Raman response of magnetic excitations in cuprate ladders and planes

Abstract

An unified picture for the Raman response of magnetic excitations in cuprate spin-ladder compounds is obtained by comparing calculated two-triplon Raman line-shapes with those of the prototypical compounds SrCu2O3 (Sr123), Sr14Cu24O41 (Sr14), and La6Ca8Cu24O41 (La6Ca8). The theoretical model for the two-leg ladder contains Heisenberg exchange couplings J and J ⊥ plus an additional four-spin interaction Jcyc. Within this model Sr123 and Sr14 can be described by x := J /J ⊥ = 1.5, xcyc := Jcyc/J ⊥ = 0.2, J Sr123 ⊥ = 1130 cm −1 and J Sr14 ⊥ = 1080 cm −1 . The couplings found for La6Ca8 are x = 1.2, xcyc = 0.2, and J La6Ca8 ⊥ = 1130 cm −1 . The unexpected sharp two-triplon peak in the ladder materials compared to the undoped two-dimensional cuprates can be traced back to the anisotropy of the magnetic exchange in rung and leg direction. With the results obtained for the isotropic ladder we calculate the Raman line-shape of a two-dimensional square lattice using a toy model consisting of a vertical and a horizontal ladder. A direct comparison of these results with Raman experiments for the two-dimensional cuprates R2CuO4 (R=La,Nd), Sr2CuO2Cl2, and YBa2Cu3O 6+δ yields a good agreement for the dominating two-triplon peak. We conclude that short range quantum fluctuations are dominating the magnetic Raman response in both, ladders and planes. We discuss possible scenarios responsible for the high-energy spectral weight of the Raman line-shape, i.e. phonons, the triple-resonance and multi-particle contributions.

Breathers and Raman scattering in a two-leg ladder with staggered Dzyaloshinskii-Moriya interaction

“…70,71 As the coupling between the magnons increases, the peak approaches the lower band edge. [69][70][71] Turning to the ladder with Dzyaloshinskii-Moriya interaction, in the weak-coupling case, the threshold at twice the spin gap will be obtained from the contribution of antisymmetric modes a / a to the Raman intensity. 67 The symmetric modes will give extra peaks and thresholds but at energy scales small compared to the gap.…”

Resonant two-phonon Raman scattering as a probe of hole crystal formation inSr14−xCaxCu24O41

“…A longstanding source of uncertainty for Sr 14 Cu 24 O 41 has been the intense high-energy spectral weight. 28 Gozar et al 29 in a study focused primarily on the two-magnon response have observed the transfer of spectral weight from the low-energy ͑Ͻ700 cm −1 ͒ region of their Raman spectra to higher energy as T is decreased. The disappearance of low-energy continuum states on cooling has also been observed Sugai and Suzuki.…”

K -edge and L3 -edge RIXS study of columnar and staggered quantum dimer phases of the square lattice Heisenberg model

“…Using this modulation one can explain the disappearance of the one-triplon excitation at q x = 0, and the emergence at q x 0. Both in Raman [39] and RIXS [68] experiments, finite two-triplon excitation intensity is observed in spin ladders at zero momentum transfer. According to Eqs.…”