Recent Submissions
The silicon vacancy centers in SiC : determination of intrinsic spin dynamics for integrated quantum photonics
(2024) Liu, Di; Kaiser, Florian; Bushmakin, Vladislav; Hesselmeier, Erik; Steidl, Timo; Ohshima, Takeshi; Son, Nguyen Tien; Ul-Hassan, Jawad; Soykal, Öney O.; Wrachtrup, Jörg
The negatively charged silicon vacancy center ( VSi-) in silicon carbide (SiC) is an emerging color center for quantum technology covering quantum sensing, communication, and computing. Yet, limited information currently available on the internal spin-optical dynamics of these color centers prevents us from achieving the optimal operation conditions and reaching the maximum performance especially when integrated within quantum photonics. Here, we establish all the relevant intrinsic spin dynamics of the VSi-center at cubic lattice site (V2) in 4H-SiC by an in-depth electronic fine structure modeling including the intersystem-crossing and deshelving mechanisms. With carefully designed spin-dependent measurements, we obtain all the previously unknown spin-selective radiative and non-radiative decay rates. To showcase the relevance of our work for integrated quantum photonics, we use the obtained rates to propose a realistic implementation of time-bin entangled multi-photon GHZ and cluster state generation. We find that up to three-photon GHZ or cluster states are readily within reach using the existing nanophotonic cavity technology.
Revealing process and material parameter effects on densification via phase-field studies
(2024) Seiz, Marco; Hierl, Henrik; Nestler, Britta; Rheinheimer, Wolfgang
Sintering is an important processing step in both ceramics and metals processing. The microstructure resulting from this process determines many materials properties of interest. Hence the accurate prediction of the microstructure, depending on processing and materials parameters, is of great importance. The phase-field method offers a way of predicting this microstructural evolution on a mesoscopic scale. The present paper employs this method to investigate concurrent densification and grain growth and the influence of stress on densification. Furthermore, the method is applied to simulate the entire freeze-casting process chain for the first time ever by simulating the freezing and sintering processes separately and passing the frozen microstructure to the present sintering model.
Thermocapillary central lamella recess during droplet impacts onto a heated wall
(2024) Palmetshofer, Patrick; Geppert, Anne K.; Steigerwald, Jonas; Arcos Marz, Tim; Weigand, Bernhard
We experimentally observe a new phenomenon, the formation of a toroidal region of lower film thickness in the center of the lamella formed during high Weber number water droplet impacts onto smooth heated walls. This region forms around the air bubble, which is entrapped during the initial impact phase at the impact center. Our study encompasses a variation of the droplet size, impact velocity, surface wettability and temperature. We show how this phenomenon can be explained considering a two-step process involving thermocapillary convection in two separate regions: The temperature gradient along the surface of the entrapped air bubble caused by heat conduction induces flow that pumps warmer liquid to the lamella-ambient interface due to the Marangoni effect. The non-uniform temperature distribution along it then causes fluid acceleration in the radial direction, depleting the fluid volume around the bubble in a self-amplifying manner. We use direct numerical simulations of a stagnant liquid film with an enclosed bubble at the wall to confirm this theory.
Realization of a classical Ruddlesden Popper type bilayer nickelate in Sr3Ni-xAlxO7-δ with unusual Ni4+
(2024) Yilmaz, Hasan; Küster, Kathrin; Starke, Ulrich; Clemens, Oliver; Isobe, Masahiko; Puphal, Pascal
The discovery of 80 K superconductivity in bilayer La3Ni2O7 at pressures greater than 14 GPa presents a unique opportunity to study a novel class of high-temperature superconductors. Therefore, other bilayer nickelates following the classical (T4+) Ruddlesden-Popper (RP) series of Sr3Ni2O7 would present an interesting new candidate. In this work, we study the stabilization of RP n = 2 phase in Sr3Ni2-xAlxO7-δ, via floating zone growth of crystals. With powder and single-crystal XRD, we study the stability range of the RP-type phase. Our Thermogravimetric Analysis (TGA), X-ray photoelectron spectroscopy (XPS) and gas extraction studies reveal a remarkably high oxidation state of Ni4+ stabilized by chemical strain from Al. The obtained black crystals are insulating in transport and show a magnetic transition around 12 K.
Zwei Dichter und ihre Katastrophensucht - Lukas Bärfuss und Heinrich von Kleist
(2025) Reiß, Birgitt; Bühler-Dietrich, Annette (Prof. Dr.)
Ausgehend von der Kleistrede, die Lukas Bärfuss 2011 in Thun gehalten hat, wird literaturwissenschaftlich untersucht, welche Bezugspunkte es in seinem Werk zu demjenigen von Heinrich von Kleist gibt.
