Uniaxial Pressure Control of Superconductivity via Stripe Instability in La1.64Eu0.2Sr0.16CuO4 Crystals

Uni-axial pressure dependence of the superconducting critical temperature (T _c) with a static stripe instability was examined in La_1.64Eu_0.2Sr_0.16CuO₄ single crystals. We find anisotropy in the dependence of T _c on uni-axial pressure applied along the tetragonal [001], [100], and [110] directions. The behavior of dT _c/dP _[001] and dT _c/dP _[100] is essentially the same as that in the dynamical stripe phase of Eu-free La_1.84Sr_0.16CuO₄. In contrast, novel behavior observed in dT _c/dP _[110] in the present material is attributed to the weakening of static stripe order. Our result points toward a promising new way to control the superconducting state with uni-axial pressures via a coupling of the stripe instability to the underlying lattice symmetry.     

Measurement and theory of the orientation dependence of Knoop microhardness in single-crystal mercuric iodide

Measurements were made of the orientation dependence of the Knoop microhardness H _K on the (001) and (110) faces of single crystals of red (tetragonal) mercuric iodide that were vapour-grown for use in radiation detectors. The (001) faces of the crystals are softest when the major diagonal of the Knoop indentor (called here the indentor) is parallel to the [100] crystallographic axis, and H _K increases monotonically, by about 25%, as the indentor is rotated from the [100] to the [110] axis. The (110) surfaces are hardest when the indentor is parallel to [001]; H _K decreases by about 50% as the indentor is rotated from [001] to [1¯10]; the experimental data indicate an intermediate microhardness minimum that occurs before the [1¯10] orientation is reached. Particularly interesting surface topography, including bands of slip lines, is observed in the vicinity of indentations on the (110) planes, which apparently have not previously been characterized by Knoop microhardness indentation. Theoretically, the size of a microhardness indention is presumed to depend on the volume of material in which appropriate slip systems are stressed sufficiently to cause appreciable slip. To test this concept and determine which particular slip systems dominate the indention process, the infinite flat punch model was used to calculate the orientational and volumetric variations of shear stress on various potential slip systems in mercuric iodide. For indention processes controlled by movement (i.e. slip) of material in the [001] direction, over {100} planes, these calculations predict the following (experimentally observed) results: (a) on the (001) plane, H _K is smallest at [001] and greatest at [110], with no intermediate extremum; (b) on the (110) plane, H _K has its greatest value at [001] and a minimum between [001] and [1¯10]; (c) H _K at [110] on the (001) plane is essentially the same as H _K at [1¯10] on the (110) plane; and (d) the relative variation of H _K is greater on the (110) than on the (001) surface. Finally, the expected orientational variation of H _K on the (100) and (101) surfaces was determined theoretically.     

Fermi surface deformation parameters and cyclotron effective masses in white tin

Oscillatory magnetostriction and the de Haas-van Alphen effect have been used to determine Fermi surface deformation parameters for cross sections lying normal to[001] in white tin. The temperature dependence of the amplitudes has been used to calculate effective masses for orbits lying normal to[001], [100], and[110].     

Piezomodulation of excitonic reflectance spectra in CuCl

We have investigated the piezoreflectance spectra of the 1s Z₃ and Z₁₂ excitons in single crystals of copper chloride CuCl at 95 K with linearly polarized light. The spectra were studied for the applied low pressure p (p lower than 1 MPa) directed along the [001], [111] and [110] axis with the wave vector k of the incident light parallel to the [110] direction. A strong piezo-optical response is observed for the Z₃ exciton to regard to the one observed for Z₁₂. From the stress-induced shifts and splitting of the excitonic levels, we have been able to deduce the shear excitonic deformation potentials: b = (–0.18 ± 0.02) eV and d = (0.30 ± 0.03) eV.     

Simulation of Fatigue Crack Propagation in Ductile Metals by Blunting and Re-sharpening

