We imaged various surface reconstruction on two semiconducting tellurium (0001) single crystals using a low-temperature scanning tunneling microscopy (LT-STM) system. One of the crystals was heated to the point of tellurium sublimation. It reveals a 2 × 2 reconstruction on small hexagonal terraces, predominantly a 4 × 3 and a novel, apparently compressed 6 × 2 reconstruction. The 6×2 reconstructed surface exhibits 1 Å high steps between terraces which is not feasible within the established crystal structure. We thus propose a novel modified structure which accommodates 1 Å steps. A second crystal displayed widely spaced atomic rows with a quasi-random atomic arrangement in between. Furthermore, scanning tunneling spectroscopy data show in-gap states tied to these rows. In this study, neither crystal ever showed a clear 2×1 reconstruction as expected from an earlier LEED study. Also, despite multiple rounds of preparation in ultra-high vacuum amorphous regions remained on both crystal.

We present a simple homemade solution enabling in situ radio frequency (RF) reflectometry measurements with a millikelvin scanning tunneling microscope (mk-STM). The additions described below were made using RF best practices, following similar detection schemes commonly employed in the quantum information science community. Using a Niobium STM tip to form a superconductor-insulator-normal metal tunnel junction, the evolution of coherence peaks at the SC-gap edge is carefully measured to characterize the RF losses and electron temperature. We further identify impedance matching as a crucial factor to achieve high sensitivity in the reflectometry by tuning the tip-sample capacitance as a function of approach distance. As a demonstration of this capability, we measure a 50 × 50 nm2 area of island features that have been condensed onto the surface of a gold single crystal. Position-dependent reflectometry losses allow us to image island sizes down to a total surface area of 5 nm2, given our current sensitivity.

We present a simple home made solution enabling in-situ RF reflectometry measurements with a millikelvin scanning tunneling microscope (mk-STM). The additions described below were made using RF best practices following similar detection schemes commonly employed in the quantum information science (QIS) community. Using a Niobium STM tip to form a superconductor-insulator-normal metal (SIN) tunnel junction, the evolution of coherence peaks at the SC-gap edge are carefully measured to characterize the RF losses and electron temperature. We further identify impedance matching as a crucial factor to achieve high sensitivity in the reflectometry by tuning the tip-sample capacitance as a function of approach distance. As a demonstration of this capability, we measure a 50x50 nm^2 area of island features that have been condensed onto the surface of a gold single crystal. Position dependent reflectometry losses allow us to image island sizes down to a total surface area of 5 nm^2 given our current sensitivity.

Phase-separation is of great importance in spacecraft tanks. The propulsion engines of spacecrafts require a single-phase propellant supply, which is a challenge using cryogenic liquids. In order to extend the knowledge of the capillary transport of cryogenic liquid in porous materials, we investigated wicking of liquid nitrogen at quasi-isothermal conditions in porous structures. The experiments were performed at saturation temperature in a one species system using the weight measurement technique. The setup configurations enabled to perform experiments with a sample overheat below 1 K. Porosity and permeability were calculated from the liquid nitrogen wicking experiments. The dimensionless representation of the cryogenic wicking experiments shows a good agreement compared to the wicking experiments with storable liquid. It leads to the statement that the combination of macroscopic parameters were obtained correctly for each experiment.

To enable future deep space exploration, orbital refueling of spacecraft is essential. However, transferring liquid in a microgravity environment is a complex process dependent on various factors. One of the basic and critical tasks is to separate phases to allow the supply of gas-free liquid from one tank to another. For this purpose, a liquid acquisition device is essential. In this work, a screen channel liquid acquisition device was designed and used to investigate phase separation and liquid removal from an experiment tank in a microgravity environment. The experiments were performed using the drop tower facility at the University of Bremen, with HFE-7500 as the test liquid under isothermal conditions. This investigation explored the interdependent effects of various phenomena, including the reorientation of liquid in the tank, capillary rise between parallel plates, flow through screen pressure variation, and bubble point breakthrough. Under subcritical conditions, the SC-LAD was found to supply gas-free liquid at the outlet, as long as the pressure drop across the screen was lower than the bubble point threshold. At the critical point, the screen started to ingest bubbles, resulting in a sharp peak in the differential pressure signal. The wetted area of the screen was obtained by analyzing images captured with a high-speed camera and used to calculate the analytical pressure drop. The experimental results were compared with the analytical solution and discussed in detail.

This paper presents open challenges and perspectives of propellant management for crewed deep space exploration. The most promising propellants are liquid hydrogen and liquid methane, together with liquid oxygen as an oxidizer. These fluids remain liquid only at cryogenic conditions, that is, at temperatures lower than 120 K. To extend the duration of space exploration missions, or even to enable them, the storage and refueling from a cryogenic on-orbit depot is necessary. We review reference missions, architectures, and technology demonstrators and explain the main operations that are considered as enablers for cryogenic storage and transfer. We summarize the state of the art for each of them, showing that many gaps in physical knowledge still need to be filled. This paper is based on recommendations originally proposed in a White Paper for ESA’s SciSpacE strategy.

