arXiv subjects

Andreas Suter

Publications and source records attributed to Andreas Suter.

At least 37 records · Page 2Linked to original sources

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

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

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↗

arXiv2022

Hydrogen-impurity induced unconventional magnetism in semiconducting molybdenum ditelluride

Layered transition-metal dichalcogenides are proposed as building blocks for van der Waals (vdW) heterostructures due to their graphene-like two dimensional structure. For this purpose, a magnetic semiconductor could represent an invaluable component for various spintronics and topotronics devices. Here, we combine different local magnetic probe spectroscopies with angle-resolved photoemission and density-functional theory calculations to show that 2H-MoTe2 is on the verge of becoming magnetic. Our results present clear evidence that the magnetism can be "switched on" by a hydrogen-like impurity. We also show that this magnetic state survives up to the free surface region, demonstrating the material's potential applicability as a magnetic component for thin-film heterostructures.

cond-mat.mtrl-sci↗

arXiv2022

Simulation studies for upgrading a high-intensity surface muon beamline at Paul Scherrer Institute

The $μ$E4-LEM beamline at Paul Scherrer Institute (PSI, Switzerland) is a special muon beamline combining the hyprid type surface muon beamline $μ$E4 with the low energy muon facility (LEM) and delivers $μ^{+}$ with tunable energy up to 30 keV for low-energy muon spin rotation experiments (LE-$μ$SR). We investigate a possible upgrade scenario for the surface muon beamline $μ$E4 by replacing the last set of quadrupole triplet with a special solenoid to obtain 1.4 times original beam intensity on the LEM muon moderator target. In order to avoid the muon beam intensity loss at the LEM spectrometer due to the stray magnetic field of the solenoid, three kinds of solenoid models have been explored and the stray field of the solenoid at the LEM facility is finally reduced to the magnitude of the geomagnetic field. A more radical design, "Super-$μ$E4", has also been investigated for further increasing the brightness of the low energy muon beam, where we make use of the current $μ$E4 channel and all sets of quadrupole triplets are replaced by large aperture solenoids. Together with the new slanted muon target E, at least 2.9 times the original muon beam intensity can be expected in the Super-$μ$E4 beamline. Our work demonstrates the feasibility of upgrading surface muon beamlines by replacing quadrupole magnets with normal-conducting solenoids, resulting in higher muon rates and smaller beam spot sizes.

physics.acc-ph↗

arXiv2022

Discovery of an electronic crystal in a cuprate Mott insulator

Copper oxide high temperature superconductors universally exhibit multiple forms of electronically ordered phases that break the native translational symmetry of the CuO2 planes. The interplay between these orders and the superconducting ground state, as well as how they arise through doping a Mott insulator, is essential to decode the mechanisms of high-temperature superconductivity. Over the years, various forms of electronic liquid crystal phases including charge/spin stripes and incommensurate charge-density-waves (CDWs) were found to emerge out of a correlated metallic ground state in underdoped cuprates. Early theoretical studies also predicted the emergence of a Coulomb-frustrated 'charge crystal' phase in the very lightly-doped, insulating limit of the CuO2 planes. Here, we use resonant X-ray scattering, electron transport, and muon spin rotation measurements to fully resolve the electronic and magnetic ground state and search for signatures of charge order in very lightly hole-doped cuprates from the RBa2Cu3O7-d family (RBCO; R: Y or rare earth). X-ray scattering data from RBCO films reveal a breaking of translational symmetry more pervasive than was previously known, extending down to the Mott limit. The ordering vector of this charge crystal state is linearly connected to the charge-density-waves of underdoped RBCO, suggesting that the former phase is a precursor to the latter as hole doping is increased. Most importantly, the coexistence of charge and spin order in RBCO suggests that this electronic symmetry-breaking state is common to the CuO2 planes in the very lightly-doped regime. These findings bridge the gap between the Mott insulating state and the underdoped metallic state and underscore the prominent role of Coulomb-frustrated electronic phase separation among all cuprates.

cond-mat.str-el↗

arXiv2022

Intrinsic magnetism in superconducting infinite-layer nickelates

The discovery of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ [1] introduced a new family of layered nickelate superconductors that has now been extended to include a range of Sr-doping [2, 3], Pr or La in place of Nd [4-6], and the 5-layer Nd$_6$Ni$_5$O$_{12}$ [7]. A number of studies indicate that electron correlations are strong in these materials [8-14], and hence a central question is whether or not magnetism is present as a consequence of these interactions. Here we report muon spin rotation/relaxation studies of a series of superconducting infinite-layer nickelates. In all cases we observe an intrinsic magnetic ground state, regardless of the rare earth ion or doping, arising from local moments on the nickel sublattice. The coexistence of magnetism - which is likely to be antiferromagnetic and short-range ordered - with superconductivity is reminiscent of some iron pnictides [15] and heavy fermion compounds [16], and qualitatively distinct from the doped cuprates [17].

