Superconductivity and magnetism are often antagonistic in quantum matter, although their intertwining has long been considered in frustrated-lattice systems. Here we utilize scanning tunnelling microscopy and muon spin resonance to demonstrate time-reversal symmetry-breaking superconductivity in kagome metal Cs(V, Ta)3Sb5, where the Cooper pairing exhibits magnetism and is modulated by it. In the magnetic channel, we observe spontaneous internal magnetism in a fully gapped superconducting state. Under the perturbation of inverse magnetic fields, we detect a time-reversal asymmetrical interference of Bogoliubov quasi-particles at a circular vector. At this vector, the pairing gap spontaneously modulates, which is distinct from pair density waves occurring at a point vector and consistent with the theoretical proposal of an unusual interference effect under time-reversal symmetry breaking. The correlation between internal magnetism, Bogoliubov quasi-particles and pairing modulation provides a chain of experimental indications for time-reversal symmetry-breaking kagome superconductivity. The authors use scanning tunnelling microscopy and muon spin resonance to demonstrate time-reversal symmetry-breaking superconductivity in Cs(V, Ta)3Sb5. The Cooper pairing in this state exhibits magnetism and is modulated by it.

FeSe_1-xS_x superconductors are commonly considered layered van der Waals materials with negligible inter-layer coupling. Here, using inelastic neutron scattering to study spin excitations in single-crystal samples, we reveal that the magnetic coupling between adjacent Fe layers is not only significant, as it affects excitations up to 15 meV, but also ferromagnetic in nature, making the system different from most unconventional superconductors including iron pnictides. Our observation provides a new standpoint to understand the absence of magnetic order in FeSe_1-xS_x. Since intercalating between the Fe layers is known to enhance superconductivity and suppress the inter-layer coupling, superconductivity appears to be a more robust phenomenon in the two-dimensional limit than antiferromagnetic order.

We present a comprehensive microscopic insight into the spin configurations within the bond-frustrated cubic spinel compound MnCr2S4 directly unveiled through extensive single-crystal neutron diffraction studies carried out in zero magnetic field and in fields up to 35 T. While our zero field results confirm the ferrimagnetic structure with an antiparallel arrangement of the magnetic Cr3+ and Mn2+ sublattices below T FiM approximate to 65 K, as well as the presence of the exotic Yafet-Kittel phase below T YK approximate to 5 K, our data measured in fields enable us to precisely determine the field-induced magnetic structures and their evolution across the phase transitions at mu 0 H 1 approximate to 11 T and mu 0 H 2 approximate to 25 T and beyond that towards mu 0 H 3 (approximate to 50 T). Additionally, combining our experimental findings with mean-field-theory calculations reveals a complex field dependence of the Mn-Mn and Mn-Cr exchange interactions across the different phases, highlighting the significant influence of spin-lattice coupling in this material.

As many kagome metals, the topological superconductor AV3Sb5 with (A = K,Rb,Cs) hosts a charge density wave. A related chiral flux phase that breaks the time-reversal symmetry has been further theoretically predicted in these materials. The flux phase is associated with loop currents that produce ordered orbital magnetic moments, which would occur at the momentum points, M, characterizing the charge-density wave state. Polarized neutron-diffraction experiments have been performed on an assembly of single crystals of CsV3Sb5 to search for such orbital magnetic moments. No evidence for the existence of a three-dimensionally ordered moment is found at any temperature at the first M1 = (1/2,0,0) point in the Brillouin zone within an excellent experimental uncertainty, i.e., m = 0 +/- 0.01 mu B per vanadium atom. However, the data might be suggestive of a putative magnetic orbital moment in the second Brillouin zone at M2 = (1/2,1/2,0) at the detection limit of the experiment. Some loop currents patterns flowing only on vanadium triangles are able to account for this finding suggesting an ordered orbital magnetic moment of, at most, <^>0.02 +/- 0.01 mu B per vanadium triangle.

FeSe1-xSx superconductors are commonly considered layered van der Waals materials with negligible interlayer coupling. Here, using inelastic neutron scattering to study spin excitations in single-crystal samples, we reveal that the magnetic coupling between adjacent Fe layers is ferromagnetic in nature, making the system different from most unconventional superconductors including iron pnictides. The weak interlayer coupling is estimated to be Jc 0.2 meV, in agreement with the short spin-spin correlation length xi c 0.2c along the c axis. The results provide an experimental basis for establishing a microscopic theoretical model to describe the absence of magnetic order in FeSe1-xSx.

