Traditionally, superconductivity has only manifested in materials cooled to temperatures hovering near absolute zero, rendering widespread application impractical and costly. Breakthroughs began to emerge in the late 20th century, with discoveries of high-temperature superconductors, such as copper-oxide ceramics, which can superconduct at temperatures above the boiling point of liquid nitrogen (77 K). Although this was a significant leap, the ultimate goal remained elusive: superconductivity at or near room temperature under manageable conditions.

A transformative development came with the advent of hydrogen-rich metallic compounds, particularly hydrogen sulfide (H₃S) and lanthanum decahydride (LaH₁₀). These materials demonstrate superconductivity at unprecedentedly high temperatures of 203 Kelvin (-70°C) and 250 Kelvin (-23°C), respectively, when subjected to enormous pressures exceeding one million times atmospheric pressure. These transition temperatures, well above that of liquid nitrogen, have captivated researchers worldwide, ushering in a new class of "high-temperature" superconductors that hint at the possibility of room-temperature superconductivity.

Central to understanding this phenomenon is the superconducting gap, a quantum mechanical property that defines the energy required to break the electron pairs—known as Cooper pairs—that facilitate resistance-free conductivity. This gap acts as a fingerprint of the superconducting state, offering critical information on the strength and nature of the interaction between electrons and lattice vibrations (phonons). Unraveling the precise characteristics of this gap is vital for decoding the mechanism underpinning superconductivity in these sophisticated materials.

However, probing the superconducting gap in hydrogen-rich compounds like H₃S presents a formidable challenge. Their synthesis demands extremely high pressures, conditions that render conventional measurement techniques such as scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy ineffective. The extraordinary environment makes direct experimental access to the superconducting state tremendously difficult, limiting understanding of these materials’ microscopic properties.

Addressing this barrier, scientists at the Max Planck Institute in Mainz developed a novel planar electron tunneling spectroscopy method capable of operating under such extreme conditions. This breakthrough technique was successfully applied to H₃S, marking the first direct observation of its superconducting gap. This accomplishment not only provides vital experimental validation for theoretical models but also opens the door to comprehensive studies of other complex hydride superconductors created under ultrahigh pressures.

The experimental data reveal that H₃S possesses a fully open superconducting gap measuring approximately 60 millielectronvolts (meV), a value that strongly signifies a robust pairing mechanism. In comparison, the deuterium analogue D₃S exhibits a smaller gap around 44 meV. Deuterium’s heavier isotope nature confirms that electron-phonon coupling is the driving force behind superconductivity in these systems, affirming longstanding theoretical predictions regarding lattice vibrations facilitating electron pairing in hydrides.

This discovery provides pivotal insights into the fundamental mechanisms of hydrogen-based high-temperature superconductors. By confirming phonon-mediated electron pairing through isotope substitution, researchers can better understand the requisites for high critical temperatures. Importantly, this knowledge forms a solid groundwork upon which scientists can explore new hydrogen-rich materials with the potential to reach or even exceed room temperature superconductivity.

The implications of this progress extend beyond pure science. Unlocking room-temperature superconductivity could enable transformative applications, such as highly efficient power grids free from transmission losses, ultra-compact and fast quantum computers, and revolutionary magnetic levitation transport systems. The key lies in engineering materials that sustain superconductivity at ambient pressures, making them accessible and practical beyond specialized laboratory conditions.

Leading figures in the field have heralded this research as a watershed moment. The late Dr. Mikhail Eremets, a pioneer recognized for his seminal work on high-pressure superconductivity, described the study as the most significant since the initial discovery of superconductivity in H₃S in 2015. His visionary work laid the foundation for exploring hydrogen-rich compounds under pressure as promising routes toward high-temperature superconductivity, a dream now one step closer to reality thanks to these new findings.

