Diminishing Mott gap by doping electrons through depositing one monolayer thin film of Rb on Ca$_{2}$CuO$_{2}$Cl$_{2}$

TL;DR

This study investigates how electron doping via Rb monolayer deposition on Ca$_{2}$CuO$_{2}$Cl$_{2}$ affects the Mott gap, revealing complex electronic evolution that informs the understanding of high-temperature superconductivity.

Contribution

It demonstrates that Rb doping modifies the Mott insulator's electronic structure in unexpected ways, challenging the rigid band model and providing new insights into cuprate superconductivity.

Findings

Fermi energy pinned within the Mott gap near the charge transfer band edge

Increase in upper Hubbard band spectral weight upon doping

Significant reduction of the Mott gap with further doping

Abstract

Understanding the doping evolution from a Mott insulator to a superconductor probably holds the key for resolving the mystery of unconventional superconductivity in copper oxides. To elucidate the evolution of the electronic state starting from the Mott insulator, we dose the surface of the parent phase Ca$_{2}$CuO$_{2}$Cl$_{2}$ by depositing one monolayer thin film of Rb atoms which are supposed to donate electrons to the CuO$_{2}$ planes underneath. We successfully achieved the Rb thin films with periodic structures, and the scanning tunneling microscopy or spectroscopy (STM or STS) measurements on the surface show that the Fermi energy is pinned within the Mott gap but more close to the edge of the charge transfer band (CTB). However, the electron doping does not reduce the spectra weight of the upper Hubbard band (UHB) for the double occupancy as expected from the rigid model, but instead increase it; meanwhile, further doping will create a new wide spread in gap states derivative from the UHB, and the Mott gap will be significantly diminished. Our results provide new clues to understand the strong correlation effect of parent Mott insulators for cuprates and shed new light on the origin of high-temperature superconductivity.