Strain Instead of Pressure: Superconductivity in Nickelate Films express
A ceramic plate as thin as a soap film, and through it flows current with zero resistance. That's the superconductivity achieved in nickelates, relatives of the famous cuprate superconductors. Previously, this required enormous pressure, comparable to that at the ocean floor. Now scientists have found a workaround: they grew the film on a crystalline substrate that squeezes it by 2.14%. The strain acts like a vise, forcing atoms into the right arrangement.
Under this strain, the film exhibits superconductivity already at 60 kelvins (minus 213°C) and loses all resistance at 33 K. Magnetic measurements confirmed that the superconducting layer is two-dimensional — current flows only in the plane of the film, like on infinitely smooth ice. But the real surprise was how the charges move. In crystals under pressure, current is carried by "holes"—positively charged vacancies left by electrons. Here it's the opposite: the carriers are electrons themselves. This fundamental difference suggests that strain and pressure, though leading to the same goal, affect the material in different ways.
So a 2.14% strain opens a new dimension for controlling superconductivity. Instead of squeezing a crystal from all sides, you can simply choose the right "foundation" for growing the film. Then a ceramic plate, resembling a piece of colored glass, becomes a lossless conductor, a bridge to energy-efficient technologies.
Original abstract
as written by the authors arXiv:2608.01295v1
The realization of high-temperature superconductivity in bilayer nickelates under epitaxial compressive strain is widely interpreted as mimicking the effects of high hydrostatic pressure. To test the equivalence of these mechanisms, we investigated a comprehensive strain continuum ranging from compressive (-2.14%) to tensile (+0.91%). Crucially, via ozone-assisted atomic-layer epitaxy, we realized high-temperature superconductivity in as-grown La2PrNi2O7 films on NdAlO3 substrates, which induce the most extreme compressive strain in this material system. Under extreme compression (-2.14%), these films exhibit a Tc_onset of 60 K, zero resistance at 33 K, and a diamagnetic response at 20 K, with magnetotransport measurements confirming a quasi-two-dimensional superconducting nature. Comparing our phase diagram with reported data reveals distinct lattice responses: unlike in pressurized crystals, the superconducting window in epitaxial films diverges significantly in the out-of-plane parameter c (or c/ap ratio) but remains consistent with the bulk regarding the in-plane parameter ap. Crucially, while superconductivity in both systems emerges from the suppression of spin-density waves (SDW), Hall measurements reveal a fundamental electronic dichotomy: optimal superconducting films are intrinsically electron-like (exhibiting a negative Hall coefficient), in stark contrast to the hole-like nature (positive Hall coefficient) of high-pressure bulk crystals and non-superconducting tensile films. Ultimately, both tuning strategies effectively modulate the underlying correlation landscape - the true driver of superconductivity - transcending the constraints of specific Fermi surface topologies. This work establishes a macroscopic platform for probing the multi-orbital physics of nickelates, offering a new dimension for investigating high-temperature superconductivity.
A temperature of 60 kelvins is colder than the surface of Pluto (about 44 K), yet this superconductivity is still considered "high-temperature" because conventional superconductors work only near absolute zero (below 30 K).