Unmasking the record-setting sulfur hydride superconductor

Hydrogen sulfide (H₂S) is the noxious byproduct of anaerobic digestion that gives swamps their characteristic smell. In 2014 Mikhail Eremets of the Max Planck Institute for Chemistry and his colleagues set out to test a prediction of pressure-induced metallization and superconductivity in the malodorous compound. ¹

At a pressure of 100 GPa, the Max Planck group observed the hoped-for disappearance of resistance on cooling their sample below 60 K. But when they further raised the pressure to 150 GPa, the superconducting transition temperature T_c, instead of dropping as theory predicted, shot up to 190 K. Not only had the researchers shattered the previous record for T_c, held by a cuprate superconductor, by 30 K, but they had done so with a conventional phonon-mediated superconductor.

News of the group’s discovery set off a flurry of theoretical work to explain the finding. ² In the meantime, Eremets and his colleagues extended their work ³ to include magnetic measurements that demonstrated a Meissner effect—the expulsion of external magnetic fields that is taken as a definitive sign of superconductivity. They also raised T_c to 203 K, 8 K greater than the sublimation point of dry ice.

Despite the discovery, the identity of the superconductor was in doubt. The surprising rise in T_c at pressures greater than 100 GPa made Eremets and his colleagues suspect their superconductor was not H₂S but a different sulfur hydride left from the pressure-induced dissociation of H₂S.

As it turned out, Tian Cui and his team at Jilin University in China had independently predicted—before the Max Planck group’s announcement—that H₃S should harbor superconductivity around 200 K at 200 GPa. ⁴ Other theorists quickly came to the same conclusion.

To experimentally identify the superconductor, Eremets’s group teamed up with Katsuya Shimizu and colleagues at Osaka University to perform simultaneous resistance and x-ray diffraction measurements. ⁵ Their results confirm the theorists’ prediction that the superconductor is indeed H₃S.

Unconventionally conventional

Intriguingly, superconductivity in H₃S is of the conventional variety. That is, it’s described by the venerable Bardeen-Cooper-Schrieffer (BCS) theory, which explains how pairs of electrons are nudged together by lattice vibrations (phonons) to become Cooper pairs. When the Max Planck researchers replaced hydrogen with deuterium, they observed a telltale sign of BCS superconductivity, an isotope effect: The greater mass of the deuterium atom lowers the phonon frequency and, with it, T_c.

The BCS theory has a deceptively simple recipe for achieving high T_c: Create a high density of conduction-electron states and couple the conduction electrons to high-frequency phonons. But before H₃S, the highest-temperature BCS superconductor was magnesium diboride, with a T_c of 40 K. (See the article by Paul Canfield and George Crabtree, Physics Today, March 2003, page 34 .)

High-frequency phonons come from light elements, and hydrogen is the lightest of all elements. The search for metallic hydrogen has been ongoing since 1935, when Eugene Wigner and Hillard Huntington predicted that hydrogen would become metallic under pressure. Neil Ashcroft added extra motivation in 1968 when he used BCS theory to argue that metallic hydrogen would be a high-temperature superconductor. Signs of a metallic fluid phase of hydrogen have been spotted (see Physics Today, September 2015, page 12 ) but metallicity in solid hydrogen remains an elusive goal.

In 2004 Ashcroft chimed in again with the idea of looking at crystals of hydrogen-rich molecules. He reasoned that the hydrogen atoms in compounds such as silicon hydride are chemically precompressed, so the pressure required to metallize the hydrogen would be substantially less. Following Ashcroft’s prescription, researchers have studied various hydrides in search of metallized hydrogen and possibly high-temperature superconductivity. For the moment, H₃S with T_c = 203 K at 150 GPa is the hottest find from that search.

The big squeeze

High-pressure experiments are notoriously difficult. Eremets and company discovered that H₂S, a gas at room temperature, couldn’t simply be loaded into a diamond anvil cell and pressurized. The sample would decompose before reaching the required high pressures, and all the researchers would detect was elemental sulfur.

To make a stable sample, the diamond anvil cell had to first be cooled to 200 K. Then, as shown in figure 1, H₂S gas was passed through a capillary into the cell, where it liquefied. The top anvil was then pushed down to clamp some of the liquid inside a gasket. The cell was heated to 220 K to evaporate away the H₂S outside the gasket. Only then could the researchers increase the pressure.

For the x-ray diffraction experiments, the Max Planck team prepared four samples in Germany—two for H₂S and two for D₂S. Eremets hand-delivered the loaded diamond anvil cells to Japan.

Figure 2 shows the experimental setup at Japan’s SPring-8 synchrotron facility, where the x-ray diffraction measurements were performed. Because the sample is polycrystalline, the measured pattern comprises rings that correspond to particular scattering angles.

Integrating the intensities around the rings and plotting the result as a function of the scattering angle gives the graph shown in figure 3. The diffraction pattern contains the signatures of H₃S and elemental sulfur, the other byproduct of the dissociation of H₂S. By analyzing the scattering angles at which the peaks occur, the researchers could identify the crystal structure of H₃S.

At high pressure, superconducting H₃S has the cubic structure shown in the inset of figure 3. Because hydrogen atoms scatter x rays weakly, their positions in the crystal lattice couldn’t be precisely determined. That information will have to await other measurements, such as NMR, that can probe the hydrogen positions at high pressure.

Theory and experiment

Mari Einaga, the lead author of the new x-ray paper, sees renewed interest in BCS theory, which, she explains, “was largely abandoned because of the discovery of cuprates and other unconventional superconductors.” Theorists have developed sophisticated computational tools based on density functional theory to search for stable compounds and predict their crystal structures. And unlike unconventional superconductivity, whose origin remains uncertain, BCS superconductivity is understood well enough that its likelihood and T_c can be predicted.

To appreciate the accuracy with which theorists can calculate the electronic structure of hypothetical materials, one need only revisit the early predictions for H₃S. “Amazingly, all the calculations predicted T_c around 200 K,” says Eremets. “Basically, all our results were quickly and consistently explained.”

Einaga notes, “Our work was initiated by theory, and further understanding of the superconductivity is the result of close interplay between theory and experiment.” That successful partnership raises hopes that theory can guide future searches for new high-temperature superconductors.