Calculations of the Electronic Structure ofLa2−xBaxCu

Abstract

We present the first calculations of the electronic structure of the high-temperature superconductors in the disordered phase using the tight-binding coherent-potential approximation. These calculations are used to assess the influence of increasing oxygen vacancies when La is alloyed with M (~Sr or Ba) in the system \,2L2x M x Q\xO\y . Contrary to simple doping arguments, it is found that deoxygenation does not raise the Fermi level CEF). The increase in T c in the range 0.05 <x < 0.15 and the decrease in T c beyond the peak at x =0.15 follow the density of states at E?. PACS numbers: 74.70.Vy, 71.20.-b, 71.25.PiThe L&i-xMxCuOt-y system (M=Ba or Sr) displays an unusual assortment of phases, including normal and superconducting, metallic and nonmetallic, antiferromagnetic and paramagnetic, tetragonal and orthorhombic, and possibly spin-glass or other complex phases. Since this compound is the ancestor of all the high-T c cuprates and itself shows superconductivity up to 40 K (for x=0.15 and small y), our understanding the concentration dependence of T c is of special interest because it could provide clues to the pairing mechanism in these materials.A brief but excellent review of the properties of this system has been presented by Jorgensen. l Although initial reports of T c (x) for x>0.15 were ambiguous because of sample dependence, it has been established 1 that, although at equilibrium the system phase separates for x > 0.15, rapid cooling can produce single-phase material to much larger x. Samples become superconducting for x as small as 0.06, and T c peaks at x =0.15, then decreases to zero at x =0.3. The interpretation of this result is complicated by the discovery that, although the O vacancy concentration y is small (<0.03) for x < 0.15, for larger concentrations of M y increases, to near 0.10 for x =0.3 and to y =0.2 for x =0.6 [ Fig. (4) ofRef. 11. This complex, double disorder, behavior makes it unclear whether it is the increased alloying or the development of oxygen vacancies which kills T c . One interpretation 1 which is consistent with these data is that (l) alloying with the alkaline-earth ion lowers the Fermi level E? into the van Hove singularity, 2 " 4 increasing the density of states N(Ep) and enhancing T c , then (2) introduction of O vacancies overcompensates the Af-atom hole doping by electron doping, raising E? out of the van Hove singularity. We show below that this interpretation is inconsistent with calculations which account for the disorder.During the intense investigation of this system, several studies 2 " 4 have dealt with the band structure of the stoichiometric compound La2CuC>4 as well as the ordered compound 2 LaBaCuC>4. For the stoichiometric material the paramagnetic calculations do not correspond directly to the antiferromagnetic, and nonmetallic, 5 phase which is observed, but it has been assumed that a rigid-band extrapolation of the band structure into the metallic regime is realistic. This rigid-band picture predicts a peak in N(EF) precisely where T c peaks, as long a...

Importance of Local Band Effects for Ferromagnetism in Hole-DopedLa2CuO4Cuprate Superconductors

“…While the main DOS features with random distribution of Ba sites are close to a rigid band picture given by our VCA calculations and by the CPA results for x ¼ 0:14 [25] and for x ¼ 0:30 [26], the DOS for clustered configurations can be different. The results for the clustered cases are compared in Fig.…”

“…1:5), ferromagnetism appears in VCA calculations. Random oxygen vacancies, described by coherent potential approximation (CPA) calculations, will increase the DOS at E F [25] and might help Stoner magnetism.…”

Dielectric Function of La2CuO4

“…Obviously, this hope is shared by several groups, who recently presented band structure investigations about density of states, e.g. [3], and the optical conductivity [4] or dielectric function [5,6] of La,CuO, or YBa,Cu,O, being in fair agreement with experiments. Our investigations, although much less elaborate, yield similar results.…”

“…To evaluate the required matrix element we only took into account on-site and first inter-site contributions. In addition, for the on-site term we approximated the radial component of the Wannier functions by exp (-r/3,), 3, w 2/a. Inter-site terms are merely important for q < ./a were they are described by an adjustable parameter t = (p,(r) 1x1 dX2-Jra)}.…”

Different doping from apical and planar oxygen vacancies in Ba2CuO4−δand La2Jarlborg1, Barbiellini2, Markiewicz3 et al. 2012Phys. Rev. B160190View full textAdd to dashboardCite

“…Despite the unavoidable noise in the DOS, there is a clear dband shift to lower energy (with respect the Fermi level) for vacancies on apical sites compared to the pure case (BCO), while the shift for vacancies in the planes is much smaller with even some small enhancement of the DOS above E F . A DOS enhancement produced by O vacancies was found in coherent-potential-approximation (CPA) calculations 18,19 where all the oxygen sites (planar and apical) had equivalent random occupation. Notably, rigid band like pictures 20,21 have often been invoked in describing the doping evolution of the overdoped and optimally doped cuprates.…”