116 research outputs found
The Anomalous Hall Effect
This article reviews the main contributions to the anomalous Hall effect and
its resurgence in the past three decades, which has been accompanied by the
rise of topological phenomena and topological materials. I will show that the
anomalous Hall effect stems from the interplay of magnetism, spin-orbit
coupling, disorder scattering and driving electric fields, it spans the
classical, quantum and relativistic worlds, and has generated controversy for
nearly one and a half centuries. It has become the smoking gun for detecting
magnetic order and, extraordinarily, it continues to reveal new physics, with
several novel varieties reported in the past decade.Comment: Book chapter for Elsevier Encyclopedia of Condensed Matter Physic
External gates and transport in biased bilayer graphene
We formulate a theory of transport in graphene bilayers in the weak momentum
scattering regime in such a way as to take into account contributions to the
electrical conductivity to leading and next-to-leading order in the scattering
potential. The response of bilayers to an electric field cannot be regarded as
a sum of terms due to individual layers. Rather, interlayer tunneling and
coherence between positive- and negative-energy states give the main
contributions to the conductivity. At low energies, the dominant effect of
scattering on transport comes from scattering within each energy band, yet a
simple picture encapsulating the role of collisions in a set of scattering
times is not applicable. Coherence between positive- and negative-energy states
gives, as in monolayers, a term in the conductivity which depends on the order
of limits. The application of an external gate, which introduces a gap between
positive- and negative-energy states, does not affect transport. Nevertheless
the solution to the kinetic equation in the presence of such a gate is very
revealing for transport in both bilayers and monolayers.Comment: 6 pages, accepted for publication in Physical Review
Dominance of extrinsic scattering mechanisms in the orbital Hall effect: graphene, transition metal dichalcogenides and topological antiferromagnets
The theory of the orbital Hall effect (OHE), a transverse flow of orbital
angular momentum in response to an electric field, has concentrated
overwhelmingly on intrinsic mechanisms. Here, using a quantum kinetic
formulation, we determine the full OHE in the presence of short-range disorder
using 2D massive Dirac fermions as a prototype. We find that, in doped systems,
extrinsic effects associated with the Fermi surface (skew scattering and side
jump) provide of the OHE. This suggests that, at experimentally
relevant transport densities, the OHE is primarily extrinsic
Dephasing of Si spin qubits due to charge noise
Spin qubits in Silicon quantum dots can have long coherence times, yet their
manipulation relies on the exchange interaction, through which charge noise can
induce decoherence. Charge traps near the interface of a Si heterostructure
lead to fluctuations in the quantum-dot confinement and barrier potentials,
which cause gating errors and two-spin dephasing. We quantify these effects in
Si double quantum dots using a realistic model of noise. Specifically, we
consider both random telegraph noise from a few traps (good for dots grown on
submicron wafers) and 1/f noise from many traps (good for larger wafers
appropriate for quantum dot arrays). We give estimates of gate errors for
single-spin qubit architectures and dephasing in singlet-triplet qubits
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