A minimum single-band model for low-energy excitations in superconducting K$_x$Fe$_2$Se$_2$

TL;DR

This paper introduces a simplified single-band model for K$_x$Fe$_2$Se$_2$ that captures low-energy excitations and explains its high-temperature superconductivity through spin fluctuations and Fermi surface topology.

Contribution

A minimal single-band model is developed that aligns with complex multi-orbital models and explains superconductivity in K$_x$Fe$_2$Se$_2$.

Findings

Identification of $(rac{ ext{pi}}{2},rac{ ext{pi}}{2})$ spin excitation

Discovery of $d_{x^2-y^2}$ pairing symmetry

Explanation of high $T_c$ via Fermi surface and spin interactions

Abstract

We propose a minimum single-band model for the newly discovered iron-based superconducting K$_x$Fe$_2$Se$_2$. Our model is found to be numerically consistent with the five-orbital model at low energies. Based on our model and the random phase approximation, we study the spin fluctuation and the pairing symmetry of superconducting gap function. The $(\pi/2,\pi/2)$ spin excitation and the $d_{x^2-y^2}$ pairing symmetry are revealed. All of the results can well be understood in terms of the interplay between the Fermi surface topology and the local spin interaction, providing a sound picture to explain why the superconducting transition temperature is as high as to be comparable to those in pnictides and some cuprates. A common origin of superconductivity is elucidated for this compound and other high-T$_c$ materials.