48 research outputs found
Mirror protected Dirac fermions on a Weyl semimetal NbP surface
The first Weyl semimetal was recently discovered in the NbP class of
compounds. Although the topology of these novel materials has been identified,
the surface properties are not yet fully understood. By means of scanning
tunneling spectroscopy, we find that NbPs (001) surface hosts a pair of Dirac
cones protected by mirror symmetry. Through our high resolution spectroscopic
measurements, we resolve the quantum interference patterns arising from these
novel Dirac fermions, and reveal their electronic structure, including the
linear dispersions. Our data, in agreement with our theoretical calculations,
uncover further interesting features of the Weyl semimetal NbPs already exotic
surface. Moreover, we discuss the similarities and distinctions between the
Dirac fermions here and those in topological crystalline insulators in terms of
symmetry protection and topology
Atomic-scale visualization of quasiparticle interference on a type-II Weyl semimetal surface
We combine quasiparticle interference simulation (theory) and atomic
resolution scanning tunneling spectro-microscopy (experiment) to visualize the
interference patterns on a type-II Weyl semimetal MoWTe for
the first time. Our simulation based on first-principles band topology
theoretically reveals the surface electron scattering behavior. We identify the
topological Fermi arc states and reveal the scattering properties of the
surface states in MoWTe. In addition, our result reveals
an experimental signature of the topology via the interconnectivity of bulk and
surface states, which is essential for understanding the unusual nature of this
material.Comment: To appear in Phys. Rev. Let
Negative flat band magnetism in a spin-orbit coupled correlated kagome magnet
It has long been speculated that electronic flat band systems can be a
fertile ground for hosting novel emergent phenomena including unconventional
magnetism and superconductivity. Although flat bands are known to exist in a
few systems such as heavy fermion materials and twisted bilayer graphene, their
microscopic roles and underlying mechanisms in generating emergent behavior
remain elusive. Here we use scanning tunneling microscopy to elucidate the
atomically resolved electronic states and their magnetic response in the kagome
magnet Co3Sn2S2. We observe a pronounced peak at the Fermi level, which is
identified to arise from the kinetically frustrated kagome flat band.
Increasing magnetic field up to +-8T, this state exhibits an anomalous
magnetization-polarized Zeeman shift, dominated by an orbital moment in
opposite to the field direction. Such negative magnetism can be understood as
spin-orbit coupling induced quantum phase effects tied to non-trivial flat band
systems. We image the flat band peak, resolve the associated negative
magnetism, and provide its connection to the Berry curvature field, showing
that Co3Sn2S2 is a rare example of kagome magnet where the low energy physics
can be dominated by the spin-orbit coupled flat band. Our methodology of
probing band-resolved ordering phenomena such as spin-orbit magnetism can also
be applied in future experiments to elucidate other exotic phenomena including
flat band superconductivity and anomalous quantum transport.Comment: Nature Physics onlin
Observation of Weyl fermions in a magnetic non-centrosymmetric crystal
Characterized by the absence of inversion symmetry, non-centrosymmetric
materials are of great interest because they exhibit ferroelectricity, second
harmonic generation, emergent Weyl fermions, and other fascinating phenomena.
It is expected that if time-reversal symmetry is also broken, additional
magneto-electric effects can emerge from the interplay between magnetism and
electronic order. Here we report topological conducting properties in the
non-centrosymmetric magnet PrAlGe. By photoemission spectroscopy, we observe an
arc parametrizing surface-localized states---a topological arc. Using the
bulk-boundary correspondence, we conclude that these arcs correspond to
projected topological charges of in the surface Brillouin zone,
demonstrating the presence of magnetic Weyl quasiparticles in bulk. We further
observe a large anomalous Hall response, arising from diverging bulk Berry
curvature fields associated with the magnetic Weyl band structure. Our results
demonstrate a topological phase with robust electronic surface states and
anomalous transport in a non-centrosymmetric magnet for the first time,
providing a novel material platform to study the interplay between magnetic
order, band topology and transport.Comment: To appear in Nature Communications (2020
Observation of sixfold degenerate fermions in PdSb
Three types of fermions have been extensively studied in topological quantum
materials: Dirac, Weyl, and Majorana fermions. Beyond the fundamental fermions
in high energy physics, exotic fermions are allowed in condensed matter systems
residing in three-, six- or eightfold degenerate band crossings. Here, we use
angle-resolved photoemission spectroscopy to directly visualize
three-doubly-degenerate bands in PdSb. The ultrahigh energy resolution we
are able to achieve allows for the confirmation of all the sixfold degenerate
bands at the R point, in remarkable consistency with first-principles
calculations. Moreover, we find that this sixfold degenerate crossing has
quadratic dispersion as predicted by theory. Finally, we compare sixfold
degenerate fermions with previously confirmed fermions to demonstrate the
importance of this work: our study indicates a topological fermion beyond the
constraints of high energy physics