Lattice Dynamics of Solid Cubane within the Quasi-Harmonic Approximation

Solid cubane, which is composed of weakly interacting cubic molecules, exhibits many unusual and interesting properties, such as a very large thermal expansion and a first-order phase transition at T$_{p}$=394 K from an orientationally-ordered phase of R$\bar{3}$ symmetry to a {\it non-cubic} disordered phase of the same symmetry with a volume expansion of 5.4%, among the largest ever observed. We study the lattice dynamics of solid cubane within the quasi-harmonic and rigid-molecule approximation to explain some of these unusual dynamical properties. The calculated phonon density of states, dispersion curves and thermal expansion agree surprisingly well with available experimental data. We find that the amplitude of thermally excited orientational excitations (i.e. librons) increases rapidly with increasing temperature and reaches about 35$^{\rm o}$ just before the orientational phase transition. Hence, the transition is driven by large-amplitude collective motions of the cubane molecules. Similarly the amplitude of the translational excitations shows a strong temperature dependence and reaches one tenth of the lattice constant at T=440 K. This temperature is in fair agreement with the experimental melting temperature of 405 K, indicating that the Lindemann criterion works well even for this unusual molecular solid.Comment: 15 pages, 6 figures (devoted to Prof. Ciraci in honor of his sixtieth birthday

Origin of the ~150 K Anomaly in LaOFeAs; Competing Antiferromagnetic Superexchange Interactions, Frustration, and Structural Phase Transition

From first principles calculations we find that the nearest and next nearest neighbor superexchange interactions between Fe ions in LaOFeAs are large, antiferromagnetic (AF), and give rise to a frustrated magnetic ground state which consists of two interpenerating AF square sublattices with M(Fe)=0.48$\mu_B$. The system lowers its energy further by removing the frustration via a structural distortion. These results successfully explain the magnetic and structural phase transitions in LaOFeAs recently observed by neutron scattering. The presence of competing strong antiferromagnetic exchange interactions and the frustrated ground state suggest that magnetism and superconductivity in doped LaOFeAs may be strongly coupled, much like in the high-T$_c$ cuprates.Comment: 4.2 pages, 5 figures, see http://www.ncnr.nist.gov/staff/taner/laofeas for more informatio

The Unprecedented Giant Coupling of Fe-spin State and the As-As Hybridization in Iron-Pnictide

From first principles calculations we unravel surprisingly strong interactions between arsenic ions in iron-pnictides, the strength of which is controlled by the Fe-spin state. Reducing the Fe-magnetic moment, weakens the Fe-As bonding, and in turn, increases As-As interactions, causing giant reduction in the c-axis. For CaFe$_2$As$_2$ system, this reduction is as large as 1.4 \AA. Since the large c-reduction has been recently observed only under high-pressure\cite{cTphase}, our results suggest that the iron magnetic moment should be present in Fe-pnictides at all times at ambient pressure. Finally, the conventional electron-phonon coupling in the collapsed phase of CaFe$_2$As$_2$ gives a maximum $T_c$ of 0.6 K and can not explain the $\sim12$ K superconductivity observed in some experiments. Implications of these findings on the mechanism of superconductivity in iron-pnictides are discussed.Comment: Published version with updated references. It has new results such as conventional e-ph coupling in the collapsed phase of CaFe2As2 gives a maximum Tc of 0.6 K and cannot explain the ~12 K superconductivity observed in some experiments. Implications of these findings on the mechanism of superconductivity in iron pnictides are discusse

Genetic structure of Pleurobranchaea maculata in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Genetics, The New Zealand Institute for Advanced Study, Massey University, Auckland, New Zealand

The grey side-gilled sea slug (Pleurobranchaea maculata), which is native to the western and south Pacific, is known to contain high concentrations of tetrodotoxin (TTX). P. maculata populations around New Zealand exhibit individual, spatial and temporal differences in TTX concentration, but the origin of TTX in P. maculata is not fully understood. The main goal of my PhD project was to examine the genetic structure and demographic history of P. maculata populations from different regions in New Zealand and to clarify whether there is a correlation between variability in TTX concentrations and genetic structure. A sample of 146 P. maculata individuals were collected from three populations from the north-eastern North Island (Ti Point-TP, Auckland-AKL and Tauranga-TR), one population from the southern North Island (Wellington-WL) and one population from the northern South Island (Nelson-NL). TP, AKL and TR were designated the “Northern cluster”, whereas the WL and NL population were labelled as the “Southern cluster” due to the relative geographical locations of these clusters. Twelve nuclear microsatellite markers that were developed based on shotgun sequence were obtained from the genome of P. maculata. The markers were used to analyse the genetic structure of P. maculata populations. The mitochondrial cytochrome c oxidase subunit I (1153 bp) and cytochrome b (1060 bp) genes were also partially sequenced in P. maculata individuals. The microsatellite data reveal high genetic diversity and lead to the rejection of the hypothesis of panmixia: populations from the Northern cluster are highly connected but significantly differentiated from the Sothern cluster. A weak differentiation was also observed between the WL and NL populations. The two populations correlate with regional variations in TTX concentrations: the Northern cluster populations contain highly toxic individuals, whereas the Southern cluster (WL and NL) populations harbour either slightly toxic or non-toxic populations. The disjunction between the Northern and Southern clusters can be explained by biogeographical barriers specific to New Zealand but also with a stepping stone model. The geographical gap between the sampling locations made it impossible to draw firm conclusions as to the origin of the disjunction. The mtDNA sequence data reveal high haplotype diversity, low nucleotide diversity and a star-shaped haplotype network. These data can be explained by a population expansion dating back to the Pleistocene era. All the sampling locations are significantly differentiated from each other according to mtDNA data. Given that microsatellite and mitochondrial sequences evolve at different rates, incomplete linkage sorting is expected to be completed for mtDNA before, which should be reflected in a more pronounced structure for mtDNA markers where members of the populations have diverged recently. Although this may explain the geographical conflict between the microsatellite and mtDNA data, it is necessary to consider the possibility that the discordance between microsatellite markers and mtDNA may be in part attributable to the relatively small sample size