Magnetic excitations and their anisotropy in YBCO: slave-boson mean-field analysis of bilayer t-J model

Abstract

We perform a comprehensive analysis of the dynamical magnetic susceptibility χ(q,ω) in the slave-boson mean-field scheme of the bilayer t-J model. In the d-wave pairing state, the strongest magnetic spectral weight appears at q=Q=(pi,pi) and ω=ω_{Q}^{res}, and spreads into a diamond-shaped shell around Q in q space for ω<ω_{Q}^{res}. This weight is due to a collective mode, namely a particle-hole bound state, which has a downward ωversus q dispersion around Q. Within the high intensity shell, the incommensurate (IC) signals at q=(pi,pi\pm 2piη) and (pi\pm 2piη,pi) tend to be stronger than the diagonal incommensurate (DIC) signals at q=(pi\pm 2piη',pi\pm 2piη'), especially for a large hole density δ. For ω\ll ω_{Q}^{res} the IC signals completely disappear and the weight remains only around the DIC positions. For ω> ω_{Q}^{res} strong signals of Imχ(q,ω) tracing an upward dispersion are found and interpreted as an overdamped collective mode near ω_{Q}^{res}. In the normal state, Imχ(q,ω) has a broad peak at q=Q. That is, the IC and DIC signals appear only in the d-wave pairing state. We also study effects of a small orthorhombic anisotropy, which is intrinsic in untwinned YBCO crystals. Because of electron-electron correlations favoring d-wave shaped Fermi surface deformations (dFSD), we expect an enhanced anisotropy of magnetic excitation spectra. This effect is particularly pronounced for low δand at relatively high temperature. The present theory provides a rather detailed microscopic explanation of the most salient properties of magnetic excitations observed in YBCO.

50 pages, 20 figures

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• Theory of reduced singlet pairing without the underlying state of charge stripes in the high-temperature superconductor YBa_2Cu_3O_6.45
• Generalized Pomeranchuk instabilities in graphene
• Self-masking of spontaneous symmetry breaking in layer materials
• Magnetic excitations in La-based cuprate superconductors: slave-boson mean-field analysis of the two-dimensional t-J model
• Spin susceptibility in bilayered cuprates: resonant magnetic excitations
• Spin-stiffness of anisotropic Heisenberg model on square lattice and possible mechanism for pinning of the electronic liquid crystal direction in YBCO
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• Coexistence of magnetism and superconductivity in a t-J bilayer
• Phenomenological theory of spin excitations in La- and Y-based cuprates
• Difficulty in the Fermi-Liquid-Based Theory for the In-Plane Magnetic Anisotropy in Untwinned High-T_c Superconductor
• Theory of the in-plane anisotropy of magnetic excitations in YBa_{2}Cu_{3}O_{6+y}
• Reply to 'Difficulty in the Fermi-Liquid-Based Theory for the In-Plane Magnetic Anisotropy in Untwinned High-$T_c$ Superconductors'