Doping-Induced Spectral Shifts in Two Dimensional Metal Oxides

TL;DR

This paper investigates doping effects in layered metal oxides, revealing significant spectral shifts that challenge traditional models and impact electronic properties, especially in the context of superconductivity.

Contribution

It demonstrates that doping induces large spectral shifts in layered oxides, altering carrier character and reducing expected superconductivity, which refines understanding of doping effects in these materials.

Findings

Spectral shifts oppose the rigid band model.

Doping leads to O-Cu-O molecule-based carriers.

Weak electron-phonon coupling explains absence of high Tc.

Abstract

Doping of strongly layered ionic oxides is an established paradigm for creating novel electronic behavior. This is nowhere more apparent than in superconductivity, where doping gives rise to high temperature superconductivity in cuprates (hole-doped) and to surprisingly high Tc in HfNCl (Tc=25.5K, electron-doped). First principles calculations of hole-doping of the layered delafossite CuAlO2 reveal unexpectedly large doping-induced shifts in spectral density, strongly in opposition to the rigid band picture that is widely used as an accepted guideline. These spectral shifts, of similar origin as the charge transfer used to produce negative electron affinity surfaces and adjust Schottky barrier heights, drastically alter the character of the Fermi level carriers, leading in this material to an O-Cu-O molecule-based carrier (or polaron, at low doping) rather than a nearly pure-Cu hole as in a rigid band picture. First principles linear response electron-phonon coupling (EPC) calculations reveal, as a consequence, net weak EPC and no superconductivity rather than the high Tc obtained previously using rigid band expectations. These specifically two-dimensional dipole-layer driven spectral shifts provides new insights into materials design in layered materials foe functionalities besides superconductivity.