Visualizing Fermi surface anisotropy underpinning a unidirectional charge density wave*

Transition metal dichalcogenide 2H-NbSe2 has emerged as a fruitful playground for investigating charge density wave (CDW) transitions, with several different charge ordering wave vectors and geometries reported in this system. Here, we use spectroscopic-imaging scanning tunneling microscopy (SI-STM) to visualize and compare the underlying band structure of a unidirectional (1Q) and a triangular (3Q) 2a₀ CDW phase in strained NbSe₂. In the 1Q CDW state, we discover a small anisotropy in the scattering of electrons along the CDW direction, indicative of a distorted Fermi surface. Our results provide an additional input for a microscopic theory of CDW transitions in NbSe₂.