Protein stability and enzyme activity at extreme biological temperatures

in Journal of Physics : Condensed Matter (2010), 22

Raman scattering intensities in BaTiO3 and PbTiO3 prototypical ferroelectrics from density functional theory

in Journal of Physics : Condensed Matter (2009), 21

Nonlinear optical susceptibilities and Raman scattering spectra of the ferroelectric phases of BaTiO3 and PbTiO3 are computed using a first-principles approach based on density functional theory and ... [more ▼]

Nonlinear optical susceptibilities and Raman scattering spectra of the ferroelectric phases of BaTiO3 and PbTiO3 are computed using a first-principles approach based on density functional theory and taking advantage of a recent implementation based on the nonlinear response formalism and the 2n + 1 theorem. These two prototypical ferroelectric compounds were chosen to demonstrate the accuracy of the Raman calculation based both on their complexity and their technological importance. The computation of the Raman scattering intensities has been performed not only for the transverse optical modes, but also for the longitudinal optical ones. The agreement between the measured and computed Raman spectra of these prototypical ferroelectrics is remarkable for both the frequency position and the intensity of Raman lines. This agreement presently demonstrates the state-of-the-art in the computation of Raman responses on one of the most complex systems, ferroelectrics, and constitutes a step forward in the reliable prediction of their electro-optical responses. [less ▲]

A study of the high temperature spin reorientation in YCoFe/sub 3/B

in Journal of Physics : Condensed Matter (2009), 21(18), 1860017-1860017

The iron-57 Mossbauer spectra of YCoFe /sub 3/ B have been measured between 4.2 and 480 K and reveal that YCoFe /sub 3/ B exhibits an axial orientation of the iron magnetic moments below 450 K and a basal ... [more ▼]

The iron-57 Mossbauer spectra of YCoFe /sub 3/ B have been measured between 4.2 and 480 K and reveal that YCoFe /sub 3/ B exhibits an axial orientation of the iron magnetic moments below 450 K and a basal orientation above 450 K. This spin reorientation, also observed in the thermomagnetic curves, results from the different signs of the contributions to the magnetic anisotropy of the 2c and 6i sites that are occupied by iron. The neutron diffraction patterns of YCoFe /sub 3/ B have been measured at 2 K and between 290 and 770 K and have been successfully analyzed with a model compatible with the magnetic orientation obtained from the Mossbauer spectra. The hybridization between the cobalt or iron 3d orbitals and the boron 2p orbitals leads to a larger magnetic moment and hyperfine field on the 2c site as compared to the 6i site. [less ▲]

First-principles study of the dielectric and dynamical properties of orthorhombic CaMnO3

in Journal of Physics : Condensed Matter (2008), 20(25),

The structural, dielectric and dynamical properties of the low temperature antiferromagnetic orthorhombic phase of CaMnO3 have been computed from first principles, using a density functional theory ... [more ▼]

The structural, dielectric and dynamical properties of the low temperature antiferromagnetic orthorhombic phase of CaMnO3 have been computed from first principles, using a density functional theory approach within the local spin density approximation. The theoretical structural parameters are in good agreement with experiment. The full set of zone-center phonons is reported, allowing new assignment of experimental Raman data and providing reference values for the interpretation of future infrared phonon measurements. It is shown that the static dielectric constant is very large and comparable in amplitude to that of isostructural CaTiO3. In contrast to the pseudocubic structure, it is also highly anisotropic. These features are discussed in relationship to the anomalous Born effective charges and the presence of low frequency polar modes. [less ▲]

First-principles study of filled and unfilled antimony skutterudites

in Journal of Physics : Condensed Matter (2007), 19(9),

Using a first-principles approach based on density-functional theory, the electronic, dielectric and dynamical properties of the skutterudites CoSb3 and TlFeCo3Sb12 are studied. In particular, the ... [more ▼]

Using a first-principles approach based on density-functional theory, the electronic, dielectric and dynamical properties of the skutterudites CoSb3 and TlFeCo3Sb12 are studied. In particular, the electron localization tensor, static and dynamic effective charges, static and optical dielectric constants and phonon dispersion curves are computed. The Born effective charges are found to be significantly larger than the static charges of the ions. Moreover, the static dielectric constant of TlFeCo3Sb12 is found to be significantly larger than that of CoSb3. The analysis of the phonon dispersion curves reveals a low-energy mode due to coupled vibrations of Tl and Sb. This mode is at the origin of a well-defined peak in the phonon density of states of TlFeCo3Sb12 and its mode effective charge is related to the increase of the dielectric constant in TlFeCo3Sb12. Our results are compared to recent experiments performed on CoSb3 and TlFeCo3Sb12, and differences between the lattice dynamics of TlFeCo3Sb12 and other filled skutterudites are highlighted. [less ▲]

