  We have investigated electronic and structural properties of thallium (Tl)
nanoclusters formed on the Si(111)-7$\times$7 surface at room temperature (RT)
by utilizing photoemission spectroscopy (PES) and high-resolution
electron-energy-loss spectroscopy (HREELS) combined with first principles
calculations. Our PES data show that the state S2 stemming from Si restatoms
remains quite inert with Tl coverage $\theta$ while S1 from Si adatoms
gradually changes, in sharp contrast with the rapidly decaying states of Na or
Li nanoclusters. No Tl-induced surface state is observed until $\theta$=0.21 ML
where Tl nanoclusters completely cover the faulted half unit cells (FHUCs) of
the surface. These spectral behaviors of surface states and a unique loss peak
L$_2$ associated with Tl in HREELS spectra indicate no strong Si-Tl bonding and
are well understood in terms of gradual filling of Si dangling bonds with
increasing $\theta$. Our calculational results further reveal that there are
several metastable atomic structures for Tl nanoclusters at RT transforming
from each other faster than 10$^{10}$ flippings per second. We thus conclude
that the highly mobile Tl atoms form self-trapped nanoclusters within FHUC at
RT with several metastable phases. The mobile and multi-phased nature of Tl
nanoclusters not only account for all the existing experimental observations
including the fuzzy scanning tunneling microscope images and a dynamical model
proposed by recent x-ray study but also provides an example of self-trapping of
atoms in a nanometer-scale region.
