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Competition among exchange interactions is able to induce novel spin correlations on a bipartite lattice without geometrical frustration. A prototype example is the spiral spin liquid, which is a correlated paramagnetic state characterized by sub-dimensional degenerate propagation vectors. Here, using spectral graph theory, we show that spiral spin liquids on a bipartite lattice can be approximated by a further-neighbor model on the corresponding line-graph lattice that is non-bipartite, thus broadening the space of candidate materials that may support the spiral spin liquid phases. As illustrations, we examine neutron scattering experiments performed on two spinel compounds, ZnCr$_2$Se$_4$ and CuInCr$_4$Se$_8$, to demonstrate the feasibility of this new approach and expose its possible limitations in experimental realizations., Comment: 20 pages, 18 figures

Phonon-derived behaviors are important indicators of novel phenomena in transition metal trihalides, including spin liquid behavior, two-dimensional magnetism, and spin-lattice coupling. However, phonons and their dependence on spin structure and excitations have not been adequately explored. In this work, we probe and critically examine the vibrational properties of bulk $CrCl_3$ using inelastic neutron scattering and density functional theory. We demonstrate that magnetic and van der Waals interactions are essential to describing the structure and phonons in $CrCl_3$; however, the specific spin configuration is unimportant. This provides context for understanding thermal transport measurements as governed by dynamical spin-lattice couplings. More importantly, we introduce an efficient dynamic method that exploits translational symmetries in large conventional unit cells that generates insights into phonon dispersions, interactions, and measured spectra in terms of quantum phase interference conditions. This work opens new avenues for understanding phonons in layered magnets and more generally in conventional cell geometries of a variety of materials., Main text: 21 pages, 7 figures. The manuscript is currently under review for a journal publication

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 YbCl3. 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 YbCl3 as a benchmark material for quantum magnetism on the honeycomb lattice., Honeycomb lattices with interacting spins can host rich magnetic behaviour; however, typically features are complicated by additional interactions. Here, the authors perform neutron scattering on YbCl3, which exhibits near perfect two-dimensional magnetism, providing a benchmark for other materials.

Spiral spin-liquids are correlated paramagnetic states with degenerate propagation vectors forming a continuous ring or surface in reciprocal space. On the honeycomb lattice, spiral spin-liquids present a novel route to realize emergent fracton excitations, quantum spin liquids, and topological spin textures, yet experimental realizations remain elusive. Here, using neutron scattering, we show that a spiral spin-liquid is realized in the van der Waals honeycomb magnet FeCl$_3$. A continuous ring of scattering is directly observed, which indicates the emergence of an approximate U(1) symmetry in momentum space. Our work demonstrates that spiral spin-liquids can be achieved in two-dimensional systems and provides a promising platform to study the fracton physics in spiral spin-liquids., Comment: 16 pages, 14 figures

Quantum magnets admit more than one classical limit and $N$-level systems with strong single-ion anisotropy are expected to be described by a classical approximation based on SU($N$) coherent states. Here we test this hypothesis by modeling finite temperature inelastic neutron scattering (INS) data of the effective spin-one antiferromagnet \bfso{}. The measured dynamic structure factor is calculated with a generalized Landau-Lifshitz dynamics for SU($3$) spins. Unlike the traditional classical limit based on SU($2$) coherent states, the results obtained with classical SU($3$) spins are in good agreement with the measured temperature-dependent spectrum. The SU($3$) approach developed here provides a general framework to understand the broad class of materials comprising weakly coupled antiferromagnetic dimers, trimers, or tetramers, and magnets with strong single-ion anisotropy., 8 pages 4 figures

The kagome lattice is a fertile platform to explore topological excitations with both Fermi-Dirac and Bose-Einstein statistics. While relativistic Dirac Fermions and flat-bands have been discovered in the electronic structure of kagome metals, the spin excitations have received less attention. Here we report inelastic neutron scattering studies of the prototypical kagome magnetic metal FeSn. The spectra display well-defined spin waves extending up to 120 meV. Above this energy, the spin waves become progressively broadened, reflecting interactions with the Stoner continuum. Using linear spin wave theory, we determine an effective spin Hamiltonian that reproduces the measured dispersion. This analysis indicates that the Dirac magnon at the K-point remarkably occurs on the brink of a region where well-defined spin waves become unobservable. Our results emphasize the influential role of itinerant carriers on the topological spin excitations of metallic kagome magnets., 6 pages, 4 figures

