a new type of a magnetic semimetal with nontrivial topology

Electrical transport measurements were performed on high-quality GdB₄ single crystal with high residual resistivity ratio (RRR) > 300 under applying magnetic fields up to 14 T. In the absence of magnetic field, the temperaturedependent electrical resistivity ρ(T) shows a metallic behavior with antiferromagnetic and hidden transitions at TN = 43 K and Tt = 6 K, respectively. With increasing magnetic fields, this metallic behavior turns into a seemingly “insulating” one with saturation at Tt < 6 K, which indicates partial gapping of the Fermi surface. At Tt < 6 K, evidences for a topologically non-trivial metallic state are found, such as extremely large magneto-resistance (73,000 % at 2 K, 9 T), non-zero Berry phase, anomalous contribution in Hall resistivity, and negative longitudinal magnetoresistance (LMR). Here we argue that the anomalous contribution in Hall resistivity and non-zero Berry phase are manifestations of a topological metallic state realizable when either spin chirality in the real space or topological node in the k space exists. The observation of negative LMR with weak antilocalization whose origin is Adler-Bell-Jackiw anomaly further supports the former scenario.