Sinead Griffin

Scaling Behaviour and Beyond Equilibrium in the Hexagonal Manganites

We show that the improper ferroelectric phase transition in the multiferroic hexagonal manganites displays the same symmetry-breaking characteristics as those proposed in early-universe theories. We present an analysis of the Kibble-Zurek theory of topological defect formation applied to the hexagonal manganites, discuss the conditions determining the range of cooling rates in which KibbleZurek behavior is expected, and show that recent literature data are consistent with our predictions. We explore experimentally for the first time to our knowledge the cross-over out of the Kibble-Zurek regime and find a surprising “anti-Kibble-Zurek” behavior.

Detection of sub-MeV dark matter with three-dimensional Dirac materials

We propose the use of three-dimensional Dirac materials as targets for direct detection of sub-MeV dark matter. Dirac materials are characterized by a linear dispersion for low-energy electronic excitations, with a small band gap of O(meV) if lattice symmetries are broken. Dark matter at the keV scale carrying kinetic energy as small as a few meV can scatter and excite an electron across the gap. Alternatively, bosonic dark matter as light as a few meV can be absorbed by the electrons in the target. We develop the formalism for dark matter scattering and absorption in Dirac materials and calculate the experimental reach of these target materials. We find that Dirac materials can play a crucial role in detecting dark matter in the keV to MeV mass range that scatters with electrons via a kinetically mixed dark photon, as the dark photon does not develop an in-medium effective mass. The same target materials provide excellent sensitivity to absorption of light bosonic dark matter in the meV to hundreds of meV mass range, superior to all other existing proposals when the dark matter is a kinetically mixed dark photon.

Global formation of topological defects in the multiferroic hexagonal manganites

Author(s): Meier, QN; Lilienblum, M; Griffin, SM; Conder, K; Pomjakushina, E; Yan, Z; Bourret, E; Meier, D; Lichtenberg, F; Salje, EKH; Spaldin, NA; Fiebig, M; Cano, A | Abstract: The spontaneous transformations associated with symmetry-breaking phase transitions generate domain structures and defects that may be topological in nature. The formation of these defects can be described according to the Kibble-Zurek mechanism, which provides a generic relation that applies from cosmological to interatomic length scales. Its verification is challenging, however, in particular at the cosmological scale where experiments are impractical. While it has been demonstrated for selected condensed-matter systems, major questions remain regarding, e.g., its degree of universality. Here, we develop a global Kibble-Zurek picture from the condensed-matter level. We show theoretically that a transition between two fluctuation regimes (Ginzburg and mean field) can lead to an intermediate region with reversed scaling, and we verify experimentally this behavior for the structural transition in the series of multiferroic hexagonal manganites. Trends across the series allow us to identify additional intrinsic features of the defect formation beyond the original Kibble-Zurek paradigm.

Observation of a two-dimensional Fermi surface and Dirac dispersion in YbMnSb2

We present the crystal structure, electronic structure, and transport properties of the material

A density functional theory study of the influence of exchange-correlation functionals on the properties of FeAs

{\mathrm{YbMnSb}}_{2}

Thermodynamic signature of Dirac electrons across a possible topological transition in ZrTe5

, a candidate system for the investigation of Dirac physics in the presence of magnetic order. Our measurements reveal that this system is a low-carrier-density semimetal with a two-dimensional Fermi surface arising from a Dirac dispersion, consistent with the predictions of density-functional-theory calculations of the antiferromagnetic system. The low temperature resistivity is very large, suggesting that scattering in this system is highly efficient at dissipating momentum despite its Dirac-like nature.

Ab initio investigation of FeAs/GaAs heterostructures for potential spintronic and superconducting applications

We use density functional theory within the local density approximation (LDA), LDA  +  U, generalised gradient approximation (GGA), GGA  +  U, and hybrid-functional methods to calculate the properties of iron monoarsenide. FeAs, which forms in the MnP structure, is of current interest for potential spintronic applications as well as being the parent compound for the pnictide superconductors. We compare the calculated structural, magnetic and electronic properties obtained using the different functionals to each other and to experiment, and investigate the origin of a recently reported magnetic spiral. Our results indicate the appropriateness or otherwise of the various functionals for describing FeAs and the related Fe-pnictide superconductors.

Duality of Topological Defects in Hexagonal Manganites

Nityan L. Nair, 2 Philipp T. Dumitrescu, Sanyum Channa, Sinéad M. Griffin, 4 Jeffrey B. Neaton, 4, 5, 2 Andrew C. Potter, and James G. Analytis 2 Department of Physics, University of California, Berkeley, California 94720, USA Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Department of Physics, University of Texas at Austin, Austin, TX 78712, USA Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Kavli Energy NanoScience Institute at Berkeley, Berkeley, CA 94720, USA (Dated: August 11, 2017)

Defect Chemistry as a Crystal Structure Design Parameter: Intrinsic Point Defects and Ga Substitution in InMnO3

Ultra-thin FeAs is of interest both as the active component in the newly identified pnictide superconductors, and in spintronic applications at the interface between ferromagnetic Fe and semiconducting GaAs. Here we use first-principles density functional theory to investigate the properties of FeAs/GaAs heterostructures. We find that the Fermi surface is modified from that characteristic of the pnictide superconductors by interactions between the FeAs layer and the As atoms in the GaAs layers. Regardless of the number of FeAs layers, the Fe to As ratio, or the strain state, the lowest energy magnetic ordering is always antiferromagnetic, suggesting that such heterostructures are not promising spintronic systems, and offering an explanation for the failure of spin injection across Fe/GaAs interfaces.