Antiferromagnetic Ground State, Charge Density Waves and Oxygen Vacancies Induced Metal-Insulator Transition in Pressurized La$_{3}$Ni$_{2}$O$_{7}$

TL;DR

This study uses comprehensive calculations to reveal the antiferromagnetic ground state, charge density wave ordering, and oxygen vacancy effects in pressurized La$_{3}$Ni$_{2}$O$_{7}$, explaining its complex phase transitions and superconductivity.

Contribution

The paper provides a detailed theoretical analysis of the magnetic, charge density wave, and vacancy-induced phenomena in La$_{3}$Ni$_{2}$O$_{7}$, linking these to its superconducting properties.

Findings

La$_{3}$Ni$_{2}$O$_{7}$ has an antiferromagnetic ground state under various pressures.

Charge density wave orders with oxygen distortions are identified, explaining metal-insulator transitions.

Oxygen vacancies lead to distinct phases and pressure-induced metal-insulator transitions.

Abstract

La$_{3}$Ni$_{2}$O$_{7}$ has garnered widespread interest recently due to its high-temperature superconductivity under pressure, accompanied by charge density wave (CDW) ordering and metal-insulator (MI) transitions in the phase diagram. Here, we reveal with comprehensive calculations that La$_{3}$Ni$_{2}$O$_{7}$ possesses an antiferromagnetic ground state under both low and high pressures, with the strong Fermi surface nesting contributed by the flat band that leads to phonon softening and electronic instabilities. Several stable CDW orders with oxygen octahedral distortions are identified, which can trigger the MI transitions. The estimated CDW transition temperature ($\approx$120 K) at ambient pressure agrees nicely with experimental results. In the presence of apical oxygen vacancies, we identify two different phases, say, half distortion and full distortion phases, respectively, and their competition can lead to a pressure-induced MI transition, in good agreement with experimental observations. In addition, we find that the electron-phonon coupling is too small to contribute to superconductivity. These results appear to indicate an unconventional superconducting pairing mechanism mediated by antiferromagnetic fluctuations. A phase diagram that is consistent with the experimental results is given. The present results not only explain the origins of experimentally observed CDW and MI transitions, but also provide insight for deeply understanding the properties like superconductivity, CDW and the role of oxygen vacancies in pressurized La$_{3}$Ni$_{2}$O$_{7}$.