For topological magnon spectra, determining and explaining the presence of a gap at a magnon crossing point is a key to characterize the topological properties of the system. An inelastic neutron scattering study of a single crystal is a powerful experimental technique that is widely employed to probe the magnetic excitation spectra of topological materials. Here, we show that when the scattering intensity rapidly disperses in the vicinity of a crossing point, such as a Dirac point, the apparent topological gap size is extremely sensitive to experimental conditions including sample mosaic, resolution, and momentum integration range. We demonstrate these effects using comprehensive neutron-scattering measurements of CrCl$_3$. Our measurements confirm the gapless nature of the Dirac magnon in CrCl$_3$, but also reveal an artificial, i.e. extrinsic, magnon gap unless the momentum integration range is carefully controlled. Our study provides an explanation of the apparent discrepancies between spectroscopic and first-principles estimates of Dirac magnon gap sizes, and provides guidelines for accurate spectroscopic measurement of topological magnon gaps., 6 pages and 4 figures

The structurally simplest high-temperature superconductor FeSe exhibits an intriguing superconducting nematic paramagnetic phase with unusual spin excitation spectra that are different from typical spin waves; thus, determining its effective magnetic exchange interactions is challenging. Here we report neutron scattering measurements of spin fluctuations of FeSe in the tetragonal paramagnetic phase. We show that the equal-time magnetic structure factor, $\mathcal{S}(\textbf{Q})$, can be effectively modeled using the self-consistent Gaussian approximation calculation with highly frustrated nearest-neighbor ($J_{1}$) and next-nearest-neighbor ($J_{2}$) exchange couplings, and very weak further neighbor exchange interaction. Our results elucidate the frustrated magnetism in FeSe, which provides a natural explanation for the highly tunable superconductivity and nematicity in FeSe and related materials.

Swedenborgite Ca${}_{0.5}$Y${}_{0.5}$BaCo${}_{4}$O${}_{7}$, with geometrically frustrated kagome and triangular lattices, exhibits a disordered ground state in spite of strong antiferromagnetic coupling between the Co spins. Here, the authors perform single-crystal diffuse neutron scattering experiments to fully characterize the short-range spin correlations in this compound and determine that long-range incommensurate spin correlations are suppressed by exchange disorder. This result reveals a general correspondence between unequal interactions and incommensurate spin correlations on the swedenborgite lattice.

Quantum critical points (QCPs) are widely accepted as a source of a diverse set of collective quantum phases of matter. The basic nature of a QCP is manifested in the critical fluctuation spectrum which in turn is determined by the adjacent phases and associated order parameters. Here we show that the critical fluctuation spectrum of CeCu5.8Ag0.2 can not be explained by fluctuations associated with a single wave vector. Interestingly, when the critical fluctuations at wave vectors corresponding to the incommensurate antiferromagnetic order adjacent to the QCP are separated they are found to be three dimensional and to obey the scaling behavior expected for long wavelength fluctuations near an itinerant antiferromagnetic QCP. Without this separation, E/T scaling with a fractional exponent is observed. Together these results demonstrate that a multicomponent fluctuation spectrum is a previously unexplored route to obtaining E/T scaling at a QCP.

Recent DFT calculations for Ba2CoO4 (BCO) and neutron scattering experiments for SrRuO3 (SRO) have shown that oxygen develops a magnetic polarization. Moreover, DFT calculations for these compounds also unveiled unexpected nodes in the spin density, both along Co-O and Ru-O. For BCO, the overall antiferromagnetic state in its triangular lattice contains unusual zigzag spin patterns. Here, using simple model calculations supplemented by DFT we explain and extend these results. We predict that ligands that in principle should be spinless, such as O$^{2-}$, will develop a net polarization when they act as electronic bridges between transition metal (TM) spins ferromagnetically ordered, regardless of the number of intermediate ligand atoms. The reason is the hybridization between atoms and mobility of the electrons with spins opposite to those of the closest TM atoms. Moreover, for bonds with TMs antiferromagnetically ordered, counterintuitively our calculations show that oxygens should also have a net magnetization for the super-super-exchange cases TM-O-O-TM while for only one oxygen, as in Cu-O-Cu, the O-polarization should cancel. Our simple model also allows us to explain the presence of nodes based on the antibonding character of the dominant singly occupied molecular orbitals along the TM-O bonds. Finally, the zigzag pattern order becomes the ground state mainly due to the influence of the Hubbard $U$, that creates the moments, in combination with a robust easy-axis anisotropy that suppresses the competing 120$^{\circ}$ degree antiferromagnetic order of a triangular lattice. Our predictions are generic and should be applicable to any other compound with characteristics similar to those of BCO and SRO., 15 pages, 17 figures

