Effect of the Oxidation State of Pb on the Superconducting Transition Temperature of Bi2 – x Pb x Sr2CaCu2O8 + δ

A procedure is developed for determining the Pb⁴⁺ concentration in Pb-substituted Bi-based 2212 superconductors. The effect of the Pb⁴⁺ : Pb²⁺ ratio on the superconducting transition temperature of 2212 is studied. The results indicate that the highest T _c is offered by Bi_1.8Pb_0.2Sr₂CaCu₂O_{8 +} containing the smallest amounts of Pb and Cu in their higher oxidation states.     

Vortex Lattice Melting into Pinned Liquid State in La2−x Sr x CuO4

Magnetization measurements in La_2–x Sr _x CuO₄ (LSCO) crystals indicate vortex order-disorder transition manifested by a sharp kink in the second magnetization peak. The transition field exhibits unique temperature dependence, namely a strong decrease with temperature in the entire measured range. This behavior rules out the conventional interpretation of a disorder-driven transition into an entangled vortex solid phase. We argue that the transition in LSCO is driven by both thermally- and disorder-induced fluctuations. The resulting pinned liquid disordered state is characterized by large thermal fluctuations and irreversible magnetic behavior. We extend these results and postulate that melting, solid-solid, and solid to pinned liquid transitions, are special cases of a unified order-disorder phase transition driven by both thermally- and disorder-induced fluctuations. The temperature dependence of the transition line and the nature of the disordered phase (solid, liquid, or pinned liquid) are determined by the relative contributions of these fluctuations and by the pinning mechanism. By varying the pinning mechanism and the pinning strength one obtains a spectrum of transition lines. In order to test our numerical results we present studies on a series of irradiated LSCO crystals, and show that increase of pinning, causes a depression of the transition line as well as a decrease in the curvature as the transition line, in agreement with our calculations.     

Optical Properties of the Cu–O Plane in the Bi2Sr2−x La x CuO6 Family

The ab-plane optical conductivity of seven single crystals, belonging to the family Bi₂Sr_2?x La _x CuO₆, has been measured for hole concentrations per Cu site 0.03≤p≤0.18, and for 6 K≤T≤300 K (500 K for p=0.16). At low doping, ten phonon lines are detected, which are due to the removal of the degeneracy of five E _u modes (out of the predicted six). They are superimposed to a far-infrared band, which as doping increases, closes the insulating gap thus building up the Drude term. The insulator-to-metal transition occurs between p=0.7 and p=0.10 consistently with a Mott mechanism. In the metallic phase, a multiband analysis identifies a Drude term plus a mid-infrared band, which weakly depends on temperature and softens as p increases, like in other cuprates. The optical response of the crystal at optimum doping has been analyzed also in the superconducting phase. The Ferrel–Glover–Tinkham sum rule requires an integration up to Ω?6Δ and the penetration depth is 290 nm. The bosonic spectral function includes a strong peak around 50 meV, which survives up to 500 K and, therefore, might be assigned to an electron–phonon interaction.     

Electrical and microstructural properties of Bi2−x Sb x Te and Bi2−x Sb x Te2 foils obtained by the ultrarapid quenching process

This paper is concerned with the feasibility and reproducibility of the ultrarapid quenching process used to fabricate Bi_2–x Sb _x Te and Bi_2–x Sb _x Te₂ alloys for thermoelectric applications. Microstructural properties of the materials, obtained in the shape of foils, were studied concerning the phase analysis, cell parameters, texture, and microstructure observations. The Bi_2–x Sb _x Te alloys were found to have the (2 0 3) texture. The (2 0 4) texture, with an additional (1 1 0) component for x values greater than 0.4, was predominant for Bi_2–x Sb _x Te₂ foils. The electrical properties of these materials were then characterized by measuring the Seebeck coefficient, Hall coefficient, and electrical resistivity. It was found that Bi_2–x Sb _x Te foils changed from n- to p-type for an x-value of about 1.2. A maximum Seebeck coefficient, ||, of 36×10^–6 V K^–1 was measured for Bi₂Te. In the case of Bi_2–x Sb _x Te₂ foils, the change from n- to p-type was observed for an x value of about 1. A maximum Seebeck coefficient, ||, of 32×10^–6 V K^–1, was measured for Bi_1.4Sb_0.6Te₂. Measurements of the temperature-dependent electrical resistivity, Hall and Seebeck coefficients of the foils were carried out and the analysis revealed a semi-metallic behavior.     

