Structure Responsible for the Superconducting State in La₃Ni₂O₇ at High-Pressure and Low-Temperature Conditions

Area of Science:

• Condensed Matter Physics
• Materials Science
• Superconductivity

Background:

• A recent report identified a new superconductor, La3Ni2O7, with a critical temperature (Tc) of 80 K at high pressures.
• This nickelate represents the second class of unconventional superconductors, alongside cuprates, exhibiting Tc above liquid nitrogen temperature.
• The initial phase diagram was based on transport measurements and room-temperature X-ray diffraction (XRD), necessitating further investigation.

Purpose of the Study:

• To determine the specific crystallographic phase responsible for the high-temperature superconductivity in La3Ni2O7.
• To conduct in situ high-pressure and low-temperature synchrotron XRD experiments.
• To correlate structural evolution with the superconducting properties.

Main Methods:

• In situ high-pressure and low-temperature synchrotron X-ray diffraction (XRD) experiments were performed.
• The study involved compressing the La3Ni2O7 sample to approximately 19 GPa at 40 K.
• Electronic structure calculations were performed based on the determined crystal structure.

Main Results:

• A phase transition from orthorhombic (Amam) to orthorhombic (Fmmm) structures was observed.
• A novel tetragonal phase (space group I4/mmm) was discovered at 19 GPa and 40 K, coinciding with the superconducting state.
• Electronic structure calculations indicated that eg orbitals of Ni atoms dominate the electronic states near the Fermi energy.

Conclusions:

• The tetragonal phase of La3Ni2O7 is identified as the likely phase responsible for high-temperature superconductivity.
• The study highlights the importance of structural details, such as Ni-O octahedra regularity and Ni-O-Ni bonding angles, in understanding superconductivity.
• This research provides crucial insights into the mechanism of superconductivity in double-layered nickelates.