Quantum-confinement effects in InAs-InP core-shell nanowires

in Journal of Physics : Condensed Matter (2007), 19(29),

We report the detection of quantum confinement in single InAs–InP core–shell nanowires. The wires, having an InAs core with ~25 nm diameter, are characterized by emission spectra in which two peaks are ... [more ▼]

We report the detection of quantum confinement in single InAs–InP core–shell nanowires. The wires, having an InAs core with ~25 nm diameter, are characterized by emission spectra in which two peaks are identified under high excitation intensity conditions. The peaks are caused by emission from the ground and excited quantized levels, due to quantum confinement in the plane perpendicular to the nanowire axis. We have identified different energy contributions in the emission spectra, related to the wurtzite structure of the wires, the strain between the wurtzite core and the shell, and the confinement energy of the InAs core. Calculations based on six-band strain-dependent theory allow the theoretical estimation of the confined energy states in such materials, and we found these results to be in good agreement with those from the photoluminescence studies. [less ▲]

The effect of electron-phonon interaction in iron-doped III-V cubic semiconductors

in Journal of Physics : Condensed Matter (2000), 12(12), 2691-2699

A theoretical study of optical absorption and emission measurements of Fe2+ as a substitutional impurity in InP and GaP is presented. A new interpretation of the low-temperature absorption spectrum is ... [more ▼]

A theoretical study of optical absorption and emission measurements of Fe2+ as a substitutional impurity in InP and GaP is presented. A new interpretation of the low-temperature absorption spectrum is proposed based on a weak Jahn-Teller interaction between the electronic excited states and a local gap mode of Gamma(5) symmetry. The model also includes the crystal potential, hybridization with the orbitals of the ligands of the host crystal, spin-orbit interaction and a weak dynamic Jahn-Teller coupling of the orbital ground state of Fe2+ with transverse acoustic phonons of Gamma(3) symmetry. The theoretical model describes with good accuracy the measured positions and relative intensities of the spectral lines. In addition, the mass dependence of the local gap mode of Gamma(5) symmetry reproduces the general features of the fine structures associated with the isotopic shifts of the zero-phonon line and the contribution to the isotopic shifts arising from the difference in zero-point energy between the initial and final states of the transition is evaluated. [less ▲]

Local order of the high-pressure metallic phase of liquid selenium: a diffraction study

in Journal of Physics : Condensed Matter (1999), 11(50), 10243-1024910249

Liquid selenium undergoes a phase transition towards a metallic phase when sufficient pressure is applied. We performed an x-ray diffraction experiment at the 1D30 beamline of the ESRF at the wavelength ... [more ▼]

Liquid selenium undergoes a phase transition towards a metallic phase when sufficient pressure is applied. We performed an x-ray diffraction experiment at the 1D30 beamline of the ESRF at the wavelength of 0.149 Aring to investigate the local order of the metallic liquid. The diffraction pattern of liquid Se has been recorded with the large-volume Paris-Edinburgh cell between (20degC, 3 GPa) and (1650degC, 4.1 GPa). The local order of the metallic liquid selenium is found to be close to that of liquid tellurium. In particular, the coordination number Z increases with temperature from 2.6 to 3.0 [less ▲]

THERMAL-CONDUCTIVITY OF PURE OR IRON-DOPED YBA2CU3O7-DELTA WITH OR WITHOUT AN EXCESS OF CUO

in Journal of Physics : Condensed Matter (1994), 6(31), 6305-6316

The thermal conductivities of YBa2(Cu1-xFex)3O7-delta + y% CuO ceramics for 0 < x < 0.03 and 0 < y < 5 have been measured. The thermal conductivities of all samples exhibit a minimum in the vicinity of ... [more ▼]

The thermal conductivities of YBa2(Cu1-xFex)3O7-delta + y% CuO ceramics for 0 < x < 0.03 and 0 < y < 5 have been measured. The thermal conductivities of all samples exhibit a minimum in the vicinity of the critical temperature T(c) and a maximum near T(c)/2. These results are interpreted with the help of an electronic model, i.e. supposing that the main contribution to the thermal conductivity below T(c) is due to electron scattering in the CuO2 planes. Within a simple two-fluid model derived from kinetic theory, taking into account the temperature dependence of the electronic relaxation time and the normal charge carrier concentration, we obtain theoretical curves which reproduce the experimental results quite well. We also take into account the porosity of the samples (in term of an intergrain contribution) to derive the electronic thermal conductivity. The parameter values of the model and the observed minimum are explained in terms of physical properties. In particular, the contribution of superconductivity fluctuations to the thermal conductivity is shown to be negligible with respect to other mechanisms. [less ▲]