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.      

Activation energy for anion disordering in LnF<Subscript>3</Subscript> (Ln = La,Ce, Pr) superionic crystals

The energetics of lattice disordering in crystals with the LaF₃ structure has been studied by Raman spectroscopy. Analysis of the width of Raman lines indicates that, for T < T _c (in the dielectric phase of LaF₃), the activation energy for anion disordering is E _d ≃ 0.20 eV. This value agrees well with the 0.22 eV obtained as the difference between the energies of a fluoride ion in lattice and interstitial sites in a finite crystalline LaF₃ cluster.     

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.     

Microwave Surface Impedance and Complex Conductivity of High-T c Single Crystals: Current State and Unsolved Problems

Common and distinctive features of the temperature dependences of microwave surface impedance Z(T)=R(T)+iX(T) and conductivity σ(T) in the ab-plane and along the c-axis of high-T _c single crystals (HTSC) are discussed. The main attention is focused on an evolution of these dependences in YBaCuO crystal with the oxygen deficiency. Comparison of YBaCuO with other HTSC crystals reveals a number of peculiarities, namely, the linear dependence R_ab(T) ∞ T up to T _c/2 in HTSC single crystals with tetragonal lattice and up to T _c/3 in YBaCuO where R _ab (T) shows also a broad peak at T∼ T_c/2; breakdown of normal skin effect in some HTSC crystals; dramatic effect of pseudogap on the superfluid density in the heavily underdoped YBCO; several orders of magnitude higher residual surface resistance in HTSC when compared to conventional superconductors; etc. Possible explanations are discussed in the context of the specific features of HTSC structure and in the framework of recent models of quasiparticles’ c-transport and pseudogap state in HTSC.     

Orientation and temperature dependence of superelasticity caused by reversible γ-α′ martensitic transformations in FeNiCoAlTa single crystals

Reversible thermoelastic γ-α′ martensite transformations (MTs) have been studied in Fe-28%Ni-17%Co-11.5%Al-2.5%Ta (at. %) single crystals under cooling/heating and loading conditions. It is established that the precipitation of dispersed γ′ phase particles with an average size of d ≤ 5 nm leads to thermoelastic γ-α′ MTs with a small temperature hysteresis (ΔT = 20 K). The [001] oriented crystals favor the attaining of maximum superelasticity (SE) ɛ_SE = 6.8%, minimum mechanical hysteresis Δσ = 130 MPa, and large SE temperature interval ΔT _SE = 130 K. In contrast, in the [111] oriented crystals, small values of ɛ_SE = 2.0%, high values of Δσ = 350–400 MPa and narrow SE interval ΔT _SE = 55 K are found.     

Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveT _c on a series of polycrystalline samples of YBa₂Cu₃O_7−y (YBCO) reveal a number of regularities. The size of ΔC(T _c )/T _c for the samples [ΔC(T _c ) is the jump in the specific heat atT _c ] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andT _c induced by the application of a 7 ^Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveT _c appears to be essentially independent of ΔC(T _c )/T _c . These results are consistent with the idea that samples of YBCO belowT _c are a mixture of superconducting and normal phases. Supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Dept. of Energy under Contract DE-AC03-76SF00098, and by an EXXON Education Grant from the Research Corporation.     

Vortex State Microwave Resistivity in Tl-2212 Thin Films

We present measurements of the field induced changes in the 47 GHz complex resistivity, Δρ~(H, T), in Tl₂Ba₂CaCu₂O_8+x (TBCCO) thin films with T _c ≃ 105 K, prepared on CeO₂ buffered sapphire substrates. At low fields (μ₀ H < 10 mT) a very small irreversible feature is present, suggesting a little role of intergranular phenomena. Above that level Δρ~(H, T exhibits a superlinear dependence with the field, as opposed to the expected (at high frequencies) quasilinear behaviour. We observe a crossover between predominantly imaginary to predominantly real (dissipative) response with increasing temperature and/or field. In addition, we find the clear scaling property Δρ~(H, T = Δρ~[H/H ^* (T)], where the scaling field H ^∗ (T) maps closely the melting field measured in single crystals. We discuss our microwave results in terms of loss of flux lines rigidity.     

