Lead chalcogenides have exceptional thermoelectric properties and intriguing anharmonic lattice dynamics underlying their low thermal conductivities. An ideal material for thermoelectric efficiency ...is the phonon glass-electron crystal, which drives research on strategies to scatter or localize phonons while minimally disrupting electronic-transport. Anharmonicity can potentially do both, even in perfect crystals, and simulations suggest that PbSe is anharmonic enough to support intrinsic localized modes that halt transport. Here, we experimentally observe high-temperature localization in PbSe using neutron scattering but find that localization is not limited to isolated modes - zero group velocity develops for a significant section of the transverse optic phonon on heating above a transition in the anharmonic dynamics. Arrest of the optic phonon propagation coincides with unusual sharpening of the longitudinal acoustic mode due to a loss of phase space for scattering. Our study shows how nonlinear physics beyond conventional anharmonic perturbations can fundamentally alter vibrational transport properties.
In quantum magnets, magnetic moments fluctuate heavily and are strongly entangled with each other, a fundamental distinction from classical magnetism. Here, with inelastic neutron scattering ...measurements, we probe the spin correlations of the honeycomb lattice quantum magnet YbCl
. A linear spin wave theory with a single Heisenberg interaction on the honeycomb lattice, including both transverse and longitudinal channels of the neutron response, reproduces all of the key features in the spectrum. In particular, we identify a Van Hove singularity, a clearly observable sharp feature within a continuum response. The demonstration of such a Van Hove singularity in a two-magnon continuum is important as a confirmation of broadly held notions of continua in quantum magnetism and additionally because analogous features in two-spinon continua could be used to distinguish quantum spin liquids from merely disordered systems. These results establish YbCl
as a benchmark material for quantum magnetism on the honeycomb lattice.
In conventional quasi-one-dimensional antiferromagnets with quantum spins, magnetic excitations are carried by either magnons or spinons in different energy regimes: they do not coexist ...independently, nor could they interact with each other. In this Letter, by combining inelastic neutron scattering, quantum Monte Carlo simulations, and random phase approximation calculations, we report the discovery and discuss the physics of the coexistence of magnons and spinons and their interactions in Botallackite-Cu2(OH)3Br. This is a unique quantum antiferromagnet consisting of alternating ferromagnetic and antiferromagnetic spin1 / 2 chains with weak interchain couplings. Our study presents a new paradigm where one can study the interaction between two different types of magnetic quasiparticles: magnons and spinons.
The crystal structures of two Delafossites, Cu3Ni2SbO6 and Cu3Co2SbO6, are determined by high-resolution synchrotron powder X-ray diffraction. The Ni and Co are ordered with respect to Sb in the ...layer of edge sharing octahedra, forming magnetic layers with honeycomb geometry. High-resolution electron microscopy confirms ordering, and selected-area electron diffraction patterns identify examples of the stacking polytypes. Low temperature synthetic treatments result in disordered stacking of the layers, but heating just below their melting points results in nearly fully ordered stacking variants. The major variant in both cases is a monoclinic distortion of a 6-layer Delafossite polytype, but a significant amount of a 2-layer polytype is also present for the Ni case. The antiferromagnetic ordering with transitions, at 22.3 and 18.5 K for Ni and Co variants, respectively, is investigated by temperature and field dependent magnetization, as well as specific heat. The sharp magnetic transitions support the presence of well developed 2:1 ordering of the Co:Sb or Ni:Sb ions in the honeycomb layers. Neutron diffraction measurements at 4 K are used to determine the magnetic structures. For both the Ni and Co phases, the propagation vector is k = 100, and can be described as alternating ferromagnetic chains in the metal-oxide plane giving an overall antiferromagntic “zigzag” alignment. While orientation of the magnetic moments of the Co is along the b-axis, the Ni moments are in the ac plane, approximately parallel to the stacking direction. Bulk magnetization properties are discussed in terms of their magnetic structures.
The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials. Various mechanisms producing MITs have been extensively considered, including ...the Mott (electron localization via Coulomb repulsion), Anderson (localization via disorder), and Peierls (localization via distortion of a periodic one-dimensional lattice) mechanisms. One additional route to a MIT proposed by Slater, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention. Using neutron and x-ray scattering we show that the MIT in NaOsO(3) is coincident with the onset of long-range commensurate three dimensional magnetic order. While candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT.
