AVS 46th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS1+EM-TuA

Paper SS1+EM-TuA9
Structure and Dynamics of KTaO@sub 3@(001) via Elastic and Inelastic Helium Atom Scattering@footnote 1@

Tuesday, October 26, 1999, 4:40 pm, Room 606

Session: Oxides: Growth and Structure
Presenter: J.G. Skofronick, Florida State University
Authors: J.A. Li, Florida State University
T.W. Trelenberg, Florida State University
E.A. Akhadov, Florida State University
S.A. Safron, Florida State University
J.G. Skofronick, Florida State University
L.A. Boatner, Oak Ridge National Laboratory
Correspondent: Click to Email
Although bulk KTaO@sub 3@ is considered an incipient ferroelectric which remains cubic at all temperatures, He diffraction on the (001) surface, cleaved in situ, shows reconstruction of the (1x1) surface to (2x1). The surface behavior, however, seems to depend on its thermal history as, for example, in the following observations: 1)After the sample is cleaved at about 300K and cooled to about 190K, He diffraction reveals a (1x1) surface which appears to be stable indefinitely. 2)If the target is then warmed from 190K to about 220K, reconstruction to the (2x1) phase takes place gradually over a period of about 2 days, whereas if it is warmed to about 260K, the (2x1) phase becomes evident after about 3 hours. 3)When the target is instead taken from 190K to approximately 60K and then to about 260K, the (2x1) surface is apparent immediately. 4)Restoration to (1x1) can be achieved (a) in a few hours or (b) immediately by warming the surface to about 330K or to about 360K, respectively. In addition, there is considerable hysteresis in the intensities of the He diffraction spots for the (2x1) surface over the thermal cycle from 260 K to 50 K to 260 K. Helium time-of-flight (TOF) spectra were obtained to determine the surface phonon dispersions of the (1x1) and (2x1) surfaces for several surface temperatures between 80K and 300K. These appear very similar over the entire surface Brillouin zone. The dominant peaks in the TOF spectra correspond to a dispersionless mode at about 13.5meV, which stiffens slightly as the temperature is raised. However, the lowest energy branch which is quite flat at low temperatures seems to soften near the zone center as the temperature increases. @FootnoteText@ @footnote 1@Supported by U. S. Department of Energy grant number DE-FG02-97ER45635.