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 LiB_2O_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_2O_4 and heavy fermion behavior in LiV_2O_4. To date, however, LiTi_2O_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_2O_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_2O_4 from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi_2O_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.

ABSTRACT We report the dielectric and magnetic properties of epitaxial thin films of the high entropy oxide (HEO) perovskite Nd(Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Ni 0.2 )O 3, which orders magnetically below T mag ≈190 K. At T ≫ T mag , the dielectric response reveals a Debye‐type frequency dependence with a zero‐frequency dielectric constant of ≈230–250. The dc bias voltage loops of are reversible but exhibit three distinct peaks centred at zero and finite positive and negative voltage. We provide evidence that the zero‐bias peak is governed by the oxygen sublattice while the finite bias peaks originate from cationic dipoles. The maximal response of the latter appears to be shifted to finite bias by a static uncompensated electric field due to a vertical gradient of the oxygen content. Below T mag , this anomalous dielectric response is strongly suppressed, presumably by magnetostriction that counteracts and freezes the ionic displacements. These findings indicate a unique correlation between configurational entropy, dielectric response, and magnetic properties. In combination with a large dielectric strength, it enables a non‐hysteretic tuning of the dielectric response of magnetoelectronic devices with multiple parameters like temperature, electric, and magnetic field. This HEO is equally interesting for fundamental studies of competing electric and magnetic orders in strongly disordered materials.

Here we study the range of keV positive muons mu+ implanted in Nb2O5(xnm)/Nb(ynm)/SiO2(300nm)/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 <= E <= 23.3 keV, we compare the measured diamagnetic mu+ signal fraction fdia. against predictions derived from implantation profile simulations using the TRIM.SP Monte Carlo code. Treating the implanted mu+ as light protons p+, 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.

Muon-spin spectroscopy at continuous sources has stagnated at a stopped muon rate of ∼40kHz for the last few decades. The major limiting factor is the requirement of a single muon in the sample during the typical 10µs data gate window. To overcome this limit and to be able to perform muon-spin relaxation/rotation (µSR) measurements on millimeter-sized samples, one can use vertex reconstruction methods to construct µSR 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 (vx-μSR) spectrum. The unique capabilities and benefits of such a vx-µSR spectrometer are discussed.

We report the dielectric and magnetic properties of epitaxial thin films of the high entropy oxide (HEO) perovskite Nd(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3, which orders magnetically below T mag approximate to 190 K. At T >> T mag, the dielectric response reveals a Debye-type frequency dependence with a zero-frequency dielectric constant of approximate to 230-250. The dc bias voltage loops of are reversible but exhibit three distinct peaks centred at zero and finite positive and negative voltage. We provide evidence that the zero-bias peak is governed by the oxygen sublattice while the finite bias peaks originate from cationic dipoles. The maximal response of the latter appears to be shifted to finite bias by a static uncompensated electric field due to a vertical gradient of the oxygen content. Below T mag, this anomalous dielectric response is strongly suppressed, presumably by magnetostriction that counteracts and freezes the ionic displacements. These findings indicate a unique correlation between configurational entropy, dielectric response, and magnetic properties. In combination with a large dielectric strength, it enables a non-hysteretic tuning of the dielectric response of magnetoelectronic devices with multiple parameters like temperature, electric, and magnetic field. This HEO is equally interesting for fundamental studies of competing electric and magnetic orders in strongly disordered materials.

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 (μSR) 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

Chemical disorder in compositionally complex perovskite oxides generates a broad distribution of exchange pathways and spin states, but the microscopic origin and spatial homogeneity of the resulting magnetic phases remain debated. Here, we tune the Mn fraction (x = 0.2-0.6) in epitaxial La(Cr, Mn, Fe, Co, Ni)O-3 thin films and resolve the coupled evolution of valence, spin state, and magnetism using element-specific x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD). Mn enrichment drives an internal redistribution of charge, in which Mn evolves toward a Mn3+-rich mixed valence, while Co converts from predominantly Co3+ to high-spin Co2+. This valence/spin-state coupling amplifies the Mn- and Co-derived ferromagnetic response by nearly an order of magnitude while increasing the magnetic onset temperature to at least 250 K, whereas Fe and Cr remain essentially trivalent with weak dichroism. Depth-resolved low-energy muon spin spectroscopy (LE-mu SR) shows magnetic homogeneity through the film thickness, with a secondary relaxation maximum near 25 K indicating a low-temperature dynamical crossover consistent with frustrated magnetism in a strongly disordered spin lattice.

Abstract Repeated recharging of lithium-ion batteries can lead to the deposition of metallic lithium ( m -Li) on the anode surface, which may trigger thermal runaway and degrade battery safety. However, in situ visualization of m -Li remains challenging with existing techniques. Inside batteries, not only metallic lithium but also salt-state lithium ( s -Li) and carrier lithium ( c -Li) are present near the anode surface. Therefore, to quantitatively analyze m -Li, it is essential to selectively detect metallic lithium while excluding signals from these other lithium species. To address this issue, we are developing a novel in situ imaging method for metallic lithium based on muonic X-ray analysis. In this method, both s -Li and c -Li can contribute to the lithium-derived background (LDB). While the LDB from c -Li has been previously reported, that from s -Li has not yet been evaluated. As a fundamental study, we performed muonic X-ray measurements at the Paul Scherrer Institute (PSI) in Villigen, Switzerland, comparing the muonic X-ray intensities of m -Li and s -Li compounds such as LiF and Li 2 CO 3 . Our results show that the muonic X-ray intensity of s -Li is about one-fifteenth that of m -Li, and its LDB contribution is less than 2% of that from c -Li in the anode.

