&Bullet; physics 14, 46

Theorists propose new strategies for designing high-temperature superconductors that operate at greatly reduced pressures – a step towards superconductors under ambient conditions.

One of the hottest physical results of 2020 was evidence that a hydrogen-rich material with no electrical resistance at. can guide

$15{\phantom{\rule{0.3em}{0ex}}}^{\circ }\text{C.}$

, the average temperature of a spring day in Paris. However, this performance required another condition that is not met on the banks of the Seine: a pressure of 267 GPa, which comes close to that of the Earth’s core. Researchers, including Lilia Boeri from the Sapienza University in Rome, are optimistic about easing this requirement. At the recent March meeting of the American Physical Society, Boeri presented two possible ways to achieve superconductivity in closer environmental conditions.

The path to room temperature superconductivity harks back to a 1960s prediction that hydrogen, if pressed sufficiently, could turn into a metal that is superconducting at high temperatures. While the pressure for hydrogen metallization is exceptionally high, the hunters of superconductivity sought similar effects at lower pressures with hydrogen-rich compounds called hydrides. The first hydride superconductor was finally discovered in 2015 and triggered a “hydride rush” that continues to break record after record for superconducting temperatures.

According to Boeri, much of this progress has been driven by theoretical advances over the past 15 years. Equipped with a new density functional theory for superconductors and with increasingly powerful crystal structure prediction methods, theorists have guided experiments by using the first principles to calculate which potential superconductors can be synthesized and at which critical temperatures their superconductivity begins.

These theories have guided efforts to increase the superconducting temperatures of hydrides and identify some key components. For example, highly symmetrical crystal structures, high-frequency phonons and “stiff” covalent bonds between the atoms can increase the strength of the electron-phonon coupling in the material and raise the critical temperature. Metallic hydrogen should have all these properties, but only at impermissible pressures. But with hydrides, the introduction of atoms into the lattice creates a “chemical” pressure that can compress the hydrogen lattice from within, reducing the external pressure required to synthesize the crystal and achieve superconductivity.

This chemical pressure can be achieved by combining hydrogen with larger atoms, and researchers have previously made hydride superconductors using elements such as lanthanum, sulfur, and yttrium. Theorists have already predicted critical temperatures for binary hydrides, which are made by combining hydrogen with almost any other element. But none of these materials are supposed to work at dramatically lower pressures, so Boeri and her team developed two new directions of research.

The first is to add a third element to the hydride, which increases the number of possible structures and thus the possibilities for optimizing chemical pressure. Boeri and co-workers studied a ternary hydride synthesized from lanthanum and boron. Their predictions suggest that such a compound should be both stable at 40 GPa and superconducting up to a temperature of 100 K.

That’s still massive pressure, but crossing the 100 GPa threshold would be a boon to the field, Boeri says. “I could just go to the lab next door and ask a chemist to synthesize this compound for me,” while few facilities in the world can do this when pressures of 100 GPa are required. That would speed up the conduct of experiments and allow a faster exchange of ideas between theorists and experimenters, which would make the hunt much faster, she says.

The second direction Boeri is pursuing is to replace the hydrogen with other light atoms. Boron and carbon materials also form covalent bonds and harbor phonons with frequencies suitable for high temperature superconductivity. Boeri’s team is studying a large database of boride and carbide structures with the aim of developing reliable formulas for predicting their critical temperatures. The projected temperatures are lower than those of hydrides, but these materials form stable crystals at ambient pressure, which makes processing easier.

This line of research is “fundamentally interesting,” says Marvin Cohen, a physicist at the University of California at Berkeley. Cohen has long believed that the secret to increasing superconductor temperature lies in covalent bonds, and says these new connections will allow this idea to be tested. And if the microscopic theories hold true for these systems and the researchers show a reduction in pressure to 40 GPa, as Boeri predicts, “there is no fundamental reason why they couldn’t go down to ambient,” he says.

– Matteo Rini

Matteo Rini is the editor of physics.

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