Scanning probe microscopy (SPM) has become an essential surface characterization technique in research and development. By concept, SPM performance crucially depends on the quality of the nano-probe element, in particular, the apex radius. Now, with the development of advanced SPM modes beyond morphology mapping, new challenges have emerged regarding the design, morphology, function, and reliability of nano-probes. To tackle these challenges, versatile fabrication methods for precise nano-fabrication are needed. Aside from well-established technologies for SPM nano-probe fabrication, focused electron beam-induced deposition (FEBID) has become increasingly relevant in recent years, with the demonstration of controlled 3D nanoscale deposition and tailored deposit chemistry. Moreover, FEBID is compatible with practically any given surface morphology. In this review article, we introduce the technology, with a focus on the most relevant demands (shapes, feature size, materials and functionalities, substrate demands, and scalability), discuss the opportunities and challenges, and rationalize how those can be useful for advanced SPM applications. As will be shown, FEBID is an ideal tool for fabrication/modification and rapid prototyping of SPM-tipswith the potential to scale up industrially relevant manufacturing.

A well-known feature of magnetic field driven dynamics of domain walls in ferromagnets is the existence of a threshold driving force at which the internal magnetization of the domain wall starts to precess -a phenomenon known as the Walker breakdown -resulting in an abrupt drop of the domain wall propagation velocity. Here, we report on micromagnetic simulations of magnetic field driven domain wall dynamics in thin ferromagnetic strips with perpendicular magnetic anisotropy which demonstrate that in wide enough strips Walker breakdown is a multistep process: It consists of several distinct velocity drops separated by short linear parts of the velocity vs field curve. These features originate from the repeated nucleation, propagation and annihilation of an increasing number of Bloch lines within the domain wall as the driving field magnitude is increased. This mechanism arises due to magnetostatic effects breaking the symmetry between the two ends of the domain wall.

We demonstrate a cryo-compatible, fully fiber-integrated, alignment-free optical microresonator. The compatibility with low temperatures expands its possible applications to the wide field of solid-state quantum optics, where a cryogenic environment is often a requirement. At a temperature of 4.6 K we obtain a quality factor of (9.9 ± 0.7) × 10 6 . In conjunction with the small mode volume provided by the nanofiber, this cavity can be either used in the coherent dynamics or the fast cavity regime, where it can provide a Purcell factor of up to 15. Our resonator is therefore suitable for significantly enhancing the coupling between light and a large variety of different quantum emitters and due to its proven performance over a wide temperature range, also lends itself for the implementation of quantum hybrid systems. IntroductionAchieving efficient coupling between quantum emitters and light is an important goal of research in quantum communication and computation, sensor applications and fundamental studies. Such an efficient interaction can for example be realized by means of an optical cavity or by reducing the mode area of the light field to the size of the emitter's interaction cross-section. Optical nanofibers offer such a strong transverse confinement of the light field.This makes them a versatile tool for interfacing different types of emitters such as atoms [1-3], single molecules [4], quantum dots [5][6][7] and color centers in diamond [8,9]. For many proof-of-principle experiments, cold atoms were the emitters of choice as they represent a well-controlled and isolated system. However, the experimental overhead for a cold-atom set-up is significant and solid-state emitters are much more suitable for practical and scalable platforms for quantum networks or nanosensors [10,11].Yet, the optical transitions of solid state emitters also couple to the phononic degrees of freedom of the system, leading to dephasing and inelastic scattering. In order to avoid this problem, the phonons of the system have to be frozen out and the branching ratio of emission into the coherent zero-phonon line has to be maximized. Further, cryogenic temperatures may be required to be able to spectrally address individual solid state emitters in the case, where many are present in the same host system [4]. This calls for a cryo-compatible optical microresonator with high quality factor, Q, that selectively accelerates the desired optical transition via the Purcell effect [12][13][14][15].Here, we show that a fully fiber-based optical microresonator that consists of a tapered optical fiber with two integrated fiber Bragg gratings (FBGs) [16,17], as demonstrated in [18], can be employed at cryogenic temperatures. In particular, we confirm that a high Q factor prevails after contact gas-cooling from room temperature to liquid helium temperature. Consequently, the resonator is still compatible with reaching the strong coupling regime. Furthermore, for usage at cryogenic temperatures, the alignment-free character of our resonator represen...

The (111) facet of magnetite (Fe 3 O 4 ) has been studied extensively by experimental and theoretical methods, but controversy remains regarding the structure of its low-energy surface terminations. Using density functional theory (DFT) computations, we demonstrate three reconstructions that are more favorable than the accepted Fe oct2 termination under reducing conditions. All three structures change the coordination of iron in the kagome Fe oct1 layer to be tetrahedral. With atomically resolved microscopy techniques, we show that the termination that coexists with the Fe tet1 termination consists of tetrahedral iron capped by 3-fold coordinated oxygen atoms. This structure explains the inert nature of the reduced patches.