Abstract
A series of iron based Fe-Te-Se superconductor thin films depositing on 0.7wt% Nb-doped SrTiO3 at substrate temperatures in the 250°C -450°C range by pulsed laser ablation of a constituents well defined precursor FeTe0.55Se0.55 target sample. We study the possible growth mechanism and its influence on the superconductor properties. Experimental results indicate the superconductive and non-superconductive properties are modulated only by the thickness of the thin films through the temperature range. The films appear as superconductor whenever the thickness is above a critical value ∼30nm and comes to be non-superconductor below this value. Relative ratios of Fe to (Te+Se) in the films retained Fe/(Te+Se)<1 for superconductor and Fe/(Te+Se)>1 for non-superconductor no matter what the film growth temperature was. The effect of film growth temperature takes only the role of modulating the ratio of Te/Se and improving crystallinity of the systems. According to the experimental results we propose a sandglass film growth mechanism in which the interfacial effect evokes to form a Fe rich area at the interface and Se or Te starts off a consecutive filling up process of chalcogenide elements defect sides, the process is significant before the film thickness reaches at ∼30nm.
Highlights
Far apart from the copper oxide family, the discovery of superconductivity in the iron-based oxypnictides LaFeAsO1−XFX1 at TC=26K added a new family to the superconductive systems, and the TC of the oxypnictides family was soon pushed to 57.3K by Sm substitution of La in Sm0.95La0.05O0.85F0.15FeAs.[2]
According to the experimental results we propose a sandglass film growth mechanism in which the interfacial effect evokes to form a Fe rich area at the interface and Se or Te starts off a consecutive filling up process of chalcogenide elements defect sides, the process is significant before the film thickness reaches at ∼30nm
Following the PLD the chalcogenide elements Se or Te may tend to fill up oxygen defect sides in substrate surface, in-plain strain may play a role to create a Fe rich area at the interface, this process leading to a consecutive migration of the Se or Te elements from upper layer to fill up the “holes” in layers next to uppermost one until the thickness arrives at ∼30nm
Summary
Elemental portions, and secondly by modulating single element composition that assists to generate the superconducting behavior should be of help to simplify experimental complexity for pin down microscopic origins enduing the system with the observed macroscopic transport property. These binary chalcogenide systems attracted great attention due to their simple planar crystal structure and strong TC dependence on external conditions.[4,5] Understanding substrate-film interface effect and its role in the superconducting phase transition has been an important research field of these systems. Following the PLD the chalcogenide elements Se or Te may tend to fill up oxygen defect sides in substrate surface, in-plain strain may play a role to create a Fe rich area at the interface, this process leading to a consecutive migration of the Se or Te elements from upper layer to fill up the “holes” in layers next to uppermost one until the thickness arrives at ∼30nm
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