Fermi surface reconstruction of superoxygenated La2CuO4superconductors with ordered oxygen interstitials

Abstract

Novel imaging methods show that the mobile dopants in optimum doped La 2 CuO 4+y (LCO) get self-organized, instead of randomly distributed, to form an inhomogeneous network of nanoscale metallic puddles with ordered oxygen interstitials interspersed with oxygen-depleted regions. These puddles are expected to be metallic, being far from half filling because of high dopant density, and to sustain superconductivity having a size in the range 5-20 nm. However, the electronic structure of these heavily doped metallic puddles is not known. In fact the rigid-band model fails because of ordering of dopants and supercell calculations are required to obtain the Fermi surface reconstruction. We have performed advanced band calculations for a large supercell La 16 Cu 8 O 32+N where N = 1 or 2 oxygen interstitials form rows in the spacer La 16 O 16+N layers intercalated between the CuO 2 layers as determined by scanning nano x-ray diffraction. The additional occupied states made by interstitial oxygen orbitals sit well below the Fermi level (E F ) and lead to hole doping as expected. The unexpected results show that in the heavily doped puddles the altered Cu(3d)-O(2p) band hybridization at E F induces a multiband electronic structure with the formation of multiple Fermi surface spots: (a) Small gaps appear in the folded Fermi surface, (b) three minibands cross E F with reduced Fermi energies of 60, 150, and 240 meV, respectively, (c) the density of states and band mass at E F show substantial increases, and (d) spin-polarized calculations show a moderate increase of antiferromagnetic spin fluctuations. All calculated features are favorable to enhance superconductivity; however, the comparison with experimental methods probing the average electronic structure of cuprates will require the description of the electronics of a network of multigap superconducting puddles.

A Study of Superconducting La<sub>2</sub>CuO<sub>4</sub> via Generalized BCS Equations Incorporating Chemical Potential

“…By including μ in GBCSEs as applicable to LCO, appealing to an idea inspired by the work of Tacon et al [9] and using as input any of the observed values of the gap-to-T c ratio for 36 ≤ T c ≤ 40 K, we have been led to solutions for μ and λ that are "sensible". Sensible because we found a) μ to have a low value-in the meV range (recall that for elemental SCs, μ is in ≈2 -10 eV range), which is in accord with the assertion made in [14] and the values reported in [15], and b) λ to be less than 0.5, which is in accord with the Bogoliubov constraint as discussed in [5]. (18) and (19), respectively.…”

“…We now draw attention to the fact that Fermi energies of HTSCs have of late been attracting considerable interest, partly because their low values are believed [14] to be the cause of high-T c s and partly because they shed light on the gap-structures of HTSCs. In the context of LCO it seems significant that, on the basis of a thorough theoretical investigation, Jarlborg and Bianconi [15] have recently reported three values for its Fermi energy: 60, 150, and 240 meV. We now recall that both the usual BCS equations and GBCSEs, Equations (5) and (8) …”

“…which is the second equation after Equation (15), with E F replaced by μ and the factor of ħ 2 inserted. However, before we can use this equation we need to find the band effective mass of electrons in LCO.…”

Electronic Structure of HgBa2CuO4+δ with Self-organized Interstitial Oxygen Wires in the Hg Spacer Planes

“…The details of the methods have been published earlier [47,53,65,66]. The supercell Hg 12 Ba 24 Cu 12 O 48+2 is extended 6-and 2-lattice constants along x and y, respectively.…”

Electronic Structure and Properties of Superconducting Materials with Simple Fermi Surfaces

“…The idea that a higher N (E F ) provides a larger gain in electronic energy from a superconducting gap, and hence stronger coupling constants and higher T C , is still valid at large disorder. How to modify the cuprate band structures in order to get higher DOS without destroying other properties might be difficult, perhaps ordering of impurities into stripes is a promising way [50].…”