| Since the discovery of superconductivity,the exploration of superconducting materials and the study of the mechanism of superconductivity have always been the central research frontier in the field of condensed matter physics.In particular,the discovery of copper oxides and iron-based high-temperature superconductivity has injected hopes for the realization of room-temperature superconductors,and at the same time it presents new challenges for the study of superconducting mechanisms.Since then,the reduced dimensional materials containing transition metals attract significant interests for the exploration of novel high-Tc superconductors.Recently,a series of Cr-based superconductors A2Cr3As3?A=K,Rb,Cs?have attracted wide attention due to their quasi-one-dimensional?Q1D?lattice structure,strong electron-associated interactions,and possible spin-triplet superconducting pairs.In this thesis,we mainly explored and discovered a series of Q1D Cr and Mo system superconductors,and studied the influence of physical pressure and chemical doping effect on its superconductivity.A2Cr3As3 belong to a Q1D hexagonal lattice structure,in which the alkali metal element A occupies two non-equivalent crystallographic positions,namely the 3k-site and the 1c-site,and the atomic molar ratio of the two positions is 3:1.Since A2Cr3As3superconductors have positive chemical pressure effects,in order to increase the Tc of the Cr-based superconductor,we replaced the K in K2Cr3As3 with Na,which has a smaller ionic radius,to apply a positive chemical pressure.We prepared a series of polycrystalline(K1-xNax)2Cr3As3?x=01?using a solid-state reaction method.From the crystal structure and chemical composition characterizations of(K1-xNax)2Cr3As3,we found two distinct chemical phases with the same hexagonal lattice structure but distinguished by different site occupations of Na atoms at the two kinds of K-site in the K2Cr3As3 lattice,and Na2Cr3As3 cannot be obtained by high-temperature solid-state reaction method.Our experimental results present that when x<0.5,Na successively replaces the K atom and forms a solid solution phase,which is denoted as?-phase.When x?29??28?0.7,only the novel individual phase of(K0.25Na0.75)2Cr3As3exists,in which Na completely replaces the 3k-site K,and the 1c-site is still occupied by the K atom?denoted as?-phase?.The refined lattice constants of?-phase are a=9.408?8??,c=4.215?6??.In the middle region,a mixture of two phases exists,and the relative content of?-phase in the mixture increases with x increasing.Resistivity and susceptibility measurements suggest that superconductivity in?-phase is suppressed with lower Tc and shielding volume fraction,while superconductivity in?-phase is enhanced significantly with Tc 7.6 K and shielding volume fraction close to100%from ZFC mode.Furthermore,we substituted Cr with Mn in K2Cr3As3 to study the effect of impurity scattering on its superconductivity.The Mn-doped K2Cr3As3 singlecrystals were grown by a high-temperature melting method.EDS analysis of K2(Cr1-xMnx)3As3?x=0.050.15?single crystals shows that Mn can replace Cr,but the actual doping amount is much lower than the nominal component.Magnetic susceptibility measurements show that Mn doping can destroy the superconductivity of K2Cr3As3,and the superconductivity Tc and shielding volume fraction are significantly reduced.Unlike other A2Cr3As3 superconductors,Na2Cr3As3 cannot be obtained by high-temperature solid-state reaction method,so we tried various methods to prepare Na2Cr3As3.Finally,we obtained(K1-xNax)2Cr3As3 through ion-exchange reaction in sodium naphthalenide solution using KCr3As3 or K2Cr3As3 as precursor.In this thesis,we successfully prepared Na2Cr3As3 single crystals by ion-exchange reaction with excess sodium naphthalenide solution using K2Cr3As3 single crystal as the precursor.Chemical composition characterization confirms the atomic ration of 2:3:3,and no K element was detected.Similar with K2Cr3As3,the crystal structure of Na2Cr3As3belongs to a Q1D noncentrosymmetric hexagonal lattice structure with space group P-6m2?No.187?,in which the?Cr3As3?2–linear chains are separated by Na+ions,and the refined lattice parameters are a=9.239?2??,shrinking about 7.6%,and c=4.209?6??.The resistance,susceptibility,and specific heat measurements confirm the superconductivity of Na2Cr3As3 at 8.6 K,the highest Tc in Cr-based superconductors.The upper critical field derived from G-L formula is 54 T,much larger than the Pauli limit,and the Sommerfeld coefficient is 76.5 mJ mol–1 K–2 comparable with that of K2Cr3As3.This suggests that Na2Cr3As3 can be spin-triplet superconductors with strong electron correlations.The Tc of Na2Cr3As3 decreases upon hydrostatic pressure slowly,revealed by resistivity measurements under different pressures.ACr3As3 have a similar