Coexistence of Superconductivity and Antiferromagnetism in Topological Magnet MnBi2Te4 Films.

The interface of two materials can harbor unexpected emergent phenomena. One example is interface-induced superconductivity. In this work, we employ molecular beam epitaxy to grow a series of heterostructures formed by stacking together two nonsuperconducting 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 coexistence of 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.

Interface-induced superconductivity in magnetic topological insulators.

The interface between two different materials can show unexpected quantum phenomena. In this study, we used molecular beam epitaxy to synthesize heterostructures formed by stacking together two magnetic materials, a ferromagnetic topological insulator (TI) and an antiferromagnetic iron chalcogenide (FeTe). We observed emergent interface-induced superconductivity in these heterostructures and demonstrated the co-occurrence of superconductivity, ferromagnetism, and topological band structure in the magnetic TI layer-the three essential ingredients of chiral topological superconductivity (TSC). The unusual coexistence of ferromagnetism and superconductivity is accompanied by a high upper critical magnetic field that exceeds the Pauli paramagnetic limit for conventional superconductors at low temperatures. These 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.

GermanIum array for non-destructive testing (GIANT) setup for muon-induced x-ray emission (MIXE) at the Paul Scherrer Institute.

The usage of muonic x-rays to study elemental properties like nuclear radii ranges back to the seventies. This triggered the pioneering work at the Paul Scherrer Institute (PSI), during the eighties on the Muon-induced x-ray emission (MIXE) technique for a non-destructive assessment of elemental compositions. In recent years, this method has seen a rebirth, improvement, and adoption at most muon facilities around the world. Hereby, the PSI offers unique capabilities with its high-rate continuous muon beam at the Swiss Muon Source (SµS). We report here the decision-making, construction, and commissioning of a dedicated MIXE spectrometer at PSI, the GermanIum Array for Non-destructive Testing (GIANT) setup. Multiple campaigns highlighted the outstanding capabilities of MIXE at PSI, e.g., resolving down to 1 at. % elemental concentrations with as little as 1 h data taking, measuring isotopic ratios for elements from iron to lead, and characterizing gamma rays induced by muon nuclear capture. On-target beam spots were characterized with a dedicated charged particle tracker to be symmetric to 5% with an average σ = 22.80(25) and 14.41(8) mm for 25 and 45 MeV/c, respectively. Advanced analysis of the high-purity germanium signals further allows us to improve energy and timing resolutions to ∼1 keV and 20 ns at 1 MeV, respectively. Within the GIANT setup, an average detector has a photopeak efficiency of ϵE=0.11% and an energy resolution of σE=0.8keV at E = 1000 keV. The overall performance of the GIANT setup at SµS allowed us to start a rich user program with archaeological samples, Li-ion battery research, and collaboration with the industry. Future improvements will include a simulation-based analysis and a higher degree of automation, e.g., automatic scans of a series of muon momenta and automatic sample changing.

Efficient magnetic switching in a correlated spin glass.

The interplay between spin-orbit interaction and magnetic order is one of the most active research fields in condensed matter physics and drives the search for materials with novel, and tunable, magnetic and spin properties. Here we report on a variety of unique and unexpected observations in thin multiferroic Ge1-xMnxTe films. The ferrimagnetic order parameter in this ferroelectric semiconductor is found to switch direction under magnetostochastic resonance with current pulses many orders of magnitude lower as for typical spin-orbit torque systems. Upon a switching event, the magnetic order spreads coherently and collectively over macroscopic distances through a correlated spin-glass state. Utilizing these observations, we apply a novel methodology to controllably harness this stochastic magnetization dynamics.

Emergent Magnetism with Continuous Control in the Ultrahigh-Conductivity Layered Oxide PdCoO2.

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

Spin-orbit driven superconducting proximity effects in Pt/Nb thin films.

