Superconductivity near 80 Kelvin in single crystals of La3Ni2O7 under pressure

Abstract: High-transition-temperature (high-T_c) superconductivity in cuprates has been discovered for more than three decades, but the underlying mechanism remains a mystery. Cuprates are the only unconventional superconducting family that host bulk superconductivity with T_cs above the liquid nitrogen boiling temperature at 77 Kelvin. Here we report an observation of superconductivity in single crystals of La3Ni2O7 with a maximum T_c of 80 Kelvin at pressures between 14.0-43.5 gigapascals using high-pressure resistance and mutual inductive magnetic susceptibility measurements. The superconducting phase under high pressure exhibits an orthorhombic structure of Fmmm space group with the 3d_(x^2-y^2 ) and 3d_(z^2 ) orbitals of Ni cations strongly interacting with oxygen 2p orbitals. Our density functional theory calculations suggest the superconductivity emerges coincidently with the metallization of the {\sigma}-bonding bands under the Fermi level, consisting of the 3d_(z^2 ) orbitals with the apical oxygens connecting Ni-O bilayers. Thus, our discoveries not only reveal important clues for the high-T_c superconductivity in this Ruddlesden-Popper double-layered perovskite nickelates but also provide a new family of compounds to investigate the high-T_c superconductivity mechanism.

• The paper demonstrates superconductivity near 80 K in La3Ni2O7 single crystals when high pressures (14–43.5 GPa) trigger structural changes.
• It employs high-pressure resistance and magnetic susceptibility measurements alongside DFT calculations to elucidate the superconducting mechanism.
• The study reveals a strange metallic state above Tc and highlights Ni valence tuning comparable to cuprates, offering new insights into high-Tc physics.

Superconductivity at High Temperatures in LaNi$_3$O$_7$ Under High Pressure

The pursuit of new materials that exhibit high-temperature superconductivity continues to be a focal point of condensed matter physics. This study introduces single crystals of LaNi$_3$O$_7$, demonstrating superconductivity at around 80 K, achieved under pressures ranging from 14.0 to 43.5 GPa. This observation situates these crystals among the few unconventional superconductors operating above the liquid nitrogen boiling point.

The authors employ high-pressure resistance and mutual inductive magnetic susceptibility measurements to probe the superconductivity characteristics. The phase transition of LaNi$_3$O$_7$ into a superconducting state is marked by distinct structural transformations. Under high pressure, the material transitions to an orthorhombic structure with an Fmmm space group. This structural reconfiguration is supported by density functional theory (DFT) calculations, which suggest that metallization of 3d$_{x²-y²}$ orbitals occurs as they mix with oxygen 2p orbitals, forming $\sigma$-bonding bands below the Fermi level. Such structural transformations and electronic reconfigurations provide compelling evidence that applying pressure induces significant changes in both bonding and electronic properties conducive to high-temperature superconductivity.

The theoretical insight is complemented by experimental evidence showing not only a superconducting transformation at elevated pressures but also the emergence of a strange metallic state above the transition temperature. Intriguingly, the resistance displays a linear dependence on temperature up to 300 K in this state, comparable to behaviors noted in cuprate superconductors. This underscores the potential of LaNi$_3$O$_7$ as a representative model for studying the unconventional mechanisms underlying high-temperature superconductivity.

From a structural standpoint, the presence of voids for metal valence state tuning is significant. This study discusses the role of Ni$^{2.5+}$, whose configuration is akin to Cu$^{2+}$ in high-T$_c$ cuprates, suggesting that strong inter-layer interactions facilitated by apical oxygen anions play a pivotal role. This particular configuration supports the formulation of $\sigma$-bonding and anti-$\sigma$-bonding bands, reminiscent of those seen in MgB$_2$ and other high-T superconductors, hence motivating the exploration of superconductivity within similar contexts.

The observed T$_c$ of 80 K, surpassing many known limits of nickel-based and iron-based superconductors, represents a notable benchmark, albeit without the need for elemental hydrogen enrichment known to characterize other high-pressure superconductors like LaH$_x$. The distinctive behavior of LaNi$_3$O$_7$ crystals, particularly under the external variable of pressure, contributes to the broader narrative detailing the indispensable role of structural and electronic interplays in achieving high-T superconductivity. This research potentially amplifies the role of Ruddlesden-Popper phases as enabling frameworks for superconductivity exploration, providing a fertile ground for theoretical and experimental advancements.

Future research directions may encompass extending this paradigm to other compounds within the Ruddlesden-Popper nickelate series, exploring varying elemental substitutions as well as pressure conditions to uncover the detailed parameter space defining optimal superconducting behaviors. By synthesizing further insights regarding the interrelations between structural and electronic modifications under pressure, the goal of better understanding and harnessing high-temperature superconductivity could move closer to fruition.