Valence-skipping and quasi-two-dimensionality of superconductivity in a van der Waals insulator
Caorong Zhang, Junwei Huang, Kun Zhai, Keivan Akhtari, Zhiwei Shen, Lingyi Ao, Zeya Li, Feng Qin, Yukai Chang, Ling Zhou, Ming Tang, Xueting Dai, Caiyu Qiu, Yi Zhang, Lin Wang, Zhongyuan Liu, Yongjun Tian, M. S. Bahramy, Hongtao Yuan
Abstract
Valence fluctuation of interacting electrons plays a crucial role in emergent quantum phenomena in correlated electron systems. The theoretical rationale is that this effect can drive a band insulator into a superconductor through charge redistribution around the Fermi level. However, the root cause of such a fluctuating leap in the ionic valency remains elusive. Here, we demonstrate a valence-skipping-driven insulator-to-superconductor transition and realize quasi-two-dimensional superconductivity in a van der Waals insulator GeP under pressure. This is shown to result from valence skipping of the Ge cation, altering its average valency from 3+ to 4+, turning GeP from a layered compound to a three-dimensional covalent system with superconducting critical temperature reaching its maximum of 10 K. Such a valence-skipping-induced superconductivity with a quasi-two-dimensional nature in thin samples, showing a Berezinskii-Kosterlitz-Thouless-like character, is further confirmed by angle-dependent upper-critical-field measurements. These findings provide a model system to examine competing order parameters in valence-skipping systems.