Laird and Smith [(1962). Philosophical Magazine 8, 847–857] proposed a plastic sliding-off mechanism for the stage II fatigue crack growth via striation formation. In their view, the fatigue crack extension results solely from the changing character of deformation at the crack tip during loading and unloading. In particular, the crack tip blunts during the loading stage and folds into a double notch during the unloading stage, resulting in striation formation. In order to verify Laird’s plastic blunting mechanism for ductile polycrystals as well as for ductile fcc single crystals, FE calculations were performed for a rectangular plate with an initially sharp crack under plane strain conditions. The plate was subjected to a fully reversed tension-to-pressure cyclic load perpendicular to the crack plane (Mode 1). In the single crystal case the crack propagation simulations were carried out for cracks with crack plane (001) for two different crack growth orientations [110] and [100]. No initial radius for the crack tip was assumed. The actual shape of the crack tip followed from an initially sharp crack by repeated remeshing. To model the constitutive behavior typical for polycrystalline ductile metals, J₂ hypo-elasto-plasticity model with Armstrong–Frederick kinematic hardening was used. To model the constitutive behavior typical for ductile fcc single crystals, a geometrically nonlinear version of Cailletaud’s model based on the multiplicative elasto-plastic decomposition of the deformation gradient was implemented into the FE program ABAQUS. For simplicity, only octahedral slip systems were considered. Using repeated remeshing for severely distorted elements at the advancing crack tip, deformation patterns in the sense of Laird’s mechanism for fatigue crack propagation with striation formation were obtained in the case of the polycrystal simulation as well as in the case of the single crystal simulation for [110] crack growth direction. The simulation for [100] crack growth direction with the same stress level as for [110] direction also yielded crack extension by progressive large deformations but without striation formation. The dependence of the fatigue striation formation on the crack growth direction as predicted by the simulation of crack propagation in single crystals is verified by the experimental results of Neumann [(1974). Acta Metallurgica 22, 1155–1165] on pure copper single crystals.     

Electric resistance and magnetoresistance of 30-nm-Thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films elastically compressed in the (110) plane

We have studied the structure, electric resistance, and magnetoresistance of 30-nm-thick (110)La_0.67Ca_0.33MnO₃ (LCMO) films grown by laser deposition on (001)-oriented LaAlO₃ substrates. The unit cell parameters a and b (along the [100] and [010]LCMO axes, respectively) of these manganite films are significantly (by ∼1.2%) increased as compared to the corresponding values in the pseudocubic unit cell of bulk stoichiometric LCMO crystals. At T < 150 K, the temperature dependence of the resistivity of LCMO films is well described by the relation ρ = ρ₁ + ρ₂ (H) T ^4.5. The value of ρ 2 decreases with increasing magnetic field and is close to the analogous coefficient for manganite films grown on substrates with small lattice misfit.     

Load and directional effects on microhardness and estimation of toughness and brittleness for flux-grown LaBO3 crystals

Results of microhardness measurements on (100) and (110) planes of flux-grown LaBO₃ crystals, in the applied load range of 10–100g, are presented. The microhardness was found to decrease with increasing load in a non-linear manner. By applying Hays and Kendall's law, the materials resistance pressure and other constants of the equation could be calculated. Hardness anisotropy, showing periodic variation of H _v with the maxima and minima repeating at every 15° change in orientation of the indentor, is described and discussed. H _max/H_min are estimated as 1.14 and 1.06 for (100) and (110) planes, respectively. The fracture toughness values, K _c, determined from measurements of crack lengths, are estimated to be 1.6, 1.7 MN m^–3/2 (for (100) planes) and 1.2, 1.5 MN m^–3/2 (for (110) planes) at 90 and 100g loads, respectively. The brittleness index, B _i, is estimated as 4.6, 4.0 m^–1/2 (for (100) planes) 6.0, 4.6 m^–1/2 (for (110) planes) at 90 and 100g, loads respectively.     

Growth kinetics of spherulitic apatite in some MgO-CaO-SiO2-P2O5 glasses

Crystallization of glasses with compositions (wt%) of 11.2 MgO, 33.3 SiO₂, (55.5–x) CaO, and xP₂O₅ (x=18.3, 16.65, 15.825 and 15.0) resulted in a spherulitic apatite phase with different crystal morphologies. An ellipsoidal morphology was observed for x=18.3, 16.65 and 15.825, and an anomalous morphology was observed for x=15.0. A metastable phase, which was similar in some characteristics to apatite, was also found for x=15.0. The growth kinetics of the spherulitic apatite crystals were investigated to explain the above observations. Both the dendrite arms along the [0001] and [1 1¯20] directions of the apatite crystals showed constant growth rates in each glass. Growth-rate anisotropy was found between these two directions. The ellipsoidal shape of the apatite crystals is explained by this growth-rate anisotropy. The growth rates, and the growth-rate anisotropy, varied with the P₂O₅ content in such a manner that the changes in phase formation behaviour can be explained on the basis of the kinetic results.     

Dielectric properties of Be(IO<Subscript>3</Subscript>)<Subscript>2</Subscript>⋅4H<Subscript>2</Subscript>O crystals

The article studies the dielectric properties, dc conductivity and ac conductivity of Be(IO₃)₂⋅4H₂O single crystals. The dielectric constant ε has been defined for the three directions of the vectors a, b and c in the crystals in the temperature interval 280–340 K and frequency range 100 Hz–10⁶ Hz. The crystals show strongly expressed anisotropy, at 20 ^∘C and frequency 100 Hz ε_a = 235, ε_b = 30 and ε_c = 85. The frequency dependence of ε is evidence of the presence of low-frequency relaxation polarization in the crystals. The activation energies of the three directions in the crystals have been derived from the temperature dependence of dc conductivity, and they are 1.03 eV, 0.836 eV and 1.2 eV respectively.     

