Exotic superconducting state lurks at an astonishingly high magnetic field

Superconductivity and magnetism don’t usually mix. When a superconductor is placed in a magnetic field, it expels the field from its bulk through the Meissner effect; a strong enough field destroys the superconducting state entirely. In the vast majority of superconductors, electrons form spin-singlet pairs, with s– or d-wave symmetry, that are twisted apart by the field. Even the rare p-wave, spin-triplet superconductors (such as strontium ruthenate; see Physics Today, December 2006, page 23 ) are limited in how strong a magnetic field they can tolerate.

Last year the list of unusual superconductors grew by one, when Nicholas Butch and colleagues at NIST and the University of Maryland discovered spin-triplet superconductivity in uranium telluride, or UTe₂. (The paper reporting their results, although submitted in October 2018, wasn’t published until this August; in the intervening time, the discovery was confirmed by a team of researchers at Tohoku University in Japan and Grenoble Alps University in France.)

It was clear from the start that something unconventional was going on. When Butch and colleagues applied a magnetic field along the crystalline material’s a– or c-axis, superconductivity broke down, as expected. But for a field along the b-axis, the material remained superconducting up to 20 T, the strongest field they could create in their lab.

To continue their experiments, the researchers turned to the National High Magnetic Field Laboratory facilities in Tallahassee, Florida, and Los Alamos, New Mexico, where they could study the effects of fields up to 65 T—a field so strong it can be sustained for pulses of only 25 ms at a time without destroying the magnet. Their results are shown in the figure. The material’s high critical field along its b-axis turned out to be a sign of a separate, “reentrant” superconducting phase, so called because the field destroys superconductivity and then immediately reestablishes it.

But the surprises kept coming. At higher fields still, UTe₂ undergoes a magnetic phase transition in which its magnetization along the b-axis abruptly doubles. And inside that field-polarized phase hides yet another superconducting phase. (The French–Japanese group has also reported the reentrant phase and the magnetic phase transition , but not the field-polarized superconducting phase.)

It’s not yet known how or why the field-polarized superconducting phase exists, by what mechanism the electrons pair up at such a high field, or why they do so only for fields at a particular range of angles between the b– and c-axes. But Butch and colleagues have one more curious observation: The critical temperatures for the field-polarized superconducting phase and UTe₂‘s ordinary low-field superconducting phase are almost the same, at about 1.6 K. (The reentrant phase has a somewhat lower critical temperature.) Whatever the high-field pairing mechanism, it happens on an energy scale similar to to the onset of zero-field superconductivity. (S. Ran et al., Nat. Phys., 2019, doi:10.1038/s41567-019-0670-x ; thumbnail image credit: Emily Edwards/University of Maryland.)