The Nodal SDW Gap and the Superconducting Gap in?BaFe2?x Co x As2

The spin density wave (SDW) transition in BaFe₂As₂ and the superconducting transition in BaFe_1.84Co_0.16As₂ were investigated by Raman scattering. The symmetries of the nodal SDW gap at 400?cm^?1 and the superconducting gap at 75?cm^?1 are both?B _2g. The superconducting coherent peak energy is smaller than the gap energy of the hole pocket, indicating that the peak is the resonant peak in the S _?? superconductor. The superconducting symmetry is given by B _2g in the orbital combination and A _1g (S _??) in the momentum space. The exchange interaction energies are estimated from the two-magnon peak.     

The magnetic instability in the heavy fermion compounds Ce1−x La x Ru2Si2

The magnetization and the specific heat of Ce_1–x La _x Ru₂Si₂ with x0.13 are reported with special attention to the effect of magnetic field and the role of lanthanum doping. Evidence is given of differences between the undoped (x=0) and the solid solution (x0) cases. A common feature is the occurrence of well-defined anomalies in the magnetization at the metamagnetic fieldH _M independently of whether the ground state is one of long-range order or Pauli paramagnetism. Forx=0, the ground state appears to be a Pauli paramagnet for any strength of the magnetic field; quantum fluctuations or deviations from an ideal lattice may prevent the occurrence of a true static magnetic transition.Laboratoire associé à l'Université Joseph Fourier, Grenoble.     

Real Space Imaging of the Electronic States in Underdoped Ca2−x Na x CuO2Cl2 Single Crystals

Scanning tunneling microscopy (STM) has been performed on underdoped Ca_2–x Na _x CuO₂Cl₂ (Na-CCOC) single crystals to investigate the electronic states of doped Mott insulators near the metal-insulator transition. STM images taken at 7 K show patch-like or river-like irregular features superposed on the atomic corrugations. The irregular structure has a characteristic length scale of 20 Å, which is unchanged in different samples and at different doping levels studied (0.08<x<0.12). Bias voltage dependence of the STM image suggests that Na-CCOC consists of two distinct phases with different electronic states.     

Upper critical fields of the superconducting layered compounds Nb1−x Ta x Se2

The upper critical fieldsH _c2 (T) of the superconducting layered compounds Nb_1–x Ta _x Se₂ (0x0.20) have been measured with the magnetic field oriented either perpendicular or parallel to the layer plane. Just belowT _c , positive (upward) curvature is seen inH _c2 (T) for both field directions and for all compositions. The amount of positive curvature is not a function of crystal quality. At low temperatures,H _c2 (T) displays enhanced linearity as compared to the prediction of the isotropic theory, and shows no evidence of Pauli paramagnetic limiting for either field orientation. The critical field slopesdH _c2 /dT nearT _c change only slightly with composition.H _c2 (T) has been calculated for two Fermi surface models for NbSe₂ using the band structure calculated by Wexler and Woolley. TheseH _c2 (T) calculations indicate that Fermi surface anisotropy can explain the positive curvature and enhanced linearity seen in the experimental data. The calculations also show that theH _c2 (T) data are better explained with a Fermi surface that consists of open, undulating cylinders with an additional smaller closed piece, than with a Fermi surface model that consists only of the open, undulating cylinders of the original Wexler and Woolley model.     

Antiferromagnetic Ordering in the One-Dimensional Edge-Sharing CuO2 Chain System Ca2+x Y2−x Cu5O10

We have investigated the spin state of the one-dimensional edge-sharing chain system Ca_2+x Y_2–x Cu₅O₁₀ with 0x1.667 by means of the magnetic susceptibility and specific heat measurements, using the large-size single-crystals. It has been found that the magnetic ground state is an antiferromagnetically ordered state for 0x1.0, while a spin-gap state seems to be formed for x1.5. Accordingly, it appears that the quantum critical point is located between x=1.0 and 1.5 in Ca_2+x Y_2–x Cu₅O₁₀.     

