Zicheng Tao

Despite their importance for exotic quantum effects, the surface electronic structure of magnetic topological insulators MnBi 2 Te 4 and MnBi 4 Te 7 remains poorly understood. Using high-efficiency spin- and angle-resolved photoemission spectroscopy, we directly image the spin-polarization and orbital character of the surface states in both compounds and map our observations onto a model wave function to describe the complex spin-orbital texture, which solidifies our understanding of the surface band structure by establishing the single-band nature of the most prominent states. Most importantly, our analysis reveals a new mechanism for reducing the magnetic gap of the topological surface states based on the orbital composition of the wave function.

Altermagnetism (AM), a newly discovered magnetic state, ingeniously integrates the properties of ferromagnetism and antiferromagnetism, representing a significant breakthrough in the field of magnetic materials. Despite experimental verification of some typical AM materials, such as MnTe and MnTe_{2}, the pursuit of AM materials that feature larger spin splitting and higher transition temperature is still essential. Here, our research focuses on CrSb, which possesses Néel temperature of up to 700 K and giant spin splitting near the Fermi level (E_{F}). Utilizing high-resolution angle-resolved photoemission spectroscopy and density functional theory calculations, we meticulously map the three-dimensional electronic structure of CrSb. Our photoemission spectroscopic results on both (0001) and (101[over ¯]0) cleavages of CrSb collaboratively reveal unprecedented details on AM-induced band splitting, and subsequently pin down its unique bulk g-wave symmetry through quantitative analysis of the angular and photon-energy dependence of spin splitting. Moreover, the observed spin splitting reaches the magnitude of 0.93 eV near E_{F}, the most substantial among all confirmed AM materials. This Letter not only validates the nature of CrSb as a prototype g-wave-like AM material but also underscores its pivotal role in pioneering applications in spintronics.

The realization of intrinsic magnetic topological insulators offers an ideal platform to investigate high-temperature quantum anomalous Hall effect as well as quantum devices. The family of MnBi2Te4(Bi2Te3)n has been confirmed that it belongs to this system. However, whether there is a bandgap in the surface states remains controversial. In this work, the ferromagnetism in Sb-doped MnBi4Te7 is confirmed by magnetic transport. Utilizing angle-resolved photoemission spectroscopy, we demonstrate the nontrivial topology in Mn(Bi0.7Sb0.3)4Te7 with an energy gap on the MnBi2Te4 termination. As well, we detect distinct topological surface states on two different terminations. Our results provide spectral evidence of an energy gap in the topological surface states.

The weak antilocalization (WAL) effect is known as a quantum correction to the classical conductivity, which never appeared in two-dimensional magnets. In this work, we reported the observation of a WAL effect in the van der Waals ferromagnet Fe5-xGeTe2 with a Curie temperature Tc ~ 270 K, which can even reach as high as ~ 120 K. The WAL effect could be well described by the Hikami-Larkin-Nagaoka and Maekawa-Fukuyama theories in the presence of strong spin-orbit coupling (SOC). Moreover, A crossover from a peak to dip behavior around 60 K in both the magnetoresistance and magnetoconductance was observed, which could be ascribed to a rare example of temperature driven Lifshitz transition as indicated by the angle-resolved photoemission spectroscopy measurements and first principles calculations. The reflective magnetic circular dichroism measurements indicate a possible spin reorientation that kills the WAL effect above 120 K. Our findings present a rare example of WAL effect in two-dimensional ferromagnet and also a magnetotransport fingerprint of the strong SOC in Fe5-xGeTe2. The results would be instructive for understanding the interaction Hamiltonian for such high Tc itinerant ferromagnetism as well as be helpful for the design of next-generation room temperature spintronic or twistronic devices.