The Synthesis And Transport Properties Of Iron-based Superconductor And Transition Metal Dichalcogenides

Early 2008,a new kind of superconductor - iron-based superconductor was discovered and the superconducting transition temperature(T_c) was raised from 26K of LaO_1-xF_xFeAs to higher than 50K rapidly.The T_c was higher than the theoretical value(39K) predicted from Bardeen-Cooper-Schrieffer(BCS) theory,and this provided a strong argument for considering layered oxypnictide superconductors as unconventional superconductors.Physicists around the world are hailing the discovery of the new iron-based compounds as a major advance,as the only other high-temperature superconductors are the copper-oxygen compounds,or cuprates,that were discovered in 1986.These discoveries have provided a kind of material base for studying the origin of high temperature superconductivity.In this dissertation,we discovered a series of iron-based superconductor and the single crystal of 122-phase was grown successfully.The magnetic and transport prosperities were also studied.We synthesized the single crystal of transition metal dichalcogenides.The structure and trausport properties were also investigated.1.Brief overview of the iron-based superconductor and transition metal dichalcogenidesIn this chapter,the author reviews the discovery of iron-based superconductor. The structure,transport properties,magnetic properties and phase diagram are also introduced.The recent development in theoretical and experiment is also mentioned. For transition metal dichalcogenides,the structure and charge-density-wave of these compounds are reviewed.We also introduce the method of the single crystal growth - chemical vapor transport method.2.Superconductivity induced by oxygen deficiency in La_0.85Sr_0.15FeAsO_1-δIn this chapter,We synthesized single phase La_0.85Sr_0.15FeAsO_1-δ samples and systematically studied the effect of oxygen deficiency in this system.It is found that partial substitution of Sr for La induces the hole carrier evidenced by positive thermoelectric power(TEP) but no bulk superconductivity is observed.The superconductivity can be realized by annealing the as-grown sample in vacuum to produce the oxygen deficiency.With increasing oxygen deficiency,the spin-density-wave(SDW) is suppressed,the superconducting transition temperature(T_c) increases and the maximum Tc reaches 26 K-the same as that observed in the LaFeAsO_1-xF_x system. TEP changes the sign from positive for the nonsuperconducting as-grown sample to negative for the superconducting samples with oxygen deficiency,while Hall coefficient (R_H) keeps negative for all samples.It suggests that the dominated carrier in La_0.85Sr_0.15FeAsO_1-δ is electronlike.3.Transport properties and superconductivity in M_1-xSm_xFFeAs(M=Sr,Ba and Eu) systemsIn this chapter,We synthesized the samples M_1-xSm_xFFeAs(M=Sr,Ba and Eu) with ZrCuSiAs-type structure.These samples were characterized by resistivity and susceptibility.It is found that substitution of rare earth metal for alkaline earth metal in the two systems suppresses the anomaly in resistivity caused by the SDW order, and induces superconductivity.Superconductivity at 56K in nominal composition Sr_0.5Sm_0.5FFeAs,54 K for Ba_0.5Sm_0.5FFeAs and at 51 K for Eu_0.5Sm_0.5FFeAs is realized, indicating that the superconducting transition temperatures in the iron arsenide fluorides is the same as that in oxypnictides with the same structure.4.Transport properties and superconductivity in Ba_1-xM_xFe₂As₂ (M=La and K) with double FeAs layersIn this chapter,we synthesized the samples Ba_1-xM_xFe₂As₂(M= La and K) with a ThCr₂Si₂ - type structure.These samples were systematically characterized by resistivity,thermoelectric power(TEP) and Hall coefficient(R_H).BaFe₂As₂ shows an anomaly in resistivity at about 140 K which can be attributed to SDW order.The substitution of La for Ba leads to a shift of the anomaly to low temperature,but no superconducting transition is observed.Potassium doping leads to the suppression of the anomaly in resistivity and induces superconductivity at 38K.The Hall coefficient and TEP measurements indicate that the TEP is negative for BaFe₂As₂ and La-doped BaFe₂As₂,indicating an n-type carrier;while potassium doping leads to a change of the sign in R_H and TEP.It definitely indicates a p-type carrier in the superconducting Ba_1-xK_xFe₂As₂.5.Different resistivity response to spin-density wave and superconductivity at 20 K in Ca_1-xNa_xFe₂As₂In this chapter,We report intrinsic transport and magnetic properties and their anisotropy in high-quality single-crystal CaFe₂As₂.The resistivity anisotropy(ρ_c/ρ_ab) is～50,less than the 150 of BaFe₂As₂,which arises from the strong coupling along the c-axis due to an apparent contraction of about 0.13 nm compared to BaFe₂As₂.In sharp contrast to the case of parent compounds ROFeAs(R = rare earth) and MFe₂As₂ (M = Ba and Sr),spin-density-wave(SDW) ordering(or structural transition) leads to a steep increase of resistivity in CaFe₂As₂.Such a different resistivity response to SDW ordering is helpful to understand the role played by SDW ordering in Febased high-T_c superconductors.The susceptibility behavior is very similar to that observed in single-crystal BaFe₂As₂.A linear temperature-dependent susceptibility occurs above the SDW transition of about 165 K.Partial substitution of Na for Ca suppresses the SDW ordering(anomaly in resistivity) and induces superconductivity at～20 K.6.Transport properties of single-crystalline Cu_xTiSe₂(0.015≤x≤0.110)In this chapter,Transport properties are systematically studied for single crystals of Cu_xTiSe₂(0.015≤x≤0.110).Both the in-plane and out-of-plane resistivity show a wide peak due to charge density waves(CDWs) for single crystals with x=0.025.After the CDW state is completely suppressed around x=0.055,the superconductivity is apparently enhanced by Cu doping.No superconducting transition is observed above 1.8 K for Cu_0.11TiSe₂.The anisotropy in the resistivity increases with increasing Cu content,and is nearly T independent.The CDW state has a strong effect on the Hall coefficient and thermopower.Large thermopower,comparable to that of the triangular lattice Na_xCoO₂,is observed in Cu_xTiSe₂.Intercalation of Cu induces a negative magnetoresistance due to the interaction between conducting carriers and localized magnetic moments.7.Transport properties and large anisotropic magnetoresistance in Cu_xNbS₂ Single CrystalsIn this chapter,Transport properties of Cu_xNbS₂(x=0.09,0.44 and 0.55) single crystals were systematically studied.In-plane and out-of-plane resistivities decrease with increasing Cu content,and a transition with hysteresis shows up for the crystals with x=0.44 and 0.55.The thermopower and Hall coefficient of Cu_xNbS₂ show opposite sign,indicating two kinds of carrier in this system.Angular dependence of in-plane magnetoresistance(MRab=(ρ_ab(H)-ρ_ab(O))/ρ_ab(O)×100%) at different temperature were also studied.The single crystals with x=0.44 and 0.55 show a strong anisotropic MR_ab.For the x=0.55 sample,the MR_ab reaches 80%with a magnetic field of 14 T applied along the c-axis,while the MR_ab is less than 5%for the magnetic field applied within ab-plane.These results can be well understood in light of the anisotropie Fermi surface in the multi-band system.