Quality over quantity: Ge-doping-induced phase separation and the formation of ineffective pinning landscape in FeSe 0.4 Te 0.6 single crystals

Abstract This study successfully prepared a series of Fe 1- x Ge x Se 0.4 Te 0.6 ( x = 0–0.20) single crystals using an optimized self-flux method. Through systematic structural characterization and superconducting property measurements, we reveal the unique phase separation induced by Ge doping and its regulatory effect on flux pinning behavior. X-ray diffraction and scanning electron microscopy analyses indicate that Ge doping drives the decomposition of the FeSeTe parent phase into a coexisting system of β -FeSe and FeGe, accompanied by significant structural disorder and lattice distortion. Superconducting measurements show that with increasing Ge content, the superconducting properties exhibit a systematic concentration-dependent evolution: the superconducting transition temperature decreases gradually from 14.5 K ( x = 0), and a double superconducting transition emerges at x = 0.05; the critical current density at 4.2 K drops by up to 65%, and the pinning force density shows systematic attenuation. The underlying mechanism lies in the ‘quality’ defects of the pinning centers caused by the magnetic properties of the FeGe phase, the insufficient superconducting performance of the β -FeSe phase, and their spatially inhomogeneous distribution, collectively rendering the abundant phase interfaces introduced by phase separation ineffective for pinning. The most significant finding of this work is the revelation of the decisive role of pinning center ‘quality-over-quantity’, providing a new perspective for understanding the relationship between complex defects and superconducting performance in iron-based superconductors, as well as an important theoretical basis for the design of high-performance superconducting materials.