arXiv subjects

Thomas Prokscha

Publications and source records attributed to Thomas Prokscha.

At least 37 records · Page 2Linked to original sources

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 μs data gate window. To overcome this limit and to be able to perform muon-spin relaxation (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 μSR (VR-μSR) spectrum. The unique capabilities and benefits of such a spectrometer are discussed.

physics.ins-det↗

arXiv2024

Emergence of interfacial magnetism in strongly-correlated nickelate-titanate superlattices

Strongly-correlated transition-metal oxides are widely known for their various exotic phenomena. This is exemplified by rare-earth nickelates such as LaNiO$_{3}$, which possess intimate interconnections between their electronic, spin, and lattice degrees of freedom. Their properties can be further enhanced by pairing them in hybrid heterostructures, which can lead to hidden phases and emergent phenomena. An important example is the LaNiO$_{3}$/LaTiO$_{3}$ superlattice, where an interlayer electron transfer has been observed from LaTiO$_{3}$ into LaNiO$_{3}$ leading to a high-spin state. However, macroscopic emergence of magnetic order associated with this high-spin state has so far not been observed. Here, by using muon spin rotation, x-ray absorption, and resonant inelastic x-ray scattering, we present direct evidence of an emergent antiferromagnetic order with high magnon energy and exchange interactions at the LaNiO$_{3}$/LaTiO$_{3}$ interface. As the magnetism is purely interfacial, a single LaNiO$_{3}$/LaTiO$_{3}$ interface can essentially behave as an atomically thin strongly-correlated quasi-two-dimensional antiferromagnet, potentially allowing its technological utilisation in advanced spintronic devices. Furthermore, its strong quasi-two-dimensional magnetic correlations, orbitally-polarized planar ligand holes, and layered superlattice design make its electronic, magnetic, and lattice configurations resemble the precursor states of superconducting cuprates and nickelates, but with an $S \rightarrow 1$ spin state instead.

cond-mat.str-el↗

arXiv2024

Magnetic precursor to the structural phase transition in V$_2$O$_3$

The coupling between structural, electronic and magnetic degrees of freedom across the metal-insulator transition in V2O3 makes it hard to determine the main driving mechanism behind the transition. Specifically, the role of magnetism has been debated and its interplay with the other transitions has not been established. To address this issue, we use a combination of muon spin relaxation/rotation, electrical transport and reciprocal space mapping which allows to correlate magnetic, electronic and structural degrees of freedom in strain-engineered V2O3 thin films. Evidence is found for a magnetic instability in the vicinity of the structural transition. This is manifested as a decrease in the antiferromagnetic moment with temperature leading to a virtual Néel transition temperature which coincides with that of the structural and electronic transitions. Moreover, we find evidence for an onset of antiferromagnetic (AF) fluctuations in the rhombohedral phase even without a structural transition to the monoclinic phase. The non-congruence of the structural and magnetic transitions increases as the transition temperature is reduced by strain. In samples where the transition is most strongly suppressed by strain, a depth-dependent magnetic state is observed. These results reveal the importance of an AF instability in the paramagnetic phase in triggering the metal-insulator transition and the crucial role of the structural transition in allowing for the formation of an ordered AF state.

cond-mat.str-el↗

arXiv2024

Observation and manipulation of charge carrier distribution at the SiO$_2$/Si interface

Using low-energy muons, we map the charge carrier concentration as a function of depth and electric field across the \SiOSi interface up to a depth of \SI{100}{\nano\meter} in Si-based MOS capacitors. The results show that the formation of the anisotropic bond-centered muonium \MuBCz state in Si serves as a direct measure of the local changes in electronic structures. Different band-bending conditions could be distinguished, and the extension of the depletion width was directly extracted using the localized stopping and probing depth of the muons. Furthermore, electron build-up on the Si side of the \SiOO/Si interface, caused by the mirror charge induced by the fixed positive charge in the oxide and the image force effect, was observed. Our work represents a significant extension of the application of the muon spin rotation technique ($μ$SR) and lays the foundation for further research on direct observation of charge carrier density manipulation at technologically important semiconductor device interfaces.

cond-mat.other↗

arXiv2024

Absence of magnetic order in RuO$_2$: insights from $μ$SR spectroscopy and neutron diffraction

