Physics Colloquium: Magneto-Raman Spectroscopy to Identify Spin Structure in Low-Dimensional Quantum Materials
Dr. Angela Hight Walker, NIST
Abstract: Raman spectroscopy, imaging, and mapping are powerful non-contact, non-destructive optical probes of fundamental physics in graphene and other related two-dimensional (2D) materials, including layered, quantum materials. An amazing amount of information can be quantified from the Raman spectra, including layer thickness, disorder, edge and grain boundaries, doping, strain, thermal conductivity, magnetic ordering, and unique excitations such as magnons and charge density waves. Most interestingly for quantum materials is that Raman efficiently probes the evolution of the electronic structure and the electron-phonon, spin-phonon, and magnon-phonon interactions as a function of temperature, laser energy, and polarization. Our unique magneto-Raman spectroscopic capabilities will be detailed, enabling diffraction-limited, spatially-resolved Raman measurements while simultaneously varying the temperature (1.6 K to 400 K), laser wavelength (tunability from visible to near-infrared), and magnetic field (up to 9 T) to study the photo-physics of nanomaterials. Additionally, coupling to a triple grating spectrometer provides access to low-frequency (down to 6 cm-1, or 0.75 meV) phonon and magnon modes, which are most sensitive to coupling. Current results will be presented to highlight our capabilities and research directions, specifically on CoTiO3, a proposed Dirac topological magnon material. Interestingly, we observed a quasi-particle soup in CoTiO3: phonons, magnons, and spin-orbit excitons! We combine our experimental observations with theoretical models to explain the magnetic field dependence of all these modes. Finally, we suggest that the ring exchange of the six Co2+ within the hexagon plane is a mechanism to open a gap in the magnon spectrum at the Brillouin zone center.