Showing papers on "Single crystal published in 1987"

Growth of single-crystal TiN/VN strained-layer superlattices with extremely high mechanical hardness

Abstract: Single‐crystal TiN/VN strained‐layer superlattices (SLS’s) with layer thicknesses lTiN =lVN =λ/2 (where λ is the period of the superlattice) ranging from 0.75 to 16 nm have been grown on MgO(100 ) substrates by reactive magnetron sputtering. Cross‐sectional transmission electron microscopy (TEM) and x‐ray diffraction examinations showed that the films were single crystals exhibiting coherent interfaces and several orders of superlattice reflections. There was no evidence in either plan‐view or cross‐sectional TEM analyses of misfit interfacial dislocation arrays. The primary defects observed were dislocation loops with a diameter of 8–10 nm extending through several layers and small defects with a diameter of 1–2 nm that were confined within single layers. Microindentation hardness values H, measured as a function of λ in films with a total thickness of 2.5 μm, increased from 2035±280 kg mm−2 for Ti0.5V0.5N alloys (i.e., λ=0) to reach a maximum of 5560±1000 kg mm−2 at λ=5.2 nm and then decreased rapidly t...

Direct observation of electronic anisotropy in single-crystal Y1Ba2Cu3O7-x.

Abstract: We report direct observation of the anisotropic electronic behavior of the high-temperature superconductor ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$. Critical-field and critical-current measurements performed on single crystals show anisotropies of 10 and greater. Critical supercurrent densities in favorable directions in single crystals are 3\ifmmode\times\else\texttimes\fi{}${10}^{6}$ A/${\mathrm{cm}}^{2}$ in low fields at 4.5 K and remain above ${10}^{6}$ A/${\mathrm{cm}}^{2}$ to beyond 40 kG.

Structure of the 100 K Superconductor Ba2YCu3O7 between (5 ÷ 300) K by Neutron Powder Diffraction

Abstract: Recently Siegrist et al. proposed a structure for a high-Tc superconductor Ba2YCu3O(9-∂) based on an orthorhombic (a = ap, b = ap, c = 3ap) perovskitelike model containing Ba and Y cations ordered over the A-sites of the ABO3 structure. This ordering is responsible for the tripling of the c-axis. Half of the oxygen vacancies (at z = 1/2) are ordered, while the other half (at z = 0) are disordered over two sites. Using profile refinement of high-resolution neutron powder data at six different temperatures between 5 K and 300 K, we have refined the structure of a pure and well-characterised powder sample with onset of superconductivity at 100 K. At all temperatures we confirmed the previous model except that in our structure, all oxygen vacancies are ordered. Two-thirds of the copper cations have a pyramidal coordination and one-third has a square coordination. In our structure all squares are parallel to the (b, c)-plane, while in the one reported by Siegrist et al. the squares are disorderedly parallel to either the (a, c)- or (b, c)-plane. The difference between the two models is probably due to the fact that the single crystal used in the earlier work was highly twinned. Empirical calculations of the copper valences show that the Cu+++ cations are almost equally distributed over the two sites. No structural change has been detected at the transition.

Anisotropic Nature of High-Temperature Superconductivity in Single-Crystal Y 1 Ba 2 Cu 3 O 7 − x

Abstract: We report the first contact-free measurements of the upper critical fields, ${\mathrm{H}}_{\mathrm{c}2}$(T), of single-crystal ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$. In contrast to resistive measurements reported by others, we find that the anisotropy near ${\mathrm{T}}_{\mathrm{c}}$ is temperature independent in agreement with the expectations of anisotropic Ginzburg-Landau theory. Estimates of the anisotropic Ginzburg-Landau and London parameters are reported. These indicate that despite the large anisotropy in ${\mathrm{H}}_{\mathrm{c}2}$ (5:1), the inferred low-temperature interplanar coherence length (${\ensuremath{\xi}}_{\mathrm{z}}$=7 A\r{}) remains larger than the Cu-O layer spacing of 3.9 A\r{}. The superconductivity in ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$ thus remains fundamentally three dimensional in nature for a substantial temperature range below ${\mathrm{T}}_{\mathrm{c}}$.

