arXiv subjects

Andreas Schilling

Publications and source records attributed to Andreas Schilling.

At least 19 recordsLinked to original sources

arXiv2026

Differences in Detection: Explainability Where it Matters

We propose Differences in Detection (DnD), an intuitive method to compare two object detection models. Based on the same matching algorithm, it complements the standard metrics of mean Average Precision ($mAP$) and TIDE error analysis with the ability to compare two models directly. More specifically, we calculate the intersection of ground truth labels that are recognized by both models, followed by the corresponding difference sets and the complement set of ground truth labels that are missed by both models. The resulting comparison is more direct and intuitive than a comparison of independent summary statistics. It reveals individual and shared mistakes and becomes particularly interesting when combined with error types. In this case, the differences in detection errors can be analyzed naturally in a standard confusion matrix. While valuable in itself, we believe that one of the best applications of DnD is to guide explainability methods such as ODAM towards metric-relevant examples, grounded in structured subsets. The code for our method is available here: https://github.com/JohannesTheo/differences-in-detection

cs.CV↗

arXiv2026

Multiple Magnetic Transitions in the Trilayer Nickelate Pr$_4$Ni$_3$O$_{10}$ Revealed by Muon-Spin Rotation

A muon-spin rotation/relaxation ($\mu$SR) study of the trilayer Ruddlesden--Popper nickelate Pr$_4$Ni$_3$O$_{10}$ was performed at ambient pressure and under hydrostatic pressure up to 2.2 GPa. Three magnetic transitions were identified at ambient pressure: the onset of spin-density-wave (SDW) order at $T_{\rm SDW} \simeq 158$ K, an intermediate-temperature transition at $T^{\ast} \simeq 90$--100 K, and a low-temperature transition at $T_{\rm SDW}^{\rm Pr} \simeq 25$--27 K. While the intermediate transition at $T^{\ast}$ induces only minor changes in the internal-field distribution, the transition at $T_{\rm SDW}^{\rm Pr}$ is accompanied by a pronounced reconstruction of the magnetic structure, consistent with previous reports attributing enhanced interlayer coherence to the ordering of the Pr sublattice. The high-temperature transition at $T_{\rm SDW}$ is characterized by the sharp development of static internal magnetic fields with a narrow transition width of $0.65(4)$ K. Weak-transverse-field measurements reveal a finite thermal hysteresis of $0.27(6)$ K, with $T_{\rm SDW}^{\rm warming} > T_{\rm SDW}^{\rm cooling}$, indicating weakly first-order-like behavior. Hydrostatic pressure suppresses $T_{\rm SDW}$ linearly and reduces the ordered Ni magnetic moment $M$, with corresponding rates of ${\rm d}T_{\rm SDW}/{\rm d}p = -4.9(1)$ K/GPa and ${\rm d}\ln M/{\rm d}p = -2.0(5)\times10^{-2}$ GPa$^{-1}$, respectively, thereby demonstrating a gradual weakening of the spin-density-wave instability under compression.

cond-mat.supr-con↗

arXiv2025

Magnetic-field dependent vortex dynamics and critical currents in superconducting microwires with regular large-area perforation by pinholes

We report on results of simulations and experiments of vortex states in superconducting micro-wires with periodic rectangular pinhole structures. The simulations have been performed by means of numerically solving the time-dependent Ginzburg-Landau (TDGL) equations. With increasing bias current and for different values of the external magnetic field applied normal to the structure plane, we observe at first a vortex free Meissner state, followed by a resistive vortex-flow mixed state and a state with a more complex vortex pattern. The resulting dependence of the critical current Ic on magnetic field exhibits two plateaus with distinctly different vortex dynamics. Corresponding experimentally measured magnetic-field dependences of Ic of WSi microwires with periodic pinhole structures and varying hole spacing confirmed the predictions of these simulations, showing two ranges of magnetic field with almost field-independent critical currents. The experimentally determined critical currents are larger for a smaller pinhole spacing, in agreement with the results of the TDGL simulations. The good agreement of the simulations with the experimental results provides a convenient strategy for the optimization of single-photon detectors with or without artificial and natural defects.

