Single Crystal Growth And Electronic Transport Properties Of 112-type Iron Pnictide Superconductors Ca_1-xLa_xFeAs₂

In 2013,novel 112-type iron-based superconductors?IBSs??Ca,RE?FeAs₂?RE=Rare earth,112-type?were discovered and provided a new platform to study the unconventional superconductivity in IBSs due to its peculiar properties.Unlike the other intensively studied IBSs,it is crystallized in a low-symmetry crystal structure(Monoclinic,Space group P₂₁)with an additional metallic spacer-layer,containing one-dimensional zigzag As chains.The FeAs layers have been proven to be responsible for the High-T_c superconductivity,while the unique As zigzag bond layers could generate anisotropic Dirac cones,an additional three-dimensional?3D?hole pocket which is absent in other IBSs,and may also be possible for the realization of Majorana related physics.In this dissertation,we mainly focus on the high-quality single crystal growth and electronic transport properties of 112-type IBSs.Firstly,we investigated the Co doping effects on 112-type Ca_0.8La_0.2FeAs₂superconductors.It is found that the single crystal quality can be improved significantly by small amount of Co co-doping in Ca_0.8La_0.2Fe_0.98Co_0.02As₂.As the Co doping level increases,T_c is gradually suppressed and chemical phase separation emerges due to the formation of?Ca,La?Fe₂As₂ phase.Next,we performed systematical transport study in both normal and superconducting mixed-state of high quality Ca_0.8La_0.2Fe_0.98Co_0.02As₂ single crystal.In the normal state,the nonmonotonic T-dependent coefficient and the violation of Kohler's rule suggest either the change of carriers with T or the exotic anisotropic scattering below T<175 K.In the mixed-state,the field-dependent activation energy U₀ shows a power law dependence^with a crossover at around 2 T in both H?ab and H//ab,which is ascribed to the different pinning mechanisms.Moreover,the vortex phase transition from vortex-glass to vortex-liquid is confirmed according to the vortex-glass model.Finally,the results of mixed-state hall effect show that no sign reversal of?_xy?H?is detected,but?_xy?H?and?_xx?H?follow the relation|?_xy?H?|=A?_xx?H?^?well with an exponent?².0,which is in line with the results in theories or experiments previously reported on some high-T_c cuprates.Then,we comprehensively studied the behavior of upper critical field?₀H_c2?T?and its anisotropy?of 112-type IBSs.The anisotropy?close to T_c was firstly checked within the framework of the anisotropic Ginzburg-Landau?GL?theory.Afterwards,in order to study the intrinsic behavior of?₀H_c2?T?under high magnetic fileds and low temperatures,we measured the field-dependent resistivity with magnetic fields up to60 T.Detailed analysis suggests that two-band model is required to fully reproduce the behavior of?₀H_c2?T?for H//c,while the effect of spin paramagnetic effect is responsible for the behavior of?₀H_c2?T?for H//ab.The anisotropy?values show nonmonotonic temperature-dependent,which first increase and then decrease with decreasing temperature,consistent with the GL scaling results.After that,we studied the angular-dependent magnetoresistance?AMR?on the“parent”compound Ca_0.73La_0.27FeAs₂.The results show that the Fermi surface above the structural/antiferromagnetic transition?T_s/T_N?is quasi-two-dimensional?quasi-2D?,as revealed by the 2D scaling behavior of the AMR,??/??0??H,??=??/??0???₀Hcos??,?being the magnetic field angle with respect to the c axis.While such a2D scaling becomes invalid at temperatures below T_s/T_N,three-dimensional?3D?scaling approach by inclusion of the anisotropy of Fermi surface is efficient,indicating that the appearance of 3D Fermi surface contributed to the anisotropic electronic transport.Compared with other experimental observations,we suspect that the additional 3D hole pocket?generated by the Ca d orbital and As1 p_z orbital?around the?38?point in CaFeAs₂ will be disappeared in the heavily electron doped regime,and moreover,the Fermi surface should be reconstructed across the structural/AFM transition.Besides,a quasi-linear in-plane magnetoresistance with H//ab is observed at low temperatures and its possible origins are also discussed.Lastly,4d transition metal Pd doped single crystals of Ca_0.755La_0.245Fe_1-xPd_xAs₂?0?x?0.08?were grown using the self-flux method and characterized by x-ray diffraction,resistivity,magnetic susceptibility and Hall effect measurements,mapping out the T-x phase diagram.Bulk superconductivity up to T_c²8 K was reemerged by only a small amount of Pd doping?x⁰.013?in over-doped non-superconducting Ca_0.755La_0.245FeAs₂.The magnetization measurement reveals a fish-tail effect and relatively high critical current density J_c exceeding 10⁵ A/cm² at low temperatures.The magneto-optical images reveal a homogenous current flow within the crystal.Moreover,according to the analyses of the vortex pinning force density vs magnetic field,we found that the normal point defects are the dominant pinning sources,which probably originate from the variations of Pd dopant.Lastly,the vortex diagram of Ca_0.755La_0.245Fe_1-xPd_xAs₂?x⁰.013?was also established by combination of the magnetization and magneto-transport measurements.