In-plane ordering of oxygen vacancies in a high- Tc cuprate superconductor with compressed Cu-O octahedrons: An automated cluster expansion study

Abstract

A recently discovered high-Tc cuprate superconductor Ba2CuO4-δ exhibits exceptional Jahn-Teller distortion, wherein the CuO6 octahedrons are compressed along the c axis. As a consequence, the O vacancies prefer to reside in the CuO2 plane, but the exact structure is not known. By combining first-principles total energy calculation with the automated structure inversion method, the effective cluster interactions of O vacancies are mapped out. Around δ=0.8, where the 73K superconductivity was observed experimentally, we predict that the ordered O vacancies slice the CuO2 plane into not only 1D chains and but also two-leg ladders. A Monte Carlo simulation is performed based on the effective cluster interaction model, showing that such an ordering pattern is stable up to ~900 K. Our results put forth a concrete structural basis to discuss the underlying superconducting mechanism.2 Main Text:High-Tc superconductivity in cuprates is commonly spawned in the intact two-dimensional CuO2 plane [1][2][3][4][5][6]. However, a new cuprate superconductor Ba2CuO4-δ appears to be an exception [7]. Ba2CuO4-δ crystallizes into the typical 214 layered perovskite structure [ Fig. 1(a)], but the CuO6 octahedrons are largely compressed along the c axis, making the in-plane Cu-O bonds weaker than the out-of-plane ones. Consequently, in contrast to the apical substitution as typically observed in other high-Tc cuprates [3][4][5][6], here the O vacancies prefer to be created in the CuO2 plane [8,9]. Surprisingly, superconductivity emerges at a very high concentration of in-plane O vacancies, when the 2D parent lattice has been strongly disrupted, and the superconducting transition temperature reaches as high as 73 K around δ=0.8.At present, the in-plane O vacancy structure is still largely unknown except for the δ=1 limit, i.e. Ba2CuO3 [8]. This stoichiometric compound as a quasi-1D Mott insulator consists of paralleled (-O-Cu-O-) chains. One can reversely consider that superconductivity emerges from Ba2CuO3+γ around γ=0.2, when excess O atoms link the 1D chains, which appears to play an important role in the 73 K superconductivity, because in other quasi-1D cuprates with charge doping only, such a high Tc has never been observed.[9-12] An interesting observation is that the atomic structure of Ba2CuO3 can be viewed as long-range ordering of in-plane O vacancies in Ba2CuO4. Ba2CuO3 shares exactly the same lattice as Ba2CuO4, with only slight changes of the lattice constants (Tab. I). The O atoms are missing along all the unidirectional (-O-Cu-O-) arrays, like cutting either the warp or the weft of a fabric. The coordinates of the remaining atoms stay almost unchanged.

Two-band Conduction and Nesting Instabilities in Superconducting Ba$_2$CuO$_{3+δ}$: a First Principles Study

“…2.4 Å in La 2 CuO 4 . A cluster expansion study [26] of compositions around δ ∼ 0.2 found that ordered supercells containing Cu-O P chains and ladders are favored up to 900 K. Our choices are guided by the chain structure of isovalent Sr 2 CuO 3 for δ=0, and by the simplest choices of added in-plane oxygen atoms (described below) for δ=1/4. These choices allow the assessment of differences due to coordination (there are four-, five-, and six-fold Cu ions), which seems to be a more useful viewpoint than crystal field splitting.…”

“…To describe models of ordered planar oxygens in the superconducting doped regime, we investigated three structures of doubled or quadrupled supercells (as necessary) having two or three Cu sites, with the z = c/2 layers pictured in Fig. 2 [18,25,26]. All structures are derived from the Ba 2 CuO 3 layers of Cu-O P chains in which each Cu has an apical O above and below.…”

“…Based on the inferred vacancies in the planar oxygens in Ba 2 CuO 3+δ , several lattices [12,18,25,26] with ordered planar oxygens have been suggested to approximate the superconducting phase. Among them, we have investigated structures with brickwall [25] and two-type ladder plus chain lattices [18,26], which have Cu ions in up to three types of coordination. At δ = 1/4 doping, the bilayer structure of a ladder layer plus a chain layer is found to be the ground state among the three structures.…”

Superconducting mechanism for the cuprate Ba2CuO3+δ based on a multiorbital Lieb lattice model

“…These features are all in strong contradiction with the high-T c condition for the conventional cuprates, which suggests that an alternative pairing mechanism may be at work in this new material. Indeed, a number of theoretical studies have proposed various pairing mechanisms based on various lattice structures [9][10][11][12][13][14]. The experimental finding of Ba 2 CuO 3+δ may also shed a light on the previous finding of Sr 2 CuO 3+δ [15][16][17], which also possesses a large amount of oxygen deficiencies and a T c as high as 90 K but a much lower superconducting fraction than in the Ba 2 CuO 3+δ .…”

“…Liu et al propose a chain-type structure, which actually exists in Sr 2 CuO 3 [10,[18][19][20]. Li et al predict a ladder-type lattice based on an automated structure inversion method [12]. Le et al propose a structure where a matrix of Ba 2 CuO 4 with CuO 2 planes is embedded in the background of Ba 2 CuO 3 [11].…”