Showing 1–50 of 79 results for author: Suter, A

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

Authors: Alexander J. Grutter, Nurul Fitriyah, Brian B. Maranville, Saurav Prakash, Andreas Suter, Jochen Stahn, Gianluca Janka, Xing Gao, King Yau Yip, Zaher Salman, Thomas Prokscha, Julie A. Borchers, Ariando Ariando

Abstract: 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 reflectom… ▽ More 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. △ Less

Submitted 19 December, 2025; originally announced December 2025.

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

Authors: Ryan M. L. McFadden, Jonathan W. Angle, Eric M. Lechner, Michael J. Kelley, Charles E. Reece, Matthew A. Coble, Thomas Prokscha, Zaher Salman, Andreas Suter, Tobias Junginger

Abstract: We report direct, simultaneous measurements of the London penetration depth ($λ_L$) and Bardeen-Cooper-Schrieffer (BCS) coherence length ($ξ_0$) in oxygen-doped niobium, with impurity concentrations spanning the "clean" to "dirty" limits. Two depth-resolved techniques - low-energy muon spin spectroscopy (LE-$μ$SR) and secondary-ion mass spectrometry (SIMS) - were used to quantify the element's Mei… ▽ More We report direct, simultaneous measurements of the London penetration depth ($λ_L$) and Bardeen-Cooper-Schrieffer (BCS) coherence length ($ξ_0$) in oxygen-doped niobium, with impurity concentrations spanning the "clean" to "dirty" limits. Two depth-resolved techniques - low-energy muon spin spectroscopy (LE-$μ$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 $λ_L = 29.1(10)$ nm and $ξ_0 = 39.9(25)$ nm, corresponding to a Ginzburg-Landau (GL) parameter of $κ= 0.70(5)$. The obtained $λ_L$ and $κ$ 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. △ Less

Submitted 14 January, 2026; v1 submitted 4 November, 2025; originally announced November 2025.

Comments: Main manuscript: 8 pages, 3 figures, 2 tables. Supporting material: 17 pages, 6 figures, 7 tables

Journal ref: Phys. Rev. B 113, L060508 (2026)

Cu-spin Correlation in the Electron-overdoped High-Tc Cuprate Thin Films of La_2-x_Ce_x_CuO_4_ Probed by Low-energy Muons

Authors: S. E. Park, Y. Kawai, A. Suter, H. Okabe, J. G. Nakamura, H. Kuwahara, Z. Salman, T. Prokscha, R. Kadono, T. Adachi

Abstract: We investigated the Cu-spin correlation in the overdoped regime of the electron-doped high-Tc cuprate thin films of La_2-x_Ce_x_CuO_4_, 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… ▽ More We investigated the Cu-spin correlation in the overdoped regime of the electron-doped high-Tc cuprate thin films of La_2-x_Ce_x_CuO_4_, 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 La$_2-x_Ce_x_CuO_4_ with x~0.18. △ Less

Submitted 1 November, 2025; originally announced November 2025.

Comments: 6 pages, 6 figures, J. Phys. Soc. Jpn., in press

Journal ref: J. Phys. Soc. Jpn. 94, 124710 (2025)

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

Authors: Akashdeep Akashdeep, Sachin Krishnia, Jae-Hyun Ha, Siyeon An, Maik Gaerner, Thomas Prokscha, Andreas Suter, Gianluca Janka, Günter Reiss, Timo Kuschel, Dong-Soo Han, Angelo Di Bernardo, Zaher Salman, Gerhard Jakob, Mathias Kläui

Abstract: 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 sputter… ▽ More 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. △ Less

Submitted 6 February, 2026; v1 submitted 9 October, 2025; originally announced October 2025.

Journal ref: Appl. Phys. Lett. 128, 022406 (2026)

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

Authors: Ryan M. L. McFadden, Andreas Suter, Leon Ruf, Angelo Di Bernardo, Arnold M. Müller, Thomas Prokscha, Zaher Salman, Tobias Junginger

Abstract: Here we study the range of keV positive muons $μ^+$ 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-$μ$SR). At implantation energies 1.3 keV $\leq E \leq$ 23.3 keV, we compare the measured diamagnetic $μ^+$ signal fraction $f_{\mathrm{dia.}}$ against predictions derived from implant… ▽ More Here we study the range of keV positive muons $μ^+$ 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-$μ$SR). At implantation energies 1.3 keV $\leq E \leq$ 23.3 keV, we compare the measured diamagnetic $μ^+$ signal fraction $f_{\mathrm{dia.}}$ against predictions derived from implantation profile simulations using the TRIM.SP Monte Carlo code. Treating the implanted $μ^+$ as light protons, we find that simulations making use of updated stopping cross section data are in good agreement with the LE-$μ$SR measurements, in contrast to parameterizations found in earlier tabulations. Implications for other studies relying on accurate $μ^+$ stopping information are discussed. △ Less

Submitted 4 July, 2025; originally announced July 2025.

Comments: 13 pages, 11 figures

Journal ref: Nucl. Instrum. Methods Phys. Res., Sect. B 570, 165954 (2026)

First High-Throughput Evaluation of Dark Matter Detector Materials

Authors: Sinéad M. Griffin, Yonit Hochberg, Benjamin V. Lehmann, Rotem Ovadia, Kristin A. Persson, Bethany A. Suter, Ruo Xi Yang, Wayne Zhao

Abstract: We perform the first high-throughput search and evaluation of materials that can serve as excellent low-mass dark matter detectors. Using properties of close to one thousand materials from the Materials Project database, we project the sensitivity in dark matter parameter space for experiments constructed from each material, including both absorption and scattering processes between dark matter an… ▽ More We perform the first high-throughput search and evaluation of materials that can serve as excellent low-mass dark matter detectors. Using properties of close to one thousand materials from the Materials Project database, we project the sensitivity in dark matter parameter space for experiments constructed from each material, including both absorption and scattering processes between dark matter and electrons. Using the anisotropic materials in the dataset, we further compute the level of daily modulation in interaction rates and the resulting directional sensitivities, highlighting materials with prospects to detect the dark matter wind. Our methods provide the basic tools for the data-driven design of dark matter detectors, and our findings lay the groundwork for the next generation of highly optimized direct searches for dark matter as light as the keV scale. △ Less

Submitted 24 June, 2025; originally announced June 2025.

Comments: 6+7 pages, 3+5 figures

Report number: MIT-CTP/5862

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

Authors: Zubia Hasan, Grace A. Pan, Harrison LaBollita, Austin Kaczmarek, Suk Hyun Sung, Shekhar Sharma, Purnima P. Balakrishnan, Edward Mercer, Vivek Bhartiya, Zaher Salman, Thomas Prokscha, Andreas Suter, Alexander J. Grutter, Mirian Garcia-Fernandez, Ke-Jin Zhou, Jonathan Pelliciari, Valentina Bisogni, Ismail El Baggari, Darrell G. Schlom, Matthew R. Barone, Charles M. Brooks, Katja C. Nowack, Antia S. Botana, Brendan D. Faeth, Alberto de la Torre , et al. (1 additional authors not shown)

Abstract: 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 frust… ▽ More 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. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Experimental Observation of Short-Range Magnetic Correlations in Amorphous Nb$_2$O$_5$ and Ta$_2$O$_5$ Thin Films

Authors: Y. V. Krasnikova, A. A. Murthy, D. Bafia, F. Crisa, A. Clairmont, Z. Sung, J. Lee, A. Cano, M. Shinde, D. M. T. van Zanten, M. Bal, A. Romanenko, A. Grassellino, R. Dhundhwal, D. Fuchs, T. Reisinger, I. M. Pop, A. Suter, T. Prokscha, Z. Salman

