arXiv subjects

Thomas Prokscha

Publications and source records attributed to Thomas Prokscha.

At least 19 recordsLinked to original sources

arXiv2026

Doping dependence of local moments in infinite layer nickelates

The infinite layer nickelates are notable for their lack of long-range antiferromagnetic ordering, in contrast to the parent compounds of the superconducting cuprates. Instead, the nickelates show evidence of short-range glassy behavior in both the undoped and optimally-doped regimes, implying that local electronic moments exist independent of superconductivity. However, the systematic doping-dependent magnetic behavior is not yet fully resolved, and characterizing it could uncover the relationship between local moments and the superconducting dome. In this work, we use muon spin rotation ($\mu$SR) on a (La,Sr)NiO$_2$ doping series from the undoped parent compound, through the superconducting dome, to the over-doped normal state (Sr substitution 0% $\leq$ x $\leq$ 25%) to probe the magnetic ground state and the temperature-dependent static and dynamic behavior. We find that local moments experience spin freezing into a glassy state at temperatures on the order of a few tens of kelvin regardless of the doping level. We also observe a subtle destabilization of the glassy state with increased hole doping. These observations suggest that magnetism and superconductivity are largely decoupled phenomena with indirect interactions described in a multi-orbital framework.

cond-mat.supr-con↗

arXiv2026

Modernization and Statistical Validation of a Multilayer TRIM.SP Code for Low-Energy Muon and Ion Implantation

We describe the staged modernization of a local multilayer TRIM.SP implementation, denoted TRIM.SP-NL. The work included behavior-preserving refactoring, correction of localized bookkeeping and initialization defects, support for more than five elements per layer, reproducible build modes, and a runtime-selectable random-number-generator (RNG) interface. The RANLUX RNG remains the default reference backend, while PCG32 and xoshiro256** provide faster alternatives without changing the input-deck format. The code validation combined exact regression testing with statistical comparisons over 13 representative target multilayer configurations, comprising 143 energy-configuration points per backend and 429 simulations in total. Across the test suite, PCG32 and xoshiro256** reduced runtime by factors of 1.98 and 2.01, respectively. The implanted, backscattered, and transmitted fractions, as well as the mean implantation depths, range straggling, and representative depth profiles showed no systematic RNG dependence. Together with a Node.js-based graphical interface for setup, scans, and result collection, the Fortran engine forms the open-source TRIM.SP-NL Workbench.

physics.acc-ph↗

arXiv2026

Muonium dynamics as a probe for depth-resolved properties of 4H-SiC

This study establishes a baseline for muonium (Mu) charge-exchange dynamics in n-type 4H-SiC through a detailed low-energy muon spin rotation (LE-uSR) investigation. Epitaxially grown and ion-implanted samples with nitrogen and phosphorus donors were characterized to assess the effect of carrier concentration and doping method on defect formation. LE-uSR enabled nanometer scale depth profiling of near-surface and implanted regions, revealing variations in charge carrier concentration due to fixed surface charges. The temperature dependence of the diamagnetic fraction and phase provided direct evidence of the Mu0 to Mu- transition, with extracted activation energies consistent with known donor ionization energies. Additionally, high-field uSR was used to analyze the Mu dynamics, and Monte-Carlo simulations to model the Mu0 electron capture process. The simulation results offer a quantitative method to extract free electron concentrations from LE-uSR data, enhancing its capability to characterize the activation of dopants and carrier depth profiles. We demonstrate that LE-uSR is a powerful depth-resolved tool that can provide insights for optimizing the fabrication of reliable SiC devices for power electronics.

cond-mat.mtrl-sci↗

arXiv2026

Secondary electron yield from aluminium-coated foils for muon tagging and beam monitoring up to 60 MeV/c

