Widely Tunable Berry curvature in the Magnetic Semimetal Cr1+dTe2

Abstract

Magnetic semimetals have increasingly emerged as lucrative platforms hosting spin-based topological phenomena in real- and momentum spaces. Of particular interest is the emergence of Berry curvature, whose geometric origin, accessibility from Hall transport experiments, and material tunability, bodes well for new physics and practical devices. Cr1+δTe2, a self-intercalated magnetic transition metal dichalcogenide (TMD), exhibits attractive natural attributes relevant to such applications, including topological magnetism, tunable electron filling, magnetic frustration etc. While recent studies have explored real-space Berry curvature effects in this material, similar considerations of momentum-space Berry curvature are lacking. Here, we systematically investigate the electronic structure and transport properties of epitaxial Cr1+δTe2 thin films over a wide range of doping, δ (0.33 – 0.71). Spectroscopic experiments reveal the presence of a characteristic semi-metallic band region near the Brillouin Zone edge, which shows a rigid band like energy shift as a function of δ. Transport experiments show that the intrinsic component of the anomalous Hall effect (AHE) is sizable, and undergoes a sign flip across δ. Finally, density functional theory calculations establish a causal link between the observed doping evolution of the band structure and AHE: the AHE sign flip is shown to emerge from the sign change of the Berry curvature, as the semi-metallic band region crosses the Fermi energy. Our findings underscore the increasing relevance of momentum-space Berry curvature in magnetic TMDs and provide a unique platform for intertwining topological physics in real and momentum spaces.

Layer-Number-Independent Two-Dimensional Ferromagnetism in Cr3Te4

“…Such a nonmonotonous variation of R H with a peak-like structure with an increase in the doping level should not appear in a simple single-band hole-type conductor, suggesting that there should be another electronic band existing near the Fermi level, most likely an electron-like band, and the system enters into a different regime above T a = 350 °C. Importantly, this behavior is in accord with the recent ARPES results on the same material system, which proved that a hole-like band and an electron-like band cross with each other to form a crossing point near the Fermi level . Moreover, very interestingly, we found that T C starts to increase exactly when the system enters into this regime above T a = 350 °C beyond a crossing point in the momentum space.…”

“…When considering a relationship between T C and a doping level, one might wonder if the Stoner model would be relevant to our system as is the case with the ion-gating experiments on Fe 3 GeTe 2 flakes, where the authors observed the nonmonotonous variation of T C against a doping level, potentially associated with the peak structures in the density of states (DOS) of Fe 3 GeTe 2 near the Fermi level. However, we consider that this “conventional” Stoner picture should be irrelevant to the current system based on the fact that there seems to be no such singularity of the DOS existing near the Fermi level in this material system as revealed by the ARPES measurements . Moreover, we confirmed by the magnetization measurements that the M sat values of the HTC- and LTC-phase samples were similar to each other, which also does not agree with the Stoner picture, where M sat should be proportional to T C .…”

“…On the other hand, recent studies on Cr–Te epitaxial thin films grown by molecular-beam epitaxy (MBE) demonstrated a large variation of their magnetic properties depending on the growth condition, ,,− implying their sensitive nature to the doping level of the system. Furthermore, a very recent study by angle-resolved photoemission spectroscopy (ARPES) measurements verified the existence of a characteristic semimetallic band region near the Fermi level, demonstrating a unique feature of this material system from a fundamental materials science viewpoint as well.…”

“…First, we examined the impact of the vacuum-annealing process by systematically changing the vacuum-annealing temperature ( T a ). According to previous studies on the selenium-based epitaxial thin films, , a postgrowth annealing process under vacuum tends to shift the Fermi level to a higher energy via electron doping without modification of the band structure (i.e., rigid band shift), which was in fact proven to be the case for the MBE-grown Cr–Te films as well . We prepared a series of samples treated at different T a values and systematically examined a relationship between T C and the doping level of the system by transport measurements.…”