Dr. Vasily Minkov, head of High-Pressure Chemistry and Physics at the Max Planck Institute for Chemistry, emphasized that this advancement aligns perfectly with Eremets’ decades-long vision of pragmatic superconductors operating at manageable pressures and temperatures. The refined tunneling technique is poised to become a critical tool for future explorations, enabling systematic investigations across a broader range of hydrides and beyond.

Fundamentally, the superconducting gap encapsulates the quantum essence of the superconducting phase. When electrons form Cooper pairs at temperatures below the critical temperature (T_c), they condense into a macroscopic quantum state with zero electrical resistance. The gap quantifies the energy threshold to disrupt these pairs, directly linking to the material’s superconducting robustness. Its symmetry and magnitude provide vital clues about the nature of electron interactions and pairing mechanisms, insights that are indispensable for material design.

Since the initial discovery of superconductivity in mercury by Heike Kamerlingh Onnes in 1911, scientific understanding has continuously advanced. The high-temperature cuprates discovered by Bednorz and Müller in the 1980s shattered earlier paradigms but still fell short of room temperature operation. Hydrogen-rich hydrides, through their distinctive lattice dynamics and electron-phonon interactions under pressure, represent the cutting edge of this quest, suggesting that room-temperature superconductivity may finally emerge within reach.

Looking ahead, researchers aim to extend the new tunneling spectroscopy technique to study additional hydride superconductors and other exotic compounds synthesized at ultrahigh pressures. The nuanced information gained from detailed gap measurements will illuminate the pathways to optimize electron pairing and material stability. This scientific journey holds the promise of uncovering materials with the ideal balance of temperature, pressure, and practical usability for future technologies.

In essence, this pioneering study heralds a new chapter in superconductivity research by providing the first direct measurement of the superconducting gap in H₃S under extreme conditions. It validates theoretical frameworks and solidifies our understanding of the quantum state in high-temperature hydrides. The aspiration for widespread, ambient-condition superconductors, once a distant dream, edges closer to tangible reality, promising profound technological impact in the upcoming decades.

Subject of Research: Not applicable

Article Title: Superconducting gap of H3S measured by tunnelling spectroscopy

News Publication Date: 23-Apr-2025

Web References: http://dx.doi.org/10.1038/s41586-025-08895-2

References: Nature, DOI: 10.1038/s41586-025-08895-2

Image Credits: Not provided

Keywords

Superconduction, Room temperature, Hydrogen

Hydrogen sulfide, a simple molecule when composed as H₃S under high pressures, made headlines several years ago owing to its remarkable superconducting transition temperature (T_c) exceeding 200 K. This stunning discovery created a paradigm shift by illustrating that conventional electron-phonon mechanisms, long thought incapable of producing superconductivity at such formidable scales, might indeed underlie these unprecedented critical temperatures. However, despite theoretical predictions and indirect experimental indications, the direct spectroscopic characterization of the superconducting gap — a hallmark of the ordered state — remained elusive until now.

The superconductor’s energy gap, often dubbed the order parameter, embodies the energy scale at which electrons pair up to form Cooper pairs and condense into a superconducting phase. Detecting and measuring this gap with high precision stands as a cornerstone to confirming the nature of superconductivity, distinguishing between conventional phonon-mediated mechanisms and more exotic pairing scenarios such as those involving spin fluctuations or unconventional symmetries. In the case of hydrogen sulfide, tunneling spectroscopy, which probes electronic states near the Fermi level with exquisite energy resolution, has successfully captured this spectral fingerprint for the first time.

According to the recent report by Du, Drozdov, Minkov, and collaborators, tunneling measurements revealed a superconducting gap value of approximately 30 millielectronvolts (meV) for the H₃S sample designated as S1. This value, although substantial, intriguingly falls below the magnitude anticipated by current theoretical frameworks, which had predicted larger gap amplitudes based on strong electron-phonon coupling models. Additionally, the so-called 2Δ/k_BT_c ratio — a dimensionless parameter that scales the gap with respect to the critical temperature — was measured at 3.54, aligning closely with the classic Bardeen-Cooper-Schrieffer (BCS) theory expectation for weak to moderate coupling superconductors.