Specific heat and enthalpy of lattice disordering of LaF3 superionic crystals

Specific heat (C _p ) data obtained near the phase transition of LaF₃ are used to estimate the activation enthalpy for anion disordering, H _d. At the critical point T _c = 263 K, the concentration of disordered fluoride ions is n _c = 2.86 × 10²⁰ g⁻¹. In the dielectric phase of LaF₃ (T < T _c), H _d is 0.24–0.26 eV. In the superionic phase (T ≥ T _c), where the concentration of disordered fluoride ions exceeds n _c, the enthalpy of disordering drops to H _d ≃ 0.04 eV.     

Superconductivity in ZrxNb1−2xMox alloys: Possible dominance of a soft phonon mode

Normal-state specific heat and superconducting transition temperature measurements were performed on pure polycrystalline Nb and isoelectronic bcc Zr_xNb_1−2xMo_x alloys (x = 0.02, 0.05, and 0.10). Measured T _c, γ and θ _D all decline nearly linearly from the pure Nb values with increasing x. The changes are 15, 10, and 6%, respectively, for x = 0.1. T _cH calculated by Hopfield's theory, using data and linearly interpolated values of η and θ _D /〈θ²〉_linear ^1/2, increases with x by 28% for x = 0.1. By relaxing the linear interpolation of θ_D/〈θ²〉^1/2, Hopfield's theory can be used to evaluate an average phonon frequency 〈θ²〉_emp ^1/2 from data. It is found to be nearly constant at about 229 K, in contrast to 〈θ²〉_linear ^1/2, which declines by 8% at x = 0.1. We suggest that one possible explanation for this behavior is the dominance of incipient soft-phonon longitudinal modes which occur in Nb and presumably in the alloys. Miedema's method has been used to calculate the variation of both γ and T _c with alloying. The calculated γ is nearly constant, while the calculated T _c declines more quickly than the measured variation. The band-structure electron density of states calculated with the aid of McMillan's equation for these isoelectronic alloys is found to decline by 8% at x = 0.1, in contrast to the prediction of the rigid band model. Normal-state low-temperature specific heat measurements on a pure annealed polycrystalline sample of Nb yield γ = 7.80 ± 0.02 mJ mole⁻¹ K⁻² and θ_D = 276 ± 2 K, in excellent agreement with previous measurements. The break in slope of C _p/T vs. T ² at 3.2 K is discussed. Work supported in part by a National Science Foundation Departmental Science Development Grant No. NSF-GU2603.     

Dielectric anomalies in monoclinic TlInS<Subscript>2</Subscript> polytypes

Comparison of room-temperature x-ray diffraction data and temperature-dependent (190–220 K) permittivity data for different polytypes of monoclinic TlInS₂ demonstrates that the anomalies in (T) at the phase transition correlate with the c cell parameter of the polytypes. The (T) curve of single-phase crystals with c 15 Å shows a single, sharp peak at about 205 K. The peaks in the of the polytypes with c 30 and 60 Å occur at other temperatures, indicating that permittivity data near the phase transition can be used to identify the polytypes present in TlInS₂ crystals.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1423–1426.Original Russian Text Copyright © 2004 by Alekperov, Nadjafov     

Pb5Ge3O11 ferroelectric films prepared by a modified sol—gel method

Single-crystal Pb₅Ge₃O₁₁ films 5–105 μm thick, adhering well to platinum substrates, were prepared by pyrolysis of metalorganic precursors in combination with in situ sol-gel processing. The best films consisted of densely packed crystallites ranging in size from 200 to 300 μm, with hexagonal habits. A welldefined ferroelectric transition was revealed at 170–180°C. The typical parameters of the films are ɛ₂₀ = 30–40, tan δ ≃0.02, ɛ_max ≃200, P_s = 3.2 μC/cm²,E _c = 16 kV/cm, and ρ = 10⁸-10⁹ Ω cm, in agreement (except for ɛ_max) with those of lead germanate crystals     

Subcritical crack growth,surface energy,fracture toughness,stick-slip and embrittlement

