The lead-halide perovskites, including CH3NH3PbBr3, are components in cost effective, highly efficient photovoltaics, where the interactions of the molecular cations with the inorganic framework are suggested to influence the electronic and ferroelectric properties. CH3NH3PbBr3 undergoes a series of structural transitions associated with orientational order of the CH3NH3 (MA) molecular cation and tilting of the PbBr3 host framework. We apply high-resolution neutron scattering to study the soft harmonic phonons associated with these transitions, and find a strong coupling between the PbBr3 framework and the quasistatic CH3NH3 dynamics at low energy transfers. At higher energy transfers, we observe a PbBr6 octahedra soft mode driving a transition at 150 K from bound molecular excitations at low temperatures to relatively fast relaxational excitations that extend up to ∼ 50-100 meV. We suggest that these temporally overdamped dynamics enables possible indirect band gap processes in these materials that are related to the enhanced photovoltaic properties.Organic-inorganic, hybrid perovskites (OIPs) are materials based upon an inorganic perovskite host framework with an organic molecular cation occupying the interstitial space. These materials have been studied for quite some time [1], but interest has recently surged owing to their use in photovoltaic devices and to possible ferroelectricity.[2] While earlier work centered on Snbased OIPs as possible sensor materials [3, 4], recent interest has been focused towards Pb-based OIPs, due to their potential advantages in inexpensive photovoltaic devices, with efficiencies of the order of 20%.[5] We apply neutron scattering to study the coupled dynamics of the host PbBr 3 framework and the methylamonium (MA) cation in CH 3 NH 3 PbBr 3 , and discuss the possible relation with the photovoltaic properties. We map out the soft phonons and low-temperature quasistatic molecular rotations, and show that the low-temperature, harmonic fluctuations cross over to temporally overdamped dynamics at high temperature.The OIPs are composed of two sublattices ( Fig. 1 (a) for CH 3 NH 3 PbBr 3 ): the inorganic sublattice, consisting of a fully corner-bonded framework of octahedra (PbBr − 6 ); and the organic sublattice consisting of the MA molecular cation, (CH 3 NH + 3 ). At high temperatures, the structure of CH 3 NH 3 PbBr 3 is cubic (space group Pm3m), and below 235 K has a tetragonal structure in symmetry I4/mcm. [6,7] At 150 K, a transition to an unknown structure (believed to be incommensurate [8]) occurs followed by further distortion to orthorhombic (Pnma) at 148 K. [9-12] Neutron diffraction measurements have suggested that these transitions originate from tilting of the PbBr 6 octahedra and orientational ordering of the MA cation. [6,13] We note that despite minor differences in the phase diagrams, for all MAPbX 3 (X=Cl,Br,I), a transition to an ordered phase, in which the octahedra are tilted and the MA cations have relatively well-defined orientations, occurs in the temperatu...

We study the spin dynamics in two variants of the high-anisotropy Mn6 nanomagnet by inelastic neutron scattering, magnetic resonance spectroscopy and magnetometry. We show that a giant-spin picture is completely inadequate for these systems and that excited S multiplets play a key role in determining the effective energy barrier for the magnetization reversal. Moreover, we demonstrate the occurrence of tunneling processes involving pair of states having different total spin.

Sr3Cr2O8 consist of a lattice of spin-1/2 Cr 5+ ions, which form hexagonal bilayers and which are paired into dimers by the dominant antiferromagnetic intrabilayer coupling. The dimers are coupled three-dimensionally by frustrated interdimer interactions. A structural distortion from hexagonal to monoclinic leads to orbital order and lifts the frustration giving rise to spatially anisotropic exchange interactions. We have grown large single crystals of Sr3Cr2O8 and have performed DC susceptibility, high field magnetisation and inelastic neutron scattering measurements. The neutron scattering experiments reveal three gapped and dispersive singlet to triplet modes arising from the three twinned domains that form below the transition thus confirming the picture of orbital ordering. The exchange interactions are extracted by comparing the data to a Random Phase Approximation model and the dimer coupling is found to be J0 = 5.55 meV, while the ratio of interdimer to intradimer exchange constants is J ′ /J0 = 0.64. The results are compared to those for other gapped magnets.

Inelastic neutron scattering (INS), electron spin (ESR) and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly holedoped La 1−x Sr x CoO 3 , x ∼ 0.002. Both, INS and ESR low temperature spectra show intense excitations with large effective g-factors ∼ 10 − 18. NMR data indicate the creation of extended magnetic clusters. From the Q-dependence of the INS magnetic intensity we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions.

A knee-shaped feature observed earlier in light scattering spectra of Ca 0.4 K 0.3 ͑NO 3 ͒ 1.4 ͑CKN͒ below T c is used as a strong argument in favor of mode-coupling theory of the glass transition ͑MCT͒. Our careful measurements reveal no ''knee'' in the spectra of two glass forming liquids, CKN and ortho-terphenyl. Instead of the knee the spectra show nontrivial broadening and an increase of the intensity with a temperature increase. Both variations are confirmed by neutron scattering measurements on CKN and are neither expected in the asymptotic MCT predictions nor in any other model.

We present results from complementary characterizations of the primary relaxation rate of a room temperature ionic liquid (RTIL), 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl} imide, [C6mim][Tf2N], over a wide temperature range. This extensive data set is successfully merged with existing literature data for conductivity, viscosity, and NMR diffusion coefficients thus providing, for the case of RTILs, a unique description of the primary process relaxation map over more than 12 decades in relaxation rate and between 185 and 430 K. This unique data set allows a detailed characterization of the VTF parameters for the primary process, that are: B=890 K, T0=155.2 K, leading to a fragility index m=71, corresponding to an intermediate fragility. For the first time neutron spin echo data from a fully deuteriated sample of RTIL at the two main interference peaks, Q=0.76 and 1.4 A(-1) are presented. At high temperature (T>250 K), the collective structural relaxation rate follows the viscosity behavior; however at lower temperatures it deviates from the viscosity behavior, indicating the existence of a faster process.

We present results of parallel quasielastic neutron scattering (QENS) experiments and molecular dynamics numerical simulations for the dynamics of a prototype ionic liquid, 1-ethyl-3-methyl-imidazolium bromide. Differences and similarities with those from the crystal phase are also discussed. Both experiment and simulation demonstrate that, in the length and time scales being probed here (fractions of a nm and a few ps), the dynamics are dominated by activated translational diffusion in the liquid phase and reorientations of the ethyl groups in both solid and liquid.

We have extended the exploration of microscopic dynamics of supercooled liquids to small wave numbers Q corresponding to the scale of intermediate range order, by developing a new experimental approach for precise data correction for multiple scattering noise in inelastic coherent neutron scattering. Our results in supercooled Ca0.4K0.6(NO3)(1.4) reveal the first direct experimental evidence, after a decade of controversy, that the so-called picosecond process around the glass transition corresponds to a predicted first, faster stage of the structural relaxation. In addition, they show that this process takes the spatial form of fast heterogeneous collective flow of correlated groups of atoms.