The new versatile general purpose surface-muon instrument (GPS) based on silicon photomultipliers for μSR measurements on a continuous-wave beam

Tuning of the flat band and its impact on superconductivity in Mo5Si3-xPx

The superconductivity in systems containing dispersionless (flat) bands is seemingly paradoxical, as traditional Bardeen-Cooper-Schrieffer theory requires an infinite enhancement of the carrier masses. However, the combination of flat and steep (dispersive) bands within the multiple band scenario might boost superconducting responses, potentially explaining high-temperature superconductivity in cuprates and metal hydrides. Here, we report on the magnetic penetration depths, the upper critical field, and the specific heat measurements, together with the first-principles calculations for the Mo5Si3-xPx superconducting family. The band structure features a flat band that gradually approaches the Fermi level as a function of phosphorus doping x, reaching the Fermi level at x ≃ 1.3. This leads to an abrupt change in nearly all superconducting quantities. The superfluid density data placed on the 'Uemura plot' results in two separated branches, thus indicating that the emergence of a flat band enhances correlations between conducting electrons.

Tuneable Short-Range Antiferromagnetic Correlation in Fe-Containing Entropy Stabilized Oxides

To elucidate the impact of a high entropy elemental distribution of the lattice site on the magnetic properties in oxide compounds, a series of complex perovskites BaBO3 (B = Y, Fe, Ti, Zr, Hf, Nb, and Ta) with different Fe content ratios (0, 0.2, 0.3, and 0.4) have been synthesized and thoroughly characterized. In this complex oxide series, superconducting quantum interference device magnetometry reveals a gradual change of a well-defined magnetic phase transition and B-site magnetic moment, which correlates with the Fe content. More importantly, a comprehensive analysis of the sample with a 0.4-Fe content (40% on the B-site) including magnetization, heat capacity, neutron diffraction, and muon-spin rotation measurements suggests that in the low-temperature state, a short-range antiferromagnetic correlation may exist, which could result from the magnetic interaction of Fe ions and consequent redistribution of associated d-electrons.

Kondo screening in a Majorana metal

Kondo impurities provide a nontrivial probe to unravel the character of the excitations of a quantum spin liquid. In the S = 1/2 Kitaev model on the honeycomb lattice, Kondo impurities embedded in the spin-liquid host can be screened by itinerant Majorana fermions via gauge-flux binding. Here, we report experimental signatures of metallic-like Kondo screening at intermediate temperatures in the Kitaev honeycomb material α-RuCl3 with dilute Cr3+ (S = 3/2) impurities. The static magnetic susceptibility, the muon Knight shift, and the muon spin-relaxation rate all feature logarithmic divergences, a hallmark of a metallic Kondo effect. Concurrently, the linear coefficient of the magnetic specific heat is large in the same temperature regime, indicating the presence of a host Majorana metal. This observation opens new avenues for exploring uncharted Kondo physics in insulating quantum magnets.

Unconventional superconductivity in topological Kramers nodal-line semimetals

Crystalline symmetry is a defining factor of the electronic band topology in solids, where many-body interactions often induce a spontaneous breaking of symmetry. Superconductors lacking an inversion center are among the best systems to study such effects or even to achieve topological superconductivity. Here, we demonstrate that TRuSi materials (with T a transition metal) belong to this class. Their bulk normal states behave as three-dimensional Kramers nodal-line semimetals, characterized by large antisymmetric spin-orbit couplings and by hourglass-like dispersions. Our muon-spin spectroscopy measurements show that certain TRuSi compounds spontaneously break the time-reversal symmetry at the superconducting transition, while unexpectedly showing a fully gapped superconductivity. Their unconventional behavior is consistent with a unitary (s + ip) pairing, reflecting a mixture of spin singlets and spin triplets. By combining an intrinsic time-reversal symmetry-breaking superconductivity with nontrivial electronic bands, TRuSi compounds provide an ideal platform for investigating the rich interplay between unconventional superconductivity and the exotic properties of Kramers nodal-line/hourglass fermions.

Entanglement between Muon and I>1/2 Nuclear Spins as a Probe of Charge Environment

We report on the first example of quantum coherence between the spins of muons and quadrupolar nuclei. We reveal that these entangled states are highly sensitive to a local charge environment and thus, can be deployed as a functional quantum sensor of that environment. The quantum coherence effect was observed in vanadium intermetallic compounds which adopt the A15 crystal structure, and whose members include all technologically pertinent superconductors. Furthermore, the extreme sensitivity of the entangled states to the local structural and electronic environments emerges through the quadrupolar interaction with the electric field gradient due to the charge distribution at the nuclear (I>1/2) sites. This case study demonstrates that positive muons can be used as a quantum sensing tool to also probe structural and charge-related phenomena in materials, even in the absence of magnetic degrees of freedom.

