[Paper Review] Modulation of the Octahedral Structure and Potential Superconductivity of La$_3$Ni$_2$O$_7$ through Strain Engineering

Zihao Huo, Luo, Zhihui+11 more|arXiv (Cornell University)|Apr 17, 2024

Inorganic Chemistry and MaterialsChemistry3 citations

TL;DR

This study proposes that applying ~2 GPa uniaxial compressive strain along the c-axis in La₃Ni₂O₇ can induce superconductivity by modulating the Ni-dz² electron density at the Fermi level (n_z^EF), which correlates strongly with the observed 'right-triangle' superconducting dome in the P-T phase diagram. First-principles calculations reveal that strain-induced lattice distortion alters the octahedral structure, enhancing band nesting and Fermi surface coherence, thereby promoting superconductivity.

ABSTRACT

The recent transport measurement of La$_3$Ni$_2$O$_7$ uncover a "right-triangle" shape of the superconducting dome in the pressure-temperature (P-T) phase diagram. Motivated by this, we perform theoretical first-principles studies of La$_3$Ni$_2$O$_7$ with the pressure ranging from 0 to 100 GPa. Notably, we reveal a pressure dependence of the Ni-$d_{z^2}$ electron density at the Fermi energy ($n_z^{EF}$) that highly coincides with such shape. On this basis, we further explore the electronic structure under uniaxial stress. By tracking the stress response of $n_z^{EF}$, we propose that superconductivity can be achieved by applying only about 2 GPa of compression along the c axis. The idea is further exemplified from the perspectives of lattice distortion, band structure, Fermi surface and superconducting phase coherence. We also discuss the possible charge modulation under the stress and provide an insight to the relation between n_z^EF and the superconducting Tc in La$_3$Ni$_2$O$_7$ system. Our study provides a helpful guide to the future experiment.

Motivation & Objective

• To understand the origin of the 'right-triangle' superconducting dome observed in the pressure-temperature phase diagram of La₃Ni₂O₇.
• To investigate how strain engineering, particularly uniaxial stress along the c-axis, can modulate the electronic structure and promote superconductivity.
• To establish a quantitative link between the Ni-dz² electron density at the Fermi level (n_z^EF) and the superconducting transition temperature (Tc).
• To explore the role of lattice distortion, band structure evolution, and Fermi surface reconstruction under strain in enabling superconducting phase coherence.

Proposed method

• First-principles density functional theory (DFT) calculations were performed to study La₃Ni₂O₇ under hydrostatic pressure from 0 to 100 GPa.
• Uniaxial stress was applied along the c-axis to simulate strain effects, with electronic structure and lattice parameters analyzed under varying strain levels.
• The Ni-dz² orbital contribution to the Fermi-level density of states (n_z^EF) was tracked as a key electronic descriptor for superconducting propensity.
• Lattice distortion, band structure, Fermi surface topology, and superconducting phase coherence were evaluated as functions of applied strain.
• Charge modulation patterns under strain were examined to assess their potential role in enhancing electron pairing.

Experimental results

Research questions

• RQ1 How does the Ni-dz² electron density at the Fermi level (n_z^EF) vary with applied pressure and strain in La₃Ni₂O₇, and does it correlate with the observed superconducting dome?
• RQ2 Can uniaxial compressive strain along the c-axis induce superconductivity in La₃Ni₂O₇, and if so, at what strain magnitude?
• RQ3 What is the role of lattice distortion and octahedral structure modulation in enhancing superconducting phase coherence?
• RQ4 How do changes in band structure and Fermi surface topology under strain influence the superconducting pairing mechanism?
• RQ5 What is the relationship between charge modulation and n_z^EF in the context of superconductivity in La₃Ni₂O₇?

Key findings

• Applying approximately 2 GPa of uniaxial compressive strain along the c-axis induces a significant enhancement in the Ni-dz² electron density at the Fermi level (n_z^EF), correlating with the onset of superconductivity.
• The pressure dependence of n_z^EF closely matches the 'right-triangle' shape of the superconducting dome in the P-T phase diagram, suggesting n_z^EF as a key control parameter.
• Strain-induced lattice distortion leads to a reconstruction of the Fermi surface and enhanced band nesting, promoting superconducting phase coherence.
• The evolution of the band structure under strain reveals increased hybridization and nesting features favorable for electron pairing.
• Charge modulation patterns emerge under strain, potentially stabilizing the superconducting state by enhancing electron correlation effects.
• The study identifies a feasible experimental pathway—uniaxial strain at ~2 GPa—to achieve superconductivity in La₃Ni₂O₇ without requiring extreme pressures.