Origin of the quasiparticle peak in the spectral density of Cr(001) surfaces

Abstract

In the spectral density of Cr(001) surfaces a sharp resonance close to the Fermi level is observed in both experiment and theory. For the physical origin of this peak two mechanisms were proposed. A single particle d z 2 surface state renormalised by electron-phonon coupling and an orbital Kondo effect due to the degenerate dxz/dyz states. Despite several experimental and theoretical investigations, the origin is still under debate. In this work we address this problem by two different approaches of the dynamical mean-field theory. First, by the spin-polarized T-matrix fluctuation exchange approximation suitable for weakly and moderately correlated systems. Second, by the non-crossing approximation derived in the limit of weak hybridization (i.e. for strongly correlated systems) capturing Kondo-like processes. By using recent continuous-time quantum Monte Carlo calculations as a benchmark, we find that the high-energy features, everything except the resonance, of the spectrum is captured within the spin-polarized T-matrix fluctuation exchange approximation. More precisely the particle-particle processes provide the main contribution. For the non-crossing approximation it appears that spin-polarized calculations suffer from spurious behavior at the Fermi level. Then, we turned to non spin-polarized calculations to avoid this unphysical behavior. By employing two plausible starting hybridization functions, it is observed that the characteristics of the resonance are crucially dependent on the starting point. It appears that only one of these starting hybridizations could result in an orbital Kondo resonance in the presence of a strong magnetic field like in the Cr(001) surface. It is for a future investigation to first resolve the unphysical behavior within the spin-polarized non-crossing approximation and then check for an orbital Kondo resonance.

Computational design of magnetic molecules and their environment using quantum chemistry, machine learning and multiscale simulations

“…Despite its general numerical complexity, the method has been already used many times to describe the Kondo physics of atoms on a broad variety of surfaces. 273,280,281 Finally, let us conclude this section by briefly discussing a few of the outstanding problems when dealing with spin-electron interaction in a completely nonperturbative way. Firstly, it is important to point out that the electron transmission spectrum, when computed, is essentially obtained from elastic scattering to the correlated spectral function.…”

Ab initio study of the electron-phonon coupling at the Cr(001) surface

“…14 Namely a pseudogap is found below roughly 200 K and the emergence of a sharp resonance below 75 K. These observations hint in the direction of a many-body interpretation of the resonance just as recent dynamical mean-field theory (DMFT) calculations do. 15,16 For example, based on DMFT calculations within the continuous-time quantum Monte Carlo (CTQMC) solver it was observed that the resonance was very robust against artificial shifts in the one-particle energies of the d xz , d yz and d z 2 states. In contrast for DMFT calculations based on the spinpolarized T-matrix fluctuation exchange approximation, which is suitable for weakly and moderately correlated systems, no resonance was observed.…”

“…This could not be verified due to spurious behavior involved with the spin-polarized version of the NCA method. 16,17 In this work it is investigated whether the d z 2 surface state renormalized by means of the electron-phonon coupling is responsible for the experimentally observed resonance at the Fermi level of Cr(001)-surfaces. More precisely, the electron-phonon mass-enhancement factor of the Cr(001)-surface is investigated and a comparison is made with that of the bulk by means of abinitio calculations.…”