Modeling the X-ray absorption of Neodymium ions in cuprate superconductors

Abstract

Electronic nematicity, a breaking of the rotational symmetry of the electronic structure beyond what is expected from the ionic displacements of atoms, has been identified in the (La, M)₂CuO₄ family of the cuprate superconductors. This was achieved by measuring the temperature dependence of the (001) Bragg reflection peak intensity at resonant x-ray photon energies corresponding to different atoms within the unit cell. Recently, however, it has been identified that x-ray scattering of Nd M4,5 edge in (La, Sr, Nd)₂CuO₄ exhibits unusual increase in (001) Bragg reflection intensity when the temperature is increased from 10 to 70 K, seemingly at odds with the understanding of electronic nematicity in these systems. It was hypothesized that this change may arise from the single-ion phenomena. To test this hypothesis, polarization and temperature dependence of the x-ray absorption spectrum are analyzed. The spectrum exhibits temperature-dependent linear dichroism, which was hypothesized to be connected to the temperature dependence of the (001) scattering intensity. A numerical modeling software, Quanty, was utilized to create a two-shell model and study its properties via Monte-Carlo method. We establish that the temperature-dependent excitation into low-energy crystal field states results in significant temperature dependence to the single-ion scattering tensor. A crystalline electric field of C2v symmetry with realistic parameters captures the temperature dependencies of the x-ray absorption spectrum, while also being consistent with excitation energies measured by inelastic neutron scattering. However, the calculated (001) scattering intensity decreases with temperature for all probed crystal field parameter sets, contrary to the experimentally observed increase. We speculate at future refinements to our model that may rectify this discrepancy.