Optimal feedback improves behavioral focus during self-regulated computer-based work
(2024) Wirzberger, Maria; Lado, Anastasia; Prentice, Mike; Oreshnikov, Ivan; Passy, Jean-Claude; Stock, Adrian; Lieder, Falk
Distractions are omnipresent and can derail our attention, which is a precious and very limited resource. To achieve their goals in the face of distractions, people need to regulate their attention, thoughts, and behavior; this is known as self-regulation . How can self-regulation be supported or strengthened in ways that are relevant for everyday work and learning activities? To address this question, we introduce and evaluate a desktop application that helps people stay focused on their work and train self-regulation at the same time. Our application lets the user set a goal for what they want to do during a defined period of focused work at their computer, then gives negative feedback when they get distracted, and positive feedback when they reorient their attention towards their goal. After this so-called focus session, the user receives overall feedback on how well they focused on their goal relative to previous sessions. While existing approaches to attention training often use artificial tasks, our approach transforms real-life challenges into opportunities for building strong attention control skills. Our results indicate that optimal attentional feedback can generate large increases in behavioral focus, task motivation, and self-control-benefitting users to successfully achieve their long-term goals.
A volume of fluid method for the predictive simulation of three-dimensional immiscible liquids' droplet collisions
(2025) Potyka, Johanna; Schulte, Kathrin (Dr.-Ing.)
The interaction of two immiscible liquids in a gaseous environment is relevant for different technical applications. An example is the water injection into engines which can help to reduce emissions and increase efficiency. Another application is the rapid production of equal-sized capsules for the precise dosage and for the protection of the active substance of medicine or of food additives. The outcome of two colliding immiscible liquids' droplets as an elementary process has to be predicted for the design of such applications. Binary collisions of droplets can either result in merging or separation into two or more daughter droplets. The consideration of different properties of the liquids for both droplets breaks the symmetry of the collision process. Merging, crossing, single reflex and reflexive separation are reported in the literature as the possible outcomes of head-on collisions of immiscible liquids' droplets. These differences of immiscible compared to identical liquids' droplet collisions are only scarcely investigated in the literature. Numerical simulations allow for detailed insights into the collision process, but methods suitable for the prediction of three-dimensional and immiscible liquids' droplet collisions combined with relevant topological changes during the droplet collision process were not yet described. Therefore, a simulation tool for fully three-dimensional direct numerical simulations (DNS) of immiscible liquids' droplet collisions has been developed within this thesis and successfully applied to predictions of the immiscible liquids' droplet collision process and outcome. Existing and new methods were enhanced, developed and combined to enable massively parallel and efficient three-dimensional DNS of the interaction of two immiscible liquids of arbitrary three-dimensional geometry in a gaseous continuous environment.
A main challenge of modelling immiscible three-phase flow, as compared to two-phase flow, is the representation of the geometry of thin films and contact lines, especially in three-dimensional domains. Therefore, the Piecewise Linear Interface Calculation (PLIC) and the Volume of Fluid (VOF) advection were adapted for three-phase cells. Additionally, a modified Continuous Surface Stress (CSS) model was introduced to model the surface and interfacial forces at liquid-liquid-gas contact lines and thin films. This model allows for simulations of immiscible liquids' interactions with topological changes. The modified CSS model requires an adaptation of the density and viscosity computation throughout the entire momentum equation solver. This consistent use of properties has been key to enabling momentum and energy conserving simulations of immiscible three-phase flow.
The methods for the simulation of immiscible liquids' interaction were implemented in the multiphase flow solver Free Surface 3D (FS3D). The simulation results of several validation test cases were compared to analytical solutions. First order convergence was achieved for the PLIC reconstruction and VOF advection as well as for the liquid lens test. Furthermore, experimentally investigated cases of droplet-droplet and droplet-jet interactions have been reproduced by DNS. The morphology of selected cases as well as full regime maps for merging and separation agree with experiments. This shows that the developed methods enable the prediction of a wide range of three-dimensional interactions of immiscible liquids. Additionally, the increased efficiency of the new methods has been a key aspect to enable parameter studies within feasible compute times and simulations of large systems like droplet-jet interactions.
Based on the analysis of immiscible liquids' droplet collision DNS resulting in different liquids' distributions after separation, a scaling approach was developed. This approach employs two dimensionless numbers to classify the distribution after the separation from quantities known before the collision. Furthermore, a new distribution resulting after the separation of two droplets of partially wetting liquids was discovered through the DNS. The simulation results provide detailed information on the whole collision process. Simulation results of immiscible and identical liquids' droplet collisions have been compared. The evaluation of the energy budgets throughout representative droplet collisions underlined the increased complexity in the collision process of immiscible liquids compared to identical liquids. A larger number of influencing factors on the collision process and outcome originates from different viscosities, densities and the wetting behaviour of the two interacting liquids, which should be accounted for in future analytical models.
Summarised, this thesis provides numerical methods for the predictive three-dimensional simulation and a basis for a universal description of three-dimensional liquids' interactions with or without topological changes in gaseous environments.