Independent Control of Low-Energy Resonant States and Polaron States by the Zn-Doping and the Structural Transition in La2−x Sr x CuO4 and La2−x Ba x CuO4 (x=0.11)

The superconductivity depression mechanisms at x≈1/8 in La_2−x Ba _x CuO₄ and Zn substitution in La_2−x Sr _x CuO₄ were investigated by Raman scattering. About 80% of low-energy electronic states are two-dimensional at x≈1/8 and form the Fermi arc around (π/2,π/2). The low-energy states are composed of the resonant peak relating to the insulator–metal transition and the polaron states of the B _3u phonons. Zn substitution depresses the resonant peak, while the LTT structure depresses the polaron states. The superconductivity is suppressed if one of them is reduced.     

Influence of the cation sublattice defectness on the electronic thermoelectric power of Li x Cu(2−x)−δ (x ≤ 0.25)

This paper presents an experimental study of the electronic thermoelectric power as a function of copper content for Li _x Cu_(2?x)?δS (x ≤ 0.25) electronic-ionic mixed conductors. Using a vacancy model for lattice defects, we calculate their electronic thermoelectric power as a function of x. By adjusting the hole effective mass, we reach satisfactory agreement with experimental data.     

Systematic Evolution of the Magnetotransport Properties of Bi2Sr2−x La x CuO6 with Carrier Concentration

We report that it is possible to obtain a series of high-quality crystals of Bi ₂ Sr _2–x La _x CuO ₆ , of which the transport properties have been believed to be dirtier than those of other cuprates. In our crystals, the normal-state transport properties display behaviors which are in good accord with other cuprates; for example, in the underdoped region the in-plane resistivity _ab shows the pseudogap feature and in the overdoped region the T dependence of _ab changes to T ⁿ with n>1. The characteristic temperatures of the pseudo-gap deduced from the resistivity and the Hall coefficient data are presented.     

Theoretical Study on the Spin-State Transition of Electron-Doped Sr2−x La x CoO4 Compounds

The spin-state transition is an interesting and unresolved problem in layered perovskite Sr_2?x La _x CoO₄ under doping. The unrestricted Hartree-Fock approximation on a realistic multiband lattice model has been applied to study the various spin states of electron-doped Sr_2?x La _x CoO₄ compounds. In the doping range 0.0≤x≤0.9, various spin states in an enlarged double cell are investigated and four of them are the ground states at different doping concentration. The magnetic ground state of the doped system takes a low-spin–intermediate-spin ferromagnetically ordered state for x<0.26, a high-spin–intermediate-spin ferromagnetically ordered state for 0.26≤x<0.47, an intermediate-spin state for 0.47≤x<0.65, and followed by an high-spin antiferromagnetically ordered state for 0.65≤x≤0.9. The densities of states of all magnetic ground states are computed and their electronic and magnetic properties are discussed.     

Investigation of the lithium-rich boundary of the Li1+x Mn2−x O4 cubic spinel phase in air

Several compositions of the cubic spinel Li_1+x Mn_2?x O_4?δ phase in the lithium–manganese–oxygen (Li–Mn–O) system were synthesized at 700, 750, and 800 °C in air ( $ p_{{{\text{O}}_{2} }} $  = 0.2 atm) to investigate the Li-rich boundary of the cubic spinel phase at these temperatures. The lattice parameters of the several compositions were determined by Rietveld analysis of the measured X-ray patterns, and the Li and Mn contents of the samples were measured using inductively coupled plasma with optical emission spectroscopy (ICP-OES). A Vegard-like dependence of the measured lattice parameter of the cubic spinel phase with Li to Mn ratio exists in the homogeneity range of the cubic spinel. This dependence could be used to derive the boundary of the single phase cubic spinel field in the Li–Mn–O system at 700 and 750 °C at $ p_{{{\text{O}}_{2} }} $  = 0.2 atm and to estimate the Li-rich boundary at 800 °C. The results of the present study are compared with two other experimental studies on the homogeneity range of the cubic spinel phase in an attempt to resolve the contradiction between these two studies.     

