Superconducting order parameter in bilayer cuprates: Occurrence of π phase shifts in corner junctions

Abstract

We study the order parameter symmetry in bi-layer cuprates such as YBaCuO, where interesting π phase shifts have been observed in Josephson junctions. Taking models which represent the measured spin fluctuation spectra of this cuprate, as well as more general models of Coulomb correlation effects, we classify the allowed symmetries and determine their associated physical properties. π phase shifts are shown to be a general consequence of repulsive interactions, independent of whether a magnetic mechanism is operative. While it is known to occur in d-states, this behavior can also be associated with (orthorhombic) s-symmetry when the two sub-band gaps have opposite phase. Implications for the magnitude of Tc are discussed.PACS numbers: 74.20.Mn, 74.50.+r, The observation in YBCO of unusual Josephson junction behavior 1-4 is one of the most important experimental results to emerge from the cuprate literature in recent years. Here in a Josephson SQUID experiment the two junctions are configured so that their normals lie along the a and b axes of the CuO 2 plane. This measurement has been widely interpreted as support for a d-symmetry of the order parameter, as well as for a magnetic mechanism for superconductivity. In this paper we show that both of these inferences may be inappropriate. For notational precision, throughout this paper we use the terms s-( or d-) symmetry to correspond to states which have the same (or opposite) sign under a π/2 rotation. Thus the d-states under consideration are not necessarily of the specific d x 2 −y 2 form.The gap equation for bi-layer systems has been studied earlier in the context of a magnetic mechanism for superconductivity 5,6 . There it was observed that the d-symmetric state of the single layer problem, is transformed to a pair of in-phase d states on each of the two sub-bands, and that these compete with a pair of outof-phase s-states. Here we take the problem to a greater level of generality, establishing that this situation persists for a wide class of repulsive interactions, which are unrelated to the antiferromagnetic spin fluctuation picture. Alternate classes of the order parameter symmetry are also generated. These correspond to in-phase s-states and out-of-phase d-states. We establish how the relative stability of the two competing states is affected by changes in bandstructure, orthorhombicity, and hole filling.It should not be surprising that d-states have a more general origin beyond the antiferromagnetic spin exchange models. In a one layer cuprate, the lattice symmetry requires that all gap states are either even (s-) or odd (d-) under a π/2 rotation. In bi-layer systems, these one layer states generalize naturally to a pair of even or odd, in-phase or out-of-phase states, associated with each of the two sub-bands. Thus, as one of two allowed states, d-symmetry should be widespread, and independent of the microscopic details of the model.In the presence of both intra-and inter-layer interactions (V and V ⊥ ), the weak coupling BCS gap equation becomes a set...

Hole dispersion and symmetry of the superconducting order parameter for underdopedCuO2bilayers and the three-dimensional antiferromagnets

“…This is in agreement with Ref. [21] where a similar phenomenological in-plane susceptibility derived from the neutron scattering data was used to construct the effective potential with an arbitrary relation between the in-plane and inter-plane coefficients. The above mentioned RPA model is certainly relevant to the more phenomenological approach used in this paper if one considers the expansion of χ 0 (q) in real space, assuming that the effective coupling between quasiparticles is proportional to χ 0 .…”

Arguments and evidence for a node-containing anisotropics-wave gap form in the cuprate superconductors

“…1, where the normal to the twin boundary is at 45°. volving competition between gaps of opposite signs for the different bands ͑bonding and antibonding bands͒ of a bilayer [12][13][14][15][16] or of a bilayer plus chain 16,17 system, but in the present case these gaps have the same sign and the bilayer character of YBCO does not play an essential role. The chain gap is also assumed here to be unimportant for symmetrysensitive experiments.…”

“…Perhaps the most relevant of these for the present context are ͑a͒ the idea of a pair of similar but not identical anisotropic s-like gap forms for the bonding and antibonding bands of a bilayer cuprate model system, with opposite signs for the gaps of these two bands, [12][13][14][15][16] and ͑b͒ a related idea which assumes a major role for the gap on the chains of YBCO. 16,17 The former efforts [12][13][14][15][16] were motivated partly by the fact that most of the data concerning cuprate gap symmetry have come from the bilayer materials YBCO and Bi-2212 (Bi 2 Ba 2 CaCu 2 O 8ϩ␦ ), and they were also motivated partly by the so-called resonance, the resonance at or near 41 meV and at ͑,,͒ in the Brillouin zone, found below T c in inelastic neutron scattering for these same two bilayer materials. 18 It has been argued 15 that this type of gap form ͑and/or the presence of a major chain contribution with opposite sign 16,17 ͒ can explain the phase difference found in corner-junction and corner-SQUID tunnelling experiments, the phase difference which is commonly viewed as the signature of the d-wave gap form.…”

Effects of interlayer interaction on the superconducting state inYBa2Cu3O'Donovan1, Ćarbotte2 1997Phys. Rev. B301220View full textAdd to dashboardCite

“…The effect of an interlayer hopping has been extensively studied in works by Atkinson and Carbotte [13] as well as others. [14,15,[17][18][19] We solve the coupled bcs equations (6) using a fft technique. [4] In Fig.…”