Manipulating the spin state of thin layers of superconducting material is a promising route to generate dissipationless spin currents in spintronic devices. Approaches typically focus on using thin ferromagnetic elements to perturb the spin state of the superconducting condensate to create spin-triplet correlations. We have investigated simple structures that generate spin-triplet correlations without using ferromagnetic elements. Scanning tunneling spectroscopy and muon-spin rotation are used to probe the local electronic and magnetic properties of our hybrid structures, demonstrating a paramagnetic contribution to the magnetization that partially cancels the Meissner screening. This spin-orbit generated magnetization is shown to derive from the spin of the equal-spin pairs rather than from their orbital motion and is an important development in the field of superconducting spintronics.

Discovery of charge order in a cuprate Mott insulator.

Copper oxide superconductors universally exhibit multiple forms of electronically ordered phases that break the native translational symmetry of the CuO2 planes. In underdoped cuprates with correlated metallic ground states, charge/spin stripes and incommensurate charge density waves (CDWs) have been experimentally observed over the years, while early theoretical studies also predicted the emergence of a Coulomb-frustrated 'charge crystal' phase in the very lightly doped, insulating limit of CuO2 planes. Here, we search for signatures of CDW order in very lightly hole-doped cuprates from the 123 family RBa2Cu3O7 - δ (RBCO; R: Y or rare earth), by using resonant X-ray scattering, electron transport, and muon spin rotation measurements to resolve the electronic and magnetic ground states fully. Specifically, Pr is used to substitute Y at the R-site to systematically suppress the superconductivity and access the extremely low hole-doping regime of the cuprate phase diagram without changing the oxygen stoichiometry. X-ray scattering data taken on Pr-doped YBCO thin films reveal an in-plane CDW order that follows the same linear evolution of wave vector versus hole concentration as oxygen-underdoped YBCO but extends all the way to the insulating and magnetically ordered Mott limit. Combined with the recent observation of charge crystal phase on an insulating surface of Bi2Sr2CaCu2O8 + z, our results in RBCO suggest that this electronic symmetry breaking is universally present in very lightly doped CuO2 planes. These findings bridge the gap between the Mott insulating state and the underdoped metallic state and underscore the prominent role that Coulomb-frustrated electronic phase separation plays among all cuprates.

Metal-insulator transition in composition-tuned nickel oxide films.

Thin films of the solid solution Nd1-xLaxNiO3are grown in order to study the expected 0 K phase transitions at a specific composition. We experimentally map out the structural, electronic and magnetic properties as a function ofxand a discontinuous, possibly first order, insulator-metal transition is observed at low temperature whenx= 0.2. Raman spectroscopy and scanning transmission electron microscopy show that this is not associated with a correspondingly discontinuous global structural change. On the other hand, results from density functional theory (DFT) and combined DFT and dynamical mean field theory calculations produce a 0 K first order transition at around this composition. We further estimate the temperature-dependence of the transition from thermodynamic considerations and find that a discontinuous insulator-metal transition can be reproduced theoretically and implies a narrow insulator-metal phase coexistence withx. Finally, muon spin rotation (µSR) measurements suggest that there are non-static magnetic moments in the system that may be understood in the context of the first order nature of the 0 K transition and its associated phase coexistence regime.

The non-destructive investigation of a late antique knob bow fibula (Bügelknopffibel) from Kaiseraugst/CH using Muon Induced X-ray Emission (MIXE).

A knob bow fibula (Bügelknopffibel) of the Leutkirch type, which typologically belongs to the second half of the 4th and early 5th century CE, was excavated in 2018 in the Roman city of Augusta Raurica, present-day Kaiseraugst (AG, Switzerland). This was analyzed for the first time for its elemental composition by using the non-destructive technique of Muon Induced X-ray Emission (MIXE) in the continuous muon beam facility at the Paul Scherrer Institute (PSI). In the present work, the detection limit is 0.4 wt% with ∼ 1.5 hours of measurement time. The fibula was measured at six different positions, at a depth of 0.3-0.4 mm inside the material. The experimental results show that the fibula is made of bronze, containing the main elements copper (Cu), zinc (Zn), tin (Sn) and lead (Pb). The compositional similarities/differences between different parts of the fibula reveal that it was manufactured as two "workpieces". One workpiece consists of the knob (13.0±0.6 wt% Pb), bow (11.9±0.4 wt% Pb) and foot (12.5 ± 0.9 wt% Pb). These show a higher Pb content, suggesting a cast bronze. The spiral (3.2 ± 0.2 wt% Pb), which is part of the other workpiece, has a comparatively lower Pb content, suggesting a forged bronze.

