Disorders in a superconductor trigger strange quantum behavior

A new study reveals that highly disordered indium oxide superconductors undergo a first-order quantum phase transition — a sudden shift from a superconducting to an insulating state. 

This finding is quite incredible because such sudden shifts are not observed in superconductors. They typically experience second-order transitions, which are slow and gradual. 

The rare and abrupt phase transitions observed in this study could guide the design of materials to enhance the stability and efficiency of quantum systems. 

“We show a departure from the general paradigm, in which a discontinuous first-order quantum phase transition is tuned by the disorder,” the study authors note. 

An unexpected drop in superfluid stiffness

Phase transitions are changes in the state of a material, like from solid to liquid or from superconducting to insulating. Such changes happen when certain conditions, such as temperature or pressure, cross a critical point.

However, there’s one more property that affects phase transition, called superfluid stiffness. It is the measure of how resistant a material’s superconducting state is to changes in phase, playing a key role in understanding how superconductivity breaks down during phase transitions.

Generally, when superconductors undergo phase transition, superfluid stiffness decreases continuously and smoothly. However, the researchers observed something unusual when they examined amorphous indium oxide films.

Indium oxide has several structural, chemical, and atomic-level disorders. To understand how these disorders could be fine-tuned, they measured how the material behaved as it became more disordered. 

For this, they employed microwave spectroscopy, a technique that uses microwaves to study the internal properties and behavior of a material. The spectroscopy helped them accurately measure the superfluid stiffness of indium oxide. 

Surprisingly, instead of a gradual change, they observed an unexpectedly sharp drop in the superfluid stiffness of indium oxide films.  

The film entered a special state

A material behaves like a superconductor when a pair of electrons called a Cooper pair moves together in a coordinated fashion. During their study, the researchers found that when disorder is introduced into the material, it causes Cooper pairs to behave unusually. 

Normally, Cooper pairs help a material conduct electricity without resistance (superconductivity), but with enough disorder, the Cooper pairs start to compete with each other. This results in a conflict between the superconducting state and another state called insulating Cooper-pair glass.

The study authors also noticed that the temperature at which the films lost their superconducting ability (the critical temperature) was no longer determined by how strongly electrons paired up but by the superfluid stiffness itself. 

This means that the material entered a pseudogap regime, a special state in which electron pairs form but don’t behave in a coordinated way to maintain superconductivity. 

“We show that the critical temperature of the films no longer relates to the pairing amplitude but aligns with the superfluid stiffness, consistent with the pseudogap regime of preformed Cooper pairs,” the study authors said

The pseudogap state is a critical phase in some quantum materials, particularly high-temperature superconductors, because it helps explain their behavior and potential for use in quantum technologies.

The study is published in the journal Nature Physics.