In laboratories across the globe, modern alchemists pursue what may be the materials science equivalent of the philosopher's stone—a substance that conducts electricity without resistance at ordinary temperatures. The implications of such a discovery would ripple through civilization, from lossless power grids to quantum computers that don't require cryogenic cooling. Yet after more than a century of research since Heike Kamerlingh Onnes first observed superconductivity in mercury at 4.2 Kelvin, the ultimate prize remains tantalizingly out of reach.
"Superconductivity at room temperature would be to condensed matter physics what the Holy Grail was to medieval knights—an almost mythical object of desire."
Recent years have seen controversial claims and counter-claims about materials that might achieve this feat through unconventional mechanisms. Two approaches in particular have generated both excitement and skepticism:
The story begins in 2015, when researchers at the Max Planck Institute reported superconductivity at 203 Kelvin (-70°C) in hydrogen sulfide (H3S) under pressures of 150 gigapascals—about 1.5 million times atmospheric pressure. While still cryogenic, this temperature was significantly higher than previous records. The discovery sparked renewed interest in hydrogen-rich materials under pressure.
The theoretical framework for hydride superconductors was laid by Neil Ashcroft in the 1960s. His reasoning was elegant:
Since pure metallic hydrogen remains elusive at achievable pressures, attention turned to hydrogen-rich compounds where the hydrogen sublattice might mimic metallic hydrogen's behavior.
In 2018, another milestone was reached with lanthanum hydride (LaH10), which exhibited superconductivity up to 250 Kelvin (-23°C) at pressures around 170 GPa. The material forms a peculiar clathrate structure where lanthanum atoms create a framework containing hydrogen molecules.
| Material | Highest Reported Tc (K) | Required Pressure (GPa) | Year of Discovery |
|---|---|---|---|
| H3S | 203 | 150 | 2015 |
| LaH10 | 250 | 170 | 2018 |
| C-S-H system | 288 (15°C) | 267 | 2020* |
*Note: The carbonaceous sulfur hydride (C-S-H) result remains controversial and has not been independently replicated.
The Achilles' heel of hydride superconductors is their requirement for extreme pressures. Current diamond anvil cell technology can achieve these conditions only in microscopic samples for short durations. Practical applications would require:
Parallel to the high-pressure hydride efforts, another community explores entirely different materials—organic polymers that might achieve superconductivity through unconventional pairing mechanisms.
The field began in earnest with the discovery of superconductivity in (TMTSF)2PF6 in 1980, with a Tc of just 1.2 K under pressure. Progress was slow until the discovery of fullerene-based superconductors in the 1990s and charge-doped polymers more recently.
In the late 2000s, several groups reported anomalous conductivity in heavily doped polyacetylene and related polymers. The most provocative claim came from a Japanese group in 2017, reporting possible superconducting fluctuations up to 120 K in iodine-doped polyacetylene fibers. However, these results were met with skepticism because:
More promising results have emerged from other polymer systems:
The mechanisms in these materials are poorly understood but may involve:
The history of superconductivity research is littered with retracted claims and unreplicated results. The bar for proving superconductivity is appropriately high, requiring multiple measurement techniques to confirm:
| Criterion | Measurement Technique | Challenge in Novel Materials |
|---|---|---|
| Zero resistance | Four-point probe resistivity | Distinguishing from metallic behavior or measurement artifacts |
| Meissner effect | SQUID magnetometry | Small signal in granular or inhomogeneous samples |
| Heat capacity jump | Calorimetry | Difficult in high-pressure or small samples |
| Energy gap | Tunneling spectroscopy, ARPES | Surface vs bulk effects in complex materials |
The most controversial claims often fail on one or more of these criteria. For instance, the famous C-S-H room-temperature claim showed a resistance drop but no definitive Meissner effect, leaving open questions about whether the observation represented true superconductivity or some other electronic transition.
The field faces increasing scrutiny about reproducibility. A 2021 analysis found that:
This has led to calls for more rigorous standards in reporting new superconducting materials, including:
The Bardeen-Cooper-Schrieffer (BCS) theory has successfully explained conventional superconductors for over half a century. However, many researchers suspect that room-temperature superconductivity—if achievable—may require different mechanisms.
The hydride superconductors appear to fit an extended BCS picture where exceptionally strong electron-phonon coupling occurs in the hydrogen sublattice. Calculations suggest coupling constants (λ) as high as 2-3 in these materials compared to ~1 in conventional superconductors.
In polymers and other low-dimensional materials, excitonic or plasmonic mechanisms might mediate pairing without relying solely on phonons. These could involve:
A more exotic possibility involves topological protection of superconducting states, where edge or surface states remain superconducting even if the bulk does not. This could explain some observations in granular or inhomogeneous samples.
A key challenge is moving from serendipitous discovery to rational design. Promising directions include:
Theoretical work suggests certain design motifs may be particularly promising: