Protonation And Physical Properties Of Iron-based Superconductor FeSe

Among iron based superconductors,FeSe with the simplest crystal structure has attractted great interest of researchers in recent years due to its unique properties.One of the important topics is the tunability of its superconductivity,which is very vital not only for the mechanism of high temperature superconductivity but also for practical applications.The superconducting transition temperature（T_c）of FeSe can be significantly enhanced by high pressure,electron doping,intercalation and dimensionality reduction.With the appearance of High-T_c superconductivity,various electron orders,such as nematic phase and spin density wave,have been observed.Meanwhile,the evolution of the electronic structure with T_c is very crucial for understanding the electronic behavior and high-temperature superconductivity mechanism of FeSe-derived superconductors.Recently,a new developed hydrogen intercalation technology（protonation）has been applied to tune the carrier doping of materials,which has realized the enhancement of superconductivity in various materials including FeSe.Based on the previous research results,we used the protonation method to continuously tune the carrier concentration of FeSe single crystals,and systematically studied the superconducting state and normal state properties of H_x-FeSe single crystal after protonation.The optimal H_x-FeSe single crystal with T_c 44 K was obtained by the protonation,and its upper critical field anisotropy and normal state transport properties were systematically studied.Iron-based superconductors show great potential in practical applications due to high T_c and extremely high upper critical field.The high critical current density and low magnetic relaxation are necessary for the practical application of high temperature superconductors.For electron-doped FeSe superconductors,critical current density and flux pinning are the most important properties in practical applications.Due to the advantages of protonation method,we systematically studied J_c enhancement,flux pinning and vortex phase diagram in the optimal protonated H_x-FeSe single crystal.In addition,magnetic flux motion and fluctuation are very important issues about pinning mechanism and practical application,and strong electric application has a high requirement on magnetic flux creep rate.Therefore,the magnetic flux pinning and dynamics of Ca₁₀（Pt₃As₈）（Fe2-_xPt_xAs₂）₅ single crystal are also studied in this dissertation.The specific contents and conclusions are as follows:1.Protonation-induced discrete superconducting phases in bulk FeSe single crystals.The superconducting transition temperature,T_c,of FeSe can be significantly enhanced several-fold by applying pressure,electron doping,intercalating spacing layer,and reducing dimensionality.In this dissertation,we have found a series of discrete superconducting phases,with a maximum T_c up to 44 K,in H⁺-intercalated FeSe single crystals using an ionic liquid gating method.Accompanied with the increase of T_c,suppression of the nematic phase and evolution from non-Fermi-liquid to Fermi-liquid behavior was observed.An abrupt change in the Fermi surface topology was proposed to explain the discrete superconducting phases.A band structure that favors the high-T_c superconducting phase was also revealed.2.Upper critical field anisotropy and normal state transport properties of the H_x-FeSe single crystal.The optimal H_x-FeSe single crystal with T_c 44 K was obtained by the protonation,and its upper critical field anisotropy and normal state transport properties were systematically studied.The introduction of H⁺ions does not change the layer distance of H_x-FeSe,but leads to extremely large anisotropy.Hall measurement data show that electron doping is dominant in H_x-FeSe,but it cannot be scaled by Kohler’s rule.Meanwhile,we study the superconducting fluctuation of H_x-FeSe,which can be scaled by the two-dimensional lowest Landau energy level（LLL）,showing obvious two-dimensional characteristics.Comprehensive analysis shows that the large upper critical field anisotropy of optimal H_x-FeSe with T_c of 44 K is probably the result of the weakening of inter-layer coupling and the relatively two-dimensional Fermi surface topology.3.Significant enhancement of critical current density and vortex pinning in H⁺-intercalated FeSe single crytal.Here,we present a systematic study of J_c and vortex properties of H⁺-intercalated（H_x-FeSe）single crystals.The J_c of H_x-FeSe crystal is significantly enhanced,exceeding 1.3×10⁶ A/cm² at 4 K,which is more than two orders of magnitude larger than 1.1×10⁴ A/cm² of pristine FeSe.The pinning mechanism of H_x-FeSe is found to be surface pinning,which is different from the dominant strong point-like pinning in FeSe.The systematic study of the vortex phase transition and the underlying mechanism provides a wealth of information for the vortex phase diagram of H_x-FeSe single crystals.Our results confirm that the introduction of H⁺intercalation into FeSe not only enhance the T_c,but also significantly increase the value of J_c,which is favorable for practical applications.4.The magnetic relaxation of Ca₁₀（Pt₃As₈）（Fe2-_xPt_xAs₂）₅ single crystal.Ca₁₀（Pt₃As₈）（Fe2-_xPt_xAs₂）₅ shows an extremely high Gi comparable to that of cuprate,with a relatively low S in IBSs.The correlation between creep rate S and Ginzburg number Gi is conducive to understand the interplay between vortex dynamics and relevant parameters.Combined with a universal lower limit S～Gi^1/2（T/T_c）,this dissertation provides new clues to design high-temperature superconductors with slow creep and predict their application potentials.We present a comprehensive study of the vortex pinning and dynamics in a Ca₁₀（Pt₃As₈）（Fe2-_xPt_xAs₂）₅single crystal via the magnetization measurements.According to the magneto-transport and magnetization data,the vortex phase diagram of Ca₁₀（Pt₃As₈）（Fe2-_xPt_xAs₂）₅ is finally constructed.Such a large Gi and slow creep caused th breakthrough of the lower limit of magnetic relaxation in Ca₁₀（Pt₃As₈）（Fe2-_xPt_xAs₂）₅ single crystals.This helps to deepen our understanding of vortex dynamics and practical applications.