Anisotropy of Superconducting Single Crystal SmFeAsO0.8F0.2 Studied by Torque Magnetometry

Single crystals of the oxypnictide superconductor SmFeAsO_0.8F_0.2 with T _c≃45(1) K were investigated by torque magnetometry. The crystals of mass ≤0.1 μg were grown by a high-pressure cubic anvil technique. The use of a high-sensitive piezoresistive torque sensor made it possible to study the anisotropic magnetic properties of these tiny crystals. The anisotropy parameter γ was found to be field independent, but varies strongly with temperature ranging from γ≃8 at T≲T _c to γ≃23 at T≃0.4T _c. This unusual behavior of γ signals unconventional superconductivity.      

SmFeAsO<Subscript>0.8</Subscript>F<Subscript>0.2</Subscript> Wires Prepared Using Different Sheaths by the Powder-in-Tube Method

SmFeAsO_0.8F_0.2 wires with composite Cu/Nb or Ta sheaths have been fabricated using an in situ powder-in-tube method. Cu diffusion into the core was observed, which led to a fuzzy boundary between the sheath and core for Cu/Nb-sheathed SmFeAsO_0.8F_0.2 wires, and destroyed the superconducting phase of SmFeAsO_0.8F_0.2. However, superconductivity has been observed in the Ta-sheathed SmFeAsO_0.8F_0.2 wires with T _c=52.5 K. A severe weak-link effect depressed the development of global supercurrent. A peak effect with a strongly temperature-dependent peak field H _peak has been observed in the J _c–H curves over the range 10–40 K. The H–T phase diagram was similar to that of a previous report.     

Nano-crystalline powders and microwave dielectric properties of Zr0.8Sn0.2TiO4 ceramics derived using deep eutectic solvents

Choline chloride-malonic acid deep eutectic solvent (Cm-DES) was employed to synthesize Zr_0.8Sn_0.2TiO₄ nano-powders using Zr(CH₃COO)₄, SnCl₂?2H₂O, and Ti(OCH₃CH₃)₄ as raw materials. The Zr_0.8Sn_0.2TiO₄ nano-powder with the mean grain size of 10.7 nm was obtained at 450 °C, significantly lower than that by conventional mixed oxide routes. Meantime, the densification of Zr_0.8Sn_0.2TiO₄ ceramics was improved using the as-synthesized powder. The Zr_0.8Sn_0.2TiO₄ ceramics with relative density of 96.8?% were obtained at 1350 °C and it exhibited good microwave dielectric properties (ε_r?=?37.8, Qf?=?42,600 GHz, and τ_f =???4.1 ppm/°C). These results show that Cm-DES route is suitable for the synthesis of Zr_0.8Sn_0.2TiO₄ nano-powders, is beneficial to the low-temperature firing of Zr_0.8Sn_0.2TiO₄ ceramics, and helps reduce energy consumption and time costs.

     

Anisotropic Superconducting Properties of MgB2 and Related Compounds

Anisotropic superconducting properties of inter-metallic compounds MgB₂, Y₂PdGe₃, and CaAlSi with AlB₂ structure are studied by detailed angular dependent transport measurements. MgB₂ and CaAlSi shows appreciable anisotropy of the upper critical field, H _c2, with mass anisotropy parameter =3 and 2, respectively, while Y₂PdGe₃ is almost isotropic. In-plane anisotropy of H _c2 in these hexagonal superconductors is very small, consistent with the prediction based on GL theory.     

Preparation of spherical Zr0.8Sn0.2TiO4 powder by ultrasonic spray pyrolysis

Fine, spherical Zr_0.8Sn_0.2Tio₄ powders were prepared by spray pyrolysis, using an ultrasonic transducer, from an aqueous solution of metal chlorides. The synthesized powders had a spherical morphology and many shell fragments. The observed shell fragments were attributed to the impermeable surface crust formed during thermal decomposition of the droplets. The raw materials for the preparation of Zr_0.8Sn_0.2TiO₄ were analysed prior to any treatments because the properties of the surface crust could be related to those of the raw materials. ZrOCl₂·8H₂O was supposed to affect the shell fragments as well. Spherical Zr_0.8Sn_0.2TiO₄ powders without shell fragments could be prepared using ZrO(CH₃COO)₂ in place of ZrOCl₂·8H₂O.     

