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Scientific Report 2007-2009
Condensed matter physics and biophysics
C33. Electron-phonon interaction and electron correlation effects in
low-dimensional structures
Low-dimensional structures present peculiar electronic
properties associated to the reduced dimensions, where
the electron-phonon interactions play a leading role. Exemplary
low-D systems are the surfaces of single crystals
when they present a reduced symmetry with respect
the projected bulk-like geomtery or nanochains assembled
on nanostructured templates. In the last few years
we have analysed these systems, like interacting organic
molecules on nanotemplates [1,2], or electron-phonon interaction
in the prototypical semiconductor surfaces [3],
and of reconstructued phases on metal surfaces [4]. Different
phase structures in low dimensions systems can be
a direct consequence of electronic instability versus lattice
distortion, due to dimension reduction. Interplay between
electronic properties and atomic geometry is then
a key issue to characterize quasi-2D systems displaying
charge density waves (CDW) and strong electron-phonon
coupling, as for example sp-metals deposited on fcc(001)
systems. Furthermore, strong electron-phonon coupling
Fig. 2, but we do not observe interface states crossing
the Fermi level and the formation of a CDW. We observe
a strong damping of the electronic features approaching
the Fermi level due to a strong electron-phonon coupling.
The transition from the c(2x2) phase to the p(10x10)
phase strongly damps the Bi induced electronic states in
the energy region close to the Fermi level, because of confinement
effects induced by the domain wall formation
(arrays of dislocations).
Figure 2: Theoretical electronic state dispersion for Cu(100)
(a) and Bi/Cu(100) (b) along the M ′ Γ ′ X ′ symmetry directions.
S i indicate the Cu surface states, I i the Bi induced
states. Comparison between measured (black lines) and theoretical
I i states (dashed lines) in the right panel.
Figure 1: Electron diffraction patterns for Bi/Cu(100) system
A) p(1×1) Cu(100); B) p(1×1) 0.2ML Bi; C) c(2×2)
0.5ML Bi; D) c(9 √ 2 × √ 2)R45 ◦ 0.7ML Bi; E) p(10×10)
0.8ML Bi; F) Bi bidomain hexagonal structure at about
140ML Bi.
(EPC) has been observed in Bi surfaces, where the competition
between spin orbit effects and electron phonon
interaction can inhibit the formation of a CDW. We have
performed ARPES measurements in our LOTUS laboratory
to characterize the electronic state dispersion of the
Bi induced electronic states of the c(2×2), and p(10×10)
phases, due to a strain-induced 2D array of dislocations.
The periodicity in the different structural phases of a
single layer of Bi deposited on the Cu(100) surface is
reported in Fig. 1. The electronic state dispersion reproduces
well the predicted band structure reported in
A detailed analysis of the electron-phonon interaction
has been performed from RT down to 8K. The results
shows a linear dependence on the temperature, expected
in a Debye model of phonon modes. By a linear fit of the
electron mass enhancement parameter λ, as a function
of temperature, we obtain equal λ = 0.32, much higher
than the one reported for Cu either (bulk 0.14, surface
0.09) , thus indicating a stronger coupling due to the Bi
overlayer. This is a prototypical example of a 2D system
where the electron-phonon coupling is strongly enhanced
with respect to the bulk value.
.
References
1. A. Ferretti et al., Phys. Rev. Lett. 99, 046802 (2007).
2. A. Calabrese et al. Phys. Rev. B 79, 115446 (2009)
3. G. Bussetti et al., Surf. Sci. 602, 1423 (2008).
Authors
M.G. Betti, C. Mariani, P. Gargiani, A. Calabrese
http://server2.phys.uniroma1.it/gr/lotus/index.htm
Sapienza Università di Roma 86 Dipartimento di Fisica