166 research outputs found
Chiral topological insulator of magnons
We propose a magnon realization of 3D topological insulator in the AIII
(chiral symmetry) topological class. The topological magnon gap opens due to
the presence of Dzyaloshinskii-Moriya interactions. The existence of the
topological invariant is established by calculating the bulk winding number of
the system. Within our model, the surface magnon Dirac cone is protected by the
sublattice chiral symmetry. By analyzing the magnon surface modes, we confirm
that the backscattering is prohibited. By weakly breaking the chiral symmetry,
we observe the magnon Hall response on the surface due to opening of the gap.
Finally, we show that by changing certain parameters the system can be tuned
between the chiral topological insulator (mcTI), three dimensional magnon
anomalous Hall (3D-mAH), and Weyl magnon phases.Comment: 6 page
Spin Hall and Nernst effects of Weyl magnons
In this paper, we present a simple model of a three-dimensional insulating
magnetic structure which represents a magnonic analog of the layered electronic
system described in [Phys. Rev. Lett. {\bf 107}, 127205 (2011)]. In particular,
our model realizes Weyl magnons as well as surface states with a Dirac
spectrum. In this model, the Dzyaloshinskii-Moriya interaction is responsible
for the separation of opposite Weyl points in momentum space. We calculate the
intrinsic (due to the Berry curvature) transport properties of Weyl and
so-called anomalous Hall effect (AHE) magnons. The results are compared with
fermionic analogs.Comment: 9 pages, 5 figures, published versio
Theory of magnon motive force in chiral ferromagnets
We predict that magnon motive force can lead to temperature dependent,
nonlinear chiral damping in both conducting and insulating ferromagnets. We
estimate that this damping can significantly influence the motion of skyrmions
and domain walls at finite temperatures. We also find that in systems with low
Gilbert damping moving chiral magnetic textures and resulting magnon motive
forces can induce large spin and energy currents in the transverse direction
Magnon spin Nernst effect in antiferromagnets
We predict that a temperature gradient can induce a magnon-mediated spin Hall
response in an antiferromagnet with non-trivial magnon Berry curvature. We
develop a linear response theory which gives a general condition for a Hall
current to be well defined, even when the thermal Hall response is forbidden by
symmetry. We apply our theory to a honeycomb lattice antiferromagnet and
discuss a role of magnon edge states in a finite geometry.Comment: 17 page
Spin glass reflection of the decoding transition for quantum error correcting codes
We study the decoding transition for quantum error correcting codes with the
help of a mapping to random-bond Wegner spin models.
Families of quantum low density parity-check (LDPC) codes with a finite
decoding threshold lead to both known models (e.g., random bond Ising and
random plaquette gauge models) as well as unexplored earlier generally
non-local disordered spin models with non-trivial phase diagrams. The decoding
transition corresponds to a transition from the ordered phase by proliferation
of extended defects which generalize the notion of domain walls to non-local
spin models. In recently discovered quantum LDPC code families with finite
rates the number of distinct classes of such extended defects is exponentially
large, corresponding to extensive ground state entropy of these codes.
Here, the transition can be driven by the entropy of the extended defects, a
mechanism distinct from that in the local spin models where the number of
defect types (domain walls) is always finite.Comment: 15 pages, 2 figure
Fault-Tolerance of "Bad" Quantum Low-Density Parity Check Codes
We discuss error-correction properties for families of quantum low-density
parity check (LDPC) codes with relative distance that tends to zero in the
limit of large blocklength. In particular, we show that any family of LDPC
codes, quantum or classical, where distance scales as a positive power of the
block length, , , can correct all errors with
certainty if the error rate per (qu)bit is sufficiently small. We specifically
analyze the case of LDPC version of the quantum hypergraph-product codes
recently suggested by Tillich and Z\'emor. These codes are a finite-rate
generalization of the toric codes, and, for sufficiently large quantum
computers, offer an advantage over the toric codes.Comment: 4.5 pages, 1 figur
Hybrid skew scattering regime of the anomalous Hall effect in Rashba systems: unifying Keldysh, Boltzmann, and Kubo formalisms
We present the analytical description of the anomalous Hall effect (AHE) in a
2DEG ferromagnet within the Keldysh formalism. These results unify all three
linear response approaches to anomalous Hall transport and close a long
standing debate. We are able to identify a new extrinsic AHE regime dominated
by a hybrid skew scattering mechanism. This new contribution is inversely
proportional to the impurity concentration, resembling the normal skew
scattering, {\em but} independent of the impurity-strength, resembling the
side-jump mechanism. Within the Kubo formalism this regime is captured by
higher order diagrams which, although weak, dominate when both subbands are
occupied; this regime can be detected by variable remote doping experiments
that we describe.Comment: 5 pages, 2 figure
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