Polar catastrophe and the structure ofKTa1−xNbxO3surfaces: Resu

Helium atom scattering from potassium tantalate/niobate(001) surfaces

“…4 For the KTN targets, the data for the phonons below the Einstein mode appear to be dispersive, but the data are quite noisy, and surface phonon modes could be identified only over less than a half of the SBZ. [5][6][7] We believe that the existence of the Einstein mode for all these materials is consistent with the picture of the surface noted above, composed of potassium and oxygen ions. 7 However, the DS data for the 30% Nb targets suggest a more complex picture.…”

“…This mode is essentially the same for all (001) surfaces studied in both the h10i and h11i azimuths. 7 For the pure KTaO 3 surface, a second lower energy dispersionless branch was measured in the h10i azimuth at 80 K and in the h11i azimuth at 140 K. However, at higher temperatures in both h10i and h11i, these lower energy phonon modes are found to be dispersive, but the curves extend over less than half the SBZ. 4 For the KTN targets, the data for the phonons below the Einstein mode appear to be dispersive, but the data are quite noisy, and surface phonon modes could be identified only over less than a half of the SBZ.…”

“…Inelastic He atom scattering experiments were also carried out to determine the surface phonon dispersion curves for these materials. [1][2][3][4][5][6][7] The most general result for all the samples studied, is that every surface examined showed a nearly dispersionless, Einstein-like phonon branch across the whole Surface Brillouin Zone (SBZ) at B13.5 meV. This mode is essentially the same for all (001) surfaces studied in both the h10i and h11i azimuths.…”

“…Since the characterization of the surface layer of this material is important in understanding its behavior, helium atom diffraction and inelastic scattering experiments from perovskite KTa 1Àx Nb x O 3 (001) surfaces (where x = 0, 6, 10, 15, 20, and 30%) were carried out in this laboratory. [1][2][3][4][5][6][7][8] We refer to the samples with x 4 0 generally as KTNs. Several interesting surface behaviors for these materials have been observed and reported.…”

“…As in the previously reported work on KTNs, 4 the KTa 0.48 Nb 0.52 O 3 (001) surface was prepared from the single crystal samples by cleaving in situ under UHV conditions. Several types of measurements were carried out: [1][2][3][4][5][6][7][8] angular distributions of the scattered He atoms, by rotating the manipulator holding the target; drift spectra by measuring the He specular reflection as the He wave vector is varied; surface temperature scans by measuring the He specular reflection as the surface temperature is decreased and increased; and inelastic He atom scattering at various scattering angles (various surface momentum transfers) by time-of-flight methods.…”