Sharp Drop in Superfluid Stiffness Observed During Phase Transition in Indium Oxide Films

A team of physicists from Université Grenoble Alpes and CNRS in France, in collaboration with a colleague from Karlsruhe Institute of Technology in Germany, has recently made an intriguing discovery in the field of quantum physics. Their study, published in the prestigious journal Nature Physics, explores the odd quantum phase transition that occurs in indium oxide films when transitioning between superconducting and insulating states. Using advanced microwave spectroscopy, the research team delved into the internal properties and behaviors of indium oxide films as they underwent this phase transition, uncovering a surprising anomaly that challenges previous expectations.

The concept of quantum phase transitions is fundamental to the study of condensed matter physics, and it typically describes a system’s shift from one phase to another due to changes in external conditions like temperature, pressure, or magnetic fields. In the case of superconductors, phase transitions usually occur between superconducting and insulating states. Superfluid stiffness, a crucial property used to measure how resistant a material is to phase transitions, has been a key factor in understanding these changes. Past research has shown that when a superconductor experiences a phase transition to an insulating state, the change in superfluid stiffness is typically smooth and continuous. This gradual transition is well-understood and has been observed in numerous superconducting materials. However, the research conducted by this team challenges this conventional understanding in the case of indium oxide films.

Indium oxide is a material with unique properties that make it an interesting subject of study. When cooled to low temperatures, it becomes a superconductor, allowing electrons to move without resistance, a phenomenon that has fascinated scientists for decades. Furthermore, indium oxide exhibits multiple types of disorders at different levels, which contribute to its unusual behavior. The research team sought to explore these properties further by creating thin films of indium oxide and applying microwave spectroscopy to measure the superfluid stiffness as the material underwent a phase transition.

Their results were striking. Unlike the gradual, continuous change in superfluid stiffness that is typically seen when superconductors transition to insulating states, the team observed a sharp and unexpected drop in superfluid stiffness during the phase transition in indium oxide films. This sharp change deviates from the expected smooth curve and presents an intriguing puzzle for the researchers. To add to the mystery, the critical temperature at which the phase transition occurred was not determined by how strongly the Cooper pairs—pairs of electrons that form during the superconducting state and move in a coordinated fashion—were bound together. Instead, the researchers found that the critical temperature was governed by the superfluid stiffness, a relationship that had not been previously observed.

Cooper pairs are at the heart of the phenomenon of superconductivity. These electron pairs, which form under certain conditions, move in sync and allow for the superconducting state. In most superconducting materials, the transition from superconductivity to an insulating state is thought to depend largely on the interactions between these pairs. However, the behavior of indium oxide films in this study suggests that the phase transition may be governed by a different mechanism—superfluid stiffness, rather than the motion of Cooper pairs.

This new finding has profound implications for the study of quantum materials. For one, it highlights the potential role of superfluid stiffness in determining the properties of superconducting materials, something that has not been extensively explored in the past. Additionally, the sharp, unexpected transition observed in the indium oxide films opens up new avenues for research into the stability of quantum materials. The unusual behavior of the material during phase transitions could point to new ways of manipulating and controlling quantum systems, with applications in quantum computing, material science, and condensed matter physics.

While the researchers were not able to provide a concrete explanation for why superfluid stiffness dictates the critical temperature in this case, the discovery itself is significant. It presents an anomaly that could lead to deeper insights into the behavior of quantum materials. The study of phase transitions is central to the understanding of quantum systems, and the team’s work with indium oxide films offers a fresh perspective on how materials might behave under extreme conditions.

Furthermore, this research could potentially lead to new techniques for stabilizing quantum materials. Stability is one of the key challenges in the development of quantum technologies, such as quantum computers, which require extremely stable environments to function properly. Understanding the role of superfluid stiffness in phase transitions could offer a pathway to more stable quantum materials, enhancing their practical applications in technology.

Reference: Thibault Charpentier et al, First-order quantum breakdown of superconductivity in an amorphous superconductor, Nature Physics (2025). DOI: 10.1038/s41567-024-02713-8