Electronic Structure and Emergent Orders in Correlated Nickelates

Abstract

Strongly correlated quantum materials encompass a class of materials with novel functionalities and exotic physical properties that escapes the conventional description of quantum mechanics. Rare earth nickelate compounds have been established as archetypal correlated quantum materials with rich diversity of ground-state properties that emerge due to the interplay between the charge, spin, orbital, and lattice degrees of freedom. Despite decades of experimental scrutiny, their ground state properties still remain debated and elusive. This doctoral work presents a study of the ground state electronic and magnetic structures of nickelate compounds using a combination of soft X-ray techniques based on scattering, spectroscopy and imaging. The research efforts in this thesis unravel the physical properties of nickelates from three aspects. In the first part, we unveiled that the nanoscale electronic phase separation phenomena in nickelate are closely tied to phase transition criticality, highlighting that the scale-invariant inhomogeneity is an essential ingredient for the ground state description. In the second part, we have discovered a macroscopic chiral polarization of the non-collinear magnetic structure in rare earth nickelates which indicates a possible ferroelectric polarization by magneto-electric coupling. In the third part, we seek to understand the ground state electronic landscapes in nickelates via carrier dopings. We have identified the doping dependent electronic properties of electron-doped RENiO₃₋ₓ and observed a sudden collapse of ordered magnetism, which may provide a new mechanism for solid-state magnetoionic switching and new applications in antiferromagnetic spintronics.