Localization Effects in Bi2Sr2Ca(Cu,Co)2O8+y High Temperature Superconductors

TL;DR

This study investigates how Co doping induces a superconductor-insulator transition in Bi2Sr2CaCu2O8+y, linking resistivity changes and electronic structure, and suggests localization effects as the main cause of TC suppression.

Contribution

It provides evidence that carrier localization, not pair-breaking, explains TC reduction in Co-doped high-temperature superconductors.

Findings

Co doping causes a transition from metallic to insulating behavior.

Disappearance of dispersing band-like states correlates with resistivity changes.

Localization due to impurity potential explains the superconductor-insulator transition.

Abstract

Doping Bi2Sr2Ca1Cu2O8+y with Co causes a superconductor-insulator transition. We study correlations between changes in the electrical resistivity RHOab(T) and the electronic bandstructure using identical single crystalline samples. For undoped samples the resistivity is linear in temperature and has a vanishing residual resistivity. In angle resolved photoemission these samples show dispersing band-like states. Co-doping decreases TC and causes and increase in the residual resistivity. Above a threshold Co-concentration the resistivity is metallic (drab/dT >0) at room temperature, turns insulating below a characteristic temperature Tmin and becomes super- conducting at even lower temperature. These changes in the resistivity correlate with the disappearance of the dispersing band-like states in angle resolved photoemission. We show that Anderson localization caused by the impurity potential of the doped Co-atoms provides a consistent explanation of all experimental features. Therefore the TC reduction in 3d-metal doped high- temperature superconductors is not caused by Abrikosov Gor'kov pair- breaking but by spatial localization of the carriers. The observed suppression of TC indicates that the system is in the homogenous limit of the superconductor-insulator transition. The coexistance of insulating (dRHOab/dT <0) normal state behavior and super- conductivity indicates that the superconducting ground state is formed out of spatially almost localized carriers.