Nanoscale phase separation in deep underdoped Bi$_{2}$Sr$_{2}$CuO$_{6+\delta}$ and Ca$_2$CuO$_2$Cl$_2$

TL;DR

This study reveals nanoscale phase separation in deeply underdoped cuprates, showing how it causes the gap to fill with doping and influences the metal-insulator transition, supported by tunneling spectra analysis.

Contribution

It provides direct evidence of nanoscale phase separation in underdoped cuprates and models the nodal gap as a Coulomb gap due to impurity-pinned charged stripes.

Findings

Nanoscale phase separation extends from half filling to x~0.09.

The gap fills with doping rather than closes.

Impurity pinning of stripes drives the metal-insulator transition.

Abstract

We demonstrate that tunneling spectra in deeply underdoped Bi$_{2}$Sr$_{2}$CuO$_{6+\delta}$ (Bi2201) and Ca$_2$CuO$_2$Cl$_2$ (CCOC) provide clear evidence for nanoscale phase separation (NPS), causing the gap to fill with doping rather than close. The phase separation extends over a doping range from half filling to approximately $x\sim 0.09$. Assuming the NPS is in the form of stripes, then the nodal gap -- which we model as a Coulomb gap -- arises from impurity pinning of the charged stripes, ultimately driving a metal-insulator transition.