Weyl semimetal | EPFL Graph Search

Weyl fermions are massless chiral fermions embodying the mathematical concept of a Weyl spinor. Weyl spinors in turn play an important role in quantum field theory and the Standard Model, where they are a building block for fermions in quantum field theory. Weyl spinors are a solution to the Dirac equation derived by Hermann Weyl, called the Weyl equation. For example, one-half of a charged Dirac fermion of a definite chirality is a Weyl fermion. Weyl fermions may be realized as emergent quasiparticles in a low-energy condensed matter system. This prediction was first proposed by Conyers Herring in 1937, in the context of electronic band structures of solid state systems such as electronic crystals. Topological materials in the vicinity of band inversion transition became a primary target in search of topologically protected bulk electronic band crossings. The first (non-electronic) liquid state which is suggested, has similarly emergent but neutral excitatio

Magnetic structure of the topological semimetal Co3Sn2S2

Henrik Moodysson Rønnow, Jian Rui Soh, Ivica Zivkovic

Co3Sn2S2 has recently been predicted to be a Weyl semimetal in which magnetic order is key to its behavior as a topological material. Here, we report unpolarized neutron diffraction and spherical neutron polarimetry measurements, supported by magnetization and transport data, which probe the magnetic order in Co3Sn2S2 below TC = 177 K. The results are fully consistent with ferromagnetic order in which the spins on the Co atoms point along the crystal c axis, although we cannot rule out some canting of the spins. We find no evidence for a type of long-ranged (k = 0) in-plane 120?? antiferromagnetic order which had previously been considered as a secondary phase present at temperatures between -90 K and TC. A discontinuous change in bulk properties and neutron polarization observed at T = 125 K when samples are cooled in a field and measured on warming is found to be due to a sudden reduction in ferromagnetic domain size. Our results lend support to the theoretical predictions that Co3Sn2S2 is a magnetic Weyl semimetal.

AMER PHYSICAL SOC2022

Controlling magnetic order, magnetic anisotropy, and band topology in the semimetals Sr(Mn0.9Cu0.1)Sb-2 and Sr(Mn0.9Zn0.1)Sb-2

Yong Liu

Neutron diffraction and magnetic susceptibility studies show that orthorhombic single-crystals of topological semimetals Sr(Mn0.9Cu0.1)Sb-2 and Sr(Mn0.9Zn0.1)Sb-2 undergo three-dimensional C-type antiferromagnetic (AFM) ordering of the Mn2+ moments at T-N = 200 +/- 10 and 210 +/- 12 K, respectively, significantly lower than that of the parent SrMnSb2 with T-N = 297 +/- 3 K. Magnetization versus applied magnetic field (perpendicular to MnSb planes) below T-N exhibits slightly modified de Haas van Alphen oscillations for the Zn-doped crystal as compared to that of the parent compound. By contrast, the Cu-doped system does not show de Haas van Alphen magnetic oscillations, suggesting that either Cu substitution for Mn changes the electronic structure of the parent compound substantially, or that the Cu sites are strong scatterers of carriers that significantly shorten their mean free path thus diminishing the oscillations. Density functional theory (DFT) calculations including spin-orbit coupling predict the C-type AFM state for the parent, Cu-, and Zn-doped systems and identify the a-axis (i.e., perpendicular to the Mn layer) as the easy magnetization direction in the parent and 12.5% of Cu or Zn substitutions. In contrast, 25% of Cu content changes the easy magnetization to the b-axis (i.e., within the Mn layer). We find that the incorporation of Cu and Zn in SrMnSb2 tunes electronic bands near the Fermi level resulting in different band topology and semimetallicity. The parent and Zn-doped systems have coexistence of electron and hole pockets with opened Dirac cone around the Y-point whereas the Cu-doped system has dominant hole pockets around the Fermi level with a distorted Dirac cone. The tunable electronic structure may point out possibilities of rationalizing the experimentally observed de Haas van Alphen magnetic oscillations.

2020

Magnetoresistance from Fermi surface topology

Yi Liu, Quansheng Wu, Oleg Yazyev, Shengnan Zhang

Extremely large nonsaturating magnetoresistance has recently been reported for a large number of both topologically trivial and nontrivial materials. Different mechanisms have been proposed to explain the observed magnetotransport properties, yet without arriving to definitive conclusions or portraying a global picture. In this work, we investigate the transverse magnetoresistance of materials by combining the Fermi surfaces calculated from first principles with the Boltzmann transport theory approach relying on the semiclassical model and the relaxation time approximation. We first consider a series of simple model Fermi surfaces to provide a didactic introduction into the charge-carrier compensation and open-orbit mechanisms leading to nonsaturating magnetoresistance. We then address in detail magnetotransport in three representative materials: (i) copper, a prototypical nearly free-electron metal characterized by the open Fermi surface that results in an intricate angular magnetoresistance, (ii) bismuth, a topologically trivial semimetal in which very large magnetoresistance is known to result from charge-carrier compensation, and (iii) tungsten diphosphide WP2, a recently discovered type-II Weyl semimetal that holds the record of magnetoresistance in compounds. In all three cases our calculations show excellent agreement with both the field dependence of magnetoresistance and its anisotropy measured at low temperatures. Furthermore, the calculations allow for a full interpretation of the observed features in terms of the Fermi surface topology. Our study thus establishes guidelines to clarifying the physical mechanisms underlying the magnetotransport properties in a broad range of materials. These results will help addressing a number of outstanding questions, such as the role of the topological phase in the pronounced large nonsaturating magnetoresistance observed in topological materials.

2019