Angela R. Hight Walker
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 that are candidates for use in the next quantum revolution. 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 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 sensitive to coupling. By utilizing electrical feedthroughs, studying the strain-dependent effects on magnetic materials utilizing MEMs devices is also a novel opportunity. Current results on intriguing quantum materials will be presented to highlight our capabilities and research directions. One example leverages the Raman spectra from α-RuCl3 to probe this Kitaev magnet and possible quantum spin liquid1. Within a single layer, the honeycomb lattice exhibits a small distortion, reducing the symmetry from hexagonal to orthorhombic. We utilize polarization-dependent Raman spectroscopy to study this distortion, including polarizations both parallel and perpendicular to the c-axis. Coupling of the phonons to a continuum is also investigated. Using Raman spectroscopy to probe magnetic phenomena in the antiferromagnetic metal phosphorus trichalcogenide family2, we highlight FePS3 and MnPSe3. Using magneto-Raman spectroscopy as an optical probe of magnetic structure, we show that in FePS3 one of the Raman-active modes in the magnetically ordered state is actually a magnon with a frequency of ≈3.7 THz (122 cm−1). In addition, the surprising symmetry behavior of the magnon is studied by polarization-dependent Raman spectroscopy and explained using the magnetic point group of FePS3. Using resonant Raman scattering, we studied the Neel-type antiferromagnet MnPSe3 through its ordering temperature and also as a function of applied external magnetic field. Surprisingly, the previously assigned one-magnon scattering peak showed no change in frequency with an increasing in-plane magnetic field. Instead, its temperature dependence revealed a more surprising story. Combined with first-principle calculations, the potential origin of this Raman scattering will be discussed.
Finally, the magnetic field- and temperature-dependence of an exciting ferromagnetic 2D material, CrI3, will be presented3. We report a magneto-Raman spectroscopy study on multilayered CrI3, focusing on two new features in the spectra which appear below the magnetic ordering temperature and were previously assigned to high frequency magnons. Instead, we conclude these modes are actually zone-folded phonons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI3.
[1] PHYSICAL REVIEW B 100, 134419 (2019)[2] PHYSICAL REVIEW B 101, 064416 (2020)
[3] https://arxiv.org/abs/1910.01237 (in press @Nature Comm)