NMR of 89Y in the Copper-Oxide Spin-Chain Compound Ca2+x Y2−x Cu5O10

We report NMR lineshape, spin-lattice relaxation time T ₁, and spin-spin relaxation time T ₂ data at 17 MHz (8.07 T) for ⁸⁹Y in the copper-oxide spin-chain compound Ca_2+x Y_2–x Cu₅O₁₀. For x=0, a broad, asymmetric line with width 90 kHz is observed for T=250–300 K. The spectra exhibit an appreciable average shift (H/H+0.7%) and sharpen at lower temperature, possibly due to increasing intrachain ferromagnetic correlations. T ₁ and T ₂ decrease with decreasing temperature. The Tl data imply a short correlation-time limit, with _e=3–5×10^–11 s. The T ₂ data apparently include a contribution from dipolar interactions with copper nuclei. Relaxation time data for a doped (x=0.5) compound surprisingly show more rapid relaxation.     

In-Plane Anisotropy of the Upper Critical Field of La2−x Sr x CuO4 in the Underdoped and Overdoped Regimes

For La_2–x Sr _x CuO₄ single-crystals in the underdoped and overdoped regimes, we have measured the electrical resistivity along the c-axis, _c , under constant magnetic fields, changing the field direction in the ab-plane. At x=0.08, 0.12 and 0.16 in the underdoped and almost optimally-doped regimes, clear anisotropy of _c with the fourfold symmetry in the ab-plane has been observed. This symmetry is regarded as the fourfold symmetry of the upper critical field, H _c2, and explained as being mainly due to the anisotropy of the superconducting energy gap owing to the d_x ^{2-y 2} pairing. The magnitude of the fourfold component of H _c2, H _c2/H _c2, is largest in x=0.08 among the three crystals. At x=0.20 in the overdoped regime, no clear fourfold symmetry has been observed. It appears that the fourfold symmetry is easy to observe in the underdoped regime rather than in the overdoped regime.     

Thermodynamic properties of Ca1 − x Er x F2 + x and Ca1 − x Yb x F2 + x heterovalent solid solutions

The heat capacity of single crystals of the Ca_{1 ? x} Er _x F_{2 + x} (x = 0.05, 0.10) and Ca_0.95Yb_0.05F_2.05 fluorite solid solutions was determined by adiabatic calorimetry in the temperature range 55–300 K. The results were used to obtain temperature dependences of the Debye characteristic temperature, entropy, and enthalpy for the solid solutions.     

Density of Ni-Cr Alloy in the Mushy State

The density of Ni-Cr alloy in the mushy state has been measured using the modified sessile drop method. The density of Ni-Cr alloy in the mushy state was found to decrease with increasing temperature and Cr concentration in alloy. The molar volume of Ni-Cr alloy in the mushy state therefore increases with increasing the Cr concentration in alloy.The ratio of the difference of density divided by the temperature difference between liquidus and solidus temperatures decreases with increasing Cr concentration. The density of the alloy increased with the precipitation of a solid phase in alloy during the solidification process. The temperature dependence of the density of alloy in the mushy state was not linear but biquadratic.     

Comment on the magnetic properties of the system Sr2−xCaxFeMoO6, 0≤x≤2

The structural and magnetic properties of the n-type-metallic double-perovskite system Sr_2−xCa_xFeMoO₆ are reported. At 5 K, the saturation magnetization remains M_s(5 K)=3.5 μ_B/formula unit compared to a theoretical spin-only moment of 4.0 μ_B/formula unit for a perfectly ordered compound. From the shape of the M–H hysteresis loops below T_c, it is suggested that ferromagnetic long-range ordered domains are coupled antiferromagnetically across antiphase boundaries; random disorder within domains may be small. An unprecedented dependence of the paramagnetic susceptibility on the applied magnetic field is reported and attributed to an overlapping Fe³⁺/Fe²⁺ and Mo⁶⁺/Mo⁵⁺ redox couple, first suggested by Sleight and Weiher [Sleight AW, Weiher JF. J. Phys Chem Solids 1972; 33: 679.], which results in localized S=5/2 spins on the iron atoms coexisting with itinerant minority-spin electrons in a narrow π* band.