Strain-engineered La₃Ni₂O₇ thin films reach 40 K superconductivity without extreme pressure

For a century, superconductivity has been something physicists find, not something they build. Strain engineering in nickelate thin films is changing that premise — not as a theoretical prediction but as a measurable, reproducible experimental result. The implications reach from the quantum scale to the grid scale: if transition temperatures continue rising through deliberate lattice design, the infrastructure of modern civilization stands to be rebuilt around materials that transmit power without loss.

The conventional picture of superconductivity, codified in the Bardeen-Cooper-Schrieffer theory of 1957, describes a precise and elegant mechanism: electrons, ordinarily repelling one another, are persuaded by interactions with the ionic lattice to form bound pairs — Cooper pairs — which then condense into a frictionless quantum fluid below a critical temperature. The theory is spectacularly successful for conventional metals and has guided semiconductor device physics for decades. Its limitation is equally precise: phonon-mediated pairing cannot, within its own framework, produce superconductivity much above 30 to 40 K. Real-world technological deployment of superconductors demands performance near or above 77 K — the boiling point of liquid nitrogen, an industrially cheap and widely available coolant. The gap between what BCS theory can deliver and what engineering requires is where the entire modern program of unconventional superconductivity research operates.

Cuprate superconductors — copper-oxide layered compounds — obliterated the BCS ceiling when they were discovered to superconduct above 130 K in mercury-based variants. But they brought new difficulties in exchange: they are brittle ceramics, chemically unstable under many processing conditions, and their superconducting mechanism, after nearly four decades of investigation, remains disputed at the level of fundamental theory. The pairing symmetry appears to be d-wave, driven by antiferromagnetic spin fluctuations, but the precise origin of those fluctuations and the role of competing electronic orders — charge density waves, pseudogap phases, strange metal behavior — continues to generate contested interpretations. Cuprates demonstrated that high-temperature superconductivity is possible. They did not explain why.

The recognition that nickel, immediately adjacent to copper in the periodic table, might host analogous physics dates to theoretical proposals in the early 1990s. Ni¹⁺ in an infinite-layer oxide structure carries a 3d⁹ electronic configuration — the same occupation as Cu²⁺ in the cuprates — and the structural family of nickel oxides that can be synthesized in square-planar, oxygen-deficient geometries seemed a plausible hunting ground. Synthesis proved exceptionally difficult, requiring topotactic reduction of perovskite precursors under carefully controlled conditions. When a Stanford University group succeeded in producing superconducting Nd₀.₈Sr₀.₂NiO₂ thin films in 2019, the confirmation that nickelates could superconduct opened a global research effort — but the transition temperatures in the infinite-layer systems remained modest, below 20 K, and synthesis challenges kept the field fragmented.

The trajectory changed with the bilayer Ruddlesden-Popper compound La₃Ni₂O₇. Unlike the infinite-layer systems, this material contains coupled pairs of NiO₂ planes bridged by apical oxygen atoms, creating strong interlayer exchange pathways. Under hydrostatic pressure exceeding 14 gigapascals, La₃Ni₂O₇ bulk crystals enter a superconducting state with transition temperatures approaching 80 K — a record for the nickelate family. The structural transformation under pressure involves a symmetry change to the I4/mmm phase, which reshapes the Fermi surface and enhances the density of states at the Fermi level. The critical observation was that this structural and electronic transformation is not unique to pressure: it can be induced by a different route entirely.

Strain engineering exploits a fundamental principle of thin-film physics: when a crystalline film is grown on a substrate with a slightly different lattice parameter, the film must accommodate the mismatch. Under in-plane compressive strain — where the substrate lattice is smaller than the film’s natural spacing — the film is squeezed laterally and expands vertically, deforming the unit cell in a manner that closely mimics the effect of hydrostatic pressure on the electronic structure. The key distinction is that substrate-induced strain is a static, ambient-pressure condition. No diamond-anvil cell is required. No extreme force must be maintained during measurement or operation. The electronic phase that was previously only accessible under geological pressures becomes a permanent feature of the film’s ground state, encoded at the moment of growth.

