Probing the Order Parameter and thec-Axis Coupling of High-TcCuprates by Penetration Depth Measurements

Abstract

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

Evidence for a generic quantum transition in high-Tccuprates

“…3͒, which shows precisely the same trend with T g and T f →0 as p→0. 19. We note that this similarity does not rule out possible effects of striped phases in our LSCO data.…”

“…Both T g and T f are found to decrease with increasing doping and tend to zero at pӍ0. 19. Their behavior resembles that of the pseudogap 10,11 which vanishes at the same doping.…”

“…3 summarizes is the apparent convergence of both T g (p) and T f (p) to zero, for all Zn concentrations, at the critical doping p c Ӎ0. 19. Although we do not have data for exactly pϭ0.19, the presence of finite values of T g and T f for pϭ0.18 but their absence for 0.20, indicates that the two magnetic scales go to zero somewhere in between and in particular very near 0.19.…”

“…Analysis of specific heat data, for example, suggests that the pseudogap energy decreases with doping and falls to zero at a critical doping of p c Ӎ0. 19, just beyond optimal doping, 10,11 a behavior rather analogous to the quantum-critical heavy-fermion materials. 9 Many fundamental physical quantities such as the superconducting condensation energy, 10,11 the superfluid density, 12,13 and the quasiparticle weight, 10,14 show abrupt changes as p→p c .…”

“…12,15,19 In total, 16 samples were investigated for each doping content. We note that grain agglomerates can be a cause of poor alignment, and to eliminate these, powders were ball milled in ethanol and dried after adding a defloculant.…”