arXiv subjects
Publications and source records attributed to Antoine Khater.
A rebuttal of a recent article by Hirsch J.E. is presented below. To begin with, the main assumption, regarding the Meissner effect, which the whole argument relies upon, is disproved. Besides, the subsequent analysis misconstrues an original view of the Meissner effect by other authors [2]. Finally, the discussion of the Joule effect turns out to violate the first law of thermodynamics.
Antiferromagnetic two-dimensional (2D) materials are currently under intensive theoretical and experimental investigations in view of their potential applications in antiferromagnet-based magnonic and spintronic devices. Recent experimental studies revealed the importance of magnetic anisotropy and Dzyaloshinskii-Moriya interactions (DMI) on the ordered ground state and the magnetic excitations in these materials. Here we present a robust classical field theory approach to analyze the effect of magnetic anisotropy and Dzyaloshinskii-Moriya interactions (DMI) on the edge and bulk spin waves in 2D antiferromagnetic nanoribbons. We predict the existence of a new class of nonreciprocal edge spin waves characterized by opposite polarizations in opposite directions. These novel edge spin waves are induced by the DMI and are fundamentally different from conventional nonreciprocal spin waves for which the polarization is independent of the propagation direction. Aside this breakthrough in the field of antiferromagnetic spin waves, the study further analysis the effect of the edges structure on the magnetic excitations. In particular, we show that anisotropic bearded edges nanoribbons act as magnetic topological insulators with exceptionally interesting potentials for applications in magnonics.
We report on the theoretical prediction of asymmetric edge spin waves, propagating in opposite directions on the edges of honeycomb nanoribbons with zigzag and bearded edges boundaries. The simultaneous propagation of edge spin waves in the same direction on both edges of these nanoribbons is hence predicted to be forbidden. These asymmetric edge exchange spin waves are analogous to the nonreciprocal surface spin waves reported in magnetic thin films. Their existence is related to the nontrivial symmetry underlying the nanoribbons under study. The edge and discretized bulk exchange spin waves are calculated in the long wavelength part of the Brillouin zone using the classical spin wave theory approach and appropriate boundary conditions. In the absence of an external magnetic field in our study, the asymmetric edge spin waves propagate with equal frequencies and opposite directions, since the energy dispersion relation is independent of the sign of the wavevector components in the long wavelength part of the Brillouin zone. The edge spin waves are characterized by linear dispersion relations for magnetically isotropic nanoribbons. Introducing magnetic anisotropy in the calculation significantly enhances the energy gap between the edge and bulk spin waves in both types of nanoribbons. Based on our calculation, the large energy gap allows separate excitation of bulk and edge modes as their energies are no more overlapping.
We develop a field theory to study the dynamics of long wavelength exchange spin wave excitations on honeycomb nanoribbons characterized by armchair edge boundaries and the Néel antiferromagnetic ordering state. Appropriate boundary conditions are established by requiring that the bulk and edge spins precess with the same frequency for any given spin wave eigenmode in these systems. A set of characteristic boundary equations, common for bulk and edge spin wave modes, are hence derived. The equations of motion for the spin dynamics are then solved to determine the propagating and evanescent exchange spin wave modes. We prove in general that the bulk spin wave dynamics is discretized due to the finite width of the nanoribbon. For an isotropic magnetic nanoribbon, the Dirac cone is reduced to a single linear dispersion curve due to this discretization. The number and wavelengths of allowed bulk modes for isotropic and anisotropic nanoribbons are determined from the derived characteristic boundary equations. As witnessed by our numerical results for different examples it is shown that the characteristics of these modes depend on the width of the nanoribbon and its antiferromagnetic anisotropy. Further, anisotropic nanoribbons, even those with the slightest anisotropy, present evanescent modes with non-linear dispersion relations. The spatial variation of the amplitudes of the evanescent exchange spin waves across the finite widths of the nanoribbons, is found to be strongly dependent on the system magnetic anisotropy and its width. The developed theoretical approach is general and can be applied for nanoribbons with all types of boundary edges.
The Meissner effect is studied by using an approach based on Newton and Maxwell's equations. The objective is to assess the relevance of London's equation and shed light on the connection between the Meissner and skin effects. The properties of a superconducting cylinder, cooled in a magnetic field, are accounted for within the same framework. The radial Hall effect is predicted. The energy, associated with the Meissner effect, is calculated and compared with the binding energy of the superconducting phase with respect to the normal one.
The skin effect is analyzed to provide the numerous measurements of the penetration depth of the electromagnetic field in superconducting materials with a theoretical basis. Both the normal and anomalous skin effects are accounted for within a single framework. The emphasis is laid on the conditions required for the penetration depth to be equal to London's length, which enables us to validate an assumption widely used in the interpretation of all current experimental results.
The Meissner effect is analysed by using an approach based on Newton and Maxwell's equations, in order to assess the relevance of London's equation. The Hall effect is predicted. Two test experiments are proposed in detail to check the validity of this theory and to measure London's length.