The feasibility of foil-based muon tagging is investigated in the momentum range below 60 MeV/c, with particular focus on its applicability to the low-momentum range spanning approximately 2.5 MeV/c to 20 MeV/c, where no efficient and minimally invasive detection scheme is currently established for continuous beams. Secondary electron emission from a 7 um Mylar foil coated with 50 nm aluminium is investigated using a continuous negative muon beam with nominal momenta between 12 MeV/c and 60 MeV/c at the piE1 beamline at PSI. The emitted electrons are detected with position-sensitive microchannel plate detectors, enabling particle tagging and spatial characterization of the beam. The detection efficiency and corresponding secondary electron yield are extracted and benchmarked against literature data for protons, showing good agreement and confirming reliable muon tagging. The observed trend for negative muons is consistent with the well-established increase in ion-induced secondary electron emission toward lower particle velocities, suggesting improved performance in the low-momentum regime. A proof-of-principle reconstruction of the muon beam profile is demonstrated by correlating detected electron positions with their emission point at the foil. These results establish foil-based tagging as a viable approach for combined timing and minimally invasive beam monitoring, bridging the gap between high- and low-energy muon instrumentation.

physics.ins-det↗

arXiv2026

High-Resolution Timing for Vertex-Reconstructed Muon-Spin Spectroscopy Using Plastic Scintillators and MuTRiG

Vertex-reconstructed muon-spin spectroscopy (vx-{\mu}SR) based on silicon pixel detectors has recently demonstrated unprecedented lateral resolution and operation at muon stop rates exceeding 400 kHz. However, the intrinsic timing resolution of current silicon pixel detector technology limits the accessible frequency range and restricts {\mu}SR measurements with fast relaxation rates. In this work, we report on the integration of plastic scintillator detectors (PSD) read out with the MuTRiG ASIC into the MuSiP vx-{\mu}SR spectrometer. This complements the spatial resolution achieved by using silicon pixel detectors with high-precision timing information for incoming muons and decay positrons. We demonstrate stable operation of MuTRiG in vacuum and achieve sub-300 ps time resolution after time-walk correction. Standard transverse-field {\mu}SR measurements on a SiO$_2$ sample confirm that the combined MuTRiG-PSD system resolves precession frequencies beyond 50 MHz, far exceeding the capabilities of silicon pixel detectors alone. These results establish a viable and scalable path towards high-rate, high-resolution {\mu}SR with both excellent spatial and temporal performance.

physics.ins-det↗

arXiv2026

A comparison of simulation tools for Muon-Induced X-ray Emission (MIXE) in thin films: a study case with lithium batteries

We present a comparative study of three Monte Carlo simulation frameworks -SRIM, GEANT4, and PHITS- for modeling the transport, stopping, and atomic cascade of negative muons in micrometer-scale, multilayer systems relevant to Muon-Induced X-ray Emission (MIXE) experiments at the Paul Scherrer Institute (PSI). Using a lithium-ion battery as a benchmark target, simulated implantation profiles are compared with experimental data from the GIANT spectrometer. All three codes reproduce the overall muon depth distributions with good consistency, even across sharp density contrasts. SRIM provides reliable implantation estimates for compact geometries, whereas PHITS reproduces GEANT4 results with comparable accuracy and additionally generates muonic X-ray spectra. These spectra, however, exhibit a systematic energy offset in the K-line transitions of medium- and high-Z elements relative to theoretical and experimental values. Despite this bias, PHITS accurately captures relative intensities and spectral shapes, enabling element-specific line identification. The results demonstrate that SRIM and PHITS constitute practical tools for rapid estimation of muon implantation and stopping profiles, and that PHITS holds strong potential for predictive MIXE spectroscopy once its transition-energy bias is corrected.

hep-ex↗

arXiv2025

Net Magnetization and Inhomogeneous Magnetic Order in a High-Tc Nickelate Superconductor

High-temperature and high-magnetic-field-induced re-entrant superconductivity has been discovered in the infinite-layer nickelate $\mathrm{Sm_{1-x-y} Eu_x Ca_y Ni O_2}$ (SECNO). Infinite-layer nickelates are the closest known analogues of high-$\mathrm{T}_c$ cuprate superconductors, yet they host distinct magnetic ground states. Using low-energy muon spin relaxation and polarized neutron reflectometry, we reveal the magnetic order in SECNO. We find that magnetic freezing occurs at a higher-temperature than in other nickelate compounds, and that a substantial net magnetization of 55 $\,\mathrm{kA}\,\mathrm{m}^{-1}$ $\pm10 \,\mathrm{kA}\,\mathrm{m}^{-1}$ emerges and remains largely unchanged across the superconducting transition. The magnetism in SECNO is disordered and nonuniform.

cond-mat.supr-con↗

arXiv2025

Niobium's intrinsic coherence length and penetration depth revisited using low-energy muon spin spectroscopy and secondary-ion mass spectrometry

