Large Transport Properties of Bi$_2$Sr$_{2-x}$La$_x$CaCu$_2$O$_{8+\delta}$ Single Crystals Grown by a Floating-Zone Method

TL;DR

This study successfully grew Bi-based high-$T_c$ superconductor crystals and found that the parent insulator exhibits metallic behavior with itinerant holes, challenging the traditional insulator classification.

Contribution

It demonstrates the growth of Bi$_2$Sr$_{2-x}$La$_x$CaCu$_2$O$_{8+\delta}$ crystals via a floating zone method and reveals their unusual transport properties.

Findings

In-plane resistivity shows metallic behavior near room temperature.

Hall mobility is similar to that of optimally doped samples.

The system is not a simple insulator; holes are itinerant upon doping.

Abstract

A parent insulator of the Bi-based high-$T_c$ superconductor Bi$_2$Sr$_{2-x}$La$_x$CaCu$_2$O$_{8+\delta}$ was successfully grown by a traveling solvent floating zone method. The in-plane resistivity $\rho_{ab}$ shows metallic behavior ($d\rho_{ab}/dT >$ 0) near room temperature, although the magnitude of the resistivity is far above the Mott limit for metallic conduction. The Hall mobility of the parent insulator does not so much differ from that of an optimally doped sample. These results suggest that the system is not a simple insulator with a well-defined gap, and the hole in the parent insulator is essentially itinerant, as soon as it is doped.