The Phase Diagram And Isotope Effect Of Iron-based Superconductors And Properties Of Layered Oxypnictides Or Chalcogenides

Since the firrst discovery of high temperature cuprate superconductors in 1986,high temperature superconductivity（HTSC） has been a very hot topic in condensedmatter physics. However, over twenty years later, there is still no consensus on themechanism for superconducting pairing in HTSC cuprates. In early 2008, the discov-ery of LaFeAsO_1-xF_x boosted HTSC into the iron age of superconductivity. Physicistsaround the world devoted extensive efforts to searching and exploring the same un-derlying physics between iron-based and cuprate superconductors and hoped to findthe origin of high temperature superconductivity.In this dissertation, we systematically studied the anomalous transport propertiesand phase diagram in FeAs-based SmFeAsO_1-xF_x, which provide the evidence forcoexistence and competition of magnetism and superconductivity in SmFeAsO_1-xF_x.We also investigated the effect of oxygen and iron isotope substitution on supercon-ducting transition temperature （T_C） and the spin density wave （SDW） transitiontemperature （TSDW） in the SmFeAsO_1-xF_x and Ba_1-xK_xFe₂As₂ systems. Besides,we successfully synthesized several new layered transition metal oxypnictides or oxy-chalcogenides, which were characterized by X-ray diffraction, resistivity, susceptibility,Hall coe±cient（RH）, thermoelectric power（TEP） and heat capacity.1. Brief Overview of the Cuprates and Iron-Based SuperconductorsIn this chapter, we review the discovery of high temperature cuprate supercon-ductors and iron-based superconductors. The crystal structure, magnetic structure,phase diagram between magnetism and superconductivity and isotope effect in thesehigh-T_C superconductors are introduced in detail. The charge density wave （CDW）and SDW ground states and their collective excitations and electrodynamics are alsomentioned. There are coexistence and competition among CDW, SDW and supercon-ductivity in most superconductors.2. Anomalous Transport Properties and Phase Diagram of the FeAs-Based SmFeAsO_1-xF_x SuperconductorsIn this chapter, the detailed phase diagram and anomalous transport propertiesof Fe-based high-T_C superconductors SmFeAsO_1-xF_x were studied. It is found thatsuperconductivity emerges at x- 0.07, and optimal doping takes place in the x-0.20 sample with the highest T_C = 54 K. T_C increases monotonically with doping; theanomaly in resistivity arising from structural phase transition or SDW order is rapidly suppressed, suggesting a quantum critical point around x- 0.14. As manifestations, alinear temperature dependence of the resistivity shows up at high temperatures in thex < 0.14 regime but at low temperatures just above T_C in the x > 0.14 regime; a dropin carrier density evidenced by a pronounced rise in the Hall coe±cient is observedbelow the temperature of the anomalous peak in resistivity. A scaling behavior isobserved between the Hall angle and temperature: cotμH / T^1.5 for all samples withdifferent x in SmFeAsO_1-xF_x system.3. A Large Iron Isotope Effect in SmFeAsO_1-xF_x and Ba_1-xK_xFe₂As₂In this chapter, the effect of oxygen and iron isotope substitution on T_C and TSDWin the SmFeAsO_1-xF_x and Ba_1-xK_xFe₂As₂ systems was studied in detail. The oxygenisotope effect on T_C and TSDW is very small, while the iron isotope exponent-C =?dlnT_C=dlnM is about 0.35 （0.5 corresponds to the full isotope effect）. Surprisingly,the iron isotope exchange shows the same effect on TSDW as T_C. The strong sensitivityof superconductivity and magnetism to the lattice may be responsible for the largeisotope effect. The iron isotope effect on TSDW and T_C is much larger than the oxygenisotope effect. The reason could be that the iron–arsenide plane is the conductinglayer and thus responsible for the superconductivity, and the SDW ordering originatesfrom the Fe moment. These results indicate that electron-phonon interaction playssome role in the superconducting mechanism, but a simple electron-phonon couplingmechanism seems unlikely because a strong magnon-phonon coupling is included. Theinterplay between the lattice and magnetic degrees of freedom is a key challenge forour understanding the mechanism of high-T_C superconductivity.4. SDW Transition of Fe1 Zigzag Chains and Metamagnetic Transitionof Fe2 in TaFe_1+yTe₃In this chapter, we systematically study the AFM order of Fe1 zigzag chainsand spin-°op of excess Fe2 under high magnetic field H through the susceptibility,magnetoresistance （MR）, Hall effect and specific heat measurements in high-qualitysingle crystal TaFe_1+yTe₃. These properties suggest that the high temperature AFMtransition of the TaFeTe₃ layers should be a SDW-type AFM order. Below TN, Fe1antiferromangetic zigzag chains will induce a inner