Electrical and Magnetic Behaviour of PrFeAsO_{\mathbf{0.8}}F_{\mathbf{0.2}} Superconductor

The superconducting and ground state samples of PrFeAsO_0.8F_0.2 and PrFeAsO have been synthesised via the easy and versatile single step solid state reaction route. X-ray and Reitveld refine parameters of the synthesised samples are in good agreement to the earlier reported value of the structure. The ground state of the pristine compound (PrFeAsO) exhibited a metallic-like step in resistivity below 150 K followed by another step at 12 K. The former is associated with the spin density wave (SDW)-like ordering of Fe spins and later to the anomalous magnetic ordering for Pr moments. Both the resistivity anomalies are absent in case of the superconducting PrFeAsO_0.8F_0.2 sample. Detailed high field (up to 12 Tesla) electrical and magnetization measurements are carried out for the superconducting PrFeAsO_0.8F_0.2 sample. The PrFeAsO_0.8F_0.2 exhibited superconducting onset ( $T_{c}^{\mathrm{onset}}$ ) at around 47 K with T _c (ρ=0) at 38 K. Though the $T_{c}^{\mathrm{onset}}$ remains nearly invariant, the T _c (ρ=0) is decreased with applied field, and the same is around 23 K under an applied field of 12 Tesla. The upper critical field (H _c2) is estimated from the Ginzburg–Landau equation (GL) fitting, which is found to be ～182 Tesla. Critical current density (J _c ), being calculated from high field isothermal magnetization (MH) loops with the help of Beans critical state model, is found to be of the order of 10³ A/cm². Summarily, the superconductivity characterization of the single step synthesised PrFeAsO_0.8F_0.2 superconductor is presented.     

X-ray Photoelectron Spectra of La0.8 – x Ce x Sr0.2MnO3

La_{0.8 – x} Ce _x Sr_0.2MnO₃ (x= 0–0.3) mixed-conducting manganites are studied by x-ray photoelectron spectroscopy. The effect of A-site Ce substitution on the surface composition of the samples, charge states of the constituent cations, and bonding configuration is analyzed. The binding energies of the constituent elements are determined. The spectral shapes of the O 1s, La 4d, La 3d, Sr 3d, Mn 2p, Ce 4d, and Ce 3d peaks are examined as a function of Ce content.     

Preparation and properties of Mg0.2Zn0.8O:Al UV transparent conducting thin films deposited by RF magnetron sputtering at room temperature with rapid annealing

Mg_0.2Zn_0.8O:Al UV transparent conducting thin films were deposited by RF magnetron sputtering at room temperature with a rapid annealing process. Effects of sputtering power, argon gas pressure and annealing temperature on structure, optical and electrical properties of Mg_0.2Zn_0.8O:Al films were investigated. The experimental results show that Mg_0.2Zn_0.8O:Al thin films exhibit high preferred c-axis-orientation. The sputtering power, argon gas pressure and annealing temperature all exert a strong influence on the electrical resistivity of Mg_0.2Zn_0.8O:Al thin films due to the variation of carrier concentration and mobility in films derived from the change of effective doping and crystallinity. The lowest electrical resistivity of Mg_0.2Zn_0.8O:Al thin films is 3.5 × 10⁻³ Ω·cm when the sputtering power is 200 W, the argon gas pressure is 2.0 Pa and the annealing temperature is above 500 °C. The transparent spectrum range of Mg_0.2Zn_0.8O:Al thin films extend to ultraviolet band and the optical transmittance is between 80 and 90%, but the sputtering power, argon gas pressure and annealing temperature all exert little influence on optical transmittance.     