cond-mat.supr-con↗

arXiv2025

The physics of diamagnetic levitation

We formulate a problem on diamagnetic levitation, which may be suitable for specialized high-school or first-year students in physical sciences. We guide the students, step-by-step, through the physics of diamagnetic levitation. The calculations are simplified by assuming a ring-shaped geometry of the diamagnetic object residing above a magnetic dipole. This problem was originally intended for the International Physics Olympiad 2016 (IPHO 2016), but was finally deemed surplus and therefore not set.

physics.ed-ph↗

arXiv2024

First Sub-MeV Dark Matter Search with the QROCODILE Experiment Using Superconducting Nanowire Single-Photon Detectors

We present the first results from the Quantum Resolution-Optimized Cryogenic Observatory for Dark matter Incident at Low Energy (QROCODILE). The QROCODILE experiment uses a microwire-based superconducting nanowire single-photon detector (SNSPD) as a target and sensor for dark matter scattering and absorption, and is sensitive to energy deposits as low as 0.11 eV. We introduce the experimental configuration and report new world-leading constraints on the interactions of sub-MeV dark matter particles with masses as low as 30 keV. The thin-layer geometry of the system provides anisotropy in the interaction rate, enabling directional sensitivity. In addition, we leverage the coupling between phonons and quasiparticles in the detector to simultaneously constrain interactions with both electrons and nucleons. We discuss the potential for improvements to both the energy threshold and effective volume of the experiment in the coming years.

hep-ph↗

arXiv2024

Probing enhanced superconductivity in van der Waals polytypes of V$_x$TaS$_2$

Layered transition metal dichalcogenides (TMDs) stabilize in multiple structural forms with profoundly distinct and exotic electronic phases. Interfacing different layer types is a promising route to manipulate TMDs' properties, not only as a means to engineer quantum devices, but also as a route to explore fundamental physics in complex matter. Here we use angle-resolved photoemission (ARPES) to investigate a strong layering-dependent enhancement of superconductivity in TaS$_2$, in which the superconducting transition temperature, $T_c$, of its $2H$ structural phase is nearly tripled when insulating $1T$ layers are inserted into the system. The study is facilitated by a novel vanadium-intercalation approach to synthesizing various TaS$_2$ polytypes, which improves the quality of ARPES data while leaving key aspects of the electronic structure and properties intact. The spectra show the clear opening of the charge density wave gap in the pure $2H$ phase and its suppression when $1T$ layers are introduced to the system. Moreover, in the mixed-layer $4Hb$ system, we observe a strongly momentum-anisotropic increase in electron-phonon coupling near the Fermi level relative to the $2H$ phase. Both phenomena help to account for the increased $T_c$ in mixed $2H$/$1T$ layer structures.

cond-mat.supr-con↗

arXiv2024

Energy conservation and reversibility during thermodynamic changes of state in superconductors: Joule heat vs. magnetocaloric cooling

In the Meissner phase of a superconductor, an external constant magnetic field is shielded by circulating persistent zero-resistance supercurrents that are formed by Cooper pairs. However, a thermodynamic change of state within this phase, such as cooling or heating, inevitably generates normal currents of thermally excited unpaired charge carriers, induced by the time-dependent variations in the local magnetic field. They not only lead to deviations of the magnetic-field distribution from textbook Meissner profiles but also cause dissipative Joule heating. This sharply contradicts the expected reversibility of a truly thermodynamic superconducting state, a fact that has largely been overlooked in the literature. We show that these normal currents also produce a magnetocaloric cooling, which in total instantaneously and precisely compensates for the dissipated heat, thus ensuring overall energy conservation and reversibility. However, the Joule heating and magnetocaloric cooling processes are spatially distinct and should therefore lead to temperature inhomogeneities. We quantify these effects assuming realistic material parameters and conclude that they are challenging to measure with current experimental techniques. Significant temperature gradients are expected only directly at the first-order transition to the superconducting state, where the discontinuous flux expulsion should induce normal currents that are much larger than those deep in the Meissner phase. We also argue that the underlying physics in superconductors is fundamentally identical to that of thermomagnetic generators, where electromechanical work can be extracted from magnetized matter subjected to thermal cycles, and where the magnetocaloric cooling is balanced by the heat supplied from an external thermal reservoir.