Cubic yttria-stabilized zirconia has long been a ceramic material of interest for its many uses in thermal-based applications. Its very low and weakly temperature-dependent thermal conductivity has been ascribed to the large oxygen vacancies content, which introduces disorder and strongly scatters phonons. Still, despite many experimental works in the literature, phonon dynamics has not been fully understood yet, with several points to be clarified, such as the apparent absence of optic modes throughout the Brillouin zone. In this paper, we present findings on the phonon dispersions of this material, showing experimental evidence of low-lying optical branches throughout the Brillouin zone, which reduce the pure acoustic regime for some branches. Furthermore, the observed energy dependence of the intrinsic acoustic phonon linewidths clearly suggests the existence of competing Mie and Rayleigh scattering mechanisms. Our findings allow to uncover a different phonon dynamics scenario in this material and point to a deeper understanding of heat transport in yttria-stabilized zirconia, based on two different, concomitant mechanisms, generated by the large vacancy content.

Barium zirconate (BaZrO$_3$) is one of the very few perovskites that is claimed to retain an average cubic structure down to \SI{0}{\K}, while being energetically very close to an antiferrodistortive phase obtained by condensation of a soft phonon mode at the R point of the Brillouin zone boundary. In this work, we report a combined experimental and theoretical study of the temperature dependence of this soft phonon mode. Inelastic neutron and x-ray scattering measurements on single crystals show that it softens substantially from \SI{9.4}{\meV} at room temperature to \SI{5.6}{\meV} at \SI{2}{\K}. In contrast, the acoustic mode at the same R point is nearly temperature independent. The effect of the anharmonicity on the lattice dynamics is investigated non-perturbatively using direct dynamic simulations as well as a first-principles based self-consistent phonon theory, including quantum fluctuations of the atomic motion. By adding cubic and quartic anharmonic force constants, quantitative agreement with the neutron data for the temperature dependence of the antiferrodistortive mode is obtained. The quantum fluctuations of the atomic motion are found to be important to obtain the proper temperature dependence at low temperatures. The mean squared displacements of the different atoms are determined as function of temperature and are shown to be consistent with available experimental data. Adding anharmonicity to the computed fluctuations of the Ba-O distances also improves the comparison with available EXAFS data at \SI{300}{\K}.

The phase of the quantum-mechanical wave function can encode a topological structure with wide-ranging physical consequences, such as anomalous transport effects and the existence of edge states robust against perturbations. While this has been exhaustively demonstrated for electrons, properties associated with the elementary quasiparticles in magnetic materials are still underexplored. Here, we show theoretically and via inelastic neutron scattering experiments that the bulk ferromagnet Mn$_5$Ge$_3$ hosts gapped topological Dirac magnons. Although inversion symmetry prohibits a net Dzyaloshinskii-Moriya interaction in the unit cell, it is locally allowed and is responsible for the gap opening in the magnon spectrum. This gap is predicted and experimentally verified to close by rotating the magnetization away from the $c$-axis with an applied magnetic field. Hence, Mn$_5$Ge$_3$ realizes a gapped Dirac magnon material in three dimensions. Its tunability by chemical doping or by thin film nanostructuring defines an exciting new platform to explore and design topological magnons. More generally, our experimental route to verify and control the topological character of the magnons is applicable to bulk centrosymmetric hexagonal materials, which calls for systematic investigation.

In this paper, we focus on the thermal transport properties of antiferromagnetic spin chains cuprates. The chain magnetic excitations, the spinons, partake in heat transport at low temperature, but spinon heat transport decays well below room temperature, possibly because of a coupling with phonons. By means of inelastic neutron scattering, we thoroughly study the lattice dynamics of spin chain compounds Sr2CuO3, Ca2CuO3, along with double spin-chain compounds SrCuO2. We come to the conclusion that there are no obvious anomalies in the phonon dispersions, which suggests a weak spinon-phonon coupling regime.

We present a single-crystal neutron diffraction study on the Shastry-Sutherland lattice system TbB$_4$ at zero magnetic field and under pulsed high magnetic fields up to 35 T applied along the crystallographic $c$ axis. While our results confirm the magnetic structures at zero-field as well as those at the half- and full-magnetization plateaus, they offer new insight into the $\frac{2}{9}$- and $\frac{1}{3}$-magnetization plateaus observed in this system. A stripe model of polarized 4-spin-plaquettes whose stripe density proportionally increases with the macroscopic magnetization is in full agreement with the neutron diffraction data. Equally well suited alternative models exist which explain the observed Bragg peaks which are inherently limited in a pulsed high magnetic field experiment. We discuss the different intensity distribution in $Q$ space which can be used to distinguish these models in future experiments.

