Intracell Changes in Epitaxially StrainedYBa2Cu3O7−x

Abstract

The structure of high quality ͓YBCO N ͞PBCO M ͔ 1000 ± A superlattices, with N ranging between 1 and 12 unit cells and M 5 unit cells, grown by high oxygen pressure sputtering, is analyzed. Intracell atomic structure of the layers along the c axis and disorder at interfaces is investigated using an x-ray refinement technique. Negligible roughness, step disorder, and interdiffusion are found at the interfaces. Epitaxial mismatch strain results in a surprising reorganization of interatomic distances for the thinnest YBCO layers, which seems to correlated with the decrease in the critical temperature. Intracell structure is invoked as an additional source of T c changes in very thin YBCO layers. PACS numbers: 74.76.Bz, 61.10.Nz, 68.65. + g Since the discovery of the high T c superconductivity, structure has been recognized to play a crucial role towards the understanding of its nature and mechanisms. It has been known for years that distortions arising from cation substitution can produce significant changes in T c [1], and recent experiments on doped La 2 CuO 4 superconductors at constant carrier concentration show a clear dependence of T c on lattice strains [2]. A great effort has been put in structure determination under hydrostatic pressure [3]. Epitaxial stress in thin films offers a simple way to arrive at a strain pattern not attainable under hydrostatic pressure [4]: According to the Poisson effect, film growth on a substrate with slightly smaller (larger) in-plane lattice parameters may lead to a compression (expansion) in the ab plane that can result in an expansion (contraction) in the out-ofplane direction. Uniaxial epitaxial strain, together with Poisson's ratios, has been addressed before [5]. However, the general applicability of the Poisson effect to thin films is still doubtful [6], especially in these highly anisotropic materials. Anyway, Locquet et al. [7] have been able to double the critical temperature in the La 1.9 Sr 0.1 CuO 4 high T c superconductor using mismatch strain. They show that compressive epitaxial strain in-plane can generate much larger increases in T c than those obtained by comparable hydrostatic pressures, and their claim is that the distance relevant to the mechanism of the superconductivity being modified is the separation between consecutive CuO 2 planes. Mismatch strain constitutes an alternative way to change the intracell distances which may be "relevant" to the mechanism of superconductivity, but a quantitative structure analysis of strained films is necessary. X-ray diffraction is a widely used technique to analyze structure, which supplies structural information averaged over a length scale (structural coherence length) which may be around a hundred angstroms. The extraction of quantitative information requires the fit of the diffraction pattern to a structure model containing a large number of parameters in these complex materials, and, therefore, results may not be very reliable for single epitaxial films, which usually show a reduced number of diffraction peak...

Spin diffusion versus proximity effect at ferromagnet/superconductorLa0.7Ca0.3MnO3∕YBa2Cu3OPeña1, Visani2, García‐Barriocanal3 et al. 2006Phys. Rev. BSelf Cite541251View full textAdd to dashboardCite

“…34 Both samples show the critical temperature to decrease when the thickness of the superconducting layer is reduced. However, while YBCO/LCMO/STO bilayers show broad tails for the smallest YBCO thicknesses pointing to a 35,36 as a result of epitaxial strain relaxation or dimensionality effects, but the decrease is more pronounced for samples with ferromagnetic layers as a result of the strong F/S interplay observed previously in trilayers and superlattices. 28,29,37 Interestingly, LCMO/ YBCO/PBCO/STO samples with much lower magnetization than YBCO/LCMO/STO bilayers show also higher T c values for the same thickness values of the individual YBCO layers, which further supports the role played by the interplay between ferromagnetism and superconductivity in depressing the critical temperature in these samples.…”

Direct Evidence for Block-by-Block Growth in High-Temperature Superconductor Ultrathin Films

“…This randomness causes satellite broadening and overshadows the apparent splitting in most cases. However, the analysis of the XRD patterns for an integer number of YBCO layers has shown the absence of step disorder in our superlattices [12]. Hence, in the absence of step disorder, the layer thickness does not fluctuate in a random fashion, and an unusual superlattice diffraction pattern arises with sharp diffraction satellite peaks.…”

“…Figures 1(f)-1(h) show the XRD patterns of samples ͓YBCO n ͞PBCO 5 ͔ 100nm with n 0.2, 0.5, and 1. Vertical lines show that the modulation length does not change and remains at the value (7 nm) corresponding to a strained ͓YBCO 1 ͞PBCO 5 ͔ 100nm superlattice [12]. In other words, the repeating unit consists of a whole YBCO unit cell plus five PBCO cells, with the single YBCO cell not completely covering the substrate surface.…”

“…Various reports have been presented in recent years supporting both growth modes: Pennycook et al [9] and Jia et al [10] have shown the tendency of the system to complete unit cell blocks, however, more recently, Haage et al [11] YBCO͞PBCO superlattices were grown epitaxially on (100) SrTiO 3 substrates held at a temperature of 900 ± C in a high pressure (3.6 mbar pure oxygen) sputtering system. Such a high pressure oxygen atmosphere yields very thermalized growth at a very slow nominal growth rate of 0.013 nm͞s for YBCO which allows an accurate control of film thicknesses even below one unit cell (ഠ1.2 nm for YBCO) [12]. High angle x-ray diffraction (XRD) spectra were analyzed using the SUPREX9 refinement software [13] which provides complete structural characterization of the superlattice along the growth direction, including layer thickness, and the disorder-related parameters, such as interdiffusion and interface roughness.…”

Superconducting and normal-state interlayer exchange coupling inLa0.67Sr0.33MnO3−YSenapati1, Budhani2 2005Phys. Rev. B360350View full textAdd to dashboardCite

“…For this non-magnetic system, the T c drops as the thickness of the YBCO layer (d Y ) is reduced. The variation of T c with d Y in PBCO-YBCO-PBCO multilayers has been studied extensively by several groups, and various reasons have been given for the drop 33,34,35 . These include interfacial stress, a drop in c-axis coupling of the condensate as the number of CuO 2 planes is reduced etc.…”