Altermagnets are a novel class of magnetic materials besides ferro- and antiferromagnets, where the interplay of lattice and spin symmetries produces a magnetic order that is staggered both in coordinate as well as momentum space. The metallic rutile oxide RuO$_2$, long believed to be a textbook Pauli paramagnet, recently emerged as a workhorse altermagnet when resonant X-ray and neutron scattering studies reported nonzero magnetic moments and long-range collinear order. While experiments on thin films seem consistent with altermagnetic behavior, the origin and size of magnetic moments in RuO$_2$ still remain controversial. Here we show that RuO$_2$ is nonmagnetic, regardless if as bulk or thin film. Employing muon spin spectroscopy as a highly sensitive probe of local magnetic moments complemented by density functional theory, we find at most $1.4 \times 10^{-4} $ $μ_B$/Ru in bulk RuO$_2$ and at most $7.5 \times 10^{-4}$ $μ_B$/Ru in epitaxial films. In their essence, these moments reflect the detection limit of our spectrometers and are orders of magnitude smaller than previously reported neutron results, i.e., the moments previously assumed to rationalize altermagnetic behavior. Our own neutron diffraction measurements on RuO$_2$ single crystals identify multiple scattering as a likely source for this discrepancy.

cond-mat.mtrl-sci↗

arXiv2024

Observation of Mermin-Wagner behavior in LaFeO$_3$/SrTiO$_3$ superlattices

Two-dimensional magnetic materials attract a lot of attention since they potentially exhibit new magnetic properties due to, e.g., strongly enhanced spin fluctuations. However, the suppression of the long-range magnetic order in two dimensions due to long-wavelength spin fluctuations, as suggested by the Mermin-Wagner theorem, has been questioned for finite-size laboratory samples. Here we study the magnetic properties of a dimensional crossover in superlattices composed of the antiferromagnetic LaFeO$_3$ and SrTiO$_3$ that, thanks to their large lateral size, allowed examination using a sensitive magnetic probe - muon spin rotation spectroscopy. We show that the iron electronic moments in superlattices with 3 and 2 monolayers of LaFeO$_3$ exhibit a static antiferromagnetic order. In contrast, in the superlattices with single LaFeO$_3$ monolayer, the moments do not order and fluctuate to the lowest measured temperature as expected from the Mermin-Wagner theorem. Our work shows how dimensionality can be used to tune the magnetic properties of ultrathin films.

cond-mat.str-el↗

arXiv2024

Unveiling unconventional magnetism at the surface of Sr$_2$RuO$_4$

Materials with strongly correlated electrons exhibit physical properties that are often difficult to predict as they result from the interactions of large numbers of electrons combined with several quantum degrees of freedom. The layered oxide perovskite Sr$_2$RuO$_4$ is a strongly correlated electron material that has been intensively investigated since its discovery due to its unusual physical properties. Whilst recent experiments have reopened the debate on the exact symmetry of the superconducting state in Sr$_2$RuO$_4$, a deeper understanding of the Sr$_2$RuO$_4$ normal state appears crucial as this is the background in which electron pairing occurs. Here, by using low-energy muon spin spectroscopy we discover the existence of magnetism at the surface of Sr$_2$RuO$_4$ in its normal state. We detect static weak dipolar fields yet manifesting below a relatively high onset temperature larger than 50 K, which reveals the unconventional nature of the observed magnetism. We relate the origin of this phase breaking time reversal symmetry to electronic ordering in the form of orbital loop currents that originate at the reconstructed Sr$_2$RuO$_4$ surface. Our observations set a reference for the discovery of the same magnetic phase in other materials and unveil an electronic ordering mechanism that can influence unconventional electron pairing with broken time reversal symmetry in those materials where the observed magnetic phase coexists with superconductivity.

cond-mat.str-el↗

arXiv2024

Improving the low-energy muon beam quality of the LEM beamline at PSI: Characterisation of ultra-thin carbon foils

