H. Schwer

High-oxygen-pressure synthesis, structure and properties of the infinite-chain compound Sr0.73CuO2

Abstract A new low-dimensional compound has been synthesized at high oxygen pressure P O 2 = 2000 bar. X-ray single crystal structure analysis shows that Sr 0.73 CuO 2 has a semi-incommensurate structure consisting of one-dimensional corrugated edge-sharing CuO 2 chains alternating with Sr planes. From susceptibility measurements this compound is found to have an antiferromagnetic exchange coupling with a Curie-Weiss temperature of 135 K. A sharp magnetic transition occurs at 10 K which progressively disappears at applied magnetic fields larger than 1 T. While not yet understood this transition could originate from a structural instability.

Influence of Re substitution on the flux pinning in (Hg,Re)Ba2Ca2Cu3O8+δ single crystals

Abstract Flux pinning in Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8+ δ single crystals ( T c =130 K) and in HgBa 2 Ca 2 Cu 3 O 8+ δ crystals with T c equal to 130 and 120 K was studied. In the investigated crystals Re atoms substitute at the Hg site of HgBa 2 Ca 2 Cu 3 O 8+ δ and alter the defect structure of the host layer by pulling in four new oxygen atoms at (0.34, 0.34, 0) position to form an octahedron around Re. As a result a significant improvement of the irreversibility line position at the H–T phase diagram for Hg 0.77 Re 0.23 Ba 2 Ca 2 Cu 3 O 8+ δ crystal is observed only at low temperatures (below 80 K). However, magnetization measurements performed at temperatures above 100 K show insignificant change in the irreversibility line position only, as compared to that one of the parent compound with T c =130 K. This indicates insignificant influence of new pinning centers, introduced by Re substitution, in the temperature range, where thermal activation energy becomes significant in comparison with pinning energy. Furthermore, obtained data indicate that no improvement of the coupling between superconducting layers was achieved as a result of the expected increase of electrical conductivity in the blocking layer. Since only a small change in the c -axis lattice constant was observed in Re substituted crystals, we can conclude that a larger shortening of the blocking layer in the material is necessary to improve the irreversibility line position in HgBa 2 Ca 2 Cu 3 O 8+ δ crystals at high temperatures.

Effect of Pb doping on the structure of HgBa2CuO4+δ single crystals

Abstract X-ray structure analyses were performed with HgBa2CuO4+δ and Hg1−xPbxBa2CuO4+δ single crystals. The onset of the superconducting transition of the crystals ranges from 78–92 K. Hg-1201 crystallizes with space group P4/mmm and has lattice pafameters a = 3.8907(2) A and c = 9.556(1) A . The structure was refined to R = 0.023 (wR2 = 0.036) in the ideal 1201 type with an excess oxygen content of 6(2)% and an occupancy of 97% at the Hg site. Pb-doping decreases the lattice parameters to a = 3.8836(2) A and c = 9.553(1) A , and the refinement converged at R = 0.042 (wR2 = 0.088). Pb atoms are shifted by 0.5 A to (0.14, 0, 0) and the excess oxygen atom O(3) by 0.45 A to (0.38, 1 2 , 0). It has not a well defined position in the lattice, but is distributed randomly in a ring-like configuration around the ( 1 2 , 1 2 , 0) site, supporting a Pb clustering model. Rather high anisotropic temperature factors U11 and U22 of Hg and apical O(1) atoms are observed, which are also present in higher members of the Hg(Pb)-12(n − 1)n series. They are discussed in terms of corrugated OHgO bonds. Comparison with other structure refinements of 1201 shows a correlation of the transition temperature with the position of the Ba atom, which seems to be a good indicator for the hole doping evel in 1201.

