Spectroscopy of novel quantum states – Correlated Systems and Mott Physics

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  • Permanent members: Marie d’Angelo, Christophe Brun, Tristan Cren, Pascal David, François Debontridder, Marie Hervé


Electron-electron repulsion and relativistic effects at the heart of the electronic properties of an atomic layer of Pb

In a metal, the direct repulsive interaction between two electrons (Coulombian interaction) is shielded by the presence of many ions and electrons. When the electron density decreases this repulsive interaction between electrons increases. If this repulsion becomes strong enough, it can prevent the formation of a metallic state in favor of an insulating state called Mott’s insulator.

We have studied experimentally and theoretically a two-dimensional material predicted to be very close to the Mott transition consisting of a low-density lead atomic layer deposited on a silicon substrate. Thanks to a fine comparison of the electronic excitation spectrum measured by STM with that calculated by an “ab initio” approach, we have shown that in this 2D material, the electronic correlations are not strong enough to induce an insulating state but significantly alter the metallic state. An important conclusion is also that the spin-orbit coupling, due to the high atomic mass of lead, is an essential parameter to be taken into account for the correct modeling of the electronic properties of this system which thus presents spin-polarized metallic states.

Répulsion électron-électron et effets relativistes au cœur des propriétés électroniques d’une couche atomique de plomb

Figure 1 a: STM topographic image of a 3×3 domain of Pb/Si(111). b: Fermi surface calculated by density functional with Coulombic term (DFT+U) where the spin polarization is represented by a small coloured arrow. c: Fourier transform of a dI/dV conductance map (E=EF,x,y). d: Calculation of these processes from the electronic structure obtained in DFT+U.


The shape and spin texturing of these states at Fermi energy were determined by a method called quasiparticle interference. The calculated Fermi surface (Fig1b) is formed of two spin-polarized quasi-hexagons, each with a spin texturing (helicity) rotating in an opposite direction. This helical spin polarization gives rise to selective electron scattering processes, induced by the presence of non-magnetic impurities. There are interband processes characterized by qinter wave vectors and intraband processes characterized by qintra wave vectors (Fig1b). Wave vectors joining parallel spins give a strong signal while wave vectors joining opposite spins give a null signal. Experimentally, these processes were probed by Fourier transform of differential conductance maps dI/dV(V=0,x,y). The experimental result Fig1c shows a good agreement with the simulation Fig1d.

In perspective to this work, we will study the nature of the local magnetism of the 2D Mott phases existing in parent compounds. It remains debated and largely unknown because of the important spatial extension of the Coulombian repulsion.

Récent publication 

  1. C. Tresca, C. Brun, T. Bilgeri, G. Menard, V. Cherkez, R. Federicci, D. Longo, F. Debontridder, M. D’Angelo, D. Roditchev, G. Profeta, M. Calandra and T. Cren. Chiral Spin Texture in the Charge-Density-Wave Phase of the Correlated Metallic Pb/Si(111) Monolayer. Phys. Rev. Lett. 120, 196402 (2018) https://hal.archives-ouvertes.fr/hal-01792155


Electron-electron repulsion and strong relativistic effects at the heart of the fundamental state properties of and a Pb atomic layer