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, Sr0.73CuO2, Ca0.83CuO2 and Ba0.67CuO2 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 Tc (135 K) achieved up
to now, but the flux-pinning properties of these materials are
worse than for some other high-Tc superconductors
(e.g. YBa2Cu3O6+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 Hirr(T) for Hc for
various single crystals, namely underdoped Hg-1223 with Tc=120K,
almost optimally doped Hg-1223 with Tc=131K and
Hg-1223 doped with 20%Re (Tc=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.