J.L. Wagner

Structure and superconductivity of HgBa2CuO4+δ

Abstract We have used neutron powder diffraction to investigate the defect structure of HgBa 2 CuO 4+δ . An interstitial oxygen defect in the Hg plane is the primary doping mechanism. A superconducting transition temperature, T c onset , of 95 K is achieved when ≈0.06 oxygen atoms per formula unit are incorporated at this site by annealing the sample at 500°C in pure oxygen. Annealing in argon at 500°C lowers the oxygen content in this site to ≈0.01 and results in a T c of 59 K. The neutron powder diffraction data give evidence for a second defect in the Hg plane which we conclude involves the substitution of copper for about 8% of the mercury and the incorporation of additional oxygen (≈0.1 atoms per formula unit), presumably bonded to the copper defects. In the present samples, the concentration of this defect does not vary with synthesis conditions and its contribution to doping is, therefore, unclear. The structure of the compound is the same at room temperature and superconducting temperatures.

Structure, doping and superconductivity in HgBa2CaCu2O6+δ (Tc ⩽ 128 K)

Abstract We have studied the defect structure and superconducting propertiess of HgBa2CaCu2O6+δ by neutron powder diffraction, AC susceptibility and DC resistivity. An as-synthesized sample has an onset Tc of 128 K, the highest yet reported for this material. The critical temperature decreases slightly (126 K) after oxygenation, and drops to 92 K after reduction in flowing argon. Neutron diffraction data give evidence that the primary doping mechanism in this material is provided by oxygen atoms in an interstitial position, similar to the case of the one-layer compound HgBa2CuO4+δ. The occupancy of this site varies from 0.08 (1) for the argon-reduced sample to 0.22 (1) for the oxygenated sample. Unlike the case of the one-layer compound, no evidence of an additional defect site was found in these samples. HgBa2CaCu2O6+δ has remarkably long copper-apical oxygen distances and almost perfectly flat CuO2 planes.

Pressure-induced structural changes in superconducting HgBa2Can−1CunO2n+2+δ (n = 1, 2, 3) compounds

Abstract The crystal structures of superconducting HgBa 2 CuO 4+δ and HgBa 2 CaCu 2 O 6+δ have been investigated with a pressure up to 0.6 GPa and HgBa 2 Ca 2 Cu 3 O 8+δ to 9.2 GPa by neutron powder diffraction. The compressibility along the c -axis is nearly the same for the three compounds and up to two times larger than the compressibility along the a -axis. The one-layer compound, HgBa 2 CuO 4+δ , shows the largest a -axis compressibility, while HgBa 2 Ca 2 Cu 3 O 4+δ shows the smallest compressibility. The bond compressibilities of HgBa 2 CuO 4+δ and HgBa 2 CaCu 2 O 6+δ are significantly different from HgBa 2 Ca 2 Cu 3 O 8+δ bond compressibilities. In the one- and two-layer compounds the largest bond compressibility was the Cu-O2 (apical) bond distance, while for the three-layer compound it was the Hg-O2 bond distance.

Flux pinning and the irreversibility lines in the HgBa2CuO4+δ, HgBa2CaCu2O6+δ and HgBa2Ca2Cu3O8+δ compounds