The storage of propellants in space as well as the transfer and filling of spacecraft tanks is a prerequisite for future long-term space exploration missions. In this work, the vented filling of a partially filled tank, which is envisioned as a spacecraft tank, was investigated experimentally under compensated gravity in the Bremen Drop Tower. Experiments were performed with a partially filled tank and a test liquid HFE-7500. The drop tower provides 9 s of compensated gravity. The shape of the free liquid surface inside a right circular cylinder changes from the normal gravity configuration to a free fall configuration during the test. The filling was initiated after 3.5 s and continued until the end at 9 s. The interaction of the incoming liquid jet with the liquid interface was studied for different volumetric flow rates. A stable, but not steady liquid interface was characterized by a deformation due to the incoming liquid jet and the formation of a geyser. The growth of the geyser and the following disintegration into liquid droplets indicated an unstable liquid interface. Subcritical, critical and supercritical regimes of the volumetric flow rates were identified to classify stable and unstable liquid interfaces. The critical Weber number was found to be 1.04, which corresponds to a critical volumetric flow rate of 1.30 mL s -1 . This critical Weber number was compared with the existing literature. Additionally, the behaviour of the liquid interface during the reorientation of the liquid inside the tank was observed.

Launching a spacecraft with an adequate amount of propellant for long-range long-term missions is challenging. Therefore, it is imperative to have a mechanism to fill the propellant tanks in reduced gravity. Vented and no-vent filling are the two methods to fill tanks in reduced gravity. The interface stability during the vented filling of a tank under reduced gravity was investigated by carrying out 2D numerical simulations. Filling a tank partly filled with test liquid and also filling an initially empty tank were simulated for different volumetric inflow rates. Regimes of subcritical, critical, and supercritical flow were observed during the filling process. The critical inlet Weber numbers were determined. Furthermore, the reorientation of the liquid interface in reduced gravity was simulated and the final equilibrium position of the center point was found to be in good agreement with the predicted value. This paper is published with the permission of the authors granted to 3AF - Association Aeronautique et Astronautique de France (www.3AF.fr) organizer of the Space Propulsion International Conference.

Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine Cr2Te3 thin films manifests a unique temperature-dependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional Cr2Te3 epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. This Berry phase effect further introduces hump-shaped Hall peaks in pristine Cr2Te3 near the coercive field during the magnetization switching process, owing to the presence of strain-modulated magnetic domains. The versatile interface tunability of Berry curvature in Cr2Te3 thin films offers new opportunities for topological electronics.

The development of porous ceramic screens with high chemical stability, low density, and thermal conductivity can lead to promising screen channel liquid acquisition devices (SC-LADs) for propellant management under microgravity conditions in the future. Therefore, SiOC screens with aligned pores were fabricated via freeze-casting and applied as a SC-LAD. The pore window sizes and open porosity varied from 6 µm to 43 µm and 65% or 79%, depending on the freezing temperature or the solid loading, respectively. The pore window size distributions and bubble point tests indicate crack-free screens. On the one hand, SC-LADs with an open porosity of 79% removed gas-free liquid up to a volumetric flow rate of 4 mL s−1. On the other hand, SC-LADs with an open porosity of 65% were limited to 2 mL s−1 as the pressure drop across these screens was relatively higher. SC-LADs with the same open porosity but smaller pore window sizes showed a higher pressure drop across the screen and bubble ingestion at higher values of effective screen area when increasing the applied removal volumetric flow rate. The removed liquid from the SC-LADs was particle-free, thus representing a potential for applications in a harsh chemical environment or broad-range temperatures.

Depressurization and re-pressurization maneuvers are used for cryogenic propellants in spacecraft tanks to mitigate the incoming heat flow and to maintain an appropriate net positive suction head (NPSH). A depressurization maneuver can be used to transfer thermal energy from liquid to vapor and thus allow for subcooling of the liquid by a following, quick pressurization. The depressurization will lead to phase change, caused by the superheat relative to the initial saturation conditions, and the following pressurization will subcool the bulk liquid. Phase change will occur at any of the free surfaces of the bulk liquid or at nucleation sites in cavities at a wall. The mechanism which governs phase change at nucleation sites differs from nucleate boiling due to the temperature distribution in the liquid. In this study, we investigate the behavior of a single nucleation site during a depressurization under microgravity. We observed the bubble growth and evaluated the bubble radius with time. A total of five experiments were performed. The thermodynamic conditions are known and therefore can be correlated to the vapor bubble growth behavior. The superheat in the system varies with time, thus simple models are not applicable but can be used to estimate the bubble size. The data can be used as a reference for further analytical or numerical investigations.

We investigated the adsorption of iodine on silver (111) in ultra-high vacuum. Using low-temperature scanning tunneling microscopy (LT-STM) measurements we catalog the complex surface structures on the local scale. We identified three distinct phases with increasing iodine coverage which we tentatively associate with three phases previously reported in LEED experiments (sqrt(3)x sqrt(3)R30, "triangular", "hexagonal"). We used Fourier space and real space analysis to fully characterize each phase. While Fourier analysis most easily connects our measurements to previous LEED studies, the real space inspection reveals local variations in the superstructures of the "hexagonal" and "triangular" phase. The latter, observed here for the first time by LT-STM, stabilized by one or two adatoms sitting at the center of a rosette-like iodine reconstruction. The most stunning discovery is that variation in the adatom separation of the "triangular" phase reconstruct the Ag (111) surface lattice.

