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Register a new userInfluence of dipole interaction on lattice dynamics of crystalline ice
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Added by
Waheed Adeniyi Adeagbo
Publication date
2005
fields
Physics
and research's language is
English
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The Born effective charges of component atoms and phonon spectra of a tetrahedrally coordinated crystalline ice are calculated from the first principles method based on density functional theory within the generalized gradient approximation with the projected augmented wave method. Phonon dispersion relations in a 3x1x1 supercell were evaluated from Hellmann-Feynman forces with the direct method. This calculation is an additional work to the direct method in calculating the phonon spectra which does not take into account the polarization charges arising from dipole interaction of molecules of water in ice. The calculated Born effective polarization charges from linear response theory are supplied as the correction terms to the dynamical matrix in order to further investigate the LO-TO splitting of the polar modes of ice crystal at k=0 which has long been speculated for this system especially in the region between 28 and 37 meV both in the theoretical and experimental studies. Our results clearly show the evidence of splitting of longitudinal and transverse optic modes at the k=0-point in agreement with some experimental findings.
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We investigated the structural and dynamical properties of a tetrahedrally coordinated crystalline ice from first principles based on density functional theory within the generalized gradient approximation with the projected augmented wave method. First, we report the structural behaviour of ice at finite temperatures based on the analysis of radial distribution functions obtained by molecular dynamics simulations. The results show how the ordering of the hydrogen bonding breaks down in the tetrahedral network of ice with entropy increase in agreement with the neutron diffraction data. We also calculated the phonon spectra of ice in a 3x1x1 supercell by using the direct method. So far, due to the direct method used in this calculation, the phonon spectra is obtained without taking into account the effect of polarization arising from dipole-dipole interactions of water molecules which is expected to yield the splitting of longitudinal and transverse optic modes at the Gamma-point. The calculated longitudinal acoustic velocities from the initial slopes of the acoustic mode is in a reasonable agreement with the neutron scatering data. The analysis of the vibrational density of states shows the existence of a boson peak at low energy of translational region a characteristic common to amorphous systems.
The parameters influencing the band gap of tin sulphide thin nano-crystalline films have been investigated. Both grain size and lattice parameters are known to influence the band gap. The present study initially investigates each contribution individually. The experimentally determined dependency on lattice parameter is verified by theoretical calculations. We also suggest how to treat the variation of band gap as a two variable problem. The results allow us to show dependency of effective mass (reduced) on lattice unit volume.
We present high energy x-ray (67 keV) and neutron scattering measurements on a single crystal of K$_{1-x}$Li$_x$TaO$_3$ for which the Li content ($x=0.02$) is less than $x_c = 0.022$, the critical value below which no structural phase transitions have been reported in zero field. While the crystal lattice does remain cubic down to T=10 K under both zero-field and field-cooled ($E le 4$ kV/cm) conditions, indications of crystal symmetry lowering are seen at $T_C=63$ K where the Bragg peak intensity changes significantly. A strong and frequency-dependent dielectric permittivity is observed at ambient pressure, a defining characteristic of relaxors. However an extensive search for static polar nanoregions, which is also widely associated with relaxor materials, detected no evidence of elastic neutron diffuse scattering between 300 K and 10 K. Neutron inelastic scattering methods were used to characterize the transverse acoustic and optic phonons (TA1 and TO1 modes) near the (200) and (002) Bragg peaks. The zone center TO1 mode softens monotonically with cooling but never reaches zero energy in either zero field or in external electric fields of up to 4 kV/cm. These results are consistent with the behavior expected for a dipolar glass in which the local polar moments are frozen and exhibit no long-range order at low temperatures.
Raman scattering spectra of zigzag spin chain TaFe$_{1.25}$Te$_3$ single crystal are presented in a temperature range from 80 to 300 K. Nine Raman active modes of $A_g$ and $B_g$ symmetry are clearly observed and assigned by probing different scattering channels, which is confirmed by lattice dynamics calculations. Temperature dependence of the Raman modes linewidth is mainly governed by the lattice anharmonicity. The only deviation from the conventional behavior is observed for $A_g$ symmetry modes in a vicinity of the magnetic phase transition at $T_N approx 200$ K. This implies that the electron-phonon interaction weakly changes with temperature and magnetic ordering, whereas small changes in the spectra near the critical temperature can be ascribed to spin fluctuations.
This work studies the influence of microstructures and crystalline defects on the superconductivity of MgB2, with the objective to improve its flux pinning. A MgB2 sample pellet that was hot isostatic pressed (HIPed) was found to have significantly increased critical current density (Jc) at high fields than its un-HIPed counterpart. The HIPed sample had a Jc of 10000 A/cm2 in 50000 Oe (5 T) at 5K. This was 20 times higher than that of the un-HIPed sample, and the same as the best Jc reported by other research groups. Microstructures observed in scanning and transmission electron microscopy indicate that the HIP process eliminated porosity present in the MgB2 pellet resulting in an improved intergrain connectivity. Such improvement in intergrain connectivity was believed to prevent the steep Jc drop with magnetic field H that occurred in the un-HIPed MgB2 pellet at H > 45000 Oe (4.5 T) and T = 5 K. The HIP process was also found to disperse the MgO that existed at the grain boundaries of the un-HIPed MgB2 pellet and to generate more dislocations in the HIPed the pellets. These dispersed MgO particles and dislocations improved flux pinning also at H<45000 Oe. The HIPing process was also found to lower the resistivity at room temperature.
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