Experimental results show that the pseudogap long seen as a central challenge in cuprate high-temperature superconductors is directly linked to electron-pair formation. The evidence supports the possibility that electrons are already paired before superconductivity emerges, adding a new clue to a long-running debate over how high-temperature superconductivity works.

A joint research team at Ludwig Maximilian University of Munich in Germany and Utrecht University in the Netherlands measured shot noise, a minute electrical noise generated when current flows, and captured traces of electron-pair formation inside a cuprate superconductor, IT outlet Ars Technica reported on Nov. 8.

Cuprate superconductors are a class of materials that show superconductivity, in which electrical resistance disappears, at relatively high temperatures that can be achieved using liquid nitrogen cooling. About 40 years after their discovery, why superconductivity appears at higher temperatures than in conventional superconductors remains unclear.

One of the central challenges is the pseudogap. It is a phenomenon in which electron states in a certain energy range appear to vanish even at temperatures above the superconducting transition temperature. Researchers have debated whether the pseudogap arises because electrons form an ordered pattern such as a charge density wave, or because electrons pair up before entering the superconducting state.

To distinguish between the two explanations, the team focused on shot-noise measurements. Shot noise is tiny noise generated as charge carriers that make up an electric current move in discrete steps. The noise differs depending on whether electrons move one by one or whether two electrons move together as a pair. The team said analysing it can confirm whether electron pairs form.

Using a scanning tunnelling microscope, the researchers drove a current between the tip of a needle and a cuprate superconductor sample and measured changes in noise by surface location and energy. The experiment used a cuprate crystal known as BSCCO.

The results showed that the noise increase indicating electron-pair formation did not appear only in the superconducting state. Noise also rose clearly in the pseudogap energy range observed above the superconducting transition temperature. The findings suggest electrons are already paired in the region where the pseudogap forms.

The researchers also verified the signal through magnetic-field experiments. When they applied a magnetic field of up to 6 tesla to suppress electron-pair formation, the noise fell to the level seen when single electrons move. By contrast, the overall pseudogap pattern, in which electrons appear to vanish, largely remained.

This shows it is difficult to explain the pseudogap simply as a phenomenon in which charge arranges itself in a fixed pattern. The results support an interpretation that electron-pair formation is closely linked to the pseudogap.

The study also raises the possibility that in high-temperature superconductors, electron-pair formation and actual superconductivity can occur in different stages. In conventional superconductors, electron pairing and phase coherence, in which the pairs move like a single quantum state, appear together near the critical temperature.

In this experiment, however, the energy required to break electron pairs was observed even in a range far higher than the level corresponding to the actual superconducting transition temperature. The picture is that electrons form pairs first, and when the temperature drops further these pairs gain phase coherence and begin to move collectively.

This suggests that achieving high-temperature superconductivity may be constrained not only by the process of creating electron pairs but also by the process that makes already formed pairs move coherently with each other.

The researchers stressed that the results do not fully reveal the nature of the pseudogap. The possibility remains that fixed charge patterns such as charge density waves may work together with electron-pair formation.

There are also limitations in the experimental technique. With the current measurement method, sensitivity falls as temperature rises, limiting direct confirmation of electron-pair signals above the superconducting transition temperature. The researchers plan to develop technology to measure noise at higher temperatures and equipment to directly detect phase coherence, to identify factors that govern the superconducting transition process and limit the critical temperature.