Disorder and metal-insulator transitions in Weyl semimetals

The Weyl semimetal (WSM) is a newly proposed quantum state of matter. It has Weyl nodes in bulk excitations and Fermi arcs surface states. We study the effects of disorder and localization in WSMs and find three exotic phase transitions. (I) Two Weyl nodes near the Brillouin zone boundary can be annihilated pairwise by disorder scattering, resulting in the opening of a topologically nontrivial gap and a transition from a WSM to a three-dimensional (3D) quantum anomalous Hall state. (II) When the two Weyl nodes are well separated in momentum space, the emergent bulk extended states can give rise to a direct transition from a WSM to a 3D diffusive anomalous Hall metal. (III) Two Weyl nodes can emerge near the zone center when an insulating gap closes with increasing disorder, enabling a direct transition from a normal band insulator to a WSM. We determine the phase diagram by numerically computing the localization length and the Hall conductivity, and propose that the exotic phase transitions can be realized on a photonic lattice.Comment: 7 pages with appendix, 6 figure

Diaqua­bis[5-(2-pyrid­yl)-1H-tetra­zolato-κ2 N 1,N 5]cobalt(II)

In the title compound, [Co(C6H4N5)2(H2O)2], the Co atom is bonded to two water mol­ecules and two bidentate 5-(2-pyrid­yl)tetra­zolate ligands resulting in a slightly distorted octa­hedral CoN4O2 coordination geometry. The CoII cation is situated on a crystallographic center of inversion. The asymmetric unit therefore comprises one-half of the mol­ecule. The four N atoms belonging to two bidentate 5-(2-pyrid­yl)tetra­zolate ligands lie in the equatorial plane and the two associated water mol­ecules are observed in the axial coordination sites. The crystal structure exhibits a three-dimensional supra­molecular network assembled by inter­molecular O—H⋯N hydrogen bonds