Influence of the ferroelastic twin domain structure on the {100} surface morphology of LaAlO3 HTSC substrates

LaAlO₃ crystals have been investigated with differential scanning calorimetry (DSC), high-precision X-ray powder diffraction (XRD) and scanning force microscopy (SFM). The DSC measurements show the second-order phase transition of LaAlO₃ at 544°C, where LaAlO₃ changes its symmetry from the cubic Pm3m high-temperature phase to the pseudocubic rhombohedral low-temperature phase. This paraelastic to improper ferroelastic phase transition causes twinning in the {100} and {110} planes of the pseudocubic lattice. The twin angles between the surface {100}_pseudocubic planes of twin domains were measured by SFM on the surface of a macroscopic (100)_cubic cut crystal plate. The misorientation angle ω₁₀₀ between {100} twins is 0.195(8)°, while {110} twinning gives an angle of ω₁₁₀=0.276(7)°. The two twin kink angles correspond to a rhombohedral angle of the pseudocubic cell of the phase as ₁=90.0973(40)° and ₂=90.0975(30)°, respectively. The XRD result for this rhombohedral angle is =90.096(1)°. The orientation of the misfit steps formed during annealing after mechanical surface polishing depends on the domain orientation and pattern during polishing. Any heating close to or above T_c changes the domain pattern. Footprints of previous domain patterns can thus be found on the surface in the form of surface corrugation and changes in the shape and orientation of misfit steps.     

The temperature dependence of the short wavelength transmittance limit of vacuum ultraviolet window materials—I. Experiment

The short wavelength transmittance limit or cut-off wavelength, λ_co, of LiF, MgF₂, CaF₂, LaF₃, BaF₂, sapphire, synthetic crystal quartz and fused quartz has been measured from about 100°C to about 10°K. λ_co is not a well denned quantity, so for the purpose of this experiment it has been arbitrarily taken as the wavelength where transmittance could just be measured, usually 0.1-0.5 per cent. With one exception λ_co shifted to shorter wavelengths as the sample was cooled; the shift varied from about 40 to 80 Å over the temperature range from 100°C to 10°K, depending on the material, with the largest shift occurring in BaF₂;. The exception was LaF₃ which showed no measurable change in λ_co wth temperature. Over the temperature range from 20° to 100°C the slope of λ_co, with temperature for all materials was fairly constant, but below 20°C it decreased, approaching zero as the temperature approached 20°-10°K. In the case of synthetic crystal quartz, for example, the slope changed from about 0.28 Å/°K at room temperature to about 0.055 Å/°K at 80°K.     

Surface morphology and electrical properties of Bi---Sr---Ca---Cu---O thin films deposited by reactive sputtering from multiple targets

Smooth, superconducting films of Bi₂Sr₂CaCu₂O_8+° have been prepared by reactive sputtering from elemental targets in the presence of ozone. The influence of substrate temperature, deposition rate, and ozone pressure on the resulting films are discussed. Films deposited on SrTiO₃ substrates are c-axis oriented and featureless for substrate temperatures below 710°C. Above this temperature, small inclusions of CuO appear. Films on MgO exhibit mixed a-axis and c-axis orientation below about 710°C, and inclusions of CuO above this temperature. The temperature at which this transition occurs increases with increasing deposition rate. T_c increases and then decreases sharply with decreasing oxygen content. The oxygen partial pressure corresponding to the maximum T_c of 77 K is well above the thermodynamic stability limit for Bi₂Sr₂CaCu₂O_8+°, suggesting that an optimum carrier concentration has been achieved for these films.     

Specific heat of the cubic high-Tc superconductor Ba0·6K0·4BiO3

We report magnetic susceptibility and specific heat measurements on polycrystalline samples of the 30 K superconductor Ba_0·6K_0·4BiO₃. Normal-state magnetization measurements indicate a Pauli-paramagnetic susceptibility of χ_pauli = 2.3 × 10⁻⁵ emu/mole, from which we infer a value for the density of states at the Fermi level of N(0) = 8.6 × 10 ²¹ev⁻¹cm.⁻³ Specific heat measurements performed between 1.6 K and 40 K indicate that considerable lattice softening occurs at low temperatures; the effective Debye temperature drops from 280 K at 35 K to 210 K at 4 K, implying that soft phonon modes are present in this compound. This result indicates that conventional phonon-mediated interactions may be responsible for the high transition temperature exhibited by Ba_0·6K_0·4BiO₃.     

