Exploring Orbital Physics with Angle-Resolved Photoemission Spectroscopy
As an indispensable electronic degree of freedom, the orbital character dominates some of the key energy scales in the solid state -- the electron hopping, Coulomb repulsion, crystal field splitting, and spin-orbit coupling. Recent years have witnessed the birth of a number of exotic phases of matter where orbital physics plays an essential role, e.g. topological insulators, spin- orbital coupled Mott insulators, orbital-ordered transition metal oxides, etc. However, a direct experimental exploration is lacking concerning how the orbital degree of freedom affects the ground state and low energy excitations in these systems.
Angle-resolved photoemission spectroscopy (ARPES) is an invaluable tool for observing the electronic structure, low-energy electron dynamics and in some cases, the orbital wavefunction. Here we perform a case study of (a) the topological insulator and (b) the J1/2 Mott insulator Sr2IrO4 using ARPES.
For the prototype topological insulator Bi2X3 (X=Se, Te), our studies reveal the topological surface state has a spin-orbital coupled wavefunction asymmetric to the Dirac point, and it is possible to manipulate the spin of the photoelectron using polarized photons.
We also discover there are multiple features, including pseudogaps, Fermi "arcs", and marginal- Fermi-liquid-like electronic scattering rates in the effectively hole doped Sr2IrO4, which have been reported in the high-TC cuprates. Due to the relatively simple phase diagram of these doped iridates, we find the aforementioned low energy features are not exclusive to preformed Cooper pairs, or the existence of Quantum Critical Points as suggested in influential theories. Instead, the short-range antiferromagnetic correlation might be vital to the description of the Mott-metal crossover.