Ambient pressure growth of bilayer nickelate single crystals with superconductivity over 90 K under high pressure

Abstract: Recently, the Ruddlesden-Popper bilayer nickelate $La_3Ni_2O_7$ has emerged as a superconductor with a transition temperature ($T_c$) of ~ 80 K above 14 GPa$^{[1-4]}$. Efforts to search for nickelate superconductors with higher $T_c$$^{[5,6]}$, to grow reproducible high-quality single crystals$^{[2,7-10]}$, and to eliminate reliance on demanding high gas pressure synthesis conditions$^{[11]}$, remain significant challenges. Here we report superconductivity up to 92 K under high pressure in single crystals of bilayer nickelates synthesized at ambient pressure using flux methods. High quality $La_2SmNi_2O_{7-{\delta}}$ single crystals with dimensions up to 220 {\mu}m on edge were successfully grown. At ~ 15 GPa, these crystals exhibit superconductivity with an onset transition temperature ($T_c^{onset}$) of 68 K and zero-resistance temperature ($T_c^{zero}$) of 47 K. Increasing pressure further enhances both transition temperatures, reaching record values for nickelates: $T_{c,max}^{onset}$ = 92 K and $T_{c,max}^{zero}$ = 73 K @ 21 GPa. Notably, higher $T_c$ correlates with larger in-plane lattice distortion at ambient conditions for bilayer nickelates. Furthermore, we observed a structural transition from monoclinic $P2_1/a$ to tetragonal $I4/mmm$ at ~ 18 GPa, indicating that tetragonal structure is not a prerequisite for superconductivity to appear in this bilayer nickelate. This study provides an easy-to-access method for growing reproducible high-quality bilayer nickelate single crystals and offers new insights into achieving higher Tc superconductivity.