We have measured the low-temperature ͑T ͒ dependence of the anisotropic penetration depth l͑T ͒ of magnetically aligned powders of HgBa 2 Ca n21 Cu n O 2n121d (n 1 and 3) down to 1.2 K. For both members the T dependence of the in-plane penetration depth l ab ͑T ͒ is strongly linear, whereas the outof-plane component l c ͑T͒, for n 1 and 3, varies as T 5 and T 2 , respectively. For comparison, we also report l c ͑T͒ data for grain-aligned YBa 2 Cu 3 O 72d (d 0.0 and 0.43) which vary as T and T 2 , respectively. The results are discussed in terms of d x 2 -y 2 -wave symmetry of the order parameter in cuprates.[S0031-9007 (97)04087-8] PACS numbers: 74.25.Nf, 74.72.Bk, 74.72.GrThe high-T c superconductors (HTSC) containing CuO 2 planes are layered materials with anisotropic physical properties. Evidence is now growing that the energy gap in HTSC has "d-wave" symmetry with nodes in the order parameter in certain directions in k space [1,2]. Detailed knowledge of the behavior of the superfluid density and the symmetry of the energy gap are important for understanding various properties of the superconducting state including the pairing mechanism in these materials. One of the best probes of the superfluid density and the energy gap morphology at the Fermi surface is the penetration depth ͑l͒. Many early studies of the temperature ͑T ͒ dependence of l on single crystals, films, and aligned powders of yttrium barium copper oxide (YBCO) gave evidence for non-s-wave pairing, usually in the form of a T 2 behavior for l͑T ͒ at low temperatures rather than exponential dependence expected for s wave. However, this was not widely accepted until the work of Hardy et al. [3] on high quality YBCO crystals which showed a linear temperature dependence in the in-plane penetration depth from 1.3 to 20 K. We have found that in some cases, structural and chemical defects near the surface of YBCO crystallites have significant effects on l at low temperatures and that heat treating the crystallites after grinding in air, or grinding under argon, helped obtain higher quality surfaces and thus intrinsic information from aligned powder composites [4]. Most studies of l have concentrated on YBCO [1,3,5] and Zn doped YBCO [5,6]. Relatively few studies have examined other materials such as La 22x

The temperature (T) dependence of the anisotropic magnetic penetration depth (T) of magnetically aligned powders of crystalline HgBa 2 Ca 2 Cu 3 O 8ϩ␦ is reported. Measurements were performed in the Meissner state using the ac-susceptibility technique. The temperature dependences of the in-plane, ab (T), and out-of-plane, c (T), penetration depths are markedly different. This is believed to arise from the large anisotropy ratio ␥ϭ͓ c ͑0͒/ ab ͑0͔͒Ӎ30. The behavior of ab (T) is indicative of d-wave superconductivity while c (T) is similar to the behavior expected for a superconductor with intrinsic Josephson coupling between the CuO 2 planes. Similar measurements were performed on Ba 0.6 K 0.4 BiO 3 powders for comparison.

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We have investigated the thermal decomposition of the mercurocuprate phases HgBa2 Ca2 Cu3 O8+ , HgBa2 CuO4+ and (Hg0.8 Re0.2 )Sr2 CuO4+ . In each compound, decomposition is initiated by the breaking of weak Hg-O bonds, but is driven to completion by the formation of thermodynamically stable barium carbonate phases (BaCO3 , BaCuO2 .(CO3 )x ). We show that the barium-free mercurocuprate (Hg0.8 Re0.2 )Sr2 CuO4+ is significantly more thermally stable than HgBa2 CuO4+ , allowing heat treatments at temperatures of at least 800 °C for moderate periods of time and with only very minor decomposition observed. Further, the mechanism of decomposition for the Ba-free compound appears to be kinetically controlled, not suffering from the thermodynamic sinks (highly thermodynamically stable barium carbonates) available to Ba-based compounds.