Vacuum entanglement governs the bosonic character of magnons

It is well known that magnons, elementary excitations in a magnetic material, behave as bosons when their density is low. We study how the bosonic character of magnons is governed by the amount of a multipartite entanglement in the vacuum state on which magnons are excited. We show that if the multipartite entanglement is strong, magnons cease to be bosons. We also consider some examples, such as ground states of the Heisenberg ferromagnet and the transverse Ising model, the condensation of magnons, the one-way quantum computer, and Kitaev's toric code. Our result provides insights into the quantum statistics of elementary excitations in these models, and into the reason why a non-local transformation, such as the Jordan-Wigner transformation, is necessary for some many-body systems.Comment: 4 pages, no figur

Spin pumping by parametrically excited exchange magnons

We experimentally show that exchange magnons can be detected using a combination of spin pumping and inverse spin-Hall effect (iSHE) proving its wavelength integrating capability down to the sub-micrometer scale. The magnons were injected in a ferrimagnetic yttrium iron garnet film by parametric pumping and the iSHE-induced voltage was detected in an attached Pt layer. The role of the density, wavelength, and spatial localization of the magnons for the spin pumping efficiency is revealed. This study opens the field of the magnon-based information processing to magnons with nano-scale wavelengths

The Force Between Giant Magnons

We compute the force and torque between well-separated, slowly-moving Giant Magnons with arbitrary orientations on S^5. We propose an effective Hamiltonian for Giant Magnons in this regime

Ballistic magnon transport and phonon scattering in the antiferromagnet Nd2_2CuO4_4

The thermal conductivity of the antiferromagnet Nd$_2$CuO$_4$ was measured down to 50 mK. Using the spin-flop transition to switch on and off the acoustic Nd magnons, we can reliably separate the magnon and phonon contributions to heat transport. We find that magnons travel ballistically below 0.5 K, with a thermal conductivity growing as $T^3$, from which we extract their velocity. We show that the rate of scattering of acoustic magnons by phonons grows as $T^3$, and the scattering of phonons by magnons peaks at twice the average Nd magnon frequency.Comment: 4 pages, 3 figures, one figure modifie

Topological thermal Hall effect due to Weyl magnons

We present the first theoretical evidence of zero magnetic field topological (anomalous) thermal Hall effect due to Weyl magnons. Here, we consider Weyl magnons in stacked noncoplanar frustrated kagom\'e antiferromagnets recently proposed by Owerre, [arXiv:1708.04240]. The Weyl magnons in this system result from macroscopically broken time-reversal symmetry by the scalar spin chirality of noncoplanar chiral spin textures. Most importantly, they come from the lowest excitation, therefore they can be easily observed experimentally at low temperatures due to the population effect. Similar to electronic Weyl nodes close to the Fermi energy, Weyl magnon nodes in the lowest excitation are the most important. Indeed, we show that the topological (anomalous) thermal Hall effect in this system arises from nonvanishing Berry curvature due to Weyl magnon nodes in the lowest excitation, and it depends on their distribution (distance) in momentum space. The present result paves the way to directly probe low excitation Weyl magnons and macroscopically broken time-reversal symmetry in three-dimensional frustrated magnets with the anomalous thermal Hall effect.Comment: 9 pages, 4 figures. Revised version. The first proposal of Weyl magnons in this system can be found here arXiv:1708.04240. The present paper proposes the first transport properties induced by Weyl magnons at the lowest excitatio