Nature of charge density waves and metal-insulator transition in pressurized <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>La</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:msub> <mml:mi>Ni</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> <mml:msub> <mml:mi mathvariant="normal">O</mml:mi> <mml:mn>7</mml:mn> </mml:msub> </mml:math>

Abstract

${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{O}}_{7}$ 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 explore the nature of CDW and MI transitions using comprehensive first-principles calculations. Our findings reveal that ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{O}}_{7}$ 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. Accordingly, several stable CDW orders with oxygen octahedral distortions are identified. In the presence of apical oxygen vacancies, we identify two different phases exhibiting distortions similar to CDW phases, and their competition can lead to a pressure-induced MI transition. The estimated CDW transition temperature and MI transition pressure agree nicely with experiments. In addition, we find that the electron-phonon coupling is too weak to contribute to superconductivity. These results suggest an unconventional superconducting pairing mechanism mediated by antiferromagnetic fluctuations. Finally, we present a phase diagram consistent with the experimental results. Our findings offer crucial insights into the interplay of superconductivity, CDW, and the role of oxygen vacancies in pressurized ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{O}}_{7}$.

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