We report inelastic neutron scattering (INS) measurements on the magnetically frustrated S = 12 sawtoothchain compound atacamite Cu2Cl(OH)3 featuring inequivalent Cu(1) and Cu(2) sites. Transverse to the sawtooth chains, INS reveals two dispersive spin-wave modes and a gap of at least 0.75 meV. This behavior is rationalized within a zigzag-chain model of Cu(2) spins in an effective magnetic field of Cu(1) spins. The model is compatible with first-principles calculations and accounts for INS dispersions within linear spin-wave theory calculations. Our results reveal a unique case of an effective separation of energy scales between two differently oriented onedimensional chains, with the zigzag-chain model being essential to fully characterize the low-energy magnetism of atacamite.

Two-dimensional kagome metals consisting of corner-sharing triangles offer a unique platform for studying strong electron correlations and band topology due to its geometrically frustrated lattice structure. The similar energy scales between spin, lattice, and electronic degrees of freedom in these systems give rise to competing quantum phases such as charge density wave (CDW), magnetic order, and superconductivity. For example, kagome metal FeGe first exhibits A-type collinear antiferromagnetic (AFM) order at T_N ~ 400 K, then establishes a CDW phase coupled with AFM ordered moment below T_CDW ~ 100 K, and finally forms a $c$-axis double cone AFM structure around T_Canting ~ 60 K. Here we use neutron scattering to demonstrate the presence of gapless incommensurate spin excitations associated with the double cone AFM structure at temperatures well above T_Canting and T_CDW that merge into gapped commensurate spin waves from the A-type AFM order. While commensurate spin waves follow the Bose population factor and can be well described by a local moment Heisenberg Hamiltonian, the incommensurate spin excitations first appear below T_N where AFM order is commensurate, start to deviate from the Bose population factor around T_CDW, and peaks at T_Canting, consistent with a critical scattering of a second order magnetic phase transition, as a function of decreasing temperature. By comparing these results with density functional theory calculations, we conclude that the incommensurate magnetic structure arises from the nested Fermi surfaces of itinerant electrons and the formation of a spin density wave order. The temperature dependence of the incommensurate spin excitations suggests a coupling between spin density wave and CDW order, likely due to flat electronic bands near the Fermi level around T_N and associated electron correlation effects.

To understand the 2D triangular Heisenberg antiferromagnetic system, we investigated the magnetic structures and the dynamics of LuyY1-yMnO3 in detail. The substitutions are adjusted to the Mn atomic position close to xMn = 3 . The neutron powder diffraction data claims that the magnetic structure of LuyY1-yMnO3 is described 1 as a mixture of Gamma 3 (P6'3cm') and Gamma 4 (P6'3c'm) at the xMn position for y = 0.15, 0.30, and 0.45. The ratio of Gamma 3 and Gamma 4 depends on temperature and composition and the fraction of Gamma 3 increases upon cooling, while no clear trimerization was observed at the xMn position. We estimated exchange parameters from the analysis of the low-energy part of the spin waves. The results showed a weak trimerization effect on cooling because the nearest -neighbor exchange interaction is slightly enhanced. The temperature dependence of the spin-wave dispersion around the Gamma point shows that the spin gap closes with increasing temperature because the exchange interactions in the nearest Mn-Mn neighbor become smaller. Gapless diffusive magnetic excitation from a Mn triangular lattice has been observed in a wide range in Q and E space of LuyY1-yMnO3. We found that Lu0.7Y0.3MnO3 could be an ideal case to investigate the trimerization, frustrated magnetism, and magnetoelastic coupling often observed in two-dimensional triangular lattice Heisenberg antiferromagnet systems.

The inelastic neutron scattering spectra recorded in this study and elsewhere provide a useful set of crystal-field (CF) energy levels for the groundJ= 6 term of Ho3+in HoFeO3. The resolution of the low-energy, temperature-dependent pseudo-quadrupole ground state splitting and magnon peaks is consistent with the self-ordering of the Ho3+sublattice atTHo∼ 8-10 K and supports earlier electron spin resonance investigations of the Ho3+magnon behaviour. Systematic analysis of the grouped singlet CF levels of Ho3: HoFeO3, in conjunction with the CF Kramers doublet levels of the neighbouring Er3+: ErFeO3, has yielded possible sets of CF parameters for the two systems.

We report the magnetic structure and anisotropy of the quasi-one-dimensional S = 12 antiferromagnet Na2CuSO4Cl2 obtained by single-crystal neutron scattering, electron spin resonance (ESR), and magnetization measurements, following an earlier study of its dynamics [M. Fujihala etal., Phys. Rev. B 101, 024410 (2020)]. A Neel-type spin structure is formed within the chain of this compound, where the spins point along the b axis, and ESR data indicate an antisymmetric exchange with a uniform Dzyaloshinskii-Moriya (DM) vector pointing along the b axis. The anisotropy g factor and magnetic structure are strong indicators of magnetic anisotropy originating from a symmetric anisotropic exchange interaction and/or a magnetic dipole interaction. These results suggest that these terms of the anisotropic spin Hamiltonian counteract the effect of the DM interaction and stabilize the Neel-type structure in Na2CuSO4Cl2.