The Low-Energy Muon beamline (LEM) at the Paul Scherrer Institute currently stands as the world's only facility providing a continuous beam of low-energy muons with keV energies for conducting muon spin rotation experiments on a nanometer depth scale in heterostructures and near a sample's surface. As such, optimizing the beam quality to reach its full potential is of paramount importance. One of the ongoing efforts is dedicated to improving the already applied technique of single muon tagging through the detection of secondary electrons emerging from an ultra-thin carbon foil. In this work, we present the results from installing a thinner foil with a nominal thickness of 0.5 $μg~cm^{-2}$ and compare its performance to that of the previously installed foil with a nominal thickness of 2.0 $μg~cm^{-2}$. Our findings indicate improved beam quality, characterized by smaller beam spots, reduced energy loss and straggling of the muons, and enhanced tagging efficiency. Additionally, we introduce a method utilizing blue laser irradiation for cleaning the carbon foil, further improving and maintaining its characteristics

physics.ins-det↗

arXiv2024

Coexistence of Superconductivity and Antiferromagnetism in Topological Magnet MnBi2Te4 Films

The interface of two materials can harbor unexpected emergent phenomena. One example is interface-induced superconductivity. In this work, we employ molecular beam epitaxy to grow a series of heterostructures formed by stacking together two non-superconducting antiferromagnetic materials, an intrinsic antiferromagnetic topological insulator MnBi2Te4 and an antiferromagnetic iron chalcogenide FeTe. Our electrical transport measurements reveal interface-induced superconductivity in these heterostructures. By performing scanning tunneling microscopy and spectroscopy measurements, we observe a proximity-induced superconducting gap on the top surface of the MnBi2Te4 layer, confirming the interaction between superconductivity and antiferromagnetism in the MnBi2Te4 layer. Our findings will advance the fundamental inquiries into the topological superconducting phase in hybrid devices and provide a promising platform for the exploration of chiral Majorana physics in MnBi2Te4-based heterostructures.

cond-mat.supr-con↗

arXiv2023

Interface-Induced Superconductivity in Magnetic Topological Insulator-Iron Chalcogenide Heterostructures

When two different electronic materials are brought together, the resultant interface often shows unexpected quantum phenomena, including interfacial superconductivity and Fu-Kane topological superconductivity (TSC). Here, we use molecular beam epitaxy (MBE) to synthesize heterostructures formed by stacking together two magnetic materials, a ferromagnetic topological insulator (TI) and an antiferromagnetic iron chalcogenide (FeTe). We discover emergent interface-induced superconductivity in these heterostructures and demonstrate the trifecta occurrence of superconductivity, ferromagnetism, and topological band structure in the magnetic TI layer, the three essential ingredients of chiral TSC. The unusual coexistence of ferromagnetism and superconductivity can be attributed to the high upper critical magnetic field that exceeds the Pauli paramagnetic limit for conventional superconductors at low temperatures. The magnetic TI/FeTe heterostructures with robust superconductivity and atomically sharp interfaces provide an ideal wafer-scale platform for the exploration of chiral TSC and Majorana physics, constituting an important step toward scalable topological quantum computation.

cond-mat.mes-hall↗

arXiv2023

Evidence for current suppression in superconductor-superconductor bilayers

Superconducting radio frequency (SRF) cavities, which are critical components in many particle accelerators, need to be operated in the Meissner state to avoid strong dissipation from magnetic vortices. For a defect-free superconductor, the maximum attainable magnetic field for operation is set by the superheating field, $B_{\mathrm{sh}}$, which directly depends on the surface current. In heterostructures composed of different superconductors, the current in each layer depends not only on the properties of the individual material, but also on the electromagnetic response of the adjacent layers through boundary conditions at the interfaces. Three prototypical bilayers [$\mathrm{Nb_{1-x}Ti_xN}$(50 nm)/Nb, $\mathrm{Nb_{1-x}Ti_xN}$(80 nm)/Nb, and $\mathrm{Nb_{1-x}Ti_xN}$(160 nm)/Nb] are investigated here by depth-resolved measurements of their Meissner screening profiles using low-energy muon spin rotation (LE-$μ$SR). From fits to a model based on London theory (with appropriate boundary and continuity conditions), a magnetic penetration depth for the thin $\mathrm{Nb_{1-x}Ti_xN}$ layers of $λ_\mathrm{Nb_{1-x}Ti_xN} =$ 182.5(31) nm is found, in good agreement with literature values for the bulk alloy. Using the measured $λ_\mathrm{Nb_{1-x}Ti_xN}$, the maximum vortex-free field, $B_{\mathrm{max}}$, of the superconductor-superconductor (SS) bilayer structure was estimated to be 610(40) mT. The strong suppression of the surface current in the $\mathrm{Nb_{1-x}Ti_xN}$ layer suggests an optimal thickness of $\sim 1.4 λ_{\mathrm{Nb_{1-x}Ti_xN}} =$ 261(14) nm.

