QUASI-ONE-DIMENSIONAL CUPRATES AND MERCURY-BASED SUPERCONDUCTING CRYSTALS SYNTHESIZED AT HIGH PRESSURE

J.Karpinski, H.Schwer, G.I.Meijer, R.Molinski, E.Kopnin, M.Angst

Laboratorium fuer Festkoerperphysik ETH 8093 Zuerich

Magnetic properties of low-dimensional materials have attracted much attention in the field of solid state physics. This is because of the wide variety of interesting effects observed in solids with reduced dimensionality. We investigate magnetic properties of the new low-dimensional Cu-O spin systems which have been synthesized at high oxygen pressure. These materials, Sr_0.73CuO₂, Ca_0.83CuO₂ and Ba_0.67CuO₂ consist of 1D Cu-O spin chains separated by Sr, Ca or Ba planes and are incommensurate in chain direction. The Cu atoms in the chain are connected through two O sites by about 90 degree bonds. Our 1D compounds are insulating and characterized by anti­ferromagnetic spin 1/2 Heisenberg chains. In the low temperature limit we observe a broad maximum in the susceptibility data which might originate from a dimerization gap in the spin chain. Surprisingly, these quasi-1D materials exhibits magnetic ordering at T<=12 K despite their substantial hole doping. Weak-ferromagnetic ordering is found for the Sr and antiferromagnetic for Ca and Ba compounds. If a spin gap is present in these compounds this will be the second example of a material with coexistence of a spin gap and long-range magnetic order.

Hg-1223 exhibits the highest T_c (135 K) achieved up to now, but the flux-pinning properties of these materials are worse than for some other high-T_c superconductors (e.g. YBa₂Cu₃O_6+x). Chemical substitutions can enhance the effectiveness of the pinning through reducing the anisotropy by decreasing of the blocking layer thickness or creating points defects acting as pinning centers. Rhenium can partially substitute Hg. It is coordinated octahedrally by oxygen and forms short bonds leading to a decrease of the lattice parameters. The maximum substitutions of Re for Hg is 25% because higher Re concentrations would cause short O-O distances which are not possible. Several authors reported recently significant improvements of the flux pinning when Hg is partially replaced by Re and Ba by Sr in ceramic samples. However, due to the high anisotropy of HTSC and additional phases in ceramic samples only measurements on single crystals are reliable for the determination of true intrinsic properties of these compounds. Polycrystalline material has randomly oriented grains, which makes the measurements of superconducting parameters of highly anisotropic materials difficult. Grain boundaries limit the macroscopic critical current. The grain boundaries may act as pinning centers. Therefore, to measure "intrinsic" parameters of HTSC with any appreciable accuracy, one has to use single crystals. We have investigated the temperature dependence of the irreversibility field H_irr(T) for Hc for various single crystals, namely underdoped Hg-1223 with T_c=120K, almost optimally doped Hg-1223 with T_c=131K and Hg-1223 doped with 20%Re (T_c=130K). The irreversibility field of the Re substituted HgRe-1223 compound is lower than the one of optimally doped Hg-1223. It can also be clearly seen that the oxygen doping level has a much greater impact on the position of the irreversibility line than the Re substitution. Thus, the possible metallization of the blocking layer by Re substitution alone does not seem to improve the irreversibility region significantly. Rather, a shortening of the blocking layer thickness (e.g. by substitution of Ba by Sr) seems to achieve this goal.