# Fermi surface, pressure-induced antiferromagnetic order, and superconductivity in FeSe

## TL;DR

This study models how pressure affects the electronic structure, magnetic, and superconducting properties of FeSe, revealing the roles of orbital interactions and Fermi surface changes in its phase transitions.

## Contribution

It introduces a comprehensive 16-band $d$-$p$ model to explain pressure-induced phase transitions and the double-dome superconducting behavior in FeSe.

## Key findings

- Shallow hole pocket disappearance suppresses antiferromagnetism at ambient pressure.
- Orbital-polarization interaction drives structural transition without magnetic order.
- Fermi surface evolution explains the opposite pressure dependence of $T_s$ and $T_m$.

## Abstract

The pressure dependence of the structural ($T_s$), antiferromagnetic ($T_m$), and superconducting ($T_c$) transition temperatures in FeSe is investigated on the basis of the 16-band $d$-$p$ model. At ambient pressure, a shallow hole pocket disappears due to the correlation effect, as observed in the angular-resolved photoemission spectroscopy (ARPES) and quantum oscillation (QO) experiments, resulting in the suppression of the antiferromagnetic order, in contrast to the other iron pnictides. The orbital-polarization interaction between the Fe $d$ orbital and Se $p$ orbital is found to drive the ferro-orbital order responsible for the structural transition without accompanying the antiferromagnetic order. The pressure dependence of the Fermi surfaces is derived from the first-principles calculation and is found to well account for the opposite pressure dependences of $T_s$ and $T_m$, around which the enhanced orbital and magnetic fluctuations cause the double-dome structure of the eigenvalue $\lambda$ in the Eliashberg equation, as consistent with that of $T_c$ in FeSe.