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