High gas pressure crystal growth and properties of HgBa2Can−1CunO2n+2+δ, Sr0.73CuO2 and CaxA1−xCuO2

Abstract An overview is presented of crystal growth and physical investigations of: HgBa 2 Ca n−1 Cu n O 2n+2+δ , Ca 1−x A x CuO 2 and Sr 1−x CuO 2 compounds synthesized at high pressure. The Hg-12(n−1)n (n=1–5) single crystals have a T c onset up to 130K. Thin films of Hg-12(n−1)n have been grown from a melt. At a high oxygen pressure a new phase Sr 0.73 CuO 2 has been synthesized. It contains one-dimensional edge-sharing CuO 2 infinite chains instead of corner-sharing infinite layers, typical for cuprate superconductors.

Ca1−xRxCuO2 (R = Sr, La) single crystals with infinite-layer structure: high Ar gas pressure synthesis and properties.

Abstract Single crystals CaCuO 2 , Ca 1−x La x CuO 2 and Sr 0.53 Ca 0.47 CuO 2 with inifinite-layer structure were synthesized using high Ar pressure. Magnetization measurements showed weak diamagnetism only in CaCuO 2 probably due to some A-deficient domains. A weak diamagnetic response was also revealed in a Sr 0.53 Ca 0.47 CuO 2 powder sample. However, superconductivity was not observed by resistivity measurements.

Single crystal of the 1223/1234 intergrowth phase Hg1.44Re0.5Ba4Ca5Cu7O20 : structure and properties

Abstract A single crystal of Hg 1.44 Re 0.5 Ba 4 Ca 5 Cu 7 O 20 (HgRe-2457) has been grown with a gas-phase high pressure technique at 10 kbar. X-ray single-crystal structure analysis showed that the crystal is composed of alternating blocks of 1223 and 1234 units. Until now, intergrowth structures in the Hg based series have only been observed in a small submicron scale or as a defect. Rhenium substitutes 25% of mercury and forms short bonds of 1.98 A to oxygen atoms. They coordinate Re octahedrally and are shifted off their ideal positions to (0.36, 0.36, z ). The bond lengths are in the range of pure 1223 or 1234 compounds. The stacking sequence is disturbed by double or triple layers of 1223 or 1234, like …3-4-3-4-4-3-4-3…. This feature is visible in Fourier maps and included in the refinement. The transition temperature of this 2457 crystal is 83 K.

X-ray single crystal structure analysis of Sr0.73CuO2 and Hg1−xPbxBa2Can−1CunO2n+2+δ compounds (n= 1−5,x= 0−0.5)

Abstract The crystal structures of five members of the homologous series HgPb-12(n−1)n (n= 1 – 5) have been determined and refined by X-ray single crystal diffraction. The effects of Pb doping and of stacking faults are discussed. The high oxygen pressure phase Sr 0.73 CuO 2 has a simple semi-incommensurate Vernier type structure, containing infinite chains with edge-sharing planar CuO 4 squares sandwiched by Sr atoms.

Flux pinning in HgBa2CuO4+x single crystals

Abstract Magnetization studies on single crystals of HgBa 2 CuO 4+x , with T c onset≈ 81 K , were performed in a wide range of temperature and magnetic field. The fishtail shape of M ( H ) loop was observed even at 5 K. The irreversibility field and the peak position are well described by the power law dependence H irr/peak ( T ) = H irr/peak (0)(1 - T / T c ) α with the same exponent α = 1.8±0.1. The pinning force in the fishtail region scales with reduced magnetic field H / H irr in the wide temperature range. The normalized pinning force is well described by the relation: F p / F p(max) = A · h 2.9 ·(1 - h ) 3.8 , where h = H / H irr and A is a numerical parameter.

Improvement of the flux pinning properties of Hg-based single crystals by a substitution of Ba by Sr and Hg by Pb or Re

Single crystals of mercury based cuprate superconductors were grown under high pressure. In some crystals, Hg was partially substituted by Re or Pb and Ba by Sr. The flux pinning properties of the grown crystals were measured by SQUID magnetometry, and their irreversibility lines were determined and compared. While substitutions at the mercury site did not change the irreversibility line significantly, a substitution of Ba by Sr enhanced the irreversibility field considerably. The effective mass anisotropy was determined from torque magnetometry measurements performed in the region of reversible magnetic behavior. Relations between the irreversibility line, the effective mass anisotropy and structural parameters are discussed.