Abstract: We use muon spin rotation/relaxation/resonance ($μ$SR) to investigate the magnetic properties of niobium pentoxide (Nb$_2$O$_5$) and tantalum pentoxide (Ta$_2$O$_5$) thin films. In both oxides, we observe a magnetic response at the lowest available temperature of 2.8 K. This response appears to be structurally dependent: thermally oxidized Ta$_2$O$_5$ with low crystallinity demonstrates suppressed… ▽ More We use muon spin rotation/relaxation/resonance ($μ$SR) to investigate the magnetic properties of niobium pentoxide (Nb$_2$O$_5$) and tantalum pentoxide (Ta$_2$O$_5$) thin films. In both oxides, we observe a magnetic response at the lowest available temperature of 2.8 K. This response appears to be structurally dependent: thermally oxidized Ta$_2$O$_5$ with low crystallinity demonstrates suppressed magnetism, while fully amorphous Ta$_2$O$_5$ demonstrates local static magnetism. In contrast, amorphous Nb$_2$O$_5$ is dominated by magnetic fluctuations and is strongly magnetically disordered compared to Ta$_2$O$_5$. Our results suggest that these fundamental differences in the magnetism of Ta and Nb oxides could explain the performance limitations in superconducting qubits and resonators. △ Less

Submitted 7 May, 2026; v1 submitted 12 May, 2025; originally announced May 2025.

Comments: 15 pages, 15 figures

Report number: FERMILAB-PUB-25-0325-SQMS

Journal ref: Phys. Rev. B 113, 134446 (2026)

Complex Magnetic Ordering in Candidate Topological Superconductors

Authors: Purnima P. Balakrishnan, Hemian Yi, Zi-Jie Yan, Wei Yuan, Andreas Suter, Christopher J. Jensen, Pascal Manuel, Fabio Orlandi, Takayasu Hanashima, Christy J. Kinane, Andrew J. Caruana, Brian B. Maranville, Zaher Salman, Thomas Prokscha, Cui-Zu Chang, Alexander J. Grutter

Abstract: 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 w… ▽ More 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. △ Less

Submitted 14 March, 2025; originally announced March 2025.

Comments: Main text: 9 pages, 3 figures, 1 table. Supplementary materials: 18 pages, 17 figures

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

Authors: Anton Zubayer, Artur Glavic, Naureen Ghafoor, Yuqing Ge, Yasmine Sassa, Martin Månsson, Andreas Suter, Thomas Prokscha, Zaher Salman, Wai-Tung Lee, Kristbjörg Anna Thórarinsdóttir, Arnaud Le Febvrier, Per Eklund, Jens Birch, Fridrik Magnus, Sean Langridge, Andrew Caruana, Christy Kinane, Fredrik Eriksson

Abstract: 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 magnetizatio… ▽ More 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. △ Less

Submitted 13 April, 2026; v1 submitted 11 February, 2025; originally announced February 2025.

Pressure induced transition from chiral charge order to time-reversal symmetry-breaking superconducting state in Nb-doped CsV$_3$Sb$_5$

Authors: J. N. Graham, S. S. Islam, V. Sazgari, Y. Li, H. Deng, G. Janka, Y. Zhong, O. Gerguri, P. Kral, A. Doll, I. Bialo, J. Chang, Z. Salman, A. Suter, T. Prokscha, Y. Yao, K. Okazaki, H. Luetkens, R. Khasanov, Z. Wang, J. -X. Yin, Z. Guguchia

Abstract: The experimental realisation of unconventional superconductivity and charge order in kagome systems \textit{A}V$_3$Sb$_5$ is of critical importance. We conducted a highly systematic study of Cs(V$_{1-x}$Nb$_x$)$_3$Sb$_5$ with $x$=0.07 (Nb$_{0.07}$-CVS) by employing a unique combination of tuning parameters such as doping, hydrostatic pressure, magnetic fields, and depth, using muon spin rotation,… ▽ More The experimental realisation of unconventional superconductivity and charge order in kagome systems \textit{A}V$_3$Sb$_5$ is of critical importance. We conducted a highly systematic study of Cs(V$_{1-x}$Nb$_x$)$_3$Sb$_5$ with $x$=0.07 (Nb$_{0.07}$-CVS) by employing a unique combination of tuning parameters such as doping, hydrostatic pressure, magnetic fields, and depth, using muon spin rotation, AC susceptibility, and STM. We uncovered tunable magnetism in the normal state of Nb$_{0.07}$-CVS, which transitions to a time-reversal symmetry (TRS) breaking superconducting state under pressure. Specifically, our findings reveal that the bulk of Nb$_{0.07}$-CVS (at depths greater than 20 nm from the surface) experiences TRS breaking below $T^*=40~$K, lower than the charge order onset temperature, $T_\mathrm{CO}$ = 58 K. However, near the surface (within 20 nm from the surface), the TRS breaking signal doubles and onsets at $T_\mathrm{CO}$, indicating that Nb-doping decouples TRS breaking from charge order in the bulk but synchronises them near the surface. Additionally, Nb-doping raises the superconducting critical temperature $T_\mathrm{C}$ from 2.5 K to 4.4 K. Applying hydrostatic pressure enhances both $T_\mathrm{C}$ and the superfluid density by a factor of two, with a critical pressure $p_\mathrm{cr}$ ${\simeq}$ 0.85 GPa, suggesting competition with charge order. Notably, above $p_\mathrm{cr}$, we observe nodeless electron pairing and weak internal fields below $T_\mathrm{C}$, indicating broken TRS in the superconducting state. Overall, these results demonstrate a highly unconventional normal state with a depth-tunable onset of TRS breaking at ambient pressure, a transition to TRS-breaking superconductivity under low hydrostatic pressure, and an unconventional scaling between $T_\mathrm{C}$ and the superfluid density. △ Less

Submitted 27 November, 2024; originally announced November 2024.

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

Authors: Chubin Huang, Abhishek Rakshit, Gianluca Janka, Zaher Salman, Andreas Suter, Thomas Prokscha, Benjamin A. Frandsen, Yoav Kalcheim

Abstract: 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,… ▽ More 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. △ Less

Submitted 30 October, 2024; originally announced October 2024.

Comments: main text: 14 pages, 4 figures. Supplementary: 12 pages, 12 figures

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

Authors: Philipp Keßler, Laura Garcia-Gassull, Andreas Suter, Thomas Prokscha, Zaher Salman, Dmitry Khalyavin, Pascal Manuel, Fabio Orlandi, Igor I. Mazin, Roser Valentı, Simon Moser

Abstract: 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 scatterin… ▽ More 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. △ Less

Submitted 17 May, 2024; originally announced May 2024.

Comments: 12 pages, 13 figures

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

Authors: Michal Kiaba, Andreas Suter, Zaher Salman, Thomas Prokscha, Binbin Chen, Gertjan Koster, Adam Dubroka

Abstract: 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 t… ▽ More 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. △ Less

Submitted 7 May, 2024; originally announced May 2024.

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

Authors: Teguh Citra Asmara, Robert J. Green, Andreas Suter, Yuan Wei, Wenliang Zhang, Daniel Knez, Grant Harris, Yi Tseng, Tianlun Yu, Davide Betto, Mirian Garcia-Fernandez, Stefano Agrestini, Yannick Maximilian Klein, Neeraj Kumar, Carlos William Galdino, Zaher Salman, Thomas Prokscha, Marisa Medarde, Elisabeth Müller, Yona Soh, Nicholas B. Brookes, Ke-Jin Zhou, Milan Radovic, Thorsten Schmitt

Abstract: 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 phe… ▽ More 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. △ Less

Submitted 5 November, 2024; v1 submitted 1 March, 2024; originally announced March 2024.

