Unusual Effects of Be doping in the Iron Based Superconductor FeSe

TL;DR

This study investigates how minimal Be doping in FeSe affects its structural, electronic, and superconducting properties, revealing unusual effects such as increased Tc and structural transition temperature, contrasting with other dopants.

Contribution

It demonstrates that 6% Be doping in FeSe causes unique enhancements in superconductivity and structural stability, differing from effects of other dopants in the same material.

Findings

6% Be doping shrinks lattice by 2.4% and increases Tc by nearly 10%.

Be doping raises the structural transition temperature TS by 1.9 K.

Residual resistivity ratio increases nearly fourfold with Be doping.

Abstract

Recent superconducting transition temperatures (Tc) over 100 K for monolayer FeSe on SrTiO3 have renewed interest in the bulk parent compound. In KCl:AlCl3 flux-transport-grown crystals of FeSe0.94Be0.06, FeSe0.97Be0.03 and, for comparison, FeSe, this work reports doping of FeSe using Be, among the smallest of possible dopants, corresponding to an effective chemical pressure. According to lattice parameter measurements, 6% Be doping shrank the tetragonal FeSe lattice equivalent to a physical pressure of 0.75 GPa. Using this flux-transport method of sample preparation, 6% of Be was the maximum amount of dopant achievable. At this maximal composition of FeSe0.94Be0.06, the lattice unit cell shrinks by 2.4%, Tc - measured in the bulk via specific heat - increases by almost 10%, the Tc vs pressure behavior shifts its peak Tconset downwards by ~1 GPa, the high temperature structural transition around TS = 89 K increases by 1.9 K (in contrast to other dopants in FeSe which uniformly depress TS), and the low temperature specific heat gamma increases by 10 % compared to pure FeSe. Also, upon doping by 6% Be the residual resistivity ratio, rho(300K)/rho(T->0), increases by almost a factor of four, while rho(300K)/rho(T=Tc+) increases by 50%.