Andreas K. Schmid

Magnetic skyrmionic structures, including magnetic skyrmions and antiskyrmions, are characterized by swirling spin textures with non-trivial topologies. They are featured with specific topological charges, Q, which are of crucial importance in determining their topological properties. Owing to the invariance of the chiral nature, it is generally believed that Q is conserved in a given magnetic skyrmionic structure and is hard to alter. Here, we experimentally realize the control of Q of magnetic skyrmionic structures at room temperature in a Dzyaloshinskii-Moriya interaction (DMI) platform with spatially alternating signs. Depending on how many times it crosses the interfaces between DMI regions with opposite signs, the magnetic skyrmionic structures possess different Q. Modifying the DMI energy landscape through chemisorbed oxygen, a magnetic topological transition is realized. This creation and manipulation of magnetic skyrmionic structures with controllable Q, in particular the DMI-stabilized thin-film antiskyrmions and high-Q skyrmionic structures, enables a new degree of freedom to control their dynamics via a novel DMI confinement effect. Our findings open up an unexplored avenue on various topological magnetic skyrmionic structures and their potential applications.

The Dzyaloshinskii-Moriya antisymmetric exchange interaction (DMI) stabilises topological spin textures with promising future spintronics applications. According to crystal symmetry, the DMI can be categorized as four different types that favour different chiral textures. Unlike the other three extensively-investigated types, out-of-plane DMI, as the last type that favours in-plane chirality, remained missing so far. Here we apply point-group-dependent DMI matrix analysis to show that out-of-plane DMI exists under reduced crystal symmetry. Through strain and structure engineering, we show how C_s symmetry is realized in ultrathin magnets and observe the out-of-plane DMI stabilised in-plane chirality using spin-polarized electron microscopy. Our results show that extremely low out-of-plane DMI strengths at µeV/atom are sufficient to stabilise topological spin textures, including merons and bimerons. We also demonstrate field-induced reversible control of the in-plane chirality and merons. Our findings open up untapped paths on topological magnetic textures and their potential applications.

We investigate the electronic properties of a graphene and α-ruthenium trichloride (α-RuCl₃) heterostructure using a combination of experimental techniques. α-RuCl₃ is a Mott insulator and a Kitaev material. Its combination with graphene has gained increasing attention due to its potential applicability in novel optoelectronic devices. By using a combination of spatially resolved photoemission spectroscopy and low-energy electron microscopy, we are able to provide a direct visualization of the massive charge transfer from graphene to α-RuCl₃, which can modify the electronic properties of both materials, leading to novel electronic phenomena at their interface. A measurement of the spatially resolved work function allows for a direct estimate of the interface dipole between graphene and α-RuCl₃. Their strong coupling could lead to new ways of manipulating electronic properties of a two-dimensional heterojunction. Understanding the electronic properties of this structure is pivotal for designing next generation low-power optoelectronics devices.

We report on the measurement of the lateral distribution of the junction current of an electrical biased p-n GaN diode by electron emission microscopy using a low-energy electron microscope. The vacuum level at the surface of the diode was lowered by deposition of cesium to achieve negative electron affinity, allowing overflow electrons at the surface of the biased diodes to be emitted and their spatial distribution imaged. The results were compared to the literature, and a good match with analytical solutions by Joyce and Wemple [J. Appl. Phys. 41, 3818 (1970)] was obtained.

Modern electron microscopy and spectroscopy are key technologies for studying the structure and composition of quantum and biological materials in fundamental and applied sciences. High-resolution spectroscopic techniques and aberration-corrected microscopes are often limited by the relatively large energy distribution of currently available beam sources. This can be improved by a monochromator, with the significant drawback of losing most of the beam current. Here, we study the field-emission properties of a monocrystalline niobium-tip electron field emitter at 5.2 K, well below the superconducting transition temperature. The emitter fabrication process can generate two tip configurations, with or without a nanoprotrusion at the apex, strongly influencing the field-emission energy distribution. The geometry without the nanoprotrusion has nanoampere beam currents, long-term stability, and an energy width of around 100 meV. The beam current can be increased by 2 orders of magnitude by xenon-gas adsorption. We also study the emitter performance up to 82 K and demonstrate that the energy width of the beam can be below 40 meV with high emitter brightness even at liquid-nitrogen cooling temperatures when an apex nanoprotrusion is present. Furthermore, the spatial and temporal electron-electron correlations of the field emission are studied at normal and superconducting temperatures and the influence of Nottingham heating is discussed. This monochromatic source will allow exceptional accuracy and resolution in electron microscopy, spectroscopy, and high-coherence quantum applications.

Modern electron microscopy and spectroscopy is a key technology for studying the structure and composition of quantum and biological materials in fundamental and applied sciences. High-resolution spectroscopic techniques and aberration-corrected microscopes are often limited by the relatively large energy distribution of currently available beam sources. This can be improved by a monochromator, with the significant drawback of losing most of the beam current. Here, we study the field emission properties of a monocrystalline niobium tip electron field emitter at 5.2 K, well below the superconducting transition temperature. The emitter fabrication process can generate two tip configurations, with or without a nano-protrusion at the apex, strongly influencing the field-emission energy distribution. The geometry without the nano-protrusion has a high beam current, long-term stability, and an energy width of around 100 meV. The beam current can be increased by two orders of magnitude by xenon gas adsorption. We also studied the emitter performance up to 82 K and demonstrated the beam's energy width can be below 40 meV even at liquid nitrogen cooling temperatures when an apex nano-protrusion is present. Furthermore, the spatial and temporal electron-electron correlations of the field emission are studied at normal and superconducting temperatures and the influence of Nottingham heating is discussed. This new monochromatic source will allow unprecedented accuracy and resolution in electron microscopy, spectroscopy, and high-coherence quantum applications.