Abstract
Nematicity and magnetism are two key features in Fe-based superconductors, and their interplay is one of the most important unsolved problems. In FeSe, the magnetic order is absent below the structural transition temperature , in stark contrast to the fact that the magnetism emerges slightly below in other families. To understand such amazing material dependence, we investigate the spin-fluctuation-mediated orbital order () by focusing on the orbital-spin interplay driven by the strong-coupling effect, called the vertex correction. This orbital-spin interplay is very strong in FeSe because of the small ratio between the Hund’s and Coulomb interactions () and large , -orbital weight at the Fermi level. For this reason, in the FeSe model, the orbital order is established irrespective of the fact that the spin fluctuations are very weak, so the magnetism is absent below . In contrast, in the LaFeAsO model, the magnetic order appears just below both experimentally and theoretically. Thus, the orbital-spin interplay due to the vertex correction is the key ingredient in understanding the rich phase diagram with nematicity and magnetism in Fe-based superconductors in a unified way.
- Received 25 October 2015
DOI:https://doi.org/10.1103/PhysRevX.6.021032
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Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Strongly correlated electron systems exhibit a rich variety of spontaneous symmetry-breaking phenomena associated with charge, orbital, and spin degrees of freedoms. So-called “electronic nematic order,” which refers to the spontaneous violation of the rotational symmetry in electronic states, has attracted attention as a fundamental phenomenon in strongly correlated metals. The origin of the electronic nematic state is a hotly debated topic in Fe-based superconductors since it is an important phenomenon in solving the mystery of high- superconductivity. Here, we study the origin of the nematicity by paying special attention to the “nonmagnetic nematic order” in FeSe, which is characterized by .
We apply orbital-spin fluctuation theory to first-principles Hubbard models, and we show that the nematic orbital order is due to the strong correlation effect in both FeSe and LaFeAsO models. In particular, we focus on the ratio between the Hund’s and Coulomb interactions as well as the size of the Fermi surfaces. The microscopic origin of the orbital order is the orbital-spin interplay. Since this orbital-spin interplay is very strong in FeSe, the ferro-orbital order is established even when the spins are very small, so the magnetism is absent in the nematic phase in FeSe. Our work shows that the rich variety of the phase diagrams in Fe-based superconductors, such as the nonmagnetic/magnetic nematic phase in FeSe/LaFeAsO, can be understood in terms of the present orbital-order scenario. These theoretically predicted strong orbital-spin fluctuations are expected to play important roles in the pairing mechanism in Fe-based superconductors.
We expect that our findings will pave the way for a better understanding of high- superconductivity in Fe-based superconductors.
