Two-dimensional superconducting nature of Bi 2 Sr 2 CaCu 2 O 8+ δ thin films revealed by BKT transition.

High-quality Bi 2 Sr 2 CaCu 2 O 8+ δ superconducting thin films are successfully grown on a SrTiO 3 substrate by the Pulsed Laser Deposition technique. Superconducting critical transition temperatures T c,zero have reached up to 85 K by using optimized growth parameters. In addition, we demonstrated the two-dimensional nature of the superconductivity of thin films by virtue of exhibiting Berezinskii-Kosterlitz-Thouless (BKT) physics and anisotropic magnetic response. Furthermore, three distinct regimes are identified based on the analysis of direct current resistance. The non-Fermi liquid phase and BKT phase fluctuation zone almost perfectly merge together, which implies that the system undergoes a unique topological state that is determined by the BKT phase fluctuation preceding the onset of the superconducting state. The emergence of such a topological state radically differentiates from the three-dimensional superconducting transition, which spontaneously breaks the gauge symmetry. The current studies on the Bi 2 Sr 2 CaCu 2 O 8+ δ superconducting thin films provide some new insights for understanding the rich quantum states of matter that emerge in the vicinity of the superconducting phase transition and highlight the significant role of BKT fluctuation on two-dimensional superconducting transition.