We report direct, simultaneous measurements of the London penetration depth ($\lambda_L$) and Bardeen-Cooper-Schrieffer (BCS) coherence length ($\xi_0$) in oxygen-doped niobium, with impurity concentrations spanning the "clean" to "dirty" limits. Two depth-resolved techniques - low-energy muon spin spectroscopy (LE-$\mu$SR) and secondary-ion mass spectrometry (SIMS) - were used to quantify the element's Meissner screening profiles, analyzed within a framework that accounts for nonlocal electrodynamics. The analysis indicates intrinsic length scales of $\lambda_L = 29.1(10)$ nm and $\xi_0 = 39.9(25)$ nm, corresponding to a Ginzburg-Landau (GL) parameter of $\kappa = 0.70(5)$. The obtained $\lambda_L$ and $\kappa$ values, accurately quantified at the nanoscale, are smaller than values commonly used in applications and modeling, and indicate that clean niobium lies at the boundary between type-I and type-II superconductivity, supporting the contemporary view that its intrinsic state may be type-I.

cond-mat.supr-con↗

arXiv2025

Surface-Localized Magnetic Order in RuO2 Thin Films Revealed by Low-Energy Muon Probes

Ruthenium dioxide (RuO2) has recently emerged as a candidate altermagnet, yet its intrinsic magnetic ground state, particularly in thin films, remains debated. This study aims to clarify the nature and spatial extent of the magnetic order in RuO2 thin films grown under different conditions. Thin films of RuO2 with thicknesses of 30 nm and 33 nm are fabricated by pulsed laser deposition and sputtering onto TiO2(110) and Al2O3(1-102) substrates, respectively. Low-energy muon spin rotation/relaxation (LE-muSR) with depth-resolved sensitivity measurements is performed in transverse magnetic fields (TF) from 4 K to 290 K. The muSR data collected with a muon implantation energy of 1 keV reveal that magnetic signals originate from the near-surface region of the film (<10 nm), and the affected volume fraction is at most about 8.5%. The localized magnetic response is consistent across different substrates, growth techniques, and parameter sets, suggesting a common origin related to surface defects and dimensionality effects. The combined use of TF-muSR and the study of depth-dependent implantation with low-energy muons provides direct evidence for surface-confined, inhomogeneous static magnetic order in RuO2 thin films, helping reconcile discrepancies. These findings underscore the importance of considering reduced-dimensional contributions and motivate further investigation into the role of defects, strain, and stoichiometry on the magnetic properties of RuO2, especially at the surface.

cond-mat.mes-hall↗

arXiv2025

Microwave Spectroscopy of the Muonium $2S_{1/2}-2P_{3/2}$ Fine Structure Interval

We report a microwave spectroscopy measurement of the muonium $2S_{1/2}-2P_{3/2}$ fine structure transition, yielding a transition frequency of $9871.0 \pm 7.0~\mathrm{MHz}$, in agreement with state-of-the-art QED predictions within one standard deviation. In combination with the recent Lamb shift result, the $2P_{1/2}-2P_{3/2}$ splitting is determined, improving the spectroscopic characterization of the $n=2$ manifold in muonium. These results provide a stringent test of bound-state QED in a purely leptonic system and establish a path toward future searches for Lorentz violation and muon-specific new physics.

physics.atom-ph↗

arXiv2025

Implantation studies of low-energy positive muons in niobium thin films

Here we study the range of keV positive muons $\mu^+$ implanted in Nb$_2$O$_5$($x$ nm)/Nb($y$ nm)/SiO$_2$(300 nm)/Si [$x$ = 3.6 nm, 3.3 nm; $y$ = 42.0 nm, 60.1 nm] thin films using low-energy muon spin spectroscopy (LE-$\mu$SR). At implantation energies 1.3 keV $\leq E \leq$ 23.3 keV, we compare the measured diamagnetic $\mu^+$ signal fraction $f_{\mathrm{dia.}}$ against predictions derived from implantation profile simulations using the TRIM.SP Monte Carlo code. Treating the implanted $\mu^+$ as light protons, we find that simulations making use of updated stopping cross section data are in good agreement with the LE-$\mu$SR measurements, in contrast to parameterizations found in earlier tabulations. Implications for other studies relying on accurate $\mu^+$ stopping information are discussed.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional polaronic ground state in superconducting LiTi$_2$O$_4$

