In-Plane Anisotropy of the Penetration Depth Due to Superconductivity on the Cu-O Chains inYBa2Cu3

Abstract

Muon spin relaxation studies on a variety of polycrystalline YBa2Cu30», Y2Ba&Cu70» &, and YBa2Cu40, samples show that the 8-dependent depolarization rate (and hence n, /m") is greatly enhanced when the Cuo chains are free of disorder (8 0). Ca substitution allows the doping on the planes and chains to be independently controlled and the enhanced depolarization rate can be unambiguously attributed to condensation of mobile carriers on the chains as Cooper pairs, thus increasing the total condensate density n, . The in-plane London penetration depth is highly anisotropic: = 155 nm but Ab falls as low as 80 nm when 6 0.PACS numbers: 74.25.Ha, 74.62.Dh, 74.72.8k, 76.75.+i In contrast to all other high-T, superconducting cuprates (HTSC), the compound YBa2Cu307 s (1-2-3), together with its homologs 2-4-7 and 1-2-4, has linear Cu-0 chains additional to the Cu02 planes. These chains have a relatively high carrier density and a metallic character as evidenced by nuclear quadrupole resonance relaxation times [1], field-dependent heat capacity [2], infrared conductivity [3], and dc transport properties [4,5]. From muon spin relaxation (p, SR) data we show that, when these chains are free of disorder, the superconducting condensate density originating in the Cu02 planes is greatly enhanced due to the additional carriers on these chains condensing as Cooper pairs. That is, the CuQ chains become superconducting by proximity to the intrinsically superconducting planes providing significant enhancement in the condensate density, condensate energy, in-plane anisotropy, interlayer coupling, and flux pinning. Moreover, we argue that pair breaking on oxygen-deficient chains could provide the low-energy excitations deduced from several studies, without having to invoke d-wave symmetry.The HTSC are characterized by a generic phase behavior in which the transition temperature T, has an approximately parabolic dependence on hole concentration p, rising to a maximum T, " then falling on the overdoped side. In La2 Sr, Cu04 (2-1-4) the superconducting phase curve is conveniently approximated by [6] T,/T, '" = 1 -82. 6(p -0.16),and there is some evidence that this also well describes other, if not all, HTSC [5 -7]. Recently we have proposed from p, SR [8) and heat capacity [9] data that the decline in T"condensate density, and pair density in the overdoped region is due to pair-breaking interactions which reduce the lifetime of superconducting pairs. The p-dependent condensate density can be measured from the muon spin n, (0) = 0 5n/(1 + be. /Zvb),(3) and the curve in Fig. 1(a) is obtained by combining Eqs. (1) and (3).The reflex loop shown in Fig. 1(a) is the behavior to be expected for all HTSC with Cuo, planes only. It has been confirmed for several other systems [11 -13]. For 1-2-3, depolarization rate o, which is related to the London penetration depth AL as follows: tr~AL2 = poe'n, /m' Here n, is the superconducting condensate density and

Flux-Line Lattice Structures in UntwinnedYBa2Cu3O7−δJohnson1, Forgan2, Lloyd3 et al. 1999Phys. Rev. Lett.5012460View full textAdd to dashboardCite

“…The value of λ a provides a useful check of the consistency of our results, as it is known to be insensitive to the state of the chain layers, and is therefore reproducible across samples. It is typically quoted as 155nm for optimal doping [26] which agrees within experimental error.…”

Condensation, excitation, pairing, and superfluid density in high-Tc superconductors: the magnetic resonance mode as a roton analogue and a possible spin-mediated pairing

“…We see that T c increases with increasing carrier doping, following a nearly linear relationship with n s /m * in the underdoped region: the results from different series of cuprates share a common slope in this initial increase of T c . Systems in the optimally doped region deviate from this linear line, showing a "plateau" like behavior, which was later found out to be due to the effect of the CuO chains in the case of the YBCO systems [29]. We see that the nearly linear trend is followed also by systems with Cu/Zn substitution [8], spontaneous formation of static magnetic regions (shown with the "stripe" symbols), and even with overdoping (Tl2201) [24], the implications of which will be discussed in later sections.…”

Superfluid Anisotropy in YBCO: Evidence for Pair Tunneling Superconductivity

“…Introduction Recent measurements of the electromagnetic response of clean untwinned single crystals of Y Ba 2 Cu 3 O 7−δ and Y Ba 2 Cu 4 O 8 reveal a large ab-plane anisotropy [1,2]. The ratio of the superfluid density in b and a directions (CuO chains are along the b-axis) at zero temperature is about ≈ 2.4 for the one-chain compound Y Ba 2 Cu 3 O 6.95 and rather larger, ≈ 6 for the two-chain compound Y Ba 2 Cu 4 O 8 [1].…”

“…Taking the BCS mean-field approximation, each term in (4) can be decoupled, for example for the first term in (4), as∆ † 1,r,δ∆2,r+ĉ 2 …”