cond-mat.supr-con↗

arXiv2024

Trapped in texture bias? A large scale comparison of deep instance segmentation

Do deep learning models for instance segmentation generalize to novel objects in a systematic way? For classification, such behavior has been questioned. In this study, we aim to understand if certain design decisions such as framework, architecture or pre-training contribute to the semantic understanding of instance segmentation. To answer this question, we consider a special case of robustness and compare pre-trained models on a challenging benchmark for object-centric, out-of-distribution texture. We do not introduce another method in this work. Instead, we take a step back and evaluate a broad range of existing literature. This includes Cascade and Mask R-CNN, Swin Transformer, BMask, YOLACT(++), DETR, BCNet, SOTR and SOLOv2. We find that YOLACT++, SOTR and SOLOv2 are significantly more robust to out-of-distribution texture than other frameworks. In addition, we show that deeper and dynamic architectures improve robustness whereas training schedules, data augmentation and pre-training have only a minor impact. In summary we evaluate 68 models on 61 versions of MS COCO for a total of 4148 evaluations.

cs.CV↗

arXiv2023

Effective suppression of dark counts in superconducting microstructures with grid of pinholes in a magnetic field

In a magnetic field, vortices significantly contribute to the dark counts of single-photon detectors made of superconducting wires, and they are also limiting the critical current of such devices. To address this issue, we prepared superconducting microwires with a pinhole grid from WSi thin films and report on corresponding critical-current and count-rate measurements in an external magnetic field B. When compared to corresponding devices without pinholes, the critical current only weakly depends on the magnetic field at B < 16 mT and it is even larger already at B > 10 mT. Moreover, dark counts are not only suppressed in zero field, but particularly in magnetic fields B < 16 mT, while photon counts are virtually field insensitive in the same range of the magnetic field.

cond-mat.supr-con↗

arXiv2023

Taking a step back and looking at the superconducting dome from a distance

In my short essay in honor of Karl-Alex Mueller, I would like to deal with some aspects that have been on my mind since the beginning of my own research. Some of the facts and questions mentioned here have been known or clarified for a long time, others not. For example, I think that the fact that superconductivity occurs almost spontaneously in many cuprates close to the optimum doping level has been insufficiently (or not at all) investigated. Much research has been done in the regions outside the optimal doping, be it underdoped or overdoped. I would like to deviate from the "mainstream" for a moment and ask whether these regions are really so relevant for the occurrence of superconductivity, or whether they simply serve to interfere with it - a thought which I am sure has been expressed by others and which would certainly lead (or had already led) to lively discussions with Karl-Alex Mueller.

cond-mat.supr-con↗

arXiv2023

Competing spin-glass and spin-fluctuation states in NdxPr4-xNi3O8

Neodymium nickelates have attracted research interest due to their strongly correlated behaviour and remarkable magnetic properties. More importantly, superconductivity has recently been confirmed in thin-film samples of Sr-doped NdNiO2, bringing the layered rare earth nickel oxides into the research spotlight. In this report, we present results on a series of NdNiO2 analogues, NdxPr4-xNi3O8 (x = 0.1, 0.25, 1, 2, and 4) obtained by topotactic reduction, in which we observe systematic changes in the magnetic behaviour. As the Nd3+ content increases, the initially large spin-freezing region with magnetic frustration becomes smaller and gradually shifts to low temperatures, while the magnetic response gradually increases. The muon-spin spectroscopy measurements on Nd4Ni3O8 show that this phenomenon is likely due to the enhancement of spin fluctuations in NdxPr4-xNi3O8, which weakens the spin frustration behaviour for high Nd3+ contents and at low temperatures. These spin fluctuations can be caused by both Nd and Ni ions and could be one of the factors determining the occurrence of possible superconductivity.

