Superconducting Gap and Strong In-Plane Anisotropy in UntwinnedYBa2Cu3O7−

D. H. Lu

¹

,

D. L. Feng

²

,

N. P. Armitage

³

et al.

Abstract

With significantly improved sample quality and instrumental resolution, we clearly identify in the (π,0) ARPES spectra from YBa2Cu3O6.993, in the superconducting state, the long-sought 'peakdip-hump' structure. This advance allows us to investigate the large a-b anisotropy of the in-plane electronic structure including, in particular, a 50% difference in the magnitude of the superconducting gap that scales with the energy position of the hump feature. This anisotropy, likely induced by the presence of the CuO chains, raises serious questions about attempts to quantitatively explain the YBa2Cu3O 7−δ data from various experiments using models based on a perfectly square lattice.PACS numbers: 74.25.Jb, 74.72.Bk, 79.60.Bm High-temperature superconductivity (HTSC) is intimately related to the CuO 2 plane, which is the only common structural feature in all cuprates. This fact has led most of the proposed microscopic theories to assume a CuO 2 square planar structure. However, for the practical reason of sample quality, some of the most important and defining experiments have been performed on YBa 2 Cu 3 O 7−δ (Y123), which does not have a square lattice, but rather an orthorhombic structure (b/a ≈ 1.015), caused by the presence of a CuO chain layer [1]. This orthorhombicity, according to LDA calculation [2], should result in significant anisotropy in the in-plane electronic structure (this term will be used throughout this paper to refer to the electronic states associated with the CuO 2 plane), making it problematic to compare theories based on a square lattice with experimental data from Y123. Therefore, it is crucial to quantify the effect of orthorhombicity, if any, on the in-plane electronic structure in Y123. The problem is that angle-resolved photoemission spectroscopy (ARPES), being a uniquely powerful tool for this important task, has, until now, not been particularly effective for the study of Y123 [3]. The important 'peak-dip-hump' structure, which is seen routinely in Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) [4] has never been observed in Y123. This absence, together with the presence of a surface state [5], raises questions about ARPES data from Y123, and the universality of the superconducting peak in the cuprates.This paper reports a breakthrough in this important issue, made possible by significantly improved sample quality and instrumental resolution. By isolating a surface state peak near the Fermi energy (E F ), we can clearly resolve a 'peak-dip-hump' structure in the ARPES spectra around (π, 0) in Y123 that resembles the superconducting peak observed in Bi2212 [4]. More significantly, we find a strong a-b asymmetry of the in-plane electronic structure, such as the superconducting gap magnitude, which differs by about 50%. We argue that such a strong inplane a-b anisotropy should be taken into account when interpreting experiments performed on Y123.ARPES experiments were carried out at beamline 5-4 at SSRL, which is equipped with a normal-incidencemonochromator and a SCIENTA SES-200 analyzer. Untwinned Y...

s-Wave Symmetry Along the c-Axis and s+d In-plane Superconductivity in Bulk YBa2Cu4O8

“…However, along the crystallographic b-direction this is much more pronounced than along a, and the analysis of both data sets in terms of s + d wave gaps using (5) reveals that along the a direction the s-wave order parameter contributes 14%, whereas along the b direction the s-wave contribution is 42%. This assignment is supported by photoemission experiments on YBa 2 Cu 3 O 6.993 , where a 50% difference in the magnitude of the superconducting gaps was reported [29]. In addition, these data provide evidence that this anisotropy is not a consequence of the chain related electronic structure, but intrinsic to the CuO 2 planes.…”

Distribution of spectral weight in a system with disordered stripes

“…It is also noteworthy that y directed stripes not only lead to substantial low energy spectral weight in the anti-nodal region near (0, π), as naively expected, but also in the reciprocal region, near (π, 0), as well. (This observation is important in systems with macroscopic stripe orientational order, including, potentially, YBCO [12].) We also note that for the ordered array, there are oscillations in the spectral weight as a function of k x .…”

Phase separation models for cuprate stripe arrays

“…Such a large anisotropy is not consistent with tunneling measurements of the gap; it is possible that the anisotropy is reduced by strong interstripe coupling. On the other hand, a large gap anisotropy has been found in YBCO 70 , where the stripes are aligned along the chain direction 3 . In Ref.…”