Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: When electrons moving in two-dimensions (2D) are subjected to a strong uniform magnetic field, they form flat bands called Landau levels, which are the basis for the quantum Hall effect. Landau levels can also arise from pseudomagnetic fields (PMFs) induced by lattice distortions; for example, mechanically straining graphene causes its Dirac quasiparticles to form a characteristic set of unequally-spaced Landau levels, including a zeroth Landau level. In three-dimensional (3D) systems, there has thus far been no experimental demonstration of Landau levels or any other type of flat band. For instance, applying a uniform magnetic field to materials hosting Weyl quasiparticles, the 3D generalizations of Dirac quasiparticles, yields bands that are non-flat in the direction of the field. Here, we report on the experimental realization of a flat 3D Landau level in an acoustic crystal. Starting from a lattice whose bandstructure exhibits a nodal ring, we design an inhomogeneous distortion corresponding to a specific pseudomagnetic vector potential (PVP) that causes the nodal ring states to break up into Landau levels, with a zeroth Landau level that is flat along all three directions. These findings point to the possibility of using nodal ring materials to generate 3D flat bands, to access strong interactions and other interesting physical regimes in 3D.

Abstract: As one of the fundamental magnonic devices, a magnonic splitter device has been proposed and spin wave propagation in this device has been studied numerically and experimentally. In the present work, we fabricated a T-shaped magnonic splitter with 6 $\mu$m-wide three arms using a 100 nm-thick yttrium iron garnet film and, using time-resolved magneto-optic Kerr microscopy, observed that spin waves split into both, the vertical and the horizontal direction at the junction. Analyzing the results, we found that spin wave modes are converted into another during the splitting process and the splitting efficiency is dominantly dependent on the 1st order of incoming spin waves.

Abstract: Since the discovery of the fascinating properties in magic-angle graphene, the exploration of moir\'e systems in other two-dimensional materials has garnered significant attention and given rise to a field known as 'moir\'e physics'. Within this realm, magnetic van der Waals heterostructure and the magnetic proximity effect in moir\'e superlattices have also become subjects of great interest. However, the spin-polarized transport property in this moir\'e structures is still a problem to be explored. Here, we investigate the spin-polarized transport properties in a moir\'e superlattices formed by a two-dimensional ferromagnet CrI_3 stacked on a monolayer BAs, where the spin degeneracy is lifted because of the magnetic proximity effect associated with the moir\'e superlattices. We find that the conductance exhibits spin-resolved miniband transport properties at a small twist angle because of the periodic moir\'e superlattices. When the incident energy is in the spin-resolved minigaps, the available states are spin polarized, thus providing a spin-polarized current from the superlattice. Moreover, only a finite number of moir\'e period is required to obtain a net spin polarization of 100\%. In addition, the interlayer distance of the heterojunction is also moir\'e modifiable, so a perpendicular electric field can be applied to modulate the intensity and direction of the spin polarization. Our finding points to an opportunity to realize spin functionalities in magnetic moir\'e superlattices.

Abstract: We investigate magnon magnon hybrid states using a non Hermitian two band Hamiltonian and the concept of wavefunction hybridization. By comparing our model with micromagnetic simulations conducted on a synthetic antiferromagnet with strong magnon magnon coupling, we successfully reproduce not only the resonance frequencies and linewidths but also the phases and amplitudes of the magnon wavefunction. The hybridization effect influences the dissipation rate, leading to the crossing of linewidths. Additionally, we quantify the magnon hybridization within a magnonic Bloch sphere, which enhances the ability to manipulate hybrid magnons for coherent information processing.

Abstract: Magnetic nanoparticles (MNPs) have many applications which require MNPs to be captured and immobilized for their manipulation and sensing. For example, MNP sensors based on detecting changes to the ferromagnetic resonances of an antidot nanostructure exhibit better performance when the nanoparticles are captured within the antidot inclusions. This study investigates the influence of microfluidics upon the capture of MNPs by four geometries of antidot array nanostructures hollowed into 30 nm-thick Permalloy films. The nanostructures were exposed to a dispersion of 130 nm MNP clusters which passed through PDMS microfluidic channels with a 400 {\mu}m circular cross-section fabricated from wire molds. With the microfluidic flow of MNPs, the capture efficiency - the ratio between the number of nanoparticles captured inside of the antidot inclusions to the number outside the inclusions - decreased for all four geometries compared to previous results introducing the particles via droplets on the film surface. This indicates that most MNPs were passing over the nanostructures, since there were no significant magnetophoretic forces acting upon the particles. However, when a static magnetic field is applied, the magnetophoretic forces generated by the nanostructure are stronger and the capture efficiencies are significantly higher than those obtained using droplets. In particular, circular antidots demonstrated the highest capture efficiency among the four geometries of almost 83.1% when the magnetic field is parallel to the film plane. In a magnetic field perpendicular to the film, the circle antidots again show the highest capture efficiency of about 77%. These results suggest that the proportion of nanoparticles captured inside antidot inclusions is highest under a parallel magnetic field. Clearly, the geometry of the nanostructure has a strong influence on the capture of MNPs.