Comments: 41 pages, 14 figures

Journal ref: Advanced Materials 36, 2310668 (2024)

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

Authors: Rosalba Fittipaldi, Roman Hartmann, Maria Teresa Mercaldo, Sachio Komori, Anders Bjørlig, Wonshik Kyung, Yuuki Yasui, Takuto Miyoshi, Linde Olde-Olthof, Carla Palomares-Garcia, Veronica Granata, Itai Keren, Wataru Higemoto, Andreas Suter, Thomas Prokscha, Alfonso Romano, Canio Noce, Changyoung Kim, Yoshiteru Maeno, Elke Scheer, Beena Kalisky, Jason W. A. Robinson, Mario Cuoco, Zaher Salman, Antonio Vecchione , et al. (1 additional authors not shown)

Abstract: 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 p… ▽ More 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. △ Less

Submitted 28 February, 2024; originally announced February 2024.

Comments: 20 pages, 5 figures

Journal ref: Nat. Commun. 12, 5792 (2021)

Depth-dependent study of time-reversal symmetry-breaking in the kagome superconductor $A$V$_{3}$Sb$_{5}$

Authors: J. N. Graham, C. Mielke III, D. Das, T. Morresi, V. Sazgari, A. Suter, T. Prokscha, H. Deng, R. Khasanov, S. D. Wilson, A. C. Salinas, M. M. Martins, Y. Zhong, K. Okazaki, Z. Wang, M. Z. Hasan, M. Fischer, T. Neupert, J. -X. Yin, S. Sanna, H. Luetkens, Z. Salman, P. Bonfa, Z. Guguchia

Abstract: The breaking of time-reversal symmetry (TRS) in the normal state of kagome superconductors $A$V$_{3}$Sb$_{5}$ stands out as a significant feature. Yet the extent to which this effect can be tuned remains uncertain, a crucial aspect to grasp in light of the varying details of TRS breaking observed through different techniques. Here, we employ the unique low-energy muon spin rotation technique combi… ▽ More The breaking of time-reversal symmetry (TRS) in the normal state of kagome superconductors $A$V$_{3}$Sb$_{5}$ stands out as a significant feature. Yet the extent to which this effect can be tuned remains uncertain, a crucial aspect to grasp in light of the varying details of TRS breaking observed through different techniques. Here, we employ the unique low-energy muon spin rotation technique combined with local field numerical analysis to study the TRS breaking response as a function of depth from the surface in single crystals of RbV$_{3}$Sb$_{5}$ with charge order and Cs(V$_{0.86}$Ta$_{0.14}$)$_{3}$Sb$_{5}$ without charge order. In the bulk (i.e., > 33 nm from the surface) of RbV$_{3}$Sb$_{5}$, we have detected a notable increase in the internal magnetic field width experienced by the muon ensemble. This increase occurs only within the charge ordered state. Intriguingly, the muon spin relaxation rate is significantly enhanced near the surface (i.e., < 33 nm from the surface) of RbV$_{3}$Sb$_{5}$, and this effect commences at temperatures significantly higher than the onset of charge order. Conversely, in Cs(V$_{0.86}$Ta$_{0.14}$)$_{3}$Sb$_{5}$, we do not observe a similar enhancement in the internal field width, neither in the bulk nor near the surface. These observations indicate a strong connection between charge order and TRS breaking on one hand, and on the other hand, suggest that TRS breaking can occur prior to long-range charge order. This research offers compelling evidence for depth-dependent magnetism in $A$V$_{3}$Sb$_{5}$ superconductors in the presence of charge order. Such findings are likely to elucidate the intricate microscopic mechanisms that underpin the TRS breaking phenomena in these materials. △ Less

Submitted 16 February, 2024; originally announced February 2024.

Comments: 16 pages, 13 figures

Journal ref: Nature Communications 15, 8978 (2024)

Coexistence of Superconductivity and Antiferromagnetism in Topological Magnet MnBi2Te4 Films

Authors: Wei Yuan, Zi-Jie Yan, Hemian Yi, Zihao Wang, Stephen Paolini, Yi-Fan Zhao, Ling-Jie Zhou, Annie G. Wang, Ke Wang, Thomas Prokscha, Zaher Salman, Andreas Suter, Purnima P. Balakrishnan, Alexander J. Grutter, Laurel E. Winter, John Singleton, Moses H. W. Chan, Cui-Zu Chang

Abstract: 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.… ▽ More 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. △ Less

Submitted 14 February, 2024; originally announced February 2024.

Comments: 24 pages, 4 figures, comments are welcome

Journal ref: Nano Lett. 24, 7962 (2024)

Magnetic Fluctuations in Niobium Pentoxide

Authors: Y. Krasnikova, A. A. Murthy, F. Crisa, M. Bal, Z. Sung, J. Lee, A. Cano, D. M. T. van Zanten, A. Romanenko, A. Grassellino, A. Suter, T. Prokscha, Z. Salman

Abstract: Using a spin-polarized muon beam we were able to capture magnetic dynamics in an amorphous niobium pentoxide thin film. Muons are used to probe internal magnetic fields produced by defects. Magnetic fluctuations could be described by the dynamical Kubo-Toyabe model considering a time-dependent local magnetic field. We state that observed fluctuations result from the correlated motion of electron s… ▽ More Using a spin-polarized muon beam we were able to capture magnetic dynamics in an amorphous niobium pentoxide thin film. Muons are used to probe internal magnetic fields produced by defects. Magnetic fluctuations could be described by the dynamical Kubo-Toyabe model considering a time-dependent local magnetic field. We state that observed fluctuations result from the correlated motion of electron spins. We expect that oxygen vacancies play a significant role in these films and lead to a complex magnetic field distribution which is non-stationary. The characteristic average rate of magnetic field change is on the order of 100~MHz. The observed dynamics may provide insight into potential noise sources in Nb-based superconducting devices, while also highlighting the limitations imposed by amorphous oxides. △ Less

Submitted 17 December, 2023; originally announced December 2023.

Comments: 8 pages, 7 figures

Report number: FERMILAB-PUB-23-642-SQMS

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

Authors: Hemian Yi, Yi-Fan Zhao, Ying-Ting Chan, Jiaqi Cai, Ruobing Mei, Xianxin Wu, Zi-Jie Yan, Ling-Jie Zhou, Ruoxi Zhang, Zihao Wang, Stephen Paolini, Run Xiao, Ke Wang, Anthony R. Richardella, John Singleton, Laurel E. Winter, Thomas Prokscha, Zaher Salman, Andreas Suter, Purnima P. Balakrishnan, Alexander J. Grutter, Moses H. W. Chan, Nitin Samarth, Xiaodong Xu, Weida Wu , et al. (2 additional authors not shown)

Abstract: 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 antiferr… ▽ More 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. △ Less

Submitted 7 December, 2023; originally announced December 2023.

