# Magnetic and Superconducting Properties of Single Crystals of Fe$_{1+\delta}$Te$_{1-x}$Se$_x$ System

## TL;DR

This study investigates the magnetic and superconducting properties of Fe$_{1+ ext{delta}}$Te$_{1-x}$Se$_x$ single crystals, revealing strong antiferromagnetic spin fluctuations and unconventional superconductivity with high upper critical fields.

## Contribution

It provides new insights into the relationship between spin fluctuations and superconductivity in Fe-based chalcogenides, highlighting their strong electron correlations and unconventional nature.

## Key findings

- Superconducting transition temperatures range from 10 to 14 K.
- Magnetic susceptibility shows nearly linear-in-T behavior in superconducting samples.
- Upper critical field exceeds the Pauli limit, indicating unconventional superconductivity.

## Abstract

The spin-fluctuation effect in the Se-substituted system Fe$_{1+\delta}$Te$_{1-x}$Se$_x$ ($x$ = 0, 0.05, 0.12, 0.20, 0.28, 0.33, 0.45, 0.48 and 1.00; $0<\delta < 0.12$) has been studied by the measurements of the X-ray diffraction, the magnetic susceptibility under high magnetic fields and the electrical resistivity under magnetic fields up to 14 T. The samples with $x$ = 0.05, 0.12, 0.20, 0.28, 0.33, 0.45 and 0.48 show superconducting transition temperatures in the ranger of 10 K$\sim$14 K. We obtained their intrinsic susceptibilities by the Honda-Owen method. A nearly linear-in-$T$ behavior in magnetic susceptibility of superconducting samples was observed, indicating the antiferromagnetic spin fluctuations have a strong link with the superconductivity in this series. The upper critical field $\mu_0H_{c2}^{orb}$ for $T\to$ 0 was estimated to exceed the Pauli paramagnetic limit. The Kadowaki-Woods and Wilson ratios indicate that electrons are strongly correlated in this system. Furthermore, the superconducting coherence length and the electron mean free path were also discussed. These superconducting parameters indicate that the superconductivity in the Fe$_{1+\delta}$Te$_{1-x}$Se$_x$ system is unconventional.