The hierarchy of the coupling strengths in a physical system often engenders an effective model at low energies where the decoupled high-energy modes are integrated out. Here, using neutron scattering, we show that the spin excitations in the breathing pyrochlore lattice compound CuInCr$_4$S$_8$ are hierarchical and can be approximated by an effective model of correlated tetrahedra at low energies. At higher energies, intra-tetrahedron excitations together with strong magnon-phonon couplings are observed, which suggests the possible role of the lattice degree of freedom in stabilizing the spin tetrahedra. Our work illustrates the spin dynamics in CuInCr$_4$S$_8$ and demonstrates a general effective-cluster approach to understand the dynamics on the breathing-type lattices., Comment: 11 pages, 9 figures

CoSn is a Pauli paramagnet with relatively flat d-bands centered about 100 meV below the Fermi energy Ef. Single crystals of CoSn lightly doped with Fe, In, or Ni are investigated using x-ray and neutron scattering, magnetic susceptibility and magnetization, ac susceptibility, specific heat and resistivity measurements. Within the rigid band approximation, hole doping with a few percent of Fe or In should move the flat bands closer to Ef, whereas electron doping with Ni should move the flat bands further away from Ef. We provide evidence that this indeed occurs. Fe and In doping drive CoSn toward magnetism, while Ni doping suppresses CoSn's already weak magnetic response. The resulting ground state is different for Fe versus In doping. For Fe-doped crystals, Co1-xFexSn, with 0.02 < x < 0.27, the magnetic and specific heat data are consistent with the formation of a spin glass, with a glass transition temperature, Tg, ranging from 1 K for x=0.02 to 10 K for x= 0.27. Powder and single crystal neutron diffraction found no evidence of long-range magnetic order below Tg with x = 0.17. For In-doped crystals, CoSn1-yIny, both the magnetic susceptibility and the Sommerfeld coefficient, gamma, increase substantially relative to pure CoSn, but with no clear indication of a magnetic transition for 0.05 < y < 0.2. CoSn crystals doped with Ni (Co0.93Ni0.07Sn) have a significantly smaller magnetic susceptibility and gamma than pure CoSn, consistent with the flat bands further from Ef., 20 pages, 10 figures

Charge density waves (CDW) are modulations of the electron density and the atomic lattice that develop in some crystalline materials at low temperature. We report an unusual example of a CDW in BaFe$_2$Al$_9$ below 100 K. In contrast to the canonical CDW phase transition, temperature dependent physical properties of single crystals reveal a first-order phase transition. This is accompanied by a discontinuous change in the size of the crystal lattice. In fact, this large strain has catastrophic consequences for the crystals causing them to physically shatter. Single crystal x-ray diffraction reveals super-lattice peaks in the low-temperature phase signaling the development of a CDW lattice modulation. No similar low-temperature transitions are observed in BaCo$_2$Al$_9$. Electronic structure calculations provide one hint to the different behavior of these two compounds; the d-orbital states in the Fe compound are not completely filled. Iron compounds are renowned for their magnetism and partly filled d-states play a key role. It is therefore surprising that BaFe$_2$Al$_9$ develops a structural modulation instead at low temperature instead of magnetic order., 34 pages, 8 figures Edits to text as well as figures 2 and 5. Added neutron diffraction (Fig. 6)

