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High intensity pulsed neutron scattering reveals a new set of magnetic excitations in the pinwheel valence bond solid state of the distorted kagome lattice antiferromagnet Rb$_2$Cu$_3$SnF$_{12}$. The polarization of the dominant dispersive modes (2 meV $<\hbar\omega< $7 meV) is determined and found consistent with a dimer series expansion with strong Dzyaloshinskii-Moriya interactions ($D/J=0.18$). A weakly dispersive mode near 5 meV and shifted "ghosts" of the main modes are attributed to the enlarged unit cell below a $T=215$ K structural transition. Continuum scattering between 8 meV and 10 meV might be interpreted as a remnant of the kagome spinon continuum [T.-H. Han et al., Nature 492, 406 (2012)], Comment: 7 pages, 5 figures

The spin glass behavior of Y2Mo2O7 has puzzled physicists for nearly three decades. Free of bulk disorder within the resolution of powder diffraction methods, it is thought that this material is a rare realization of a spin glass resulting from weak disorder such as bond disorder or local lattice distortions. Here, we report on the single crystal growth of Y2Mo2O7. Using neutron scattering, we present unique isotropic magnetic diffuse scattering arising beneath the spin glass transition despite having a well-ordered structure at the bulk level. Despite our attempts to model the diffuse scattering using a computationally exhaustive search of a class of simple spin Hamiltonians, we were unable to replicate the experimentally observed energy-integrated (diffuse) neutron scattering. A T^2-temperature dependence in the heat capacity and density functional theory calculations hint at significant frozen degeneracy in both the spin and orbital degrees of freedom resulting from spin-orbital coupling (Kugel-Khomskii type) and random fluctuations in the Mo environment at the local level., Comment: 35 pages, 14 figures, 1 table, supplementary information not included, accepted for publication in Phys. Rev. B

Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO$_{0.5}$F$_{0.5}$BiS$_{2}$, and its nonsuperconducting parent compound LaOBiS$_{2}$. The crystal structures and vibrational modes have been examined. Upon F-doping, while the lattice contracts significantly along $c$ and expands slightly along $a$, the buckling of the BiS$_2$ plane remains almost the same. In the inelastic measurements, a large difference in the high energy phonon modes was observed upon F substitution. Alternatively, the low energy modes remain almost unchanged between non-superconducting and superconducting states either by F-doping or by cooling through the transition temperature. Using density functional perturbation theory we identify the phonon modes, and estimate the phonon density of states. We compare these calculations to the current measurements and other theoretical studies of this new superconducting material., Comment: 5 pages, 4 figures, 1 table

Two different microscopic phases have been observed via inelastic neutron scattering for superfluid helium confined within highly monodisperse, smooth, and unidimensional silica pores. When the helium forms a thin film on the pore walls, it supports a dramatically modified phonon-roton spectrum as well as a 2D surface roton. The energies of these modified phonon-roton modes are consistent with predictions for a dilute, low-density film, while the energy of the 2D surface roton corresponds to that of a dense film. The relatively smooth and weak surface potential of the substrate permits the formation of a film with large dilute regions and only small compressed regions. When the pores are saturated with liquid, the modified phonon-roton spectrum disappears, and bulk-like modes coexist with the 2D surface roton. Presumably, bulk-like liquid occupies the core volume of the pores and high density liquid layers are present at the liquid-solid interface. These findings clearly connect the nature of the excitations to the local density of the liquid., Comment: 4 figures

We present results of inelastic neutron scattering study on two bond disordered quasi twodimensional quantum magnets (C$_4$H$_12$N$_2$)Cu$_2$(Cl$_{1-x}$Br$_x$)$_6$ with x=0.035 and 0.075. We observe the increase of spin gap, reduction of magnon bandwidth and a decrease of magnon lifetimes compared to x=0 sample. Additional magnon damping is observed at higher energies away from zone center which is found to follow the density of single particle states.

We report the structural and magnetic properties of a new class of cobaltates with the chemical formula (BaSr)4-xLa2xCo4O15 (x = 0, 0.5 and 1). These compounds crystallize in a hexagonal structure in which cobalt ions are distributed among two distinct crystallographic sites with different oxygen coordination. Three Co-O tetrahedra and one octahedron are linked by shared oxygen atoms to form Co4O15 clusters, which are packed together into a honeycomb-like network. Partial substitution of Sr and/or Ba atoms by La allows one to adjust the degree of Co valence mixing, but all compositions remain subject to a random distribution of charge. Magnetic susceptibility together with neutron scattering measurements reveal that all studied specimens are characterized by competing ferro- and antiferro-magnetic exchange interactions that give rise to a three dimensional Heisenberg spin-glass state. Neutron spectroscopy shows a clear trend of slowing down of spin-dynamics upon increasing La concentration, suggesting a reduction in charge randomness in the doped samples.

