In conventional superconductors like Pb, the determination of the Eliashberg function in the superconducting state by tunneling experiment played a decisive role in confirming the BCS superconductivity theory in that the electron pairing is realized by exchanging phonons. In high temperature cuprate superconductors, the superconductivity mechanism remains elusive after extensive investigations in the past three decades. It has been a profound problem in modern condensed matter physics to discover and understand the nature of fluctuations and their coupling to fermions in cuprates, which lead to high temperature superconductivity and the invariably associated strange metal behaviors.

Prof. ZHOU Xingjiang’s group in the Institute of Physics, Chinese Academy of Sciences, in collaboration with Prof. Chandra VARMA from UC Riverside in US and Prof. Han-Yong CHOI from SungKyunKwan University in Korea, has made a significant progress in unveiling the pairing mechanism in high temperature cuprate superconductors. By performing high-resolution angle-resolved photoemission measurements on a high temperature superconductor Bi2212 with unprecedented accuracy and stability, combined with a precise inversion procedure, both the effective interactions in the attractive d-wave symmetry (pairing Eliashberg function) and the repulsive part in the full symmetry (normal Eliashberg function) are determined quantitatively for the first time. Several salient points of the experimental results have been revealed: (1). The normal Eliashberg function is nearly momentum-independent; (2). Near Tc, both the normal and pairing Eliahsberg functions are nearly independent of frequency and have almost the same magnitude over the complete energy range of up to about 0.4 eV, except for a low-energy feature at around 50 meV; (3). Well below Tc, the two interactions both change similarly, with superconductivity-induced features at low energies.

Besides finding the pairing self-energy and the attractive interactions for the first time, these results expose the central paradox of the problem of high Tc: how the same frequency-independent fluctuations can dominantly scatter at angles ±p/2 in the attractive channel to give d-wave pairing and lead to angle-independent repulsive scattering. The extracted pairing interactions responsible for high-Tc superconductivity allow a clear discrimination among theories based on phonons, antiferromagnetic fluctuations, the Hubbard model, and quantum-critical fluctuations of the loop-current order.

This study entitled “Quantitative determination of pairing interactions for high-temperature superconductivity in cuprates” was published on Science Advances.

The study was supported by the National Natural Science Foundation, the Ministry of Science and Technology of China, and the Chinese Academy of Sciences.