Phonons in Ultrathin Oxide Films: 2D to 3D Transition in FeO on Pt(111).

The structural and magnetic properties of ultrathin FeO(111) films on Pt(111) with thicknesses from 1 to 16 monolayers (MLs) were studied using the nuclear inelastic scattering of synchrotron radiation. A distinct evolution of vibrational characteristics with thickness, revealed in the phonon density of states (PDOS), shows a textbook transition from 2D to 3D lattice dynamics. For the thinnest films of 1 and 2 ML, the low-energy part of the PDOS followed a linear ∝E dependence in energy that is characteristic for two-dimensional systems. This dependence gradually transforms with thickness to the bulk ∝E^{2} relationship. Density-functional theory phonon calculations perfectly reproduced the measured 1-ML PDOS within a simple model of a pseudomorphic FeO/Pt(111) interface. The calculations show that the 2D PDOS character is due to a weak coupling of the FeO film to the Pt(111) substrate. The evolution of the vibrational properties with an increasing thickness is closely related to a transient long-range magnetic order and stabilization of an unusual structural phase.

Calibration of the isomer shift for iodine resonant transitions by ab initio calculations.

The isomer shift calibration constants have been calculated for 57.60 keV in 127I and for 27.72 keV in 129I resonant transitions by density functional theory. The full-potential linearized augmented plane-wave method (FLAPW) was applied in the scalar-relativistic approach. The NaI compound was used to set the origin of the scales in both cases. On the basis of the existing experimental data, the following values for the calibration constants were obtained: alpha = -0.057(2) mm s(-1) au3 for 127I and alpha = +0.164(4) mm s(-1) au3 for 129I. The ratio of the calibration constants of alpha127/alpha129 = -0.35(1) was established. Spectroscopic electric quadrupole moments for the ground state of the above nuclei have been calculated as byproduct. The quadrupole moments Q(g)(127) = -0.764(30) b and Q(g)(129) = -0.731(3) b were obtained for 127I and 129I, respectively. Errors quoted are due to the linear regression fit, and real errors might be as large as about 10% of the quoted absolute value.

Lattice dynamics of Fe-doped CoO from first principles.

The vibrational dynamics of substitutional Fe and its influence on the vibrational properties of the host CoO matrix have been investigated using ab initio calculated Hellmann-Feynman forces. Corrections for the strong on-site interaction have been taken into account via the Hubbard potential U and the local exchange interaction J. Calculations were performed with constant U on Co and variable U on Fe. It was found that Fe impurities exhibit higher values of effective force constant than the original Co. New localized modes are created in the host CoO matrix due to force constant defect between Fe and Co. Iron impurities affect the optical phonon vibrations, while the long wavelength acoustic phonons do not experience changes upon the doping. Mean-squared vibrational amplitudes of cations and anions in the ideal and Fe-doped CoO are compared to the available experimental data. The calculated mean-squared displacements of Fe remain in very good agreement with those measured by Mössbauer spectroscopy.

Calibration of the isomer shift for the 77.34 keV transition in 197Au using the full-potential linearized augmented plane-wave method.

The isomer shift calibration constant has been calculated for the 77.34 keV Mossbauer transition connecting the ground state of the (197)Au nucleus with the first excited state of this nucleus. The full-potential linearized augmented plane-wave method was used in the fully relativistic approach, albeit without taking into account the spin-orbit coupling. The final assignment of the calibration constant was based on calculations performed for AuCN, AuCl(3), AuBr(3), KAuCl(4), KAuBr(4), and metallic gold. It is found that the calibration constant takes on the following value alpha = +0.0665(4) mm s(-1) a.u.(3). The error quoted is due to the linear regression fit, and the real error might be as large as 10%. The spectroscopic electric quadrupole moment for the ground state of the (197)Au nucleus was calculated as the by-product. It was found that this moment equals Q(g) = +0.566(1)b in fair agreement with the accepted value based on the muonic hyperfine spectroscopy results. The error quoted is again due to the linear regression fit and the real error might be as large as 10%. The final assignment of the value for the quadrupole moment is based on the calculations for the following compounds: AuCl, AuBr, AuI, AuCN, and AuMn(2). Results for the magnetically ordered Au(2)Mn were applied to determine the sign of the quadrupole moment.