The Study Of Superconductivity And Magnetism In Iron Chalcogenide Superconductors

A superconducting material can conduct electric current with zero-energy loss due to the absence of electrical resistance below its superconducting transition temperature Tc; such behavior is clearly of great practical use. Research on high-T_c superconductivity gained new momentum in the year 2008 with the discovery of iron-pnictide superconductors. These materials are considered as a new kind of unconventional superconductors and this family of superconductors is the second family of high-T_c superconductors whose T_c beyond the McMillan limit predicted by the BCS theory. The iron-based superconductors provide new opportunities to address the long-standing challenge of high-T_c superconductivity, and the investigation on properties of this family of superconductors will surely give impetus to the progress in many fields of condensed matter physics.Compared to iron pnictides, FeSe has a more simple structure of only FeSe4 layers and no toxic arsenic. In addition, high-quality, large-size single crystals are relatively easy to grow for this system. All these characteristics have generated some particular attention in the iron-chalcogenide system. In this dissertation, we select the iron-chalcogenide superconductors as the research object and systematically investigate the structure, the transport properties, the magnetic properties, the pressure effect on the superconductivity in these materials, mainly focusing on the interplay between magnetism and superconductivity. The main contents are as follows:1. The pressure effect on superconductivity and magnetism has been investigated in FeSe_x (x = 0.80, 0.88). The magnetization curves display anomaly at Ts1～106 K and Ts2～78 K except for the superconducting diamagnetic transition around T_c～8 K. The magnetic anomaly at Ts1 and Ts2 can be related to a ferromagnetic and an antiferromagnetic phase transition, respectively, as revealed by specific heat measurements. The application of pressure not only raises Tc, but also increases both Ts1 and Ts2. It suggests that superconductivity in this compound comes about in a situation characterized by strong magnetic fluctuations.2. High quality FeSe thin films with different ratios of Fe to Se have been grown on GaAs and Si substrates by changing the flow rate of Fe(CO)5 in low-pressure metal organic chemical vapor deposition (MOCVD) system. For both substrates, the films grown at low flow rate of Fe(CO)5 show non-ferromagnetic and exhibit superconductivity at low temperature, while those grown at high flow rate of Fe(CO)5 display ferromagnetic and semiconducting behaviors. Our results suggest that the superconducting phase does exist in a narrow range of Fe and Se concentration near stoichiometry. The introduction of excess Fe favors ferromagnetism and leads to the suppression of superconductivity.3. We have examined the chemical effect generated by Se substitution by applying external pressure on Fe_1+yTe_1-xSe_x single crystals. With increasing pressure, the superconducting volume increases; however, the onset temperature of the superconducting diamagnetic signal, Tcmag, does not change monotonically. The inconsistent pressure effect on superconducting volume and Tcmag is in agreement with Se substitution, which suggests that the chemical pressure effect dominates the superconductivity. Our results also imply that the superconducting and magnetic phases coexist in the form of phase separation.4. High-quality single crystals of K_0.7Fe_1.6Se_1.6S_0.4 are successfully synthesized by self-flux method with the superconducting transition temperatures T_c^onset = 32.8 K and T_c^zero = 31.2 K. In contrast to external pressure effect on superconductivity, the substitution of S for Se does not suppress Tc, which suggests that chemical doping may mainly modulate the anion height from Fe-layer rather than compressing interlayer distance. The investigation of electron spin resonance shows clear evidence for strong spin fluctuation at temperatures above Tc. Accompanied by the superconducting feature spectra, a novel resonance signal develops gradually upon cooling below Tc, indicating the coexistence of superconductivity and magnetism in K_0.7Fe_1.6Se_1.6S_0.4 crystal.