Low-dimensional systems attract a lot of attention due to the exotic quantum phenomena such as quantum well states as well as enhanced many-body interactions, which include charge or spin density waves, phase transitions, and pairing interactions. In particular, self-assembled metallic nanowires on semiconducting substrates, which exhibits the fascinating one-dimensional (1D) surface band structure, have been a nice playground to observe the intrinsic instability of electrons confined in one dimension as the earlier prediction of Peierls. [1,2] However, recent studies for Pb nanowires on stepped Si(557) have showed a distinctive temperature-dependence, a stable 1D metallic conductivity down to the temperature as low as 4 K with a drastic dimensional crossover (2D-1D) at 78 K. [3] In the present work, we unveil the Fermi surface (FS) and the underlying band dispersion using angle-resolved photoemission.
We have found strong 2D Fermi contours, diamond-shaped and circular ones, which are one-dimensionally modulated. This 1D modulation is consistent with the step superstructure. As anticrossing gaps open at the band crossing points, the overlapped 2D Fermi contours are split into the multiple metallic bands, whose dimensionality corresponds to various intermediate stages between one and two. Thus, it is concluded that these nanowires are basically governed by 2D nearly-free electrons, although the surface structure shows the 1D characteristic. Through the quantitative analyses using the appropriate combination of a 2D tight binding and an 1D nearly-free electron model, we succeed to reproduce the observed FS topology as well as to obtain both the intra and inter-wire couplings. The temperature-dependence and the phase transition suggested before are discussed in terms of the observed band structure.
References:
[1] H. W. Yeom et al, Phys. Rev. Lett., 82, 4898 (1999).
[2] J. R. Ahn et al, Phys. Rev. Lett., 95, 196402 (2005).
[3] C. Tegenkamp et al, Phys. Rev. Lett., 95, 176804 (2005).
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