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Phase transitions

Phase transitions

Mott transition and charge order in Sn/Ge(111)

The search for two dimensional Mott insulators has begun on the 1990s with K/Si(111):B and SiC(0001). In 2006, we have discovered a new metal-insulator transition for 1/3 ML Sn on Ge(111) under 30 K. We have interpreted these observations in terms of a Mott transition. It was the first time that both phases (metallic and insulating) were observed in a such a surface transition.

In Sn/Ge(111), the metallic (3x3) reconstruction that appears around 200 K was considered as the fundamental state. However, our complementary experiments of low energy electron diffraction, angle-resolved photoemission and scanning tunneling microscopy have allowed us to discover two new phases. Below 60 K the system becomes a bad metal while preserving the (3x3) surface symmetry. It is a charge ordered insulating phase. Under 30 K, the surface symmetry changes to (√3 x √3)R30°, where the vertical distortion of Sn atoms in the (3x3) phase is lost. All the Sn atoms are equivalent. Photoemission using synchrotron radiation shows that the structural phase transition, also observed in the core levels, is concomitant to a band gap opening. This ground state has been explained by the onset of a Mott insulating phase.

Phase transitions in Sn/Ge(111). Left column, Sn 4d core levels for three temperatures characteristic of each phase. Middle column, STM images showing the charge order in the intermediate phase and the equivalence of all the Sn atoms at the low temperature phase. Right column, electronic structure showing the bad metallicity of the intermediate phase and the large gap at the low temperature phase. Figures from R. Cortés et al., Phys. Rev. B 88, 125113 (2013). Copyright 2013 by the American Physical Society." http://dx.doi.org/10.1103/PhysRevB.88.125113

D’autres références :

Observation of a Mott insulating ground state for Sn/Ge(111) at low temperature R. Cortés, A. Tejeda, J. Lobo, C. Didiot, B. Kierren, D. Malterre, E.G. Michel and A. Mascaraque. Physical Review Letters 96, 126103 (2006).
http://dx.doi.org/10.1103/PhysRevLett.96.126103

Highlight in Nature News & Views 441, 295 (2006).

Transition in Sn/Si(111):B by ultrafast collective diffusion

The Sn/Si(111) interface doped with B is a good candidate to look for Mott insulators, as it is hybrid between Sn/Si(111) and K/Si:B interfaces, both prone to exhibit electronic correlation effects. On Sn/Si(111):B we have found a new insulating phase at room temperature that exhibits a reversible phase transition at 520 K, giving rise to a different surface symmetry. This phase transition is explained by a the collective fluctuation of Sn surface atoms at the picosecond scale. These fluctuations correspond to Sn tetramers exploring the 24 energy-degenerated configurations as a function of time.

Phase transition in Sn/Si(111):B. On the top, STM image showing the order of the Sn/Si(111):B surface at room temperature. On the bottow, low energy electron diffraction patterns showing the reversible symmetry change at the surface around 520 K. Figure from W. Srour et al., Phys. Rev. Lett. 114, 196101 (2015). Copyright 2015 by the American Physical Society." http://dx.doi.org/10.1103/PhysRevLett.114.196101

Collective diffusion of Sn atoms. Atomic positions of surface atoms calculated by ab-initio molecular dynamics. Film from W. Srour et al., Phys. Rev. Lett. 114, 196101 (2015). Copyright 2015 by the American Physical Society." http://dx.doi.org/10.1103/PhysRevLett.114.196101