X-ray anomalous scattering investigations on the charge order in α′\alpha^\prime-NaV2_2O5_5

Anomalous x-ray diffraction studies show that the charge ordering in $\alpha^\prime$-NaV$_2$O$_5$ is of zig-zag type in all vanadium ladders. We have found that there are two models of the stacking of layers along \emph{c-}direction, each of them consisting of 2 degenerated patterns, and that the experimental data is well reproduced if the 2 patterns appears simultaneously. We believe that the low temperature structure contains stacking faults separating regions corresponding to the four possible patterns.Comment: Submitted to Phys. Rev. Lett., 4 pages, 4 eps figures inserted in the tex

Synthesis and Characterisation of Lithium Silicides

Thermal batteries are primary (non-rechargeable) batteries. To activate the battery, a pyrotechnic heat source melts the solid electrolyte to a molten salt at high temperature (typically around 500°C). The battery activation starts by a pyrotechnic source such as Fe/KClO4. Thermal batteries are made from a positive electrode material such as FeS2, a molten salt electrolyte such as LiCl:KCl and a negative electrode material. Li13Si4 is the preferred anode material for thermal batteries. The electrolyte is mixed with a binder material. MgO is a typical binder. The positive electrode material attracted attention with the aim of having a high capacity, a high voltage and good thermal stability. Previous work focused on new cathode materials and investigated battery discharge mechanisms. The negative electrode material is of interest because the high temperature structures and phase transitions have not been studied in the current literature. In this work, lithium-silicon phases were synthesised by a solid-state reaction between lithium metal and silicon powder inside evacuated quartz ampoules. The phases were characterised by powder neutron diffraction, carried out on the Polaris diffractometer at ISIS facility, Rutherford Appleton Laboratory, UK, differential scanning calorimetry (DSC) and magnetic measurements on the superconducting quantum interference device (SQUID). The lithium-silicon phases are remarkably stable at high temperature and remains crystalline, with phase transitions only occurring below room temperature.AW

Probing the Structure and Evolution of Anode Materials in Thermal Batteries

High-temperature thermal batteries use lithium-silicon alloys as the anode material. Li13Si4, Li7Si3 and Li12Si7 alloys are studied to determine if phase transitions occur or if the alloys become amorphous between room temperature and 500◦C (the typical operating temperature of thermal batteries). These alloys are synthesised by reacting lithium metal and silicon powder at elevated temperature inside an evacuated quartz ampoule. The samples’ structural changes are investigated at elevated temperatures using in situ powder neutron diffraction. This is carried out on the Polaris diffractometer at ISIS facility, Rutherford Appleton Laboratory, UK. The results of the neutron scattering experiment seem to imply that the alloys do not become amorphous at 500◦C and no phases transitions occur in the temperature range. Further work is required to determine if phase transitions occur below room temperature. The work so far has presented a simple method of synthesising these alloys and gives information on the lack of phase transitions between room temperature and 500◦C.AW

Magnon splitting induced by charge ordering in NaV_2O_5

We consider the effects of charge ordering in NaV_2O_5 (below T_SP) on the exchange constants and on the magnon dispersion. We show that the experimentally observed splitting of the magnon branches along the a direction is induced by charge ordering. We find that one can distinguish between the proposed 'zig-zag' and 'in-line' patterns of charge ordering. Only the zig-zag ordering is consistent with the experimental results regarding (i) the unusual intensity modulation observed in magnetic neutron scattering, (ii) the reduction in the intra-ladder exchange constant below T_SP, and (iii) the magnon dispersion along a. We estimate the inter-ladder exchange constant to be 1.01meV=11.7K for T>T_SP.Comment: final version for PR

Electron localisation in static and time-dependent one-dimensional model systems

Electron localization is the tendency of an electron in a many-body system to exclude other electrons from its vicinity. Using a new natural measure of localization based on the exact manyelectron wavefunction, we find that localization can vary considerably between different ground-state systems, and can also be strongly disrupted, as a function of time, when a system is driven by an applied electric field. We use our new measure to assess the well-known electron localization function (ELF), both in its approximate single-particle form (often applied within density-functional theory) and its full many-particle form. The full ELF always gives an excellent description of localization, but the approximate ELF fails in time-dependent situations, even when the exact Kohn-Sham orbitals are employed.Comment: 7 pages, 4 figure

January 7, 1928

The Breeze is the student newspaper of James Madison University in Harrisonburg, Virginia

Corrugated flat band as an origin of large thermopower in hole doped PtSb2_2

The origin of the recently discovered large thermopower in hole-doped PtSb$_2$ is theoretically analyzed based on a model constructed from first principles band calculation. It is found that the valence band dispersion has an overall flatness combined with some local ups and downs, which gives small Fermi surfaces scattered over the entire Brillouin zone. The Seebeck coefficient is calculated using this model, which gives good agreement with the experiment. We conclude that the good thermoelectric property originates from this "corrugated flat band", where the coexistence of large Seebeck coefficient and large electric conductivity is generally expected.Comment: 4 pages, 4 figure