Geometrically frustrated lattices can display a range of correlated phenomena, ranging from spin frustration and charge order to dispersionless flat bands due to quantum interference. One particularly compelling family of such materials is the half-valence spinel Li$B_2$O$_4$ materials. On the $B$-site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi$_2$O$_4$ and heavy fermion behavior in LiV$_2$O$_4$. To date, however, LiTi$_2$O$_4$ has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic groundstates. Here, we present a multimodal investigation of LiTi$_2$O$_4$, combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body theoretical calculations. Our data reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations also found in the lightly doped cuprates. The cooperation between the two interaction scales distinguishes LiTi$_2$O$_4$ from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi$_2$O$_4$. Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems. In particular, our work identifies the geometrically frustrated, mixed-valence spinel family as an under-explored platform for discovering unconventional, correlated ground states.

cond-mat.supr-con↗

arXiv2025

New Frontiers in Muon-Spin Spectroscopy Using Si-Pixel Detectors

The study of novel quantum materials relies on muon-spin rotation, relaxation, or resonance (\mSR) measurements. Yet, a fundamental limitation persists: many of these materials can only be synthesized in extremely small quantities, often at sub-millimeter scales. While \mSR ~offers unique insights into electronic and magnetic properties, existing spectrometers lack a sub-millimeter spatial resolution and the possibility of triggerless pump-probe data acquisition, which would enable more advanced measurements. The General Purpose Surface-muon instrument (GPS) at the Paul Scherrer Institute (PSI) is currently limited to a muon stopping rate of \SI{40}{\kilo\hertz} to \SI{120}{\kilo\hertz}, a constraint that will become more pressing with the upcoming High-Intensity Muon Beam (HIMB) project. To overcome these challenges, we demonstrate the feasibility of employing ultra-thin monolithic Si-pixel detectors to reconstruct the stopping position of muons within the sample, thereby significantly enhancing the capability of measuring at higher muon rate. Additionally, we explore the first steps toward a triggerless pump-probe \mSR ~measurement scheme. Unlike conventional pump-probe techniques that require external triggers, a triggerless readout system can continuously integrate stimuli pulses into the data stream, allowing real-time tracking of ultra-fast dynamics in quantum materials. This approach will enable the study of transient states, spin dynamics, and quantum coherence under external stimuli.

physics.ins-det↗

arXiv2025

Coherent microwave control of coupled electron-muon centers

Coherent control by means of tailored excitation is a key to versatile experimental schemes for spectroscopic investigation and technological utilization of quantum systems. Here we study a quantum system which consists of a coupled electron-moun spin state, i.e., muonium, a light isotope of hydrogen. We demonstrate the most fundamental coherent control techniques by microwave excitation of spin transitions, namely driven Rabi oscillations and Ramsey fringes upon free evolution. Unprecedented performance is achieved by the microwave hardware devised for these experiments, which enables coherent spin manipulation of individual, isolated, muonium centers. For muonium formed in SiO$_2$ with strong electron-muon hyperfine interaction, a virtually undamped free precession signal is observed up to a 3.5 $\mu$s time window. For muonium formed in Si with weak and anisotropic hyperfine interaction, a strong drive at the multi-quantum transition decouples the muonium center from its magnetic environment formed by the bath of $^{29}$Si nuclear spins at natural abundance. We expect that these capabilities will provide a powerful tool to investigate the effect of the environment on isolated coupled spins, uncover the details of coupled electron-muon systems in matter and validate quantum electrodynamics in the context of muonium spectroscopy.

quant-ph↗

arXiv2025

Complex Magnetic Ordering in Candidate Topological Superconductors

The search for chiral topological superconductivity in magnetic topological insulator (TI)-FeTe heterostructures is a key frontier in condensed matter physics, with potential applications in topological quantum computing. The combination of ferromagnetism, superconductivity, and topologically nontrivial surface states brings together the key elements required for chiral Majorana physics. In this work, we examine the interplay between magnetism and superconductivity at the interfaces between FeTe and a series of TI overlayers. In superconducting MnBi$_2$Te$_4$/FeTe, any interfacial suppression of antiferromagnetism must affect at most a few nanometers. On the other hand, (Bi,Sb)$_2$Te$_3$/FeTe layers exhibit near-total suppression of antiferromagnetic ordering. Ferromagnetic Cr$_x$(Bi,Sb)$_{2-x}$Te$_3$ (CBST)/FeTe bilayers exhibit net magnetization in both CBST and FeTe layers, with evidence of interactions between superconductivity and ferromagnetism. These observations identify magnetic TI/FeTe interfaces as an exceptionally robust platform to realize chiral topological superconductivity.