Abstract: The one-dimensional (1D) Su-Schrieffer-Heeger (SSH) model is central to band topology in condensed matter physics, which allows us to understand and design topological states. The Su-Schrieffer-Heeger (SSH) model serves as a basis for topological insulators and provides insights into various topological states. In this letter, we find another mechanism to analogize the SSHmodel by introducing intrinsic spin-orbital coupling (SOC) and in-plane Zeeman field instead of relying on alternating hopping integrals. In our model, the bound states are protected by a quantizedhidden polarization andcharacterized by a weak Z2 index (0;01) due to the inversion symmetry I. When the I symmetry is broken, charge pumping is achieved by tuning the polarization. Moreover, by introducing the p + ip superconductor pairing potential, a new topological phase called weak topological superconductor (TSC) is identified. The new TSC is characterized by a weak Z2 index (0;01) and nonchiral bound states. More interestingly, these nonchiral bound states give rise to a chiral nonlocal conductance, which is different from the traditional chiral TSC. Our findings not only innovate the SSH model, but also predict the existence of weak TSC, providing an alternative avenue for further exploration of its transport properties.

Abstract: Twisted bilayers of transition metal dichalcogenides (TMDs) have revealed a rich exciton landscape including hybrid excitons and spatially trapped moir\'e excitons that dominate the optical response of the material. Recent studies have revealed that in the low-twist-angle regime, the lattice undergoes a significant relaxation in order to minimize local stacking energies. Here, large domains of low energy stacking configurations emerge, deforming the crystal lattices via strain and consequently impacting the electronic band structure. However, so far the direct impact of atomic reconstruction on the exciton energy landscape and the optical properties has not been well understood. Here, we apply a microscopic and material-specific approach and predict a significant change in the potential depth for moir\'e excitons in a reconstructed lattice, with the most drastic change occurring in TMD homobilayers. We reveal the appearance of multiple flat bands and a significant change in the position of trapping sites compared to the rigid lattice. Most importantly, we predict a multi-peak structure emerging in optical absorption of WSe$_2$ homobilayers - in stark contrast to the single peak that dominates the rigid lattice. This finding can be exploited as an unambiguous signature of atomic reconstruction in optical spectra of moir\'e excitons in twisted homobilayers.

Abstract: In the context of $\theta$ electrodynamics we find transverse electromagnetic wave solutions forbidden in Maxwell electrodynamics. Our results attest to new evidence of the topological magnetoelectric effect in topological insulators, resulting from a polarization rotation of an external electromagnetic field. Unlike Faraday and Kerr rotations, the effect does not rely on a longitudinal magnetic field, the reflected field, or birefringence. The rotation occurs due to transversal discontinuities of the topological magnetoelectric parameter in cylindrical geometries. The dispersion relation is linear, and birefringence is absent. One solution behaves as an optical fiber confining exact transverse electromagnetic fields with omnidirectional reflectivity. These results may open new possibilities in optics and photonics by utilizing topological insulators to manipulate light.

Abstract: The research on the properties of spin waves (SWs) in three-dimensional nanosystems is an innovative idea in the field of magnonics. Mastering and understanding the nature of magnetization dynamics and binding of SWs at surfaces, edges, and in-volume parts of three-dimensional magnetic systems enables the discovery of new phenomena and suggests new possibilities for their use in magnonic and spintronic devices. In this work, we use numerical methods to study the effect of geometry and external magnetic field manipulations on the localization and dynamics of SWs in crescent-shaped (CS) waveguides. It is shown that changing the magnetic field direction in these waveguides breaks the symmetry and affects the localization of eigenmodes with respect to the static demagnetizing field. This in turn has a direct effect on their frequency. Furthermore, CS structures were found to be characterized by significant saturation at certain field orientations, resulting in a cylindrical magnetization distribution. Thus, we present chirality-based nonreciprocal dispersion relations for high-frequency SWs, which can be controlled by the field direction (shape symmetry) and its amplitude (saturation).

Abstract: Artificial Kitaev chains, formed by quantum dots coupled via superconductors, have emerged as a promising platform for realizing Majorana bound states. Even a minimal Kitaev chain (a quantum dot--superconductor--quantum dot setup) can host Majorana states at discrete sweet spots. However, unambiguously identifying Majorana sweet spots in such a system is still challenging. In this work, we propose an additional dot coupled to one side of the chain as a tool to identify good sweet spots in minimal Kitaev chains. When the two Majorana states in the chain overlap, the extra dot couples to both and thus splits an even--odd ground-state degeneracy when its level is on resonance. In contrast, a ground-state degeneracy will persist for well-separated Majorana states. This difference can be used to identify points in parameter space with spatially separated Majorana states, using tunneling spectroscopy measurements. We perform a systematic analysis of different relevant situations. We show that the additional dot can help distinguishing between Majorana sweet spots and other trivial zero-energy crossings. We also characterize the different conductance patterns, which can serve as a guide for future experiments aiming to study Majorana states in minimal Kitaev chains.