Measurement of the transition frequency from 2S1/2, F = 0 to 2P1/2, F = 1 states in Muonium.

Muons are puzzling physicists since their discovery when they were first thought to be the meson predicted by Yukawa to mediate the strong force. The recent result at Fermilab on the muon g-2 anomaly puts the muonic sector once more under the spotlight and calls for further measurements with this particle. Here, we present the results of the measurement of the 2S1/2, F = 0 → 2P1/2, F = 1 transition in Muonium. The measured value of 580.6(6.8) MHz is in agreement with the theoretical calculations. A value of the Lamb shift of 1045.5(6.8) MHz is extracted, compatible with previous experiments. We also determine the 2S hyperfine splitting in Muonium to be 559.6(7.2) MHz. The measured transition being isolated from the other hyperfine levels holds the promise to provide an improved determination of the Muonium Lamb shift at a level where bound state QED recoil corrections not accessible in hydrogen could be tested. This result would be sensitive to new physics in the muonic sector, e.g., to new bosons which might provide an explanation of the g-2 muon anomaly and allow to test Lorentz and CPT violation. We also present the observation of Muonium in the n = 3 excited state opening up the possibility of additional precise microwave measurements.

Spin Glass State in Strained La2/3Ca1/3MnO3 Thin Films.

Epitaxial strain modifies the physical properties of thin films deposited on single-crystal substrates. In a previous work, we demonstrated that in the case of La2/3Ca1/3MnO3 thin films the strain induced by the substrate can produce the segregation of a non-ferromagnetic layer (NFL) at the top surface of ferromagnetic epitaxial La2/3Ca1/3MnO3 for a critical value of the tetragonality τ, defined as τ = |c - a|a, of τC ≈ 0.024. Although preliminary analysis suggested its antiferromagnetic nature, to date a complete characterization of the magnetic state of such an NFL has not been performed. Here, we present a comprehensive magnetic characterization of the strain-induced segregated NFL. The field-cooled magnetic hysteresis loops exhibit an exchange bias mechanism below T ≈ 80 K, which is well below the Curie temperature of the ferromagnetic La2/3Ca1/3MnO3 layer. The exchange bias and coercive fields decay exponentially with temperature, which is commonly accepted to describe spin-glass (SG) behavior. The signatures of slow dynamics were confirmed by slow spin relaxation over a wide temperature regime. Low-energy muon spectroscopy experiments directly evidence the slowing down of the magnetic moments below ~100 K in the NFL. The experimental results indicate the SG nature of the NFL. This SG state can be understood within the context of the competing ferromagnetic and antiferromagnetic interactions of similar energies.

Proximity-Induced Odd-Frequency Superconductivity in a Topological Insulator.

At an interface between a topological insulator (TI) and a conventional superconductor (SC), superconductivity has been predicted to change dramatically and exhibit novel correlations. In particular, the induced superconductivity by an s-wave SC in a TI can develop an order parameter with a p-wave component. Here we present experimental evidence for an unexpected proximity-induced novel superconducting state in a thin layer of the prototypical TI, Bi_{2}Se_{3} proximity coupled to Nb. From depth-resolved magnetic field measurements below the superconducting transition temperature of Nb, we observe a local enhancement of the magnetic field in Bi_{2}Se_{3} that exceeds the externally applied field, thus supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state. Our experimental results are complemented by theoretical calculations supporting the appearance of such a component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer state it originates from. To the best of our knowledge, these findings present a first observation of bulk odd-frequency superconductivity in a TI. We thus reaffirm the potential of the TI-SC interface as a versatile platform to produce novel superconducting states.

