Magnetism, when combined with an unconventional electronic band structure, can give rise to forefront electronic properties such as the quantum anomalous Hall effect, axion electrodynamics, and Majorana fermions. Here we report the characterization of high-quality crystals of EuSn 2 P 2 , a new quantum material specifically designed to engender unconventional electronic states plus magnetism. EuSn 2 P 2 has a layered, Bi 2 Te 3 -type structure. Ferromagnetic interactions dominate the Curie–Weiss susceptibility, but a transition to antiferromagnetic ordering occurs near 30 K. Neutron diffraction reveals that this is due to two-dimensional ferromagnetic spin alignment within individual Eu layers and antiferromagnetic alignment between layers—this magnetic state surrounds the Sn–P layers at low temperatures. The bulk electrical resistivity is sensitive to the magnetism. Electronic structure calculations reveal that EuSn 2 P 2 might be a strong topological insulator, which can be a new magnetic topological quantum material (MTQM) candidate. The calculations show that surface states should be present, and they are indeed observed by angle-resolved photoelectron spectroscopy (ARPES) measurements.

We survey the electrical transport properties of the single-crystalline, topological chiral semimetal CoSi which was grown via different methods. High-quality CoSi single crystals were found in the growth from tellurium solution. The sample's high carrier mobility enables us to observe, for the first time, quantum oscillations (QOs) in its thermoelectrical signals. Our analysis of QOs reveals two spherical Fermi surfaces around the R point in the Brillouin zone corner. The extracted Berry phases of these electron orbits are consistent with the −2 chiral charge as reported in DFT calculations. Detailed analysis on the QOs reveals that the spin-orbit coupling induced band-splitting is less than 2 meV near the Fermi level, one order of magnitude smaller than our DFT calculation result. We also report the phonon-drag induced large Nernst effect in CoSi at intermediate temperatures.

No abstract

Superconductivity was first observed more than a century ago, but the search for new superconducting materials remains a challenge. The Cooper pairs in superconductors are ideal embodiments of quantum entanglement. Thus, novel superconductors can be critical for both learning about electronic systems in condensed matter and for possible application in future quantum technologies. Here two previously unreported materials, NbIr 2 B 2 and TaIr 2 B 2 , are presented with superconducting transitions at 7.2 and 5.2 K, respectively. They display a unique noncentrosymmetric crystal structure, and for both compounds the magnetic field that destroys the superconductivity at 0 K exceeds one of the fundamental characteristics of conventional superconductors (the "Pauli limit"), suggesting that the superconductivity may be unconventional. Supporting this experimentally based deduction, first-principle calculations show a spinsplit Fermi surface due to the presence of strong spin-orbit coupling. These materials may thus provide an excellent platform for the study of unconventional superconductivity in intermetallic compounds.

Recently, orthorhombic CuMnAs has been proposed to be a magnetic material where topological fermions exist around the Fermi level. Here we report the magnetic structure of the orthorhombic Cu0.95MnAs and Cu0.98Mn0.96As single crystals. While Cu0.95MnAs is a commensurate antiferromagnet (C-AFM) below 360 K with a propagation vector of k = 0, Cu0.98Mn0.96As undergoes a second-order paramagnetic to incommensurate antiferromagnetic (IC-AFM) phase transition at 320 K with k = (0.1,0,0), followed by a second-order IC-AFM to C-AFM phase transition at 230 K. In the C-AFM state, the Mn spins order parallel to the b-axis but antiparallel to their nearest-neighbors with the easy axis along the b axis. This magnetic order breaks Ry gliding and S2z rotational symmetries, the two crucial for symmetry analysis, resulting in finite band gaps at the crossing point and the disappearance of the massless topological fermions. However, the spin-polarized surface states and signature induced by non-trivial topology still can be observed in this system, which makes orthorhombic CuMnAs promising in antiferromagnetic spintronics.Dirac cones have been proposed and observed in many non-magnetic materials, including Cd 3 As 2 [1, 2] and Na 3 Bi [3,4]. By breaking inversion symmetry (P) or time-reversal symmetry (T ), a Dirac point can be split into a pair of Weyl points. To break T , we can either apply an external magnetic field or use the spontaneous magnetic moment inside the material. For the latter case, the correlation between spontaneous magnetism and Weyl fermions has been studied in the AMnPn 2 (A = rare earth or alkali earth and Pn = Sb or Bi) system [5][6][7][8][9][10][11][12] and the half-Heusler compound GdPtBi [13,14]. Recently, CuMnAs was proposed to be an interesting material with non-trivial topology. CuMnAs has two polymorphs; the tetragonal (TET) CuMnAs, which crystalizes in the space group P 4/nmm, and the orthorhombic (ORT) CuMnAs crystalizing in the non-symmorphic P nma space group. The TET phase consists of alternating layers of edge-sharing CuAs 4 and MnAs 4 tetrahedra. It has been proposed to be a candidate with favourable applications in spintronics [18,19] and a topological metal-insulator transition driven by the Néel vector [17]. On the other hand, the ORT phase consists of a 3D network of edge-sharing CuAs 4 and MnAs 4 tetrahedra (Fig. 2(c)), where the Mn atoms form a 3D distorted honeycomb lattice (Fig. 2(d)). ORT CuMnAs was proposed to be an antiferromagnetic topological semimetal when the spin-orbit coupling is fully considered [15,17]. In such a system, two gapless points, named as coupled Weyl fermions, are robust if the combination of PT is * Corresponding author: nini@physics.ucla.edu reserved and the non-symmorphic screw symmetry S 2z is not broken. Thus, the anti-ferromagnetic ORT CuMnAs provides an ideal system to study the interplay between antiferromagnetism (AFM) and Dirac fermions [15]. In this paper, we will focus on the ORT CuMnAs. We experimentally determine its magnetic order, which breaks th...