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.     

The interaction of hydrogen with polycrystalline zirconium I. Sticking coefficients and binding states

Values for the sticking coefficient of hydrogen on polycrystalline zirconium have been obtained using a computer-controlled flash desorption technique. Sticking coefficients exhibit a maximum of about 6.5 × 10^-4 near 700 K and fall to about 4 × 10^-5 below 350 K and above 1000 K. Such low values are surprising in view of the well-known effectiveness of zirconium as a “getter” material for hydrogen. Plots of desorption rate versus temperature show two binding states below the phase transition temperature and at least one (and possibly two) above that temperature.     

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.     

Superconducting oxide formation in the Yb---Ba---Cu---O system

Conditions for forming YbBa₂Cu₄O₈, Yb₂Ba₂Cu₃O_7−δ and YbBa₂Cu₃O_7−δ were determined in oxygen at 1.0 atm pressure by experiments with oxidized Yb---Ba---Cu---Ag alloys. YbBa₂Cu₃O_7−δ formed in less than 10 s when oxidized alloy ribbons were baked in the 805°C–890°C range. Large amounts of Ba₂Cu₃O₅ formed below 870°C. Yb₂Ba₄Cu₇O_15−δ and YbBa₂Cu₄O ₈ then formed in the 10²s–10³s range at temperatures in the 840°C–870°C and <840°C ranges, respectively. Ba₂Cu₃O₅ decomposition supplied Cu²⁺ and O²⁻ for the rapid transformation of YbBa₂Cu₃O_7−δ grains into Yb₂Ba₄Cu₇O_15−δ grains by intercalation. Intercalation was much slower in the absence of Ba₂Cu₃O₅. All of these transformation products underwent coarsening with increasing bake time. Rapid YbBa₂Cu₄O₈ and Yb₂Ba₄Cu₇O_15−δ formation, compared to the slow formation of these phase types when Yb was substituted by Y, Is due to the differing effects of Y and Yb on formation kinetics. Superconducting oxide-silver composite ribbons containing mainly YbBa₂Cu₃O_7−δ, Yb₂Ba₄Cu₇O_15−δ or YbBa₂Cu₄O₈ can be prepared from Yb---Ba ---Ba---Cu---Ag alloys.     

Diffusion of W on A W(211) plane

The diffusion of W on a (211) plane of a W field emitter has been re-examined by means of the fluctuation autocorrelation method. Diffusion along channels yielded E = 16.8 ± 0.5 kcal, D₀ = (3 ± 1) × 10⁻⁵ cm² s⁻¹. For diffusion across channels E =6.6 kcal, D₀ = 4 × 10⁻⁹cm² s⁻¹ at T < 752 K, and E = 24 kcal, D₀ = 5 × 10⁻⁴ cm² s⁻¹ at T > 752 K. The results for diffusion along channels yield E and D₀ values intermediate between recent results by Wang and Ehrlich [Surf. Sci. 206 (1988) 451] using field ion microscopy (E = 19 kcal, D₀ = 7.7 × 10⁻³ cm² s⁻¹) and Tringides and Gomer [J. Chem. Phys. 84 (1986) 4049], using the same method as the present work but a larger slit (E = 13.3 kcal, D₀ = 7 × 10⁻⁷ cm² s⁻¹). The results for cross channel diffus good agreement with those of Tringides and Gomer below 752 K, where these authors stopped. The new high temperature results suggest that the channel wall exchange mechanism postulated by Tringides and Gomer for cross channel diffusion at low T gives way to diffusion by climbing over the channel walls with higher E but also higher D₀ above 752 K. Possible reasons for the discrepancies between these three sets of results and the absence of cross channel diffusion in the work of Wang and Ehrlich are briefly discussed.     

