Antiferromagnetic magnons as highly squeezed Fock states underlying quantum correlations

Abstract

Employing the concept of two-mode squeezed states from quantum optics, we demonstrate a revealing physical picture for the antiferromagnetic ground state and excitations. Superimposed on a Néel ordered configuration, a spin-flip restricted to one of the sublattices is called a sublatticemagnon. We show that an antiferromagnetic spin-up magnon is comprised by a quantum superposition of states with n+1 spin-up and n spin-down sublattice-magnons, and is thus an enormous excitation despite its unit net spin. Consequently, its large sublattice-spin can amplify its coupling to other excitations. Employing von Neumann entropy as a measure, we show that the antiferromagnetic eigenmodes manifest a high degree of entanglement between the two sublattices, thereby establishing antiferromagnets as reservoirs for strong quantum correlations. Based on these insights, we outline strategies for exploiting the strong quantum character of antiferromagetic (squeezed-)magnons and give an intuitive explanation for recent experimental and theoretical findings in antiferromagnetic magnon spintronics. arXiv:1904.04553v3 [cond-mat.mes-hall]

Magnon-mediated superconductivity on the surface of a topological insulator

“…As the phase space is still small, a strong potential is needed in order to produce a nontrivial solution of the gap equation. Taking Ω = 0 and sufficiently small easy-axis anisotropy, fully exploiting the boosting effect [22], the potential is found to be strong enough to provide a solution. As expected, the solution has a p-wave character, as displayed in Fig.…”

“…Such an asymmetric coupling has been predicted to significantly enhance the critical temperature for magnon mediated superconductivity in a NM/AFMI heterostructure [16]. This enhancement can be understood from the picture of antiferromagnetic magnons as squeezed states, revealing that an antiferromagnetic magnon is associated with a large spin located at each sublattice [22]. Coupling to only one of the two sublattices of an AFMI thereby involves coupling to a large spin, leading to a strong enhancement of the coupling interaction [16,23].…”

Ellipticity and dissipation effects in magnon spin valves

“…Semiclassically, a Bogoliubov quasiparticle created by β † X,k corresponds to an elliptical spin wave; in contrast, a magnon created by b † X,k corresponds to a circular spin wave. The Bogoliubov quasiparticles are often also referred to as magnons, or as elliptical or squeezed magnons 19,20 . For an elliptical spin wave, the z-component of the spin is not conserved and hence not a good quantum number, unlike for a circular spin wave.…”

“…in the antiparallel configuration. Note that their contributions to currents (19) and (20) vanishes if both leads are at the same temperature and chemical potential. On the other hand,D P/AP E/S ( ) are anomalous tunneling densities of state that arise because the Gilbert damping breaks the number conservation of the Bogoliubov quasiparticles.…”

“…The ellipticity of precession has been shown to strongly affect the parametric excitation of magnons in ferromagnetic resonance experiments 17 , and plays a role in Rayleigh-Jeans condensation of pumped magnons in thin-film ferromagnets 18 . Moreover, at the quantummechanical level, the ellipticity leads to squeezed ground states and, in the case of antiferromagnets, entanglement between different sublattice magnons 19,20 . Meanwhile, a low Gilbert damping has also been demonstrated to enable long-distance spin transport in magnetic insulators such as yttrium iron garnet 4 .…”