The Study of Hole Distribution in High-Tc Tl- and Hg-based 1212 Type Cuprates via High-Resolution X-Ray Absorption Near Edge Structure

High resolution O K-edge and Cu L₃-edge X-ray absorption near-edge-structure (XANES) spectra of the high-T_c cuprates of (Tl_0.5Pb_0.5)Sr₂(Ca_1?xY_x)Cu₂O₇ (Tl-1212) and (Hg_0.5Pb_0.5)Sr₂(Ca_1?xY_x)Cu₂O₇ (Hg-1212) in powder form were measured using a bulk sensitive total-fluorescence-yield technique. Near the O 1s edge, the pre-edge peak with maxima at ～ 528.3 eV is ascribed to the transitions to O 2p holes located in the CuO₂ planes. The intensity of this pre-edge peak increases with increasing doping level of Ca²⁺ into the Y³⁺ sites in Tl-1212 and Hg-1212. In the Cu L-edge absorption spectra, high-energy shoulders at around 933 eV are attributed to the transitions to the Cu(2p_3/2)^?13d¹⁰L states in the CuO₂ layers, where L denotes the O 2p ligand hole. The behavior of these shoulders in Tl-1212 and Hg-1212 correlates with that of the pre-edge peak at ～ 528.3 eV in the O K-edge absorption spectra. The results can lead us to understand the hole distribution in high-T_c cuprates which will give a direction to find new high-T_c materials.     

Superconductivity at 103 K in CdBa2(Ca0.7Y0.3)Cu2Oy

A cadmium analogue of the mercury system with nominal composition CdBa₂(Ca_1–xY_x)Cu₂O_y has been synthesized. Thex=0 samples contain about 12 vol.% of the 1212 phase but are not superconducting. Thex=0.3 samples are superconducting atT _on = 103 K. The EDX analysis of 18 microcrystals shows a broad cationic distribution of the different components. The observed broad superconducting transition is attributed to the variousT _c of the different microcrystals.     

A new high-Tc superconductor containing thallium and its crystal structure: The “1212” phase (Tl1-xBix)1.33Sr1.33Ca1.33Cu2O6.667+δ

Joining YBa₂Cu₃O_6.5+ (123 phase) and Bi₄Sr₄Ca₂Cu₄O₁₆₊ (4424 phase) as structurally characterized high-T _c , superconductors, the thallium-containing superconductor (Tl_.75Bi_.25)_1.33Sr_1.33Ca_1.33Cu₂O_6.667+ with the ideal stoichiometry (Tl,Bi)₁Sr₂Ca₁Cu₂O_6.5+ (1212 phase) is reported here. As prepared from the component oxides, 1212 has an initial deviation from resistance linearity at 120 K, a superconducting onset temperature of 92 K, and zero resistance at 75 K. The tetragonal unit cell (P4/mmm, a=3.800 Å;c=12.072 Å, deduced from powder data) contains double copper oxygen sheets (like 4424 and 123) that alternate withsingle thallium-bismuth oxygen sheets (in contrast to 4424, which containsdouble bismuth oxygen sheets), resulting in a total of three stacked perovskite-like cells (as in 123). The copper oxide sheets (with intersheet spacing 3.38 Å) are separated by Ca²⁺ and the Cu oxide sheets and (Tl,Bi) oxide sheets (with spacing 4.35 Å) are separated by Sr²⁺, Ca²⁺, and excess (Tl,Bi)³⁺. The 1212 cell constitutes the building block for the centered, more complex 4424 cell. The 1212 structure persists to Bi contents as low as 1% and can also be stabilized by Pb instead of Bi; Tl cuprates also form other superconductors with lowerT _c .     

Structural variation and recovery of superconductivity by Ca substitution in Y0.4Pr0.6Bs2?xCaxCu3

Y_0.4Pr_0.6Ba_2–xCa_xCu₃O_7– polycrystalline samples were prepared and investigated. The Ca substitution led to a structural variation toward tetragonal symmetry, possibly due to the structural modification within the Cu(1)O layer. Superconductivity and metallic conduction were recovered whenx0.2, giving one more evidence for the existence of the ion-size effect at the Ba site.     

