Interplane Electronic Spin Polarization Transfer in the Superconducting State ofY2Ba4Cu7

Abstract

We report a spin-echo double resonance study of the plane Cu sites in the superconducting state of nearly optimally doped Y 2 Ba 4 Cu 7 O 15 employing nuclear quadrupole resonance. The detailed analysis of the temperature dependence of the indirect in-plane and interplane spin-spin relaxation rate reveals two results: (i) In the normal conducting state, strong antiferromagnetic correlations exist between adjacent CuO 2 layers. (ii) In the superconducting state, the electronic spin polarization transfer between these layers is drastically reduced for decreasing temperature. Both findings are consistent with the interlayer tunneling model. [S0031-9007(99)08388-X] PACS numbers: 76.60. Gv, 74.20.Mn, 74.72.Bk Superconductivity in Y-Ba-Cu-O high-temperature superconductors takes place in the CuO 2 planes which form double planes (or bilayers). Experimentally, one finds that the maximum of the critical temperature, T c , of bilayer compounds is systematically higher than the one of the corresponding single layer material. This clearly indicates that the charge carrier correlation between adjacent CuO 2 layers has important implications for any kind of model that tries to explain superconductivity. The most prominent ansatz that involves explicitly the interlayer mechanism is the interlayer tunneling model by Anderson and co-workers [1]. In this model, the coherent single-particle tunneling between adjacent layers is blocked, resulting in a 2D normal state charge carrier confinement that is lifted by the superconducting transition through interlayer Josephson tunneling. As a result, this leads to an amplification of the pairing mechanism within the single layer and thus to an increased T c .Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) methods which probe the electronic system in its ground state, i.e., at frequencies close to zero, are well suited to test the interlayer tunneling model. Among the many bilayer compounds Y 2 Ba 4 Cu 7 O 15 is the ideal substance to perform such an investigation. Its structure consists of a sequence of alternating YBa 2 Cu 3 O 7 (1-2-3 for short) and YBa 2 Cu 4 O 8 (1-2-4) blocks with plane Cu sites Cu(2) and Cu(3), respectively. As in YBa 2 Cu 3 O 7 and YBa 2 Cu 4 O 8 , the CuO 2 planes in Y 2 Ba 4 Cu 7 O 15 form double planes separated by Y ions. However, because of the alternation of 1-2-3 and 1-2-4 blocks the individual planes of a CuO 2 bilayer in Y 2 Ba 4 Cu 7 O 15 are inequivalent and hence can separately be monitored by NMR and NQR methods. This can be accomplished because the plane Cu NQR lines of both blocks are quite narrow and well separated [2][3][4].In this Letter, we report on new measurements that extend our earlier NQR spin-echo double resonance (SE-DOR) experiments [2] into the superconducting (SC) state of Y 2 Ba 4 Cu 7 O 15 . We show that, in the normal conducting (NC) state with decreasing temperature, strong antifer-romagnetic correlations and an increasing electronic spin polarization transfer between the two inequivalent CuO 2 planes tak...

Origin of the Enhanced Copper Spin Echo Decay Rate in the Pseudogap Regime of the Multilayer High-TcCuprates

“…One can see that χ 12 (Q)/χ 11 (Q) rapidly increases in the pseudogap T region, indicating that χ 12 (Q) rapidly grows there. This is consistent with the SE-DOR results [7,8,9] and the theoretical calculations [5,6,20,21,22]. Note that χ 12 (Q)/χ 11 (Q) ≈ T −1 2S /T −1…”

“…4(c), where is adjusted so that 1 at high T. One can see that 12 Q= 11 Q rapidly increases in the pseudogap T region, indicating that 12 Q rapidly grows there. This is consistent with the SEDOR results [14,15] and the theoretical calculations [12,13,26,27]. Note that 12 Q= 11 Q T ÿ1 2S =T ÿ1 2G where T 2S is the SEDOR decay time between the sites on the different layers.…”

“…From Fig. 4(c), we find that 12 Q= 11 Q 0:28 at T c , while T ÿ1 2S =T ÿ1 2G was reported to be about 0.25 at T c in Y 2 Ba 4 Cu 7 O 15 [15]. The consistent explanation for both T ÿ1 2M and T ÿ1 2S indicates that they can be interpreted on the same basis of the interlayer spin correlations.…”

“…Simultaneous decreases of T 1 T ÿ1 and T ÿ2 2G are observed in a single layer system [10]. One proposed mechanism for multilayer systems is spin correlations between adjacent layers [11][12][13], which had been observed in spin echo double resonance (SEDOR) experiments [14,15]. In bilayer systems, the two CuO 2 layers in a unit cell are equivalent, so that the Cu nuclei on one of the layers behave as like-spins to the others in the echo decay process and contribute to T ÿ2 2G through the interlayer spin correlations.…”

“…A key to resolve the problem is that the phenomenon is observed only in multilayer systems. One proposed mechanism is spin correlations between adjacent layers [4,5,6], which had been observed in spin echo double resonance (SEDOR) experiments [7,8,9]. They are expected to play essential roles in the systematic increase of T c with increasing the number of layers because they can provide attractive forces between layers and stabilize the superconductivity [10].…”