Investigations on the Optical Spectra and EPR Parameters for Tetragonal Sm<Superscript>3+</Superscript> Center in SmBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript>

It is of interest to study the optical spectra and EPR parameter of the rare-earth ion in the typical high T _c superconductors. The proper interpretation for these values is helpful to understand the optical and magnetic characteristics of the superconductors. In this paper, we calculate the optical spectra and EPR parameters (g factors g _∥ ,g _⊥ and hyperfine structure constants A _∥ , A _⊥ ) of tetragonal Sm³⁺ center in YBa₂Cu₃O_7−δ from the perturbation formulas of EPR parameters for 4f⁵ ion in tetragonal symmetry. In these formulas, the crystal-field J-mixing of the first and second excited-state multiplets ⁶H_7/2 and ⁶H_9/2 into the ground state, ⁶H_5/2, the mixtures among the states with the same J value via spin-orbit coupling interaction and the interactions between the ground Kramers doublet Γ γ and the same irreducible representation as Γ γ in other 11 Kramers doublets Γ x within ⁶H _J (J=5/2, 7/2, 9/2) states via crystal field and orbital angular momentum (or hyperfine structure) are considered. The calculated results are in reasonable agreement with the observed values. The results are discussed.     

Possible Multigap Superconductivity in SmFeAsO0.8F0.2: A Point-contact Andreev-reflection Spectroscopy Study

We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in SmFeAsO_0.8F_0.2 state-of-the-art polycrystals, with critical temperature T _c≈53 K. The low-temperature conductance curves show clear peaks at about 7 meV and additional shoulders at 16–20 meV. Their shape is similar to that of PCAR spectra in MgB₂ and suggests the presence of two superconducting energy gaps even in this Fe-based superconductor. The fit of the conductance curves with a two-band BTK model, up to T _c, further supports this indication in spite of a marked asymmetry in the conductance curves for positive/negative bias. The gaps obtained from the fit are Δ₁=6.15±0.45 meV and Δ₂=18±3 meV, and they follow a nice BCS-like temperature dependence, closing both at the same T _c. Our results are discussed in comparison with experimental and theoretical results in this and other Fe-based superconductors.     

Induced of zeeman coherence in an ensemble of cesium atoms interacting with a two-frequency (microwave+rf) magnetic field

Interference between Zeeman states corresponding to the forbidden magnetic-dipole transition ΔF=0, Δm _F=2 is reported in connection with the simultaneous interaction of 6² S _1/2 cesium atoms with a resonant microwave field and an rf field which varies at twice the Larmor frequency and is directed perpendicular to a static magnetic field H ₀=0.2 Oe. Pis’ma Zh. Tekh. Fiz. 24, 89–93 (July 26, 1998)     

Processing of submicrometric CaTi0.8Fe0.2O3-δ ceramics by mechanical activation

Effects of mechanical activation of precursors in order to obtain CaTi_0.8Fe_0.2O_3-δ are reported. Mixtures of CaCO₃, TiO₂ and Fe₂O₃ were dry-grinded in Teflon or zirconia containers using planetary ball-mills at different rotations (200, 300 and 500 rpm) and for various periods of time. The perovskite is not formed just by milling and a subsequent heat treatment at 800–1,000 °C was necessary. This still represents a significant advantage over the classical ceramic route as a result of the improved reaction kinetics due to the smaller grain size (<100 nm) of the activated powders and a modification of the thermodynamic initial state. Dense ceramic samples showing a bimodal submicrometric grain-size distribution (100–200 and 250–500 nm) were obtained after sintering the activated powders at 1,150 °C TEM revealed homogeneous samples, free from inhomogeneities such as core-shell grains typically observed in ceramics obtained from non-activated precursors sintered at 1,320–1,350 °C.     

