31 research outputs found
Diffusion of Dirac fermions across a topological merging transition in two dimensions
A continuous deformation of a Hamiltonian possessing at low energy two Dirac
points of opposite chiralities can lead to a gap opening by merging of the two
Dirac points. In two dimensions, the critical Hamiltonian possesses a
semi-Dirac spectrum: linear in one direction but quadratic in the other. We
study the transport properties across such a transition, from a Dirac
semi-metal through a semi-Dirac phase towards a gapped phase. Using both a
Boltzmann approach and a diagrammatic Kubo approach, we describe the
conductivity tensor within the diffusive regime. In particular, we show that
both the anisotropy of the Fermi surface and the Dirac nature of the
eigenstates combine to give rise to anisotropic transport times, manifesting
themselves through an unusual matrix self-energy.Comment: 15 pages, 14 figure
Spin dynamics of frustrated easy-axis triangular antiferromagnet 2H-AgNiO2 explored by inelastic neutron scattering
We report inelastic neutron scattering measurements of the spin dynamics in
the layered hexagonal magnet 2H-AgNiO2 which has stacked triangular layers of
antiferromagnetically-coupled Ni2+ spins (S=1) ordered in a collinear
alternating stripe pattern. We observe a broad band of magnetic excitations
above a small gap of 1.8 meV and extending up to 7.5 meV, indicating strongly
dispersive excitations. The measured dispersions of the boundaries of the
powder-averaged spectrum can be quantitatively explained by a linear spin-wave
dispersion for triangular layers with antiferromagnetic nearest- and weak
next-nearest neighbor couplings, a strong easy-axis anisotropy and additional
weak inter-layer couplings. The resulting dispersion relation has global minima
not at magnetic Bragg wavevectors but at symmetry-related soft points and we
attribute this anomalous feature to the strong competition between the
easy-axis anisotropy and the frustrated antiferromagnetic couplings. We have
also calculated the quantum corrections to the dispersion relation to order 1/S
in spin-wave theory by extending the work of Chubukov and Jolicoeur [Phys. Rev.
B v46, 11137 (1992)] and find that the presence of easy-axis anisotropy
significantly reduces the quantum renormalizations predicted for the isotropic
model.Comment: 17 pages and 15 figures. To appear in Physical Review
Core Loss During a Severe Accident (COLOSS)
See abstract in the article attachedJRC.E.2-Hot cell