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Confinement of Bose–Einstein magnon condensates in adjustable complex magnetization landscapes
(2022)
Coherent wave states such as Bose–Einstein condensates (BECs), which spontaneously form in an overpopulated magnon gas even at room temperature, have considerable potential for wave-based computing and information processing at microwave frequencies. The ability to control the transport properties of magnon BECs plays an essential role in their practical use. Here, we demonstrate the spatiotemporal control of the BEC density distribution through the excitation of magnon supercurrents in an inhomogeneously magnetized yttrium iron garnet film. The BEC is created by microwave parametric pumping and probed by Brillouin light scattering spectroscopy. The desired magnetization profile is prepared by heating the film with optical patterns projected onto its surface using a phase-based wavefront modulation technique. Specifically, we observe a pronounced spatially localized magnon accumulation caused by magnon supercurrents flowing toward each other originating in two heated regions. This accumulation effect increases the BEC lifetime due to the constant influx of condensed magnons into the confinement region. The shown approach to manipulate coherent waves provides an opportunity to extend the lifetime of freely evolving magnon BECs, create dynamic magnon textures, and study the interaction of magnon condensates formed in different regions of the sample.
We present a robust, fiber-based endoscope with a silver direct-laser-written structure for radio frequency (RF) emission next to the optical fiber facet. Thereby, we are able to excite and probe a sample, such as nitrogen-vacancy (NV) centers in diamond, with RF and optical signals simultaneously and specifically measure the fluorescence of the sample fully through the fiber. At our targeted frequency range of around 2.9 GHz, the facet of the fiber core is in the near-field of the RF-guiding silver structure, which comes with the advantage of an optimal RF intensity decreasing rapidly with the distance. By creating a silver structure on the cladding of the optical fiber, we achieve the minimal possible distance between an optically excited and detected sample and an antenna structure without affecting the optical performance of the fiber. This allows us to realize a high RF amplitude at the sample’s position when considering an endoscope solution with integrated optical and RF access. The capabilities of the endoscope are quantified by optically detected magnetic resonance (ODMR) measurements of an NV-doped microdiamond that we probe as a practical use case. We demonstrate a magnetic sensitivity of our device of 17.8 nT/Hz when measuring the ODMR exclusively through our fiber and compare the sensitivity to a measurement using a confocal microscope. Moreover, the application of our device is not limited to NV centers in diamonds. Similar endoscope-like devices combining optical excitation and detection with radio frequency or microwave antenna could be used as a powerful tool for measuring a variety of fluorescent particles that have so far only been investigated with bulky and large optical setups. Furthermore, our endoscope points toward precise distance measurements based on Rabi oscillations.
Schlafschwierigkeiten und Insomnie stellen ein gesellschaftlich zunehmendes Phänomen dar. Studierende sind hiervon überdurchschnittlich häufig betroffen, mehr als ein Drittel beklagt eine niedrige Schlafqualität. In diesem Kontext erwiesen sich vor allem achtsamkeitsbasierte Interventionen als hilfreich. Wirksame, neuere Forschungsansätze widmen sich der Untersuchung mobiler Gesundheitsanwendungen. Daran anknüpfend setzt sich die Studie zum Ziel, die Effektivität einer achtsamkeitsbasierten Intervention mittels Meditations-App („7Mind“) in Bezug auf Schlafschwierigkeiten zu evaluieren. Studierende wurden randomisiert der Treatment- oder Wartelisten-Kontrollgruppe zugeordnet. Baseline-Messung, Post-Messung (2 Wochen) und Follow-up-Messung (4 Wochen) wurden mit Selbstberichtfragebogen zu den Variablen Schlafqualität, Insomnie-Schweregrad, Achtsamkeit, Tagesmüdigkeit, Stress und Lebensqualität untersucht. Die Intervention sollte 14 Tage 20 Minuten täglich absolviert werden. Von 53 Studierenden mit Baseline-Messung füllten 35 die Post-Messung und 28 alle Messzeitpunkte aus. Im Anschluss an das Training zeigten sich zwischen den Gruppen signifikante Unterschiede bezüglich der Schlafparameter (p < .001), wohingegen die Achtsamkeit unverändert blieb. Dafür wies letztere eine signifikante Zeit x Gruppe Interaktion auf (p = .018). In Bezug auf den Schlaf ergaben sich hohe Effektstärken, welche auch zum Follow-up-Zeitpunkt persistierten (d= 2.36). Die Variablen Tagesmüdigkeit,
Stress und Lebensqualität waren hochsignifikant mit der Schlafqualität korreliert (p < .001).
Mobile achtsamkeitsbasierte Interventionen stellen eine ökonomische Alternative dar, um Studierende mit Schlafschwierigkeiten zu unterstützen. Zukünftige Schritte inkludieren die Überprüfung über einen längeren Zeitraum und mit einer größeren Reichweite.
Interfacial properties of binary azeotropic mixtures of Lennard-Jones truncated and shifted fluids were studied by molecular dynamics (MD) simulation and density gradient theory (DGT) in combination with an equation of state. Three binary mixtures were investigated, which differ in the energetic cross interaction parameter that yields different types of azeotropic behavior. This study covers a wide temperature and composition range. Mixture A exhibits a heteroazeotrope at low temperatures, which changes to a low-boiling azeotrope at high temperatures, mixture B exhibits a low-boiling azeotrope, and mixture C exhibits a high-boiling azeotrope. The phase behavior and fluid interfacial properties as well as their relation were studied. Vapor–liquid, liquid–liquid, and vapor–liquid–liquid equilibria and interfaces were considered. Density profiles, the surface tension, the interfacial thickness, as well as the relative adsorption and enrichment of the components at the interface were studied. The results obtained from the two independent methods (MD and DGT) are overall in good agreement. The results provide insights into the relation of the phase behavior, particularly the azeotropic behavior, of simple fluid mixtures and the corresponding interfacial properties. Strong enrichment was found for the mixture with a heteroazeotrope in the vicinity of the three-phase equilibrium, which is related to a wetting transition.
