Unusual effects of Be doping in the iron-based superconductor FeSe

Recent superconducting transition temperatures (T _c) over 100 K for monolayer FeSe on SrTiO₃ have renewed interest in the bulk parent compound. In KCl:AlCl₃ flux-transport-grown crystals of FeSe_0.94Be_0.06, FeSe_0.97Be_0.03 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 FeSe_0.94Be_0.06, the lattice unit cell shrinks by 2.4%, T _c-measured in the bulk via specific heat-increases by almost 10%, the T _c versus pressure behavior shifts its peak [Formula: see text] downwards by ~1 GPa, the high temperature structural transition around T _S  =  89 K increases by 1.9 K (in contrast to other dopants in FeSe which uniformly depress T _S), and the low temperature specific heat γ increases by 10% compared to pure FeSe. Also, upon doping by 6% Be the residual resistivity ratio, ρ(300 K)/ρ(T  →  0), increases by almost a factor of four, while ρ(300 K)/ρ([Formula: see text]) increases by 50%.