Hardness anisotropy in niobium carbide

Measurements of hardness anisotropy by Knoop diamond indentation on the {100} surfaces of Nb₆C₅ crystals show that the hardness is determined by crystallographic slip on {111} 〈1¯10〉 and {110} 〈1¯10〉 systems. {111} is the preferred slip plane for Nb₆C₅ and crystals with higher carbon content which show a marked decrease in Knoop hardness. The carbon atom/vacancy arrangement in these crystals is shown, by electron diffraction, to possess short-range order. Crystals annealed at low temperatures contain domains of non-cubic long-range order which increase the Knoop hardness and eliminate the anisotropy in hardness. Dislocation arrangements around Knoop indentations have been directly observed by electron microscopy in an attempt to confirm the slip processes deduced from hardness anisotropy.     

Microstructure of Epitaxial La0.7Ca0.3MnO3 Thin Films Deposited by Direct Current Magnetron Sputtering on LaAlO3 Substrate

Transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) have been used to study the microstructural properties of La_0.7Ca_0.3MnO₃ films on (001) LaAlO₃ substrates prepared by direct current magnetron sputtering technique. The as-grown thin films with different thickness are perfectly coherent with the substrates. The film suffers a tetragonal deformation in the area near the interface between the film and the substrate. With increasing thickness, the film is partially relaxed. It was found that La_0.7Ca_0.3MnO₃ films consist of two types of oriented domains described as: (1) (110)_f [001]_f||(001)_s[100]_s and (1¹10)_f [001]_f||(001)_s[100]_s and (2) (110)_f [001]_f||(001)_s[010]_s and (1¹10)_f [001]_f//(001)_s[010]_s. Upon annealing, the film is relaxed by the formation of mis¯t dislocations. Other than mis¯t dislocations, two types of threading dislocations with Burgers vector of <100> and <110> were also identified.     

Orientation dependence of electromechanical properties of relaxor based ferroelectric single crystals

The orientation dependence of electromechanical properties of relaxor based ferroelectric single crystals Pb(Zn_1/3Nb_2/3)O₃–(6–7)%PbTiO₃ and Pb(Mg_1/3Nb_2/3)O₃–33%PbTiO₃ has been calculated by coordinate transformation. Different from previous studies, the optimum cutting orientations have been predicted in terms of their piezoelectric responses in the corresponding crystal planes. The calculation results indicated that the anisotropic piezoelectric effects of [001] _c and [011] _c poled multi-domain crystals mainly come from the intrinsic contribution. However, the strong dielectric anisotropy of [001] _c poled multi-domain crystals mainly comes from extrinsic domain and domain wall contributions. For [011] _c poled multi-domain crystals, the intrinsic orientation effect enhances the dielectric anisotropy.     

H c2 anisotropy in the system niobium-nitrogen

Systematic experiments on the impurity and temperature dependence of the H _c2anisotropy in niobium are reported. A combined evaluation of these two dependences within the framework of a recent nonlocal theory is shown to permit a separation of different sources contributing to the H _c2anisotropy. Whereas the l = 4 component of the anisotropy is mainly due to band-structure anisotropies of the normal state, the l = 6 component in pure niobium originates predominantly from anisotropies of the energy gap. The rms anisotropy of the energy gap in niobium is estimated to be 0.08. The anisotropy of the flux line lattice predicted on the basis of the microscopic anisotropy parameters as deduced from the H _c2measurements is in agreement with experimental observations.     

The microstructure and fracture properties of MgO crystals containing a dispersed phase

MgO crystals containing up to 40% by volume of magnesioferrite and up to 2% by volume of iron and nickel were produced by a diffusion technique followed by appropriate heat treatments. Magnesioferrite precipitate did not significantly change the effective surface energy of a crack as measured by the double cantilever beam technique. Iron and nickel precipitate was produced in the form of platelets lying on {100}_MgO planes whose orientation relationships were [001]_MgO ∥[001]_Fe, [110]MgO ∥[100]_Fe with a spread of +10°, approximately, and [001]_MgO ∥[001]_Ni, [010]_MgO ∥[010]_Ni with negligible spread. Despite the crystallographic orientation relationships, the metal-MgO interface appeared to be very weak; the reasons for this are discussed. The effect of the metal precipitate on crack propagation was to markedly increase the density of cleavage steps. For a volume fraction of precipitate of 0.02, this led to a small increase in the effective surface energy, on the order of 1 Jm⁻².     

