Gap structure of FeSe determined by field-angle-resolved specific heat measurements

TL;DR

This study reveals the complex multigap structure of FeSe through detailed field-angle-resolved specific heat measurements, identifying three distinct superconducting gaps with unique symmetry and anisotropy properties.

Contribution

The paper provides the first detailed experimental determination of the gap structure and symmetry in FeSe using angle-resolved specific heat, supported by theoretical calculations.

Findings

FeSe has three distinct superconducting gaps with different symmetries.

The smallest gap has vertical-line nodes or minima along the $k_z$ direction.

The gaps exhibit specific anisotropic behaviors related to their Fermi surface bands.

Abstract

Quasiparticle excitations in FeSe were studied by means of specific heat ($C$) measurements on a high-quality single crystal under rotating magnetic fields. The field dependence of $C$ shows three-stage behavior with different slopes, indicating the existence of three gaps ($\Delta_1$, $\Delta_2$, and $\Delta_3$). In the low-temperature and low-field region, the azimuthal-angle ($\phi$) dependence of $C$ shows a four-fold symmetric oscillation with sign change. On the other hand, the polar-angle ($\theta$) dependence manifests as an anisotropy-inverted two-fold symmetry with unusual shoulder behavior. Combining the angle-resolved results and the theoretical calculation, the smaller gap $\Delta_1$ is proved to have two vertical-line nodes or gap minima along the $k_z$ direction, and is determined to reside on the electron-type $\varepsilon$ band. $\Delta_2$ is found to be related to the electron-type $\delta$ band, and is isotropic in the $ab$-plane but largely anisotropic out of the plane. $\Delta_3$ residing on the hole-type $\alpha$ band shows a small out-of-plane anisotropy with a strong Pauli-paramagnetic effect.