Spectroscopic signatures of collective excitations in spin-orbit coupled systems

Systems without inversion in the unit cell generally have spin-split electronic structures due to the presence of parity-breaking spin-orbit coupling. This modification leads to new effects, some of which are generically expected due to the spin-splitting (such as the existence of chiral-spin collective modes). This talk will focus on the novel and unconventional effects of spin-orbit coupling which have very clear and detectable spectroscopic signatures. These happen both at low and high energies of excitation (relative to the fermi level), creating new scattering channels that couple the excitations of the system to an external probe (here photons). While the low energy modifications alter the nature of electronic correlations, the high energy ones alter how the system interacts with external probes. As a result, previously "invisible" excitations can now become prominent. One such effect that we will discuss in the talk is a novel spin-mediated photon-plasmon scattering whose origin lies in the breaking of the SU(2) invariance which in turn arises from the breaking of parity. This scattering renders the plasmon of a 3D system visible to spectroscopic tools when it has been known for decades that it has a negligible spectral weight unless aided by external sources such as phonons, gratings, etc. Another unconventional effect, which only involves the low energy modifications, is the resonant charge-spin conversion in 2D spin-orbit coupled systems. We will discuss the experimental relevance in each case.