Flux free growth of superconducting FeSe single crystals

TL;DR

This paper demonstrates a simple flux-free method to grow superconducting FeSe single crystals, revealing their structural phases and superconducting properties, including a high upper critical field of around 50 Tesla.

Contribution

It introduces an easy high-temperature melt and slow cooling technique for flux-free growth of FeSe crystals, providing detailed structural and superconducting characterizations.

Findings

Superconductivity observed below 10K in the crystals.

High upper critical field (around 50 Tesla) estimated from measurements.

Presence of both tetragonal and hexagonal FeSe phases confirmed.

Abstract

We report flux free growth of superconducting FeSe single crystals by an easy and versatile high temperature melt and slow cooling method for first time. The room temperature XRD on the surface of the piece of such obtained crystals showed single 101 plane of Beta-FeSe tetragonal phase. The bulk powder XRD, being obtained by crushing the part of crystal chunk showed majority tetragonal and minority FeSe hexagonal crystalline phases. Detailed HRTEM images along with SAED (selected area electron diffraction) showed the abundance of both majority and minority FeSe phases. Both transport (RT) and magnetization (MT) exhibited superconductivity at below around 10K. Interestingly, the magnetization signal of these crystals is dominated by the magnetism of minority magnetic phase, and hence the isothermal magnetization (MH) at 4K was seen to be ferromagnetic (FM) like. Transport (R-T) measurements under magnetic field showed superconductivity onset at below 12K, and R = 0 (Tc) at 9K. Superconducting transition temperature (Tc) decreases with applied field to around 6K at 7Tesla, with dTc/dH of 0.4K/Tesla, giving rise to an Hc2 value of around 50 Tesla, 30 Tesla and 20 Tesla for Rn = 90, 50 and 10 percent respectively. FeSe single crystal activation energy is calculated from Thermally Activated Flux Flow (TAFF) model which is found to decreases with field.