cond-mat.supr-con↗

arXiv2020

Proximity-Induced Odd-Frequency Superconductivity in a Topological Insulator

At an interface between a topological insulator (TI) and a conventional superconductor (SC), superconductivity has been predicted to change dramatically and exhibit novel correlations. In particular, the induced superconductivity by an $s$-wave SC in a TI can develop an order parameter with a $p$-wave component. Here we present experimental evidence for an unexpected proximity-induced novel superconducting state in a thin layer of the prototypical TI, Bi$_2$Se$_3$, proximity coupled to Nb. From depth-resolved magnetic field measurements below the superconducting transition temperature of Nb, we observe a local enhancement of the magnetic field in Bi$_2$Se$_3$ that exceeds the externally applied field, thus supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state. Our experimental results are complemented by theoretical calculations supporting the appearance of such a component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer state it originates from. To the best of our knowledge, these findings present a first observation of bulk odd-frequency superconductivity in a TI. We thus reaffirm the potential of the TI-SC interface as a versatile platform to produce novel superconducting states.

cond-mat.supr-con↗

arXiv2018

Kubo spins in nano-scale aluminum grains: A muon spin relaxation study

We report muon spin relaxation rate measurements on films composed of aluminum grains having a size of a few nm, with a large energy level splitting of the order of 100 K. The films range from weakly metallic to insulating. In the insulating case the low temperature relaxation rate is consistent with the presence of single electron spins in grains having an odd number of electrons. The relaxation rate temperature dependence follows an activation law having an energy scale in agreement with the average level splitting. In weakly metallic films the relaxation rate is smaller and decreases faster with temperature. Overall our observations are in line with the presence of a Kubo spin in Al nano-size grains due to quantum size effects.

cond-mat.str-el↗

arXiv2017

Spectroscopic perspective on the interplay between electronic and magnetic properties of magnetically doped topological insulators

We combine low energy muon spin rotation (LE-$μ$SR) and soft-X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to study the magnetic and electronic properties of magnetically doped topological insulators, (Bi,Sb)$_2$Te$_3$. We find that one achieves a full magnetic volume fraction in samples of (V/Cr)$_x$(Bi,Sb)$_{2-x}$Te$_3$ at doping levels x $\gtrsim$ 0.16. The observed magnetic transition is not sharp in temperature indicating a gradual magnetic ordering. We find that the evolution of magnetic ordering is consistent with formation of ferromagnetic islands which increase in number and/or volume with decreasing temperature. Resonant ARPES at the V $L_3$ edge reveals a nondispersing impurity band close to the Fermi level as well as V weight integrated into the host band structure. Calculations within the coherent potential approximation of the V contribution to the spectral function confirm that this impurity band is caused by V in substitutional sites. The implications of our results on the observation of the quantum anomalous Hall effect at mK temperatures are discussed.

cond-mat.mtrl-sci↗

arXiv2017

Superconducting properties of Cu intercalated Bi$_2$Se$_3$ studied by Muon Spin Spectroscopy

We present muon spin rotation measurements on superconducting Cu intercalated Bi$_2$Se$_3$, which was suggested as a realization of a topological superconductor. We observe a clear evidence of the superconducting transition below 4 K, where the width of magnetic field distribution increases as the temperature is decreased. The measured broadening at mK temperatures suggests a large London penetration depth in the $ab$ plane ($λ_{\mathrm{eff}}\sim 1.6$ $\mathrmμ$m). We show that the temperature dependence of this broadening follows the BCS prediction, but could be consistent with several gap symmetries.

cond-mat.supr-con↗

arXiv2016

Real-Time Computation of Parameter Fitting and Image Reconstruction Using Graphical Processing Units

In recent years graphical processing units (GPUs) have become a powerful tool in scientific computing. Their potential to speed up highly parallel applications brings the power of high performance computing to a wider range of users. However, programming these devices and integrating their use in existing applications is still a challenging task. In this paper we examined the potential of GPUs for two different applications. The first application, created at Paul Scherrer Institut (PSI), is used for parameter fitting during data analysis of muSR (muon spin rotation, relaxation and resonance) experiments. The second application, developed at ETH, is used for PET (Positron Emission Tomography) image reconstruction and analysis. Applications currently in use were examined to identify parts of the algorithms in need of optimization. Efficient GPU kernels were created in order to allow applications to use a GPU, to speed up the previously identified parts. Benchmarking tests were performed in order to measure the achieved speedup During this work, we focused on single GPU systems to show that real time data analysis of these problems can be achieved without the need for large computing clusters. The results show that the currently used application for parameter fitting, which uses OpenMP to parallelize calculations over multiple CPU cores, can be accelerated around 40 times through the use of a GPU. The speedup may vary depending on the size and complexity of the problem. For PET image analysis, the obtained speedups of the GPU version was more than x40 larger compared to a single core CPU implementation. The achieved results show that it is possible to improve the execution time by orders of magnitude.

cs.DC↗

arXiv2016

Intrinsic Paramagnetic Meissner Effect due to s-wave Odd-Frequency Superconductivity

