Element Substitution Effect Of 1111-phase LnFeAsO Iron Based Superconductors

Element substitution effect is a powerful method to explore new superconducting ma-terials and probe superconducting(SC) mechanism. A lot of new iron based superconduc-tors were discovered by chemical doping methods. Furthermore, chemical doping on dif-ferent sites of crystal lattice can not only reveal the intrinsic factor crucial to superconduc-tivity, but also provide insights on the pairing symmetry.In this dissertation, the studies mainly focus on two aspects of iron based supercon-ductors:(1) The effect of element substitution on the four different sites of crystal lattice in 1111 phase SmFeAsO. The variations of superconductivity and transport properties with doping levels were investigated, and the systematical electronic phase diagram were ob-tained. (2) The effect of non-magnetic Zn impurities on the SDW order of parent compound LaFeAsO, and on Tc in under-and optimal-and over-dope regime of LaFeAsO1-xFx was studied, and these results suggest a possible scenario of SC paring symmetry. The main original results are listed as the following.(1) For the 1111 phase compound SmFeAsO, we performed the chemical dopings of Th on Sm site, Co or Ni on Fe site, P on As site, and F on O site. We found that all the four types of chemical doping can lead to the suppression of SDW order and consequentially superconductivity is induced. Our group reported independently the Co-doping induced superconductivity, and for the first time we observed the superconductivity induced by Ni-doping and P-doping. These results have attracted intensive attention of the scientists in this field. In the SmFe1-xTxAsO (T=Co,Ni) systems, due to the itinerant nature of Co/Ni 3d electrons, the electron-type charge carriers were induced by Co or Ni doping, and it was further confirmed by the measurements of thermopower and Hall coefficient. Furthermore, we found that the absolute value of normal state thermopower changes regularly with dop-ing level. We suggest that the thermopower should consist of two different contributions: one is the normal background term and the other is anomalous term. The norma term in- creases slightly and monotonously with the electron doping level and it just results from the change of charge carrier density. The latter anomalous term firstly increases with the doping level, and reaches a maximum at optimal doping level, and then it drops, indicating a close correlation with the variation of Tc. We suggest that this close correlation between Tc and the thermopower could be a universal feature for all the electron-type iron-based su-perconductors. In the SmFeAsOi-xPx system, the maximum Tc is about 4.1 K, far smaller than that of LaFeAsO1-xPx (Tc of 13 K). The lower Tc may be ascribed to the magnetism of Sm ion.(2) We firstly reported the doping effect of non magnetic Zn impurity in both par-ent and F-doped LaFeAsO compounds. For the parent compound, non magnetic Zn doping severely suppresses SDW order. Only 5% Zn doping can completely destroy the long range SDW order and the system becomes semiconductor-like behavior. Meanwhile, Zn doping does not affect or even enhances Tc of optimal-doped LaFeAsO0.9F0.1, in contrast to the case of high-Tc cuprates.We further studied the F-doping dependence of Zn impurity effect in LaFe1-xZnxAsO1-yFy system, and found that Tc gradually enhances with the increase of Zn content in the underdoped regime (y=0.05). However, Tc sharply drops in the over-doped regime (y=0.15), and only 2% Zn doping can destroy superconductivity. Combined with the theoretic works, we suggest that the SC paring symmetry may vary with the carrier density (doping level). In the under-and optimal-doped regime, the SC paring symmetry should be an s++wave state which is not sensitive to the non magnetic impurities according to the Anderson theory. Because the SDW order is competing with superconductivity, the increase in Tc with Zn content in the underdoped regime could be due to the further sup-pression of SDW order by Zn impurity. In the over-doped regime, the SC paring symmetry may become the s+-or d-wave and therefore the non magnetic impurity can cause strong magnetic pair-breaking effect. Thus significant suppression of Tc is observed even with the addition of slight Zn content. These interesting conclusions are helpful to understanding the iron-based SC mechanism.(3) Finally, the magnetic properties of cobalt-based oxyarsenide SmCoAsO are inves- tigated by measuring magnetization, magnetoresistance and specific heat. The compound undergoes three magnetic transitions:a PM to FM transition of Co 3d electrons at Tc of about 80 K, a FM to AFM meta-magnetic transition around 45 K where the transition tem-perature decreases with the increasing magnetic field, and finally an AFM order of the local magnetic moment of rare earth Sm ion at 5.6 K. SmCoAsO is an itinerant ferromagnetic compound, and RKKY interaction should play a key role in the magnetism. Below 45 K, the magnetic structure should be the A-type AFM order. The FM order remains within the CoAs layer, and the AFM coupling between the CoAs layers was favored due to the sign change of RKKY interaction which is sensitive to the c-axis length. However, the AFM coupling is very weak, and a magnetic field of about 2 T can induce an AFM-FM meta-magnetic transition. In a summary, a rich magnetic phase diagram is established and the interplay between the 3d electrons of Co and 4f electron of Sm is discussed.