[Paper Review] Electronic correlations and Fermi liquid behavior of intermediate-band states in titanium-doped silicon

A. Östlin, L. Chioncel|arXiv (Cornell University)|Nov 5, 2021

Semiconductor Quantum Structures and Devices52 references3 citations

TL;DR

This study investigates electronic correlations and localization in titanium-doped silicon using a combined typical medium theory (TMT) and dynamical mean-field theory (DMFT) approach within density functional theory. It finds that intermediate-band states remain metallic across experimentally relevant doping concentrations, exhibiting Fermi liquid behavior with coherent quasiparticles and a strongly reduced density of states at the Fermi level due to correlations.

ABSTRACT

We study the nature of the electronic states in the intermediate band formed by interstitial titanium in silicon. Our single-site description combines effects of electronic correlations, captured by dynamical mean-field theory, and disorder, modeled using the coherent potential approximation and the typical medium mean-field theory. For all studied concentrations an extended metallic state with a strongly depleted density of states at the Fermi level is obtained. The self-energy is characteristic to Fermi-liquids and for certain temperatures reveals the existence of coherent quasi-particles.

Motivation & Objective

• To determine the electronic nature of intermediate-band states in titanium-doped silicon.
• To disentangle the effects of electronic correlations and disorder on metal-insulator transitions.
• To assess whether the intermediate band remains metallic or becomes insulating at high doping concentrations.
• To evaluate the validity of Fermi liquid behavior in correlated, disordered intermediate-band systems.

Proposed method

• Combines TMT and DMFT within a DFT framework to treat both disorder and electron-electron correlations.
• Uses geometric averaging of the path operator in TMT to model disorder, replacing arithmetic averaging in standard CPA.
• Applies dynamical mean-field theory to capture strong electronic correlation effects in the 3d shell of Ti impurities.
• Performs full charge self-consistency between DFT, TMT, and DMFT in a coupled iterative loop.
• Calculates spectral functions and self-energies to probe quasiparticle coherence and localization.
• Employs the typical medium mean-field theory to compute the typical density of states as a localization indicator.

Experimental results

Research questions

• RQ1 Does the intermediate band in Ti-doped silicon remain metallic at high doping concentrations despite strong electronic correlations?
• RQ2 To what extent do electronic correlations suppress the density of states at the Fermi level in the intermediate band?
• RQ3 Are coherent quasiparticles present in the intermediate band, indicating Fermi liquid behavior?
• RQ4 How do disorder and correlation compete to influence the metal-insulator transition in this system?

Key findings

• The system remains metallic for all studied Ti concentrations, with no metal-to-insulator transition observed.
• The self-energy exhibits a characteristic Fermi liquid form, indicating the presence of coherent quasiparticles at certain temperatures.
• The density of states at the Fermi level is strongly depleted due to electronic correlations, consistent with a Fermi liquid with reduced quasiparticle weight.
• The typical medium theory order parameter remains non-zero, indicating the absence of Anderson localization in the intermediate band.
• Spectral functions show a clear quasiparticle peak near the Fermi level, supporting coherent Fermi liquid behavior.
• The results suggest that the intermediate band in Ti-doped silicon is metallic and correlated, with properties consistent with a Fermi liquid despite disorder and strong correlations.