Structural Studies of the Superconducting La2.5Y0.5CaBa3(Cu1?xFex)7Oz (0 ≤ x ≤ 0.1) Perovskite System by Neutron Diffraction

We have investigated the effect of Fe substitution on the structural and superconducting properties of La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z system by Rietveld refinement of the neutron diffraction patterns of three samples with x = 0.02 (labelled B₁), x = 0.06 (B₂), and x = 0.10 (B₃) along with X-ray diffraction, resistivity, AC susceptibility, and oxygen-content measurements. Samples B₁, B₂, and B₃ are superconducting with T _c ^R=0 values of 73, 62, and 41 K, respectively. Neutron diffraction studies confirm (i) the formation of a single phase tetragonal structure (space group P4/mmm) for all three samples, (ii) Ca and Y ions substitution at the La site concomitantly displaces La onto Ba sites, and (iii) increasing x from 0.02 to 0.10 increases oxygen content (the amount of oxygen per unit cell), as well as Cu(1)— O(4) and Cu(1)— O(1) bond lengths whereas Cu(2)— O(4) bond length decreases with corresponding decrease in T _c to 41 K due to increasing occupancy of Fe ions at Cu(2) site. The change in bond lengths with oxygen content are essentially the same as those of Fe content (x). Present studies establish a correlation between the bond lengths (Cu(1)— O(1), Cu(1)— O(4), and Cu(2)— O(4)) and the measured T _c values of three samples.     

Effect of Ca Substitution on the Superconductivity of La2.5Y0.5CaBa3(Cu0.88Fe0.12)7Oz

The structural and superconducting properties of single-phase La_2.5–y Y_0.5Ca_1+y Ba₃ (Cu_0.88Fe_0.12)₇O _z (LYCaBCuFe) (y= 0.0–1.0) compounds with triple perovskite structure are investigated using X-ray diffraction, resistivity, a.c. susceptibility, and oxygen content measurements. Increasing Ca substitution for La resulted in a decrease in unit cell axes and volume. T _c ^R=0 shows a marginal increase from 31 K to 37 K for y = 0.0–0.21 and thereafter it decreases with increasing y leading to zero T _c ^R=0 at y 0.84. This shows that the suppression of T _c from 80 K to 31 K by Fe doping at x = 0.12 La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.     

Extraordinary behaviour of the Y1?xPrxBa2Cu3O7?δ system

Investigations of Y_1–x M _x Ba₂Cu₃O_7– (M=Ce, Th)c-axis oriented thin film specimens show that the rate of depression ofT _c withx is larger for M=Th, than for M=Ce and Pr, and suggest that Ce, like Th, is tetravalent in this compound. Hall effect measurements on Y_1–x Pr _x Ba₂Cu₃O_7– single crystals reveal aT ² dependence of the cotangent of the Hall angle in the normal state and a negative Hall anomaly belowT _c in the superconducting state, in agreement with recent reports. Our research shows that the depth, , of the negative Hall signal scales withT/T _c and that the maximum value of decreases linearly withx and vanishes atx0.24. Magnetoresistance measurements on Y_1–x Pr _x Ba₂Cu₃O_7– single crystals indicate that the irreversibility lineH(T ^*) obeys a universal scaling relation characterized by anm=3/2 power law nearT _c, with a crossover to a more rapid temperature dependence of belowT/T _c 0.6, similar to that observed for polycrystalline specimens.     

A new family of superconductors containing carbonate group

Three new oxycarbonate superconductors recently obtained in our laboratory are described. (1) Sr₂CuO₂(CO₃)_1–x (BO₃) _x (T _c = 35 K) was synthesized under ambient pressure, providing a new method for carrier doping on the layered copper oxycarbonate system. (2) Ca _n (Ca, Sr)₂Cu _n+1(CO₃)O _y (n = 1, 2, 3) was synthesized under high pressure. Superconductivity was achieved by substituting (BO₃)^3– for a part of (CO₃)^2–. (3) A new Hg system HgBa₂Sr₂Cu₂(CO₃)O₆₊ was synthesized.T _c obtained from the magnetic susceptibility is about 66 K.     

Enhancement of Pinning Role by Nonstoichiometry in YBa2Cu3O7?y Superconductors

Considerably improved flux pinning and critical current density J c values have been achieved in Y-deficient Y-123 superconductors by directional solidification in air. In comparison with the regular Y-123 composition, Y-deficient one also has an orthorhombic structure and Y-123 main crystal phase remains in it. Whereas with the shortage of Y, Y_1–x Ba₂Cu₃O_7–y can be regarded as (YBa₂Cu₃)_1–x O_7–t(Ba₂Cu₃) _x O _t–y or (YBa₂Cu₃O_7–z )( _x ^YO _z–y ), so there may develop several kinds of microstructure defects as pinning sites in the system, such as highly dense, fine-scale, and faultlike defects, as well as localized superstructure, which are able to induce the increasing in flux pinning and J c values in higher external magnetic fields. This kind of simple nonstoichiometric route could lead to a commercial technique for flux-pinning enhancement in Y-123 bulk materials.     

