The kagome lattice is a two-dimensional network of corner-sharing triangles that is known to host exotic quantum magnetic states. Theoretical work has predicted that kagome lattices may also host Dirac electronic states that could lead to topological and Chern insulating phases, but these states have so far not been detected in experiments. Here we study the d-electron kagome metal FeSn, which is designed to support bulk massive Dirac fermions in the presence of ferromagnetic order. We observe a temperature-independent intrinsic anomalous Hall conductivity that persists above room temperature, which is suggestive of prominent Berry curvature from the time-reversal-symmetry-breaking electronic bands of the kagome plane. Using angle-resolved photoemission spectroscopy, we observe a pair of quasi-two-dimensional Dirac cones near the Fermi level with a mass gap of 30 millielectronvolts, which correspond to massive Dirac fermions that generate Berry-curvature-induced Hall conductivity. We show that this behaviour is a consequence of the underlying symmetry properties of the bilayer kagome lattice in the ferromagnetic state and the atomic spin-orbit coupling. This work provides evidence for a ferromagnetic kagome metal and an example of emergent topological electronic properties in a correlated electron system. Our results provide insight into the recent discoveries of exotic electronic behaviour in kagome-lattice antiferromagnets and may enable lattice-model realizations of fractional topological quantum states.

The quantum mechanical (Berry) phase of the electronic wavefunction plays a critical role in the anomalous 1,2 and spin Hall e ects 3,4 , including their quantized limits 5-7 . While progress has been made in understanding these e ects in ferromagnets 8 , less is known in antiferromagnetic systems. Here we present a study of antiferromagnet GdPtBi, whose electronic structure is similar to that of the topologically non-trivial HgTe (refs 9-11), and where the Gd ions o er the possibility to tune the Berry phase via control of the spin texture. We show that this system supports an anomalous Hall angle Θ AH > 0.1, comparable to the largest observed in bulk ferromagnets 12 and significantly larger than in other antiferromagnets 13 . Neutron scattering measurements and electronic structure calculations suggest that this e ect originates from avoided crossing or Weyl points that develop near the Fermi level due to a breaking of combined timereversal and lattice symmetries. Berry phase e ects associated with such symmetry breaking have recently been explored in kagome networks 14-17 ; our results extend this to half-Heusler systems with non-trivial band topology. The magnetic textures indicated here may also provide pathways towards realizing the topological insulating and semimetallic states 9-11,18,19 predicted in this material class.The ordinary Hall effect is due to the Lorentz force bending of charge carriers perpendicular to a magnetic field. In systems where time-reversal symmetry (TRS) is spontaneously broken, it typically can be overwhelmed by a different class of mechanisms for transverse velocity. In such systems, there are contributions to transverse velocity from both extrinsic effects due to spindependent scattering 13 and intrinsic effects related to real space 20,21 and momentum space 2 Berry phase mechanisms. The former is relevant in systems with non-coplanar spin textures with finite scalar spin chirality χ ijk = S i · (S j × S k ), where S n are spins, while the latter generically occurs in TRS-broken systems originating from the spin-orbit-interaction-induced Berry curvature of the filled bands. The anomalous Hall effect (AHE) due to magnetic texture is most often associated with finite χ ijk and tends to exhibit relatively small anomalous Hall angles Θ AH 0.01 (such as SrFeO 3 (ref. 22) or Pr 2 Ir 2 O 7 (ref. 23)), while intrinsic band-structure-based effects are common in ferromagnetic systems and can be significantly larger 13 . Recent theoretical work has suggested that effects that rely on both magnetic texture and strong spin-orbit coupling may exist in noncollinear antiferromagnets that lead to significant Hall responses 14 . Single-crystal studies of Mn 3 Sn and Mn 3 Ge have been shown to support Θ AH 0.02 and 0.05, respectively, originating from its inverse triangular spin structure and electronic structure 16,17 .Here we study single crystals of GdPtBi, a member of the family RPtBi (R is a rare earth element) known to exhibit antiferromagnetic ordering 24 . As shown in Fig. 1a, this syste...

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We studied the structural properties of an orbital-spin-coupled spinel oxide, MnV2O4, mainly by single-crystal x-ray diffraction measurement. It was found that a structural phase transition from cubic to tetragonal and ferrimagnetic ordering occur at the same temperature (Ts,TN=57 K). The structural phase transition was induced also by magnetic field above Ts. In addition, magnetic-field-induced alignment of tetragonal domains results in large magnetostriction below Ts. We also found that the structural phase transition is caused by the antiferro-type ordering of the V t2g orbitals.

Strong spin-orbit coupling (SOC) can result in ground states with non-trivial topological properties. The situation is even richer in magnetic systems where the magnetic ordering can potentially have strong influence over the electronic band structure. The class of AMnBi2 (A = Sr, Ca) compounds are important in this context as they are known to host massive Dirac fermions with strongly anisotropic dispersion, which is believed to be due to the interplay between strong SOC and magnetic degrees of freedom. We report the optical conductivity of YbMnBi2, a newly discovered member of this family and a proposed Weyl semi-metal (WSM) candidate with broken time reversal symmetry. Together with density functional theory (DFT) band structure calculations, we show that the complex conductivity can be interpreted as the sum of an intra-band Drude response and inter-band transitions. We argue that the canting of the magnetic moments that has been proposed to be essential for the realization of the WSM in an otherwise antiferromagnetically ordered system is not necessary to explain the optical conductivity. We believe our data is explained qualitatively by the uncanted magnetic structure with a small offset of the chemical potential from strict stochiometry. We find no definitive evidence of a bulk Weyl nodes. Instead we see signatures of a gapped Dirac dispersion, common in other members of AMnBi2 family or compounds with similar 2D network of Bi atoms. We speculate that the evidence for a WSM seen in ARPES arises through a surface magnetic phase. Such an assumption reconciles all known experimental data. INTRODUCTION Correlated electron systems with strong SOC have been the subject of intensive research in recent years. The interplay of electronic correlations and SOC can result in emergent topological phases and has opened up a completely new direction in condensed matter physics. This interplay can be very different depending on the specifics of the electronic correlation. In weakly to moderately interacting electron systems, SOC can lead to non-trivial band topology as observed in conventional topological insulators [1], Dirac and Weyl semi-metals [2-4], axion insulators [5] and topological superconductors [6]. More recently, the effects of SOC on strongly correlated systems are being explored with the realization of new material systems with heavy 4d/5d transition metal compounds [7]. The iridates deserve special mention in this category and have been instrumental in exploring much of this uncharted territory [8, 9]. In addition to the emergence of topologically non-trivial ground states, the interplay between SOC and magnetic degrees of freedom themselves is also quite interesting. The family of AMnBi 2 (A = Sr, Ca) compounds are particularly important in this context. Being structurally similar to iron based superconductors, they are referred to as manganese pnictides, which contain layers of Mn-Bi edge sharing tetrahedra and a Bi square net separated by a layer of A atoms [10]. These compounds were expected from firs...