Anisotropic Structure of the Order Parameter in FeSe_{0.4}Te_{0.6} Revealed by Angle Resolved Specific Heat

TL;DR

This study uses angle-resolved specific heat measurements to reveal the anisotropic structure of the superconducting gap in FeSe_{0.4}Te_{0.6}, providing critical insights into its complex gap symmetry and challenging existing models.

Contribution

First bulk, angle-resolved specific heat measurement on Fe-based superconductor revealing gap anisotropy and minima, constraining theoretical models.

Findings

Observed fourfold oscillation in specific heat with magnetic field orientation.

Identified locations of gap minima or nodes on the Fermi surface.

Placed severe restrictions on existing theoretical models of the gap structure.

Abstract

The symmetry and structure of the superconducting gap in the Fe-based superconductor are the central issue for understanding these novel materials. So far the experimental data and theoretical models have been highly controversial. Some experiments favor two or more constant or nearly-constant gaps, others indicate strong anisotropy and yet others suggest gap zeros ("nodes"). Theoretical models also vary, suggesting that the absence or presence of the nodes depends quantitatively on the model parameters. An opinion that has gained substantial currency is that the gap structure, unlike all other known superconductors, including cuprates, may be different in different compounds within the same family. A unique method for addressing this issue, one of the very few methods that are bulk and angle-resolved, calls for measuring the electronic specific heat in a rotating magnetic field, as a function of field orientation with respect to the crystallographic axes. In this Communication we present the first such measurement for an Fe-based high-Tc superconductor (FeBSC). We observed a fourfold oscillation of the specific heat as a function of the in-plane magnetic field direction, which allowed us to identify the locations of the gap minima (or nodes) on the Fermi surface. Our results place severe restrictions on the gap structure and on the existing theoretical models.