Real-space observation of low-temperature charge density wave in layered itinerant ferromagnet Fe₅GeTe₂

Charge density wave, a periodic modulation of electronic charge density often accompanied by a periodic lattice distortion, plays a vital role to induce exotic phenomena in condensed matter physics. In non-magnetic quantum materials, contrast inversion in scanning tunneling microscopy images, observed between opposite bias polarity, serves as a hallmark of the charge density wave. However, in itinerant ferromagnetic systems, charge density wave formation competes with magnetism: A charge density wave order typically reduces the density of states at the Fermi level, while the Stoner criterion for spontaneous spin polarization requires a high density of states at Fermi level. Therefore, direct real-space observation of such polarity-dependent contrast inversion in ferromagnetic materials remains elusive and experimentally challenging. Here, we demonstrate the observation of a charge density wave in itinerant ferromagnet Fe₅GeTe₂ associated with $\sqrt{3}$ × $\sqrt{3}$ superlattice, revealed through polarity-dependent scanning tunneling microscopy imaging. Importantly, we observe a gap-like dip at the Fermi level in tunneling spectra, serving additional evidence for the emergence of charge density wave in Fe₅GeTe₂. Interestingly, the strength of charge modulation can be systematically tuned by Fe1 vacancies and impurities, while the spectroscopic intensity shows a high sensitivity to surface degradation. Our finding provides an inspiring insight to charge density wave on the van der Waals ferromagnetic materials.