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Spin and orbital excitations beyond the nearest-neighbor interactions: spinon pairs and dispersing dds in infinite-layer $CaCuO_{2}$.

Martinelli L., Betto D., Kummer K., Arpaia R., Di Castro D., Moretti Sala M., Brookes N.B., Devereaux T.P., Braicovich L., Ghiringhelli G.
  Martedì 13/09   09:00 - 13:30   Aula D - Marianna Ciccone   II - Fisica della materia   Presentazione
The core of cuprate high-$T_{c}$ superconductors are $CuO_{2}$ planes, which show a ground state of $x^{2}-y^{2}$ ferro-orbital symmetry and Néel antiferromagnetic order. The elementary excitations are usually well described as local (non-collective) $dd$ excitations, which show no dispersion with momentum, and $\Delta S=1$ magnons (plus multi-magnons), respectively. However, longer-range interactions cannot always be neglected. We have performed Resonant Inelastic X-ray Scattering (RIXS) measurements on the infinite-layer compound $CaCuO_{2}$, where the absence of apical oxygens leads to larger than usual hopping integrals $t$ and $t^{\prime}$, superexchange $J$. In the orbital spectrum we find dispersive orbital excitations and the lifting of the $xz/yz$ degeneracy, which indicates a partially delocalized nature. Moreover, the magnetic spectral region shows a clear anomaly close to $(1/2,0)$. With an innovative combination of polarimetric and ultra-high-resolution RIXS measurements, we reveal that it is more consistent with a spinon-pairs continuum than with a multi-magnon one. We therefore find clear signatures of exotic physics beyond the canonical ferro-orbital, AF Néel ground state.