cond-mat.supr-con↗

arXiv2023

Emergent magnetism with continuous control in the ultrahigh conductivity layered oxide PdCoO2

The current challenge to realizing continuously tunable magnetism lies in our inability to systematically change properties such as valence, spin, and orbital degrees of freedom as well as crystallographic geometry. Here, we demonstrate that ferromagnetism can be externally turned on with the application of low-energy helium implantation and subsequently erased and returned to the pristine state via annealing. This high level of continuous control is made possible by targeting magnetic metastability in the ultra-high conductivity, non-magnetic layered oxide PdCoO2 where local lattice distortions generated by helium implantation induce emergence of a net moment on the surrounding transition metal octahedral sites. These highly-localized moments communicate through the itinerant metal states which triggers the onset of percolated long-range ferromagnetism. The ability to continuously tune competing interactions enables tailoring precise magnetic and magnetotransport responses in an ultra-high conductivity film and will be critical to applications across spintronics.

cond-mat.mtrl-sci↗

arXiv2022

Collective topological spin dynamics in a correlated spin glass

The interplay between spin-orbit interaction (SOI) and magnetic order is currently one of the most active research fields in condensed matter physics and leading the search for materials with novel and tunable magnetic and spin properties. Here we report on a variety of unexpected and unique observations in thin multiferroic \Ge$_{1-x}$Mn$_x$Te films. The ferrimagnetic order in this ferroelectric semiconductor is found to reverse with current pulses six orders of magnitude lower as for typical spin-orbit torque systems. Upon a switching event, the magnetic order spreads coherently and collectively over macroscopic distances through a correlated spin-glass state. Lastly, we present a novel methodology to controllably harness this stochastic magnetization dynamics, allowing us to detect spatiotemporal nucleation of topological spin textures we term ``skyrmiverres''.

cond-mat.mtrl-sci↗

arXiv2022

Depth-resolved measurements of the Meissner screening profile in surface-treated Nb

We report depth-resolved measurements of the Meissner screening profile in several surface-treated Nb samples using low-energy muon spin rotation (LE-$μ$SR). In these experiments, implanted positive muons, whose stopping depths below Nb's surface were adjusted between ~10 nm to ~150 nm, reveal the field distribution inside the superconducting element via their spin-precession (communicated through their radioactive decay products). We compare how the field screening is modified by different surface treatments commonly employed to prepare superconducting radio frequency (SRF) cavities used in accelerator beamlines. In contrast to an earlier report [A. Romanenko et al., Appl. Phys. Lett. 104 072601 (2014)], we find no evidence for any "anomalous" modifications to the Meissner profiles, with all data being well-described by a London model. Differences in screening properties between surface treatments can be explained by changes to the carrier mean-free-paths resulting from dopant profiles near the material's surface.

cond-mat.supr-con↗

arXiv2022

Thin film and surface preparation chamber for the low energy muons spectrometer

We have designed and constructed a thin film preparation chamber with base pressure of $<2 \times 10^{-9}$~mbar. Currently, the chamber is equipped with two large area evaporators (a molecular evaporator and an electron-beam evaporator), an ion sputtering gun, a thickness monitor and a substrate heater. It is designed such that it can handle large area thin film samples with a future possibility to transfer them in vacuum directly to the low energy muons (LEM) spectrometer or to other advanced characterization facilities in the Quantum Matter and Materials Center (QMMC) which will be constructed in 2024. Initial commissioning of the chamber resulted in high quality, large area and uniform molecular films of CuPc and TbPc$_2$ on various substrate materials. We present first results from low energy $μ$SR (LE-$μ$SR) measurements on these films.

cond-mat.mtrl-sci↗

arXiv2022

Investigation of the SiO2-SiC interface using low energy muon spin rotation spectroscopy