The susceptibility of NO in β-quinol clathrates

Susceptibility measurements on polycrystalline samples of NO in β-quinol clathrates between 1°K and room temperature are described. The results show in particular that below 200°K the susceptibility becomes systematically larger than that expected for a free gas and that within an experimental error of about 5 per cent the susceptibility tends to a constant value of 13.9 × 10⁻⁵ e.m.u./g NO below about 15°K. The presence of a 1/T term at low temperatures suggested by Van Vleck could not be detected. These results are compared to those obtained by Cooke and Duffus, which show an appreciable temperature dependence of the susceptibility even below 30°K. The discrepancy is attributed to paramagnetic impurities in the earlier sample.     

Thermal expansion of solid CH4 and CD4

The linear expansion coefficients of solid light methane CH₄ and deuteromethane CD₄ have been denned in the temperature range 10°–24°K and 6°–58°K, respectively. The question about the nature of rotational motion of molecules in CH₄ and CD₄ crystals and about the existence of low temperature phase transition in solid methane is discussed.     

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.     

Luminescence and circularly polarized luminescence of macrocyclic Eu(III) and Tb(III) complexes embedded in xerogel and sol–gel SiO2 glasses

Luminescence, time-resolved luminescence, circularly polarized luminescence (CPL) and decay profiles of Ln(III)(15-crownether-5) (Ln=Ce, Sm, Eu, Tb) and Tb(III)-(R),(S)-cyclen derivative complexes doped in xerogel and sol–gel silica glasses are measured at temperatures down to 10 K to characterize luminescence properties and the electronic structure in the excited states. Luminescence spectral profiles and calculation of crystal field parameters (B₀⁽²⁾,B₂⁽²⁾) in the ⁵D₀→⁷F_J(J=1,2) transition give evidence of the fact that the pentagonal and planar structure of Eu(III) (15-crownether-5) does hold in xerogel and sol–gel glasses prepared at temperatures below 100°C. As annealing temperatures are increased from 80°C to 750°C, Eu(III) complexes in sol–gel glasses are found to decompose gradually to SiO₂:Eu³⁺. Tb(III)-(R) and (S)-cyclen derivative complexes in xerogel reveal at room temperature and 10 K sharp CPL spectra with luminescence dissymmetry factors g_lum=−0.1 and 0.1, respectively. These complexes doped in sol–gel glasses represent luminescence characteristics of rare earth ions encapsulated in the nano-porous host.     

Surface properties of Hafnium diboride(0 0 0 1) as determined by X-ray photoelectron spectroscopy and scanning tunneling microscopy

The surface structure and properties of the HfB₂(0 0 0 1) (Hafnium diboride, HfB₂) surface have been investigated with X-ray photoelectron spectroscopy, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Annealing temperatures above 1900°C produce a sharp (1×1) LEED pattern, which corresponds to STM images showing flat (0 0 0 1) terraces with a very low contamination level separated by steps 3.4 Å in height, corresponding to the separation of adjacent Hf planes in the HfB₂ bulk structure. For lower annealing temperatures, extra p(2×2) spots were observed with LEED, which correspond to intermediate terraces of a p(2×1) missing row structure as observed with STM.     

O2 interaction with Pt{100}-hex-R0.7°: scattering, sticking and saturating

The interaction of oxygen with the Pt{100}-hex-R0.7° surface has been studied using supersonic molecular beams at incident translational energies from 0.06 to 0.9 eV and surface temperatures from 300 to 600 K. Scattering measurements show the existence of both intrinsic and extrinsic precursor states, and the trapping probability into these states is high at low incident energies. However, sticking probability measurements on the clean surface indicate that O₂ dissociative adsorption on Pt{100}-hex-R0.7° is a direct activated process, in contrast to that on Pt{100}-(1 × 1) or Pt{111}. Strong temperature enhancement of the initial sticking probability has been observed and accounted for partly by a dynamical barrier model. The sticking probability varies strongly with oxygen coverage, which is explained through computer simulations of island formation. The formation of small islands is demonstrated by TEAS measurements. Thermal desorption measurements show that, at high incident energies above 0.5 eV, new states are populated and higher coverages, up to a full monolayer, are reached.     

