Ambipolar doping-induced surface in-gap state on Mott-insulating Ca$_2$RuO$_4$

TL;DR

This study investigates how surface doping with Cs atoms and C60 molecules induces in-gap states on Ca2RuO4, revealing a new coherent Ru 4d state within the Mott gap without significant chemical potential shifts.

Contribution

It demonstrates that surface doping can create in-gap states in a Mott insulator through hybridization, offering new insights into surface-induced metal-insulator transitions.

Findings

Emergence of a new Ru 3d photoemission peak with doping

Development of coherent states within the Mott gap

No significant chemical potential jump observed

Abstract

We report an x-ray photoemission spectroscopy study of Ca$_2$RuO$_4$ surface-dosed with Cs alkali atoms and C$_{60}$ molecules. Due to its small ionization energy (large electron affinity), deposited Cs atoms (C$_{60}$ molecules) are expected to provide a solid surface with electrons (holes). Upon dosing the dopants to Mott-insulating Ca$_2$RuO$_4$, we found a new Ru $3d$ photoemission peak emerging on the lower binding-energy side, suggesting the creation of a core-hole screening channel associated with coherent Ru $4d$ states around the Fermi level. For both the Cs and C$_{60}$ dosing, this change occurred without an appreciable chemical potential jump. The coherent state, therefore, develops within the Mott gap through hybridization with the impurity level of the dopants. The present work highlights the flexibility of Mott-insulator surfaces as a playground for metal-insulator transitions.