Comments: 14 pages, 4 figures. Accepted by Science. Comments are welcome

Journal ref: Science 383, 634 (2024)

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

Authors: Matthew Brahlek, Alessandro R. Mazza, Abdulgani Annaberdiyev, Michael Chilcote, Gaurab Rimal, Gábor B. Halász, Anh Pham, Yun-Yi Pai, Jaron T. Krogel, Jason Lapano, Benjamin J. Lawrie, Gyula Eres, Jessica McChesney, Thomas Prokscha, Andreas Suter, Seongshik Oh, John W. Freeland, Yue Cao, Jason S. Gardner, Zaher Salman, Robert G. Moore, Panchapakesan Ganesh, T. Zac Ward

Abstract: 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 v… ▽ More 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. △ Less

Submitted 27 August, 2023; v1 submitted 25 July, 2023; originally announced July 2023.

Comments: https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.3c01065

Journal ref: Nano Letters 23, 16, 7279-7287 (2023)

Evidence for current suppression in superconductor-superconductor bilayers

Authors: Md Asaduzzaman, Ryan M. L. McFadden, Anne-Marie Valente-Feliciano, David R. Beverstock, Andreas Suter, Zaher Salman, Thomas Prokscha, Tobias Junginger

Abstract: 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 heterost… ▽ More 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. △ Less

Submitted 28 August, 2023; v1 submitted 18 April, 2023; originally announced April 2023.

Comments: 13 pages and 8 figures

Journal ref: Supercond. Sci. Technol. 37 (2024) 025002

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

Authors: Ryan M. L. McFadden, Md Asaduzzaman, Thomas Prokscha, Zaher Salman, Andreas Suter, Tobias Junginger

Abstract: 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… ▽ More 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. △ Less

Submitted 21 December, 2022; originally announced December 2022.

Comments: 15 pages, 5 figures, 2 tables

Journal ref: Phys. Rev. Appl. 19, 044018 (2023)

Coupling of Magnetic Phases at Nickelate Interfaces

Authors: C. Domínguez, J. Fowlie, A. B. Georgescu, B. Mundet, N. Jaouen, M. Viret, A. Suter, A. J. Millis, Z. Salman, T. Prokscha, M. Gibert, J. -M. Triscone

Abstract: In this work we present a model system built out of artificially layered materials, allowing us to understand the interrelation of magnetic phases with that of the metallic-insulating phase at long length-scales, and enabling new strategies for the design and control of materials in devices. The artificial model system consists of superlattices made of SmNiO$_3$ and NdNiO$_3$ layers -- two members… ▽ More In this work we present a model system built out of artificially layered materials, allowing us to understand the interrelation of magnetic phases with that of the metallic-insulating phase at long length-scales, and enabling new strategies for the design and control of materials in devices. The artificial model system consists of superlattices made of SmNiO$_3$ and NdNiO$_3$ layers -- two members of the fascinating rare earth nickelate family, having different metal-to-insulator and magnetic transition temperatures. By combining two complementary techniques -- resonant elastic x-ray scattering and muon spin relaxation -- we show how the magnetic order evolves, in this complex multicomponent system, as a function of temperature and superlattice periodicity. We demonstrate that the length scale of the coupling between the antiferromagnetic and paramagnetic phases is longer than that of the electronic metal-insulator phase transition -- despite being subsidiary to it. This can be explained via a Landau theory -- where the bulk magnetic energy plus a gradient cost between magnetic and non magnetic phases are considered. These results provide a clear understanding of the coupling of magnetic transitions in systems sharing identical order parameters. △ Less

Submitted 12 November, 2022; originally announced November 2022.

Depth-dependent magnetic crossover in a room-temperature skyrmion-hosting multilayer

Authors: T. J. Hicken, M. N. Wilson, Z. Salman, S. L. Zhang, S. J. R. Holt, T. Prokscha, A. Suter, F. L. Pratt, G. van der Laan, T. Hesjedal, T. Lancaster

Abstract: Skyrmion-hosting multilayer stacks are promising avenues for applications, although little is known about the depth dependence of the magnetism. We address this by reporting the results of circular dichroic resonant elastic x-ray scattering (CD-REXS), micromagnetic simulations, and low-energy muon-spin rotation (LE-$μ^+$SR) measurements on a stack comprising [Ta/CoFeB/MgO]$_{16}$/Ta on a Si substr… ▽ More Skyrmion-hosting multilayer stacks are promising avenues for applications, although little is known about the depth dependence of the magnetism. We address this by reporting the results of circular dichroic resonant elastic x-ray scattering (CD-REXS), micromagnetic simulations, and low-energy muon-spin rotation (LE-$μ^+$SR) measurements on a stack comprising [Ta/CoFeB/MgO]$_{16}$/Ta on a Si substrate. Energy-dependent CD-REXS shows a continuous, monotonic evolution of the domain-wall helicity angle with incident energy, consistent with a three-dimensional hybrid domain-wall-like structure that changes from Néel-like near the surface to Bloch-like deeper within the sample. LE-$μ^+$SR reveals that the magnetic field distribution in the trilayers near the surface of the stack is distinct from that in trilayers deeper within the sample. Our micromagnetic simulations support a quantitative analysis of the $μ^+$SR results. By increasing the applied magnetic field, we find a reduction in the volume occupied by domain walls at all depths, consistent with a crossover into a region dominated by skyrmions above approximately 180 mT. △ Less

Submitted 27 March, 2024; v1 submitted 12 October, 2022; originally announced October 2022.

Comments: 9 pages, 4 figures

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

Authors: Gesa Welker, Martin de Wit, Tjerk H. Oosterkamp, John A. Mydosh, Andreas Suter, Thomas Prokscha, Lucia Bossoni

Abstract: 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… ▽ More 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. △ Less

Submitted 5 September, 2022; originally announced September 2022.

Hydrogen-impurity induced unconventional magnetism in semiconducting molybdenum ditelluride

Authors: Jonas A. Krieger, Daniel Tay, Igor P. Rusinov, Sourabh Barua, Pabitra K. Biswas, Lukas Korosec, Thomas Prokscha, Thorsten Schmitt, Niels B. M. Schröter, Tian Shang, Toni Shiroka, Andreas Suter, Geetha Balakrishnan, Evgueni V. Chulkov, Vladimir N. Strocov, Zaher Salman

Abstract: 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… ▽ More 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. △ Less

Submitted 7 June, 2022; originally announced June 2022.

Comments: 17 pages, 13 figures, including supplementary

Journal ref: Phys. Rev. Materials 7, 044414 (2023)

Discovery of an electronic crystal in a cuprate Mott insulator

Authors: Mingu Kang, Charles Zhang, Enrico Schierle, Stephen McCoy, Jiarui Li, Ronny Sutarto, Feizhou He, Andreas Suter, Thomas Prokscha, Zaher Salman, Eugen Weschke, Shane Cybart, John Y. T. Wei, Riccardo Comin

Abstract: 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 yea… ▽ More 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. △ Less

Submitted 16 March, 2022; originally announced March 2022.

Intrinsic magnetism in superconducting infinite-layer nickelates

Authors: Jennifer Fowlie, Marios Hadjimichael, Maria M. Martins, Danfeng Li, Motoki Osada, Bai Yang Wang, Kyuho Lee, Yonghun Lee, Zaher Salman, Thomas Prokscha, Jean-Marc Triscone, Harold Y. Hwang, Andreas Suter

Abstract: 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 whe… ▽ More 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]. △ Less

Submitted 28 January, 2022; originally announced January 2022.

