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

Abstract: Graphene quantum dots are promising candidates for qubits due to weak spin-orbit and hyperfine interactions. The hyperfine interaction, controllable via isotopic purification, could be the key to further improving the coherence. Here, we use isotopically enriched graphite crystals of both $^{12}$C and $^{13}$C grown by high-pressure-high-temperature method to exfoliate graphene layers. We fabricated Hall bar devices and performed quantum transport measurements, revealing mobilities exceeding $10^{5}$$\textrm{cm}^{2}/Vs$ and a long mean free path of microns, which are as high as natural graphene. Shubnikov-de Haas oscillations, quantum Hall effect up to the filling factor of one, and Brown-Zak oscillations due to the alignment of hBN and graphene are observed thanks to the high mobility. These results constitute a material platform for physics and engineering of isotopically-enriched graphene qubits.

Abstract: We conduct a first-principles study of Hall effects in rare-earth dichalcogenides, focusing on monolayers of the H-phase EuX$_2$ and GdX$_2$, where X = S, Se, and Te. Our predictions reveal that all EuX$_2$ and GdX$_2$ systems exhibit high magnetic moments and wide bandgaps. We observe that while in case of EuX$_2$ the $p$ and $f$ states hybridize directly below the Fermi energy, the absence of $f$ and $d$ states of Gd at the Fermi energy results in $p$-like spin-polarized electronic structure of GdX$_2$, which mediates $p$-based magnetotransport. Notably, these systems display significant anomalous, spin, and orbital Hall conductivities. We find that in GdX$_2$ the strength of correlations controls the relative position of $p$, $d$ and $f$-states and their hybridization which has a crucial impact on $p$-state polarization and the anomalous Hall effect, but not the spin and orbital Hall effect. Moreover, we find that the application of strain can significantly modify the electronic structure of the monolayers, resulting in quantized charge, spin and orbital transport in GdTe$_2$ via a strain-mediated orbital inversion mechanism taking place at the Fermi energy. Our findings suggest that rare-earth dichalcogenides hold promise as a platform for topological spintronics and orbitronics.

Abstract: The hybrid skyrmion, a type of magnetic skyrmion with intermediate helicity between Bloch and N\'eel skyrmion, has gained more attraction. It is tolerant toward the skyrmion Hall effect and a potential candidate for quantum bits. We investigated the stabilization and helicity control of the hybrid skyrmion in a two-dimensional magnetic system using an analytical model and micromagnetic simulation. We look at the interplaying factors of the bulk ($D_b$) and interfacial ($D_i$) Dzyaloshinskii-Moriya (DM) interactions along with the dipolar interaction. We show that the hybrid skyrmion can stabilize through the interplay between interfacial DM and either bulk DM or dipolar interaction. We can also control the helicity of the hybrid skyrmion by tuning the ratio of $D_i/D_b$ when there is no dipolar interaction, or simply by adjusting the $D_i$ when the $D_b$ is absent. Our results suggest that hybrid skyrmions can exist within $0 < |D_i| < 0.4$ mJ/m$^2$ for Co-based magnetic systems.

Abstract: Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature ($T$=5~K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), $i.e.$ MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, $i.e.$ in-plane E$'$ and out-of-plane A$'_1$, but the appearance of 1$^{st}$, 2$^{nd}$, and higher-order phonon modes is recognised. The intensity profiles of the A$'_1$ modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe$_2$ ML, the A$'_1$ mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, $i.e.$ bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.

Abstract: It is shown micromagnetic and atomistic spin dynamics simulations can use multiple GPUs in order to reduce computation time, but also to allow for a larger simulation size than is possible on a single GPU. Whilst interactions which depend on neighbouring spins, such as exchange interactions, may be implemented efficiently by transferring data between GPUs using halo regions, or alternatively using direct memory accesses, implementing the long-range demagnetizing interaction is the main difficulty in achieving good performance scaling, where the data transfer rate between GPUs is a significant bottleneck. A multi-GPU convolution algorithm is developed here, which relies on single-GPU FFTs executed in parallel. It is shown that even for micromagnetic simulations where the demagnetizing interaction computation time dominates, good performance scaling may be achieved, with speedup factors up to 1.8, 2.5, and 3.1, for 2, 3, and 4 GPUs respectively. The code developed here can be used for any number of GPUs in parallel, with performance scaling strongly dependent on inter-GPU data transfer rate and connection topology. This is further improved in micromagnetic simulations which include a spin transport solver, obtaining speedup factors up to 1.96, 2.8, and 3.7, for 2, 3, and 4 GPUs respectively. The best case scenario is obtained for atomistic spin dynamics simulations, where the demagnetizing interaction is implemented with spin-averaged cells. Using a single workstation with 4 GPUs, it is shown atomistic spin dynamics simulations with up to 1 billion spins, and atomistic Monte Carlo simulations with up to 2 billion spins are possible, with a near-ideal performance scaling.

Abstract: Soft magnetic dots in the form of thin rings have unique topological properties. They can be in a vortex state with no vortex core. Here, we study the magnon modes of such systems both analytically and numerically. In an external magnetic field, magnetic rings are characterized by easy-cone magnetization and shows a giant splitting of doublets for modes with the opposite value of the azimuthal mode quantum number. The effect of the splitting can be refereed as a magnon analog of the topology-induced Aharonov-Bohm effect. For this we develop an analytical theory to describe the non-monotonic dependence of the mode frequencies on the azimuthal mode number, influenced by the balance between the local exchange and non-local dipole interactions.