Magnetic excitations and their anisotropy inYBa2Cu3O6+x: Slave-boson mean-field analysis of the bilayer

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. We use model parameters appropriate for YBa 2 Cu 3 O 6+x (YBCO), a typical bilayer high-T c cuprate compound well studied by neutron scattering experiments. In the d-wave pairing state, the strongest magnetic spectral weight appears at q = Q ≡ (π, π) and ω = ω res Q , and spreads into a diamond-shaped shell around Q in q space for ω < ω res Q . This weight is due to a collective mode, namely a particle-hole bound state, which has a downward ω versus q dispersion around Q. Within the high intensity shell, the incommensurate (IC) signals at q = (π, π ± 2πη) and (π ± 2πη, π) tend to be stronger than the diagonal incommensurate (DIC) signals at q = (π ± 2πη ′ , π ± 2πη ′ ), especially for a large hole density δ. For ω ≪ ω res Q the IC signals completely disappear and the weight remains only around the DIC positions. For ω > ω res Q strong signals of Imχ(q, ω) tracing an upward dispersion are found and interpreted as an overdamped collective mode near ω res Q . 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 dwave 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 δ and at relatively high temperature. The present theory provides a rather detailed microscopic explanation of the most salient properties of magnetic excitations observed in YBCO.

Nernst-effect anisotropy as a sensitive probe of Fermi-surface distortions from electron-nematic order

“…24 Nematic order is expected to strongly amplify this anisotropy, and theoretical treatments have suggested anisotropies up to |ǫ| = 20%. 10 Fig . 2 shows the anisotropic elements of the response tensorsσ andα, together with the Nernst coefficient, for In both panels, the white area corresponds to an empty band.…”

“…Nematic order distorts the band structure, which, on the mean-field level, can be captured by anisotropic hop-ping parameters. 8,10 Here we shall focus on the case of a d x 2 −y 2 electron nematic, as may arise from a Pomeranchuk instability in the l = 2 channel or as precursor of a stripe phase. We introduce an anisotropy parameter ǫ, such that the hopping matrix elements obey t 1x,y = (1 ± ǫ/2)t 1 and t 3x,y = (1 ± ǫ/2)t 3 .…”

“…[5][6][7] Microscopically, it is one of the known instabilities of the two-dimensional (2d) Hubbard model. [8][9][10] The first clear-cut signature of electron-nematic order in cuprates was found in neutron scattering experiments on YBa 2 Cu 3 O 6.45 , 1 where the spin-fluctuation spectrum was found to develop a distinct anisotropic incommensurability below a temperature T of about 150 K. Earlier transport measurements on YBa 2 Cu 3 O 6+δ (Ref. 11) also detected resistivity anisotropies ρ a /ρ b of order 2 in the underdoped regime, but remained less conclusive.…”

Cuprate superconductors in the vicinity of a Pomeranchuk instability

“…2 for ω = 0.30J and δ = 0.12 in the odd channel; a similar result is obtained in the even channel. [15] The strong intensity region forms a deformed diamond shape at low T [ Fig. 2(a)], and the spectral weight inside the diamond gradually increases with T [ Fig.…”

Coexistence of magnetism and superconductivity in at−Jbilayer

“…A large volume of work exists that focuses on the various details of the coexistence phenomenon. Some of the theoretical attempts include mean field studies of t-J and Hubbardlike models which confirm the coexistence of magnetic and superconducting order 4,5 . The coexistence is suggestive of the presence of short range AF correlations in SC state that are probed by inelastic neutron scattering experiments via an enhanced scattering intensity near the AF wavevector (π, π) 6,7,8,9 .…”