Mesotaxy: single-crystal growth of buried CoSi2 layers

Abstract: Buried single‐crystal CoSi2 layers in silicon have been formed by high dose implantation of cobalt followed by annealing. These layers grow in both the (100) and (111) orientations—those in (111) have better crystallinity, but those in (100) are of higher electrical quality. Electrical transport measurements on the layers give values for the resistance ratios and superconducting critical temperatures that are better than the best films grown by conventional techniques and comparable to bulk CoSi2.

Synthesis and structure of ( cis )-[1-ferrocenyl-2-(4-nitrophenyl)ethylene], an organotransition metal compound with a large second-order optical nonlinearity

Abstract: There is considerable interest in materials with nonlinear optical properties. Two important manifestations of light interacting with a nonlinear optical material are, the process of second-harmonic generation (SHG) which is relevant to new laser technology, and the electro-optic effect which has applications in telecommunications and in integrated optics1. Several inorganic and organic materials with unusually large second-order nonlinearities are known1. However, reports of organometallic compounds with second-order nonlinear optical properties are few2–4 and they have relatively small values for SHG efficiencies. Here we describe a new compound (cis)-[1-ferrocenyl-2-(4-nitrophenyl)ethylene] (compound 2) exhibits solvatochromic behaviour. A Kurtz-powder measurement, using 1.907 µm light from a Raman shifted Nd-YAG laser operating at 1.064 µm, shows the compound provides highly efficient second harmonic generation, with a signal some 62 times that from a urea reference sample. The single crystal X-ray structure of (cis)-[1-ferrocenyl-2-(4-nitrophenyl)ethylene] has been determined. The large nonlinear optical property of this compound demonstrates that organotransition metal compounds can play an important role in the development of the nonlinear optical properties of materials.

Properties of Fe single‐crystal films grown on (100)GaAs by molecular‐beam epitaxy

Abstract: Single‐crystal (100)Fe films 90–330 A thick have been grown on etch‐annealed (100)GaAs substrates by molecular‐beam‐epitaxy techniques. Ferromagnetic resonance data indicate that the two in‐plane 〈110〉 directions are inequivalent and, together with magnetometry data, show that the average film magnetization decreases as the thickness decreases. The inequivalence is attributed to the nature of the interface bonding at a (100) zinc‐blende surface. The decreased magnetization is attributed to the formation of Fe2As microclusters in the film due to As diffusion which is supported by Auger and electron diffraction studies. In general, the Fe films grown to date on etch‐annealed (100)GaAs substrates are significantly inferior to those grown on (110)GaAs.

Kinetics of the activated dissociative adsorption of methane on the low index planes of nickel single crystal surfaces

Abstract: We have measured the kinetics of the methane decomposition reaction on Ni(111), Ni(100), and Ni(110) single crystal surfaces under the high incident flux conditions of 1 Torr methane. We find for these processes apparent activation energies of 12.6, 6.4, and 13.3 kcal mol−1, respectively. Initial methane sticking coefficients at 500 K vary with the Ni surface, but are all ∼10−8 to 10−7. The Ni(110) surface is the most active, followed by Ni(100) and Ni(111). A large (∼ factor of 20) kinetic isotope effect is seen for CH4 vs CD4 on the Ni(100) surface, whereas none is seen on the Ni(110) surface. A comparison is made between measured thermal sticking coefficients and those calculated from the results of recent molecular beam experiments of CH4 on Ni(111) and Ni(100) surfaces. Agreement of our results with the Ni(100) beam results is poor, whereas agreement with the Ni(111) beam results is very good. A comparison is also made between our results and rates of the catalytic steam reforming reaction of methane.

