Local order and orientational correlations in liquid and crystalline phases of carbon tetrabromide from neutron powder diffraction measurements

The liquid, plastic crystalline and ordered crystalline phases of CBr$_4$ were studied using neutron powder diffraction. The measured total scattering differential cross-sections were modelled by Reverse Monte Carlo simulation techniques (RMC++ and RMCPOW). Following successful simulations, the single crystal diffraction pattern of the plastic phase, as well as partial radial distribution functions and orientational correlations for all the three phases have been calculated from the atomic coordinates ('particle configurations'). The single crystal pattern, calculated from a configuration that had been obtained from modelling the powder pattern, shows identical behavior to the recent single crystal data of Folmer et al. (Phys. Rev. {\bf B77}, 144205 (2008)). The BrBr partial radial distribution functions of the liquid and plastic crystalline phases are almost the same, while CC correlations clearly display long range ordering in the latter phase. Orientational correlations also suggest strong similarities between liquid and plastic crystalline phases, whereas the monoclinic phase behaves very differently. Orientations of the molecules are distinct in the ordered phase, whereas in the plastic crystal their distribution seems to be isotropic.Comment: 19 pages, 7 figures, accepted for publication in Physical Review B (http://prb.aps.org/

(Sr_3Sc_2O_5)Fe_2As_2 as a possible parent compound for FeAs-based superconductors

A new compound with the FeAs-layers, namely (Sr_3Sc_2O_5)Fe_2As_2 (abbreviated as FeAs-32522), was successfully fabricated. It has a layered structure with the space group of I4/mmm, and with the lattice constants a = 4.069 $\AA$ and c = 26.876 $\AA$. The in-plane Fe ions construct a square lattice which is close to that of other FeAs-based superconductors, such as REFeAsO (RE = rare earth elements) and (Ba,Sr)Fe_2As_2. However the inter FeAs-layer spacing in the new compound is greatly enlarged. The temperature dependence of resistivity exhibits a weak upturn in the low temperature region, but a metallic behavior was observed above about 60 K. The magnetic susceptibility shows also a non-monotonic behavior. Interestingly, the well-known resistivity anomaly which was discovered in all other parent compounds, such as REFeAsO, (Ba,Sr)Fe_2As_2 and (Sr,Ca,Eu)FeAsF and associated with the Spin-Density-Wave (SDW)/structural transition has not been found in the new system either on the resistivity data or the magnetization data. This could be induced by the large spacing distance between the FeAs-planes, therefore the antiferromagnetic correlation between the moments of Fe ions in neighboring FeAs-layers cannot be established. Alternatively it can also be attributed to the self-doping effect between Fe and Sc ions. The Hall coefficient R_H is negative but strongly temperature dependent in wide temperature region, which indicates the dominance of electrical conduction by electron-like charge carriers and probably a multi-band effect or a spin related scattering effect. It is found that the magnetoresistance cannot be described by the Kohler's rule, which gives further support to above arguments.Comment: 5 pages, 5 figures, some contents added, and one figure adde

Quasi One-Dimensional Spin Fluctuations in YBa(2)Cu(3)O(6+x)

We study the spin fluctuation of the oxygen deficient planes of YBa(2)Cu(3)O(6+x). The Cu-O chains that constitute these planes are described by a model that includes antiferromagnetic interactions between spins and Kondo-like scattering of oxygen holes. The spectrum of magnetic excitations shows the presence of incommensurate dynamic fluctuations along the direction of the chains. The presence of itinerant holes is responsible for the existence of important differences between the spin dynamics of this system and that of a quasi-one-dimensional localized antiferromagnet. We comment on the possibility of experimental observation of these fluctuations.Comment: 22 pages, REVTEX, 3 figures, to appear in PRB55 (1May 1997

Magnetic and electrical properties and carrier doping effects on the Fe-based host compound Sr4Sc2Fe2As2O6

Additional charge carriers were introduced to the iron oxyarsenide Sr4Sc2Fe2As2O6 under a high-pressure condition, followed by measurements of electrical resistivity, Hall coefficient, and magnetic susceptibility. The host compound Sr4Sc2Fe2As2O6 shows metallic conductivity down to ~200 K and turns to show a semiconducting-like conductivity accompanied by a positive magneto-resistance (22% at 70 kOe). Although the carrier density is comparable at 300 K (5.9x1021 cm-3) with that of the other Fe-based superconductors, no superconductivity appears down to 2 K. This is primarily because the net carrier density decreases over 3 orders of magnitude on cooling and additionally a possible magnetic order at ~120 K prevents carriers from pairing. The properties were altered largely by introducing the additional carriers.Comment: 22 pages, 9 figures, 1 table, 41 references, accepted Phys. Rev. B 201

Using molecular-dynamics simulations to understand and improve the treatment of anharmonic vibrations. II. Developing and assessing new Debye-Waller factors

Two new anharmonic forms for the Debye-Waller factor, aimed at modelling curvilinear and asymmetric motion, have been introduced. These forms permit the refinement of structures with these types of anharmonic motion using a small number of additional parameters. Molecular-dynamics-derived numerical probability density functions (PDFs) have been used to assess the merit of these new functions in real space. The comparison is favourable particularly for the curvilinear PDF based on a parabolic coordinate system change of a trivariate Gaussian distribution. The initial results also suggest that high-order even terms from the Gram-Charlier series may be important for modelling methyl-group libration. The molecular-dynamics data sets provide useful insights into the nature of anharmonic thermal motion. Addressing the problem in real space allows intuitive PDFs to be developed but numerical methods may be necessary for these methods to be implemented in refinement programs as an analytical Debye-Waller factor cannot always be obtained