Smoking is associated with increased eryptosis, suicidal erythrocyte death, in a large population-based cohort
(2024) Schmitt, Marvin; Ewendt, Franz; Kluttig, Alexander; Mikolajczyk, Rafael; Kraus, F. Bernhard; Wätjen, Wim; Bürkner, Paul-Christian; Stangl, Gabriele I.; Föller, Michael
Smoking has multiple detrimental effects on health, and is a major preventable cause of premature death and chronic disease. Despite the well-described effect of inhaled substances from tobacco smoke on cell toxicity, the association between smoking and suicidal erythrocyte death, termed eryptosis, is virtually unknown. Therefore, the blood samples of 2023 participants of the German National Cohort Study (NAKO) were analyzed using flow cytometry analysis to determine eryptosis from fluorescent annexin V-FITC-binding to phosphatidylserine-exposing erythrocytes. Blood analyses were complemented by the measurement of hematologic parameters including red blood cell count, hematocrit, hemoglobin, mean corpuscular cell volume (MCV) and mean corpuscular hemoglobin (MCH). Eryptosis was higher in smokers than in non- and ex-smokers, and positively associated with the number of cigarettes smoked daily ( r = 0.08, 95% CI [0.03, 0.12]). Interestingly, despite increased eryptosis, smokers had higher red blood cell indices than non-smokers. To conclude, smokers were characterized by higher eryptosis than non-smokers, without showing any obvious detrimental effect on classic hematological parameters.
Enhancing heat transfer at low temperatures by laser functionalization of the inner surface of metal pipes
(2024) Holder, Daniel; Peter, Alexander; Kirsch, Marc; Cáceres, Sergio; Weber, Rudolf; Onuseit, Volkher; Kulenovic, Rudi; Starflinger, Jörg; Graf, Thomas
The latent heat transfer during vapour condensation in the condenser section of passive heat transport devices such as the two-phase closed thermosiphon is limited by film condensation. Dropwise condensation provides an increase of the heat transfer coefficient by up to one order of magnitude and can be achieved with a water-repellant surface. The inner surface of pipes made from stainless steel was functionalized by laser surface texturing with ultrashort laser pulses and subsequent storage in a liquid containing long-chained hydrocarbons. The pipes were separated into half-pipes by wire eroding to enable laser texturing of the inner surface, and were then joined by electron beam welding after laser texturing. As a result, superhydrophobic and water-repellent surfaces with a contact angle of 153° were obtained on the inner surface of the pipes with a length of up to 1 m. The functionalized pipes were used in the condenser section of a two-phase closed thermosiphon to demonstrate a heat transfer rate of 0.92 kW at 45 °C, which is approximately three times the heat transfer rate of 0.31 kW of a smooth reference pipe.
New examples of ferroelectric nematic materials showing evidence for the antiferroelectric smectic-Z phase
(2024) Nacke, Pierre; Manabe, Atsutaka; Klasen-Memmer, Melanie; Chen, Xi; Martinez, Vikina; Freychet, Guillaume; Zhernenkov, Mikhail; Maclennan, Joseph E.; Clark, Noel A.; Bremer, Matthias; Giesselmann, Frank
We present a new ferroelectric nematic material, 4-((4′-((trans)-5-ethyloxan-2-yl)-2′,3,5,6′-tetrafluoro-[1,1′-biphenyl]-4-yl)difluoromethoxy)-2,6-difluorobenzonitrile (AUUQU-2-N) and its higher homologues, the molecular structures of which include fluorinated building blocks, an oxane ring, and a terminal cyano group, all contributing to a large molecular dipole moment of about 12.5 D. We observed that AUUQU-2-N has three distinct liquid crystal phases, two of which were found to be polar phases with a spontaneous electric polarization Ps of up to 6 µC cm-2. The highest temperature phase is a common enantiotropic nematic (N) exhibiting only field-induced polarization. The lowest-temperature, monotropic phase proved to be a new example of the ferroelectric nematic phase (NF), evidenced by a single-peak polarization reversal current response, a giant imaginary dielectric permittivity on the order of 103, and the absence of any smectic layer X-ray diffraction peaks. The ordinary nematic phase N and the ferroelectric nematic phase NF are separated by an antiferroelectric liquid crystal phase which has low permittivity and a polarization reversal current exhibiting a characteristic double-peak response. In the polarizing light microscope, this antiferroelectric phase shows characteristic zig-zag defects, evidence of a layered structure. These observations suggest that this is another example of the recently discovered smectic ZA (SmZA) phase, having smectic layers with the molecular director parallel to the layer planes. The diffraction peaks from the smectic layering have not been observed to date but detailed 2D X-ray studies indicate the presence of additional short-range structures including smectic C-type correlations in all three phases-N, SmZA and NF-which may shed new light on the understanding of polar and antipolar order in these phases.