We report transport and inelastic neutron scattering studies on electronic properties and spin dynamics of the quasi-one-dimensional spin chain antiferromagnet RbFeS$_2$. An antiferromagnetic phase transition at $T_N\approx195$ K and dispersive spin waves with a spin gap of 5 meV are observed. By modeling the spin excitation spectra using linear spin wave theory, intra and inter-chain exchange interactions are found to be $SJ_1=100(5)$ meV and $SJ_3=0.9(3)$ meV, respectively, together with a small single-ion anisotropy of $SD_{zz}=0.04(1)$ meV. Comparison with previous results for other materials in the same class of Fe$^{3+}$ spin chain systems reveals that although the magnetic order sizes show significant variation from 1.8 to 3.0$\mu_B$ within the family of materials, the exchange interactions $SJ$ are nevertheless quite similar, analogous to the iron pnictide superconductors where both localized and delocalized electrons contribute to the spin dynamics., Comment: 7 pages, 5 figure

We investigate the physical properties of the Shastry-Sutherland lattice material BaNd$_2$ZnO$_5$. Neutron diffraction, magnetic susceptibility, and specific heat measurements reveal antiferromagnetic order below 1.65 K. The magnetic order is found to be a 2-$\boldsymbol{Q}$ magnetic structure with the magnetic moments lying in the Shastry-Sutherland lattice planes comprising the tetragonal crystal structure of BaNd$_2$ZnO$_5$. The ordered moment for this structure is 1.9(1) $\mu_B$ per Nd ion. Inelastic neutron scattering measurements reveal that the crystal field ground state doublet is well separated from the first excited state at 8 meV. The crystal field Hamiltonian is determined through simultaneous refinement of models with both the LS coupling and intermediate coupling approximations to the inelastic neutron scattering and magnetic susceptibility data. The ground state doublet indicates that the magnetic moments lie primarily in the basal plane with magnitude consistent with the size of the determined ordered moment., Comment: 13 pages, 8 figures

Spin-$\frac{1}{2}$ chains with alternating antiferromagnetic (AF) and ferromagnetic (FM) couplings exhibit quantum entanglement as the integer-spin Haldane chains and might be similarly utilized for quantum computations. Such alternating AF-FM chains have been proposed to be realized in the distorted honeycomb lattice compound ${\mathrm{Na}}_{2}{\mathrm{Cu}}_{2}{\mathrm{TeO}}_{6}$, but to confirm this picture a comprehensive understanding of the exchange interactions including terms outside of the idealized model is required. Here, we employ neutron scattering to study the spin dynamics in ${\mathrm{Na}}_{2}{\mathrm{Cu}}_{2}{\mathrm{TeO}}_{6}$ and accurately determine the coupling strengths through the random phase approximation and density functional theory approaches. We find the AF and FM intrachain couplings are the dominant terms in the spin Hamiltonian, while the interchain couplings are AF but perturbative. This hierarchy in the coupling strengths and the alternating signs of the intrachain couplings can be understood through their different exchange paths. Our results establish ${\mathrm{Na}}_{2}{\mathrm{Cu}}_{2}{\mathrm{TeO}}_{6}$ as a weakly coupled alternating AF-FM chain compound and reveal the robustness of the gapped ground state in alternating chains under weak interchain couplings.

$4f$ intermediate valence (IV) compounds are canonical hosts of correlated electron physics and can contribute to our understanding of the larger class of correlated electron materials. Here we study the prototype IV compound $\mathrm{Yb}{\mathrm{Al}}_{3}$ which exhibits a nonintegral valence with a moderately heavy fermion ground state and a large Kondo temperature (${T}_{\mathrm{K}}\ensuremath{\sim}500--600\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). To better characterize the correlated physics of $\mathrm{Yb}{\mathrm{Al}}_{3}$, we have measured the phonon and the magnetic excitation spectra on single crystals of this material by time-of-flight inelastic neutron scattering and inelastic x-ray scattering. We have also performed theoretical calculations of the phonon spectra. We present three findings of these measurements. First, we observe that the measured phonon spectra can be described adequately by a calculation based on standard DFT+$U$ density functional theory. The calculated energies, however, are 10% too low compared to the measured energies. This discrepancy may reflect a hardening of the phonons due to dynamic $4f$ correlations. Second, the low-temperature spin fluctuations on the Kondo energy scale ${k}_{\mathrm{B}}{T}_{\mathrm{K}}$ have a momentum ($\mathbit{Q}$) dependence similar to that seen recently in the IV compound $\mathrm{CeP}{\mathrm{d}}_{3}$. For that system, the $\mathbit{Q}$ dependence has been attributed to particle-hole excitations in a coherent itinerant $4f$ correlated ground state. We suggest a similar origin for the momentum dependence seen in $\mathrm{Yb}{\mathrm{Al}}_{3}$. This $\mathbit{Q}$ dependence disappears as the temperature is raised towards room temperature and the $4f$ electron band states become increasingly incoherent. Such a coherent/incoherent crossover is expected to be generic for correlated electron systems. Third, a low-temperature magnetic peak observed in the neutron scattering near 30 meV shows dispersion identical to a particular optic-phonon branch. This $4f/\mathrm{phonon}$ resonance disappears for $T\ensuremath{\ge}150\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The phonon spectrum appears to be unaffected by the resonance. We discuss several possibilities for the origin of this unusual excitation, which may be unique to $\mathrm{Yb}{\mathrm{Al}}_{3}$. We suggest that the excitation may arise from the large amplitude beating of the light Al atoms against the heavy Yb atoms, resulting in a dynamic $4f/3p$ hybridization.