In 1933, Meissner and Ochsenfeld reported the expulsion of magnetic flux, the diamagnetic Meissner effect, from the interior of superconducting lead. This discovery was crucial in formulating the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. In exotic superconducting systems BCS theory does not strictly apply. A classical example is a superconductor-magnet hybrid system where magnetic ordering breaks time-reversal symmetry of the superconducting condensate and results in the stabilisation of an odd-frequency superconducting state. It has been predicted that under appropriate conditions, odd-frequency superconductivity should manifest in the Meissner state as fluctuations in the sign of the magnetic susceptibility meaning that the superconductivity can either repel (diamagnetic) or attract (paramagnetic) external magnetic flux. Here we report local probe measurements of faint magnetic fields in a Au/Ho/Nb trilayer system using low energy muons, where antiferromagnetic Ho (4.5 nm) breaks time-reversal symmetry of the proximity induced pair correlations in Au. From depth-resolved measurements below the superconducting transition of Nb we observe a local enhancement of the magnetic field in Au that exceeds the externally applied field, thus proving the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state.

cond-mat.supr-con↗

arXiv2015

The Dynamical Kernel Scheduler - Part 1

Emerging processor architectures such as GPUs and Intel MICs provide a huge performance potential for high performance computing. However developing software using these hardware accelerators introduces additional challenges for the developer such as exposing additional parallelism, dealing with different hardware designs and using multiple development frameworks in order to use devices from different vendors. The Dynamic Kernel Scheduler (DKS) is being developed in order to provide a software layer between host application and different hardware accelerators. DKS handles the communication between the host and device, schedules task execution, and provides a library of built-in algorithms. Algorithms available in the DKS library will be written in CUDA, OpenCL and OpenMP. Depending on the available hardware, the DKS can select the appropriate implementation of the algorithm. The first DKS version was created using CUDA for the Nvidia GPUs and OpenMP for Intel MIC. DKS was further integrated in OPAL (Object-oriented Parallel Accelerator Library) to speed up a parallel FFT based Poisson solver and Monte Carlo simulations for particle matter interaction used for proton therapy degrader modeling. DKS was also used together with Minuit2 for parameter fitting, where $χ^2$ and $max-log-likelihood$ functions were offloaded to the hardwareaccelerator. The concepts of the DKS, first results together with plans for the future will be shown in this paper.

physics.comp-ph↗

arXiv2015

Geant4 simulation of the PSI LEM beam line: energy loss and muonium formation in thin foils and the impact of unmoderated muons on the $μ$SR spectrometer

The PSI low-energy $μ$SR spectrometer is an instrument dedicated to muon spin rotation and relaxation measurements. Knowledge of the muon beam parameters such as spatial, kinetic energy and arrival-time distributions at the sample position are important ingredients to analyze the $μ$SR spectra. We present here the measured energy losses in the thin carbon foil of the muon start detector deduced from time-of-flight measurements. Muonium formation in the thin carbon foil (10 nm thickness) of the muon start detector also affect the measurable decay asymmetry and therefore need to be accounted for. Muonium formation and energy losses in the start detector, whose relevance increase with decreasing muon implantation energy ($<10$ keV), have been implemented in Geant4 Monte Carlo simulation to reproduce the measured time-of-flight spectra. Simulated and measured time-of-flight and beam spot agrees only if a small fraction of so called "unmoderated" muons which contaminate the mono-energetic muon beam of the $μ$SR spectrometer is introduced. Moreover the sensitivity of the beam size and related upstream-downstream asymmetry for a specially shaped "nose" sample plate has been studied for various beam line settings, which is of relevance for the study of thermal muonium emission into vacuum from mesoporous silica at cryogenic temperatures.

physics.ins-det↗

arXiv2013

Spatially homogeneous ferromagnetism below the enhanced Curie temperature in EuO_{1-x} thin films

We have used low energy implanted muons as a volume sensitive probe of the magnetic properties of EuO_{1-x} thin films. We find that static and homogeneous magnetic order persists up to the elevated T_C in the doped samples and the muon signal displays the double dome feature also observed in the sample magnetization. Our results appear incompatible with either the magnetic phase separation or bound magnetic polaron descriptions previously suggested to explain the elevated T_C, but are compatible with an RKKY-like interaction mediating magnetic interactions above 69 K.

cond-mat.str-el↗

arXiv2013

The Meissner effect in a strongly underdoped cuprate above its critical temperature

The Meissner effect and the associated perfect "bulk" diamagnetism together with zero resistance and gap opening are characteristic features of the superconducting state. In the pseudogap state of cuprates unusual diamagnetic signals as well as anomalous proximity effects have been detected but a Meissner effect has never been observed. Here we have probed the local diamagnetic response in the normal state of an underdoped La1.94Sr0.06CuO4 layer (up to 46 nm thick, critical temperature Tc' < 5 K) which was brought into close contact with two nearly optimally doped La1.84Sr0.16CuO4 layers (Tc \approx 32 K). We show that the entire 'barrier' layer of thickness much larger than the typical c axis coherence lengths of cuprates exhibits a Meissner effect at temperatures well above Tc' but below Tc. The temperature dependence of the effective penetration depth and superfluid density in different layers indicates that superfluidity with long-range phase coherence is induced in the underdoped layer by the proximity to optimally doped layers; however, this induced order is very sensitive to thermal excitation.

cond-mat.supr-con↗