Direct correlation betweenTcandCuO2bilayer spacing in

M. Varela

¹

,

D. Arias

²

,

Z. Sefrioui

³

et al.

Abstract

We report the effects of epitaxial strain and deoxygenation on high quality ͓YBa 2 Cu 3 O 7Ϫx (YBCO) N /PrBa 2 Cu 3 O 7 (PBCO) 5 ͔ 1000 Å superlattices, with 1ϽNϽ12 unit cells. High-spatialresolution electron energy loss spectroscopy shows that strained, fully oxygenated YBCO layers are underdoped. Irrespective of whether underdoping is induced by strain or deoxygenation, x-ray diffraction analysis shows that T c correlates directly with separation of the CuO 2 bilayers. DOI: 10.1103/PhysRevB.66.134517 PACS number͑s͒: 74.76.Bz, 68.65.Cd, 74.80.Dm Since the discovery of high-T c superconductivity, the relation between critical temperature and structure has been the focus of considerable interest. Changes in the critical temperature (T c ) induced by pressure ͑physical͒ and isovalent substitution ͑chemical pressure͒ have been extensively investigated to find which interatomic distances within the unit cell are relevant for the mechanism of superconductivity ͑see, for instance, Refs. 1-3 and references therein͒. However, despite considerable experimental and theoretical effort, a definitive answer has not yet been obtained, partly due to the complex intracell structure of the cuprates. Most theories consider superconductivity primarily located on the CuO 2 planes, which are coupled by proximity or pair tunneling effects, 4 -9 and extensive experimental work has been done, directed to examine the importance of the distance between CuO 2 planes of the same and of neighboring cells for the mechanism of superconductivity. Modifications of the interlayer coupling have been explored, changing the number of copper planes, 10 their inner structure, 11,12 or intercalating nonsuperconducting spacers as iodized 13 or organic compounds.14 Concerning the separation between neighboring CuO 2 planes, substitution of the rare-earth elements is known to influence lattice dimensions ͑chemical pressure experiments͒. 1,15 An increase of the rare earth ionic radius from 0.99 Å for Yb to 1.08 for Nd is known to increase the distance between planes and also T c from 88 to 95 K. 16Despite the extensive work, the results of some of the experiments are contradictory, 13,14 and a clear picture has not emerged yet. Changes in the distance between planes or modifications in the intracell distances due to cationic substitution may have, in addition, a direct influence in the charge transfer and thus on the carrier concentration. On the other hand, it is well known that changes in oxygen content, which result in changes in the carrier concentration, also cause significant structural changes. 17,18 This interplay between structural changes and doping has obscured the relationship between superconductivity and structure.In addition to physical or chemical pressure, structural changes induced by epitaxial strain in thin films can be used to explore the relationship between structure and superconductivity. In a previous paper we have reported that significant epitaxial strain occurs in ultrathin YBa 2 Cu 3 O 7Ϫx layers in ͓YBa 2 Cu 3 O 7Ϫx (YBCO...

Broad temperature study of RE-substitution effects on the in-field critical current behavior of REBCO superconducting tapes

“…The change of T c from 85.2 K to 93.4 K with increasing r from 0.0985 nm for Yb to 0.1066 nm for Eu shows a near-linear trend, which is overall in good agreement with early studies of bulk materials [11,12]. It has been proved that the r effect on T c in mono-RE REBCO films originates from the strain-induced charge redistribution between the charge reservoir and the CuO 2 plane [11] and correlates directly with separation of the CuO 2 plane [12,39] due to the changes of lattice parameters of RE substitutions. The T c of binary RE films fit well to the trendline, evidencing the formation of a uniform mixing RE-123 phase.…”

“…However, their cost/performance ratio is still not satisfactory, especially for applications in a high magnetic field. Therefore, a variety of approaches has been studied to further increase their self-field and in-field critical currents (I c ), including off-stoichiometric compositions [4,5], rare earth substitutions and combinations [1][2][3][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23], and the addition of BaZrO 3 [24][25][26][27], BaSnO 3 [28,29], BaHfO 3 [23], Gd 2 TaO 7 [30], and Ba 2 YNbO 6 [31,32]. Some combined usage of these technologies is expected to substantially improve the performance of REBCO tapes under practical working conditions.…”

“…RE substitutions and combinations have attracted great research interest because some REBCO films have superior characteristics to YBCO film. The impacts of RE substitutions include lower processing temperatures and wider processing windows for smaller RE ions [7,13], higher critical transition temperatures (T c ) for larger RE ions [11,12], improved surface morphologies for larger RE ions owing to different film growth kinetics, which are essential for the growth of high quality thick films [14,15], and the possibility of introducing point defects arising from cation exchange between large RE ions and Ba ions [7,8]. Substituting a Y ion in YBCO with a larger RE ion can enhance the self-field and in-field critical currents at 77 K. This has been proven in GdBCO [2,3,16,17], EuBCO [14,22,23] and SmBCO [15,18] films.…”

Magnetic, Electrical Properties and Spin-Glass Effect of Substitution of Ca for Pr in Ca2-xPrxMnO4 Compounds

“…The crystal chemistry of these oxides shown in numerous studies, are complex and subject to controversy [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The Ca 2-x Ln x MnO 4 compounds (Ln = Pr, Sm, Gd, Ho) are good examples of Ruddlesden-Popper manganites.…”

Synthesis, Structure and Properties of Tetragonal Sr2M3As2O2 (M3 = Mn3, Mn2Cu and MnZn2) Compounds Containing Alternating CuO2-Type and FeAs-Type Layers

“…The high T c in the cuprates is believed to be intimately related to the crystal geometry. [14][15][16] In the FeAs-type compounds, on the other hand, the As atoms are arranged in layers on either side of the Fe square lattice layers, resulting in tetrahedral coordination of the Fe atoms by As. Strong direct Fe-Fe interactions cause the electron states at E F to be dominated by states derived from the Fe d-orbitals.…”