Helium atom scattering from KTa0.7Nb0.3O3(001): Anomalous surface reflectivity with varying surface temperature and helium wave vector

Abstract

Helium atom diffraction experiments have been carried out on the (001) surface of KTaO${}_{3}$ doped with 30% Nb. The surfaces were produced by cleaving single crystals of the material in situ. After the samples were thermally cycled, the angular distributions measured in the $\ensuremath{\langle}100\ensuremath{\rangle}$ azimuth, but not those in the $\ensuremath{\langle}110\ensuremath{\rangle}$ azimuth, revealed half-order diffraction peaks. These indicate the formation of small ($2\ifmmode\times\else\texttimes\fi{}1$) surface domains. The scans of the specular and Bragg diffraction peak intensities, as the sample temperatures were varied from about 325 to 60--80 K and back to 325 K, showed large hysteresis, particularly in the $\ensuremath{\langle}100\ensuremath{\rangle}$ azimuth. In addition, these scans showed a distinct intensity dip at about 85 K, which is far removed from any bulk phase transition temperature of this material. Most curious, the specular reflectivity of the surface was found to be a strong function of the He wave vector, decreasing rapidly as the wave vector was varied above or below an optimum value.