Structure Yttrium barium copper oxide

part of lattice structure of yttrium barium copper oxide

ybco crystallizes in defect perovskite structure consisting of layers. boundary of each layer defined planes of square planar cuo4 units sharing 4 vertices. planes can puckered. perpendicular these cuo2 planes cuo4 ribbons sharing 2 vertices. yttrium atoms found between cuo2 planes, while barium atoms found between cuo4 ribbons , cuo2 planes. structural feature illustrated in figure right.

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like many type-ii superconductors, ybco can exhibit flux pinning: lines of magnetic flux may pinned in place in crystal, force required move piece particular magnetic field configuration. piece of ybco placed above magnetic track can levitate @ fixed height.

although yba2cu3o7 well-defined chemical compound specific structure , stoichiometry, materials fewer 7 oxygen atoms per formula unit non-stoichiometric compounds. structure of these materials depends on oxygen content. non-stoichiometry denoted x in chemical formula yba2cu3o7−x. when x = 1, o(1) sites in cu(1) layer vacant , structure tetragonal. tetragonal form of ybco insulating , not superconduct. increasing oxygen content causes more of o(1) sites become occupied. x < 0.65, cu-o chains along b axis of crystal formed. elongation of b axis changes structure orthorhombic, lattice parameters of = 3.82, b = 3.89, , c = 11.68 Å. optimum superconducting properties occur when x ~ 0.07, i.e., of o(1) sites occupied, few vacancies.

in experiments other elements substituted on cu , ba sites, evidence has shown conduction occurs in cu(2)o planes while cu(1)o(1) chains act charge reservoirs, provide carriers cuo planes. however, model fails address superconductivity in homologue pr123 (praseodymium instead of yttrium). (conduction in copper planes) confines conductivity a-b planes , large anisotropy in transport properties observed. along c axis, normal conductivity 10 times smaller in a-b plane. other cuprates in same general class, anisotropy greater , inter-plane transport highly restricted.

furthermore, superconducting length scales show similar anisotropy, in both penetration depth (λab ≈ 150 nm, λc ≈ 800 nm) , coherence length, (ξab ≈ 2 nm, ξc ≈ 0.4 nm). although coherence length in a-b plane 5 times greater along c axis quite small compared classic superconductors such niobium (where ξ ≈ 40 nm). modest coherence length means superconducting state more susceptible local disruptions interfaces or defects on order of single unit cell, such boundary between twinned crystal domains. sensitivity small defects complicates fabricating devices ybco, , material sensitive degradation humidity.