Characterizing hBN-protected atomically thin Bi₂Sr₂CaCu₂O_8+x using ultrafast optical pump-probe spectroscopy

Atomically thin high-Tc superconductor Bi₂Sr₂CaCu₂O_8+x (Bi2212) has recently been isolated successfully, which opens exciting possibilities of engineering for emergent phenomena. However, Bi2212 at its thinnest limit easily deteriorates during usual nanofabrication processes, thus calling for unconventional preparation and characterization methods. Here we present an all-optical pump-probe technique for identifying superconductivity in atomically thin Bi2212 flakes with hBN protection. As the superconducting gap opens below Tc, a slowing of the recovery dynamics is observed in the transient reflection signal. The effectiveness of hBN protection is evidenced by a sharp contrast between the hBN capped and uncapped regions. We find that the Tc in a pristine Bi2212 atomically thin flake can approach the bulk limit. Our hBN protection and optical pump-probe characterization methods can be helpful for future studies of thin Bi2212-based devices.