Order parameters in the Verwey phase transition

The Verwey phase transition in magnetite is analyzed on the basis of the Landau theory. The free energy functional is expanded in a series of components belonging to the primary and secondary order parameters. A low-temperature phase with the monoclinic P2/c symmetry is a result of condensation of two order parameters X_3 and \Delta_5 . The temperature dependence of the shear elastic constant C_44 is derived and the mechanism of its softening is discussed.Comment: 4 pages, 1 figur

Tourism in an unstable and complex world? Searching for relevant a political risk paradigm and model for tourism organisations

This work has a single aim, focusing on developing a political risk model relevant for tourism organisations, which are operating in an increasingly complex and turbulent international environment. It pays particular attention to the language of risk (how risks are articulated and described), the culture of risk (how risks are viewed), and the risk process (how they are analysed and assessed). The work critically evaluates a variety of methods that can be utilised to scan, analyse and assess political hazards and risks. It finds that many of the existing methods of political and country risk assessment are limited and not sufficiently contextualised to the needs of the tourism industry. Whilst many models can have an attractive façade of using positivistic methods to calculate political risks, in practice these are fraught with problems. The study also highlights a more complex relationship between tourism and political instability, whereby tourism can be characterised as much by its robustness, as its sensitivity. A model is developed which primarily adapts a systems theory approach, whereby a language, culture and practical process is developed through which the analysis of various factors and indicators can take place. The approach adopted has a number of stages, which vary in the amount of data necessary for the analysis and assessment of political risks. The model begins by utilising existing travel advice databases, moving onto an analysis of the frequency of past events, then to the nature of the political system itself, finishing with an analysis and assessment of more complex input factors and indicators which relate to notions of causation. One of the more provocative features of the model is the argument that it is more than possible to make an assessment of the risks that the political environment can pose to a tourism organisation, without necessarily understanding theories of causation

Vibrational properties of alpha- and sigma-phase Fe-Cr alloy

Experimental investigation as well as theoretical calculations, of the Fe-partial phonon density-of-states (DOS) for nominally Fe_52.5Cr_47.5 alloy having (a) alpha- and (b) sigma-phase structure were carried out. The former at sector 3-ID of the Advanced Photon Source, using the method of nuclear resonant inelastic X-ray scattering, and the latter with the direct method [K. Parlinski et al., Phys. Rev. Lett. {78, 4063 (1997)]. The characteristic features of phonon DOS, which differentiate one phase from the other, were revealed and successfully reproduced by the theory. Various data pertinent to the dynamics such as Lamb-Mossbauer factor, f, kinetic energy per atom, E_k, and the mean force constant, D, were directly derived from the experiment and the theoretical calculations, while vibrational specific heat at constant volume, C_V, and vibrational entropy, S were calculated using the Fe-partial DOS. Using the values of f and C_V, we determined values for Debye temperatures, T_D. An excellent agreement for some quantities derived from experiment and first-principles theory, like C_V and quite good one for others like D and S was obtained.Comment: 4 pages, 3 figure

Dual fast-cycling superconducting synchrotron at Fermilab and a possible path to the future of high energy particle physics

We briefly outline shorter and longer term physics motivation for constructing a dual, fast-cycling superconducting synchrotron accelerator (DSFMR - Dual Super-Ferric Main Ring) in the Tevatron tunnel at Fermilab. We discuss using this accelerator as a high-intensity dual neutrino beam source for the long-baseline neutrino oscillation search experiments, and also as a fast, dual pre-injector accelerator for the VLHC (Very Large Hadron Collider)

Magnetic Lifshitz transition and its consequences in multi-band iron-based superconductors

In this paper we address Lifshitz transition induced by applied external magnetic field in a case of iron-based superconductors, in which a difference between the Fermi level and the edges of the bands is relatively small. We introduce and investigate a two-band model with intra-band pairing in the relevant parameters regime to address a generic behaviour of a system with hole-like and electron-like bands in external magnetic field. Our results show that two Lifshitz transitions can develop in analysed systems and the first one occurs in the superconducting phase and takes place at approximately constant magnetic field. The chosen sets of the model parameters can describe characteristic band structure of iron-based superconductors and thus the obtained results can explain the experimental observations in FeSe and Co-doped BaFe2As2 compounds