cond-mat.supr-con↗

arXiv2025

Advanced muon-spin spectroscopy with high lateral resolution using Si-pixel detectors

Muon-spin spectroscopy at continuous sources has stagnated at a stopped muons rate of ~40 kHz for the last few decades. The major limiting factor is the requirement of a single muon in the sample during the typical 10 {\mu}s data gate window. To overcome this limit and to be able to perform muon-spin relaxation ({\mu}SR) measurements on millimeter-sized samples, one can use vertex reconstruction methods to construct {\mu}SR spectra. This is now possible thanks to the availability of very thin monolithic Si-pixel chips, which offer minimal particle scattering and high count rate. Here we present results from a Si-pixel based spectrometer that utilizes vertex reconstruction schemes for the incoming muons and emitted positrons. With this spectrometer we were able to obtain a first vertex reconstructed {\mu}SR (VR-{\mu}SR) spectrum. The unique capabilities and benefits of such a spectrometer are discussed.

physics.ins-det↗

arXiv2025

Monitoring the tagging efficiency of the Low-Energy Muon beamline through background analysis: Insights into the long-term performance of ultrathin carbon foils

The efficient tagging of individual muons, along with the stability of the involved ultrathin carbon foil, is critical for ensuring fast, reliable and reproducible low-energy muon spin rotation (LE-$\mathrm{\mu}$SR) measurements. At the Paul Scherrer Institute's Low-Energy Muon (LEM) beamline, we developed a method to monitor the tagging efficiency of the beamline using routinely collected muon decay histograms. This method leverages background comparison before and after the arrival time of the muons at the sample to extract the tagging efficiency, eliminating the need for additional detectors or measurements. By analyzing data collected between 2018 and 2024, we confirm the method's reliability and validate its results using independent reference measurements. Furthermore, we establish a correlation between the tagging efficiency and foil thickness by investigating the impact of contamination and outgassing on the carbon foil, as well as the restoration effects of laser cleaning. The findings underscore the importance of monitoring the carbon foil's condition to maintain consistent beamline performance and reproducibility. This method offers a practical approach for detector efficiency monitoring, applicable to other beamlines employing similar setups.

physics.ins-det↗

arXiv2025

Persistent Uncorrelated Magnetic Domains in Fe/Si Multilayers and their suppression by incorporating 11B4C

This study investigates magnetic domains in Fe/Si and Fe/Si + B4C multilayers using spin flip off-specular polarized neutron reflectometry. The results show that Fe/Si multilayers exhibit pronounced spin flip off-specular scattering originating from magnetic domains that are uncorrelated out of plane. With increasing external magnetic field the domains progressively coalesce and their magnetization rotates toward alignment with the applied field, approaching a homogeneous magnetic state at higher fields. In contrast, Fe/Si + B4C multilayers exhibit no detectable spin flip off-specular scattering already at low fields, indicating that the multilayer reaches magnetic saturation at significantly lower applied fields. The scattering patterns are interpreted using distorted wave Born approximation simulations in BornAgain, enabled by our added code for simulating magnetic domains and magnetic ordering. To further probe the magnetic behavior, low-energy mu+SR measurements were performed, representing the first mu+SR investigation of polarizing neutron optics multilayers. Together with comparison to previously reported VSM data, these measurements provide insight into the magnetic behavior across short range, medium range, and long range length scales. The results show that incorporating approximately 15 vol.% B4C makes the magnetic configuration highly responsive to external magnetic fields, with clear sensitivity to both in-plane and out-of-plane field geometries. These results show that B4C suppresses magnetic domains and spin flip off-specular scattering, improving Fe/Si coatings for neutron polarization optics in regards to off-specular scattering, and other applications requiring easy magnetic manipulation.

cond-mat.mtrl-sci↗