The coupling of spin, charge and lattice degrees of freedom results in the emergence of novel states of matter across many classes of strongly correlated electron materials. A model example is unconventional superconductivity, which is widely believed to arise from the coupling of electrons via spin excitations. In cuprate high-temperature superconductors, the interplay of charge and spin degrees of freedom is also reflected in a zoo of charge and spin-density wave orders that are intertwined with superconductivity. A key question is whether the different types of density waves merely coexist or are indeed directly coupled. Here we use a novel neutron diffraction technique with superior beam-focusing that allows us to probe the subtle spin-density wave order in the prototypical high-temperature superconductor La1.88Sr0.12CuO4 under applied uniaxial pressure to demonstrate that it is immediately coupled with charge-density wave order. Our result shows that suitable models for high-temperature superconductivity must equally account for charge and spin degrees of freedom via uniaxial charge-spin stripe fluctuations.

High-Entropy Alloys (HEAs) are a new family of crystalline random alloys with four or more elements in a simple unit cell, at the forefront of materials research for their exceptional mechanical properties. Their strong chemical disorder leads to mass and force-constant fluctuations which are expected to strongly reduce phonon lifetime, responsible for thermal transport, similarly to glasses. Still, the long range order would associate HEAs to crystals with a complex disordered unit cell. These two families of materials, however, exhibit very different phonon dynamics, still leading to similar thermal properties. The question arises on the positioning of HEAs in this context. Here we present an exhaustive experimental investigation of the lattice dynamics in a HEA, Fe 20 Co 20 Cr 20 Mn 20 Ni 20 , using inelastic neutron and X-ray scattering. We demonstrate that HEAs present unique phonon dynamics at the frontier between fully disordered and ordered materials, characterized by long-propagating acoustic phonons in the whole Brillouin zone.

Spin-orbit coupling (SOC) is a key to understand the magnetically driven superconductivity in iron-based superconductors, where both local and itinerant electrons are present and the orbital angular momentum is not completely quenched. Here, we report a neutron scattering study on the bilayer compound CaK(Fe0.96Ni0.04)(4)As-4 with superconductivity coexisting with a noncollinear spin-vortex crystal magnetic order that preserves the tetragonal symmetry of the Fe-Fe plane. In the superconducting state, two spin resonance modes with odd and even L symmetries due to the bilayer coupling are found similar to the undoped compound CaKFe4As4 but at lower energies. Polarization analysis reveals that the odd mode is c-axis polarized, and the low-energy spin anisotropy can persist to the paramagnetic phase at high temperature, which closely resembles other systems with in-plane collinear and c-axis biaxial magnetic orders. These results provide the missing piece of the puzzle on the SOC effect in iron-pnictide superconductors, and also establish a common picture of c-axis preferred magnetic excitations below T-c regardless of the details of magnetic pattern or lattice symmetry.

Despite decades of intense research, the enigmatic pseudo-gap (PG) phase of superconducting cuprates remains unsolved. In the last 15 years, various symmetry breaking states were discovered in the PG phase, including an intra-unit cell (IUC) magnetism, which preserves the lattice translational (LT) symmetry but breaks the time-reversal and parity symmetries, and an additional incipient charge density wave breaking the LT symmetry. However, none of these states can (alone) account for the partial gapping of the Fermi surface. Here we report a hidden LT-breaking magnetism using polarized neutron diffraction. Our measurements reveal magnetic correlations, in two different underdoped YBa 2 Cu 3 O 6.6 single crystals that set in at the PG onset temperature with (i) a planar propagation wave vector ( π , 0) ≡ (0, π ), yielding a doubling or quadrupling of the magnetic unit cell and (ii) magnetic moments mainly pointing perpendicular to the CuO 2 layers. The LT-breaking magnetism is at short-range suggesting the formation of clusters of 5–6 unit cells. Together with the previously reported IUC magnetism, it yields a hidden magnetic texture of the CuO 2 unit cells hosting loop currents, forming large supercells that may be helpful for elucidating the PG puzzle.

