High-T_c superconductivity in ultrathin Bi_2Sr_2CaCu_2O_8+x down to halfunit-cell thickness by protection with graphene

TL;DR

This study demonstrates that ultrathin Bi_2Sr_2CaCu_2O_8+x films retain high critical temperatures when protected with graphene, revealing unique normal state properties and enabling exploration of 2D superconductivity.

Contribution

The paper introduces a novel fabrication method for high-quality ultrathin Bi2212 films using graphene protection, allowing detailed study of their intrinsic superconducting properties at minimal thickness.

Findings

Superconducting transition remains sharp down to half-unit-cell thickness.

Normal state resistivity slope varies significantly with thickness.

Sheet resistance increases dramatically while T_c stays constant.

Abstract

High-T_c superconductors confined to two dimension exhibit novel physical phenomena, such as superconductor-insulator transition. In the Bi_2Sr_2CaCu_2O_8+x (Bi2212) model system, despite extensive studies, the intrinsic superconducting properties at the thinness limit have been difficult to determine. Here we report a method to fabricate high quality single-crystal Bi2212 films down to half-unit-cell thickness in the form of graphene/Bi2212 van der Waals heterostructure, in which sharp superconducting transitions are observed. The heterostructure also exhibits a nonlinear current-voltage characteristic due to the Dirac nature of the graphene band structure. More interestingly, although the critical temperature remains essentially the same with reduced thickness of Bi2212, the slope of the normal state T-linear resistivity varies by a factor of 4-5, and the sheet resistance increases by three orders of magnitude, indicating a surprising decoupling of the normal state resistance and superconductivity. The developed technique is versatile, applicable to investigate other two-dimensional (2D) superconducting materials.