Journal ref: Nat. Phys. 18 1043-1047 (2022)

Sulfur-induced magnetism in FeSe$_{1-x}$S$_x$ thin films on LaAlO$_3$ revealed by muon spin rotation/relaxation

Authors: F. Nabeshima, Y. Kawai, N. Shikama, Y. Sakishita, A. Suter, T. Prokscha, S. E. Park, S. Komiya, A. Ichinose, T. Adachi, A. Maeda

Abstract: Muon spin rotation/relaxation measurements were performed to investigate magnetic properties of FeSe$_{1-x}$S$_x$ thin films on LaAlO$_3$. A drastic decrease of the initial asymmetry was observed together with the peak structure in the temperature dependence of the relaxation rate of muon spins almost at the same temperature where kink anomalies were observed in the temperature dependent resistivi… ▽ More Muon spin rotation/relaxation measurements were performed to investigate magnetic properties of FeSe$_{1-x}$S$_x$ thin films on LaAlO$_3$. A drastic decrease of the initial asymmetry was observed together with the peak structure in the temperature dependence of the relaxation rate of muon spins almost at the same temperature where kink anomalies were observed in the temperature dependent resistivity. With increasing S content, the anomaly temperature increased and the magnetic fluctuations at the lowest temperature were suppressed. These results show that the S substitution induces magnetism at low temperatures in FeSe$_{1-x}$S$_x$ thin films. Although the behaviors of the magnetic and nematic phases in FeSe films towards chemical pressure by S substitution are similar to those for bulk FeSe towards hydrostatic pressure, the behavior of $T_{\mathrm c}$ is significantly different between these systems. Our results demonstrate that the detailed comparative investigation among physical and chemical pressure effects is essentially important to understand the interplay of the magnetism, the nematicity and the superconductivity in iron chalcogenides. △ Less

Submitted 11 March, 2021; originally announced March 2021.

Comments: 19 pages, 5 figures

Journal ref: Phys. Rev. B 103, 184504 (2021)

Magnetic order of tetragonal CuO ultra-thin films

Authors: N. Ortiz Hernandez, Z. Salman, T. Prokscha, A. Suter, J. R. L. Mardegan, S. Moser, A. Zakharova, C. Piamonteze, und U. Staub

Abstract: We present a detailed low-energy muon spin rotation and x-ray magnetic circular dichroism (XMCD) investigation of the magnetic structure in ultra-thin tetragonal (T)-CuO films. The measured muon-spin polarization decay indicates an antiferromagnetic (AFM) order with a transition temperature higher than 200K. The XMCD signal obtained around the Cu $L_{2,3}$ edges indicates the presence of pinned Cu… ▽ More We present a detailed low-energy muon spin rotation and x-ray magnetic circular dichroism (XMCD) investigation of the magnetic structure in ultra-thin tetragonal (T)-CuO films. The measured muon-spin polarization decay indicates an antiferromagnetic (AFM) order with a transition temperature higher than 200K. The XMCD signal obtained around the Cu $L_{2,3}$ edges indicates the presence of pinned Cu$^{2+}$ moments that are parallel to the sample surface, and additionally, isotropic paramagnetic moments. The pinning of some of the Cu moments is caused by an AFM ordering consisting of moments that lie most likely in the plane of the film. Moreover, pinned moments show a larger orbital magnetic moment contribution with an approximate ratio of $m_{orb}/m_{spin} = 2$, indicating that these spins are located at sites with reduced symmetry. Some fractions of the pinned moments remain pinned from an AFM background even at 360K, indicating that $T_N >$ 360K. A simple model could explain qualitatively these experimental findings; however, it is in contrast to theoretical predictions, showing that the magnetic properties of ultra-thin T-CuO films differ from bulk expectations and is more complex. △ Less

Submitted 2 February, 2021; v1 submitted 30 January, 2021; originally announced February 2021.

Comments: 11 Figures

Journal ref: Phys. Rev. B 103, 224429 (2021)

Proximity-Induced Odd-Frequency Superconductivity in a Topological Insulator

Authors: Jonas A. Krieger, Anna Pertsova, Sean R. Giblin, Max Döbeli, Thomas Prokscha, Christof W. Schneider, Andreas Suter, Thorsten Hesjedal, Alexander V. Balatsky, Zaher Salman

Abstract: 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 superc… ▽ More 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. △ Less

Submitted 30 June, 2020; v1 submitted 26 March, 2020; originally announced March 2020.

Comments: Accepted version for publication in Physical Review Letters

Journal ref: Phys. Rev. Lett. 125, 026802 (2020)

Intertwined magnetic, structural, and electronic transitions in V$_2$O$_3$

Authors: Benjamin A. Frandsen, Yoav Kalcheim, Ilya Valmianski, Alexander S. McLeod, Z. Guguchia, Sky C. Cheung, Alannah M. Hallas, Murray N. Wilson, Yipeng Cai, Graeme M. Luke, Z. Salman, A. Suter, T. Prokscha, Taito Murakami, Hiroshi Kageyama, D. N. Basov, Ivan K. Schuller, Yasutomo J. Uemura

Abstract: We present a coordinated study of the paramagnetic-to-antiferromagnetic, rhombohedral-to-monoclinic, and metal-to-insulator transitions in thin-film specimens of the classic Mott insulator V$_2$O$_3$ using low-energy muon spin relaxation, x-ray diffraction, and nanoscale-resolved near-field infrared spectroscopic techniques. The measurements provide a detailed characterization of the thermal evolu… ▽ More We present a coordinated study of the paramagnetic-to-antiferromagnetic, rhombohedral-to-monoclinic, and metal-to-insulator transitions in thin-film specimens of the classic Mott insulator V$_2$O$_3$ using low-energy muon spin relaxation, x-ray diffraction, and nanoscale-resolved near-field infrared spectroscopic techniques. The measurements provide a detailed characterization of the thermal evolution of the magnetic, structural, and electronic phase transitions occurring in a wide temperature range, including quantitative measurements of the high- and low-temperature phase fractions for each transition. The results reveal a stable coexistence of the high- and low-temperature phases over a broad temperature range throughout the transition. Careful comparison of temperature dependence of the different measurements, calibrated by the resistance of the sample, demonstrates that the electronic, magnetic, and structural degrees of freedom remain tightly coupled to each other during the transition process. We also find evidence for antiferromagnetic fluctuations in the vicinity of the phase transition, highlighting the important role of the magnetic degree of freedom in the metal-insulator transition. △ Less

Submitted 23 December, 2019; v1 submitted 22 October, 2019; originally announced October 2019.

Journal ref: Phys. Rev. B 100, 235136 (2019)

Direct observation of hole carrier density profiles and their light induced manipulation at the surface of Ge

Authors: T. Prokscha, K. H. Chow, Z. Salman, E. Stilp, A. Suter

Abstract: We demonstrate that, by using low-energy positive muon ($μ^+$) spin spectroscopy as a local probe technique, the profiles of free charge carriers can be directly determined in the accumulation/depletion surface regions of p- or n-type Ge wafers. The detection of free holes is accomplished by measuring the effect of the interaction of the free carriers with the $μ^+$ probe spin on the observable mu… ▽ More We demonstrate that, by using low-energy positive muon ($μ^+$) spin spectroscopy as a local probe technique, the profiles of free charge carriers can be directly determined in the accumulation/depletion surface regions of p- or n-type Ge wafers. The detection of free holes is accomplished by measuring the effect of the interaction of the free carriers with the $μ^+$ probe spin on the observable muon spin polarization. By tuning the energy of the low-energy $μ^+$ between 1 keV and 20 keV the near-surface region between 10 nm and 160 nm is probed. We find hole carrier depletion and electron accumulation in all samples with doping concentrations up to the $10^{17}$ cm$^{-3}$ range, which is opposite to the properties of cleaved Ge surfaces. By illumination with light the hole carrier density in the depletion zone can be manipulated in a controlled way. Depending on the used light wavelength $λ$ this change can be persistent ($λ= 405, 457$ nm) or non-persistent ($λ= 635$ nm) at temperatures $< 270$ K. This difference is attributed to the different kinetic energies of the photo-electrons. Photo-electrons generated by red light do not have sufficient energy to overcome a potential barrier at the surface to be trapped in empty surface acceptor states. Compared to standard macroscopic transport measurements our contact-less local probe technique offers the possibility of measuring carrier depth profiles and manipulation directly. Our approach may provide important microscopic information on a nanometer scale in semiconductor device studies. △ Less

Submitted 21 July, 2020; v1 submitted 25 September, 2019; originally announced September 2019.