Oxygen permeability of Sr1-x Ce x Co0.2Fe0.8O3-δ ceramic membranes

Sr_1-x Ce _x Fe_0.8Co_0.2O_3-δ (x = 0, 0.05, 0.10, 0.15) ceramics were prepared by solid-state reactions, and their oxygen permeability was measured as a function of cerium content and membrane thickness at temperatures from 400 to 1000‡C and oxygen partial pressures from 10² to 10⁵ Pa. The oxygen permeability of the ceramics was found to decrease systematically with increasing Ce content. The oxygen transport was shown to be limited by volume diffusion. Doping of the ceramics with 20 wt % Ag notably improves their cracking resistance during thermal cycling.     

Microstructure and the influence of spontaneous strain in LaCoO3, La0.8Sr0.2CoO3 and La0.8Ca0.2CoO3

Transmission Electron Microscopy has been conducted on rhombohedral perovskite oxides with composition LaCoO₃, La_0.8Sr_0.2CoO₃, and La_0.8Ca_0.2CoO₃. Thin foils prepared within the first weeks of sintering showed macroscopic strain and a very high defect density, which included fault-related superlattice structures. Samples prepared several months after sintering, showed no significant macroscopic strain and the main defect was found to be domains due to reflection twinning in the pseudo-cube plane. The behaviour is likely to be related to accomodation of spontaneous strain arising upon cooling of the material from the sintering temperature. The presence of twin domains illustrates the lowering of lattice symmetry as compared to the ideal cubic perovskite structure, and is relevant for explaining recent observations of ferroelastic behaviour of these materials.     

Temperature dependence of thermal conductivity of vanadium substituted BPSCCO system between 10 and 150 K

Thermal conductivity of a set of (Bi_0.8Pb_0.2−y V _y )₂Sr₂Ca₂Cu₃O_10+δ (0 =y ≤ 0.05) pellets in the temperature range between 10 and 150 K is reported. Vanadium substitution influences strongly the magnitude of thermal conductivity (λ,) over the entire temperature range. But the nature of λ(T) dependence remains similar to that generally observed for HTSCs. The electronic contribution to the total λ in the normal state is estimated to be ∼ 25%. We have attempted to examine our data, assuming the role of both electrons and phonons in the origin of the λ(T) behaviour belowT _c. Observed temperature variation of λ(T) for the present set of samples could be explained very well assuming this electron + approach. Some of the microscopic quantities estimated from the best-fit parameters give reasonable values.     

The Activation Energy <Emphasis Type="Italic">U</Emphasis>(<Emphasis Type="Italic">T</Emphasis>,<Emphasis Type="Italic">H</Emphasis>) in Y-based Superconductors

Based on the Arrhenius equation, a method to calculate the activation energy from the resistance transition is proposed for high temperature superconductors. This method is applied to the Y-based superconductors. The activation energy is found to be U(T,H)∼(1−T/T _c )^4.8(H/H ₀)^−3.8 of YBCO crystal, and U(T,H)∼(1−T/T _c )^3.3(H/H ₀)^−2.2 of Er doped MTG YBCO crystal, respectively. With the obtained activation energy U(T,H), the lower part of the experimental curve ρ(T,H) and its derivative can be reproduced.      

Microstructure and NO decomposition behavior of sol-gel derived (La0.8Sr0.2)0.95MnO3/yttria-stabilized zirconia nanocomposite thin film

(La_0.8Sr_0.2)_0.95MnO₃ and (La_0.8Sr_0.2)_0.95MnO₃/YSZ gel films were deposited by a spin-coating technique on scandium-doped zirconia (ScSZ) substrate using the precursor solution prepared from La(Oi-C₃H₇)₃, Sr(Oi-C₃H₇)₂, Mn(Oi-C₃H₇)₂ and 2-methoxyethanol. By heat-treating the gel films, the membrane reactors, (La_0.8Sr_0.2)_0.95MnO₃|ScSZ|Pt and (La_0.8Sr_0.2)_0.95MnO₃/YSZ|ScSZ|Pt were fabricated. It was found that the pre-firing temperature affected the microstructure evolution of (La_0.8Sr_0.2)_0.95MnO₃ and (La_0.8Sr_0.2)_0.95MnO₃/YSZ thin films. Pre-firing at low temperature resulted in high porosity and large grain size of the thin films. NO decomposition characteristics of the obtained membrane reactors were investigated at 600 °C in reactant gas, 1000 ppm of NO and 2% of oxygen. By applying a direct current to the membrane reactors, NO can be decomposed at the (La_0.8Sr_0.2)_0.95MnO₃ and (La_0.8Sr_0.2)_0.95MnO₃/YSZ composite cathode. By incorporating YSZ into (La_0.8Sr_0.2)_0.95MnO₃, the required consuming power to decompose NO could be reduced.     