We present a comprehensive neutron scattering study of the breathing pyrochlore magnet LiGaCr_{4}S_{8}. We observe an unconventional magnetic excitation spectrum with a separation of high- and low-energy spin dynamics in the correlated paramagnetic regime above a spin-freezing transition at 12(2) K. By fitting to magnetic diffuse-scattering data, we parametrize the spin Hamiltonian. We find that interactions are ferromagnetic within the large and small tetrahedra of the breathing pyrochlore lattice, but antiferromagnetic further-neighbor interactions are also essential to explain our data, in qualitative agreement with density-functional-theory predictions [Ghosh et al., npj Quantum Mater. 4, 63 (2019)2397-464810.1038/s41535-019-0202-z]. We explain the origin of geometrical frustration in LiGaCr_{4}S_{8} in terms of net antiferromagnetic coupling between emergent tetrahedral spin clusters that occupy a face-centered-cubic lattice. Our results provide insight into the emergence of frustration in the presence of strong further-neighbor couplings, and a blueprint for the determination of magnetic interactions in classical spin liquids.

Single crystals of ${\mathrm{Nd}}_{2}{\mathrm{O}}_{3}$ were grown and characterized using neutron scattering and thermodynamic measurements. ${\mathrm{Nd}}_{2}{\mathrm{O}}_{3}$ has long-range antiferromagnetic order below ${T}_{\mathrm{N}}=0.55$ K and specific-heat measurements have demonstrated that a significant amount of the magnetic entropy is released above ${T}_{\mathrm{N}}$. Inelastic neutron scattering experiments reveal a magnetic mode(s) with little dispersion peaked at $\ensuremath{\approx}0.37\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ that is of greatest intensity below ${T}_{\mathrm{N}}$ but persists above $2{T}_{\mathrm{N}}$. This persistence of dynamic correlations is likely related to frustrated interactions associated with the nearly ideal stacked triangular lattice geometry of ${J}_{\text{eff}}=\frac{1}{2}$ spins on ${\mathrm{Nd}}^{3+}$ ions. The magnetization is observed to be strongly anisotropic at all temperatures due to crystal field effects, with easy-plane anisotropy observed. A noncompensated magnetic structure is inferred from the temperature dependence of the magnetization when a magnetic field of sufficient strength is applied within the basal plane near ${T}_{\mathrm{N}}$, and the evolution of the long-range order is summarized in a temperature-field phase diagram.

Understanding the spin-wave excitations of chiral magnetic order, such as the skyrmion crystal (SkX), is of fundamental interest to confirm such exotic magnetic order. The SkX is realized by competing Dzyaloshinskii-Moriya and ferromagnetic-exchange interactions with a magnetic field or anisotropy. Here we compute the dynamical spin structure factor, using Monte Carlo and spin dynamics simulations, extracting the spin-wave spectrum in the SkX, in the vicinity of the paramagnet to SkX transition. Inside the SkX, we find six spin-wave modes, which are supplemented by another mode originating from the ferromagnetic background. Above the critical temperature $T_s$ for the skyrmion crystallization, we find a diffusive regime, reminiscent of the liquid-to-crystal transition, revealing that topological spin texture of skyrmionic character starts to develop above $T_s$ as the precursor of the SkX. We discuss the opportunities for the detection of the spin waves of the SkX using inelastic-neutron-scattering experiments in manganite-iridate heterostructures., 11 pages, 7 figures