Using measurements of the magnetization hysteresis we determine the temperature and field dependence of the critical current density, the pinning force and the irreversibility lines of polycrystalline HgBa{sub 2}CuO{sub 4+{delta}}, HgBa{sub 2}CaCu{sub 2}O{sub 6+{delta}} and HgBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+{delta}}. For all three materials we observe an exponential decay of the critical current with increasing temperature and/or field. The temperature dependence of the irreversibility fields follows a power law, H{sub irr}{approx}(1-T/{Tc}){sup 4}, and is intermediate to that of YBa{sub 2}Cu{sub 3}O{sub 7} and the Bi/Tl-based superconductors. These results are analyzed in a model in which the separation between the superconducting CuO2 blocks is the important parameter. At 77 K the irreversibility field of HgBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+{delta}} is 2 T and the critical current density at 77 K in a field of 1 T is about 2 x 103 A/cm2. The plate-like nature of the grains in the double- and triple-layer compounds offers the possibility that textured materials with good coupling between the grains can be prepared.Abstract Using measurements of the magnetization hysteresis we determine the temperature and field dependence of the critical current density, the pinning force and the irreversibility lines of polycrystalline HgBa 2 CuO 4+δ , HgBa 2 CaCu 2 O 6+δ and HgBa 2 Ca 2 Cu 3 O 8+δ . For all three materials we observe an exponential decay of the critical current with increasing temperature and/or field. The temperature dependence of the irreversibility fields follows a power law, H irr ∝(1− T T c ) 4 , and is intermediate to that of YBa 2 Cu 3 O 7 and the Bi/Tl-based superconductors. These results are analyzed in a model in which the separation between the superconducting CuO 2 blocks is the important parameter. At 77 K the irreversibility field of HgBa 2 Ca 2 Cu 3 O 8+δ is 2 T and the critical current density at 77 K in a field of 1 T is about 2×10 3 A/cm 2 . The plate-like nature of the grains in the double- and triple-layer compounds offers the possibility that textured materials with good coupling between the grains can be prepared.

The irreversibility line of HgBa2CuO4+δ

Using magnetization hysteresis loops we have determined the irreversibility line of polycrystalline samples of HgBa2CuO4+δ and Hg0.8Pb0.2Ba2CuO4+δ. The irreversibility line is found at rather high temperatures. It does not show the drastic depression with increasing temperature as is observed in highly anisotropic Bi/Tl‐based CuO superconductors. At 77 K the irreversibility field is around 0.3 T, which is substantially higher than in Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10. The advantages of this material for large‐scale applications are outlined.

Low-pressure synthesis of tetragonal Sr2CuO3 + χ from a single-source hydroxometallate precursor

Abstract Synthesis of the tetragonal “high-pressure” form of Sr 2 CuO 3+χ at 370°C in 1 atm oxygen from a copper hydroxometallate precursor, Sr 2 Cu(OH) 6 is reported. Results of thermogravimetric analysis, and in-situ X-ray diffraction on the thermal decomposition of the precursor are described. The as-synthesized tetragonal material is nonsuperconducting and is irreversibly converted to the orthorhombic phase by heating at 450°C in 1 atm O 2 .

Hydrostatic presure dependence of the superconducting transition temperature of HgBa2CaCu2O6+δ and HgBa2Ca2Cu3O8+δ

The dependence of the superconducting transition temperature [Tc](P) on purely hydrostatic pressure to 0.9 GPa has been determined in ac susceptibility studies in a He-gas pressure system for optimally doped ceramic samples of HgBa[sub 2]CaCu[sub 2]O[sub 6+[delta]] and HgBa[sub 2]Ca[sub 2]Cu[sub 3]O[sub 8+[delta]] with superconducting transitions at [Tc](0) [approx equal] 126.6 K and 133.9 K, respectively. [Tc] increases reversibly under hydrostatic pressure at the rates, d[Tc]/dP [approx equal] +1.80 [+-] 0.06 K/GPa and +1.71 [+-] 0.05 K/GPa, respectively. Within experimental error, these values are the same as found previously for optimally doped single-layered HgBa[sub 2]CuO[sub 4+[delta]], where d[Tc]/dP [approx equal] +1.72 [+-] 0.05 K/GPa. Remarkably, the logarithmic volume derivative of [Tc] is nearly identical for all three compounds, dln[Tc]/dlnV [approx equal] [minus]1.20 [+-] 0.05, even though the bulk modulus differs by more than 30%. This provides strong evidence that a common mechanism is responsible for the pressure dependence of the superconducting state in all three compounds.

