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Superconductivity is an advantageous property observed in some materials, which entails the ability to conduct electricity without resistance below specific critical temperatures. One particularly fascinating phenomenon observed in some unconventional superconductors is so-called spin-triplet pairing.

In conventional superconductors, electrons form what are known as "Cooper pairs," pairs of electrons with opposite momentum and spin, a phenomenon referred to as spin-singlet pairing. In some unconventional superconductors, on the other hand, researchers observed a different state known as spin-triplet pairing, which entails the formation of pairs of electrons with parallel spins.

Researchers at Fudan University, RIKEN Center for Emergent Matter Science (CEMS) and other institutes recently observed a significantly enhanced nonreciprocal transport in a class of superconductors known as KTaO₃-based interface superconductors, which they proposed could be explained by the co-existence of spin-singlet and spin-triplet states.

Their paper, in Â鶹ÒùÔºical Review Letters, offers new insight into these materials' unconventional superconductivity and could potentially inform the future use of these materials for the development of spintronics and quantum technologies.

"KTaO₃-based interface superconductors were and quickly grabbed attention because they show a much higher superconducting transition temperature than previously known SrTiO₃-based interface superconductors," Pan He, co-senior author of the paper, told Â鶹ÒùÔº.

"Despite this interest, studies until now have focused on linear transport measurements. We realized that the nonlinear or nonreciprocal transport of these superconductors hadn't been explored yet. This was a key motivation for us, as nonreciprocal transport offers a unique lens through which to investigate superconductors, especially those with broken inversion symmetry at interfaces."

In materials science, nonreciprocal transport entails an electrical resistance that differs based on the direction in which electrical current is flowing. This phenomenon was previously found to be a powerful tool to probe parity mixing (i.e., the coexistence of spin-singlet and spin-triplet states).

Specifically, the presence of certain nonreciprocal transport in a material may indicate that both spin-singlet and spin-triplet pairings are contributing to its superconductivity. Building on this idea, He and his colleagues tried to shed more light on the underlying physics of KTaO₃-based interface superconductors, in which superconductivity arises at the interface between a potassium tantalate (KTaO₃) layer and other compounds.

"Experimentally, we probed nonreciprocal transport by measuring second-harmonic generation under a low-frequency alternating current," said Jinfeng Zhai, co-first author of the paper. "This technique, often referred to as electric harmonic measurement, allows us to detect subtle nonlinearities in the material's electrical response that are indicative of nonreciprocal transport."

The measurements collected by the researchers did in fact unveil nonreciprocal transport in KTaO₃-based interface superconductors, suggesting that parity mixing contributed to their superconductivity. As part of their study, He, Zhai and their colleagues also developed a theoretical model that explains their observations.

"We developed a model incorporating both Rashba spin-orbit coupling and hexagonal warping," explained Taekoo Oh, co-first author of the paper. "Based on this model, we then used time-dependent Ginzburg-Landau theory to calculate the expected nonreciprocal transport behavior."

Notably, the theoretical model devised by the researchers was found to be closely aligned with their experimental observations. This suggests that the superconductivity reported in KTaO₃-based interface superconductors could in fact arise from co-existing spin-singlet and spin-triplet pairings.

"We believe the most significant achievement of our study is providing strong evidence for an attractive spin-triplet pairing interaction within the KTaO₃-based interface superconductors, which we revealed through our nonreciprocal transport measurements and theoretical approaches," said Naoto Nagaosa, co-senior author of the paper.

This recent study gathered new valuable insight into KTaO₃-based interface superconductors and the processes contributing to their superconductivity, which could be further explored and validated in future research. The team at Fudan University is now planning to extend their investigation to encompass other interface superconductors that have been found to exhibit superconductivity at even higher temperatures.

"Finding and understanding these materials is crucial for moving toward real-world applications," added Jian Shen, co-senior author of the paper. "We're also eager to delve into novel spintronic phenomena within these interface superconductors. Given the evidence for spin-polarized spin-triplet Cooper pairs, this system offers an ideal platform to combine the fascinating fields of superconductivity and spintronics, potentially leading to entirely new functionalities."

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