We have performed elastic and inelastic neutron experiments on single crystal samples of the coordination polymer compound CuF2(H2O)2(pyz) (pyz=pyrazine) to study the magnetic structure and excitations. The elastic neutron diffraction measurements indicate a collinear antiferromagnetic structure with moments oriented along the [0.7 0 1] real-space direction and an ordered moment of 0.60 +/- 0.03 muB/Cu. This value is significantly smaller than the single ion magnetic moment, reflecting the presence of strong quantum fluctuations. The spin wave dispersion from magnetic zone center to the zone boundary points (0.5 1.5 0) and (0.5 0 1.5) can be described by a two dimensional Heisenberg model with a nearest neighbor magnetic exchange constant J2d = 0.934 +/-0.0025 meV. The inter-layer interaction Jperp in this compound is less than 1.5% of J2d. The spin excitation energy at the (0.5 0.5 0.5) zone boundary point is reduced when compared to the (0.5 1 0.5) zone boundary point by ~10.3 +/- 1.4 %. This zone boundary dispersion is consistent with quantum Monte Carlo and series expansion calculations which include corrections for quantum fluctuations to linear spin wave theory., Comment: 7 pages, 6 figures

The Time-of-Flight and Neutron-Spin-Echo techniques have been applied for the research of ultrasonic effect at the cold neutron diffraction in mosaic single crystals (KBr and mica). It was found that h.f. ultrasound intensifies quasi-elastic neutron scattering and this effect cannot be explained as a result of neutron scattering by thermal phonons in a sample heated by ultrasound; neither can it be interpreted in the context of standard secondary extinction theory. A new modified model is proposed based on the assumption that the angular distribution function describing mosaic block disorientation is not smooth but contains deep narrow troughs. This model is qualitatively equivalent to the preposition about existence large block structure of sample. The corresponding kinetic equations are formulated and exactly solved. The results obtained correlate well with experimental data. Our results evidence that such an approach is applicable for studying the ultrasonic effect at neutron diffraction in different technologies dealing with mosaic single crystals. PACS: 61.05F-,62.65, 63.20.dd Keywords: neutron, ultrasound, mosaic, time-of-flight, spin-echo, Comment: 24 pages, 22 figure

We review a strategy for targeted synthesis of large single crystal samples of prototype quantum magnets for inelastic neutron scattering experiments. Four case studies of organic copper halogenide S=1/2 systems are presented. They are meant to illustrate that exciting experimental results pertaining to forefront many-body quantum physics can be obtained on samples grown using very simple techniques, standard laboratory equipment, and almost no experience in in advanced crystal growth techniques., Comment: 16 pages, 10 figures

This paper presents results of a recent study of multiferroic \CCO\ by means of single crystal neutron diffraction. This system has two close magnetic phase transitions at $T\nsub{N1}=24.2$ K and $T\nsub{N2}=23.6$ K. The low temperature magnetic structure below $T\nsub{N2}$ is unambiguously determined to be a fully 3-dimensional proper screw. Between $T\nsub{N1}$ and $T\nsub{N2}$ antiferromagnetic order is found that is essentially 2-dimensional. In this narrow temperature range, magnetic near neighbor correlations are still long range in the ($H,K$) plane, whereas nearest neighbors along the $L$-direction are uncorrelated. Thus, the multiferroic state is realized only in the low-temperature 3-dimensional state and not in the 2-dimensional state.

The design and performance of the new Cold Neutron Chopper Spectrometer (CNCS) at the Spallation Neutron Source in Oak Ridge are described. CNCS is a direct geometry inelastic time-of-flight spectrometer, designed to cover essentially the same energy and momentum transfer ranges as IN5 at ILL, LET at ISIS, DCS at NIST, TOFTOF at FRM2, AMATERAS at J-PARC, PHAROS at LANSCE and NEAT at HZB, at similar energy resolution. Measured values of key figures such as neutron flux at sample position and energy resolution are compared between measurements and ray tracing Monte Carlo simulations, and good agreement (better than 20% of absolute numbers) has been achieved. The instrument performs very well in the cold and thermal neutron energy ranges, and promises to become a workhorse for the neutron scattering community for quasielastic and inelastic scattering experiments.

