21 research outputs found
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 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
Discovery of unconventional chiral charge order in kagome superconductor KV3Sb5
Intertwining quantum order and nontrivial topology is at the frontier of
condensed matter physics. A charge density wave (CDW) like order with orbital
currents has been proposed as a powerful resource for achieving the quantum
anomalous Hall effect in topological materials and for the hidden phase in
cuprate high-temperature superconductors. However, the experimental realization
of such an order is challenging. Here we use high-resolution scanning
tunnelling microscopy (STM) to discover an unconventional charge order in a
kagome material KV3Sb5, with both a topological band structure and a
superconducting ground state. Through both topography and spectroscopic
imaging, we observe a robust 2x2 superlattice. Spectroscopically, an energy gap
opens at the Fermi level, across which the 2x2 charge modulation exhibits an
intensity reversal in real-space, signaling charge ordering. At
impurity-pinning free region, the strength of intrinsic charge modulations
further exhibits chiral anisotropy with unusual magnetic field response.
Theoretical analysis of our experiments suggests a tantalizing unconventional
chiral CDW in the frustrated kagome lattice, which can not only lead to large
anomalous Hall effect with orbital magnetism, but also be a precursor of
unconventional superconductivity.Comment: Orbital magnetism calculation adde
Negative flat band magnetism in a spin–orbit-coupled correlated kagome magnet
Electronic systems with flat bands are predicted to be a fertile ground for hosting emergent phenomena including unconventional magnetism and superconductivity1,2,3,4,5,6,7,8,9,10,11,12,13,14,15, but materials that manifest this feature are rare. Here, we use scanning tunnelling microscopy to elucidate the atomically resolved electronic states and their magnetic response in the kagome magnet Co3Sn2S2 (refs. 16,17,18,19,20). We observe a pronounced peak at the Fermi level, which we identify as arising from the kinetically frustrated kagome flat band. On increasing the magnetic field up to ±8 T, this state exhibits an anomalous magnetization-polarized many-body Zeeman shift, dominated by an orbital moment that is opposite to the field direction. Such negative magnetism is induced by spin–orbit-coupling quantum phase effects21,22,23,24,25 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 a kagome magnet where the low-energy physics can be dominated by the spin–orbit-coupled flat band
Signatures of Weyl Fermion Annihilation in a Correlated Kagome Magnet.
The manipulation of topological states in quantum matter is an essential pursuit of fundamental physics and next-generation quantum technology. Here we report the magnetic manipulation of Weyl fermions in the kagome spin-orbit semimetal Co_{3}Sn_{2}S_{2}, observed by high-resolution photoemission spectroscopy. We demonstrate the exchange collapse of spin-orbit-gapped ferromagnetic Weyl loops into paramagnetic Dirac loops under suppression of the magnetic order. We further observe that topological Fermi arcs disappear in the paramagnetic phase, suggesting the annihilation of exchange-split Weyl points. Our findings indicate that magnetic exchange collapse naturally drives Weyl fermion annihilation, opening new opportunities for engineering topology under correlated order parameters