Metal-Insulator Transition and Pseudogap in Bi$_{1.76}$Pb$_{0.35}$Sr$_{1.89}$CuO$_{6+\delta}$ High-$T_c$ Cuprates

TL;DR

This study identifies the critical doping level at which a metal-insulator transition occurs in a high-$T_c$ cuprate, revealing that the pseudogap persists beyond this transition and influences the electronic properties.

Contribution

It provides bulk-sensitive evidence for the doping-dependent metal-insulator transition and the persistent pseudogap in Bi-based cuprates, highlighting their interrelation.

Findings

MI transition occurs at p=0.09(1) with zero DOS at Fermi level

Pseudogap causes a consistent reduction in DOS across doping levels

Superconductivity emerges beyond the MI critical doping

Abstract

It is inferred from bulk-sensitive muon Knight shift measurement for a Bi$_{1.76}$Pb$_{0.35}$Sr$_{1.89}$CuO$_{6+\delta}$ single-layer cuprate that metal-insulator (MI) transition (in the low temperature limit, $T\rightarrow0$) occurs at the critical hole concentration $p=p_{\rm MI}=0.09(1)$, where the electronic density of states (DOS) at the Fermi level is reduced to zero by the pseudogap irrespective of the N\'eel order or spin glass magnetism. Superconductivity also appears for $p>p_{\rm MI}$, suggesting that this feature is controlled by the MI transition. More interestingly, the magnitude of the DOS reduction induced by the pseudogap remains unchanged over a wide doping range ($0.1\le p\le0.2$), indicating that the pseudogap remains as a hallmark of the MI transition for $p>p_{\rm MI}$.