Atomically precise, δ-doped structures forming electronic devices in Si have been routinely fabricated in recent years by using depassivation lithography in a scanning tunneling microscope (STM). While H-based precursor/monatomic resist chemistries for incorporation of donor atoms have dominated these efforts, the use of halogen-based chemistries offers a promising path toward atomic-scale manufacturing of acceptor-based devices. Here, B-doped δ-layers were fabricated in Si(100) by using BCl3 as an acceptor dopant precursor in ultrahigh vacuum. Additionally, we demonstrate compatibility of BCl3 with both H and Cl monatomic resists to achieve area-selective deposition on Si. In comparison to bare Si, BCl3 adsorption selectivity ratios for H- and Cl-passivated Si were determined by secondary ion mass spectrometry depth profiling (SIMS) to be 310(10):1 and 1529(5):1, respectively. STM imaging revealed that BCl3 adsorbed readily on bare Si at room temperature, with SIMS measurements indicating a peak B concentration greater than 1.2(1) × 1021 cm-3 with a total areal dose of 1.85(1) × 1014 cm-2 resulting from a 30 langmuir BCl3 dose at 150 °C. In addition, SIMS showed a δ-layer thickness of ∼0.5 nm. Hall bar measurements of a similar sample were performed at 3.0 K, revealing a sheet resistance of ρ□ = 1.9099(4) kΩ □-1, a hole carrier concentration of p = 1.90(2) × 1014 cm-2, and a hole mobility of μ = 38.0(4) cm2 V-1 s-1 without performing an incorporation anneal. Finally, 15 nm wide B δ-doped nanowires were fabricated from BCl3 and were found to exhibit ohmic conduction. This validates the use of BCl3 as a dopant precursor for atomic-precision fabrication of acceptor-doped devices in Si and enables development of simultaneous n- and p-type doped bipolar devices.

B-doped δ-layers were fabricated in Si(100) using BCl3 as a dopant precursor in ultrahigh vacuum. BCl3 adsorbed readily at room temperature, as revealed by scanning tunneling microscopy (STM) imaging. Annealing at elevated temperatures facilitated B incorporation into the Si substrate. Secondary ion mass spectrometry (SIMS) depth profiling demonstrated a peak B concentration > 1.2(1) × 10 cm−3 with a total areal dose of 1.85(1) × 10 cm−2 resulting from a 30 L BCl3 dose at 150 ◦C. Hall bar measurements of a similar sample were performed at 3.0 K revealing a sheet resistance of Rs = 1.91 kΩ −1, a hole concentration of n = 1.90 × 10 cm−2 and a hole mobility of μ = 38.0 cm2V−1s−1 without performing an incorporation anneal. Further, the conductivity of several B-doped δ-layers showed a log dependence on temperature suggestive of a two-dimensional system. Selective-area deposition of BCl3 was also demonstrated using both Hand Cl-based monatomic resists. In comparison to a dosed area on bare Si, adsorption selectivity ratios for H and Cl resists were determined by SIMS to be 310(10):1 and 1529(5):1, respectively, further validating the use of BCl3 as a dopant precursor for atomic precision fabrication of acceptor-doped devices in Si.

Spin-momentum locking in the surface mode of topological insulators leads to the surface accumulations of spin-polarized electrons caused by bias currents through topological insulator samples. It is demonstrated in this letter that surface spin-polarized electron accumulations caused by the above bias currents can be sensed by using scanning tunneling microscopy. The experimental results of this sensing are presented for tin-doped bismuth selenide samples by employing iron-coated tungsten tips as well as nonmagnetic tungsten tips. A linear increase in the spin accumulation as a function of bias current through topological insulator samples is observed.

Phase separation in space is critical for gas-free propellant supply, life support systems, refueling of spacecraft in low earth orbit (LEO), and for deep space exploration missions. In the absence of gravity, the stability of the liquid-gas interface depends on capillary forces. High liquid flow rates, sudden accelerations, and vibrational disturbances can cause the free surface of the liquid to collapse, which results in the ingestion of gas. Propellant tanks may have screen channel liquid acquisition devices (SCLADs) to position and maintain a gas-free propellant supply to the outlet. A saturated porous screen permits liquid to pass through but acts as a barrier to the gas. We investigated phase separation in porous media integrated capillary channels during parabolic flights (33 rd DLR parabolic flight campaign in March 2019). An open side of a rectangular channel was covered with a dutch twill weave 200×1400. The liquid was ingested into the channel from its surroundings by establishing a differential pressure across the screen section. The gas-phase was blocked during the liquid withdrawal. We could show that the gas breakthrough occurs when the pressure difference across the screen exceeds the bubble point pressure. The experimental results showed good agreement with correlations from literature.