Magneto-Raman Spectroscopy of Layered Transition-Metal Dichalcogenides

Raman spectroscopy offers a non-contact, non-destructive, and high-throughput optical technique to probe the fundamental physics of two-dimensional (2D) layered materials. We discuss our unique magneto-Raman capability, which affords measurement of Raman spectra with simultaneous variation of temperature (4K to 400K), laser excitation wavelength (tunability from UV to near-IR), and magnetic field (up to 9 T). Coupling to a triple-grating spectrometer provides measurement of low-frequency phonons. Recent results on novel 2D materials will be presented to highlight instrumentation capabilities, including metallic TaSe₂, which exhibits transitions between commensurate and incommensurate charge-density wave (CDW) phases, and related, layered materials. The dependence of the observed Raman-active phonons in TaSe₂ on temperature and magnetic field will be presented and compared with earlier results on MoS₂ and calculations using density functional theory. Specifically, we observe the appearance of low-frequency, zone-folded modes in the CDW state. The phase, amplitude, and previously unanalyzed zone-folding modes are assigned to specific phonons observed in experimental spectra and DFT. Uniquely, the calculations show Ta-rich stripes emerge in the commensurate CDW phase.