Suppression of superconducting transition temperature in orthorhombic La2-xCaxCuO4

Abstract The calcium solid-solubility limit in La 2- x Ca x CuO 4 has been extended from x ≈0.10 for air-synthesized material to x =0.2 using synthesis at an oxygen pressure of 600 atm and 1110°C. The tetragonal-to-orthorhombic transition occurs at higher temperatures, or larger doping levels, than for the isostructural Sr-substituted material. The superconducting transition temperature increases with doping to 34 K at x =0.15 and is suppressed for x>0.15, similar to the Sr-substituted material. Comparison of structural and superconducting properties for Ca- and Sr-substituted materials indicated that: (1) both orthorhombic and tetragonal structures are superconducting; (2) at a fixed doping level, the optimum superconducting properties (the highest T c ) are found for a perfectly flat and square CuO 2 plane; (3) T c is suppressed by the distortions of the CuO 2 plane or by overdoping resulting in a maximum of T c found at x =0.15 under ambient conditions.

High-resolution and analytical electron microscopy of HgBa2CuO4+δ a new copper-oxide superconductor

Abstract High-resolution and analytical electron microscopy (HREM and AEM, respectively) techniques have been utilized to probe the submicroscopic details of crystallography, chemistry and electronic structure of HgBa2CuO4+δ, a recently discovered single- layer copper-oxide superconductor. Core loss EELS analysis of O K edge reveals a pre-edge similar to many p-type doped copper oxide superconductors, while a free change carrier plasmon has been identified with low loss EELS. It is discovered that the delocalized hole carrier concentration in a lower-Tc sample (argon annealed), as inferred from oscillator strengths, is about ≈60% of that of a fully oxygenated sample, which has a higher Tc. It is also discovered that there is a strong excitation just above the delocalized hole state (by ≈ 2 eV) in both lower- and higher-Tc samples. These observations are compared with other cuprate superconductors.

The Phase Diagrams and Doped-Hole Segregation in La2CuO4+δ and La2−xSrxCuO4+δ (x ≤ 0.15, δ ≤ 0.12)

The magnetic and structural phase diagrams of the title systems are reviewed, with an emphasis on our recent results obtained from magnetic and structural neutron diffraction, thermogravimetric analysis, iodometric titration, magnetic susceptibility x(T), and 139La nuclear quadrupole resonance (NQR) measurements. From measurements on electrochemically oxidized polycrystalline samples, the known miscibility gap in the system La2CuO4+δ is found to lie between δ ≈ 0.01 and 0.06 at low temperatures, with a maximum phase separation temperature T ps ≈ 415 K. Within the miscibility gap, the superconducting transition temperature T c of the oxygen-rich phase and the Neel temperature T N of the oxygen- deficient phase are constant at ≈ 32 K and 250 K, respectively. Neutron diffraction measurements showed T ps = 260(5) K and T N = 245(3) K for a single crystal. Beyond the miscibility gap, two distinct superconducting phases are found in polycrystalline samples at δ ≈ 0.06–0.08 (T c ≈ 32–34 K) and 0.11–0.12 (T c ≈ 42–45 K), separated by another two-phase region. The doped-hole concentration in the CuO2 planes is found to be p ≈ 0.08 holes/Cu and ≈ 0.16 holes/Cu for the two phases, respectively. These data suggest that a large fraction of the excess oxygen atoms participate in oxygen-oxygen bonding in both phases. Measurements of T c versus pressure suggest pressure-induced changes in the doped-hole concentration in the CuO2 layers. Superstructure reflections observed in the neutron diffraction patterns of both phases suggest spatial ordering of the excess oxygen atoms. In the La2−xSrxCuO4+δ system, the phase separation disappears by x = 0.03 for δ ≈ 0.03. Bulk superconductivity is found below T c = 40 K for 0.01 ≤ x ≤ 0.15 for maximally oxidized samples. For δ = 0 and 0 < x < 0.08, our 139La NQR and X(T) data indicate that the doped holes condense into walls separating undoped nanoscopic domains, consistent with theory based on an electronic mechanism for phase separation in these systems. In the antiferromagnetic regime (x < 0.02) below TN, the doped-hole spins are found to freeze at a temperature T f = (815 K)x, whereas in the spin-glass regime 0.02 < x < 0.08, the spin-glass transition at T g ∝ 1/x is found to arise from cooperative freezing of the dynamically-ordered mesoscopic undoped domains.