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.

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 stopping depth 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 stopping depth 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 stopping depth and stopping profiles, and that PHITS holds strong potential for predictive MIXE spectroscopy once its transition-energy bias is corrected.

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.

Ruthenium dioxide (RuO2) has recently emerged as an altermagnetic candidate, but its intrinsic magnetic ground state in thin films remains widely 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 and 33 nm are deposited by pulsed laser deposition and sputtering onto TiO2(110) and Al2O3(1¯102) substrates, respectively. Low-energy muon spin rotation/relaxation (LE-μSR) with depth-resolved sensitivity measurements is performed in transverse magnetic fields (TF) from 4 K to 290 K. The μSR 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 approximately 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-μSR 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.

We report direct, simultaneous measurements of the London penetration depth (AL) and Bardeen-CooperSchrieffer coherence length ( 0) in oxygen-doped niobium, with impurity concentrations spanning the "clean" to "dirty" limits. Two depth-resolved techniques-low-energy muon spin spectroscopy and secondary-ion mass spectrometry-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 AL = 29.1(10) nm and 0 = 39.9(25) nm, corresponding to a Ginzburg-Landau parameter kappa = 0.70(5). The obtained AL and kappa values, accurately quantified at the nanoscale, are smaller than those 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.

Vertex-reconstructed muon-spin spectroscopy (vx-& micro;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 & micro;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-& micro;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 & micro;SR measurements on a SiO2 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 & micro;SR with both excellent spatial and temporal performance.

Abstract LaAlO 3 (LAO) is, alongside SrTiO 3 (STO), and sapphire, one of the most commonly used substrates for complex metal-oxide thin film growth. LAO plays a crucial role in inducing interesting physical properties, such as in LaNiO 3 /LAO superlattices and interface superconductivity at STO/LAO interfaces. While μ SR studies have characterized STO, sapphire, and LSAT regarding Muonium formation - a critical factor when quantifying thin film LE- μ SR results - surprisingly little is known about Muonium formation in LAO. In this work, we present comprehensive bulk and low-energy μ SR measurements revealing the energy and temperature dependence of different muon charge states in LAO, effectively addressing this significant knowledge gap in the characterization of this important substrate material.

We report a microwave spectroscopy measurement of the muonium 2S1/2-2P3/2 fine-structure transition, yielding a transition frequency of 9871.0 +/- 7.0 MHz, in agreement with state-of-the-art QED predictions within one standard deviation. In combination with the recent Lamb shift result, the 2P1/2-2P3/2 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.

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.

We investigated the Cu-spin correlation in the overdoped regime of the electron-doped high-Tc cuprate thin films of La2-xCexCuO4, changing the reduction condition from muon spin relaxation using low-energy muons. The Cu-spin correlation developed at low temperatures for optimally reduced films with x = 0.13 as well as x = 0.17 where the superconductivity was almost suppressed. These results are contrary to those observed in the hole-doped high-Tc cuprates where the development of the antiferromagnetic Cu-spin correlation disappears together with the suppression of superconductivity. The Cu-spin correlation developed at low temperatures in x = 0.17 may be understood in terms of antiferromagnetism, but it may be related to a ferromagnetic order recently suggested in the nonsuperconducting heavily overdoped La2-xCexCuO4 with x similar to 0.18.

Epitaxial thin-film heterostructures of the strongly spin-orbit coupled Mott-insulator Sr2IrO4 (SIO) and the cuprate high temperature superconductor YBa2Cu3O7 - delta (YBCO) are grown with pulsed laser deposition (PLD). A high crystalline quality is confirmed with X ray diffraction. The magnetic order of single SIO layers is studied with dc magnetization and low-energy muon spin rotation measurements and resembles that of the bulk material with a canted antiferromagnetic order. The electronic normal state and superconducting properties of YBCO (10, 12, or 14 nm)-SIO (20 nm) and inversely stacked SIO (20nm)-YBCO (10, 12, or 14 nm) bilayers are studied with dc resistivity measurements and found to be strongly dependent on the sequence of the layer stacking. The YBCO-SIO bilayers with d(YBCO) = 14nm, 12 nm, and 10 nm are all metallic and superconducting with an onset temperature around 85K and zero resistivity below 65K. To the contrary, for the inversely stacked SIO-YBCO bilayers a metallic and superconducting response occurs only at d(YBCO) = 14 nm, whereas and 10 nm are electronic insulators. This highlights that a long-ranged localization and/or depletion of the YBCO charge carriers occurs at the SIO-YBCO interface that is very anomalous and remains to be understood.