Seminars Archive
Philip Hofmann
Abstract
Friday, May 18, 2001, 14:30
Seminar Room, ground floor, Building "T"
Sincrotrone Trieste, Basovizza
Surface phase transition on alpha-Ga(010)
Philip Hofmann
(Institute for Storage Ring Facilities, University of Aarhus)
ABSTRACT
alpha-Ga(010) shows a reversible transition between a (1x1) above 230
K and a (2*2 x *2)R45 below this temperature. We have studied
the low and high temperature structure by LEED, the phase transition using
SPA-LEED and the temperature-dependent electronic structure by angle-resolved
photoemission.
The geometric structure of the high temperature phase is a relaxed
truncated bulk structure in which the Ga "dimers" have been cut. In the
low temperature structure, the atoms are shifted from the high temperature
positions but now major mass transport is observed in the phase transition.
Both structures are characterized by a very low surface Debye temperature,
indicative of large vibrations or other structural fluctuations.
SPA-LEED shows that the surface phase transition takes place in a very
narrow temperature window. It also reveals a splitting in the (2*2 x *2)R45
spots which corresponds to a real-space distance about 18 times the size
of the unit cell.
Several interesting features are found in the electronic structure.
The linewidth of the surface state at the corner of the surface Brillouin
zone (SBZ) was found to decrease substantially over a small temperature
range when cooling the sample from the (1x1) to the (2*2 x *2)R45
phase. This change is accompanied by a general decrease in background and
Fermi level intensity in the projected bulk band gap. In the immediate
vicinity of the SBZ corner, however, a new state is found very close to
the Fermi level.
A detailed mapping of the Fermi level intensity at different
temperatures and photon energies was performed to investigate the
possibility of Fermi surface nesting. The data also show the presence of
substantial fluctuations in the high temperature phase. Based on these
findings, we discuss possible mechanisms for the phase transition.
