70 research outputs found
Berry phase jumps and giant nonreciprocity in Dirac quantum dots
We predict that a strong nonreciprocity in the resonance spectra of Dirac
quantum dots can be induced by the Berry phase. The nonreciprocity arises in
relatively weak magnetic fields and is manifest in anomalously large
field-induced splittings of quantum dot resonances which are degenerate at
due to time-reversal symmetry. This exotic behavior, which is governed by
field-induced jumps in the Berry phase of confined electronic states, is unique
to quantum dots in Dirac materials and is absent in conventional quantum dots.
The effect is strong for gapless Dirac particles and can overwhelm the
-induced orbital and Zeeman splittings. A finite Dirac mass suppresses the
effect. The nonreciprocity, predicted for generic two-dimensional Dirac
materials, is accessible through Faraday and Kerr optical rotation measurements
and scanning tunneling spectroscopy.Comment: 6 pages, 6 figure
Energy-driven Drag at Charge Neutrality in Graphene
Coulomb coupling between proximal layers in graphene heterostructures results
in efficient energy transfer between the layers. We predict that, in the
presence of correlated density inhomogeneities in the layers, vertical energy
transfer has a strong impact on lateral charge transport. In particular, for
Coulomb drag it dominates over the conventional momentum drag near zero doping.
The dependence on doping and temperature, which is different for the two drag
mechanisms, can be used to separate these mechanisms in experiment. We predict
distinct features such as a peak at zero doping and a multiple sign reversal,
which provide diagnostics for this new drag mechanism.Comment: 6 pgs, 3 fg
Topological Bloch Bands in Graphene Superlattices
We outline an approach to endow a plain vanilla material with topological
properties by creating topological bands in stacks of manifestly nontopological
atomically thin materials. The approach is illustrated with a model system
comprised of graphene stacked atop hexagonal-boron-nitride. In this case, the
Berry curvature of the electron Bloch bands is highly sensitive to the stacking
configuration. As a result, electron topology can be controlled by crystal axes
alignment, granting a practical route to designer topological materials. Berry
curvature manifests itself in transport via the valley Hall effect and
long-range chargeless valley currents. The non-local electrical response
mediated by such currents provides diagnostics for band topology
Quantum Noise as an Entanglement Meter
Entanglement entropy, which is a measure of quantum correlations between
separate parts of a many-body system, has emerged recently as a fundamental
quantity in broad areas of theoretical physics, from cosmology and field theory
to condensed matter theory and quantum information. The universal appeal of the
entanglement entropy concept is related, in part, to the fact that it is
defined solely in terms of the many-body density matrix of the system, with no
relation to any particular observables. However, for the same reason, it has
not been clear how to access this quantity experimentally. Here we derive a
universal relation between entanglement entropy and the fluctuations of current
flowing through a quantum point contact (QPC) which opens a way to perform a
direct measurement of entanglement entropy. In particular, by utilizing
space-time duality of 1d systems, we relate electric noise generated by opening
and closing the QPC periodically in time with the seminal S = 1/3 log L
prediction of conformal field theory
Resonant Tunneling and Intrinsic Bistability in Twisted Graphene Structures
We predict that vertical transport in heterostructures formed by twisted
graphene layers can exhibit a unique bistability mechanism. Intrinsically
bistable - characteristics arise from resonant tunneling and interlayer
charge coupling, enabling multiple stable states in the sequential tunneling
regime. We consider a simple trilayer architecture, with the outer layers
acting as the source and drain and the middle layer floating. Under bias, the
middle layer can be either resonant or non-resonant with the source and drain
layers. The bistability is controlled by geometric device parameters easily
tunable in experiments. The nanoscale architecture can enable uniquely fast
switching times.Comment: 7 pages, 4 figure
Shockley-Ramo theorem and long-range photocurrent response in gapless materials
Scanning photocurrent maps of gapless materials, such as graphene, often
exhibit complex patterns of hot spots positioned far from current-collecting
contacts. We develop a general framework that helps to explain the unusual
features of the observed patterns, such as the directional effect and the
global character of photoresponse. We show that such a response is captured by
a simple Shockley-Ramo-type approach. We examine specific examples and show
that the photoresponse patterns can serve as a powerful tool to extract
information about symmetry breaking, inhomogeneity, chirality, and other local
characteristics of the system.Comment: 7 pgs, 3 fg
Energy flows in graphene: hot carrier dynamics and cooling
Long lifetimes of hot carriers can lead to qualitatively new types of responses in materials. The magnitude and time scales for these responses reflect the mechanisms governing energy flows. We examine the microscopics of two processes which are key for energy transport, focusing on the unusual behavior arising due to graphene's unique combination of material properties. One is hot carrier generation in its photoexcitation dynamics, where hot carriers multiply through an Auger type carrier–carrier scattering cascade. The hot-carrier generation manifests itself through elevated electronic temperatures which can be accessed in a variety of ways, in particular optical conductivity measurements. Another process of high interest is electron-lattice cooling. We survey different cooling pathways and discuss the cooling bottleneck arising for the momentum-conserving electron–phonon scattering pathway. We show how this bottleneck can be relieved by higher-order collisions—supercollisions—and examine the variety of supercollision processes that can occur in graphene
Topological Valley Currents in Gapped Dirac Materials
Gapped 2D Dirac materials, in which inversion symmetry is broken by a
gap-opening perturbation, feature a unique valley transport regime. The system
ground state hosts dissipationless persistent valley currents existing even
when topologically protected edge modes are absent or when they are localized
due to edge roughness. Topological valley currents in such materials are
dominated by bulk currents produced by electronic states just beneath the gap
rather than by edge modes. Dissipationless currents induced by an external bias
are characterized by a quantized half-integer valley Hall conductivity. The
under-gap currents dominate magnetization and the charge Hall effect in a
light-induced valley-polarized state.Comment: 5pgs 3fg
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