crystal structure as A2Cr3As3,while they are semiconductors and exhibit a spin-glass transition at low temperature,and no superconductivity is observed.In order to elucidate the origin of the different physical properties between the two systems,we prepared the single crystals of KCr3As3 by the deintercalation of K ions from K2Cr3As3 crystals.However,the electrical resistivity and magnetic susceptibility measurements exhibit that KCr3As3 single crystals show weak superconducting signals.In order to verify the origin of this superconductivity,we performed a series of annealing and immersing treatments on KCr3As3.Finally,we confirm that the annealed KCr3As3 single crystal display superconducting transition at 5 K with shielding volume fraction close to 100%.The upper critical field given by G-L formula is 20.8 T,much larger than the Pauli limit,and the Sommerfeld coefficient is 81.31 mJ mol–1 K–2,the similar value with that of A2Cr3As3superconductors.These characteristics indicate that KCr3As3 has the similar superconductivity of A2Cr3As3 and may be a spin-triplet superconductor with strong-electron correlations.The chemical composition is close to 1:3:3 revealed by EDS measurements.The lattice structure is centrosymmetric with space group P63/m?No.176?,in which?Cr3As3?2–linear chains are separated by K+,and the refined lattice parameters are a=9.090?8??,c=4.182?9??.We also prepared RbCr3As3superconductor with the similar method,and the Tc is 7.3 K.Different from A2Cr3As3,ACr3As3 belongs to a centrosymmetric crystal structure,and the superconductivity of ACr3As3 keeps stable,even after exposure to air or immersed into water.The discovery of ACr3As3 superconductors provides a new platform for the investigation of the superconductivity in Cr-based superconductors.KCr3As3 will decompose at473 K,and the Tc of KCr3As3 decreases significantly with pressure increasing.The linear chain in Q1D Cr-based superconductors consists of the similar structural units with Chevrel phase,and Chevrel-phase superconductors have abundant superconducting phase diagrams,high superconducting Tc and upper critical magnetic field.In order to reveal the relationship between the two types of superconductors,we tried to prepare Q1D A-Mo-As superconductors.The polycrystalline K2Mo3As3 was synthesized with a solid-state-reaction method.It crystalized in a noncentrosymmetric hexagonal crystal structure,which was analyzed with the Rietveld refinement of powder XRD patterns using K2Cr3As3 as a prototype.In the lattice structure,the?Mo3As3?2–linear chains are separated by K+ions,and the refined lattice parameters are a=10.145?5??,c=4.453?8??.The bulk superconductivity at 10.4 K is confirmed by the resistivity,susceptibility,and heat capacity measurements.The Sommerfeld coefficient of K2Mo3As3 is 13 mJ mol–1 K–2,which is between the value of Tl2Mo6As6 and K2Cr3As3.The needlelike singlecrystals of K2Mo3As3 with size 1?1?300?m3 were grown at 1173 K.The crystal structure characterization was consistent with the results of polycrystals,and the refined lattice parameters are a=10.146?0??,c=4.442?3??.Resistivity and susceptibility measurements revealed that Tc of K2Mo3As3 singlecrystals is 10.8 K.We also prepared Rb2Mo3As3 and Cs2Mo3As3 by the similar method,of which the Tc are 10.6K and 11.5 K,respectively.Tc of A2Mo3As3 exceeds that of previously reported Q1D superconductors.The discovery of A2Mo3As3 superconductors constructs the relationship between Chevrel-phase superconductors and Q1D Cr-based superconductors,providing new opportunities for the study of superconductivity in Q1D superconductors.We have also explored the superconductivity of many layered materials containing transition elements,especially compounds containing Mn,Cu,or Pd.Among them,we discovered the LaPd2Bi2 superconductor with Ca Be2Ge2-type structure.We successfully grown LaPd2Bi2 single crystals using Pd Bi as the flux.The lattice structure was characterized with powder XRD patterns,which was analyzed with Rietveld refinement using GSAS software,and the refined lattice constants are a=4.717?2??,c=9.957?3??.The resistivity and magnetic susceptibility measurements confirm the superconductivity of LaPd2Bi2 at 2.83 K.In summary,we discovered a series of Q1D superconductors Na2Cr3As3,ACr3As3,and A2Mo3As3,and investigated the doping effect of Na or Mn on the superconductivity of K2Cr3As3.This provides new ideas and platforms for the exploration of Q1D superconducting material and the study on the superconductivity of Q1D superconductors.In addition,we explored superconductivity of layered materials containing transition elements,and discovered La Bd2Bi2 superconductor with CaBe2Ge2 structure.These explorations accumulated rich experience for exploring new superconductors. |
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