In this paper detailed neutron scattering measurements of the magnetic excitation spectrum of CuCrO$_2$ in the ordered state below $T_{\rm{N1}}=24.2$ K are presented. The spectra are analyzed using a model Hamiltonian which includes intralayer-exchange up to the next-next-nearest neighbor and interlayer-exchange. We obtain a definite parameter set and show that exchange interaction terms beyond the next-nearest neighbor are important to describe the inelastic excitation spectrum. The magnetic ground state structure generated with our parameter set is in agreement with the structure proposed for CuCrO$_2$ from the results of single crystal diffraction experiments previously published. We argue that the role of the interlayer exchange is crucial to understand the incommensurability of the magnetic structure as well as the spin-charge coupling mechanism.

CuMoO4 is a triclinic quantum magnet based on S = 1/2 moments at the Cu2+ site. It has recently attracted interest due to the remarkable changes in its chromic and volumetric properties at high temperatures, and in its magnetic properties at low temperatures. This material exhibits a first order structural phase transition at T_C ~ 190 K as well as a magnetic phase transition at T_N ~ 1.75 K. We report low temperature heat capacity measurements as well as extensive elastic and inelastic neutron scattering measurements on powder samples taken above and below T_N. We observe neutron diffraction consistent with a simple (1/2, 0, 0) antiferromagnetic structure indicating a doubling of the a-axis periodicity below T_N. In addition, inelastic neutron scattering above a spin gap of ~ 2.3 meV is consistent with triplet excitations out of paired S = 1/2 moments which form singlet dimers. Low lying spin wave excitations are also observed and these originate from ordered S = 1/2 moments below T_N. Taken together these measurements show the ground state of CuMoO4 to display both non-magnetic singlets, and ferromagnetically-coupled spins coexisting within an antiferromagnetic structure below T_N ~ 1.75 K., Comment: 7 pages, 7 figures

Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic (TO) mode and the longitudinal acoustic (LA) modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying LA phonons. The LA-TO anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.

Frustrated magnetic systems exhibit highly degenerate ground states and strong fluctuations, often leading to new physics. An intriguing example of current interest is the antiferromagnet on a diamond lattice, realized physically in A-site spinel materials. This is a prototypical system in three dimensions where frustration arises from competing interactions rather than purely geometric constraints, and theory suggests the possibility of unusual magnetic order at low temperature. Here we present a comprehensive single-crystal neutron scattering study of CoAl2O4, a highly frustrated A-site spinel. We observe strong diffuse scattering that peaks at wavevectors associated with Neel ordering. Below the temperature T*=6.5 K, there is a dramatic change in the elastic scattering lineshape accompanied by the emergence of well-defined spin-wave excitations. T* had previously been associated with the onset of glassy behavior. Our new results suggest instead that T* signifies a first-order phase transition, but with true long-range order inhibited by the kinetic freezing of domain walls. This scenario might be expected to occur widely in frustrated systems containing first-order phase transitions and is a natural explanation for existing reports of anomalous glassy behavior in other materials., Comment: 40 pages, 9 figures, Introduction and discussion altered and expanded. Additional section and figure added to Supplementary Information

We investigate the doping dependence of the nanoscale electronic and magnetic inhomogeneities in the hole-doping range 0.0020.05, dominated by Co^{3+}-Co^{3+} intracluster interactions. Our conclusions are in good agreement with a recently proposed model of the phase separation in cobalt perovskites [He et al., Europhys. Lett. 87, 27006 (2009)].

We present inelastic neutron scattering and thermodynamic measurements characterizing the magnetic excitations in a disordered non-magnetic substituted spin-liquid antiferromagnet. The parent compound Ba3Mn2O8 is a dimerized, quasi-two-dimensional geometrically frustrated quantum disordered antiferromagnet. We substitute this compound with non-magnetic vanadium for the S = 1 manganese atoms, Ba3(Mn1-xVx)2O8, and find that the singlet-triplet excitations which dominate the spectrum of the parent compound persist for the full range of substitution examined, x = 0.02 to 0.3. We also observe additional low-energy magnetic fluctuations which are enhanced at the greatest substitution values. These excitations may be a precursor to a low-temperature random singlet phase which may exist in Ba3(Mn1-xVx)2O8, Comment: 30 pages, 9 figures

