The square-lattice quantum Heisenberg antiferromagnet displays a pronounced anomaly of unknown origin in its magnetic excitation spectrum. The anomaly manifests itself only for short wavelength excitations propagating along the direction connecting n
earest neighbors. Using polarized neutron spectroscopy, we have fully characterized the magnetic fluctuations in the model metal-organic compound CFTD, revealing an isotropic continuum at the anomaly indicative of fractional excitations. A theoretical framework based on the Gutzwiller projection method is developed to explain the origin of the continuum at the anomaly. This indicates that the anomaly arises from deconfined fractional spin-1/2 quasiparticle pairs, the 2D analog of 1D spinons. Away from the anomaly the conventional spin-wave spectrum is recovered as pairs of fractional quasiparticles bind to form spin-1 magnons. Our results therefore establish the existence of fractional quasiparticles in the simplest model two dimensional antiferromagnet even in the absence of frustration.
Using low-energy projection of the one-band t-t-t-Hubbard model we derive an effective spin-Hamiltonian and its spin-wave expansion to order 1/S. We fit the spin-wave dispersion of several parent compounds to the high-temperature superconducting cupr
ates: La2CuO4, Sr2CuO2Cl2 and Bi2Sr2YCu2O8. Our accurate quantitative determination of the one-band Hubbard model parameters allows prediction and comparison to experimental results of measurable quantities such as staggered moment, double occupancy density, spin-wave velocity and bimagnon excitation spectrum and density of states, which is discussed in relation to K-edge RIXS and Raman experiments.
Using high-resolution resonant inelastic x-ray scattering (RIXS), we performed a momentum-resolved study of magnetic excitations in the model spin-1/2 2D antiferromagnetic insulator Sr_2CuCl_2O_2. We identify both a single-spin-wave feature and a mul
ti-magnon continuum, and show that the X-ray polarization can be used to distinguish these two contributions in the cross-section. The spin-waves display a large (70 meV) dispersion between the zone-boundary points ($pi$,0) and ($pi$/2,$pi$/2). Employing an extended $t$-$t$-$t$-$U$ one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions. We conclude that sizeable extended magnetic interactions are present in scoc{} and probably important in all undoped cuprates.
