Marek Barthkowiak

The universality of free fall, a cornerstone of Einstein’s theory of gravity, has so far only been tested with neutral composite states of first-generation standard model particles, such as atoms, neutrons and antihydrogen. Muonium, the bound state of a positively charged anti-muon and an electron, offers the possibility to probe gravity in the absence of the strong interaction with second-generation standard model particles. However, the short muon lifetime and the existing diffuse thermal muonium sources rendered such measurements unfeasible. Here we report the generation of a high-brightness muonium beam, which we extract from a thin layer of superfluid helium. The mean longitudinal velocity and narrow spread of the velocity distribution indicate a superthermal beam, and yields are similar to the highest-intensity diffuse sources. This beam is expected to enable muonium interferometry and a per-cent-level measurement of its gravitational acceleration. In addition, its unprecedented brightness opens the way to sub-kilohertz 1S–2S spectroscopy, enabling the precise determination of the muon mass and stringent tests of bound-state quantum electrodynamics.

The transition metal dichalcogenide 6R-TaS₂ is a rich quantum platform hosting charge density wave (CDW) order, superconductivity, and an additional temperature scale at T^* ≃ 40 K marked by pronounced magnetoresistance and a nonlinear Hall effect (NHE). However, the nature of the superconducting pairing, the origin of the NHE, and their relationship with the CDW remain unclear. Using muon-spin rotation, magnetotransport and hydrostatic pressure techniques, we identify a nodal superconducting state with low superfluid density at ambient pressure, with no spontaneous magnetic order detected below T^*. This rules out magnetism as the origin of the NHE. Under pressures up to 2 GPa, the superfluid density rises markedly in correlation with the superconducting transition temperature, the nodal pairing shifts to a nodeless state, and the CDW onset is reduced by half. Notably, the NHE is fully suppressed and magnetoresistance drops by 50% within just 0.2 GPa, highlighting the fragility of the state with NHE. These results reveal competition between superconductivity, charge order, and the nonlinear Hall effect in 6R-TaS₂, driven by weakened interlayer coupling and shared electronic states.

In quantum magnetic materials it is common to observe both static and dynamic lattice effects on the magnetic excitation spectrum. Less common is to find that the magnetic correlations have a significant impact on the phonon spectrum. Here we study the metal-organic material ( C 5 H 9 NH 3 ) 2 CuBr 4 (Cu-CPA), in which an explanation of the low-lying excitations depends crucially on a full understanding of both the spin and lattice subsystems. We report high-resolution neutron spectroscopy enabled by large, deuterated single crystals that reveal how both sectors are affected by the recently discovered structural phase transition. By measuring over several Brillouin zones, we disentangle the vibrational contribution to the spectrum in order to obtain an accurate estimate of the quasi-one-dimensional magnetic signal. The low-energy magnetic excitations are dominated by two gaps, Δ b = 0.41 meV and Δ a = 0.55 meV , which contribute with equal intensity ratios, confirming that Cu-CPA realizes a two-ladder spin Hamiltonian, and we deduce the magnetic interaction parameters of both ladders. The phonon spectrum contains highly localized modes at an anomalously low energy around 2 meV. For half of these modes, this characteristic frequency drops by approximately 5% as magnetic correlations become established with decreasing temperature, and we connect this apparent “elastomagnetic” behavior with the location and structure of the cyclopentylammonium rings.

Understanding the interplay between magnetism and superconductivity in nickelate systems is a key objective in condensed matter physics. Gaining microscopic insights into magnetism—particularly as it emerges near superconductivity—requires a synergistic approach that combines complementary experimental techniques with controlled tuning of external parameters. In this paper, we present a systematic investigation of the three-layer Ruddlesden-Popper (RP) nickelate La 4 Ni 3 O 10 using muon-spin rotation/relaxation ( μ SR ) and resistivity measurements. At ambient pressure, two incommensurate spin-density-wave (SDW) transitions are identified at T SDW ≃ 132 K and T * ≃ 80 – 90 K . Comparison of the observed internal magnetic fields with dipole-field calculations reveals a magnetic structure consistent with antiferromagnetically coupled SDW order on the outer two Ni-O layers, with smaller moments on the inner Ni-O layer. Above T * , the moments lie primarily in the a b plane, but below this temperature they undergo a subtle distortion and develop a c -axis component. The internal fields at the muon stopping sites appear abruptly at T SDW , suggesting a first-order-like nature of the SDW transition, which is closely linked to the charge-density wave (CDW) order occurring at the same temperature ( T SDW = T CDW ). Under applied pressure, all transition temperatures—including T SDW , T * , and T CDW —are suppressed at a nearly uniform rate of ≃ − 13 K/GPa. This behavior contrasts with that of the two-layer RP nickelate La 3 Ni 2 O 7 , where pressure enhances the separation between the SDW and CDW transitions. The oxygen-isotope substitution ( O 16 → O 18 ) reveals that the CDW transition temperature shifts to higher values in the O 18 -substituted samples. The isotope effect on T SDW and T * differs significantly. Specifically, when the CDW and SDW orders are intertwined, a notable isotope effect is observed on T SDW , leading to equal transition temperatures and nearly identical isotope shifts for both T CDW and T SDW . In contrast, at T * , where the SDW transition occurs independently of the CDW, no isotope effect is detected.

