A research team led by Prof. HAO Lin from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences, in collaboration with Changzhou University and South China University of Technology, has developed a method for measuring electrical transport in freestanding thin films under high pressure.
Using the method, they found that crystal dimensionality can strongly affect the pressure response of SrIrO3: three-dimensional films undergo a pressure-driven semimetal–insulator–metal transition, while two-dimensional films remain insulating even under high pressure.
The findings were published in Science China Physics, Mechanics & Astronomy.
High pressure is widely used to tune the electronic properties of materials. It has been particularly effective in studies of bulk materials, but applying pressure to thin films is much more difficult. Thin films are grown on substrates that provide mechanical support. The enormous geometric scale mismatch between the substrate and the nanoscale film makes it difficult for external pressure to act effectively on the film. Removing the substrate can solve this problem, but freestanding films are mechanically fragile and difficult to integrate with electrodes for electrical measurements under high pressure.
In this study, the research team developed a set of techniques for protecting and releasing thin films, fabricating nanoscale electrodes, and carrying out electrical transport measurements under high pressure. The resulting method works for films with different electrical properties, from metals to insulators, making it applicable to a broad range of thin-film materials.
The method has also attracted interest for its potential applications in studying other thin-film systems, including freestanding superconducting materials.
The researchers used magnetic iridate SrIrO3 as a model system to explore the effects of pressure and dimensionality on its electronic states.
They found that three-dimensional SrIrO3 films changed from a semimetal to an insulator and then to a metal as pressure increased. In contrast, SrIrO3 in the two-dimensional limit remained insulating even under high pressure.
The results show that dimensionality plays an important role in the pressure response of thin-film materials, offering a way to explore electronic states in quantum materials under extreme conditions, according to the team.
(a) In bulk materials, dimensionality is fixed by the crystal structure, whereas thin films allow flexible tuning of dimensionality through epitaxial growth. (b) Flowchart of the high-pressure strategy for freestanding films. (Image by CHEN Jingxin)
