Materials known as superconductors transmit electrical energy with 100% efficiency. They have a wide range of applications, such as magnetic resonance imaging in hospitals. However, these applications have been hampered, largely because the superconducting state exists only at temperatures well below room temperature (295 kelvin). Writing in Nature, Drozdov et al.1 report several key results that confirm that, when compressed to pressures of more than one million times Earth’s atmospheric pressure, lanthanum hydride compounds become superconducting at 250 K — a higher temperature than for any other known material.
Superconductivity was first discovered2 in 1911, in mercury cooled below 4 K. The temperature below which a material becomes superconducting is called the critical temperature. It was quickly appreciated that a state exhibiting zero electrical resistance could be tremendously useful, if materials that have critical temperatures much higher than 4 K could be found. Over the past century, as more superconductors have been discovered, the record for the highest critical temperature achieved has progressed, in fits and starts, towards the ultimate goal of room temperature.
In 2014, some of the authors of the current paper and their colleagues shattered the previous record3 of 164 K. They found that hydrogen sulfide — a chemical responsible for the odour of rotten eggs — transforms into a superconductor at a temperature of about 200 K, when compressed to nearly 2 million times atmospheric pressure4,5. Then, in 2018, two independent research groups reported, almost simultaneously, that compressed lanthanum hydride compounds might exhibit superconductivity at even higher temperatures, ranging from 215 K to perhaps as high as 280 K (refs 6–8)6–8.
The common features of these hydrogen sulfide and lanthanum hydride superconductors are that they are rich in hydrogen and that superconductivity emerges only under pressures above about one million times atmospheric pressure. Under these extreme conditions, chemical bonds can be greatly altered, inducing the formation of compounds that are otherwise unstable. In the case of lanthanum hydride, high pressure seems to stabilize the formation of a compound, LaH10, that has a much larger hydrogen content than is achievable at ambient pressure9,10.
Drozdov et al. reached these extraordinarily high pressures (roughly half the pressure at Earth’s core) using a device called a diamond anvil cell. This device could fit in the palm of your hand and generates pressure by squeezing a sample, contained by a thin metal foil, between two flattened diamonds (Fig. 1). This configuration severely limits the types of measurement that can be carried out, because the sample is tiny (of the order of 0.01 millimetres across) and is surrounded on all sides by the comparatively massive foil and diamonds. Moreover, to obtain electrical measurements, electrical leads need to make contact with the sample, but must remain electrically isolated from the foil.
The authors overcame these experimental challenges and uncovered crucial evidence that confirms the high-temperature superconductivity in lanthanum hydride compounds. To verify that a material is superconducting, researchers typically look for three features: zero electrical resistance; a reduction in the critical temperature under an applied magnetic field; and an expulsion of magnetic fields from the interior of the material on cooling (a phenomenon known as the Meissner effect). Drozdov et al. detected the first two of these signatures of superconductivity. The final criterion — observation of the Meissner effect — is currently unattainable because the samples are too small.
The search for high-temperature superconductivity in hydrogen-rich compounds can be linked to predictions11 made in 2004. The reasoning behind these predictions hinges on a theory that forecasts that, under certain circumstances, elements that have low atomic masses can contribute to high critical temperatures. Hydrogen, being the lightest element, is optimal for high critical temperatures. And, by this logic, replacing hydrogen with the heavier isotope deuterium should lower the critical temperature. Drozdov et al. observed this isotope effect and found that, compared with the lanthanum hydride samples, the critical temperature in lanthanum deuteride samples is lower by almost exactly the amount predicted by the theory.
From a scientific standpoint, these results suggest that we might be entering a transition from discovering superconductors by empirical rules, intuition or luck to being guided by concrete theoretical predictions. The superconducting critical temperature has long been considered to be one of the most difficult properties to compute accurately. But the hydrogen sulfide and lanthanum hydride experiments were motivated by computational results that anticipated both the necessary pressures and the resulting critical temperatures9,12. These remarkable successes for theory seem to be driven by innovative computational methods that were enabled by advances in computing power.
What is the practical importance of superconductivity in materials synthesized in minuscule quantities at more than one million times atmospheric pressure? The answer depends on whether the superconducting states can be recovered at ambient pressure. Diamond itself is an example of a material that forms at high pressure but is metastable at ambient pressure. The effort to manufacture synthetic diamond provided substantial motivation for the development of high-pressure methods. Today, however, synthetic diamonds are grown using a low-pressure technique called chemical-vapour deposition. Optimistically, it might eventually be possible to use similar low-pressure methods to produce metastable superconducting compounds that are initially discovered at high pressure.
In the next few years, experiments will probably focus on searching for superconductivity in other pressurized hydrogen-rich materials. Given that only a small fraction of possible hydrogen-rich systems have been subjected to experiments at these tremendous pressures, it seems more likely than ever that the dream of room-temperature superconductivity might be realized in the near future. At that point, the grand challenge will shift from pushing the necessary temperatures higher to pushing the required pressures lower.
Nature 569, 491-492 (2019)