Superconductivity in YBa2Cu3O7 single crystals

Abstract: Increased understanding of fundamental properties of the new cuprate superconductors depends on advances in the preparation of quality materials for study. Early work focused on the study of sintered polycrystalline samples. In some cases, grain growth during sintering yielded individual grains large enough (˜80 μm) for single crystal X-ray determination1. Larger single crystals allow measurements of physical properties including measurements of anisotropic behaviour. The anisotropy of upper critical fields in YBa2Cu3O7 single crystals has been reported by lye et al.2 who found critical fields characteristic of a quasi-two-dimensional superconductor, which are highest when the field is orientated perpendicular to the c-axis. Dinger et al.3 have measured critical fields and critical currents of single crystals and find anisotropies of 10 and greater3. These studies contain little detail on the crystal growth of YBa2Cu3O7. Here we report the growth of single crystals as large as 4 mm of YBa2Cu3O7 and DyBa2Cu3O7 and confirm superconductivity in them by magnetic and electrical measurements. Single crystals of other rare earth barium cuprates are also obtained although we have yet to optimize growth conditions for these phases.

Crystal-face dependence of surface melting.

Abstract: Ion-shadowing and -blocking experiments on a cylindrical single crystal of Pb reveal a strongly orientation-dependent disordering (melting) of the surface with increasing temperature. The thickness of the disordered surface layer is found to diverge logarithmically as the bulk melting point is approached. The process of disordering is shown to be driven by an orientation-dependent difference in free energy between the surface in its ordered and liquid states.

The Anisotropic Upper Critical Field of Single Crystal YBa2Cu3Ox

Abstract: We report a study of anisotropic upper critical field of single crystal samples of orthorhombic YBa2Cu3Ox. The critical field shows anisotropy characteristic of a quasi-two-dimensional superconductor, and is highest when the field is oriented perpendicular to the c-axis. For this field direction, the zero resistance state persists up to 86 K in a magnetic field of 90 kOe, which is lower by only 5 K than the zero resistance temperature in the absence of magnetic field. The anisotropy Hc2⊥/Hc2// is about 2 at 90 K, 5 at 86.5 K and tends to increase with decreasing temperature.

Soft-phonon behavior and transport in single-crystal La 2 CuO 4

Abstract: Using a flux technique we have grown sizable single crystals of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$. With x rays and neutrons we have studied both the static and dynamic aspects of the tetragonal to orthorhombic structural phase transition; classic soft-phonon behavior is observed at the ((1/2, 1) / 2 , 0) zone boundary involving rotations of ${\mathrm{CuO}}_{6}$ octahedra. The pure and lightly-doped single crystals show hopping conductivity, ln\ensuremath{\rho}\ensuremath{\sim}(${T}_{0}$/T${)}^{1/4}$, indicating that the electronic states at the Fermi energy are localized.

Growth of YBa2Cu3Ox single crystals

Abstract: We report a set of conditions for crystal growth of the high‐temperature superconductor YBa2Cu3Ox. The as‐grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.

Method for growing single crystal thin films of element semiconductor

Abstract: A method for growing a single crystal thin film of an element semiconductor which comprises repeating the successive operations of feeding a single kind of gas containing the element semiconductor as a component element onto a substrate heated in a growth chamber and then exhausting the gas in the growth chamber under controlled conditions and thereby growing the single crystal thin film of said element semiconductor in a desired thickness with a precision of monomolecular layer. In an alternate method for growing a single crystal thin film of an element semiconductor, the sole gas containing the element semiconductor is continuously fed onto the substrate for a given period of time, thereby forming a single crystal thin film of element semiconductor having a desired thickness. According to the present invention, there can be obtained single crystal thin films with high quality in a high reproducibility, by simplified operating parameters and growing apparatus.

Effect of pressure on the crystal structure of perovskite-type MgSiO3

Abstract: The crystal structure of perovskite-type MgSiO3 has been studied up to 96 kbar, using a miniature diamondanvil pressure cell and by means of single-crystal four-circle diffractometry. The observed unit cell compression gives a bulk modulus of Ko=2.47 Mbar, assuming K′o=4. The unit cell compression is controlled mainly by the tilting of SiO6 octahedra. The effect of pressure is to change Mg polyhedron towards 8-fold coordination rather than 12-fold coordination. The polyhedral bulk moduli of SiO6 and MgO8 are 3.8 Mbar and 1.9 Mbar, respectively.

