Topological matter is known to exhibit unconventional surface states and anomalous transport owing to unusual bulk electronic topology. In this study, we use photoemission spectroscopy and quantum transport to elucidate the topology of the room temperature magnet Co 2 MnGa. We observe sharp bulk Weyl fermion line dispersions indicative of nontrivial topological invariants present in the magnetic phase. On the surface of the magnet, we observe electronic wave functions that take the form of drumheads, enabling us to directly visualize the crucial components of the bulk-boundary topological correspondence. By considering the Berry curvature field associated with the observed topological Weyl fermion lines, we quantitatively account for the giant anomalous Hall response observed in our samples. Our experimental results suggest a rich interplay of strongly correlated electrons and topology in this quantum magnet.The discovery of topological phases of matter has led to a new paradigm in physics, 30 which not only explores the analogs of particles relevant for high energy physics, but also 31 offers new perspectives and pathways for the application of quantum materials [1][2][3][4][5][6][7][8][9][10]. To 32 date, most topological phases have been discovered in non-magnetic materials [6][7][8], which 33 severely limits their magnetic field tunability and electronic/magnetic functionality. Iden-34 tifying and understanding electronic topology in magnetic materials will not only provide 35 indispensable information to make their existing magnetic properties more robust, but also 36 has the potential to lead to the discovery of novel magnetic response that can be used to ex-37 plore future spintronics technology. Recently, several magnets were found to exhibit a large 38 anomalous Hall response in transport, which has been linked to a large Berry curvature in 39 their electronic structures [11][12][13][14][15]. However, it is largely unclear in experiment whether the 40 Berry curvature originates from a topological band structure, such as Dirac/Weyl point or 41 line nodes, due to the lack of spectroscopic investigation. In particular, there is no direct vi-42 sualization of a topological magnetic phase demonstrating a bulk-boundary correspondence 43 with associated anomalous transport. 44Here we use angle-resolved photoemission spectroscopy (ARPES), ab initio calculation 45 and transport to explore the electronic topological phase of the ferromagnet Co 2 MnGa [10]. 46In our ARPES spectra we discover a line node in the bulk of the sample. Taken together with 47 our ab initio calculations, we conclude that we observe Weyl lines protected by crystalline 48 mirror symmetry and requiring magnetic order. In ARPES we further observe drumhead 49 surface states connecting the bulk Weyl lines, revealing a bulk-boundary correspondence in a 50 magnet. Combining our ARPES and ab initio calculation results with transport, we further 51 find that Berry curvature concentrated by the Weyl lines accounts for the giant intrinsic 52 anomal...

While the discovery of two-dimensional (2D) magnets opens the door for fundamental physics and next-generation spintronics, it is technically challenging to achieve the room-temperature ferromagnetic (FM) order in a way compatible with potential device applications. Here, we report the growth and properties of single- and few-layer CrTe2, a van der Waals (vdW) material, on bilayer graphene by molecular beam epitaxy (MBE). Intrinsic ferromagnetism with a Curie temperature (TC) up to 300 K, an atomic magnetic moment of ~0.21 $${\mu }_{{\rm{B}}}$$ μ B /Cr and perpendicular magnetic anisotropy (PMA) constant (Ku) of 4.89 × 105 erg/cm3 at room temperature in these few-monolayer films have been unambiguously evidenced by superconducting quantum interference device and X-ray magnetic circular dichroism. This intrinsic ferromagnetism has also been identified by the splitting of majority and minority band dispersions with ~0.2 eV at Г point using angle-resolved photoemission spectroscopy. The FM order is preserved with the film thickness down to a monolayer (TC ~ 200 K), benefiting from the strong PMA and weak interlayer coupling. The successful MBE growth of 2D FM CrTe2 films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2D magnet-based spintronics devices.

Weyl semimetals are novel topological conductors that host Weyl fermions as emergent quasiparticles. In this paper, we propose a new type of Weyl semimetal state that breaks both time-reversal symmetry and inversion-symmetry in the RAlGe (R=Rare earth) family. Compared to previous predictions of magnetic Weyl semimetal candidates, the prediction of Weyl nodes in RAlGe are more robust and less dependent on the details of the magnetism, because the Weyl nodes are already generated by the inversion breaking and the ferromagnetism acts as a simple Zeeman coupling that shifts the Weyl nodes in k space. Moreover, RAlGe offers remarkable tunability, which covers all varieties of Weyl semimetals including type-I, type-II, inversion-breaking and time-reversal breaking, depending on a suitable choice of the rare earth elements. Further, the unique noncentrosymmetric and ferromagnetic Weyl semimetal state in RAlGe enables the generation of spin-currents.