We report on the resonant excitation of spin waves in micro-structured magnetic thin films by short-wavelength surface acoustic waves (SAWs). The spin waves as well as the acoustic waves are studied by micro-focused Brillouin light scattering spectroscopy. At low magnetic bias fields, a resonant phonon–magnon conversion is possible, which results in the excitation of short-wavelength spin waves. Using micromagnetic simulations, we verify that during this excitation both energy and linear momentum are conserved and fully transferred from the SAW to the spin wave. This conversion can already be detected after an interaction length of a few micrometers. Thus, our findings pave the way for miniaturized magneto-elastic spin-wave emitters for magnon computing.
We study the influence of transport effects on time- and space-resolved magnetization dynamics in a laser-excited thick nickel film. We explicitly include diffusive heat transport and spin-resolved charge transport as well as Seebeck and Peltier effects and calculate the dynamics of spin-dependent electronic temperatures, chemical potentials, lattice temperatures, and magnetization. We find that transport has an influence on the magnetization dynamics closer to the excited surface as well as in regions deeper than the penetration depth of the laser. We reveal that, for higher absorbed fluences and in the presence of transport, thick magnetic films show a quenching time nearly independent of depth, though the magnitude of quenching is depth-dependent.
We investigate ultrafast spin dynamics due to exchange, electron–phonon and Elliott–Yafet spin-flip scattering in a model with a simple band structure and ferromagnetically coupled electronic sublattices (or more generally, subsystems). We show that this incoherent model of electronic dynamics leads to sublattice magnetization changes in opposite directions after ultrashort-pulse excitation. This prominent feature on an ultrafast timescale is related to a transfer of energy and angular momentum between the subsystems due to exchange scattering. Our calculations illustrate a possible incoherent mechanism that works in addition to the coherent optically induced spin transfer mechanism.
Spin waves in yttrium iron garnet (YIG) nano-structures attract increasing attention from the perspective of novel magnon-based data processing applications. For short wavelengths needed in small-scale devices, the group velocity is directly proportional to the spin-wave exchange stiffness constant λex. Using wave vector resolved Brillouin light scattering spectroscopy, we directly measure λex in Ga-substituted YIG thin films and show that it is about three times larger than for pure YIG. Consequently, the spin-wave group velocity overcomes the one in pure YIG for wavenumbers k > 4 rad/μm, and the ratio between the velocities reaches a constant value of around 3.4 for all k > 20 rad/μm. As revealed by vibrating-sample magnetometry and ferromagnetic resonance spectroscopy, Ga:YIG films with thicknesses down to 59 nm have a low Gilbert damping (α<10−3), a decreased saturation magnetization μ0MS≈20 mT, and a pronounced out-of-plane uniaxial anisotropy of about μ0Hu1≈95 mT, which leads to an out-of-plane easy axis. Thus, Ga:YIG opens access to fast and isotropic spin-wave transport for all wavelengths in nano-scale systems independently of dipolar effects.
In this work, we present a method to microscopically investigate the liquid–vapor interfaces on the bottom side of droplets, which were placed on superhydrophobic structures, so that wetting in the Cassie–Baxter (CB) state occurred. These interfaces are hard to access optically, especially when an opaque substrate material is used, which is usually the case for technical applications. In that case, the menisci have to be observed through the droplet, which substantially deteriorates the imaging quality. Other methods that circumvent these distortions, such as optical coherence tomography, are restricted to a resolution of several micrometers. Confocal or fluorescence microscopy additionally requires a transparent substrate. To measure the liquid–vapor interfaces formed in the Cassie–Baxter state with high accuracy liquid droplets of a monomer solution that chemically reacts to form the elastomer, polydimethylsiloxane was placed on structured surfaces. Because double reentrant structures were used, wetting occurred in the Cassie–Baxter state despite the low surface tension of the monomer solution. After curing, it was possible to remove the solid droplets from the surface and investigate them using confocal microscopy, which provides an excellent height resolution of 10 nm. Test structures such as arrays of stripes and holes with variable spacing or diameter were used to investigate the impact of their geometry on the liquid–vapor interfaces formed in the CB state. Although the maximum height of the menisci on the droplet's bottom side is in the region of several 10 μm, the 10 nm resolution is required to adequately compare their topography with simplified theoretical models.
The magnetic response of a ferromagnet after an ultrafast optical excitation can be connected to the underlying electronic dynamics either via primary excitation processes during the laser pulse or via secondary collision processes. In the latter case, the information on the details of the excitation is lost and, therefore, the electron dynamics can be described using quasi-equilibrium concepts. In this work, we study the effect of the pump photon energy on the ultrafast demagnetization dynamics in ferromagnetic nickel. We find that the magnetization dynamics for similar absorbed energies for a range of pump photon energies are almost identical and depend only on the absorbed energy. This is in stark contrast to characteristic differences in the optically excited electronic distributions, as calculated from the band structure. In addition, the measured fluence-dependent dynamics can be reproduced with a model based on local temperatures. These findings indicate that it is mainly secondary processes that are responsible for the observed demagnetization dynamics.