Ultrafast optical characterization of superconductivity in atomically thin Bi₂Sr₂CaCu₂O_8+x

Recently, atomically thin van der Waals superconducting material Bi₂Sr₂CaCu₂O_8+x (Bi-2212) has been isolated and proven to inherit the properties of its bulk counterpart. However,  in  the atomically thin limit, Bi-2212 is very fragile and easily deteriorates during usual nanofabrication processes. A non-invasive characterization technique is thus highly desired. Here we present an optical pump-probe method for identifying superconductivity of atomically thin Bi-2212 flakes. We confirm its applicability by comparison with resistivity measurements in thick flakes, in which both methods yield comparable superconducting transition temperature (Tc). To further validate this technique, our pump-probe results show the expected gap closing signature with increasing applied magnetic field. Direct pump-probe measurements from micron-sized trilayer and bilayer Bi-2212 samples yield Tc values that are very close to that of bulk materials. For these thin samples, low temperature exfoliation and hBN capping are used to preserve the sample quality. Our work provides an all-optical approach for characterizing superconductivity with diffraction limited spatial resolution in atomically thin Bi-2212, which is helpful for furthering the study of 2D superconductivity.