Electron pairs are essential for superconductivity, but pairing alone is not enough: the pairs must also act together. A new study finds evidence that an electric field can shift the main constraint on superconductivity at an oxide interface, from establishing this collective coordination to forming the pairs themselves.
The research, led by Prof. Jie Wu’s group at Westlake University in collaboration with Prof. Yanwu Xie’s group at Zhejiang University, focuses on the boundary between two insulating oxides: lanthanum aluminate and potassium tantalate. At this LaAlO 3 /KTaO 3 (111) interface, superconductivity is confined to a layer only about four nanometers thick. Its low dimensionality and relatively sparse charge carriers make it difficult for different regions to maintain a shared quantum “rhythm,” known as phase coherence.
Superfluid stiffness measures how robustly the system maintains this phase coordination. But determining stiffness at an ultrathin interface is challenging because its magnetic signal is exceptionally weak. The team developed a highly sensitive two-coil mutual-inductance setup for ultralow temperatures: one coil generates an alternating magnetic field, while the other detects how effectively the sample screens it.
Varying the gate voltage from -900 to +900 volts revealed two regimes. At low stiffness, the temperature at which a detectable diamagnetic response appeared increased nearly linearly with stiffness. This is consistent with Bose–Einstein-condensate-like (BEC-like) behavior, in which establishing phase coherence limits the transition. At higher stiffness, the transition temperature became nearly independent of stiffness, suggesting Bardeen–Cooper–Schrieffer-like (BCS-like) behavior, in which pair formation becomes the main constraint.
An unusual magnetic response provided another clue. Under small static magnetic fields, the researchers observed a paramagnetic Meissner effect near the transition: a contribution that enhances rather than screens the detected magnetic flux. Together with the saturation of diamagnetic screening under weak alternating fields, this supports a picture of superconducting “islands” connected by weak Josephson coupling. This granularity refers to spatial variations in superconductivity, not necessarily to structural grains.
In this picture, an electric field strengthens the connections between islands until keeping them in step is no longer the bottleneck. The findings motivate similar measurements at related interfaces and spatially resolved probes of the superconducting gap and stiffness to test the proposed island network.
Science Bulletin
Experimental study