Abstract:

　　Ferromagnetic van der Waals (vdW) compounds, such as Fe₃GeTe₂ (FGT) and its variants with Ga, have sparked considerable interest due to their itinerant magnetism, correlated state, and high magnetic transition temperature. Experimental studies have demonstrated the tunability of FGT's Curie temperature, T_C, through adjustments in quintuple layer numbers (QL) and carrier concentrations n. However, the underlying mechanism remains elusive. In this study, employing molecular beam epitaxy, we synthesize two-dimensional (2D) FGT films with precise layer control down to the 1 QL limit, facilitating an exploration of the band structure and the evolution of itinerant carrier density. Angle-resolved photoemission spectroscopy reveals significant band structure changes at the ultra-thin limit, while density functional theory calculations elucidate the band evolution, largely governed by interlayer coupling, from 1 QL to 2 QL to multilayer; with the latter exhibiting a band structure akin to bulk FGT. We present a complete depiction of the band structure evolution in FGT from monolayer to bulk. Additionally, we find that carrier concentration n is intrinsically linked to the number of QL as well as temperature, unraveling the intricate relationships between n and the magnetic behavior of FGT, with a critical carrier concentration triggering the magnetic phase transition. Our findings underscore the pivotal role of band structure and itinerant electrons in governing magnetic phase transitions in such 2D vdW ferromagnetic materials.

Biography:

　　Professor Ke Zou has been an Assistant Professor in the Department of Physics and Astronomy & Quantum Matter Institute (QMI) at the University of British Columbia (UBC) since 2018. He previously held the position of Postdoctoral Associate in the Department of Applied Physics at Yale University. Dr. Zou earned his Ph.D. in Physics from Pennsylvania State University in 2012. He is a prolific researcher with 41 publications in high-impact journals, which have been cited approximately 2,650 times, resulting in an h-index of 20, according to Google Scholar.  In April 2024, He was awarded the "Humboldt Research Fellowship for Experienced Researchers".