Nature of the (3√2x√2)R45o↔(√2x√2)R45o phase transition of 0.5 ML Sn/Cu(100).

J.E. GAYONE; J. FUHR; A. CARRERA; E.A. SANCHEZ; O. GRIZZI; J. MARTÍNEZ BLANCO; E.G. MICHEL; H. ASCOLANI

Granada

Conferencia; 10th International Conference on Atomically Controlled Surfaces, Interfaces and Nanostructures.; 2009

It has been recently reported that the (3√2x√2)R30o phase formed at room temperature by 0.5 MLof Sn atoms on Cu(100) surfaces undergoes a reversible transition to a (√2x√2)R30o phase when thetemperature increases above 360 K. [1] Accompanying this symmetry lowering there is a gapopening at the nested regions of the 2D Fermi surface of the high-temperature phase, a typicalphenomenon of Peierls-type transitions. However, the size of the gap observed in the 3√2x√2)R30olow-temperature phase turned out to be at least 45 times larger than the value predicted for weakcharge density waves. [2] This unusual behaviour has also been reported for the phase transitionsfound in the In/ and Pb/Cu(100) systems.We have investigated the structural and electronic properties of (3√2x√2)R30o phase using acombination of scanning tunnelling microscopy, time-of-flight ion-scattering spectroscopy (TOF-ISS), and total-energy-minimization calculations. Theory and experiment support the missing-rowsurface-alloy (MR) structure [3] as the ground state for this surface. The electronic structurecalculated for the MR model reproduces almost perfectly both the experimental STM images and the observed electronic bands. [4] In addition, the existence of Cu vacancies was conclusively confirmed by our TOF-ISS experiments. Moreover, our TOF-ISS results combined withMonteCarlo calculations indicate that the amount of Cu vacancies remains approximately constant along the phase transition. In conclusion, our results support a transition of order/disorder type where the Cu vacancies form a 2D gas in the HT phase. These findings indicates a more complex process than the simple distortion expected from a weak coupling charge density wave transition and shed light on the atomistic nature of this process.[1] J. Martínez-Blanco et al. , Phys. Rev. B72, 041401(R) (2005).[2] J. Martinez-Blanco et al., Phys. Rev. B77, 195418 (2008).[3] K. Pussi et al. Surf. Science 549, 24 (2004)[4] J. Fuhr et al., submitted to Phys. Rev. B.