Unveiling phase diagram of the lightly doped high-Tc cuprate superconductors with disorder removed

TL;DR

This study investigates a disorder-free, multi-layered cuprate superconductor using ARPES and quantum oscillations, revealing a tiny Fermi pocket at low doping and a phase transition from superconducting to metallic state.

Contribution

It provides the first evidence that minimal carrier doping can induce metallicity in a disorder-free cuprate, challenging previous notions of polaronic features in underdoped cuprates.

Findings

Tiny Fermi pocket observed at less than 1% doping.

Well-defined quasiparticles without polaronic features.

Phase transition from superconducting to metallic at 4% doping.

Abstract

The currently established electronic phase diagram of cuprates is based on a study of single- and double-layered compounds. These CuO$_2$ planes, however, are directly contacted with dopant layers, thus inevitably disordered with an inhomogeneous electronic state. Here, we solve this issue by investigating a 6-layered Ba$_2$Ca$_5$Cu$_6$O$_{12}$(F,O)$_2$ with inner CuO$_2$ layers, which are clean with the extremely low disorder, by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. We find a tiny Fermi pocket with a doping level less than 1% to exhibit well-defined quasiparticle peaks which surprisingly lack the polaronic feature. This provides the first evidence that the slightest amount of carriers is enough to turn a Mott insulating state into a metallic state with long-lived quasiparticles. By tuning hole carriers, we also find an unexpected phase transition from the superconducting to metallic states at 4%. Our results are distinct from the nodal liquid state with polaronic features proposed as an anomaly of the heavily underdoped cuprates.