143 research outputs found
Weyl, Dirac and high-fold chiral fermions in topological quantum materials
Quantum materials hosting Weyl fermions have opened a new era of research in
condensed matter physics. First proposed in 1929 in particle physics, Weyl
fermions have yet to be observed as elementary particles. In 2015, Weyl
fermions were detected as collective electronic excitations in the strong
spin-orbit coupled material tantalum arsenide, TaAs. This discovery was
followed by a flurry of experimental and theoretical explorations of Weyl
phenomena in materials. Weyl materials naturally lend themselves to the
exploration of the topological index associated with Weyl fermions and their
divergent Berry curvature field, as well as the topological bulk-boundary
correspondence giving rise to protected conducting surface states. Here, we
review the broader class of Weyl topological phenomena in materials, starting
with the observation of emergent Weyl fermions in the bulk and of Fermi arc
states on the surface of the TaAs family of crystals by photoemission
spectroscopy. We then discuss some of the exotic optical and magnetic responses
observed in these materials, as well as the progress in developing some of the
related chiral materials. We discuss the conceptual development of high-fold
chiral fermions, which generalize Weyl fermions, and we review the observation
of high-fold chiral fermion phases by taking the rhodium silicide, RhSi, family
of crystals as a prime example. Lastly, we discuss recent advances in Weyl-line
phases in magnetic topological materials. With this Review, we aim to provide
an introduction to the basic concepts underlying Weyl physics in condensed
matter, and to representative materials and their electronic structures and
topology as revealed by spectroscopic studies. We hope this work serves as a
guide for future theoretical and experimental explorations of chiral fermions
and related topological quantum systems with potentially enhanced
functionalities.Comment: To appear in Nature Review Material
Tunability of the topological nodal-line semimetal phase in ZrSiX-type materials
The discovery of a topological nodal-line (TNL) semimetal phase in ZrSiS has
invigorated the study of other members of this family. Here, we present a
comparative electronic structure study of ZrSiX (where X = S, Se, Te) using
angle-resolved photoemission spectroscopy (ARPES) and first-principles
calculations. Our ARPES studies show that the overall electronic structure of
ZrSiX materials comprises of the diamond-shaped Fermi pocket, the nearly
elliptical-shaped Fermi pocket, and a small electron pocket encircling the zone
center () point, the M point, and the X point of the Brillouin zone,
respectively. We also observe a small Fermi surface pocket along the
M--M direction in ZrSiTe, which is absent in both ZrSiS and ZrSiSe.
Furthermore, our theoretical studies show a transition from nodal-line to
nodeless gapped phase by tuning the chalcogenide from S to Te in these material
systems. Our findings provide direct evidence for the tunability of the TNL
phase in ZrSiX material systems by adjusting the spin-orbit coupling (SOC)
strength via the X anion.Comment: 7 pages, 4 figure
Non-Kondo-like Electronic Structure in the Correlated Rare-Earth Hexaboride YbB
We present angle-resolved photoemission studies on the rare-earth hexaboride
YbB, which has recently been predicted to be a topological Kondo insulator.
Our data do not agree with the prediction and instead show that YbB
exhibits a novel topological insulator state in the absence of a Kondo
mechanism. We find that the Fermi level electronic structure of YbB has
three 2D Dirac cone like surface states enclosing the Kramers' points, while
the f-orbital which would be relevant for the Kondo mechanism is eV
below the Fermi level. Our first-principles calculation shows that the
topological state which we observe in YbB is due to an inversion between Yb
and B bands. These experimental and theoretical results provide a new
approach for realizing novel correlated topological insulator states in
rare-earth materials.Comment: 5 pages, 4 figures, Submitted in 2014. Published in 2015, Phys. Rev.
Lett. 114, 01640
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