Journal ref: Phys. Rev. Applied 14, 014098 (2020)

Do topology and ferromagnetism cooperate at the EuS/Bi$_2$Se$_3$ interface?

Authors: J. A. Krieger, Y. Ou, M. Caputo, A. Chikina, M. Döbeli, M. -A. Husanu, I. Keren, T. Prokscha, A. Suter, C. -Z. Chang, J. S. Moodera, V. N. Strocov, Z. Salman

Abstract: We probe the local magnetic properties of interfaces between the insulating ferromagnet EuS and the topological insulator Bi$_2$Se$_3$ using low energy muon spin rotation (LE-$μ$SR). We compare these to the interface between EuS and the topologically trivial metal, titanium. Below the magnetic transition of EuS, we detect strong local magnetic fields which extend several nm into the adjacent layer… ▽ More We probe the local magnetic properties of interfaces between the insulating ferromagnet EuS and the topological insulator Bi$_2$Se$_3$ using low energy muon spin rotation (LE-$μ$SR). We compare these to the interface between EuS and the topologically trivial metal, titanium. Below the magnetic transition of EuS, we detect strong local magnetic fields which extend several nm into the adjacent layer and cause a complete depolarization of the muons. However, in both Bi$_2$Se$_3$ and titanium we measure similar local magnetic fields, implying that their origin is mostly independent of the topological properties of the interface electronic states. In addition, we use resonant soft X-ray angle resolved photoemission spectroscopy (SX-ARPES) to probe the electronic band structure at the interface between EuS and Bi$_2$Se$_3$. By tuning the photon energy to the Eu anti-resonance at the Eu $M_5$ pre-edge we are able to detect the Bi$_2$Se$_3$ conduction band, through a protective Al$_2$O$_3$ capping layer and the EuS layer. Moreover, we observe a signature of an interface-induced modification of the buried Bi$_2$Se$_3$ wave functions and/or the presence of interface states. △ Less

Submitted 31 January, 2019; originally announced January 2019.

Microscopic effects of Dy-doping in the topological insulator Bi2Te3

Authors: L. B. Duffy, N. -J. Steinke, J. A. Krieger, A. I. Figueroa, K. Kummer, T. Lancaster, S. R. Giblin, F. L. Pratt, S. J. Blundell, T. Prokscha, A. Suter, S. Langridge, V. N. Strocov, Z. Salman, G. van der Laan, T. Hesjedal

Abstract: Magnetic doping with transition metal ions is the most widely used approach to break timereversal symmetry in a topological insulator, a prerequisite for unlocking the TIs exotic potential. Recently, we reported the doping of Bi2Te3 thin films with rare earth ions, which, owing to their large magnetic moments, promise commensurately large magnetic gap openings in the topological surface states. Ho… ▽ More Magnetic doping with transition metal ions is the most widely used approach to break timereversal symmetry in a topological insulator, a prerequisite for unlocking the TIs exotic potential. Recently, we reported the doping of Bi2Te3 thin films with rare earth ions, which, owing to their large magnetic moments, promise commensurately large magnetic gap openings in the topological surface states. However, only when doping with Dy has a sizable gap been observed in angle-resolved photoemission spectroscopy, which persists up to room-temperature. Although disorder alone could be ruled out as a cause of the topological phase transition, a fundamental understanding of the magnetic and electronic properties of Dy:Bi2Te3 remained elusive. Here, we present an X-ray magnetic circular dichroism, polarized neutron reflectometry, muon spin rotation, and resonant photoemission study of the microscopic magnetic and electronic properties. We find that the films are not simply paramagnetic but that instead the observed behavior can be well explained by the assumption of slowly fluctuating, inhomogeneous magnetic patches with increasing volume fraction as the temperature decreases. At liquid helium temperatures, a large effective magnetization can be easily introduced by the application of moderate magnetic fields, implying that this material is very suitable for proximity coupling to an underlying ferromagnetic insulator or in a heterostructure with transition metal-doped layers. However, the introduction of some charge carriers by the dopants cannot be excluded at least in these highly doped samples. Nevertheless, we find that the magnetic order is not mediated via the conduction channel in these rare earth doped samples and therefore magnetic order and carrier concentration are expected to be independently controllable. This is not generally the case for transition metal doped topological insulators. △ Less

Submitted 6 August, 2018; originally announced August 2018.

Comments: 9 pages, 5 figures

Journal ref: Phys. Rev. B 97, 174427 (2018)

Search for the magnetic monopole at a magnetoelectric surface

Authors: Q. N. Meier, M. Fechner, T. Nozaki, M. Sahashi, Z. Salman, T. Proschka, A. Suter, P. Schoenherr, M. Lilienblum, P. Borisov, I. E. Dzyaloshinskii, M. Fiebig, H. Luetkens, N. A. Spaldin

Abstract: We show, by solving Maxwell's equations, that an electric charge on the surface of a slab of a linear magnetoelectric material generates an image magnetic monopole below the surface provided that the magnetoelectric has a diagonal component in its magnetoelectric response. The image monopole, in turn, generates an ideal monopolar magnetic field outside of the slab. Using realistic values of the el… ▽ More We show, by solving Maxwell's equations, that an electric charge on the surface of a slab of a linear magnetoelectric material generates an image magnetic monopole below the surface provided that the magnetoelectric has a diagonal component in its magnetoelectric response. The image monopole, in turn, generates an ideal monopolar magnetic field outside of the slab. Using realistic values of the electric- and magnetic- field susceptibilties, we calculate the magnitude of the effect for the prototypical magnetoelectric material Cr$_2$O$_3$. We use low energy muon spin rotation to measure the strength of the magnetic field generated by charged muons as a function of their distance from the surface of a Cr$_2$O$_3$ films, and show that the results are consistent with the existence of the monopole. We discuss other possible routes to detecting the monopolar field, and show that, while the predicted monopolar field generated by Cr$_2$O$_3$ is above the detection limit for standard magnetic force microscopy, detection of the field using this technique is prevented by surface charging effects. △ Less

Submitted 21 January, 2019; v1 submitted 20 April, 2018; originally announced April 2018.

Comments: 8 pages, 9 figures

Journal ref: Phys. Rev. X 9, 011011 (2019)

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

Authors: Nimrod Bachar, Aviad Levy, Thomas Prokscha, Andreas Suter, Elvezio Morenzoni, Zaher Salman, Guy Deutscher

Abstract: 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… ▽ More 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. △ Less

Submitted 8 March, 2018; originally announced March 2018.