We use neutron scattering to show that ferromagnetic (FM) phase transition in the two-dimensional (2D) honeycomb lattice ${\mathrm{CrI}}_{3}$ is a weakly first order transition and controlled by spin-orbit coupling (SOC) induced magnetic anisotropy, instead of magnetic exchange coupling as in a conventional ferromagnet. With increasing temperature, the magnitude of magnetic anisotropy, seen as a spin gap at the Brillouin zone center, decreases in a power law fashion and vanishes at ${T}_{C}$, while the in-plane and $c$-axis spin-wave stiffnesses associated with magnetic exchange couplings remain robust at ${T}_{C}$. We also compare parameter regimes where spin waves in ${\mathrm{CrI}}_{3}$ can be described by a Heisenberg Hamiltonian with Dzyaloshinskii-Moriya interaction or a Heisenberg-Kitaev Hamiltonian. These results suggest that the SOC induced magnetic anisotropy plays a dominant role in stabilizing the FM order in single layer 2D van der Waals ferromagnets.

The S = 1/2 quasi-one-dimensional antiferromagnet K2CuSO4X2 (X = Cl, Br) exhibits peculiar Dzyaloshinskii-Moriya (DM) interactions that are uniform along its spin chains and antiparallel with respect to neighboring chains. This feature has received much attention recently because it leads to spin frustration, however, the spin dynamics around T-N and magnetic structure have not been reported. Here we report magnetic behaviors of Na2CuSO4Cl2. The orthorhombic crystal structure of Na2CuSO4Cl2, which is identical to K2CuSO4X2, was verified. The results of the thermodynamic measurements suggest that this compound has moderately strong intra- and interchain interaction for investigation of the spin state around T-N. The inelastic neutron scattering and muon spin relaxation and rotation measurements reveal the presence of a two-spinon continuum, and below T-N, the long-range order develops. However, the obtained critical exponent is beta = 0.18, which is not indicative of a three-dimensional magnetic system; instead, one-dimensional (1D) spin correlation likely affects the formation of magnetic ordering in Na2CuSO4Cl2. There is a possibility that Na(2)CuSO(4)Cl(2 )is a model compound for investigation of DM-induced frustration effects in a 1D quantum spin system.

Lebing Chen ,1 Jae-Ho Chung,2,* Tong Chen,1 Chunruo Duan,1 Astrid Schneidewind ,3 Igor Radelytskyi,3 David J. Voneshen,4 Russell A. Ewings ,4 Matthew B. Stone ,5 Alexander I. Kolesnikov ,5 Barry Winn,5 Songxue Chi,5 R. A. Mole,6 D. H. Yu,6 Bin Gao ,1 and Pengcheng Dai 1,† 1Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA 2Department of Physics, Korea University, Seoul 02841, Korea 3Forschungszentrum Jülich GmbH, Jülich Centre for Neutron Science (JCNS) at Heinz Maier-Leibnitz Zentrum (MLZ), Lichtenbergstrasse 1, 85748 Garching, Germany 4ISIS Pulsed Neutron and Muon Source, STFC Rutherford Appleton Laboratory, Harwell Campus, Didcot, Oxon, OX11 0QX, United Kingdom 5Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 6Australian Nuclear Science and Technology Organisation, Locked bag 2001, Kirrawee DC, New South Wales 2232, Australia

Analysis of inelastic neutron scattering data recorded for the rare earth intermetallic compound TmV2Al20 yields a cubic crystal field Hamiltonian with parameters x = -0.63(1) and W = +0.43(1) K (Lea, Leask & Wolf notation). This result is supported by a strong electron paramagnetic resonance signal that is consistent with magnetic splitting of the first excited (T-5((1))) state. Using this crystal field Hamiltonian, single crystal magnetisation and specific heat data are then interpreted in terms of a model that involves partial Al flux substitution of an approximately 10% depleted Tm "cage" site. (C) 2020 Elsevier B.V. All rights reserved.

This paper shows that atomic motions in ultra-thin alumina glass result in a characteristic peak in the vibrational density-of-states at 2.8 meV. The boson peak frequency measured by neutron spectroscopy is in agreement with the values calculated from molecular dynamics simulations. Confinement within the nanometer-thick layer shifts the boson peak frequency.