In this work, we investigate the evolution and settling of magnon condensation in the spin-1/2 dimer system Sr$_{3}$Cr$_{2}$O$_{8}$ using a combination of magnetostriction in pulsed fields and inelastic neutron scattering in a continuous magnetic field. The magnetic structure in the Bose-Einstein condensation (BEC) phase was probed by neutron diffraction in pulsed magnetic fields up to 39~T. The magnetic structure in this phase was confirmed to be an XY-antiferromagnetic structure validated by irreducible representational analysis. The magnetic phase diagram as a function of an applied magnetic field for this system is presented. Furthermore, zero-field neutron diffraction results indicate that dimerization plays an important role in stabilizing the low-temperature crystal structure.

The low and weakly temperature-varying lattice thermal conductivity, κL(T), in crystals with a complex unit cell such as type-I clathrates is assumed to originate from a reduced momentum and energy space available for propagative lattice vibrations, which is caused by the occurrence of low-energy optical phonon modes. In the context of ab initio self-consistent phonon (SCP) theory, it has been shown that the cubic and quartic anharmonic interactions result in a temperature-induced energy renormalization of these low-lying optical branches which contributes to the anomalous behavior of κL(T) in structurally ordered type-I clathrates [T. Tadano and S. Tsuneyuki, Phys. Rev. Lett. 120, 105901 (2018)]. By means of inelastic neutron scattering, we provide evidence for this energy renormalization in temperature, which has been resolved for transversely and longitudinally polarized phonons in the single crystal type-I clathrate Ba7.81Ge40.67Au5.33. By mapping the neutron intensity in the momentum space, we demonstrate the coherent character of the low-lying optical phonons. The overall phonon spectrum and dynamical structure factors are satisfactorily reproduced by ab initio harmonic calculations using density functional theory with the meta-GGA SCAN functional and a fully ordered structure. However, a polarization-dependent cutoff energy with opposing temperature shifts for longitudinal and transverse acoustic dispersions is experimentally observed which is not reproduced by the simulations. Anharmonicity affects the energies of the low-lying optical phonons in the transverse polarization, which compares quantitatively well with available results from SCP theory, whereas differences are observed for the longitudinal polarization.

Aperiodic crystals are long-range ordered crystals that lack periodicity.A good description of these materials is provided by the superspace approach [1,2].Although their structure is in general well described, the atomic realisations and properties of their dynamics are more debated.Phason modes that should arise from the new degrees of freedom due to the aperiodic order have been experimentally observed in very few incommensurately modulated phase and quasicrystals [1,3].Also, low thermal conductivity in those systems asks for an investigation of their dynamics.

We report polarized neutron scattering measurements of the low energy spin fluctuations of the iron-selenide superconductor Li$_{0.8}$Fe$_{0.2}$ODFeSe below and above its superconducting transition temperature $T_c=41$ K. Our experiments confirmed that the resonance mode near 21 meV is magnetic. Moreover, the spin excitations are essentially isotropic in spin space at 5$\leq E\leq$ 29 meV in the superconducting and normal states. Our results suggest that the resonance mode in iron-based superconductors becomes isotropic when the influence of spin-orbit coupling and magnetic/nematic order is minimized, similar to those observed in cuprate superconductors.

In the rare-earth pyrochlore Tb 2 Ti 2 O 7 , there are strong interactions between crystal field and phonon excitations resulting in the hybridization of crystal field excitations with both acoustic and optical phonon excitations, which may be implicated in its evasion of the expected long-range ordered states. The magnetoelastic coupling mechanisms are thought to involve large quadrupolar matrix elements between the crystal field states that allow them to couple with intersecting phonons. The character of the hybridized modes can be determined by polarized neutron scattering, as is done here for the case of a crystal field-optical phonon coupling. The coupling mechanism can be further investigated by applying a magnetic field to modify the energies of the crystal field states relative to the phonon spectrum. For a weakly dispersive optical phonon and crystal field level, this has the effect of detuning the quasidegeneracy necessary for hybridization and suppressing the magnetoelastic signal. For a strongly dispersive acoustic phonon crossing a crystal field level, the magnetic field moves the crystal field level, changing the wave vector and energy at which the modes intersect. The field-driven modification of matrix elements for dipole and quadrupole operators involved in the formation of the coupled mode results in the suppression of the magnetoelastic coupling. As the crystal field states shift to higher energy and the magnetoelastic coupling is suppressed, the spin system is driven closer to classically anticipated ordered structures.