Comments: 14 pages, 5 figures, including supplementary material

Journal ref: Phys. Rev. B 101, 024424 (2020)

Experimental study of the magnetic field distribution and shape of domains near the surface of type-I superconductors in the intermediate state

Authors: V. Kozhevnikov, A. Suter, T. Prokscha, C. Van Haesendonck

Abstract: The importance of accounting for the inhomogeneity of the magnetic field distribution and roundness of domain walls near the surface of type-I superconductors in the intermediate state for forming the equilibrium flux structure was predicted by Landau eight decades ago. Further studies confirmed this prediction and extended it to all equilibrium properties of this state. Here we report on direct d… ▽ More The importance of accounting for the inhomogeneity of the magnetic field distribution and roundness of domain walls near the surface of type-I superconductors in the intermediate state for forming the equilibrium flux structure was predicted by Landau eight decades ago. Further studies confirmed this prediction and extended it to all equilibrium properties of this state. Here we report on direct depth-resolved measurements of the field distribution and shape of domains near the surface of high-purity type-I (indium) films in a perpendicular field using Low-Energy Muon Spin Rotation spectroscopy. We find that at low applied fields (in about half of the field range of the intermediate state) the field distribution and domains' shape agrees with that proposed by Tinkham. However, for high fields our data suggest that reality differs from theoretical expectations. In particular, the width of the superconducting laminae can expand near the surface leading to formation of a maximum in the static magnetic field in the current-free space outside the sample. A possible interpretation of these experimental results is discussed. △ Less

Submitted 30 June, 2020; v1 submitted 22 February, 2018; originally announced February 2018.

Comments: 12 pages, 16 figures

Journal ref: J. Supercondactivity and Novel Magnetism, 2020

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

Authors: Jonas A. Krieger, Cui-Zu Chang, Marius-Adrian Husanu, Daria Sostina, Arthur Ernst, Mikhail M. Otrokov, Thomas Prokscha, Thorsten Schmitt, Andreas Suter, Maia Garcia Vergniory, Evgueni V. Chulkov, Jagadeesh S. Moodera, Vladimir N. Strocov, Zaher Salman

Abstract: 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 transi… ▽ More 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. △ Less

Submitted 28 November, 2017; v1 submitted 17 October, 2017; originally announced October 2017.

Journal ref: Phys. Rev. B 96, 184402 (2017)

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

Authors: Jonas A. Krieger, Amit Kanigel, Amit Riback, Ekaterina Pomjakushina, Khanan B. Chashka, Kazimierz Conder, Elvezio Morenzoni, Thomas Prokscha, Andreas Suter, Zaher Salman

Abstract: 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 penetrat… ▽ More 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. △ Less

Submitted 28 July, 2017; originally announced July 2017.

Superconductivity drives magnetism in delta-doped La2CuO4

Authors: A. Suter, G. Logvenov, A. V. Boris, F. Baiutti, F. Wrobel, L. Howald, E. Stilp, Z. Salman, T. Prokscha, B. Keimer

Abstract: The understanding of the interplay between different orders in a solid is a key challenge in highly correlated electronic systems. In real systems this is even more difficult since disorder can have a strong influence on the subtle balance between these orders and thus can obscure the interpretation of the observed physical properties. Here we present a study on delta-doped La2CuO4 superlattices.… ▽ More The understanding of the interplay between different orders in a solid is a key challenge in highly correlated electronic systems. In real systems this is even more difficult since disorder can have a strong influence on the subtle balance between these orders and thus can obscure the interpretation of the observed physical properties. Here we present a study on delta-doped La2CuO4 superlattices. By means of molecular beam epitaxy whole LaO-layers were periodically replaced through SrO-layers providing a charge reservoir, yet reducing the level of disorder typically present in doped cuprates to an absolute minimum. The induced superconductivity and its interplay with the antiferromagnetic order is studied by means of low-energy muSR. We find a quasi-2D superconducting state which couples to the antiferromagnetic order in a non-trivial way. Below the superconducting transition temperature, the magnetic volume fraction increases strongly. The reason could be a charge redistribution of the free carriers due to the opening of the superconducting gap which is possible due to the close proximity and low disorder between the different ordered regions. △ Less

Submitted 23 June, 2017; originally announced June 2017.

Comments: 4 figures

Journal ref: Phys. Rev. B 97, 134522 (2018)

Possible existence of a filamentary state in type-II superconductors

Authors: V. Kozhevnikov, A. -M. Valente-Feliciano, P. J. Curran, A. Suter, A. H. Liu, G. Richter, E. Morenzoni, S. J. Bending, C. Van Haesendonck

Abstract: The standard interpretation of the phase diagram of type-II superconductors was developed in 1960s and has since been considered a well-established part of classical superconductivity. However, upon closer examination a number of fundamental issues arise that leads one to question this standard picture. To address these issues we studied equilibrium properties of niobium samples near and above the… ▽ More The standard interpretation of the phase diagram of type-II superconductors was developed in 1960s and has since been considered a well-established part of classical superconductivity. However, upon closer examination a number of fundamental issues arise that leads one to question this standard picture. To address these issues we studied equilibrium properties of niobium samples near and above the upper critical field Hc2 in parallel and perpendicular magnetic fields. The samples investigated were very high quality films and single crystal discs with the Ginzburg-Landau parameters 0.8 and 1.3, respectively. A range of complementary measurements have been performed, which include dc magnetometry, electrical transport, muSR spectroscopy and scanning Hall-probe microscopy. Contrarily to the standard scenario, we observed that a superconducting phase is present in the sample bulk above Hc2 and the field Hc3 is the same in both parallel and perpendicular fields. Our findings suggest that above Hc2 the superconducting phase forms filaments parallel to the field regardless on the field orientation. Near Hc2 the filaments preserve the hexagonal structure of the preceding vortex lattice of the mixed state and the filament density continuously falls to zero at Hc3. Our work has important implications for the correct interpretation of properties of type-II superconductors and can also be essential for practical applications of these materials. △ Less

Submitted 9 May, 2017; originally announced May 2017.

Comments: 6 pages, 8 figures

Journal ref: Phys. Rev. B 95, 174509 (2017)

Suppression of magnetic excitations near the surface of the topological Kondo insulator SmB6

Authors: P. K. Biswas, M. Legner, G. Balakrishnan, M. Ciomaga Hatnean, M. R. Lees, D. McK. Paul, E. Pomjakushina, T. Prokscha, A. Suter, T. Neupert, Z. Salman

Abstract: We present a detailed investigation of the temperature and depth dependence of the magnetic properties of 3D topological Kondo insulator SmB6 , in particular near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk… ▽ More We present a detailed investigation of the temperature and depth dependence of the magnetic properties of 3D topological Kondo insulator SmB6 , in particular near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk SmB6 , which produce local magnetic fields of about ~1.8 mT fluctuating on a time scale of ~60 ns. We find that the excitonic fluctuations are suppressed when approaching the surface on a length scale of 40-90 nm, accompanied by a small enhancement in static magnetic fields. We associate this length scale to the size of the excitonic state. △ Less

Submitted 4 January, 2017; originally announced January 2017.

Comments: 5 pages, 5 figures, accepted for publication as a Rapid Communication in Phys. Rev. B

Journal ref: Phys. Rev. B 95, 020410 (2017)

Intrinsic Ferromagnetism in the Diluted Magnetic Semiconductor Co:TiO$_2$

Authors: H. Saadaoui, X. Luo, Z. Salman, X. Y. Cui, N. N. Bao, P. Bao, R. K. Zheng, L. Tseng, Y. H. Du, T. Prokscha, A. Suter, T. Liu, Y. R. Wang, S. Li, J. Ding, S. P. Ringer, E. Morenzoni, J. B. Yi

Abstract: Here we present a study of magnetism in \CTO\ anatase films grown by pulsed laser deposition under a variety of oxygen partial pressures and deposition rates. Energy-dispersive spectrometry and transition electron microscopy analyses indicate that a high deposition rate leads to a homogeneous microstructure, while very low rate or postannealing results in cobalt clustering. Depth resolved low-ener… ▽ More Here we present a study of magnetism in \CTO\ anatase films grown by pulsed laser deposition under a variety of oxygen partial pressures and deposition rates. Energy-dispersive spectrometry and transition electron microscopy analyses indicate that a high deposition rate leads to a homogeneous microstructure, while very low rate or postannealing results in cobalt clustering. Depth resolved low-energy muon spin rotation experiments show that films grown at a low oxygen partial pressure ($\approx 10^{-6}$ torr) with a uniform structure are fully magnetic, indicating intrinsic ferromagnetism. First principles calculations identify the beneficial role of low oxygen partial pressure in the realization of uniform carrier-mediated ferromagnetism. This work demonstrates that Co:TiO$_2$ is an intrinsic diluted magnetic semiconductor. △ Less

Submitted 7 December, 2016; originally announced December 2016.