Measurements of the phonon density of states by inelastic neutron \emph{time-of-flight} scattering and specific heat measurements along with first principles calculations, provide compelling evidence for the existence of an Einstein oscillator ("\emph{rattler}") at ${\omega}_{E1} \approx$ 5.0 meV in the filled skutterudite Yb$_{0.2}$Co$_{4}$Sb$_{12}$. Multiple dispersionless modes in the measured density of states of Yb$_{0.2}$Co$_{4}$Sb$_{12}$ at intermediate transfer energies (14 meV $\leq$ \emph{$\omega$} $\leq$ 20 meV) are exhibited in both the experimental and theoretical \emph{density-of-states} of the Yb-filled specimen. A peak at 12.4 meV is shown to coincide with a second Einstein mode at \emph{$\omega_{E2} \approx$} 12.8 meV obtained from heat capacity data. The local modes at intermediate transfer energies are attributed to altered properties of the host CoSb$_{3}$ cage as a result of Yb-filling. It is suggested that these modes are owed to a complementary mechanism for the scattering of heat-carrying phonons in addition to the mode observed at ${\omega}_{E1} \, \approx$ 5.0 meV. Our observations offer a plausible explanation for the significantly-higher \textit{dimensionless figures of merit} of filled skutterudites, compared to their parent compounds., Comment: 9 pages, 6 figures

Y0.5Ca0.5BaCo4O7 was recently introduced as a possible candidate for capturing some of the predicted classical spin kagome ground state features. Stimulated by this conjecture we have taken up a more complete study of the spin correlations in this compound with neutron scattering methods on a powder sample characterized with high-resolution neutron diffraction and the temperature dependence of magnetic susceptibility and specific heat. We have found that the frustrated near neighbor magnetic correlations involve not only the kagome planes but concern the full Co sub-lattice as evidenced by the analysis of the wavevector-dependence of the short range order. We conclude from our results that the magnetic moments are located on the Co sub-lattice as a whole and that correlations extend beyond the two-dimensional kagome planes. We identify intriguing dynamical properties, observing high-frequency fluctuations with a lorentzian line-width {\Gamma} <= 20 meV at ambient temperature. On cooling a low-frequency (~ 1 meV) dynamical component develops alongside the high-frequency fluctuations, which eventually becomes static at temperatures below T ~ 50 K. The high-frequency response with an overall line-width of ~ 10 meV prevails at T <= 2 K coincident with a fully elastic short-range ordered contribution.

Investigations of the spin dynamics of the geometrically frustrated pyrochlore (Tb_pY_{1-p})_2Ti_2O_7, using muon spin relaxation and neutron spin echo, as a function of magnetic coverage p, have been carried out. Our major finding is that paramagnetic fluctuations prevail as T->0 for all values of p, and that they are sensitive to dilution, indicating a cooperative spin motion. However, the percolation threshold p_c is not a critical point for the fluctuations. We also find that the low temperatures spectral density has a 1/f behavior, and that dilution slows down the spin fluctuations., Comment: 5 pages, 5 figure

We present new Neutron Spin Echo (NSE) results and a revisited analysis of historical data on spin glasses, which reveal a pure power-law time decay of the spin autocorrelation function $s(Q,t) = S(Q,t)/S(Q)$ at the glass temperature $T_g$, each power law exponent being in excellent agreement with that calculated from dynamic and static critical exponents deduced from macroscopic susceptibility measurements made on a quite different time scale. It is the first time that this scaling relation involving exponents of different physical quantities determined by completely independent experimental methods is stringently verified experimentally in a spin glass. As spin glasses are a subgroup of the vast family of glassy systems also comprising structural glasses, other non-crystalline systems living matter the observed strict critical scaling behaviour is important. Above the phase transition the strikingly non-exponential relaxation, best fitted by the Ogielski (power-law times stretched exponential) function, appears as an intrinsic, homogeneous feature of spin glasses., Comment: 5 pages and 4 figures

We report new zero-field muon spin relaxation and neutron spin echo measurements in ferromagnetic (FM) (La,Ca)MnO3 which taken together suggest two spatially separated regions in close proximity possessing very different Mn-ion spin dynamics. One region corresponds to an extended cluster which displays 'critical slowing down' near Tc and an increasing volume fraction below Tc. The second region possesses more slowly fluctuating spins and a decreasing volume fraction below Tc. These data are discussed in terms of the growth of small polarons into overlapping regions of correlated spins below Tc, resulting in a microscopically inhomogeneous FM transition., Comment: 10 pages, 3 figures