Explore the vast range of titles available.

Altermagnets are a novel class of magnetic materials, where magnetic order is staggered both in coordinate and momentum space. The metallic rutile oxide RuO 2 , long believed to be a textbook Pauli paramagnet, recently emerged as a putative workhorse altermagnet when resonant X-ray and neutron scattering studies reported nonzero magnetic moments and long-range collinear order. While some experiments seem consistent with altermagnetism, magnetic order in RuO 2 remains controversial. We show that RuO 2 is nonmagnetic, both in bulk and thin film. Muon spectroscopy complemented by density-functional theory finds at most 1.14 × 10 −4   μ B /Ru in bulk and at most 7.5 × 10 −4   μ B /Ru in 11 nm epitaxial films, at our spectrometers’ detection limit, and dramatically smaller than previously reported neutron results that were used to rationalize altermagnetic behavior. Our own neutron diffraction measurements on RuO 2 single crystals identify multiple scattering as the source for the false signal in earlier studies.

By harnessing the charge transfer that takes place at the interface between a metal and a layer of molecules, the usually non-magnetic materials copper and manganese are made magnetic at room temperature. Designer magnetism in copper and manganese This paper shows that thin films of non-magnetic metals such as copper and manganese can be made magnetic at room temperature by harnessing the charge transfer that takes place at the interface between the metal and a layer of molecules. Such a strategy potentially broadens the range of materials that could be used for magnetic and spintronic devices. 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 3 d 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 1 , 2 . 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 C 60 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 3 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 4 , 5 . 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 ).

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 2S , F = 0 → 2P , 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.

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. The authors study a Pt/Nb hybrid structure by scanning microscopy and muon spin rotation. They find an anomalous absence of Meissner screening near the Pt/Nb interface due to spin-triplet pair correlations driven by spin-orbit coupling alone with no ferromagnetic layer necessary.

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 Ge 1− x Mn x Te 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. GeTe is a ferroelectric semiconductor with broken inversion symmetry, which leads to a large spin-orbit interaction. When doped with small amounts of manganese, it becomes magnetoelectric. Here, Krempasky et al show that the ferrimagnetic ordering of Mn-doped GeTe can be switched with unusually small currents under specific resonant conditions, orders of magnitude smaller than typical for spin-orbit torque based switching.

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 2 S 1/2 , F  = 0 → 2 P 1/2 ,  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 2 S 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. Muonium is a hydrogen like bound system with a positive muon and an electron. Here the authors measure the Lamb shift and frequency of the transition from 2S1/2, F = 0 state to 2P1/2, F = 1 state in muonium atom and the hyperfine structure of the 2S level.

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. Magnetic metamaterials can be designed to provide models of frustrated systems that allow theoretical predictions to be experimentally tested. Here the authors realise a 2D XY model with dipolar interactions and find behaviour consistent with predictions of a low-temperature ordered state.

A method to measure the superconducting (SC) stiffness tensor ρ ¯ s , without subjecting the sample to external magnetic field, is applied to La 1.875 Sr 0.125 CuO 4 . The method is based on the London equation J = - ρ ¯ s A , 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, ρ ¯ s at T  →  T c 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 CuO 2 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  <  T c . Precise measurements of the superconducting stiffness tensor can give detailed insights into the superconductor-normal phase transition. Kapon et al. introduce the Stiffnessometer approach for sensitive magnetic-field-free measurements and find two transition temperatures in LSCO rings.

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 ground states. 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. The authors study epitaxial thin films of the pyrochlore-sublattice compound LiTi2O4 by RIXS and ARPES. They observe cooperation between strong electron correlations and strong electron-phonon coupling, giving rise to a mobile polaronic ground state in which charge motion and lattice distortions are coupled.

Epitaxial thin‐film heterostructures of the strongly spin‐orbit coupled Mott‐insulator Sr2IrO4 (SIO) and the cuprate high temperature superconductor YBa2Cu3O7 − δ (YBCO) are grown with pulsed laser deposition (PLD). A high crystalline quality is confirmed with X ray diffraction. The magnetic order of single SIO layers is studied with dc magnetization and low‐energy muon spin rotation measurements and resembles that of the bulk material with a canted antiferromagnetic order. The electronic normal state and superconducting properties of YBCO (10, 12, or 14 nm)–SIO (20 nm) and inversely stacked SIO (20nm)–YBCO (10, 12, or 14 nm) bilayers are studied with dc resistivity measurements and found to be strongly dependent on the sequence of the layer stacking. The YBCO–SIO bilayers with d(YBCO) = 14nm, 12 nm, and 10 nm are all metallic and superconducting with an onset temperature around 85K and zero resistivity below 65K. To the contrary, for the inversely stacked SIO–YBCO bilayers a metallic and superconducting response occurs only at d(YBCO) = 14 nm, whereas d(YBCO)=12nm $d({\\rm YBCO}) = {12}\\ {\\rm nm}$and 10 nm are electronic insulators. This highlights that a long‐ranged localization and/or depletion of the YBCO charge carriers occurs at the SIO–YBCO interface that is very anomalous and remains to be understood. Multilayers of insulators with a strong spin‐orbit coupling, like Sr2IrO4, and high temperature superconductors, like YBa2Cu3O7, hold great promises for new device concepts in quantum computing. Here, it is shown that the electronic properties of such multilayers are strongly affected by the mutual stacking order. This observation calls for further studies of the underlying mechanism and needs to be considered for the development of suitable devices.