An international team of physicists has observed a rare phenomenon called re-entrant superconductivity for the first time in a two-dimensional material. In this effect, an applied magnetic field first destroys superconductivity, but as the field grows stronger, the superconducting state unexpectedly reappears.
The discovery, made by researchers led by Denis Maryenko at Japan’s RIKEN Center for Emergent Matter Science, came as a complete surprise. The team was not searching for the effect. «We did not look for the RSC, and it was extremely surprising that we saw that,» Maryenko told Physics World. The findings are reported in Science Advances.
Superconductivity is a state in which certain materials conduct electricity with zero resistance. In conventional superconductors, a magnetic field suppresses this state by breaking apart the paired electrons, known as Cooper pairs, that carry the current, or by introducing magnetic flux that creates resistance as it moves through the material. In a few unconventional materials, however, competition between ferromagnetic ordering, where magnetic moments align in the same direction, and superconducting ordering, where paired electron moments point in opposite directions, can produce re-entrant superconductivity.
Previously, re-entrant superconductivity had been observed in three-dimensional ferromagnetic materials such as CeRh2As2, organic superconductors, and possibly in heavy fermion compounds like UTe2. Two-dimensional materials offer advantages for studying superconductivity, including stronger spin-orbit coupling that locks electron spins perpendicular to the sample surface, which raises the upper critical field, the maximum magnetic field at which superconductivity can persist.
The RIKEN team built on earlier observations of two-dimensional superconductivity at interfaces between KTaO3 and other materials. They grew LaTiO3 on KTaO3 because the two materials have good lattice matching, which benefits epitaxial growth. The researchers then performed magnetotransport measurements on these heterostructures to see how the samples behaved under increasingly large magnetic fields.
«We just wanted to see how the critical magnetic field behaves as a function of temperature, but suddenly we started seeing something else emerging at a rather low field: a resistive peak that seemed to separate two superconducting regions,» Maryenko said. This cusp appeared at a magnetic field of 0.9 tesla, with zero resistance measured at both lower and higher applied fields. The field value was independent of temperature and charge carrier concentration, which the team controlled by tuning a gate voltage across the interface.
To explain the effect, collaborators performed ab initio calculations of the electron band structure at the interface. These calculations revealed a Van Hove singularity, a point where electrons have a very high density of states for certain momenta. A separate group developed a model based on spin-orbit coupling, drawing on symmetry arguments and the band structure calculations.
In the absence of an applied field, the model found a symmetry where electrons with opposite momenta have the same energy, so two electrons forming a Cooper pair have a total momentum of zero. This favors the formation of spin-singlet Cooper pairs. Applying a field breaks this symmetry, reducing pairing efficiency and lowering the critical temperature for superconductivity. At the same time, the field shifts electrons closer to the Van Hove singularity, increasing the density of states and raising the critical temperature. The combination of these opposing effects produces a minimum in the critical temperature as a function of field, giving rise to re-entrant superconductivity.
The discovery establishes the LaTiO3/KTaO3 interface as a robust platform for studying unconventional superconductivity in two-dimensional systems, offering a new route to understand materials whose properties remain poorly understood.