It is proposed that the difficulties encountered with the meaning of subcritical crack growth arose from a misunderstanding of the Griffith equation. This equation is G=2γ for an equilibrium crack (stable or unstable) where γ is the intrinsic surface energy. When G>2γ the crack has a velocity v depending on the crack extension force G−2γ, even in a vacuum, and the following equation, well verified for adherence of elastomers, G−2γ=2γφ _T(v) where φ _T(v) is related to viscoelastic losses or internal friction at the crack tip, is generalized to other materials. At a critical speed v _c, dφ/dv becomes negative; as a negative branch cannot be observed the velocity jumps to high values on a second positive branch, so that G=G _c is a criterion for crack speed discontinuity, not the Griffith criterion. The multiplicative factor 2γ on the right-hand side accounts for the shift of the v-K curves with environment. No stress corrosion is needed to explain subcritical crack growth. Subcritical crack growth in glasses and ceramics and velocity jump in brittle polymers are shown to agree with this proposal. This model can also explain stick-slip motion when a mean velocity is imposed in the negative branch. Occurrence of velocity jump or stick-slip depends on the geometry tested and the stiffness of the apparatus. A second kind of stick-slip associated with cavitation in liquid-filled cracks is discussed. When the surrounding medium can reach the crack tip and reduce the surface energy, even at the critical speed v _c, the critical strain energy release rate G _c is reduced in the same proportion as γ, and a loading which would have given subcritical growth will give a catastrophic failure. Reduction of surface energy in the Rehbinder effect and in embrittlement by segregation is discussed. Finally, the evolution of ideas concerning the Irwin-Orowan formula and fracture toughness is examined.     

Revealing the Anisotropy in MgB<Subscript>2</Subscript> Superconductor and Investigation of Critical Current near Critical Temperature by Using Hall Probe Susceptibility Technique

We report the results of an investigation on critical current measurements in bulk MgB₂ superconductor by using Hall probe ac susceptibility. The temperature versus ac susceptibility has been measured as a function of external ac magnetic field with no dc part and magnitude in the range 240–1200 A/m by using the Hall probe method. The real part of the Hall probe susceptibility showed two-step transitions near critical temperature. We attributed this behavior to the anisotropic nature of the MgB₂ compound. Close to the critical temperature the estimated anisotropy parameter is γ≈2.5, a value that is in fair agreement to the ones reported in the literature for polycrystalline and single crystals MgB₂. We have also analyzed the susceptibility data within the framework of a critical state model (using the real part of the susceptibility) and an anisotropic nature of MgB₂ compound. The inter-grain critical current is found to be 4×10⁶ A/m² at 39.59 K by using the critical state model. The temperature dependence of the critical current density was found to be in the form of J _c (T)∝(1−T/T _c ) ^β , where β was calculated as 2.30 by using critical state model, respectively. Scanning electron microscopy was used for grain size estimation.     

Study of137m Ba double decay

An investigation has been made of higher-order effects in^137m Ba decay. Double decay of the isomeric state^137m Ba has been demonstrated with the emission of a γ photon and a conversion electron. The probability of this effect relative to that of ordinary conversion is T_eγ/T _c =(7.3±0.9)·10⁻⁵. The results of the measurements and calculations are discussed. Translated from Izmeritel'naya Tekhnika, No. 10, pp. 57–58, October, 1997.     

The development of forced convection heat transfer near a forward stagnation point with Newtonian heating

A mathematical model for the unsteady forced convection boundary-layer flow near a forward stagnation point is considered when there is Newtonian heating on the surface whereby the heat transfer is proportional to the local surface temperature. In a previous paper (Salleh et al. J Eng Math 69:101–110, 2011), a critical value γ _c, dependent on the Prandtl number σ, of the heat transfer coefficient γ was identified, with solutions for the corresponding steady problem possible only for γ < γ _c. The unsteady problem considered here shows that these steady states are attained at large times when γ < γ _c. For γ > γ _c, the solution still continues to large time, now growing exponentially with time. This rate of growth is determined by an eigenvalue problem which we solve numerically for general values of γ and σ and asymptotically for large γ and both large and small σ.     