The Magnetic Genome of Two-Dimensional van der Waals Materials

Magnetism in two-dimensional (2D) van der Waals (vdW) materials has recently emerged as one of the most promising areas in condensed matter research, with many exciting emerging properties and significant potential for applications ranging from topological magnonics to low-power spintronics, quantum computing, and optical communications. In the brief time after their discovery, 2D magnets have blossomed into a rich area for investigation, where fundamental concepts in magnetism are challenged by the behavior of spins that can develop at the single layer limit. However, much effort is still needed in multiple fronts before 2D magnets can be routinely used for practical implementations. In this comprehensive review, prominent authors with expertise in complementary fields of 2D magnetism (i.e., synthesis, device engineering, magneto-optics, imaging, transport, mechanics, spin excitations, and theory and simulations) have joined together to provide a genome of current knowledge and a guideline for future developments in 2D magnetic materials research.

First demonstration of tuning between the Kitaev and Ising limits in a honeycomb lattice

Recent observations of novel spin-orbit coupled states have generated interest in 4d/5d transition metal systems. A prime example is the [Formula: see text] state in iridate materials and α-RuCl₃ that drives Kitaev interactions. Here, by tuning the competition between spin-orbit interaction (λ_SOC) and trigonal crystal field (Δ_T), we restructure the spin-orbital wave functions into a previously unobserved [Formula: see text] state that drives Ising interactions. This is done via a topochemical reaction that converts Li₂RhO₃ to Ag₃LiRh₂O₆. Using perturbation theory, we present an explicit expression for the [Formula: see text] state in the limit Δ_T ≫ λ_SOC realized in Ag₃LiRh₂O₆, different from the conventional [Formula: see text] state in the limit λ_SOC ≫ Δ_T realized in Li₂RhO₃. The change of ground state is followed by a marked change of magnetism from a 6 K spin-glass in Li₂RhO₃ to a 94 K antiferromagnet in Ag₃LiRh₂O₆.

Time-reversal symmetry-breaking charge order in a kagome superconductor

The kagome lattice¹, which is the most prominent structural motif in quantum physics, benefits from inherent non-trivial geometry so that it can host diverse quantum phases, ranging from spin-liquid phases, to topological matter, to intertwined orders^2-8 and, most rarely, to unconventional superconductivity^6,9. Recently, charge sensitive probes have indicated that the kagome superconductors AV₃Sb₅ (A = K, Rb, Cs)^9-11 exhibit unconventional chiral charge order^12-19, which is analogous to the long-sought-after quantum order in the Haldane model²⁰ or Varma model²¹. However, direct evidence for the time-reversal symmetry breaking of the charge order remains elusive. Here we use muon spin relaxation to probe the kagome charge order and superconductivity in KV₃Sb₅. We observe a noticeable enhancement of the internal field width sensed by the muon ensemble, which takes place just below the charge ordering temperature and persists into the superconducting state. Notably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. We further show the multigap nature of superconductivity in KV₃Sb₅ and that the [Formula: see text] ratio (where T_c is the superconducting transition temperature and λ_ab is the magnetic penetration depth in the kagome plane) is comparable to those of unconventional high-temperature superconductors. Our results point to time-reversal symmetry-breaking charge order intertwining with unconventional superconductivity in the correlated kagome lattice.

Coexistence of structural and magnetic phases in van der Waals magnet CrI3

CrI₃ has raised as an important system to the emergent field of two-dimensional van der Waals magnetic materials. However, it is still unclear why CrI₃ which has a ferromagnetic rhombohedral structure in bulk, changed to anti-ferromagnetic monoclinic at thin layers. Here we show that this behaviour is due to the coexistence of both monoclinic and rhombohedral crystal phases followed by three magnetic transitions at T_C1 = 61 K, T_C2 = 50 K and T_C3 = 25 K. Each transition corresponds to a certain fraction of the magnetically ordered volume as well as monoclinic and rhombohedral proportion. The different phases are continuously accessed as a function of the temperature over a broad range of magnitudes. Our findings suggest that the challenge of understanding the magnetic properties of thin layers CrI₃ is in general a coexisting structural-phase problem mediated by the volume-wise competition between magnetic phases already present in bulk.

CrI₃ is a popular van der Waals magnet that exhibits anomalous magnetic properties between bulk and thin layers due to different crystal symmetry. Here, the authors report the coexistence of different magnetostructural phases over the entire range of temperatures, solving a long-standing puzzle.

Probing spin fluctuations in NaOsO3by muon spin rotation and NMR spectroscopy

We have used muon spin rotation and relaxation (μSR) and²³Na nuclear magnetic resonance (NMR) spectroscopic methods in the NaOsO₃antiferromagnetic phase to determine the temperature evolution of the magnetic order parameter and the role of the magnetic fluctuations at the Néel temperature. Additionally, we performed muon spin relaxation measurements in the vicinity ofT_A= 30 K, where the appearance of an anomaly in the electrical resistivity was suggested to be due to a progressive reduction of the Os magnetic moment associated with spin fluctuation. Our measurements suggest the absence of prominent change in the spin fluctuations frequency atT_A, within the muon probing time scale and the absence of a reduction of the localized Os magnetic moment reflected by the stability within few permille of the local magnetic field strength sensed by the muons below 50 K.

Cation Distributions and Magnetic Properties of Ferrispinel MgFeMnO4

The crystal structure and magnetic properties of the cubic spinel MgFeMnO₄ were studied by using a series of in-house techniques along with large-scale neutron diffraction and muon spin rotation spectroscopy in the temperature range between 1.5 and 500 K. The detailed crystal structure is successfully refined by using a cubic spinel structure described by the space group Fd3̅m. Cations within tetrahedral A and octahedral B sites of the spinel were found to be in a disordered state. The extracted fractional site occupancies confirm the presence of antisite defects, which are of importance for the electrochemical performance of MgFeMnO₄ and related battery materials. Neutron diffraction and muon spin spectroscopy reveal a ferrimagnetic order below T_C = 394.2 K, having a collinear spin arrangement with antiparallel spins at the A and B sites, respectively. Our findings provide new and improved understanding of the fundamental properties of the ferrispinel materials and of their potential applications within future spintronics and battery devices.

Unconventional Magnetism in Layered Transition Metal Dichalcogenides

In this contribution to the MDPI Condensed Matter issue in Honor of Nobel Laureate Professor K.A. Müller I review recent experimental progress on magnetism of semiconducting transition metal dichalcogenides (TMDs) from the local-magnetic probe point of view such as muon-spin rotation and discuss prospects for the creation of unique new device concepts with these materials. TMDs are the prominent class of layered materials, that exhibit a vast range of interesting properties including unconventional semiconducting, optical, and transport behavior originating from valley splitting. Until recently, this family has been missing one crucial member: magnetic semiconductor. The situation has changed over the past few years with the discovery of layered semiconducting magnetic crystals, for example CrI 3 and VI 2 . We have also very recently discovered unconventional magnetism in semiconducting Mo-based TMD systems 2H-MoTe 2 and 2H-MoSe 2 [Guguchia et. al., Science Advances 2018, 4(12)]. Moreover, we also show the evidence for the involvement of magnetism in semiconducting tungsten diselenide 2H-WSe 2 . These results open a path to studying the interplay of 2D physics, semiconducting properties and magnetism in TMDs. It also opens up a host of new opportunities to obtain tunable magnetic semiconductors, forming the basis for spintronics.

Magnetic-Field Control of Topological Electronic Response near Room Temperature in Correlated Kagome Magnets

Strongly correlated kagome magnets are promising candidates for achieving controllable topological devices owing to the rich interplay between inherent Dirac fermions and correlation-driven magnetism. Here we report tunable local magnetism and its intriguing control of topological electronic response near room temperature in the kagome magnet Fe_{3}Sn_{2} using small angle neutron scattering, muon spin rotation, and magnetoresistivity measurement techniques. The average bulk spin direction and magnetic domain texture can be tuned effectively by small magnetic fields. Magnetoresistivity, in response, exhibits a measurable degree of anisotropic weak localization behavior, which allows the direct control of Dirac fermions with strong electron correlations. Our work points to a novel platform for manipulating emergent phenomena in strongly correlated topological materials relevant to future applications.

Extended Magnetic Dome Induced by Low Pressures in Superconducting FeSe_{1-x}S_{x}

We report muon spin rotation and magnetization measurements under pressure on Fe_{1+δ}Se_{1-x}S_{x} with x≈0.11. Above p≈0.6  GPa we find a microscopic coexistence of superconductivity with an extended dome of long range magnetic order that spans a pressure range between previously reported separated magnetic phases. The magnetism initially competes on an atomic scale with the coexisting superconductivity leading to a local maximum and minimum of the superconducting T_{c}(p). The maximum of T_{c} corresponds to the onset of magnetism while the minimum coincides with the pressure of strongest competition. A shift of the maximum of T_{c}(p) for a series of single crystals with x up to 0.14 roughly extrapolates to a putative magnetic and superconducting state at ambient pressure for x≥0.2.

Magnetism in semiconducting molybdenum dichalcogenides

Transition metal dichalcogenides (TMDs) are interesting for understanding the fundamental physics of two-dimensional (2D) materials as well as for applications to many emerging technologies, including spin electronics. Here, we report the discovery of long-range magnetic order below T _M = 40 and 100 K in bulk semiconducting TMDs 2H-MoTe₂ and 2H-MoSe₂, respectively, by means of muon spin rotation (μSR), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The μSR measurements show the presence of large and homogeneous internal magnetic fields at low temperatures in both compounds indicative of long-range magnetic order. DFT calculations show that this magnetism is promoted by the presence of defects in the crystal. The STM measurements show that the vast majority of defects in these materials are metal vacancies and chalcogen-metal antisites, which are randomly distributed in the lattice at the subpercent level. DFT indicates that the antisite defects are magnetic with a magnetic moment in the range of 0.9 to 2.8 μ_B. Further, we find that the magnetic order stabilized in 2H-MoTe₂ and 2H-MoSe₂ is highly sensitive to hydrostatic pressure. These observations establish 2H-MoTe₂ and 2H-MoSe₂ as a new class of magnetic semiconductors and open a path to studying the interplay of 2D physics and magnetism in these interesting semiconductors.