Origin of the Verwey transition in magnetite: Group theory, electronic structure, and lattice dynamics study

The Verwey phase transition in magnetite has been analyzed using the group theory methods. It is found that two order parameters with the symmetries $X_3$ and $\Delta_5$ induce the structural transformation from the high-temperature cubic to the low-temperature monoclinic phase. The coupling between the order parameters is described by the Landau free energy functional. The electronic and crystal structure for the cubic and monoclinic phases were optimized using the {\it ab initio} density functional method. The electronic structure calculations were performed within the generalized gradient approximation including the on-site interactions between 3d electrons at iron ions -- the Coulomb element $U$ and Hund's exchange $J$. Only when these local interactions are taken into account, the phonon dispersion curves, obtained by the direct method for the cubic phase, reproduce the experimental data. It is shown that the interplay of local electron interations and the coupling to the lattice drives the phonon order parameters and is responsible for the opening of the gap at the Fermi energy. Thus, it is found that the metal-insulator transition in magnetite is promoted by local electron interactions, which significantly amplify the electron-phonon interaction and stabilize weak charge order coexisting with orbital order of the occupied $t_{2g}$ states at Fe ions. This provides a scenario to understand the fundamental problem of the origin of the Verwey transition in magnetite.Comment: 17 pages, 5 figures, 8 tables. Accepted version to be published in Phys. Rev.

Ab initio and nuclear inelastic scattering studies of Fe3_3Si/GaAs heterostructures

The structure and dynamical properties of the Fe$_3$Si/GaAs(001) interface are investigated by density functional theory and nuclear inelastic scattering measurements. The stability of four different atomic configurations of the Fe$_3$Si/GaAs multilayers is analyzed by calculating the formation energies and phonon dispersion curves. The differences in charge density, magnetization, and electronic density of states between the configurations are examined. Our calculations unveil that magnetic moments of the Fe atoms tend to align in a plane parallel to the interface, along the [110] direction of the Fe$_3$Si crystallographic unit cell. In some configurations, the spin polarization of interface layers is larger than that of bulk Fe$_3$Si. The effect of the interface on element-specific and layer-resolved phonon density of states is discussed. The Fe-partial phonon density of states measured for the Fe$_3$Si layer thickness of three monolayers is compared with theoretical results obtained for each interface atomic configuration. The best agreement is found for one of the configurations with a mixed Fe-Si interface layer, which reproduces the anomalous enhancement of the phonon density of states below 10 meVComment: 14 pages, 9 figures, 4 table

Tourism in an unstable and complex world? : searching for a relevant political risk paradigm and model for tourism organisations

This work has a single aim, focusing on developing a political risk model relevant for tourism organisations, which are operating in an increasingly complex and turbulent international environment. It pays particular attention to the language of risk (how risks are articulated and described), the culture of risk (how risks are viewed), and the risk process (how they are analysed and assessed). The work critically evaluates a variety of methods that can be utilised to scan, analyse and assess political hazards and risks. It finds that many of the existing methods of political and country risk assessment are limited and not sufficiently contextualised to the needs of the tourism industry. Whilst many models can have an attractive façade of using positivistic methods to calculate political risks, in practice these are fraught with problems. The study also highlights a more complex relationship between tourism and political instability, whereby tourism can be characterised as much by its robustness, as its sensitivity. A model is developed which primarily adapts a systems theory approach, whereby a language, culture and practical process is developed through which the analysis of various factors and indicators can take place. The approach adopted has a number of stages, which vary in the amount of data necessary for the analysis and assessment of political risks. The model begins by utilising existing travel advice databases, moving onto an analysis of the frequency of past events, then to the nature of the political system itself, finishing with an analysis and assessment of more complex input factors and indicators which relate to notions of causation. One of the more provocative features of the model is the argument that it is more than possible to make an assessment of the risks that the political environment can pose to a tourism organisation, without necessarily understanding theories of causation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo