X-ray powder diffraction (XRD), magnetic susceptibility χ, isothermal magnetization M , heat capacity Cp and in-plane electrical resistivity ρ measurements as a function of temperature T and magnetic field H are presented for CaCo1.86As2 single crystals. The electronic structure is probed by angle-resolved photoemission spectroscopy (ARPES) measurements of CaCo1.86As2 and by full-potential linearized augmented-plane-wave calculations for the supercell Ca8Co15As16 (CaCo1.88As2). Our XRD crystal structure refinement is consistent with the previous combined refinement of x-ray and neutron powder diffraction data showing a collapsed-tetragonal ThCr2Si2-type structure with 7(1)% vacancies on the Co sites corresponding to the composition CaCo1.86As2 [D. G. Quirinale et al., Phys. Rev. B 88, 174420 (2013)]. The anisotropic χ(T ) data are consistent with the magnetic neutron diffraction data of Quirianale et al. that demonstrate the presence of A-type collinear antiferromagnetic order below the Néel temperature TN = 52(1) K with the easy axis being the tetragonal c axis. However, no clear evidence from the ρ(T ) and Cp(T ) data for a magnetic transition at TN is observed. A metallic ground state is demonstrated from the band calculations and the ρ(T ), Cp(T ) and ARPES data, and spin-polarized calculations indicate a competition between the A-type AFM and FM ground states. The Cp(T ) data exhibit a large Sommerfield electronic coefficient reflecting a large density of states at the Fermi energy D(EF) that is enhanced compared with the band structure calculation where the bare D(EF) arises from Co 3d bands. At 1.8 K the M (H) data for H c exhibit a well-defined first-order spin-flop transition at an applied field of 3.5 T. The small ordered moment of ≈ 0.3 µB/Co obtained from the M (H) data at low T , the large exchange enhancement of χ and the lack of a self-consistent interpretation of the χ(T ) and M (H, T ) data in terms of a local moment Heisenberg model together indicate that the magnetism of CaCo1.86As2 is itinerant.
Rare-earth platinum bismuth (RPtBi) has been recently proposed to be a potential topological insulator. In this paper we present measurements of the metallic surface electronic structure in three members of this family, using angle resolved photoemission spectroscopy (ARPES). Our data shows clear spin-orbit splitting of the surface bands and the Kramers' degeneracy of spins at theΓ andM points, which is nicely reproduced with our full-potential linearized augmented plane wave calculation for a surface electronic state. Topologically non-trivial behavior is signified by band inversion in the calculated bulk electronic structures, yet no direct indication of such behavior is detected by ARPES, except for a weak Fermi crossing detected in close vicinity to theΓ point, making the total number of Fermi crossings odd. In the surface band calculation, however, this crossing is explained by a Kramers' pair of bands that are very close to each other. The classification of this family of materials as topological insulators remains an open question.
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