# Highly anisotropic superconducting gaps and possible evidence of antiferromagnetic order in FeSe single crystals

## TL;DR

This study measures specific heat in FeSe single crystals revealing highly anisotropic superconducting gaps and potential antiferromagnetic order, with detailed modeling indicating a complex gap structure and possible magnetic transition.

## Contribution

It provides the first detailed specific heat analysis suggesting highly anisotropic gaps and evidence for antiferromagnetic order in FeSe, supported by comprehensive modeling.

## Key findings

- Superconducting gaps are highly anisotropic with minima below 0.15 meV.
- A specific heat anomaly at 1.08 K may indicate antiferromagnetic order.
- The gap contributions are approximately 32% from hole pockets and 68% from electron pockets.

## Abstract

Specific heat has been measured in FeSe single crystals down to 0.414 K under magnetic fields up to 16 T. A sharp specific heat anomaly at about 8.2 K is observed and is related to the superconducting transition. Another jump of specific heat is observed at about 1.08 K which may either reflect an antiferromagnetic transition of the system or a superconducting transition arising from Al impurity. We would argue that this anomaly in low temperature region may be the long sought antiferromagnetic transition in FeSe. Global fitting in wide temperature region shows that the models with a single contribution with isotropic s-wave, anisotropic s-wave, and d-wave gap all do not work well, nor the two isotropic s-wave gaps. We then fit the data by a model with two components in which one has the gap function of $\Delta_0(1+\alpha cos2\theta)$. To have a good global fitting and the entropy conservation for the low temperature transition, we reach a conclusion that the gap minimum should be smaller than 0.15 meV ($\alpha$ = 0.9 to 1), indicating that the superconducting gap(s) are highly anisotropic. Our results are very consistent with the gap structure derived recently from the scanning tunneling spectroscopy measurements and yield specific heat contributions of about 32\% weight from the hole pocket and 68\% from the electron pockets.