Khalil Zakeri

Beyond their fundamental interest in exploring bosonic Dirac physics, Dirac magnons hold potential for the development of efficient spin-transport devices. Here, we demonstrate the experimental realization of gapped terahertz Dirac magnons in an epitaxial Co bilayer on Ru(0001) using spin-polarized electron spectroscopy. Supported by first-principles calculations, we show that although the substrate breaks sublattice symmetry, thereby opening a gap at the K ¯ points, the local valley topology is preserved. This is evidenced by a spectral weight redistribution, valley-centered curvature concentrations, and a nonzero local Berry phase. Our results establish the interface as a tuning parameter, paving the way for engineering Dirac magnons in a variety of simple ultrathin bilayer magnets.

A distinct difference between graphene-an atomic layer of carbon-and conventional semiconductors is that its electrons behave as massless Dirac fermions, giving rise to unprecedented physical properties. Magnetically ordered solids host magnons, quasiparticles associated with magnetic degrees of freedom. While Dirac magnons have recently been predicted in specific insulating or rare-earth magnets, their existence in thin 3d magnets remains elusive because of the complex nature of itinerant magnetism and dimensionality effects. Here, we demonstrate the presence of Dirac magnons in a thin itinerant elemental ferromagnet. By investigating atomically designed hexagonal close-packed cobalt films, we establish that magnons in such structures resemble the Dirac electrons in graphene. We explain the physical nature of these Dirac magnons and discuss the consequences of symmetry, dimensionality, magnetic interactions, the number of atomic layers, and cobalt's itinerant magnetism on the properties of the Dirac points. Our results pave the way for finding and engineering Dirac magnons in a variety of low-dimensional layered 3d ferromagnets and metamaterials.

Harnessing symmetry-protected terahertz Dirac magnons would enable robust, low-loss magnonic transport, offering a compelling alternative to conventional information carriers. Here we propose a novel yet remarkably simple platform for hosting terahertz Dirac magnons in low-dimensional magnetic architectures. We demonstrate, through first-principles calculations and linear spin-wave theory, that atomically designed magnetic layers possessing C_3v symmetry can exhibit Dirac magnon band crossings and a quantized Berry phase of ±π. The formation of the Dirac point, absent in their bulk counterpart, is a direct consequence of the reduced dimensionality. Our finding not only highlights the unique dimensionality-driven Dirac magnons in layered structures but also sheds light on the complex behavior of the Berry phase in such structures. We show that parameters such as surface symmetry, atomic layer composition, film thickness, epitaxial relationship, and chemical environment can be used to tune the Berry phase and the resulting symmetry-associated properties.

Chirality is one of the inherent characteristics of some objects in nature. In magnetism, chiral magnetic textures can be formed in systems with broken inversion symmetry and due to an antisymmetric magnetic interaction, known as Dzyaloshinskii–Moriya interaction (DMI). Here, aiming for a fundamental understanding of this chiral interaction on the atomic scale, several synthetic layered structures composed of alternating atomic layers of 3 d ferromagnetic metals epitaxially grown on the Ir(001) surface are designed. It is demonstrated both experimentally and theoretically that the atomistic DMI depends critically not only on the orbital occupancy of the interface magnetic layer but also on the sequence of the atomic layers. It is shown that even large atomistic DMI values can result in a small effective DMI, and conversely. Furthermore, the dependence of the effective DMI on the number of atomic layers deviates from a simple scaling law. These observations are attributed to the complexity of the electronic structure and the contributions of different orbitals to the hybridization and DMI. The results are anticipated to provide guidelines for achieving full control over both the chirality and the magnitude of the atomistic DMI in layered materials.

Utilizing ultrafast terahertz (THz) magnons, the quanta of collective magnetic excitations, as carriers may provide a promising alternative to overcome the problems associated with electrical losses in nanoelectronic devices and circuits. However, efficient excitation of propagating coherent THz magnons in magnonic nanowaveguides is an essential requirement for the development of such devices. Here, by growing ultrathin ferromagnetic nanostructures on a reconstructed surface, we create well-ordered periodic magnetic nanostripes. We demonstrate that such atomically architectured nanowaveguides not only provide a versatile platform for an efficient generation of THz magnons but also allow for their fast propagation. Our results reveal the complex nature of the spin dynamics within such designed nanowaveguides and pave the way for designing ultrafast magnon-based logic devices with THz operation frequencies.

Understanding the impact of electronic correlations and disorder is essential for an accurate description of solids. Here, we study the role of correlations, disorder, and multimagnon processes in THz spin dynamics. We reveal that a significant part of the electron self-energy, which goes beyond the adiabatic local spin density approximation, arises from the interaction between electrons and a virtual magnon gas. This interaction leads to a substantial modification of the exchange splitting and a renormalization of magnon energies, in agreement with the experimental data. Finally, we establish a quantitative hierarchy of magnon relaxation processes based on first principles.

The experimental investigations of the magnetic interactions in an atomically thin magnetic layer are essential to understand the physics of low-dimensional magnets. The dispersion relation of collective magnetic excitations (magnons) would provide an access to these fundamental interactions on the atomic scale. Here in order to be able to excite the magnons by means of spin-polarized electrons we couple a Ni monolayer to one and two atomic layers of Co and probe the magnon dispersion relation up to the Brillouin zone boundary. Comparing to the results of ab initio calculations we quantify the complex pattern of the magnetic exchange interaction in the Ni monolayer. We show that, although the magnons in this system are rather stiff, the Heisenberg exchange coupling between the Ni spins is weak. We unravel the origin of the observed large magnon stiffness constant, being a consequence of the small spin density of the Ni atoms.

The propagation characteristics of fermionic and bosonic quasiparticles determine the fundamental transport properties of solids and are of great technological relevance for designing logic devices. In particular, nonreciprocity, which describes that a quasiparticle flows preferably along a certain direction of a symmetry path, is an essential requirement to realize logic architectures, e.g., switches, diodes, transistors, etc. Here we introduce a mechanism, which leads to giant nonreciprocity of ultrafast terahertz magnons in ferromagnetic films with a large spin-orbit coupling. The mechanism is based on the competition between the exchange and spin-orbit scattering. We anticipate that the effect can be used to excite nonreciprocal or even unidirectional magnons in a large class of ultrathin films and nanostructures grown on substrates with a large spin-orbit coupling.