Regular Article

Hund-Heisenberg model in superconducting infinite-layer nickelates

¹ College of Physics and Information Technology, Shaanxi Normal University, Xi’an 710119, P.R. China
² School of Physical Science and Technology & Key Laboratory for Magnetism and Magnetic Materials of the MoE, Lanzhou University, Lanzhou 730000, P.R. China
³ Center for High-Pressure Science and Technology Advanced Research, Beijing 100094, P.R. China

^a e-mail: jun.chang@hotmail.com

Received: 9 July 2020
Received in final form: 3 October 2020
Accepted: 9 October 2020
Published online: 2 December 2020

Abstract

We theoretically investigate the unconventional superconductivity in the newly discovered infinite-layer nickelates Nd_1−xSr_xNiO₂ based on a two-band model. By analyzing the transport experiments, we propose that the doped holes dominantly enter the Ni d_xy or/and d_3z²−r² orbitals as charged carriers, and form a conducting band. Via the onsite Hund coupling, the doped holes are coupled to the Ni localized holes in the d_x²−y² orbital band. We demonstrate that this two-band model could be further reduced to a Hund-Heisenberg model. Using the reduced model, we show the non-Fermi liquid state above the critical T_c could stem from the carriers coupled to the spin fluctuations of the localized holes. In the superconducting phase, the short-range spin fluctuations mediate the carriers into Cooper pairs and establish d_x²−y²-wave superconductivity. We further predict that the doped holes ferromagnetically coupled with the local magnetic moments remain itinerant even at very low temperature, and thus the pseudogap hardly emerges in nickelates. Our work provides a new superconductivity mechanism for strongly correlated multi-orbital systems and paves a distinct way to exploring new superconductors in transition or rare-earth metal oxides.