Strain-tuned structural, electronic, and superconducting properties of thin-film La$_3$Ni$_2$O$_7$

Abstract

The recent discovery of high-temperature superconductivity in La$_3$Ni$_2$O$_7$ under ambient-pressure in strained thin films raises the question of how superconductivity can be optimized through strain. In this work, we investigate the strain-dependent electronic structure and superconducting transition temperature ($T_c$) of La$_3$Ni$_2$O$_7$ using density functional theory combined with random phase approximation spin-fluctuation calculations. We find that biaxial strain acts as a tuning parameter for Fermi surface topology and magnetic correlations. Large tensile strain drives a Lifshitz transition characterized by a $d_{z^2}$ band crossing, leading to a sharp increase in the density of states and theoretical pairing strength. However, this is accompanied by a large increase in magnetic proximity, suggesting strong competition with spin-density-wave order. Conversely, under compressive strain, we identify a structurally selective $T_c$ enhancement restricted to the high-symmetry $I4/mmm$ phase. This effect is driven by the straightening of Ni--O--Ni bonds and the emergence of a $Γ$-centered hole pocket, yielding $T_c$ values consistent with recent thin-film experiments. Our results highlight the balance between structural symmetry, electronic topology, and magnetic instability in nickelates, and provides a theoretical framework for optimizing superconductivity via strain engineering.

Figures (10)

• Figure 1: Total energy as a function of biaxial strain for each phase, referenced to the minimum-energy structure (unstrained $Amam$). Energy differences remain on the order of 10 meV/f.u. across the strain range shown.
• Figure 2: Evolution of the out-of-plane lattice parameter $c$ (top) and the in-plane Ni--O--Ni bond angle (bottom) for the strained structures. The out-of-plane parameter decreases as strain is tuned from compressive to tensile values, while the bond angle decreases with increasing tensile strain. The bond angle for the $I4/mmm$ phase (not shown) is fixed at 180$^\circ$ by symmetry.
• Figure 3: Representative validation at $-2.0\%$ strain, comparing DFT bands (black) to the Wannier interpolation (red dashed) near the Fermi level for $Amam$ (left), $Amam$ ($a = b$) (center), and $I4/mmm$ (right).
• Figure 4: Wannier band structures for the $I4/mmm$ phase (top row) and $Amam$ ($a=b$) phase (bottom row) under biaxial strain. Columns correspond to $-2.0\%$ (compressive), $0\%$ (unstrained), and $+2.0\%$ (tensile) strain. The Fermi level is at 0 eV. Note that because of the skewed and rotated $Amam$ cell, the bands do not cleanly align with the $I4/mmm$ bands.
• Figure 5: Strain dependence of the density of states at the Fermi level, $N(E_F)$. A nonmonotonic evolution is observed, with a sharp rise at large tensile strain.