This apparent discrepancy between empirical data and theoretical predictions opens a compelling dialogue within the superconductivity community. While the isotope effect observed, involving substitution with deuterium to form D₃S, and the consistency with an s-wave symmetry gap strongly point towards phonon-driven pairing interactions, the reduced gap magnitude invites more nuanced scrutiny. This could suggest that current theoretical treatments, though sophisticated, might yet lack the full complexity of the actual interactions or structural inhomogeneities present in these pressurized samples.

Moreover, the investigations uncovered evidence of a multigap superconducting scenario within inhomogeneous hydrogen sulfide specimens. Multigap superconductivity, wherein distinct gaps coexist on different parts of the Fermi surface or in spatially segregated superconducting phases, represents a richer and more intricate state than single-gap models. Its presence here highlights the heterogeneous nature of high-pressure hydrides and the necessity for comprehensive studies addressing phase separation, crystal structure variations, and their influence on superconducting parameters.

The implications of these findings extend far beyond hydrogen sulfide alone. The successful application of tunneling spectroscopy to dissect the superconducting gap in such challenging experimental conditions underscores the technique’s potency as a diagnostic tool. It paves the way for analogous explorations across the broader family of metal superhydrides and related materials. Understanding their superconducting gap structures with high fidelity is vital for decoding the mechanisms responsible for their often remarkably high critical temperatures and for guiding the discovery of new compounds operable at lower pressures.

These developments dovetail with a wider scientific pursuit to link microscopic interactions with macroscopic superconducting properties. Resolving the nature of electron-phonon coupling strength, anisotropies in the order parameter, or the presence of competing phases can decisively inform the direction of theoretical modeling and synthetic efforts. Furthermore, such insight contributes to the ultimate goal of engineering materials capable of attaining room-temperature superconductivity under ambient or technologically feasible conditions.

The interplay between theory and experiment witnessed in this work exemplifies the dynamic evolution of superconductivity research. The direct experimental detection of a superconducting gap, particularly with high resolution and under multi-megabar pressures, sets a new benchmark. Yet it also raises new questions, such as the precise origin of the discrepancy between observed and predicted gap magnitudes and the full character of the multigap phenomena manifesting in these complex hydride systems.

Expanding the scope of this research to include systematic isotopic substitution and pressure-dependent studies could unravel further subtleties governing pairing interactions. Additionally, complementary spectroscopic techniques like angle-resolved photoemission or neutron scattering may illuminate collective excitations and electronic structure modifications concomitant with superconductivity. Such multi-pronged approaches will build a more comprehensive picture of these enigmatic states.

In sum, the first unambiguous tunneling spectroscopic identification of the superconducting gap in hydrogen sulfide marks a seminal advance in high-pressure superconductivity science. It solidifies the central role of phonon-mediated Cooper pairing in sustaining superconductivity at record-high temperatures within this material family. Simultaneously, the nuanced deviations from theoretical expectations beckon deeper inquiries into material-specific complexities and the quest for materials surpassing current performance benchmarks.

As experimental methods continue to improve and theoretical models become increasingly sophisticated, the path toward realizing superconductors functional at ambient conditions grows ever clearer. Hydrogen sulfide and its hydride cousins today exemplify the remarkable achievements borne from this synergy of experimental innovation and intellectual exploration, promising a future where lossless electrical conduction could revolutionize energy, transportation, and beyond.

Subject of Research: Superconductivity and superconducting gap characterization in high-pressure hydrogen sulfide (H₃S).

Article Title: Superconducting gap of H₃S measured by tunnelling spectroscopy.

Article References:
Du, F., Drozdov, A.P., Minkov, V.S. et al. Superconducting gap of H₃S measured by tunnelling spectroscopy. Nature (2025). https://doi.org/10.1038/s41586-025-08895-2

Image Credits: AI Generated