Effect Of Hole-carrier Doping On The Low Temperature Ordering And Transport Properties In "11" Type Iron-based Superconductor

Various physical properties in iron-based superconductor make the high temperature superconducting mechanism complicated and confusing.It is found that there are multiple electronic ordered phases in iron-based superconductors at low temperature,including superconductivity,electronic nematic order,antiferromagnetic order,etc.The electronic nematic order is new and common electronic state in iron-based superconductors,close to superconductivity and antiferromagnetic state.The interplay between antiferromagnetic order,electronic nematicity and superconductivity are complex and closely associated with each other.To ascertain the superconducting mechanism of iron-based superconductor,it is necessary to understand the mechanism that how the electronic nematicity and antiferromagnetic order influence superconductivity.Due to the complex interplay between each other,its mechanism is not yet clear and there is still a lot of controversy.The iron-based superconductors of“11”system have the most simple structure,non-toxic and easy preparation,making it to be the preferred materials to syudy the fundamental physics in the iron-based superconducting materials.It is found that various doping can greatly influence the electronic ordered phases and tune superconductivity effectivly in“11”type iron-based superconductors,which is an effective research method to explore the high temperature superconducting mechanism.Based on this background,this article focus on the hole-carrier doping in"11"type iron-based superconductors,further studied the interplay between superconductivity,electronic nematicity and antiferromagnetic order,providing valuable experimental data and theoretical clues for understanding the superconducting mechanism of iron-based superconductors.Guided by this study,the superconductivity and transport properties of“11”type iron-based superconductors are improved effectively through hole-carrier doping.The main contents and conclusions are as following.In order to study the relationship between antiferromagnetism and superconductivity in"11"type iron-based superconductors and the influence of hole-carrier doping on these two ordered phases,high-quality polycrystalline FeTe_1-xS_x(0?x?0.5)samples were prepared by two-step sintering method.The lattice paramaters of tetragonal Fe(Te,S)are calculated using XRD results and the solubility of S in tetragonal FeTe is about 20%.It is found that S doping(10%)in FeTe can suppress the formation of FeTe₂ and promote the formation of tetragonal phase.At the same time,the addition of S can optimize the microstructure and improve the quality of polycrystalline samples.On the one hand,the temperature dependence of magnetization and electronic resistivity measurements together demonstrate that S doping can suppress antiferromagnetism in FeTe.When the S doping level is more than 10%,the superconducting transition appears accompanied by the disappearance of antiferromagnetism and structure transition.Hall effect measurements show that hole carriers were introduced into FeTe by S doping and leads to the strong domination of hole carriers in FeTe_1-xS_x(0.1?x?0.3).This hole-carrier doping subtly changed the Fermi surface and made the hole type Fermi surface reconstruction,which eventually led to the disappearance of collinear antiferromagnetic state and the emergence of superconductivity.Furtherly,the FeSe_1-xS_x polycrystalline samples were slected as study object to study the relationship between electronic nematicity and superconductivity in"11"type iron-based superconductors and the influence of hole-carrier doping on these two ordered phases.It is found that proper S doping(x<20%)in FeSe is benefical to the formation of tetragonal phase.At the same time,the addition of S can densify the microstructure,leading to the optimization of the the quality of FeSe_1-xS_xpolycrystalline samples.As S atoms doping into FeSe by subsitution for Se sites,positive chemical pressure is introduced.With increasment of S doping,T_c and H_c2are enhanced to 12.2 K and 25.2 T with x=0.1,respectively,both of which are improved by about 30%.In addition,the electronic nematicity was suppressed gradually and finally vanished as x>0.2.Hall effect measurements demonstrate that the transport under T_s is dominated by hole carriers,characterised by obvious hole doping in FeSe_1-xS_x.Then high-quality superconducting FeSe_1-xS_x films were fabricated via pulsed laser deposition.The results show that these films have pure phase,high quality of epitaxial growth and excellent superconductivity.In contrast to the S doped FeSe bulk,the T_c and T_s in FeSe_1-xS_x films gradually decrease with the increasing of S content and no enhanced T_c is observed after the disappearance of the nematic transition.No matter in FeSe_1-xS_x bulk or film in this work,T_c shows no abruptly enhancement after the disappearance of T_s.These results suggest that there is more complex relationship between electronic nematicity and superconductivity in FeSe system,rather than the simple dependence as previously reported.According to the hole-carrier doping effect induced by Se/Te site substitution in FeSe,the possibility of hole-carrier doping by Fesite substitution and further enhancement of the superconductivity in FeSe were also studied.Based on the first principle calculations,the hole-carrier doping by Nb partially substituting for Fesite in FeSe were analyzed theoretically.Based on this point,FeNb_xSe polycrystalline samples were prepared.It is found that Nb doping can optimize the the microstructure of tetragonal phase and improve the compactness and grain connectivity in FeSe.With the increase of Nb doping,Nb with bigger radius incorporate FeSe by substituting for Fesites,negative chemical pressure is introduced and the superconductivity is gradually enhanced.With the optimal doping concentration of Nb as x=0.04,T_c is distinctly enhanced up to 13.6 K in FeNb_0.04Se and the upper critical field is enhanced up to 28 T,both of which are enhanced by about 30%as compared with that of undoped FeSe.And FeNb_0.04Se showed good transport performance at high magnetic fields(3 T)with a J_c raised by an order of magnitude.In addition,transport performance test results show that the electronic nematicity transition in FeSe is not suppressed by Nb doping.Below the transition temperature of electronic nematicity,the transport is dominated by hole carriers,characterised by obvious hole doping in FeNb_xSe(x=0.02,0.04,0.06).These results suggest that Nb doping in FeSe can introduce hole carriers effectively,relalizing the Fermi surface reconstruction characterised by hole-type Fermi surfaces increase and electron-type Fermi surfaces decrease,which is the main reason for the improvement of superconductivity in FeSe.