Compounds with two-dimensional (2D) layers of magnetic ions weakly connected by van der Waals bonding offer routes to enhance quantum behavior, stimulating both fundamental and applied interest. ...CrPS4 is one such magnetic van der Waals material, however, it has undergone only limited investigation. Here we present a comprehensive series of neutron scattering measurements to determine the magnetic structure and exchange interactions. The observed magnetic excitations allow a high degree of constraint on the model parameters not normally associated with measurements on a powder sample. The results demonstrate the 2D nature of the magnetic interactions, while also revealing the importance of interactions along 1D chains within the layers. The subtle role of competing interactions is observed, which manifest in a nontrivial magnetic transition and a tunable magnetic structure in a small applied magnetic field through a spin-flop transition. Our results on the bulk compound provide insights that can be applied to an understanding of the behavior of reduced layer CrPS4.
We use neutron scattering and bulk property measurements to determine the single-ion crystal-field Hamiltonians of delafossites KErSe2 and CsErSe2. These two systems contain planar equilateral ...triangular Er lattices arranged in two stacking variants: rhombohedral (for K) or hexagonal (Cs). Our analysis shows that regardless of the stacking order both compounds exhibit an easy-plane ground-state doublet with large Jz=1/2 terms and the potential for significant quantum effects, making them candidates for quantum-spin-liquid or other exotic ground states.
•La0.7Ca0.3MnO3 has coexisting ferromagnetic and antiferromagnetic correlations above the magnetic ordering temperature.•Antiferromagnetic correlations persist below the magnetic ordering temperature ...but disappear deep in the ordered state.•Polarized neutron scattering confirms these correlations to be purely magnetic.
La0.7Ca0.3MnO3 (LCMO30) is a perovskite manganite with a phase transition (TC = 238 K) between a paramagnetic insulator and a ferromagnetic metal. We use single-crystal, inelastic neutron scattering to refine previous work on the low energy (ℏω < 10 meV) magnetic excitations. At 300 K the spectrum consists of a diffusive ferromagnetic component at the pseudo-cubic Brillouin zone center coexisting with a quasielastic excitation located at the pseudo-cubic Brillouin zone edge. Both components are anti-correlated with ferromagnetic long-range order, showing weakened intensity at 215 K in the ferromagnetic phase. Polarized neutron scattering shows both features to be purely magnetic in origin. This indicates the simultaneous presence of ferromagnetic and antiferromagnetic correlations (located, respectively, at the zone center and zone edge) in a three-dimensional perovskite, while this coexistence had been previously observed only in a bilayer compound.
The refinement of coarse grains in cast uranium can lead to wrought-like properties. In this study, a β-quenched process is coupled with a shock-loading technique and α-annealing to manipulate ...uranium's grain size and the resulting microstructures have been characterized. Neutron powder diffraction has been used to evaluate the residual strain accumulated from the thermo-mechanical treatments. Optical microscopy and electron backscattering (EBSD) in scanning electron microscopy have been employed to evaluate the grain size variation and the nature of deformation mechanisms, respectively. Large strain anisotropy at the lattice level was observed on shock-loaded specimens. The final stress relief annealing did not eliminate all the residual strain. Slip and twinning were observed optically on the shocked specimens while EBSD indicates that although {130}, {172}, and {112} deformation twins were identified, an unusual type of twinning ‘{176}’/ was found to be dominant. It is believed that the magnitude of energy applied favored the occurrence of the {176} twin in the polycrystalline uranium. Average grain size of cast uranium underwent a significant reduction to ∼92μm at the end of the process. The overall results indicate the shock-loading approach as a promising step toward controlling cast uranium grain size and thus its mechanical properties.
We study the correlated quantum magnet YbCl3 with neutron scattering, magnetic susceptibility, and heat capacity measurements. The crystal field Hamiltonian is determined through simultaneous ...refinements of the inelastic neutron scattering and magnetization data. The ground-state doublet is well isolated from the other crystal field levels and results in an effective spin-1/2 system with local easy plane anisotropy at low temperature. Cold neutron spectroscopy shows low-energy excitations peaked at 0.5 meV that are consistent with nearest-neighbor antiferromagnetic correlations.
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