Anisotropic electron gas in a hyperbolic van der Waals material

Electron gases in low-dimensional materials exhibit unusual optical and electronic phenomena arising from reduced dimensionality and enhanced interactions. While such states are typically engineered in quantum-confined systems and heterostructures, they rarely occur in naturally existing materials. Hyperbolic materials provide a unique platform where extreme lattice anisotropy can give rise to unconventional electronic states and electron-phonon interactions, yet experimental probes of their underlying electron dynamics remain limited. Here, we show that angle-resolved polarized Raman spectroscopy reveals anisotropic electron-phonon coupling in MoOCl2, a naturally occurring hyperbolic material whose optical response originates from a highly anisotropic electron gas. Pronounced polarization-dependent Fano line shapes reveal coherent coupling between optical phonons and an anisotropic electronic continuum. A microscopic model quantitatively reproduces the Raman spectra and identifies a distinct spectroscopic fingerprint of the anisotropic electron gas. Excitation-energy- and thickness-dependent measurements further reveal a tunable continuum with weak interlayer coupling. Our results establish Raman spectroscopy as a sensitive probe of electron-phonon interactions in naturally hyperbolic materials and anisotropic electron gases. Low-dimensional electron gases can give rise to unusual light-matter interactions, but naturally occurring examples are rare. Here, the authors show that Raman spectroscopy reveals a unique fingerprint of the strongly anisotropic electron gas in the hyperbolic material MoOCl2 through its interaction with lattice vibrations.