Andrew Boothroyd

Abstract:

Resonant elastic X-ray scattering (REXS) at the Eu M5 edge reveals an antiferromagnetic structure in layered EuCd2Sb2 at temperatures below TN = 7.4 K with a magnetic propagation vector of (0, 0, 1/2) and spins in the basal plane. Magneto-transport and REXS measurements with an in-plane magnetic field show that features in the magnetoresistance are correlated with changes in the magnetic structure induced by the field. Ab initio electronic structure calculations predict that the observed spin structure gives rise to a gapped Dirac point close to the Fermi level with a gap of ∆E ∼ 0.01 eV. The results of this study indicate that the Eu spins are coupled to conduction electron states near the Dirac point.

Abstract:

The chemical and magnetic structures of the series of compounds Ca2−xLaxRuO4 [x = 0, 0.05(1), 0.07(1), 0.12(1)] have been investigated using neutron diffraction and resonant elastic x-ray scattering. Upon La doping, the low-temperature S-P bca space group of the parent compound is retained in all insulating samples [x 0.07(1)], but with significant changes to the atomic positions within the unit cell. These changes can be characterized in terms of the local RuO6 octahedral coordination: with increasing doping, the structure, crudely speaking, evolves from an orthorhombic unit cell with compressed octahedra to a quasitetragonal unit cell with elongated ones. The magnetic structure on the other hand, is found to be robust, with the basic k = (0,0,0), b-axis antiferromagnetic order of the parent compound preserved below the critical La doping concentration of x ≈ 0.11. The only effects of La doping on the magnetic structure are to suppress the A-centred mode, favoring the B mode instead, and to reduce the Néel temperature somewhat. Our results are discussed with reference to previous experimental reports on the effects of cation substitution on the d4 Mott insulator Ca2RuO4, as well as with regard to theoretical studies on the evolution of its electronic and magnetic structure. In particular, our results rule out the presence of a proposed ferromagnetic phase, and suggest that the structural effects associated with La substitution play an important role in the physics of the system.

Abstract:

We use resonant elastic x-ray scattering to determine the evolution of magnetic order in EuCd 2 As 2 below T N = 9.5 K, as a function of temperature and applied magnetic field. We find an A-type antiferromagnetic structure with in-plane magnetic moments, and observe dramatic magnetoresistive effects associated with field-induced changes in the magnetic structure and domain populations. Our ab initio electronic structure calculations indicate that the Dirac dispersion found in the nonmagnetic Dirac semimetal Cd 3 As 2 is also present in EuCd 2 As 2 , but is gapped for T < T N due to the breaking of C 3 symmetry by the magnetic structure.