In the axially symmetric lacunar spinel GaV${}_{4}$S${}_{8}$, skyrmion vortices are confined along specific orientations, resulting in enhanced robustness against oblique magnetic fields and in altered spin dynamics as compared to other skyrmion hosts. In addition, the authors show that the strong magnetic anisotropy in GaV${}_{4}$S${}_{8}$ drives an extended crossover of the zero-field cycloid texture on approaching its ferromagnetic ground state. The time-dependent magnetic response displays anomalous nonlinearity - a signature of enhanced thermodynamic rigidity and robust domain dynamics.

We present a comprehensive neutron scattering study of the breathing pyrochlore magnet LiGaCr_{4}S_{8}. We observe an unconventional magnetic excitation spectrum with a separation of high- and low-energy spin dynamics in the correlated paramagnetic regime above a spin-freezing transition at 12(2) K. By fitting to magnetic diffuse-scattering data, we parametrize the spin Hamiltonian. We find that interactions are ferromagnetic within the large and small tetrahedra of the breathing pyrochlore lattice, but antiferromagnetic further-neighbor interactions are also essential to explain our data, in qualitative agreement with density-functional-theory predictions [Ghosh et al., npj Quantum Mater. 4, 63 (2019)2397-464810.1038/s41535-019-0202-z]. We explain the origin of geometrical frustration in LiGaCr_{4}S_{8} in terms of net antiferromagnetic coupling between emergent tetrahedral spin clusters that occupy a face-centered-cubic lattice. Our results provide insight into the emergence of frustration in the presence of strong further-neighbor couplings, and a blueprint for the determination of magnetic interactions in classical spin liquids.

An ongoing challenge in the study of quantum materials, is to reveal and explain collective quantum effects in spin systems where interactions between different modes types are important. Here we approach this problem through a combined experimental and theoretical study of interacting transverse and longitudinal modes in an easy-plane quantum magnet near a continuous quantum phase transition. Our inelastic neutron scattering measurements of Ba$_{2}$FeSi$_{2}O$_{7}$ reveal the emergence, decay, and renormalization of a longitudinal mode throughout the Brillouin zone. The decay of the longitudinal mode is particularly pronounced at the zone center. To account for the many-body effects of the interacting low-energy modes in anisotropic magnets, we generalize the standard spin-wave theory. The measured mode decay and renormalization is reproduced by including all one-loop corrections. The theoretical framework developed here is broadly applicable to quantum magnets with more than one type of low energy mode., Comment: 29 pages, 8 figures in main text

Polar materials can host a variety of topologically significant magnetic phases, which often emerge from a modulated magnetic ground state. Relatively few noncentrosymmetric tetragonal materials have been shown to host topological spin textures and new candidate materials are necessary to expand the current theoretical models. This manuscript reports on the anisotropic magnetism in the polar, tetragonal material NdCoGe$_3$ via thermodynamic and neutron diffraction measurements. The previously reported $H$-$T$ phase diagram is updated to include several additional phases, which exist for both $H$ = 0 and with an applied field H$\perp$ c. Neutron diffraction data reveal that the magnetic structures below $T_{N1}$ = 3.70 K and $T_{N2}$ = 3.50 K are incommensurate, with a ground state magnetic order that is incommensurate in all directions with the propagation vector $\vec{k}$ = (0.494, 0.0044, 0.385) at 1.8 K. A unique magnetic structure solution is not achievable, but the possible single and multi-$\vec{k}$ spin models are discussed. These results demonstrate that NdCoGe3 hosts complicated magnetic order derived from modulated magnetic moments., Comment: 11 pages, 10 figures

