Topologically protected electron states arranged artificially in real space form a highly tunable emergent atomic chain.

A new methodology is used to design robust Weyl semimetals and to identify the most robust and ideal Weyl semimetal candidate in Ta₃S₂.

Weyl physics emerges in the laboratory

Weyl fermions—massless particles with half-integer spin—were once mistakenly thought to describe neutrinos. Although not yet observed among elementary particles, Weyl fermions may exist as collective excitations in so-called Weyl semimetals. These materials have an unusual band structure in which the linearly dispersing valence and conduction bands meet at discrete “Weyl points.” Xu et al. used photoemission spectroscopy to identify TaAs as a Weyl semimetal capable of hosting Weyl fermions. In a complementary study, Lu et al. detected the characteristic Weyl points in a photonic crystal. The observation of Weyl physics may enable the discovery of exotic fundamental phenomena.

Science, this issue p. 613 and 622

Photoemission established tantalum phosphide as a Weyl semimetal, which hosts exotic Weyl fermion quasiparticles and Fermi arcs.

Nailing down the topology of a semimetal

Topological insulators are exotic materials that have a conducting surface state that can withstand certain types of material imperfection. Theoreticians have predicted a different kind of surface state in related three-dimensional topological Dirac semimetals, which do not have an energy gap in the band structure of the bulk. Xu et al. used photoemission spectroscopy to map out the band structure of the material Na₃Bi and detected the predicted surface state. Their results may lead to further insights into the physics of topological matter.

Science, this issue p. 294

Two types of bulk insulator are realized in the same family of compounds through chemical doping.

Abstract

A topologically ordered material is characterized by a rare quantum organization of electrons that evades the conventional spontaneously broken symmetry–based classification of condensed matter. Exotic spin-transport phenomena, such as the dissipationless quantum spin Hall effect, have been speculated to originate from a topological order whose identification requires a spin-sensitive measurement, which does not exist to this date in any system. Using Mott polarimetry, we probed the spin degrees of freedom and demonstrated that topological quantum numbers are completely determined from spin texture–imaging measurements. Applying this method to Sb and Bi_1–xSb_x, we identified the origin of its topological order and unusual chiral properties. These results taken together constitute the first observation of surface electrons collectively carrying a topological quantum Berry's phase and definite spin chirality, which are the key electronic properties component for realizing topological quantum computing bits with intrinsic spin Hall–like topological phenomena.