Optical and core-level X-ray spectroscopy of correlated two-dimensional materials

Abstract

The intersection of low-dimensionality and strongly correlated electrons in van der Waals (vdW) materials offers a rich landscape of ordered phases and associated excitations for potential applications in nanoelectronics. The coupling between distinct degrees of freedom in correlated materials provide routes to realize novel functional properties, which can be further manipulated by the high tunability intrinsic to vdW materials through, e.g., heterostructures and doping. However, identifying the mechanism of correlated phases poses a fundamental challenge due to coexistent and competing orders. This requires detailed knowledge of the microscopic interactions/excitation spectra, methods to disentangle the individual roles of coexistent orders, and selective probes of symmetry-breaking within different coupled degrees of freedom. In this thesis, we demonstrate the utility and complementarity of resonant X-ray spectroscopy and symmetry-selective optical probes in combination with appropriate external tuning parameters (e.g. strain, pressure, ligand substitution, layer number) for revealing the origin of correlated phases in low-dimensional vdW materials. We first investigate the triangular lattice antiferromagnet NiI₂. Frustrated exchange interactions result in a helimagnetic ground state and spin-induced ferroelectric order, making bulk NiI₂ a type-II multiferroic. Using a combination of optical spectroscopic probes, including Raman, magneto-optics, and second harmonic generation, we demonstrate the persistence of multiferroic order to the single-layer limit. We then aim to resolve the microscopic magnetic interactions and their interplay with the lattice symmetry to identify the origin of the magnetic ground state. Towards this goal, we investigate the magnetic ground state and transition temperature versus hydrostatic pressure and layer number, and directly probe the evolution of magnetic/structural orders with resonant magnetic X-ray scattering/structural diffraction, respectively. From these results, we demonstrate the central role of interlayer exchange interactions and their coupling to the structural symmetry in driving the magnetic ground state of NiI₂. We next investigate the broader class of triangular lattice nickel dihalides, NiX₂ (X = Cl, Br, I), to identify the origin of sharp optical excitations, i.e. excitons, in nickel-based vdW magnets. We employ Ni-L₃ edge resonant inelastic X-ray scattering (RIXS) to access a q-resolved and site-specific view into the excitation spectra. We identify the sharp excitons with spin-forbidden intra-configurational multiplets of octahedrally-coordinated Ni²⁺, which become renormalized by Ni-X charge transfer. We also observe a finite dispersion of these excitations, demonstrating a multiplet delocalization that is controlled by the ligand-tuned charge transfer gap in a process analogous to ground state superexchange. These results establish the microscopic origin of these excitons and provide a mechanism to explain their possible coupling to the magnetic order/excitations. Finally, we study the iron-based superconductor FeSe, which displays a rotational symmetry breaking electronic nematic phase in proximity to unconventional superconductivity without magnetic order. To understand the origin of nematicity, we investigate the ordering of the orbital degrees of freedom using X-ray linear dichroism with in-situ uniaxial strain tuning, electronic transport measurements and structural diffraction. We observe a lattice-independent orbital polarization acting as the primary nematic order parameter. This resolves the orbital origin of nematicity in FeSe and suggests that anisotropic spin fluctuations are the mechanism of unconventional superconductivity.