[Paper Review] Superconductivity near 80 Kelvin in single crystals of La3Ni2O7 under pressure

Hualei Sun, Mengwu Huo+11 more|arXiv (Cornell University)|May 16, 2023

Magnetic and transport properties of perovskites and related materials46 references12 citations

TL;DR

The authors report superconductivity in La3Ni2O7 single crystals with Tc up to ~80 K under high pressure (14.0–43.5 GPa), associated with an orthorhombic Fmmm phase and Ni 3d orbital–oxygen p orbital interactions.

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 σ-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.

Motivation & Objective

• Motivate search for high-Tc superconductivity in nickelates analogous to cuprates.
• Investigate whether La3Ni2O7 under pressure can host bulk superconductivity.
• Determine structural, electronic, and orbital conditions accompanying superconductivity in this nickelate.
• Explore implications for mechanisms of high-Tc superconductivity in layered nickelates.

Proposed method

• Measure high-pressure resistance and mutual inductive magnetic susceptibility on La3Ni2O7 single crystals.
• Identify Tc and its pressure dependence between 14.0 and 43.5 GPa.
• Determine crystal structure (orthorhombic Fmmm) in the superconducting regime.
• Perform density functional theory calculations to link metallization of sigma-bonding bands to superconductivity.
• Analyze Ni 3d (x2−y2, z2) orbital interactions with O 2p orbitals across Ni–O bilayers.

Experimental results

Research questions

• RQ1 Can La3Ni2O7 single crystals exhibit bulk superconductivity under applied pressure?
• RQ2 What structural phase and orbital configurations accompany superconductivity in this material?
• RQ3 How do the Ni 3d–O 2p interactions evolve with pressure to enable superconductivity?

Key findings

• Superconductivity with a maximum Tc of 80 K is observed under pressures of 14.0–43.5 GPa.
• The superconducting phase has an orthorhombic Fmmm structure.
• Ni cations show strong interaction between 3d(x2−y2) and 3d(z2) orbitals with oxygen 2p orbitals.
• DFT calculations link superconductivity to metallization of sigma-bonding bands at the Fermi level, involving Ni 3d(z2) orbitals and apical oxygens in Ni–O bilayers.