cond-mat.supr-con↗

arXiv2023

Superconductivity in hydrated Lix(H2O)yTaS2

We have systematically studied the structural and physical properties of the superconducting hydrated Lix(H2O)yTaS2. The powder X-ray diffraction patterns suggest that all the samples are single-phase compounds, and the crystal structure is similar to that of 2H-TaS2. The transition temperature to superconductivity shows a dome-shape dependence on the lithium content x with a maximum Tc of 4.6 K for x=0.42, which is larger than in corresponding optimally doped 2H-TaS2 superconductors without water or organic intercalants (Tc ~ 4.2 K). There are no signs for a charge-density-wave formation in hydrated Lix(H2O)yTaS2. While our magnetic data indicate a rather strongly type-II behavior, heat-capacity measurements reveal, like in other 2H-TaS2-type compounds. From the corresponding Sommerfeld constants and Debye temperatures, we can derive the parameter describing the electron-phonon coupling and the electron density of states DOS(EF) at the Fermi level as functions of x. While the variation of the DOS(EF) is consistent with that of Tc, indicating that the lithium intercalation is tuning Tc via changing the DOS(EF) in 2H-Lix(H2O)yTaS2, the simultaneous changes of electron-phonon coupling and Debye temperatures may also play a certain role.

cond-mat.supr-con↗

arXiv2021

Universal spin-glass behaviour in bulk LaNiO2, PrNiO2 and NdNiO2

Motivated by the recent discovery of superconductivity in infinite-layer nickelate thin films, we report on a synthesis and magnetization study on bulk samples of the parent compounds ${R}$NiO$_{2}$ (${R}$=La, Pr, Nd). The frequency-dependent peaks of the AC magnetic susceptibility, along with remarkable memory effects, characterize spin-glass states. Furthermore, various phenomenological parameters via different spin glass models show strong similarity within these three compounds as well as with other rare-earth metal nickelates. The universal spin-glass behaviour distinguishes the nickelates from the parent compound CaCuO$_{2}$ of cuprate superconductors, which has the same crystal structure and $d^9$ electronic configuration but undergoes a long-range antiferromagnetic order. Our investigations may indicate a distinctly different nature of magnetism and superconductivity in the bulk nickelates than in the cuprates.

cond-mat.supr-con↗

arXiv2021

Physical properties of amorphous molybdenum silicide films for single photon detectors

We systematically investigated the physical properties of amorphous Mo$_{\rm x}$Si$_{1-x}$ films deposited by the magnetron co-sputtering technique. The critical temperature $T_C$ of Mo$_{\rm x}$ Si$_{1-x}$ films increases gradually with the stoichiometry x, and the highest $T_C$=7.9 K was found in Mo$_{\rm 0.83}$ Si$_{0.17}$. Beyond $x$=0.83, preformed Cooper pairs and superconducting domains persist in the films, despite the superconducting state with perfect zero-resistivity is absent. The thick films of Mo$_{\rm 0.83}$ Si$_{0.17}$ show surprising degradation in which the onset of zero-resistivity is suppressed below 2 K. The thin Mo$_{\rm 0.83}$ Si$_{0.17}$ films, however, reveal robust superconductivity even with thickness d$\leq$1 nm. We also characterized wide microwires based on the 2 nm thin Mo$_{\rm 0.8}$ Si$_{0.2}$ films with widths 40 and 60 $μ$m, which show single-photon sensitivity at 780 nm and 1550 nm wavelength

cond-mat.supr-con↗

arXiv2021

Neutron diffraction of field-induced magnon condensation in the spin-dimerized antiferromagnet Sr$_{3}$Cr$_{2}$O$_{8}$

