A Thin Film That Switches Heat express
A thin film of nickel oxide, tens of nanometers thick, is like a sheet of paper folded hundreds of times thinner. At room temperature, it conducts electricity, but cool it slightly and it becomes an insulator. This metal-insulator transition has been known for a long time, but how heat behaves during it remained a mystery.
Conventional methods couldn't measure the thermal conductivity of such a thin film because the substrate contributed too much. The authors used frequency-domain thermoreflectance (FDTR): a laser heats the surface, and the reflected light reveals how quickly heat dissipates into the material. This allowed them to see that at the metal-insulator transition, thermal conductivity across the film changes sharply and with almost no hysteresis.
The difference in behavior along and across is linked to nanodomains — regions where metallic and insulating phases coexist. When the film thickness approaches the domain size, percolation along the layer is hindered, but across it is not. It's like a crowd trying to pass through a narrow corridor: movement along the corridor gets stuck, but across it flows freely.
Original abstract
as written by the authors arXiv:2603.21405
Perovskite oxides display correlated electrical, magnetic, and thermal properties that can be further tuned in the thin-film limit, making them contenders for next-generation electronics. Measuring thermal transport in thin films is challenging, because traditional techniques are dominated by the substrate. Here, frequency-domain thermoreflectance (FDTR) of an epitaxial NdNiO$$_3$$ thin film reveals a sharp change in out-of-plane thermal conductivity across the metal-insulator transition. Complementary frequency-domain photoreflectance (FDPR) reveals a large change in ambipolar diffusivity of photoexcited carriers. While the in-plane electrical resistance shows large hysteresis, out-of-plane thermal and charge transport shows negligible hysteresis. We attribute this discrepancy to anisotropy in the percolation of nanoscale domains across the transition as the film thickness approaches the domain length scale. We establish FDTR and FDPR as sensitive probes of quantum material phase transitions and highlight NdNiO$$_3$$ for thermal control and memory applications.
The NdNiO₃ film in the experiment was about 30 nanometers thick, a thousand times thinner than a human hair.