Modelling spin waves in noncollinear antiferromagnets: spin-flop states, spin spirals, skyrmions and antiskyrmions

Spin waves in antiferromagnetic materials have great potential for next-generation magnonic technologies. However, their properties and their dependence on the type of ground-state antiferromagnetic structure are still open questions. Here, we investigate theoretically spin waves in one- and two-dimensional model systems with a focus on noncollinear antiferromagnetic textures such as spin spirals and skyrmions of opposite topological charges. We address in particular the nonreciprocal spin excitations recently measured in bulk antiferromagnet $\alpha$--$\text{Cu}_2\text{V}_2\text{O}_7$ utilizing inelastic neutron scattering experiments [Phys.\ Rev.\ Lett.\ \textbf{119}, 047201 (2017)], where we help to characterize the nature of the detected spin-wave modes. Furthermore, we discuss how the Dzyaloshinskii-Moriya interaction can lift the degeneracy of the spin-wave modes in antiferromagnets, resembling the electronic Rashba splitting. We consider the spin-wave excitations in antiferromagnetic spin-spiral and skyrmion systems and discuss the features of their inelastic scattering spectra. We demonstrate that antiskyrmions can be obtained with an isotropic Dzyaloshinskii-Moriya interaction in certain antiferromagnets.Comment: 26 pages, 9 figure

First-principles investigation of spin wave dispersions in surface-reconstructed Co thin films on W(110)

We computed spin wave dispersions of surface-reconstructed Co films on the W(110) surface in the adiabatic approximation. The magnetic exchange interactions are obtained via first-principles electronic structure calculations using the Korringa-Kohn-Rostoker Green function method. We analyze the strength and oscillatory behavior of the intralayer and interlayer magnetic interactions and investigate the resulting spin wave dispersions as a function of the thickness of Co films. In particular, we highlight and explain the strong impact of hybridization of the electronic states at the Co-W interface on the magnetic exchange interactions and on the spin wave dispersions. We compare our results to recent measurements based on electron energy loss spectroscopy [E. Michel, H. Ibach, and C.M. Schneider, Phys. Rev. B 92, 024407 (2015)]. Good overall agreement with experimental findings can be obtained by considering the possible overestimation of the spin splitting, stemming from the local spin density approximation, and adopting an appropriate correction.Comment: 14 pages, 14 figure

Characteristic functions on the boundary of a planar domain need not be traces of least gradient functions

Given a smooth bounded planar domain, we construct a compact set on the boundary s.t. its characteristic function is not the trace of a least gradient function. This generalize the construction of Spradlin and Tamasan [ST14] on the disc

Episodic memory, the cotemporality problem, and common sense

Direct realists about episodic memory claim that a rememberer has direct contact with a past event. But how is it possible to be acquainted with an event that ceased to exist? That’s the so-called cotemporality problem. The standard solution, proposed by Sven Bernecker, is to distinguish between the occurrence of an event and the existence of an event: an event ceases to occur without ceasing to exist. That’s the eternalist solution for the cotemporality problem. Nevertheless, some philosophers of memory claim that the adoption of an eternalist metaphysics of time would be too high a metaphysical price to be paid to hold direct realist intuitions about memory. Although I agree with these critics, I will try to show two things. First, that this kind of “common sense argument” is far from decisive. Second, that Bernecker’s proposal remains the best solution to the cotemporality problem