Scientists unlocked a superconductor mystery under crushing pressure

The challenge is that superconductivity usually occurs only at very low temperatures, far below everyday conditions. This limitation has prevented widespread practical use. That picture began to change with the discovery of superconductivity in hydrogen-rich materials. Hydrogen sulfide (H₃S) becomes superconducting at 203 Kelvin (-70°Celsius), while lanthanum decahydrid (LaH10) reaches superconductivity at 250 Kelvin (-23°Celsius). These temperatures are far higher than those of earlier superconductors and are above the boiling point of liquid nitrogen, which is why scientists classify them as high temperature superconductors. Their discovery marked a major step toward the long-standing goal of room-temperature superconductivity.

The Superconducting Gap and Why It Is Crucial

At the heart of superconductivity is a feature known as the superconducting gap. This property reveals how electrons join together to form the superconducting state and serves as a clear signature that distinguishes a superconductor from an ordinary metal.

Understanding the superconducting gap is essential because it directly reflects how electrons interact inside the material. Without measuring this gap, scientists cannot fully explain why a material becomes superconducting or what mechanism makes resistance disappear.

Why Measuring Hydrogen Superconductors Is So Difficult

Despite their importance, hydrogen-rich superconductors such as H₃S have been extremely challenging to study. These materials can only be created under enormous pressures that exceed atmospheric pressure by more than a million times. Because of these extreme conditions, widely used techniques like scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy cannot be applied.

As a result, the superconducting gap in these materials had remained unmeasured, leaving a major gap in scientists’ understanding of how high-temperature superconductivity works in hydrogen-rich compounds.

A New Tunneling Technique Breaks the Barrier

To solve this problem, researchers at the Max Planck Institute in Mainz developed a planar electron tunneling spectroscopy method that can operate under these extreme pressures. This new approach made it possible to directly probe the superconducting gap in H₃S for the first time.

With this technique, the team obtained a clear picture of the superconducting state in hydrogen-rich materials, overcoming a barrier that had limited progress in the field for years.

What the Measurements Revealed

The researchers found that H₃S has a fully open superconducting gap of approximately 60 millielectronvolt (meV). They also studied its deuterium counterpart, D₃S, which showed a smaller gap of about 44 meV. Deuterium is a hydrogen isotope and has one more neutron.

This difference is significant because it confirms that superconductivity in H₃S is driven by interactions between electrons and phonons. Phonons are quantized vibrations of a material’s atomic lattice. The results support long-standing theoretical predictions about the mechanism behind superconductivity in hydrogen-rich compounds.

Why This Breakthrough Matters

For the researchers in Mainz, the achievement goes beyond technical success. It provides a foundation for uncovering the fundamental origins of high-temperature superconductivity in hydrogen-based materials. “We hope that by extending this tunneling technique to other hydride superconductors, the key factors that enable superconductivity at even higher temperatures can be pinpointed. This should ultimately enable the development of new materials that can operate under more practical conditions,” states Dr. Feng Du, first author of the now published study.

Dr. Mikhail Eremets, a leading figure in high-pressure superconductivity research who deceased in November 2024, described the study as “the most important work in the field of hydride superconductivity since the discovery of superconductivity in H₃S in 2015.” Vasily Minkov, project leader of High-Pressure Chemistry and Physics at the Max Planck Institute for Chemistry, added: “Mikhail´s vision of superconductors operating at room temperature and moderate pressures comes a step closer to reality through this work.”

A Brief History of Superconductivity

Superconductivity refers to the ability of certain materials to conduct electrical current without resistance. It was first discovered in pure mercury in 1911 by Heike Kamerlingh Onnes. For many decades, scientists believed this phenomenon could only occur at temperatures near absolute zero (-273 °C).

That assumption changed in the late 1980s when Georg Bednorz and Karl Alexander Müller discovered copper-oxide superconductors, also known as cuprates, that exhibited high-temperature superconductivity under normal atmospheric pressure. This discovery sparked worldwide research efforts.

Over time, scientists reached critical temperatures (T_c) of about 133 K at ambient pressure and 164 K under high pressure. Progress then stalled until hydrogen-rich compounds entered the picture.

Hydrogen-Rich Materials Push the Limits

The discovery of superconductivity in H₃S at megabar pressures, with a T_c = 203 K by the research group led by Dr. Mikhail Eremets, represented a turning point. Soon after, even higher critical temperatures were observed in hydrogen-rich metal hydrides such as YH9 (T_c ≈ 244 K) and LaH10 (T_c ≈ 250 K).

Current theoretical models now suggest that superconductivity above room temperature may be possible in several hydrogen-dominated systems when subjected to extreme pressure.

Cooper Pairs and the Meaning of the Superconducting Gap

In normal metals, electrons near the Fermi level can move freely. The Fermi level represents the highest energy level electrons can occupy in a solid at absolute zero. When a material becomes superconducting, electrons instead form paired states known as Cooper pairs and enter a collective quantum state.

In this state, paired electrons move together without scattering off phonons or impurities in the crystal lattice, which eliminates electrical resistance. This pairing creates an energy gap near the Fermi level called the superconducting gap. The gap represents the minimum energy required to break a Cooper pair and plays a protective role by stabilizing the superconducting state against disturbances.

The superconducting gap is a defining feature of superconductivity. Its size and symmetry provide critical insight into how electrons interact and pair, making it a key fingerprint of the underlying superconducting mechanism.