Direct Observation of d-Wave Superconducting Gap Symmetry in Pressurized La3Ni2O7-delta Single Crystals
Zi-Yu Cao, Di Peng, Seokmin Choi, Fujun Lan, Lan Yu, Enkang Zhang, Zhenfang Xing, Yuxin Liu, Feiyang Zhang, Tao Luo, Lixing Chen, Vuong Thi Anh Hong, Seung-Yeop Paek, Harim Jang, Jinghong Xie, Huayu Liu, Hongbo Lou, Zhidan Zeng, Yang Ding, Jun Zhao, Cailong Liu, Tuson Park, Qiaoshi Zeng, Ho-kwang Mao
Published: 2025/9/16
Abstract
The recent discovery of superconductivity in pressure-stabilized bulk La3Ni2O7-delta, with a critical temperature (Tc) exceeding 77 K, has opened a new frontier in high-temperature superconductivity research beyond cuprates. Yet, the superconducting gap amplitude and symmetry, the key parameters to characterize a superconductor, remain elusive due to the overwhelming challenges of gap studies under high pressure. Here, we introduce in situ directional point-contact spectroscopy conducted under truly hydrostatic pressure, enabling the direct mapping of the superconducting gap in pressurized La3Ni2O7-delta single crystals. Depending on the junction orientation, differential conductance (dI/dV) spectra exhibit distinct V-shaped quasiparticle features and a sharp zero-bias peak, indicating a predominant d-wave-like pairing symmetry. Measurement of the c-axis gap amplitude Delta yields a gap-to-Tc ratio of 2Delta/kBTc = 4.2(5), positioning La3Ni2O7-delta firmly among unconventional, nodal high-Tc superconductors. These findings set stringent constraints on theoretical models for nickelate superconductors and establish a robust spectroscopic approach for understanding superconductors under extreme pressures.