The Exploration Of Several Iron-based Or Chromium-based Superconductors With Low-dimensional Crystal Structures

Superconductors exhibit macroscopic quantum phenomena such as zero resistance,Meissner effect,and Josephson effect,which can be used for magnetic field measurement,quantum computing,and high field applications,but there is still no consensus on the microscopic mechanism of superconductivity yet.Two families of high-temperature superconductors:the cuprate superconductors and the iron-based superconductors are both layered compounds with quasi-two-dimensional crystal structures.In this essay,we introduce the discovery and the doping phase diagram of the quasi-two-dimensional iron-based compound EuFeAs₂.We also introduce the explorations of several Cr-based superconductors with quasi-one-dimensional crystal structures.The iron-based superconductors contain a unique 112-type compound Ca_1-xRE_xFeAs₂（RE=Rare earth elements）.Ca_1-xRE_xFeAs₂ has a crystal structure with an independent arsenic layer in the lattice,and the electron doping by replacing Ca with La enhances the Ferelated spin density wave（SDW）.However,Ca_1-xRE_xFeAs₂ is stabilized only when 0.10≤x≤0.27,and the undoped Ca FeAs₂ is not available.In this essay,we studied the discovery and the electron doping phase diagram of another 112phase EuFeAs₂.We have synthesized EuFeAs₂,the Pr-doped Eu_1-xPr_xFeAs₂,and the co-doped Eu_0.9Pr_0.1Fe_1-yCo_yAs₂ polycrystals,and studied their crystal structures,electrical resistivity,and magnetic susceptibility properties.EuFeAs₂ is a system containing Ferelated SDW,Eu related antiferromagnetism（AFM）,and superconductivity.In the Pr doped Eu_1-xPr_xFeAs₂,the temperature of SDW transition(T^SDW)shifts from 103 K in the undoped EuFeAs₂ to 87 K when x=0.20.And the transition temperature of the Eu related AFM(T^Eu-AFM)is lowered from 40 K to 27 K simultaneously.The optimal superconductive transition temperature（T_c=8.3 K）is achieved at x=0.15.In the superconductive region（0.035≤x≤0.20）,superconductivity,SDW,and AFM coexist.In order to fully suppress SDW and achieve a higher T_c,we synthesized a series of co-doped Eu_0.9Pr_0.1Fe_1-yCo_yAs₂ polycrystals.T^Eu-AFM remains unchanged,while T^SDW shifts to lower temperatures with increased Co content.The SDW transition disappears at y=0.07,where the highest T_c=30.6 K is achieved simultaneously.Although Pr and Co doping are both electron dopings to EuFeAs₂,they have significantly different effects on the magnetic orderings.This may be the result of the weakened couplings between the adjacent Eu layers and Fe₂As₂ layers,which infers the enhanced two-dimensionality of EuFeAs₂ by the arsenic layers.The recently discovered K₂Cr₃As₃and several other quasi-one-dimensional Cr-based superconductors exhibit unconventional superconductivity,such as the large upper critical field,the absence of the spin-lattice coherent peaks,and the detrimental to superconductivity by the non-magnetic impurities.Theoretical studies suggest that these compounds are possibly p-wave superconductors.We have successfully synthesized the 133-type KCr₃As₃（T_c=5 K）and Rb Cr₃As₃（T_c=7.3 K）single crystals by the deintercalation methods.This essay introduces the discovery and the unconventional superconductivity of the Rb Cr₃As₃ single crystals.Rb Cr₃As₃ single crystals crystallize in a centrosymmetric structure（P6₃/m（No.176））,and the resistivity shows metallic behavior at the normal state.Rb Cr₃As₃ single crystals have anisotropic upper critical fields:the upper critical field is 54.1 T when the magnetic field is applied perpendicular to the c direction,while the upper critical field is 29.9 T when the magnetic field is parallel to the c direction.The upper critical fields are higher than the Pauli limited field（μH_P（0）=1.84*T_c=13.4 T）in both directions,which indicate the unconventional superconductivity.For the 133-type Cr-based materials,the as-prepared polycrystals and the unannealed single crystals exhibit different behaviors:the as-prepared polycrystalline samples are non-superconductive,but the single crystals are superconductive without annealing.The transformation process from Rb₂Cr₃As₃ to Rb Cr₃As₃ is studied by preparing a series of samples with immersing Rb₂Cr₃As₃ in ethanol for different periods of time.The structural and magnetic susceptibility properties of these samples show a relation between the lattice parameters and superconductivity:the larger the crystal lattice,the higher the superconductive transition temperatures.The enlargement of the lattice with the prolonged immersing time may come from the relax of the crystallographical strain,and it may also arise from the intercalation of H ions in the Cr As chains.We have also prepared CsCr₃As₃ single crystal samples by the deintercalation methods.The CsCr₃As₃ single crystals show trace superconductivity at 6.6 K,and they are metallic at the normal state,but no diamagnetic signal is found in the magnetic susceptibility.We have also tried to synthesize the Li₂Cr₃As₃ with the ion exchange methods.An unknown peak is found in the X-Ray diffraction patterns,which may indicate the presence of a new chemical phase.In the last part of the essay,the doping of Sb/P in K₂Cr₃As₃ polycrystals is studied.T_c is decreased in the doped samples,which may arise from lattice distortions or disorders introduced by doping.