An unusual morphology and crystallization behavior in in situ formed polyphenylene oxide/polyamide 6 blends

Novel polyphenylene oxide/polyamide 6 (PPO/PA6) blends were synthesized via in situ polymerization of ε-caprolactam with PPO dissolved in it. The introduction of 10 wt% PPO into PPO/PA6 led to phase inversion of the blends, which was nearly completed by incorporating 15 wt% PPO into the blends. A single crystallization temperature (T _c) of PA6 was detected for PPO/PA6 with 1–4 wt% PPO, while double T _c existed in the blends with 6–15 wt% PPO. After eliminating previous thermal history, PPO/PA6 containing no more than 6 wt% PPO gave a single melting point (T _m), but the blends with 10–15 wt% PPO exhibited double T _m. Increasing PPO content in PA6 resulted in the transformation of its crystal form from α-crystal to γ-crystal, which might be attributed to hindrance of crystallization of PA6 particles in PPO-rich phase.     

Vortex Phase Diagram as a Function of Oxygen Deficiency in Untwinned YBa2Cu3O y

Magnetization and resistivity measurements were performed in un-twinned YBa ₂ Cu ₃ O _y (YBCO) single crystals with different oxygen content. We find that the vortex phase diagram is a strong function of the oxygen deficiency; (1) For fully oxidized YBCO (y 7, T _c 87.5 K), the first-order vortex lattice melting line T _m(H) is observed up to 30 T. (2) For slightly overdoped YBCO (T _c 92 K), T _m(H) and the vortex glass transition line T _g(H) terminate at the multicritical point and the field-driven disordering transition line separates the vortex solid phase into the Bragg glass and the vortex glass phases. (3) For slightly underdoped YBCO (T _c 92.3 K), T _g (H) terminates well below the critical point of T _m(H), indicating the existence of the vortex slush regime by the introduced oxygen deficiency.     

Theoretical Study of Specific Heat,Density of States,Free Energy,and Critical Field of High Temperature Cuprate Superconductors Based on Intra and Interlayer Interactions

The role of attractive interlayer and intralayer interactions in layered high T _c cuprate superconductors have been investigated using a one-band two layer tight binding Hamiltonian. Self-consistent equations for the superconducting order parameter (Δ) and critical temperature (T _c ) are derived using double time Green’s functions and equation of motion method. The expression for excitonic type correlation (γ _c ), specific heat, density of states, free energy, and critical field are obtained. The interlayer interactions play an important role in the enhancement of T _c in layered high T _c cuprates. The oxygen isotope effect is also analyzed. The agreement between theoretical and experimental results for the system YBa_2−x La _x Cu₃O₇ (0≤x≤0.5) is quite satisfactory.      

Effect of oxygen content on the electrical properties of YBa2Cu3O7−x single crystals

The effect of oxygen content in the single crystals of high-temperature superconductor YBa₂Cu₃O_7−x on the electrical resistivity, the Hall effect in the plane perpendicular to thec axis and the energy gap Δ, measured with tunnelling electron microscope, has been studied. The distribution of the gap along the surface of the crystal was also studied. The results of the study on the relationship between the magnitude of the energy gap Δ and the superconducting transition temperatureT _c of single crystals with various oxygen contents are approximated by the linear dependence 2Δ_av=4·4kT _c .     

Preparation of epitaxial YBa2Cu3O7−x films by magnetron sputtering

Thin films of highT _c superconductor YBa₂Cu₃O_7−x were obtained by magnetron sputtering. MgO, YSZ, YSH and Al₂O₃ single crystals were used as substrates. Epitaxial films with tetragonal structure havingT _c 55–60 K grow at substrate temperaturesT _s between 930 K and 980 K. Orientation of the films in thisT _s range was (100) and (001) for (100) MgO substrate, (111) and (001) for (1012) Al₂O₃ and (111) YSH and (113) or (103) on (110) YSZ and (111) YSH. Single crystalline films with orthorhombic structure and (001) orientation were grown on all the substrates whenT _s exceeded 980 K. They haveT _c>80 K.