624 research outputs found
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
and 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 and Hund's exchange . 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 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 FeSi/GaAs heterostructures
The structure and dynamical properties of the FeSi/GaAs(001) interface
are investigated by density functional theory and nuclear inelastic scattering
measurements. The stability of four different atomic configurations of the
FeSi/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 FeSi
crystallographic unit cell. In some configurations, the spin polarization of
interface layers is larger than that of bulk FeSi. 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 FeSi
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
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