Quantum phase transitions are among the most intriguing phenomena that can occur when the electronic ground state of correlated metals are tuned by external parameters such as pressure, magnetic field, or chemical substitution. Such transitions between distinct states of matter are driven by quantum fluctuations, and can give rise to macroscopically coherent phases that are at the forefront of condensed matter research. However, the nature of the critical fluctuations, and thus the fundamental physics controlling many quantum phase transitions, remain poorly understood in numerous strongly correlated metals. Here we study the model material CeCu 5.8 Ag 0.2 to gain insight into the implications of critical fluctuations originating from different regions in reciprocal space. By employing an external magnetic field along the crystallographic a and c axis as auxiliary tuning parameter, we observe a pronounced anisotropy in the suppression of the quantum critical fluctuations, reflecting the spin anisotropy of the long-range ordered ground state at larger silver concentration. Coupled with the temperature dependence of the quantum fluctuations, these results suggest that the quantum phase transition in CeCu 5.8 Ag 0.2 is driven by three-dimensional spin-density wave fluctuations.

Chiral spin textures in magnetic insulators promise magneto-electric (ME) spintronics with orders-of-magnitude lower power consumption than metallic systems. However, realizing the short magnetic periods required for high-density device integration remains difficult, as conventional Dzyaloshinskii–Moriya interaction (DMI)-based mechanisms typically constrain spiral periods to tens of nanometers. While theory predicts that strong single-ion anisotropy (SIA) on frustrated lattices can stabilize complex non-coplanar textures, the potential for using this mechanism to engineer such compact textures remains largely unexplored. Here, we report that a cubic chiral insulator Co ₅ TeO ₈ provides an experimental example of this paradigm. Comprehensive neutron scattering and magnetometry reveal helimagnetic spirals with continuously tunable pitch of 5.7 to 10 nm embedded in a complex phase diagram spanning 8 distinct phases. Capacitance anomalies throughout the phase diagram indicate ME coupling, pointing to the possibility of future E -field control of these textures. The temperature and field dependence of the helical wavevector strongly support a scenario in which site-dependent SIA provides the leading contribution to the selection of the helical period from a frustration-induced degenerate manifold. Consistent with this interpretation, ab initio calculations place SIA approximately an order of magnitude above DMI, distinct from conventional helimagnets. Co ₅ TeO ₈ thus offers an experimental realization of sub-10-nm helimagnetism and motivates a design principle for anisotropy-engineered correlated insulators.

Kagome lattice decorated with S = 1 / 2 spins is one of the most discussed ways to realize a quantum spin liquid. However, all previous material realizations of this model have suffered from additional complications, ranging from additional interactions to impurity effects. Recently, a new quantum kagome system has been identified in the form of averievite Cu 5 − x Zn x V 2 O 10 (CsCl), featuring a unique double-layer spacing between the kagome planes. Using muon spin spectroscopy we show that only a complete substitution (i.e., x = 2 ) of interplanar copper ions leads to a quantum-disordered ground state. In contrast, the parent compound ( x = 0 ) exhibits long-range magnetic order, with a phase transition around 24 K. Experiments performed on the partially substituted material ( x = 1 ) show that the transformation proceeds through an intermediate disordered, partially frozen ground state, unaffected by pressures up to 23 kbar. Our study provides a microscopic view of the magnetism of the decoupling of the kagome layers and establishes the averievite as a new material platform for the experimental study of the fully-decoupled kagome layers.

The observation of superconductivity in La₃Ni₂O_7–δ under pressure, following the suppression of a high-temperature density wave state, has attracted considerable attention. The nature of this density wave order was not clearly identified. Here we probe the magnetic response of the zero-pressure phase of La₃Ni₂O_7–δ as hydrostatic pressure is applied, and find that the apparent single density wave transition at zero applied pressure splits into two. The comparison of our muon-spin rotation and relaxation experiments with dipole-field numerical analysis reveals the magnetic structure’s compatibility with a stripe-type arrangement of Ni moments, characterized by alternating lines of magnetic moments and non-magnetic stripes at ambient pressure. When pressure is applied, the magnetic ordering temperature increases, whereas the unidentified density wave transition temperature falls. Our findings reveal that the ground state of the La₃Ni₂O_7–δ system is characterized by the coexistence of two distinct orders—a magnetically ordered spin density wave and a lower-temperature ordering that is most probably a charge density wave—with a notable pressure-enhanced separation between them.