A few semiconductor surface systems with adsorbates have attracted renewed
interests as low dimensional electronic systems where exotic phenomena, such
as Peierls transitions and Luttinger-Liquid behaviors, can be observed at
low temperature (LT).
One of the notable recent systems is the Pb nanowires on the stepped Si(557)
surface. Deposition of Pb onto Si(557) followed by annealing was reported to
yield an 1D chain structure [1] . The surface-state conductivity showed a
sudden switching between 1D metal (LT) and 2D insulator [high temperature
(HT)] at Tc=78K. This is quite unusual, considering other low-dimensional
systems which basically show metal(HT)-to-insulator(LT) transition or
power-low conductivity due to the Peierls instability or non-Fermi liquid
nature. In the previous scanning tunneling microscopy (STM) study, a
disordered chain structure was observed above Tc, which changed to an
ordered but modulated one below Tc, indicating a disorder-order transition
in relation to the conductivity switching transition [1].
Recently, we established a phase diagram of the Pb/Si(557) system through an
electron diffraction and photoemission study, which exhibits two different
1D phases, so called the α 1x2 and β 1x2 [2]. Based on the formation
condition, the α 1x2 phase was considered as the 1D chain system shown
in the previous STM [1].
In the present study, we have performed a detailed STM study for the α
1x2 phase from 140K to 4K, which are far above and below the reported Tc. We
have found that this phase consists of a chain structure formed on the (223)
terrace, whose misorientation from the (557) direction is compensated by
narrow (111) terraces. However, contrary to the previous study, our
high-resolution STM images have shown that the α 1x2 phase is completely
commensurate with the underlying lattice with a clear x2 modulation along
the chain direction above and below Tc. No indication of the long-period
modulation is found in each chain even at 4K. This rules out the
order-disorder and the commensurate-incommensurate transition suggested
before.
Possible mechanisms for the striking conductivity change are discussed.
References:
[1] C.Tegenkamp et al., PRL 95,176804(2005).
[2] K.S. Kim et al., PRB submitted.
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