Collective spin and charge excitations in cuprate materials

With the development of experimental and numerical techniques, our understanding of two-dimensional superconducting cuprates has grown rapidly over the last few decades. On the one hand, it is now clear that the simple single-band Hubbard model can reproduce many experimental observations, including the presence of competing stripe orders, the psuedogap, and unconventional d-wave superconductivity. On the other hand, the single-band Hubbard model fails to capture many experimental observations, including the recent observations of particle-hole asymmetry in the charge excitations of Nd2−xCexCuO4 and La2−xCexCuO4 and the s′-wave charge order observed in La1.875Ba0.125CuO4. It is questionable whether the single-band Hubbard model can describe the low energy physics of the copper oxide compounds.

In this work, we study the quasi-1D cuprate spin chain Sr2CuO3 by employing a multi-orbital Hubbard model and explore its various experimentally-relevant dynamical response functions. Interestingly, we find many important features, which are missed in the single-band Hubbard model. For example, we observe a clear particle-hole asymmetry in the orbital-resolved charge exciations near q = 0. This result is relevant to the particle-hole asymmetry observed in recent Cu L3-edge resonant inelastic X-ray experiments on the cuprates. Moreover, we observe a spin response existing between 6-8 eV, which cannot be measured in inelastic neutron experiments. This higher energy spin response explains the missing intensity in the inelastic neutron experiments on the copper oxide compounds. Our results imply that one must explicitly include the oxygen degrees of freedom to fully understand some experimental observations on cuprate materials.