| Due to the various kinds of compounds,the rich physical properties,and the potential application prospects,iron-based superconductors have been paid great attentions ever since its discovery.The superconducting Tc of rare earth(RE)element electron-doped 122-type Ca1-xRExFe2As2 crystals can be up to 49 K,which is the highest among various Fe-based bulks except 1111 system.This Tc value is remarkably high in comparision with hole-type Ba1-xKxFe2As2 superconductor(Tcmax?38 K)bearing the same crystalline type and also the same RE-doped iso-structrual Sr1-xRExFe2As2(Tcmax?22 K),which has stimulated strong research interests in regard of the origin of the abnormal high-Tc superconductivity.The new 112-type superconductors Ca1-xRExFeAs2 also shows superconductivity over 40 K,which has attracted attentions from many researchers for their unique monoclinic crystal structure and additional As Zig-Zag chain with possible topological properties between FeAs layers.This thesis has investigated the superconductivity related electrical and magnetic properties of 122-type Ca1-xRExFe2As2 and 112-type Ca1-xRExFeAs2 crystals in detail.The first chapter is an introduction.We first briefly introduce the basic knowledge of superconductivity,and then give an overview of the fundmental properties of Fe-based superconductors.At last,we emphasise on the current research status of both 122-type Ca1-xRExFe2As2 and112-type Ca1-xRExFeAs2 superconductors.In chapter 2,we describe the growth method of the studied crystals,the structural and elemental characterization methods,the electrical and magnetic measurement strategies,and high-presure electric and magnetic measurement techniques.In chapter 3,we first discover the two superconducting phases in 122-type Ca1-xLaxFe2As2 on basis of electrical and magnetic measurement results.Then,through rotating the angle between sample and magnetic field in resistivity measurements,the high-Tc superconductivity is demonstrated to show high magnetic anisotropy.Furthermore,it is found that the angle-dependent upper critical field can be nicely fitted by Tinkham's formula for extremely thin films,indicating a two-dimensional nature of the high-Tc superconductivity.At last,we find that transition metal element(Co.Ni)co-doping can introduce additional electron and arouse superconductivity around 35 K in under-doping Ca1-xLaxFe2As2 which doesn't show high-Tc superconductivity.Co and Ni doping phase diagrams can be scaled into a single curve.For a large range of electron doping,the high Tc value keeps essentially unvariable.This strange phenonmenon quite resembles the interface superconductivity in La2CuO4-La2-SrxCuO4 bilayer system.Combined with the two-dimensional nature for the high-Tc superconductivity,our results support the idea of interface effect induced superconductivity.In chapter 4,we study the anisotropic upper critical field,thermal activated energy,and critical current density of a 112-type superconductor.The upper critical field anisotropy is found to locate between the value in 122-type and in 1111-type Fe-based superconductors.In chapter 5,we observe the pronounced second magnetization peak effect in Co-co-doped Ca1-xLaxFeAs2 single crystal and study the vortex dynamics in detail.The second magnetization peak is found to show strong association to a crossover from elastic to plastic vortex creep.We discuss the related influence factors in this elastic to plastic vortex creep crossover induced second magnetization peak effect.In chapter 6,we investigate the high-pressure effect on 112-type crystals.For samples showing an antiferromagnetic phase transition in their normal state,pressure causes an initial decrease and a following increase of the superconducting Tc,revealing a V-shape pressure dependent phase diagram.While,for samples only showing superconductivity,Tc first increases and then decreases with the gradual increase of pressure,forming a dome-shape phase diagram.Through Hall measurement under pressure,we demonstrate that pressurizing can increase the weight of electron pocket in the Fermi surface and thus plays the similar role as La doping.However,a different point from La doping is,throughout the whole pressure range(far above the total suppression of superconductivity),there is on witness of anomalous behavior associated with the antiferromagnetic phase transition in the resistivity curves.Additionally,when the sample shows superconductivity,the normal state transport shows a serious deviation from Fermi liquid behavior.While,when the superconductivity is totally suppressed by pressure,the sample shows a recovery of conventional Fermi liquid behavior in the normal state transport.Chapter 7 is a summary of the whole research contents. |
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