106 research outputs found
When chiral photons meet chiral fermions - Photoinduced anomalous Hall effects in Weyl semimetals
The Weyl semimetal is characterized by three-dimensional linear band touching
points called Weyl nodes. These nodes come in pairs with opposite chiralities.
We show that the coupling of circularly polarized photons with these chiral
electrons generates a Hall conductivity without any applied magnetic field in
the plane orthogonal to the light propagation. This phenomenon comes about
because with all three Pauli matrices exhausted to form the three-dimensional
linear dispersion, the Weyl nodes cannot be gapped. Rather, the net influence
of chiral photons is to shift the positions of the Weyl nodes. Interestingly,
the momentum shift is tightly correlated with the chirality of the node to
produce a net anomalous Hall signal. Application of our proposal to the
recently discovered TaAs family of Weyl semimetals leads to an
order-of-magnitude estimate of the photoinduced Hall conductivity which is
within the experimentally accessible range.Comment: 9 pages, 4 figure
Absence of Magnetic Fluctuations in the Ferromagnetic/Topological Heterostructure EuS/BiSe
Heterostructures of topological insulators and ferromagnets offer new
opportunities in spintronics and a route to novel anomalous Hall states. In one
such structure, EuS/BiSe a dramatic enhancement of the Curie
temperature was recently observed. We performed Raman spectroscopy on a similar
set of thin films to investigate the magnetic and lattice excitations.
Interfacial strain was monitored through its effects on the BiSe
phonon modes while the magnetic system was probed through the EuS Raman mode.
Despite its appearance in bare EuS, the heterostructures lack the corresponding
EuS Raman signal. Through numerical calculations we rule out the possibility of
Fabry-Perot interference suppressing the mode. We attribute the absence of a
magnetic signal in EuS to a large charge transfer with the BiSe.
This could provide an additional pathway for manipulating the magnetic,
optical, or electronic response of topological heterostructures.Comment: 6 pages, 3 figure
Topology and geometry under the nonlinear electromagnetic spotlight
For many materials, a precise knowledge of their dispersion spectra is
insufficient to predict their ordered phases and physical responses. Instead,
these materials are classified by the geometrical and topological properties of
their wavefunctions. A key challenge is to identify and implement experiments
that probe or control these quantum properties. In this review, we describe
recent progress in this direction, focusing on nonlinear electromagnetic
responses that arise directly from quantum geometry and topology. We give an
overview of the field by discussing new theoretical ideas, groundbreaking
experiments, and the novel materials that drive them. We conclude by discussing
how these techniques can be combined with new device architectures to uncover,
probe, and ultimately control novel quantum phases with emergent topological
and correlated properties.Comment: Nature Materials (In Press
Coulomb blockade in an atomically thin quantum dot coupled to a tunable Fermi reservoir
Gate-tunable quantum-mechanical tunnelling of particles between a quantum
confined state and a nearby Fermi reservoir of delocalized states has
underpinned many advances in spintronics and solid-state quantum optics. The
prototypical example is a semiconductor quantum dot separated from a gated
contact by a tunnel barrier. This enables Coulomb blockade, the phenomenon
whereby electrons or holes can be loaded one-by-one into a quantum dot.
Depending on the tunnel-coupling strength, this capability facilitates single
spin quantum bits or coherent many-body interactions between the confined spin
and the Fermi reservoir. Van der Waals (vdW) heterostructures, in which a wide
range of unique atomic layers can easily be combined, offer novel prospects to
engineer coherent quantum confined spins, tunnel barriers down to the atomic
limit or a Fermi reservoir beyond the conventional flat density of states.
However, gate-control of vdW nanostructures at the single particle level is
needed to unlock their potential. Here we report Coulomb blockade in a vdW
heterostructure consisting of a transition metal dichalcogenide quantum dot
coupled to a graphene contact through an atomically thin hexagonal boron
nitride (hBN) tunnel barrier. Thanks to a tunable Fermi reservoir, we can
deterministically load either a single electron or a single hole into the
quantum dot. We observe hybrid excitons, composed of localized quantum dot
states and delocalized continuum states, arising from ultra-strong
spin-conserving tunnel coupling through the atomically thin tunnel barrier.
Probing the charged excitons in applied magnetic fields, we observe large
gyromagnetic ratios (~8). Our results establish a foundation for engineering
next-generation devices to investigate either novel regimes of Kondo physics or
isolated quantum bits in a vdW heterostructure platform.Comment: Published in Nature Nanotechnology. 7 pages + 14 supplementary
information pages. 14 figure
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