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تسجيل مستخدم جديدSuperconducting mechanism for the cuprate Ba$_2$CuO$_{3+delta}$ based on a multiorbital Lieb lattice model
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For the recently discovered cuprate superconductor $mathrm{Ba_{2}CuO_{3+delta}}$, we propose a lattice structure which resembles the model considered by Lieb to represent the vastly oxygen-deficient material. We first investigate the stability of the Lieb-lattice structure, and then construct a multiorbital Hubbard model based on first-principles calculation. By applying the fluctuation-exchange approximation to the model and solving the linearized Eliashberg equation, we show that $s$-wave and $d$-wave pairings closely compete with each other, and, more interestingly, that the intra-orbital and inter-orbital pairings coexist. We further show that, if the energy of the $d_{3z^2-r^2}$ band is raised to make it incipient with the lower edge of the band close to the Fermi level within a realistic band filling regime, $spm$-wave superconductivity is strongly enhanced. We reveal an intriguing relation between the Lieb model and the two-orbital model for the usual K$_2$NiF$_4$ structure where a close competition between $s-$ and $d-$wave pairings is known to occur. The enhanced superconductivity in the present model is further shown to be related to an enhancement found previously in the bilayer Hubbard model with an incipient band.
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First principles investigations of the high temperature superconducting system Ba$_2$CuO$_{3+delta}$, recently discovered at $deltaapprox0.2$ at $T_c=70$ K, are applied to demonstrate the effects of oxygen ordering on the electronic and magnetic prop
erties. The observed `highly over-doped superconducting phase displays stretched Cu-planar oxygen O$_{rm P}$ distances and anomalously shortened Cu-apical O$_{rm A}$ separations compared with other cuprates. The stoichiometric system $delta=0$, with its strongly one-dimensional (1D) Cu-O$_{rm P}$ chain structure, when nonmagnetic shows 1D Fermi surfaces that lead, within density functional theory, to antiferromagnetic Cu-O$_{rm P}$ chains (a spin-Peierls instability). Accounting for 1D fluctuations and small interchain coupling according to the theory of Schulz indicates this system, like Sr$_2$CuO$_3$, is near the 1D Luttinger-liquid quantum critical phase. The unusual Cu-O bond lengths per se have limited effects on other properties for $delta$=0. We find that a `doubled bilayer structure of alternating Cu-O$_{rm P}$ chains and wide rung Cu$_3$O$_4$ ladders is the energetically preferred one of three possibilities where the additional oxygen ions bridge Cu-O$_{rm P}$ chains in the superconducting phase $delta=1/4$. Nominal formal valences of the three Cu sites are discussed. The six-fold (octahedral) site is the most highly oxidized, accepting somewhat more holes in the $d_{z^2}$ orbital than in the $d_{x^2-y^2}$ orbital. The implication is that two-band physics is involved in the pairing mechanism and the superconducting carriers. The Fermi surfaces of this metallic bilayer structure show both 1D and 2D strong (incipient) nesting instabilities, possibly accounting for the lack of clean single-phase samples based on this structure and suggesting importance for the pairing mechanism.
The recently discovered cuprate superconductor Ba$_2$CuO$_{3+delta}$ exhibits a high $T_csimeq73$K at $deltasimeq0.2$. The polycrystal grown under high pressure has a structure similar to La$_2$CuO$_4$, but with dramatically different lattice paramet
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