Using positive muons as local probes implanted at low energy enables gathering information about the material of interest with nanometer depth resolution (low energy muon spin rotation spectroscopy (LE-$μ$SR). In this work, we leverage the capabilities of LE-$μ$SR to perform an investigation of the SiO$_\text{2}$-SiC interface. Thermally oxidized samples are investigated before and after annealing in nitric oxide (NO) and argon (Ar) ambience. Thermal oxidation is found to result in structural changes both in the SiC crystal close to the interface and at the interface itself. Annealing in NO environment is known to passivate the defects leading to a reduction of the density of interface traps (D$_{it}$); LE-$μ$SR further reveals that the NO annealing results in a thin layer of high carrier concentration in SiC, extending to more than 50 nm depending on the annealing conditions. We also see indications of Si vacancy (V$_{Si}$) formation in SiC after thermal oxidation. Following NO annealing, nitrogen occupies the V$_{Si}$ sites, leading to the reduction in D$_{it}$ and at the same time, creating a charge-carrier-rich region near the interface. By comparing the LE-$μ$SR data from a sample with known doping density, we perform a high-resolution quantification of the free carrier concentration near the interface after NO annealing and discuss the origin of observed near-surface variations. Finally, the depletion of carriers in a MOS capacitor in the region below the interface is shown using LE-$μ$SR. The NO annealed sample shows the narrowest depletion region, likely due to the reduced D$_{it}$ and charge-carrier-rich region near the interface. Our findings demonstrate the many benefits of utilizing LE-$μ$SR to study critical regions of semiconductor devices that have been inaccessible with other techniques while retaining nanoscale depth resolution and a non-destructive approach.

cond-mat.mtrl-sci↗

arXiv2022

GermanIum Array for Non-destructive Testing (GIANT) setup for Muon Induced X-ray Emission (MIXE) at the Paul Scherrer Institute

The usage of muonic X-rays to study elemental properties like nuclear radii ranges back to the seventies. This triggered the pioneering work at the Paul Scherrer Institute (PSI), during the eighties, on the Muon Induced X-ray Emission (MIXE) technique for a non-destructive assessment of elemental compositions. In the recent years, this method has seen a rebirth, improvement and adoption at most muon facilities around the world. Hereby, the PSI offers unique capabilities with its high-rate continuous muon beam at the Swiss Muon Source (S$μ$S). We report here the decision making, construction and commissioning of a dedicated MIXE spectrometer at PSI, the GermanIum Array for Non-destructive Testing (GIANT) setup. Multiple campaigns highlighted the outstanding capabilities of MIXE at PSI, e.g. resolving down to 1 at% elemental concentrations with as little as 1 h data taking, measuring isotopic ratios for elements from iron to lead, and characterizing gamma rays induced by muon nuclear capture. On-target beamspots were characterized with a dedicated charged particle tracker to be 22.06$\pm$0.18 and 14.45$\pm$0.06 mm for 25 and 45 MeV/c, respectively. Advanced analysis of the High Purity Germanium (HPGe) signals further allows to improve energy and timing resolutions to ~1 keV and 20 ns at 1 MeV, respectively. Within the GIANT setup, an average detector has a photopeak efficiency of $\overline{ε_E}$ =0.11% and an energy resolution of $\overline{σ_E}$ = 0.8 keV at E=1000 keV. The overall performance of the GIANT setup at S$μ$S allowed to start a rich user program with archaeological samples, Li-ion battery research, and collaboration with industry. Future improvements will include a simulation based analysis and a higher degree of automation, e.g. automatic scans of a series of muon momenta and automatic sample changing.

physics.ins-det↗

arXiv2022

Increased muon field at surface and substrate interface of palladium thin films

We performed depth-dependent low-energy muon spin spectroscopy ($μ$SR) studies on three palladium 100 nm thin films, both undoped and doped with 170 ppm of iron. Muons implanted in the surface and substrate interface region probe an increased local magnetic field compared to the inner part of the sample. The field increase extends over a few nanometers, it is temperature-independent (in the range of 3.7 - 100 K), stronger for the iron-doped samples and accompanied by an increase in local field inhomogeneity. We consider various potential origins for this magnetic surface state, such as adsorbents and supressed d-states. Our conclusion is that orbital moments induced at the surface / interface by localized spins and charges are the most likely explanation, potentially accompanied by magnetic moments due to crystal irregularities.

cond-mat.mes-hall↗