69 research outputs found
Resolving the Topological Classification of Bismuth with Topological Defects
Bulk boundary correspondence in topological materials allows to study their
bulk topology through the investigation of their topological boundary modes.
However, for classes that share similar boundary phenomenology, the growing
diversity of topological phases may lead to ambiguity in the topological
classification of materials. Such is the current status of bulk bismuth. While
some theoretical models indicate that bismuth possesses a trivial topological
nature, other theoretical and experimental studies suggest non-trivial
topological classifications such as a strong or a higher order topological
insulator, both of which hosts helical modes on their boundaries. Here we use a
novel approach to resolve the topological classification of bismuth by
spectroscopically mapping the response of its boundary modes to a topological
defect in the form of a screw dislocation (SD). We find that the edge mode
extends over a wide energy range, and withstands crystallographic
irregularities, without showing any signs of backscattering. It seems to bind
to the bulk SD, as expected for a topological insulator (TI) with non-vanishing
weak indices. We argue that the small scale of the bulk energy gap, at the time
reversal symmetric momentum , positions bismuth within the critical region
of a topological phase transition to a strong TI with non-vanishing weak
indices. We show that the observed boundary modes are approximately helical
already on the trivial side of the topological phase transition.
This work opens the door for further possibilities to examine the response of
topological phases to crystallographic topological defects, and to uniquely
explore their associated bulk boundary phenomena
Finite-temperature violation of the anomalous transverse Wiedemann-Franz law
The Wiedemann-Franz (WF) law links the ratio of electronic charge and heat
conductivity to fundamental constants. It has been tested in numerous solids,
but the extent of its relevance to the anomalous transverse transport, which
represents the topological nature of the wave function, remains an open
question. Here we present a study of anomalous transverse response in the
noncollinear antiferromagnet MnGe extended from room temperature down to
sub-Kelvin temperature and find that the anomalous Lorenz ratio remains close
to the Sommerfeld value up to 100 K, but not above. The finite-temperature
violation of the WF correlation is caused by a mismatch between the thermal and
electrical summations of the Berry curvature, rather than the inelastic
scattering as observed in ordinary metals. This interpretation is backed by our
theoretical calculations, which reveals a competition between the temperature
and the Berry curvature distribution. The accuracy of the experiment is
supported by the verification of the Bridgman relation between the anomalous
Ettingshausen and Nernst effects. Our results identify the anomalous Lorenz
ratio as an extremely sensitive probe of Berry spectrum near the chemical
potential.Comment: 9 pages,6 figures, Supplemental Material include
Observation of Spin Hall Effect in Weyl Semimetal WTe2 at Room Temperature
Discovery of topological Weyl semimetals has revealed the opportunities to
realize several extraordinary physical phenomena in condensed matter physics.
Specifically, these semimetals with strong spin-orbit coupling, broken
inversion symmetry and novel spin texture are predicted to exhibit a large spin
Hall effect that can efficiently convert the charge current to a spin current.
Here we report the direct experimental observation of a large spin Hall and
inverse spin Hall effects in Weyl semimetal WTe2 at room temperature obeying
Onsager reciprocity relation. We demonstrate the detection of the pure spin
current generated by spin Hall phenomenon in WTe2 by making van der Waals
heterostructures with graphene, taking advantage of its long spin coherence
length and spin transmission at the heterostructure interface. These
experimental findings well supported by ab initio calculations show a large
charge-spin conversion efficiency in WTe2; which can pave the way for
utilization of spin-orbit induced phenomena in spintronic memory and logic
circuit architectures
Visualizing near-coexistence of massless Dirac electrons and ultra-massive saddle point electrons
Strong singularities in the electronic density of states amplify correlation
effects and play a key role in determining the ordering instabilities in
various materials. Recently high order van Hove singularities (VHSs) with
diverging power-law scaling have been classified in single-band electron
models. We show that the 110 surface of Bismuth exhibits high order VHS with an
usually high density of states divergence . Detailed mapping
of the surface band structure using scanning tunneling microscopy and
spectroscopy combined with first-principles calculations show that this
singularity occurs in close proximity to Dirac bands located at the center of
the surface Brillouin zone. The enhanced power-law divergence is shown to
originate from the anisotropic flattening of the Dirac band just above the
Dirac node. Such near-coexistence of massless Dirac electrons and ultra-massive
saddle points enables to study the interplay of high order VHS and Dirac
fermions
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