Single-crystal absorption and reflection infrared spectroscopy of forsterite and fayalite

Abstract: Polarized single-crystal absorption and reflection spectra of fundamental modes in both the mid- and far-infrared are presented for microscopic crystals of forsterite and fayalite. All modes predicted by symmetry were observed for forsterite, but two B3u modes were not observed for fayalite. Consideration of previously determined frequency shifts for isotopically and chemically substituted olivines, along with symmetry analysis, produced a complete set of band assignments satisfying all constraints for forsterite. A plausible assingment was derived for fayalite by analogy. The frequency shifts from forsterite to fayalite are consistently small for bands assigned to SiO4 stretching and bending, moderate for rotations, and large for translations of M-site ions, suggesting that in olivine, SiO4 groups vibrate separately from the lattice. Allocating the bending and external modes among multiple continua in Kieffer's (1979c) model considerably improves prediction of quasiharmonic heat capacityC v and entropy for forsterite (∼1% discrepancy from 200–1000 K). The experimental entropy of fayalite is closely accounted for (1.8 to 0.1%) by summing lattice, electronic (from Burns' (1985) optical band assignment), and constant magnetic contributions above 200 K.S magnetic determined from the difference of the experimental and model lattice entropies shows inflection points at the two magnetic transition temperatures (23 and 66 K) and indicates that complete spin disorder is not achieved below 680 K.

The growth of single crystal Terfenol-D crystals

Abstract: Rods of the highly magnetostrictive material, Terfenol-D, Tb0.3Dy0.7Fe2, have been prepared by both Bridgman and float zone techniques. It is found that the dendritic growth front consists of parallel sheets of dendrites growing with a primary direction of and sheet planes of {111}. Unseeded rods show a strong preference for alignment of the grains. Seeding experiments have been successful for orientations but not , and the cause of this difference is discussed. The single crystal rods are seen to contain parallel {111} twin boundaries throughout their volume, growing near the central plane of the dendrite sheets. These results are analogous to those of Ge and Si. The role of the twin planes on domain wall motion and methods of eliminating the twins are discussed.

Anisotropy of the Upper Critical Magnetic Field in Single Crystal YBa2Cu3O7+y

Abstract: The upper critical magnetic field Hc2 was measured on single crystals of the recently-discovered superconducting YBa2Cu3O7+y oxide. The Hc2 along the basal plane is estimated to be larger than that along the c-axis by a factor of 5. A small anisotropy of Hc2 in the basal plane is also found. It is not clear at present whether this small anisotropy is intrinsic or due to crystal imperfectness such as micro-twinnings.

Influence of the beam spot size on ablation rates in pulsed‐laser processing

Abstract: Ablation rates in pulsed‐laser processing depend heavily on the laser beam spot size. This has been observed for the first time for a number of different materials. Detailed investigations on this effect, performed by means of XeCl excimer‐laser radiation and the example of LiNbO3, are reported in this letter. With this material, the ablation rate at constant fluence saturates at laser beam spot sizes of 2w≳80 μm and increases by about a factor of 3 when 2w is decreased to 24 μm. The effect is interpreted as being due to changes in the material transport which depend on the size of the region being processed.

Atom–surface scattering dynamics at hyperthermal energies

Abstract: We report molecular beam scattering of hyperthermal Xe atoms over an energy range 1

Antiferromagnetism and oxygen deficiency in single-crystal La2CuO4- delta.

Abstract: Two single crystals of undoped La/sub 2/CuO/sub 4/ are studied by neutron diffraction between T = 5 and 300 K. In one crystal oxygen vacancies are created by heat treatment and the magnetic properties are found to depend strongly on this treatment. While the untreated sample is found to order antiferromagnetically at T/sub N/approx. =50 K, the heat-treated crystal orders at T/sub N/approx. =150 K with a somewhat larger moment. The magnetic structure is the same for the two crystals and supports previously reported powder results.