Buffers for high temperature superconductor coatings. Low temperature growth of CeO2 films by metal–organic chemical vapor deposition and their implementation as buffers

Smooth, epitaxial cerium dioxide thin films have been grown in-situ in the 450–650°C temperature range on (001) yttria-stabilized zirconia (YSZ) substrates by metal–organic chemical vapor deposition (MOCVD) using a new fluorine-free liquid Ce precursor. As assessed by X-ray diffraction, transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM), the epitaxial films exhibit a columnar microstructure with atomically abrupt film-substrate interfaces and with only minor bending of the crystal plane parallel to the substrate surface near the interface and at the column boundaries. With fixed precursor temperature and gas flow rate, the CeO₂ growth rate decreases from 10 Å/min at 450°C to 6.5 Å/min at 540°C. The root-mean-square roughness of the films also decreases from 15.5 Å at 450°C to 4.3 Å at 540°C. High-quality, epitaxial YBa₂C₃O_7−x films have been successfully deposited on these MOCVD-derived CeO₂ films grown at temperatures as low as 540°C. They exhibit T_c=86.5 K and J_c=1.08×10⁶ A/cm² at 77.4 K.     

Anomalous temperature dependence of magnetization in single-crystal Fe65Ni35 alloy

Magnetization (0–10 Oe) and magnetic relaxation measurements were carried out in the temperature range between 4.2 and 300 K for three picture-frame samples of Fe₆₅Ni₃₅ alloy whose edges were parallel to 100, 110 and 111, respectively. The typical temperature T_g and the magnetic field H_g which correspond to the anomalous temperature in the χ-T curve and inflection field in the σ-H curve, respectively are summarized and H-T_g and H_g-T diagrams are obtained. A strong magnetic relaxation is observed along the H_g-T line. The dependence of H_g on the crystallographic direction and on the temperature are discussed by the thermal activation process of the 180° domain wall which is pinned strongly by the antiferromagnetic clusters below T_g. The anomaly of magnetization of Fe₆₅Ni₃₅ alloy can be interpreted by the macroscopic picture of the coexistence of ferromagnetic and antiferromagnetic-like regions which may be caused by a statistical fluctuation of alloy composition.     

Auger electron spectroscopy and electron energy loss spectroscopy studies on carbonization of Si(100) and (111) surfaces with ethylene

The reactions of Si(100) and Si(111) surfaces at 700 °C (973 K) with ethylene (C₂H₄) at a pressure of 1.3×10⁻⁴ Pa for various periods of time were studied by using Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS). For a C₂H₄ exposure level, the amount of C on the (111) surface was larger than that on the (100) surface. The formation of β-SiC grain was deduced by comparing the C_KLL spectra from the sample subjected to various C₂H₄ exposure levels, and from β-SiC crystal.     

Coexisting antiferromagnetism and ferromagnetism in mechanically alloyed Fe-rich Fe–Ni alloys: implications regarding the Fe–Ni phase diagram below 400°C

Fe–Ni alloys below the Invar region with compositions Fe_100−xNi_x (x=21, 24, and 27 at%) were prepared by high-energy ball milling technique (mechanical alloying). The as-milled samples, characterized by X-ray diffraction and Mössbauer spectroscopy, contain a mixture of (BCC) and γ (FCC) phases, whereas the samples annealed at 650°C for 0.5 h show a single γ (FCC) phase displaying a single line Mössbauer spectrum at room temperature (RT). At low temperature, the Mössbauer spectra of annealed Fe₇₆Ni₂₄ and Fe₇₃Ni₂₇ alloys show the existence of a magnetically split pattern together with a broad singlet, which are ascribed to a high-moment ferromagnetic Ni-rich phase and a low-moment Fe-rich phase, respectively. The Fe-rich phase in annealed Fe₇₆Ni₂₄ alloy, which is paramagnetic at RT, undergoes antiferromagnetic ordering at 40 K, estimated from the dramatic line broadening of its spectrum, giving rise to a small hyperfine field (e.g. 2 T at 6 K). The coexistence of these phases is attributed to phase segregation occurring in these alloys as a result of enhanced atomic diffusion. The stability of these alloys towards martensitic (FCC→BCC) transformation at low temperatures is discussed in connection with the Fe–Ni phase diagram below 400°C.     

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].