In this work, we investigate the evolution and settling of magnon condensation in the spin-1/2 dimer system Sr$_{3}$Cr$_{2}$O$_{8}$ using a combination of magnetostriction in pulsed fields and inelastic neutron scattering in a continuous magnetic field. The magnetic structure in the Bose-Einstein condensation (BEC) phase was probed by neutron diffraction in pulsed magnetic fields up to 39~T. The magnetic structure in this phase was confirmed to be an XY-antiferromagnetic structure validated by irreducible representational analysis. The magnetic phase diagram as a function of an applied magnetic field for this system is presented. Furthermore, zero-field neutron diffraction results indicate that dimerization plays an important role in stabilizing the low-temperature crystal structure.

cond-mat.str-el↗

arXiv2021

Size dependent nature of the magnetic-field driven superconductor-to-insulator quantum-phase transitions

The nature of the magnetic-field driven superconductor-to-insulator quantum-phase transition in two-dimensional systems at zero temperature has been under debate since the 1980s, and became even more controversial after the observation of a quantum-Griffiths singularity. Whether it is induced by quantum fluctuations of the superconducting phase and the localization of Cooper pairs, or is directly driven by depairing of these pairs, remains an open question. We herein experimentally demonstrate that in weakly-pinning systems and in the limit of infinitely wide films, a sequential superconductor-to-Bose insulator-to-Fermi insulator quantum-phase transition takes place. By limiting their size smaller than the effective penetration depth, however, the vortex interaction alters, and the superconducting state re-enters the Bose-insulating state. As a consequence, one observes a direct superconductor-to-Fermi insulator in the zero-temperature limit. In narrow films, the associated critical-exponent products diverge along the corresponding phase boundaries with increasing magnetic field, which is a hallmark of the quantum-Griffiths singularity.

cond-mat.supr-con↗

arXiv2021

Superconductivity and charge-density wave formation in lithium intercalated 2H-LixTaS2

We systematically investigated the superconducting properties and the interplay between charge-density-waves (CDW) and superconductivity in lithium-intercalated 2H-TaS2. By gradually increasing the lithium content x, the CDW formation temperature is continuously suppressed, and the onset temperature of superconductivity is increased with a maximum transition temperature Tc = 3.5 K for x = 0.096. The bulk nature of superconductivity is confirmed by a superconducting shielding fraction of the order of unity for this composition. The electronic contribution to the specific heat and Hall resistivity data demonstrate that the CDW weakens with lithium-intercalation, thereby indirectly increasing carrier density and boosting superconductivity. While the sign of the charge carriers in undoped 2H-TaS2 changes from electron-like to hole type near the CDW formation temperature around 75 K, the lithium intercalated LixTaS2 show predominantly hole-type carriers in the CDW phase even for very low lithium contents.

cond-mat.supr-con↗

arXiv2021

Two-carrier Magnetoresistance: Applications to Ca$_3$Ru$_2$O$_7$

Ambipolar transport is a commonly occurring theme in semimetals and semiconductors. Here we present an analytical formulation of the conductivity for a two-band system. Electron and hole carrier densities and their respective conductivities are mapped into a two-dimensional unit-less phase space. Provided that one of the carrier densities is known, the dimensionless phase space can be probed through magnetoresistance measurements. This formulation of the two-band model for conductivity is applied to magnetoresistance experiments on Ca$_3$Ru$_2$O$_7$. While previous such measurements focused on the low-temperature limit, we cover a broad temperature range and find negative magnetoresistance in an intermediate interval below the electronic transition at 48 K. The low-temperature magnetoresistance in Ca$_3$Ru$_2$O$_7$ is consistent with a two-band structure. However, the model fails to describe the full temperature and magnetic field dependence. Negative magnetoresistance found in an intermediate temperature range is, for example, not captured by this model. We thus conclude that the electronic and magnetic structure in this intermediate temperature range render the system beyond the most simple two-band model.

cond-mat.str-el↗