Single crystals of the single kagome layer compound FeSn are investigated using x-ray and neutron scattering, magnetic susceptibility and magnetization, heat capacity, resistivity, Hall, Seebeck, thermal expansion, thermal conductivity measurements, and density functional theory (DFT). FeSn is a planar antiferromagnet below ${T}_{\mathrm{N}}=365\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and exhibits ferromagnetic magnetic order within each kagome layer. The in-plane magnetic susceptibility is sensitive to synthesis conditions. Resistivity, Hall and Seebeck results indicate multiple bands near the Fermi energy. The resistivity of FeSn is \ensuremath{\approx}3 times lower for current along the stacking direction than in the plane, suggesting that transport and the bulk electronic structure of FeSn is not quasi-two-dimensional (2D). FeSn is an excellent metal with \ensuremath{\rho}(300 K)/\ensuremath{\rho}(2 K) values \ensuremath{\approx}100 in both directions. While the ordered state is antiferromagnetic, high temperature susceptibility measurements indicate a ferromagnetic Curie-Weiss temperature of 173 K, reflecting the strong in-plane ferromagnetic interactions. DFT calculations show a 3D electronic structure with the Dirac nodal lines along the K-H directions in the magnetic Brillouin zone about 0.3 eV below the Fermi energy, with the Dirac dispersions at the $K$ points gapped by spin-orbit coupling except at the $H$ point. The magnetism, however, is highly 2D with ${J}_{\mathrm{in}\text{\ensuremath{-}}\mathrm{plane}}/{J}_{\mathrm{out}\text{\ensuremath{-}}\mathrm{of}\text{\ensuremath{-}}\mathrm{plane}}\ensuremath{\approx}10$. The predicted spin-wave spectrum is presented.

FeSn is an itinerant antiferromagnet that hosts electronic Dirac states and ordered magnetic moments lying within its Fe Kagome-lattice planes. We present magnetization measurements of single crystals of $({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})\text{Sn}$, revealing the evolution and suppression of this magnetic order with Co substitution. We interpret the dramatic changes in magnetic anisotropy to indicate a reorientation of the moments from perpendicular to parallel to the hexagonal $c$ axis and confirm this with neutron diffraction. It has been proposed that the Dirac nodes observed in FeSn should become gapped if the moments rotate as our data suggests. We identify Co-substituted compositions that adopt both antiferromagnetic configurations at different temperatures. This system provides a unique opportunity to study how the details of magnetic order impact Dirac electron states.

Magnetic excitations and magnetic structure of EuRbFe$_4$As$_4$ were investigated by inelastic neutron scattering (INS), neutron diffraction, and random phase approximation (RPA) calculations. Below the superconducting transition temperature $T_\text{c}=36.5$~K, the INS spectra exhibit the neutron spin resonances at $Q_\text{res}=1.27(2)$~$\text{\AA}^{-1}$ and $1.79(3)$~$\text{\AA}^{-1}$. They correspond to the $\mathbf{Q}=(0.5,0.5,1)$ and $(0.5,0.5,3)$ nesting wave vectors, showing three dimensional nature of the band structure. The characteristic energy of the neutron spin resonance is $E_\text{res}=17.7(3)$~meV corresponding to $5.7(1)k_\text{B}T_\text{c}$. Observation of the neutron spin resonance mode and our RPA calculations in conjunction with the recent optical conductivity measurements are indicative of the $s_\pm$ superconducting pairing symmetry in EuRbFe$_4$As$_4$. In addition to the neutron spin resonance mode, upon decreasing temperature below the magnetic transition temperature $T_\text{N}=15$~K, the spin wave excitation originating in the long-range magnetic order of the Eu sublattice was observed in the low-energy inelastic channel. Single-crystal neutron diffraction measurements demonstrate that the magnetic propagation vector of the Eu sublattice is $\mathbf{k}=(0, 0, 0.25)$, representing the three-dimensional antiferromagnetic order. Linear spin wave calculations assuming the obtained magnetic structure with the intra- and inter-plane nearest neighbor exchange couplings of $J_1/k_\text{B}=-1.31$~K and $J_c/k_\text{B}=0.08$~K can reproduce quantitatively the observed spin wave excitation. Our results show that superconductivity and long-range magnetic order of Eu coexist in EuRbFe$_4$As$_4$ whereas the coupling between them is rather weak., Comment: 8 pages, 4 figures

Failed attempts to reproduce measurements suffer from lower resolution not ghosts., Gehring et al. argue that a splitting observed by us in the transverse acoustic (TA) phonon in the relaxor ferroelectric Pb[(Mg1/3Nb2/3)1−xTix]O3 with x = 0.30 (PMN-30PT) is caused by a combination of inelastic-elastic multiple scattering processes called ghostons. Their argument is motivated by differences observed between their measurements made on a triple-axis spectrometer and our measurements on a time-of-flight spectrometer. We show that the differences can be explained by differences in the instrument resolution functions. We demonstrate that the multiple scattering conditions proposed by Gehring et al. do not work for our scattering geometry. We also show that, when a ghoston is present, it is too weak to detect and therefore cannot explain the splitting. Last, this phonon splitting is just one part of the argument, and the overall conclusion of the original paper is supported by other results.