magnetic field Hint to interstitialFe2, which results in an AFM alignment in Fe2 sites. Moreover, the magnetic cou-pling strength between Fe1 and Fe2 is enhanced by decreasing temperature. On theother hand, the external magnetic field H_ext inclines to tune interstitial Fe2 to formFM alignment along H_ext. When H_ext arrives at the \ coercive" field HC, which isable to break the coupling between Fe1 and Fe2, these interstitial Fe2 atoms take aspin-°op from AFM to FM alignment. The local moment of Fe2 is about 4 1B/Fe. From low field （<H_C） AFM to high field （>H_C） FM for Fe2, it also induces sharpdrop on resistivity and an anomalous Hall effect. The possible magnetic structure ofTaFe_1+yTe₃ is proposed from the susceptibility and MR. The properties related to thespin-?op of Fe2 supply a good opportunity to study the coupling between Fe1 andFe2 in these TaFe_1+yTe₃ or Fe_1+yTe with interstitial Fe2 compounds.5. The Physical Properties of the Layered Pnictide-Oxides:Na₂Ti₂Pn₂O（Pn=As, Sb）In this chapter, the compounds Na₂Ti₂Pn₂O （Pn=As, Sb） with anti-K₂NiF₄structure were characterized by magnetic susceptibility, magnetoresistance （MR）, Hallcoeffcient （RH）, heat capacity and thermoelectric power （TEP）. These compoundsexhibit an anomalous transition in resistivity and susceptibility at T_S～320 K forNa₂Ti2As2O and T_S～115 K for Na₂Ti₂Sb₂O. The Hall coeffcient of Na₂Ti₂Sb₂Oshows a pronounced rise below T_S, indicating a decrease in carrier density. The sign ofTEP changes from positive to negative and magnetoresistance pronouncedly increasesat T_S for Na₂Ti₂Sb₂O. Heat capacity shows an anomalous peak at T_S～110 K, atypical behavior for SDW/CDW instability or a structure distortion. These behaviorsof Na₂Ti₂Sb₂O are quite similar to that of FeAs-based high-T_C parent compoundREFeAsO and AEFe₂As₂ with a SDW instability. Na₂Ti₂Sb₂O could be consideredas a possible parent compound for superconductivity.6. The Structure and Physical Properties of the Layered Pnictide-Oxides （SrF）₂Ti₂Pn₂O （Pn=As, Sb） and （SmO）₂Ti₂Sb₂OIn this chapter, the novel family of pnictide-oxides compounds （SrF）₂Ti₂Pn₂O（Pn=As and Sb） and （SmO）₂Ti₂Sb₂O were predicted from the stacking of the well-known ?uorite type block [Sr₂F₂]²⁺ or [Sm₂O₂]²⁺ alternating regularly with the anti-CuO2-type Ti2O square planar layer, and subsequently synthesized. The samples（SrF）₂Ti₂Pn₂O and （SmO）₂Ti₂Sb₂O with an anti-K₂NiF₄ type were characterizedby X-ray diffraction, resistivity, susceptibility, Hall coeffcient （RH）, thermoelectricpower （TEP） and heat capacity. These compounds exhibit an anomalous transitionin resistivity and susceptibility at T_S～380 K for the As analogue, at T_S～200 Kfor the Sb analogue and at T_S～230 K for （SmO）₂Ti₂Sb₂O, respectively. A drop incarrier concentration evidenced by a pronounced rise in RH is observed below T_S for（SrF）₂Ti₂Sb₂O. Additionally, TEP changes sign below T_S for （SrF）₂Ti₂Pn₂O. Heatcapacity demonstrates an anomalous peak at T_S～198 K for （SrF）₂Ti₂Sb₂O and T_S～230 K for （SmO）₂Ti₂Sb₂O, respectively, consistent with that observed in resistivityand susceptibility. Such anomaly could arise from the SDW/CDW instability or struc-ture distortion. An anomaly associated with the Sm3+ ions antiferromagnetic order is observed at 3.2 K in the specific heat for the sample （SmO）₂Ti₂Sb₂O. The effectsof La or Gd substitution and pressure on the SDW/CDW instability of RTi2Sb2O（R=Sr₂F₂, Sm₂O₂） were also studied. The SDW/CDW transition is suppressed withincreasing external pressure in （SmO）₂Ti₂Sb₂O. For （SrF）₂Ti₂Sb₂O, the insulator be-havior below the SDW/CDW transition is also suppressed distinctly under externalpressure.7. Structural and Magnetic Properties of the Layered Manganese Oxy-chalcogenides （LaO）₂Mn₂Se₂O and （BaF）₂Mn₂Se₂OIn this chapter, the new layered manganese oxychalcogenides （LaO）₂Mn₂Se₂Oand （BaF）₂Mn₂Se₂O, isostructural to （LaO）₂Fe₂Se₂O, were synthesized by using solidstate reaction method. The single crystals of the former compound were also suc-cessfully grown using fusion method. The polycrytalline samples show the semicon-ducting behavior with the activation energy gaps of about 278 meV and 416 meV for（LaO）₂Mn₂Se₂O and （BaF）₂Mn₂Se₂O, respectively. The magnetic susceptibility andspecific heat indicate an antiferromagnetic （AFM） transition at around 160±1 K forthe former compound and 100±1 K for the second compound. The strong anisotropicmagnetic properties below TN of the former compound suggest a long-range cantedAFM ordering. A broad maximum of the susceptibility can be observed for the twocompounds at high temperatures of 360 K and 210 K, respectively, suggesting thatstrong frustrated magnetic correlation gives rise to low-dimensional AFM or short-range ordering at high temperature in these rare transition metal oxychalcogenideswith an AFM checkerboard spin lattice.