Transformation of electronic structure by crystal engineering in a rare-earth magnetic topological material

The transformation of crystal or/and electronic structure is highly unexpected, especially when the materials belong to the same family. Here, we present a transformation of both crystal and electronic structure of a member GdAlSi in the rare-earth (R) RAlX (X = Si or Ge) family of Weyl semimetals. Replacing Si with the larger isovalent Ge creates sufficiently large chemical pressure to induce a structural transition from tetragonal (GdAlSi) to orthorhombic (GdAlGe). As a result, the electronic structure changes from an inversion symmetry broken Weyl semimetal (GdAlSi) to an inversion symmetry protected nodal-line metal (GdAlGe). We find that GdAlGe hosts an antiferromagnetic ground state with two successive orderings, at T_N1 = 35 K and T_N2 = 30 K. This material shows a very weak magneto-crystalline anisotropy and a magnetic field induced metamagnetic transition at 6.2 T. Furthermore, the first principles calculations and angle-resolved photoemission spectroscopy measurements reveal a Dirac-like linear band dispersion for binding energies extending from Fermi energy to ∼ 1.5 eV. Such a large energy range of Dirac dispersion is rare. Interestingly, the Fermi velocity of this linear band is very high (∼ 10⁶ m/s), ∼ 1/300 times the speed of light.