Vickers microindentation hardness studies of β-Sn single crystals

The Vickers microindentation hardness anisotropy profile and load dependence of apparent hardness of white tin (β-Sn) single crystals having different growth directions were investigated. Indentation experiments were carried out on the (001) crystallographic plane at indentation test loads ranging from 10 to 50 mN. Examinations reveal that the degree of the hardness anisotropy decreases with increasing indentation test load. Also, the materials examined exhibit significant peak load dependence (i.e., indentation size effect (ISE)). The traditional Meyer's law, proportional specimen resistance (PSR) model and modified PSR (MPSR) model, were used to analyze the load dependence of the hardness. While Meyer's law can not provide any useful information about the observed ISE, the load-independent hardness (i.e., H_PSR and H_MPSR) values can be estimated for different crystallographic directions, using the PSR and MPSR models. Briefly, for microindentation hardness determinations of β-Sn single crystals, the MPSR model is found to be more effective than the PSR model.     

HRTEM observation of the monoclinic-to-tetragonal m-t) phase transition in nanocrystalline ZrO2

The orientation relations m(100) || t(001), m[001] || t[110]; m(011) || t(100), m[100] || t[001]; m(100) || t(110), m[001] || t[001]; m(013) || t(116), m[001] || t[001] (indices for the primitive tetragonal cell) have been found between the tetragonal (t) and monoclinic (m) domains during the electron irradiation-induced m-t phase transition observed in-situ with HREM within isolated zirconia nanoparticles. Geometric models of the m-t interfaces are proposed.     

Evidence for Two Distinct Anisotropies in the Oxypnictide Superconductors SmFeAsO0.8F0.2 and NdFeAsO0.8F0.2

Single crystals of the oxypnictide superconductors SmFeAsO_0.8F_0.2 and NdFeAsO_0.8F_0.2 with T _c in the range of 44 to 48 K were investigated by torque magnetometry. An analysis of the data in terms of a recently proposed model for the anisotropic magnetization in the superconducting state, treating the magnetic penetration depth anisotropy γ _λ differently than the upper critical field anisotropy γ _H , provides evidence that in the oxypnictide superconductors two distinct anisotropies are present. As a result γ _λ differs significantly in magnitude and in temperature dependence from γ _H , analogous to MgB₂ but with a reversed sign of slope. This scenario strongly suggests a new multi-band mechanism in the novel class of oxypnictide high-temperature superconductors.      

Microhardness and brittle fracture of garnet single crystals

Hardness and fracture toughness were measured using the Vickers microhardness test in the low load range from 25 to 100 g near to the fracture threshold for near-perfect single crystals of garnets. The influence of crystal growth parameters, calcium impurity content and crystallographic orientation of Gd₃Ga₅O₁₂ (GGG) and Ca₃Ga₂Ge₃O₁₂ (CaGeGG) samples was investigated. Fracture starts with radial cracking from indent corners followed by lateral fracture of two distinct modes. The mean hardness of [111] oriented GGG isH=13 GN m^–2, for [111] oriented CaGeGG it is 12 GN m^–2, the average fracture toughness beingK _c=1.2 and 0.8 MN m^–3/2, respectively for the two crystals. Impurity doping slightly increases the strength of the material. Among the investigated crystals (111) faces are the least strong, the (100) face has maximumH andK _c values for CaGeGG. The constraint factor,, and yield stress,Y, were deduced from the measured hardness data, giving=2.2 andY about 7 GN m^–2.     

Point-Contact Spectroscopy of the Borocarbide Superconductor YNi2B2C in the Normal and Superconducting State

Point-contact (PC) spectroscopy measurements of YNi₂B₂C single crystals in the normal and superconducting (SC) state (T _c ≃ 15.4 K) for the main crystallographic directions are reported. The PC study reveals the electron–phonon interaction (EPI) spectral function with dominant phonon maximum around 12 meV and further weak structures (hump or kink) at higher energy at about 50 meV. No “soft” modes below 12 meV are resolved in the normal state. The PC EPI spectra are qualitatively similar for the different directions. Contrary, directional study of the SC gap results in Δ_{[100] ≈ 1.5 meV for the a direction and Δ_{[001] ≈ 2.3 meV along the c axis; however the critical temperature T _c in PC in all cases is near to that in the bulk sample. The value 2Δ_[001]/k _B T _c ≈ 3.6 is close to the BCS value of 3.52, and the temperature dependence Δ_[001](T) is BCS-like, while the for small gap Δ_[100](T) is below BCS behavior at T > T _c /2 similarly as in the two-gap superconductor MgB₂. It is supposed that the directional variation Δ can be attributed to a multiband nature of the SC state in YNi₂B₂C.