The consequences for transition temperature are direct and measurable. When (La,Pr)₃Ni₂O₇ bilayer nickelate films are grown under appropriate compressive strain conditions, superconductivity with onset temperatures above 40 K emerges at ambient pressure. Density functional theory calculations reveal the mechanism: in-plane compression drives the band energy at the M-point of the Brillouin zone downward, increasing the electronic density of states at the Fermi level — an effect that runs opposite to what bulk pressure does, but arrives at a similar outcome through a different geometric pathway. When modest additional hydrostatic pressure is applied on top of the pre-strained films, onset temperatures have been pushed above 60 K, with cooperative amplification of both interlayer and intralayer magnetic fluctuations identified as the driving enhancement.

The electronic structure revealed by these experiments defies simple categorization within prior theoretical frameworks. In conventional BCS superconductors, the upper critical magnetic field obeys a well-defined Pauli limit: the field at which pair-breaking by spin polarization becomes energetically favorable. Infinite-layer nickelates have been shown to sustain superconductivity in magnetic fields more than twice the Pauli limit, a direct experimental signature that phonon-mediated pairing is not the dominant mechanism. The pairing symmetry in bilayer systems appears to involve an extended s-wave character — distinct from the d-wave pairing of cuprates — potentially arising from a Feshbach resonance between two different carrier populations, one derived from the dz² orbital and one from the dx²-y² orbital of the bilayer’s nickel atoms. The interlayer coupling that connects these two orbital channels is not a perturbation but a central feature of the superconducting state.

What strain engineering accomplishes at the materials level is to make the Fermi surface topology — previously an intrinsic property of a compound fixed by its chemistry — a tunable parameter accessible through deposition conditions. The choice of substrate, the degree of lattice mismatch, the temperature and atmosphere during growth: each becomes a lever acting on the quantum geometry of the electrons at the Fermi level. Theoretical work indicates that stabilizing the I4/mmm symmetry phase under moderate compressive strain, combined with doping to tune the γ-pocket occupancy of the Fermi surface, offers a systematic optimization pathway for raising Tc further. This converts the search for higher transition temperatures from a combinatorial survey of new chemical compounds into a controlled engineering problem within a known material family.

The industrial implications scale with the transition temperature. Superconducting power transmission cables already exist in demonstration projects using cuprate compounds cooled with liquid nitrogen, but their deployment has been constrained by materials fragility, manufacturing complexity, and cooling costs. Nickelate thin films, if their Tc can be reliably elevated toward 77 K and if their structural stability under realistic processing conditions can be confirmed, would enter the operating window of liquid nitrogen cooling without requiring the extreme pressure maintenance that bulk bilayer nickelates demand. Quantum computing hardware represents a parallel application: current superconducting qubit architectures operate at temperatures in the millikelvin range, requiring dilution refrigerators of considerable expense and complexity. A shift to higher-Tc superconductors would not eliminate cryogenics, but it would dramatically reduce the engineering burden of the quantum computing stack.

Critical challenges remain unreduced. Sample quality — particularly the control of apical oxygen vacancies and other structural disorder introduced during topotactic reduction — continues to limit the reproducibility of results across research groups. The pairing symmetry question is not yet definitively resolved: phase-sensitive experiments required to determine gap nodal structure are technically demanding in thin-film geometries, and the interpretation of existing penetration depth measurements remains contested. The question of whether the transition temperatures achievable through strain engineering alone can approach technologically decisive thresholds, or whether new pairing mechanisms involving combinations of magnetic, orbital, and phonon channels will be required, is an open and quantitatively framed problem. Theoretical analysis of known unconventional superconductor families suggests that simply maximizing existing spin-exchange interactions within standard correlated electron frameworks may be insufficient to reach room temperature.

Research programs addressing these questions have proliferated since the bilayer ambient-pressure results were first reported, with contributions from groups at Stanford University, the Institute of Physics of the Chinese Academy of Sciences, the University of Science and Technology of China, and multiple European and Japanese institutions. The collaborative structure reflects the breadth of the problem: synthesis chemists, thin-film physicists, angle-resolved photoemission spectroscopists, scanning tunneling microscopists, and theorists working with density functional methods and renormalization group techniques are all essential components of the effort, and progress in any one area restructures the constraints on all others.

What the nickelate platform has established, beyond any specific transition temperature record, is a proof of concept for a new class of materials science: the deliberate engineering of quantum phase diagrams through lattice geometry control. The Fermi surface is no longer a fixed property to be measured and accepted; it is an architectural variable to be designed. Whether this approach produces a room-temperature superconductor within a decade or confirms the necessity of fundamentally new physics, it has permanently changed the conceptual vocabulary of the field. Superconductivity, once the domain of discovery, is becoming the domain of design.