Ferro-lattice-distortions and charge fluctuations in superconducting LaO1−x_{1-x}Fx_{x}BiS2_{2}

Competing ferroelectric and charge density wave phases have been proposed to be present in the electron-phonon coupled LaO$_{1-x}$F$_{x}$BiS$_{2}$ superconductor. The lattice instability arises from unstable phonon modes that can break the crystal symmetry. Upon examination of the crystal structure using single crystal diffraction, we find a superlattice pattern arising from coherent in-plane displacements of the sulfur atoms in the BiS$_{2}$ superconducting planes. The distortions morph into coordinated ferro-distortive patterns, challenging previous symmetry suggestions including the possible presence of unstable antiferro-distortive patterns. The ferro-distortive pattern remains in the superconducting state, but with the displacements diminished in magnitude. Moreover, the sulfur displacements can exist in several polytypes stacked along the c-axis. Charge carriers can get trapped in the lattice deformations reducing the effective number of carriers available for pairing

Phonons from neutron powder diffraction

The spherically averaged structure function \soq obtained from pulsed neutron powder diffraction contains both elastic and inelastic scattering via an integral over energy. The Fourier transformation of \soq to real space, as is done in the pair density function (PDF) analysis, regularizes the data, i.e. it accentuates the diffuse scattering. We present a technique which enables the extraction of off-center phonon information from powder diffraction experiments by comparing the experimental PDF with theoretical calculations based on standard interatomic potentials and the crystal symmetry. This procedure (dynamics from powder diffraction(DPD)) has been successfully implemented for two systems, a simple metal, fcc Ni, and an ionic crystal, CaF$_{2}$. Although computationally intensive, this data analysis allows for a phonon based modeling of the PDF, and additionally provides off-center phonon information from powder neutron diffraction

Orbital and spin chains in ZnV2O4

Our powder inelastic neutron scattering data indicate that \zvo is a system of spin chains that are three dimensionally tangled in the cubic phase above 50 K due to randomly occupied $t_{2g}$ orbitals of V$^{3+}$ ($3d^2$) ions. Below 50 K in the tetragonal phase, the chains become straight due to antiferro-orbital ordering. This is evidenced by the characteristic wave vector dependence of the magnetic structure factor that changes from symmetric to asymmetric at the cubic-to-tetragonal transition

Response of armour steel plates to localised air blast load : a dimensional analysis

We report on the results of dimensional analyses on the dynamic plastic response of square armour steel plates due to detonation of proximal cylindrical charges and ensued air blast loading. By assuming a generic function for the blast load, which is multiplicative comprising its spatial and temporal parts, a set of 14 dimensionless parameters, representative of the load and plate deformation, were identified and recast in the form of dimensionless functions of stand-off to charge diameter ratio. Parametric studies were performed using commercial code ABAQUS’s module of Finite Element hydrocode using MMALE and MMAE techniques, and combined with regression analyses to quantify the dimensional parameters and the expressions for dimensionless functions. A few numerical studies with various FE mesh types were also performed to validate the transient deflections against the small-scale experiments. For pulse loading due to proximal charges of small orders of stand-off/charge diameter ratio, the magnitude of the transverse deflection increased abruptly with incremental decrease in stand-off, in contradistinction to the plate deformations at higher stand-offs where variations in displacement are smooth. This confirmed the existence of a stand-off at which a transition in behaviour takes place. For stand-off values less than charge diameter, a dimensionless energy absorbing effectiveness factor was considered to investigate the prediction of rupture in the plate corresponding to different charge masses. This factor is measured as a baseline parameter to predict, using solely numerical means, the blast loads which ensue rupture on full-scale prototypes

Investigation of the spin-glass regime between the antiferromagnetic and superconducting phases in Fe1+y_{1+y}Sex_xTe1−x_{1-x}

Using bulk magnetization along with elastic and inelastic neutron scattering techniques, we have investigated the phase diagram of Fe$_{1+y}$Se$_{x}$Te$_{1-x}$ and the nature of magnetic correlations in three nonsuperconducting samples of Fe$_{1.01}$Se$_{0.1}$Te$_{0.9}$, Fe$_{1.01}$Se$_{0.15}$Te$_{0.85}$ and Fe$_{1.02}$Se$_{0.3}$Te$_{0.7}$. A cusp and hysteresis in the temperature dependence of the magnetization for the $x=0.15$ and 0.3 samples indicates spin-glass (SG) ordering below $T_{\rm sg} = 23$K. Neutron scattering measurements indicate that the spin-glass behavior is associated with short-range spin density wave (SDW) ordering characterized by a static component and a low-energy dynamic component with a characteristic incommensurate wave vector of ${\bf Q}_m = (0.46, 0, 0.50)$ and an anisotropy gap of $\sim$ 2.5 meV. Our high ${\bf Q}$-resolution data also show that the systems undergo a glassy structural distortion that coincides with the short-range SDW order

Storage Device Sizing for a Hybrid Railway Traction System by Means of Bicausal Bond Graphs

In this paper, the application of bicausal bond graphs for system design in electrical engineering is emphasized. In particular, it is shown how this approach is very useful for model inversion and parameter dimensioning. To illustrate these issues, a hybrid railway traction device is considered as a case study. The synthesis of a storage device (a supercapacitor) included in this system is then discussed

Nano-magnetic droplets and implications to orbital ordering in La1-xSrxCoO3

Inelastic cold neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic (AFM) correlations that are dynamic follow the activation to the excited state, identified as the intermediate S=1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La1-xSrxCoO3, the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.Comment: To appear in Phys. Rev. Let

Nanoscale anisotropic structural correlations in the paramagnetic and ferromagnetic phases of Nd0.5Sr0.5 MnO3

We report x-ray scattering studies of short-range structural correlations and diffuse scattering in Nd0.5Sr0.5MnO3. On cooling, this material undergoes a series of transitions, first from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) phase, and then to a charge-ordered (CO) insulating state. Highly anisotropic structural correlations were found in both the PI and FM states. The correlations increase with decreasing temperature, reaching a maximum at the CO transition temperature. Below this temperature, they abruptly collapsed. Single-polaron diffuse scattering was also observed in both the PI and FM states suggesting that substantial local lattice distortions are present in these phases. We argue that our measurements indicate that nanoscale regions exhibiting layered orbital order exist in the paramagnetic and ferromagnetic phases of Nd0.5Sr0.5MnO3.Comment: 5 pages, 4 embedded figure

Evidence for charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction

High real-space-resolution atomic pair distribution functions of La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using high-energy x-ray powder diffraction to study the size and shape of the MnO_6 octahedron as a function of temperature and doping. In the paramagnetic insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95 and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and T=20K, we find a single Mn-O bond-length; however, as the metal-insulator transition is approached either by increasing T or decreasing x, intensity progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong evidence that charge localized and delocalized phases coexist close to the metal-insulator transition in the FM phase.Comment: 8 pages, 8 postscript figures, submitted to Phys. Rev.