Superconducting and magnetic ordered states in TlBa2Y1−xCaxCu2Oy studied byμ +SR

Hole doped high-T_c oxide of TlBa₂Y_1–xCa_xCu₂Oy, Tl-1212, has been investigated by + SR technique from an insulating to a superconducting (SC) region. Fast and slow precession components of the muon spin asymmetry which are due to an antiferromagnetic (AF) ordering of Ce-spins were observed from x=0 to x=0.5. The magnetic transition temperature (about 400 K) and local ⁺ fields were found to have no x-dependence. No sign of the magnetic transition was observed for x=1.0 down to 1.8 K. The correlation between the AF ordered state and the SC state at the boundary of the two states (x=0.6) was examined.     

Transport and Cu K-XANES Studies of (Hg,Cr)Sr2(Ca,Y)Cu2O6 + δ

The nominal (Hg_0.7Cr_0.3)Sr₂(Ca_0.7Y_0.3)Cu₂O_{6 +} , referred to as (Hg,Cr)-1212, were successfully synthesized at high temperature in partial vacuum. The samples were subsequently annealed in Ar or O₂ annealed at 300°C under ambient pressure. No T _c was observed in any of these samples down to 12 K which is in contrast to high pressure synthesized Y-free (Hg,Cr)-1212 phase where the latter exhibits a superconductivity at 80 K. The divalent state of Cu, as shown by XANES in the as-prepared (Hg,Cr)-1212, is not affected either by Ar or O₂ annealing. Depletion of holes due to trivalent Y substitution at the Ca-site, and a relatively higher 'a' lattice parameter (3.851 Å) are the apparent reasons for the non-superconducting nature of the samples.     

New Pb-Based 1222 Cuprates, (Pb0.5B0.5)Sr2(RE2−x−y Ce x Sr y )Cu2O z (RE = Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Y)

New Pb-based layered cuprates with the 1222 structure have been synthesized in the (Pb_0.5B_0.5)Sr₂ (RE_2−x−y Ce _x Sr _y )Cu₂O _z (RE = Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Y) systems. The almost-single-phase samples in the systems can be obtained for a nominal composition of x=0.7 and y=0.1. The crystal structure of the samples has a tetragonal symmetry, the lattice parameters of a and c are increasing with increasing the ionic radius of RE element. Despite treatment under high O₂ pressure of 100 atm, the samples are semiconductors with the transport process characteristic of three-dimensional variable range hopping conduction.     

Investigation on the Structure and Superconducting Properties of Mg1 ? xMxB2 (M = deficiency or Ca)

We report the preparation of Mg_{1 – x} M _x B₂ (M = deficiency or Ca) compounds and their structure and superconducting properties. For Mg_{1 – x} B₂, although nearly single-phase samples can be obtained for x = 0, MgB₄ coexists with the MgB₂ phase and some minor impurity phases, and the amount of MgB₄ increases with x for 0 < x 0.5. The lattice parameters a and c of MgB₂ decreases and increases, respectively, with the increase of x, and T _c also decreases. While for Mg_{1 – x} Ca _x B₂, the superconducting transition temperature remains unchanged for x 0.3 and loss of superconductivity occurs for x > 0.3. X-ray diffraction patterns for x 0.3 samples show that MgB₂ phase coexists with CaB₆, Mg, and MgO. With increasing x, the amount of CaB₆, Mg, and MgO increases, while the amount of MgB₂ decreases. The lattice parameters of MgB₂ phase do not show any obvious change in contrast to Mg_{1 – x} B₂. The results were discussed by considering some possible contributions.     

Comparison of Superconductivity and Structure for YBa2Cu3Oy with Potassium and Sodium Doping

The nominal composition of YBa_2–x M _x Cu₃O _y (M = K, Na) cuprates with x 0.30 were synthesized by the standard solid-state reaction technique. X-ray diffraction (XRD) and the resistivity measurements were used to characterize the structure and the superconductivity of the doped cuprates. There was no impurity phase detected within the whole doping range. The structure of the main phase (123) has the orthorhombic with P _mmm symmetry. With increasing the content of dopants, the lattice constants and some other structural parameters are almost unchanged for M = K, whereas they changed for M = Na. The refined contents of dopants are consistent with that of the nominal ones. The zero resistance temperature T _c0 decreases sharply with the increase of the content of potassium in potassium-doped samples as x 0.20. For sodium-doped YBa_2–z Na _x Cu₃O _y cuprates, T _c0 varies very little. The difference in superconductivity depression may result from the shift oxygen, which transfers conducting carriers from Cu-O chains to Cu-O₂ sheets or the structural stress effect.