Phase transition in the cuprates from a magnetic-field-free stiffness meter viewpoint.

A method to measure the superconducting (SC) stiffness tensor [Formula: see text], without subjecting the sample to external magnetic field, is applied to La1.875Sr0.125CuO4. The method is based on the London equation [Formula: see text], where J is the current density and A is the vector potential which is applied in the SC state. Using rotor free A and measuring J via the magnetic moment of superconducting rings, [Formula: see text] at T → Tc is extracted. The technique is sensitive to very small stiffnesses (penetration depths on the order of a few millimeters). The method is applied to two different rings: one with the current running only in the CuO2 planes, and another where the current must cross planes. We find different transition temperatures for the two rings, namely, there is a temperature range with two-dimensional stiffness. Additional low energy muon spin rotation measurements on the same sample determine the stiffness anisotropy at T < Tc.

Collective magnetism in an artificial 2D XY spin system.

Two-dimensional magnetic systems with continuous spin degrees of freedom exhibit a rich spectrum of thermal behaviour due to the strong competition between fluctuations and correlations. When such systems incorporate coupling via the anisotropic dipolar interaction, a discrete symmetry emerges, which can be spontaneously broken leading to a low-temperature ordered phase. However, the experimental realisation of such two-dimensional spin systems in crystalline materials is difficult since the dipolar coupling is usually much weaker than the exchange interaction. Here we realise two-dimensional magnetostatically coupled XY spin systems with nanoscale thermally active magnetic discs placed on square lattices. Using low-energy muon-spin relaxation and soft X-ray scattering, we observe correlated dynamics at the critical temperature and the emergence of static long-range order at low temperatures, which is compatible with theoretical predictions for dipolar-coupled XY spin systems. Furthermore, by modifying the sample design, we demonstrate the possibility to tune the collective magnetic behaviour in thermally active artificial spin systems with continuous degrees of freedom.

Intrinsic or Interface Clustering-Induced Ferromagnetism in Fe-Doped In2O3-Diluted Magnetic Semiconductors.

Five percent Fe-doped In2O3 films were deposited using a pulsed laser deposition system. X-ray diffraction and transmission electron microscopy analysis show that the films deposited under oxygen partial pressures of 10-3 and 10-5 Torr are uniform without clusters or secondary phases. However, the film deposited under 10-7 Torr has a Fe-rich phase at the interface. Magnetic measurements demonstrate that the magnetization of the films increases with decreasing oxygen partial pressure. Muon spin relaxation (µSR) analysis indicates that the volume fractions of the ferromagnetic phases in PO2 = 10-3, 10-5, and 10-7 Torr-deposited samples are 23, 49, and 68%, respectively, suggesting that clusters or secondary phases may not be the origin of the ferromagnetism and that the ferromagnetism is not carrier-mediated. We propose that the formation of magnetic bound polarons is the origin of the ferromagnetism. In addition, both µSR and polarized neutron scattering demonstrate that the Fe-rich phase at the interface has a lower magnetization compared to the uniformly distributed phases.

Emergent magnetism at transition-metal-nanocarbon interfaces.

Charge transfer at metallo-molecular interfaces may be used to design multifunctional hybrids with an emergent magnetization that may offer an eco-friendly and tunable alternative to conventional magnets and devices. Here, we investigate the origin of the magnetism arising at these interfaces by using different techniques to probe 3d and 5d metal films such as Sc, Mn, Cu, and Pt in contact with fullerenes and rf-sputtered carbon layers. These systems exhibit small anisotropy and coercivity together with a high Curie point. Low-energy muon spin spectroscopy in Cu and Sc-C60 multilayers show a quick spin depolarization and oscillations attributed to nonuniform local magnetic fields close to the metallo-carbon interface. The hybridization state of the carbon layers plays a crucial role, and we observe an increased magnetization as sp3 orbitals are annealed into sp2-π graphitic states in sputtered carbon/copper multilayers. X-ray magnetic circular dichroism (XMCD) measurements at the carbon K edge of C60 layers in contact with Sc films show spin polarization in the lowest unoccupied molecular orbital (LUMO) and higher π*-molecular levels, whereas the dichroism in the σ*-resonances is small or nonexistent. These results support the idea of an interaction mediated via charge transfer from the metal and dz-π hybridization. Thin-film carbon-based magnets may allow for the manipulation of spin ordering at metallic surfaces using electrooptical signals, with potential applications in computing, sensors, and other multifunctional magnetic devices.

Robust Magnetic Properties of a Sublimable Single-Molecule Magnet.

The organization of single-molecule magnets (SMMs) on surfaces via thermal sublimation is a prerequisite for the development of future devices for spintronics exploiting the richness of properties offered by these magnetic molecules. However, a change in the SMM properties due to the interaction with specific surfaces is usually observed. Here we present a rare example of an SMM system that can be thermally sublimated on gold surfaces while maintaining its intact chemical structure and magnetic properties. Muon spin relaxation and ac susceptibility measurements are used to demonstrate that, unlike other SMMs, the magnetic properties of this system in thin films are very similar to those in the bulk, throughout the full volume of the film, including regions near the metal and vacuum interfaces. These results exhibit the robustness of chemical and magnetic properties of this complex and provide important clues for the development of nanostructures based on SMMs.

Distribution of glass transition temperatures Tg in polystyrene thin films as revealed by low-energy muon spin relaxation: A comparison with neutron reflectivity results.

In a previous paper [Phys. Rev. E 83, 021801 (2011)] we performed neutron reflectivity (NR) measurements on a five-layer polystyrene (PS) thin film consisting of alternatively stacked deuterated polystyrene (dPS) and hydrogenated polystyrene (hPS) layers (dPS/hPS/dPS/hPS/dPS, ∼100 nm thick) on a Si substrate to reveal the distribution of Tg along the depth direction. Information on the Tg distribution is very useful to understand the interesting but unusual properties of polymer thin films. However, one problem that we have to clarify is if there are effects of deuterium labeling on Tg or not. To tackle the problem we performed low-energy muon spin relaxation (µSR) measurements on the above-mentioned deuterium-labeled five-layer PS thin film as well as dPS and hPS single-layer thin films ∼100 nm thick as a function of muon implantation energy. It was found that the deuterium labeling had no significant effects on the Tg distribution, guaranteeing that we can safely discuss the unusual thin film properties based on the Tg distribution revealed by NR on the deuterium-labeled thin films. In addition, the µSR result suggested that the higher Tg near the Si substrate is due to the strong orientation of phenyl rings.

Beating the Stoner criterion using molecular interfaces.

Only three elements are ferromagnetic at room temperature: the transition metals iron, cobalt and nickel. The Stoner criterion explains why iron is ferromagnetic but manganese, for example, is not, even though both elements have an unfilled 3d shell and are adjacent in the periodic table: according to this criterion, the product of the density of states and the exchange integral must be greater than unity for spontaneous spin ordering to emerge. Here we demonstrate that it is possible to alter the electronic states of non-ferromagnetic materials, such as diamagnetic copper and paramagnetic manganese, to overcome the Stoner criterion and make them ferromagnetic at room temperature. This effect is achieved via interfaces between metallic thin films and C60 molecular layers. The emergent ferromagnetic state exists over several layers of the metal before being quenched at large sample thicknesses by the material's bulk properties. Although the induced magnetization is easily measurable by magnetometry, low-energy muon spin spectroscopy provides insight into its distribution by studying the depolarization process of low-energy muons implanted in the sample. This technique indicates localized spin-ordered states at, and close to, the metal-molecule interface. Density functional theory simulations suggest a mechanism based on magnetic hardening of the metal atoms, owing to electron transfer. This mechanism might allow for the exploitation of molecular coupling to design magnetic metamaterials using abundant, non-toxic components such as organic semiconductors. Charge transfer at molecular interfaces may thus be used to control spin polarization or magnetization, with consequences for the design of devices for electronic, power or computing applications (see, for example, refs 6 and 7).