Re-entrant Superconductivity Observed in 2D Material
Physicists at the RIKEN Center for Emergent Matter Science have observed a rare phenomenon where a 2D material loses its superconductivity under a magnetic field only to regain it when the field strength is increased further. This behavior, known as re-entrant superconductivity, challenges standard expectations regarding how magnetic fields interact with superconducting states. The team led by Denis Maryenko identified the effect in LaTiO3/KTaO3 heterostructures while conducting magnetotransport measurements. The researchers did not set out to find this specific outcome, making the discovery an unexpected result of their experimental work.
Conventional superconductors typically lose their properties in the presence of strong magnetic fields. This occurs because the field either breaks the Cooper pairs of electrons or introduces magnetic flux that creates resistance. In contrast, this unconventional material exhibits a resistive peak that separates two distinct superconducting regions. The phenomenon occurs at a magnetic field of 0.9 Tesla. Crucially, this threshold remains independent of both temperature and charge carrier concentration, providing a stable platform for further investigation into 2D electronic states.
Mechanisms Behind the Re-entrant State
To understand why the material behaves this way, the team collaborated with theoretical physicists to analyze the electron band structure of the interface. Calculations performed by Igor Maznichenko, Sergey Ostanin, and Arthur Ernst confirmed the presence of a Van Hove singularity. This is a point in the electronic structure where the density of states becomes exceptionally high. Such singularities are significant because they dramatically influence the behavior of electrons near the Fermi level.
Additional modeling provided by Vitalii Dugaev and Evgeny Ya Sherman revealed that the interface possesses a symmetry between opposite momenta in its bands. In a zero-field state, electrons with opposite momenta maintain the same energy, which favors the formation of spin-singlet Cooper pairs. When a magnetic field is applied, this symmetry breaks, which initially reduces the efficiency of pair formation and lowers the critical temperature of the system.
Implications for Condensed Matter Physics
As the external magnetic field increases, a secondary effect begins to dominate. The field pushes the electrons closer to the Van Hove singularity. This shift increases the density of states and raises the critical temperature, counteracting the initial suppression caused by the field. The interplay between these competing effects creates the observed dip in the critical temperature profile. This specific mechanism allows for the reappearance of superconductivity at higher field values.
This discovery marks the first time re-entrant superconductivity has been documented in a two-dimensional system. Previously, the phenomenon was limited to three-dimensional ferromagnetic compounds or specialized heavy fermion materials. The use of 2D interfaces provides unique advantages for research, including the ability to tune material properties via gate voltage. By establishing this heterostructure as a testbed, scientists now have a tool to probe the limits of superconductivity in restricted dimensions. Future work will likely focus on how spin-orbit coupling and band structure engineering can be manipulated to maintain superconducting states at even higher magnetic thresholds.