Synthesis and Bulk Properties of
Oxychloride Superconductor Ca_2-xNaxCuO₂Cl₂

N. D. Zhigadlo⁽¹⁾, J. Karpinski⁽¹⁾, S. Weyeneth⁽²⁾, R. Khasanov⁽²⁾, S. Katrych⁽¹⁾,
P. Wägli⁽¹⁾  and H. Keller⁽²⁾

(1) Laboratory for Solid State Physics, ETH Zürich, 8093 Zurich, Switzerland
e-mail: zhigadlo@phys.ethz.ch
(2) Physik-Institut der Universität Zürich, 8057 Zürich, Switzerland

Abstract - A series of polycrystalline samples and submillimeter size single crystals of a cuprate oxychloride Ca_2-xNaxCuO₂Cl₂ (Na-CCOC) with values of Na content ranging from underdoped to optimally doped regions were synthesized at pressure of 30-55 kbar and temperature of 1250-1700 ºC. A systematic variation of the transition temperature Tc with a maximum value of 29 K for x≈0.20 has been found as a function of Na content. In order to check the role of the apical oxygen for high-temperature superconductivity, we performed muon-spin rotation and magnetization studies of the in-plane magnetic penetration depth λ_ab for Ca_2-xNaxCuO₂Cl₂ samples with x≈0.11, 0.12, 0.15, 0.18, and 0.19. The absolute value of the in-plane magnetic penetration depth at T=0 was found to increase with decreasing doping from λ_ab(0)=316(19) nm for the x≈0.19 sample to lab(0)=430(26) nm for the x≈0.11 one. Based on a comparison of the present Na-CCOC data with the data of La_2-xSr_xCuO₄ cuprate superconductors, it is concluded that replacing of apical oxygen by chlorine decreases the coupling between the superconducting CuO₂ planes, leading to an enhancement of the two-dimensional properties of Na-CCOC. The torque studies implies that the anisotropy coefficient γ=84 of Ca_1.82Na_0.18CuO₂Cl₂ single crystals is much more enhanced compared to the structurally related La_1.82Sr_0.18CuO₄γ is much lower, i.e. γ ≈ 11.

Full Text, PDF

Manuscript received Nov. 23, 2007; accepted Dec. 21, 2007. Reference No. ST14, Category 2.
Paper submitted to Proceedings of EUCAS 2007.