Layered structure BiCuSeO-based compounds have extremely low thermal conductivity about similar to 0.5-0.8 W/m.K, and the corresponding physical origin has been extensively studied by the first-principles calculations. Here we experimentally revealed the physical mechanism of extremely low thermal conductivity in BiCuSeO and BiCuTeO through inelastic neutron and Raman scattering spectroscopy. Generalized phonon density of states (PDOS) characterized by inelastic neutron scattering reveals that the average acoustic-phonon velocities of BiCuSeO and BiCuTeO are as low as 2104 and 1547 m/s, respectively, which are lower than most of normal materials (similar to 3000 m/s), and strong anharmonic effect in BiCuSeO and BiCuTeO. Strongly anharmonic effect is also verified by the large Gruneisen constant of specific optical-phonon mode of BiCuSeO and BiCuTeO (similar to 6.7 in BiCuTeO). The calculated thermal conductivities of BiCuSeO and BiCuTeO by phenomenological thermal conductivity formula, under approximation of the relaxation-time as minimum quasi-particle lifetime of optical-phonon mode, are close to experimental values. Our work sheds more light on the physical mechanism of extremely low thermal conductivity in these compounds. (C) 2020 Elsevier B.V. All rights reserved.

Bulk glasses exhibit extra vibrational modes at low energies, known as the boson peak. The microscopic dynamics in nanoscale alumina impact the performance of qubits and other superconducting devices, however the existence of the boson peak in these glasses has not been previously measured. Here we report neutron spectroscopy on Al/Al$_2$O$_{3-x}$ nanoparticles consisting of spherical metallic cores from 20 to 1000 nm surrounded by a 3.5 nm thick alumina glass. An intense low-energy peak is observed at $\omega_{BP}$ = 2.8 $\pm$ 0.6 meV for highly oxidised particles, concurrent with an excess in the density of states. The intensity of the peak scales inversely with particle size and oxide fraction indicating a surface origin, and is red-shifted by 3 meV with respect to the van-Hove singularity of $\gamma$-phase Al$_2$O$_{3-x}$ nanocrystals. Molecular dynamics simulations of $\alpha$-Al$_2$O$_{3-x}$, $\gamma$-Al$_2$O$_{3-x}$ and a-Al$_2$O$_{3-x}$ show that the observed boson peak is a signature of the ultrathin glass surface, and the frequency is softened compared to that of the hypothetical bulk glass.

We report on the magnetism of charge-stripe ordered La2NiO4.11±0.01 by neutron scattering and μSR. On going towards zero energy transfer there is an observed wave vector offset in the centring of the magnetic excitations and magnetic Bragg reflections, meaning the excitations cannot be described as Goldstone modes of the magnetic order. Weak transverse field μSR measurements determine the magnetically order volume fraction is 87% from the two stripe twins, and the temperature evolution of the magnetic excitations is consistent with the low energy excitations coming from the magnetically ordered volume of the material. We will discuss how these results contrast with the proposed origin of a similar wave vector offset recently observed in a La-based cuprate, and possible origins of this effect in La2NiO4.11.

The coupling between magnetic ordering and electric polarization in magnetoelectrics is technically relevant and not yet completely understood. The elucidation of this requires a comprehension of underlying magnetic interactions and the electronic structure. Here, the authors investigate the magnetoelectric honeycomb compound Mn${}_{4}$Ta${}_{2}$O${}_{9}$, by means of neutron scattering and electronic structure calculations, to reveal the ingredients that determine the ground state. For this compound, the ground state is determined by an electron correlation assisted magnetic dipole-dipole interaction. The investigation here offers detailed insight into the factors that determine the ground-state manifold and the role of electron correlation on the strength of anisotropy.

Recently a new one-dimensional (1D) quantum spin chain system has been reported: catena-dichloro(2-Cl-3Mpy)copper(II), (where 2-Cl-3Mpy=2-chloro-3-methylpyridine). Preliminary calculations and bulk magnetic property measurements indicate that this system does not undergo magnetic ordering down to 1.8 K and is a prime candidate for investigating frustration in a J1/J2 system (where the nearest neighbour interactions, J1, are ferromagnetic and the next nearest neighbour interactions, J2, are antiferromagnetic). Calculations predicted three possible magnetic interaction strengths for J1 below 6 meV depending on the orientation of the ligand. For one of the predicted J1 values, the existence of a quantum critical point is implied. A deuterated sample of catena-dichloro(2-Cl-3Mpy)copper(II) was synthesised and the excitations measured using inelastic neutron scattering. Scattering indicated the most likely scenario involves spin-chains where each chain consists of only one of the three possible magnetic excitations in this material, rather than the completely random array of exchange interactions within each chain as predicted by Herringer et al (2014 Chem. Eur. J. 20 8355–62). This indicates the possibility of tuning the chemical structure to favour a system which may exhibit a quantum critical point.

The stability of charge ordered phases is doping dependent, with different materials having particularly stable ordered phases. In the half filled charge ordered phases of the cuprates this occurs at one eighth doping, whereas in charge-stripe ordered La2-xSrxNiO4+delta there is enhanced stability at one third doping. In this paper we discuss the known details of the charge-stripe order in La2-xSrxNiO4+delta, and how these properties lead to the one third doping stability.