In this paper we report low-temperature magnetic properties of the rare-earth perovskite material YbAlO$_3$. Results of elastic and inelastic neutron scattering experiment, magnetization measurements along with the crystalline electrical field (CEF) calculations suggest that the ground state of Yb moments is a strongly anisotropic Kramers doublet, and the moments are confined in the $ab$-plane, pointing at an angle of $\varphi = \pm 23.5^{\circ}$ to the $a$-axis. With temperature decreasing below $T_{\rm N}=0.88$ K, Yb moments order into the coplanar, but non-collinear antiferromagnetic (AFM) structure $AxGy$, where the moments are pointed along their easy-axes. In addition, we highlight the importance of the dipole-dipole interaction, which selects the type of magnetic ordering and may be crucial for understanding magnetic properties of other rare-earth orthorhombic perovskites. Further analysis of the broad diffuse neutron scattering shows that one-dimensional interaction along the $c$-axis is dominant, and suggests YbAlO$_3$ as a new member of one dimensional quantum magnets., Comment: 8 pages, 6 figures

We study the correlated quantum magnet, YbCl$_3$, 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 that are consistent with nearest neighbor antiferromagnetic correlations of reduced dimensionality., Comment: main paper plus supplemental information

BaPr2ZnO5, BaSm2ZnO5, and BaEu2ZnO5 – a series of 4f magnetic insulators comprising the Shastry-Sutherland lattice – were synthesized via a solid-state reaction under high-pressure and high-temperature conditions. The magnetic behaviors are well characterized by the localized 4f electrons of each Ln3+ (= Pr, Sm, Eu) under the influence of the crystal-electric field of the surrounding ions. The magnetic susceptibility and heat capacity measurements of BaPr2ZnO5 (Pr3+; 4f2 3H4) indicate the splitting of the ground state J-multiplet due to the crystal-electric field. BaSm2ZnO5 (Sm3+; 4f5 6H5/2) is a strong Van Vleck paramagnet, while BaEu2ZnO5 is non-magnetic due to the 7F0 ground state (Eu3+; 4f6 7F0). The series of new oxides enriches a group of the Shastry-Sutherland lattice materials and helps to develop a range of two-dimensional quantum materials.

In ${\mathrm{Ag}}_{2}{\mathrm{CoO}}_{2}$, Co forms triangular lattice layers, which are separated by the metallic (${\mathrm{Ag}}_{2}$) block. The magnetic susceptibility and heat capacity measurements show that this material exhibits an antiferromagnetic transition at ${T}_{\mathrm{N}}=17.5$ K and the Weiss temperature (${T}_{\mathrm{\ensuremath{\Theta}}}$) and the effective moment are $\ensuremath{-}274$ K and $1.62\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, respectively, indicating that the Co ion carries spin ($S$) 1/2 and has a strongly frustrated state with ${T}_{\mathrm{\ensuremath{\Theta}}}/{T}_{\mathrm{N}}=15.7$. A density functional theory calculation confirmed that the valence state of the Co ions is $2+$ and the low-spin state with $S=1/2$ is realized at reduced on-site Coulomb interaction on Co. We performed elastic and inelastic neutron scattering experiments in a powder sample of ${\mathrm{Ag}}_{2}{\mathrm{CoO}}_{2}$. Although no noticeable magnetic Bragg peaks were observed below ${T}_{\mathrm{N}}$, distinct magnetic excitations were observed in the inelastic neutron scattering experiments. The excitations are consistent with those expected for the $S=1/2$ Heisenberg triangular lattice antiferromagnet. These results suggest that the ordered moment is reduced due to the quantum effect, which explains the absence of the magnetic Bragg peaks. Our results thus suggest that ${\mathrm{Ag}}_{2}{\mathrm{CoO}}_{2}$ is a good candidate to realize a quantum Heisenberg triangular lattice antiferromagnet.