image: (a) Schematic illustration of a Ti/LaAlO3/KTaO3 device. (b) Tunneling spectra of a device with a weak tunneling barrier at T = 0.5 K under different gate voltages. Inset: Zoom in on the left peaks. (c) Tunneling spectrum of a device with a strong tunneling barrier at T = 0.5 K, plotted as black circles. (d-f) Calculated tunneling spectra (blue lines, normalized by the normal state value of the two-dimensional electron system) for strong metal-superconductor coupling, corresponding to s-wave, chiral p-wave and dxy-wave superconductors, respectively. For comparison, the normalized density of states (DOS) for each pristine superconducting state is also plotted (black dashed lines).
Credit: ©Science China Press
Recently, Professor Changgan Zeng’s team from the University of Science and Technology of China (USTC), in collaboration with Professor Jian Li’s team from Westlake University, has made significant progress in studying superconductivity at the LaAlO3/KTaO3 interface. By building Ti/LaAlO3/KTaO3 planar junctions and systematically analyzing their tunneling spectroscopy, the researchers not only revealed evidence of potential p-wave superconductivity at the LaAlO3/KTaO3 interface but also proposed a universal strategy for identifying superconducting pairing mechanisms. Their study, titled “Contact-induced Andreev bound states in normal-metal/superconductor planar junctions,” was published in National Science Review.
The study of unconventional superconductors stands as a central theme in condensed matter physics, where fascinating physics emerges from the interplay of strong electron correlation, spin-orbit coupling, band topology, etc. Among the various types of unconventional superconductivity, 2D p-wave superconductivity is particularly captivating due to its close connections with topological phases of matter and the emergence of non-abelian quasiparticle excitations. Tunneling spectroscopy serves as an effective means for probing the pairing mechanism of superconductors, offering many candidates for d-wave or p-wave superconductivity by detecting Andreev bound states. Nonetheless, existing research has rarely addressed the characterization of Andreev bound states in planar junctions with out-of-plane tunneling into unconventional superconductors.
In this study, the team constructed planar junctions consisting of metallic titanium (Ti) and a LaAlO3/KTaO3 superconducting interface. They systematically investigated the tunneling spectroscopy of these junctions. By fine-tuning the thickness of the LaAlO3 layer to control the tunneling barrier strength, they observed distinct spectroscopic behaviors. With a weak barrier, pronounced double peaks emerged within the superconducting gap, while with a strong barrier, the in-gap peaks vanished, resulting in a softened full-gap profile. Theoretical calculations revealed that, with a weak barrier, the metallic contact strongly couples with the superconductor, leading to significant variations in tunneling spectra depending on the superconducting pairing states. Specifically, for s-wave superconductors, such strong coupling fails to induce Andreev bound states, resulting in a clean gap without appreciable in-gap features. In contrast, for p-wave and d-wave superconductors, strong coupling can induce Andreev bound states within the superconducting gap, but with distinct spectroscopic characteristics: p-wave systems show a well-defined double-peak structure, while d-wave ones show only a single peak. Combining experimental and theoretical analyses, these findings not only suggest potential p-wave superconductivity at the LaAlO3/KTaO3 interface but also offer a universal approach for identifying the most sought-after superconducting states.
Beilin Wang, a Ph.D. student, and Dr. Linhai Guo, both from USTC, equally contributed to this study as co-first authors. Professors Changgan Zeng (USTC) and Jian Li (Westlake University) served as co-corresponding authors. The research was supported by funding from the National Natural Science Foundation of China, the Ministry of Science and Technology, the Chinese Academy of Sciences, and Anhui Province.