grown by the travelling-solvent floating-zone method

A centimeter size single crystal of La_2−xSr_xCuO_4+δ (volume=1.32 cm³) with x_Sr=0.08 has been grown by the travelling-solvent floating-zone (TSFZ) method using a double ellipsoidal-type optical furnace as the heat source. The crystallised phase was checked solvent free by X-ray powder diffraction experiments, the crystal dimensions and quality being investigated by X-ray and neutron Laüe techniques. Several rocking curves of the Bragg peaks were performed by neutron diffraction giving a full width at half maximum (FWHM) of 0.200° for (006) reflection and clearly showing the presence of twin domains as expected for such an orthorhombic structure. The superconducting critical temperature of the as-grown crystal under 2 oxygen bar was determined by SQUID measurements with T_c=18–20 K. Thermal treatments at different oxygen pressures were carried out showing no significant improvement of the transition sharpness and the T_c value. Normal state susceptibility was also measured from 6 to 800 K for two different field orientations and can be interpreted as an antiferromagnetic insulating state behaviour. The resistivity measurements display an insulating behaviour perpendicular to the CuO₂ planes and a metallic behaviour in the planes, with a high anisotropy ratio R_c/R_b350 at room temperature and a zero resistivity achieved at 27 K in both directions. The specific heat measurements have revealed no anomalies in the temperature range 15–300 K.