Highly dispersive magnons with spin-gap-like features in the frustrated ferromagneticS=12chain compoundCa2Y2Cu

Abstract

We report inelastic neutron scattering experiments in Ca2Y2Cu5O10 and map out the full one magnon dispersion which extends up to a record value of 53 meV for frustrated ferromagnetic (FM) edge-sharing CuO2 chain (FFESC) cuprates. A homogeneous spin-1/2 chain model with a FM nearest-neighbor (NN), an antiferromagnetic (AFM) next-nearest-neighbor (NNN) inchain, and two diagonal AFM interchain couplings (ICs) analyzed within linear spin-wave theory (LSWT) reproduces well the observed strong dispersion along the chains and a weak one perpendicularly. The ratio α = |Ja2/Ja1| of the FM NN and the AFM NNN couplings is found as ∼0.23, close to the critical point αc = 1/4 which separates ferromagnetically and antiferromagnetically correlated spiral magnetic ground states in single chains, whereas αc > 0.25 for coupled chains is considerably upshifted even for relatively weak IC. Although the measured dispersion can be described by homogeneous LSWT, the scattering intensity appears to be considerably reduced at ∼11.5 and ∼28 meV. The gap-like feature at 11.5 meV is attributed to magnon-phonon coupling whereas based on density matrix renormalization group simulations of the dynamical structure factor the gap at 28 meV is considered to stem partly from quantum effects due to the AFM IC. Another contribution to that gap is ascribed to the intrinsic superstructure from the distorting incommensurate pattern of CaY cationic chains adjacent to the CuO2 ones. It gives rise to non-equivalent CuO4 units and Cu-O-Cu bond angles Φ and a resulting distribution of all exchange integrals. The J's fitted by homogeneous LSWT are regarded as average values. The record value of the FM NN integral J1 = 24 meV among FFESC cuprates can be explained by a non-universal Φ( = 90 • ) and Cu-O bond length dependent anisotropic mean direct FM Cu-O exchangeK pd ∼120 meV, similar to a value of 105 meV for Li2CuO2, in accord with larger values for La2CuO4 and CuGeO3 (∼110 meV) reported by Braden et al. [Phys. Rev. B 54, 1105] phenomenologically. Enhanced K pd values are also needed to compensate a significant AFM J dd ≥ 6 meV from the dd channel, generic for FFESC cuprates but ignored so far.