Superconducting State Parameters of 4d-Transition Metals Superconductors

The theoretical study of the superconducting state parameters (SSP) namely electron–phonon coupling strength λ, Coulomb pseudopotential μ ^∗, transition temperature T _C , isotope effect exponent α, and effective interaction strength N _O V of 4d-transition metals superconductors have been made extensively in the present work using a model potential formalism for the first time. In the present computation, the use of pseudo-alloy-atom model (PAA) is proposed and found successful. A considerable influence of various exchange and correlation functions on λ and μ ^∗ is found from the present study. The present results of the SSP are found in qualitative agreement with the available experimental data wherever exist.     

Tilted and crossing vortex chains in layered superconductors

In presence of the Josephson vortex lattice in layered superconductors, small c-axis magnetic field penetrates in the form of vortex chains. In general, structure of a single chain is determined by the ratio of the London [λ] and Josephson [λ _J ] lengths, α=λ/λ _J . The chain is composed of tilted vortices at large α's (tilted chain) and at small α's it consists of crossing array of Josephson vortices and pancake-vortex stacks (crossing chain). We study chain structures at the intermediate α's and found two types of phase transitions. For α≲0.6 the groud state is given by the crossing chain in a wide range of pancake separationsa≳[2−3]λ _J . However, due to attractive coupling between deformed pancake stacks, the equilibrium separation can not exceed some maximum value depending on the in-plane field and α. The first phase transition takes place with decreasing pancake-stack separation a ata≳[1−2]λ _J , and rather wide range of the ratio α, 0.4≲α≲0.65. With decreasing a, the crossing chain goes through intermediate strongly-deformed configurations and smoothly transforms into the tilted chain via the second-order phase transition. Another phase transition occurs at very small densities of pancake vortices,a ~ [20−30]λ _J , and only when α exceeds a certain critical value ∼0.5. In this case small c-axis field penetrates in the form of kinks. However, at very small concentration of kinks, the kinked chains are replaced with strongly deformed crossing chains via the first-order phase transition. This transition is accompanied by a very large jump in the pancake density.     

The Upper Critical Field <Emphasis Type="Italic">H</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis>2</Subscript> in Advanced Superconductors with Anisotropic Energy Spectrum

A brief review of works on the microscopic theory of determining the upper critical field in two-band isotropic and anisotropic superconductors is given. The research is based on a set of the Ginzburg–Landau equations for the order parameters in a magnetic field that are studied in terms of the classical approach to a superconducting system in a magnetic field. Two inequivalent energy bands with different topology of Fermi surface cavities overlapping on the Fermi surface are discussed. The cases of the direction of the external magnetic field [(H)\vec]//\vec{H}// the (ab) plane and [(H)\vec]//\vec{H}// the crystallographic c axis are studied. The equations for determining H _c2(ab) and H _c2(c) for a pure superconductor and a superconductor doped with electrons and holes are derived. The analytical solutions to these equations in the vicinity of the superconducting transition temperature (T _c −T≪T _c ) and in the vicinity of zero (T≪T _c ) are found. The temperature and impurity dependences of the upper critical fields H _c2(ab) and H _c2(c), as well as the anisotropy coefficient γ _H, are studied. The resulting theory is applied to determine the dependences of the above magnetic characteristics of intermetallic compound MgB₂. The theory agrees qualitatively with experimental data.