Comments: 6 pages, 4 figures, Supplemental Material (not included): 17 pages, 9 Figures

Journal ref: Physical Review Letters, 117, 227202 (2016)

Probing current-induced magnetic fields in Au|YIG heterostructure with low-energy muon spectroscopy

Authors: A. Aqeel, I. J. Vera-Marun, Z. Salman, T. Prokscha, A. Suter, B. J. van Wees, T. T. M. Palstra

Abstract: We investigated the depth dependence of current-induced magnetic fields in a bilayer of a normal metal (Au) and a ferrimagnetic insulator (Yttrium Iron Garnet - YIG) by using low energy muon spectroscopy (LE-muSR). This allows us to explore how these fields vary from the Au surface down to the buried Au|YIG interface, which is relevant to study physics like the spin-Hall effect. We observed a maxi… ▽ More We investigated the depth dependence of current-induced magnetic fields in a bilayer of a normal metal (Au) and a ferrimagnetic insulator (Yttrium Iron Garnet - YIG) by using low energy muon spectroscopy (LE-muSR). This allows us to explore how these fields vary from the Au surface down to the buried Au|YIG interface, which is relevant to study physics like the spin-Hall effect. We observed a maximum shift of 0.4 G in the internal field of muons at the surface of Au film which is in close agreement to the value expected for Oersted fields. As muons are implanted closer to the Au|YIG interface the shift is strongly suppressed, which we attribute to the dipolar fields present at the Au|YIG interface. Combining our measurements with modelling, we show that dipolar fields caused by the finite roughness of the Au|YIG interface consistently explains our observations. Our results, therefore, gauge the limits on the spatial resolution and the sensitivity of LE-muSR to the roughness of the buried magnetic interfaces, a prerequisite for future studies addressing current induced fields caused by the spin-Hall effect. △ Less

Submitted 16 August, 2016; originally announced August 2016.

Comments: 5 pages, 4 figures

Journal ref: Appl. Phys. Lett. 110, 062409 (2017)

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

Authors: Angelo Di Bernardo, Zaher Salman, Xiaolei Wang, Mario Amado, Mehmet Egilmez, Machiel Flokstra, Andreas Suter, Steve Lee, Jianhua Zhao, Thomas Prokscha, Elvezio Morenzoni, Mark Blamire, Jacob Linder, Jason W. A. Robinson

Abstract: 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 magnet… ▽ More 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. △ Less

Submitted 7 May, 2016; originally announced May 2016.

Journal ref: Physical Review X 5, 041021 (2015)

Equilibrium properties of the mixed state in superconducting niobium in a transverse magnetic field: Experiment and theoretical model

Authors: V. Kozhevnikov, A. -M. Valente-Feliciano, P. J. Curran, G. Richter, A. Volodin, A. Suter, S. Bending, C. Van Haesendonck

Abstract: Equilibrium magnetic properties of the mixed state in type-II superconductors were measured with high purity bulk and film niobium samples in parallel and perpendicular magnetic fields using dc magnetometry and scanning Hall-probe microscopy. Equilibrium magnetization data for the perpendicular geometry were obtained for the first time. It was found that none of the existing theories is consistent… ▽ More Equilibrium magnetic properties of the mixed state in type-II superconductors were measured with high purity bulk and film niobium samples in parallel and perpendicular magnetic fields using dc magnetometry and scanning Hall-probe microscopy. Equilibrium magnetization data for the perpendicular geometry were obtained for the first time. It was found that none of the existing theories is consistent with these new data. To address this problem, a theoretical model is developed and experimentally validated. The new model describes the mixed state in an averaged limit, i.e. %without detailing the samples' magnetic structure and therefore ignoring interactions between vortices. It is quantitatively consistent with the data obtained in a perpendicular field and provides new insights on properties of vortices. % and the entire mixed state. At low values of the Ginzburg-Landau parameter, the model converts to that of Peierls and London for the intermediate state in type-I superconductors. It is shown that description of the vortex matter in superconductors in terms of a 2D gas is more appropriate than the frequently used crystal- and glass-like scenarios. △ Less

Submitted 2 March, 2018; v1 submitted 13 March, 2016; originally announced March 2016.

Comments: 8 pages, 9 figures

Journal ref: J. Supercondactivity and Novel Magnetism 31, 3433 (2018)

Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi

Authors: T. Lancaster, F. Xiao, Z. Salman, I. O. Thomas, S. J. Blundell, F. L. Pratt, S. J. Clark, T. Prokscha, A. Suter, S. L. Zhang, A. A. Baker, T. Hesjedal

Abstract: We present the results of transverse-field muon-spin rotation measurements on an epitaxially grown 40 nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around $T\approx 40$ K an… ▽ More We present the results of transverse-field muon-spin rotation measurements on an epitaxially grown 40 nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around $T\approx 40$ K and 30 K. Our results suggest that the cone phase is not the only magnetic texture realized in film samples for out-of-plane fields. △ Less

Submitted 26 April, 2016; v1 submitted 16 November, 2015; originally announced November 2015.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 93, 140412 (2016)

Remotely induced magnetism in a normal metal using a superconducting spin-valve

Authors: M. G. Flokstra, N. Satchell, J. Kim, G. Burnell, P. J. Curran, S. J. Bending, J. F. K. Cooper, C. J. Kinane, S. Langridge, A. Isidori, N. Pugach, M. Eschrig, H. Luetkens, A. Suter, T. Prokscha, S. L. Lee

Abstract: Superconducting spintronics has emerged in the last decade as a promising new field that seeks to open a new dimension for nanoelectronics by utilizing the internal spin structure of the superconducting Cooper pair as a new degree of freedom. Its basic building blocks are spin-triplet Cooper pairs with equally aligned spins, which are promoted by proximity of a conventional superconductor to a fer… ▽ More Superconducting spintronics has emerged in the last decade as a promising new field that seeks to open a new dimension for nanoelectronics by utilizing the internal spin structure of the superconducting Cooper pair as a new degree of freedom. Its basic building blocks are spin-triplet Cooper pairs with equally aligned spins, which are promoted by proximity of a conventional superconductor to a ferromagnetic material with inhomogeneous macroscopic magnetization. Using low-energy muon spin rotation experiments, we find an entirely unexpected novel effect: the appearance of a magnetization in a thin layer of a non-magnetic metal (gold), separated from a ferromagnetic double layer by a 50 nm thick superconducting layer of Nb. The effect can be controlled by either temperature or by using a magnetic field to control the state of the remote ferromagnetic elements and may act as a basic building block for a new generation of quantum interference devices based on the spin of a Cooper pair. △ Less

Submitted 13 May, 2015; originally announced May 2015.

Comments: 17 pages, 4 figures