On the origin of the quasi-particle peak in Cr(001) surfaces

TL;DR

This study investigates the origin of the sharp resonance in Cr(001) surfaces using dynamical mean-field theory, comparing two approaches, and highlights the importance of initial conditions and methodological limitations in capturing the Kondo effect.

Contribution

The paper compares spin-polarized T-matrix and non-crossing approximations in DMFT to clarify the resonance's origin, emphasizing the impact of initial hybridization and methodological issues.

Findings

High-energy spectral features are well captured by the T-matrix approximation.

Non-crossing approximation shows unphysical behavior at the Fermi level in spin-polarized calculations.

Only specific initial hybridizations may produce an orbital Kondo resonance under strong magnetic fields.

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 dz2 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.