Single Crystal Growth And Physical Properties Of Iron-based Superconductors With Alkali Metal

Superconductivity is a macroscopic quantum phenomenon that shows both the vanishing of resistance and exclusion of magnetic field. It has been extensively studied since its discovery in1911, because of its rich physical properties and huge practical application potential. The discovery of cuprate superconductors opened a new era of high temperature superconductors. However, the mechanism of high temperature superconductivity remains unrevealed yet. Another breakthrough in the field of su-perconductivity took place in2008, when Hosono et al. reported the discovery of superconductivity at Tc=26K in LaFeAsO1-xFx. The discovery of iron-based super-conductors gave another opportunity to investigate the physics of high temperature superconductivity besides the cuprates. Scientists hope to find out the general fea-tures of high temperature superconductors through comparative studies on iron-based superconductors and cuprate superconductors, and eventually reveal the mechanism of high temperature superconductivity.In this dissertation, We grew high-quality single crystal of Rb0.88Fe1.81Se2, and found its superconductivity at32K through resistivity and magnetic measurements. We also grew a series of high-quality NaFe1-xCOx;As single crystals, and systematically studied their physical properties through structural, transport, thermodynamic, and high-pressure measurements. We mapped out the phase diagram of NaFe1-xCoxAs. We also found a crossover line in the phase diagram resembles the pseudogap phase in cuprates, at which Hall angle deviates from the power-law temperature dependence. Cu doping effect in NaFeAs was also carefully studied. Moreover, we investgated the gap symmetry of AFe2As2(A=K, Cs) through Co doping effect and specific heat study.This dissertation is divided into the following five chapters:1. Brief overview of iron-based superconductorsIn this chapter, we briefly review the historical development of superconductivity, basic physical properties of iron-based superconductors, and some experimental meth-ods. We mainly review the crystalline structure, phase diagram, band structure, and pairing symmetry of iron-based superconductors. We briefly introduce the methods of single crystal growth, specific heat, and high pressure measurements, and discussed their applications in the studies of iron-based superconductors.2. Superconductivity at32K in single-crystalline RbxFe2-ySe2In this chapter, we successfully grew high-quality single crystal of Rb0.88Fe1.81Se2, which shows a clear superconducting transition in magnetic susceptibility and electri-cal resistivity. Resistivity shows the onset superconducting transition (Tc) at32.1K and zero resistivity at30K. Prom low-temperature iso-magnetic-field magnetoresis-tance, the large upper critical field HC2(0) was estimated to be as high as180T for the field applied in-plane and59T for the field applied along the c axis. The anisotropy Hc2ab(0)/Hc2c(0) is around3.0, lying right between those observed in KxFe2-y/Se2and CsxFe2-y/Se2.3. Phase diagram and physical properties of NaFe1-xCox;As single crys-talsIn this chapter, we grew a series of high-quality NaFex-xCoxAs single crystals, and studied their physical properties through structural, transport, thermodynamic, and high-pressure measurements. Based on the data of resistivity and magnetic sus-ceptibility, we mapped out the phase diagram of NaFe1-xCoxAs. Replacement of Fe by Co suppresses both the structural and magnetic transitions; however, it en-hances superconducting transition temperature and superconducting component frac-tion. Superconductivity at20K is observed at optimally doped component x=0.028. Superconductivity is suppressed by further Co doping, and eventually form a dome-like superconducting region. Through the Hall measurements, we found that the cotangent of the Hall angle follows T4-dependence for the parent compound with filamentary superconductivity and T2for the heavily overdoped non-superconducting sample. However, it exhibits approximately T3-dependence in all the samples with bulk superconductivity, suggesting that this behavior is associated with bulk supercon-ductivity in ferropnictides. A deviation develops below characteristic temperature T*well above the structural and superconducting transitions, accompanied by a depar-ture from power-law temperature dependence in resistivity. The doping dependence of T*resembles the crossover line of the pseudogap phase in cuprates. All super-conducting samples show large positive pressure coefficients and temperature-linear dependence in high temperature up to500K, which is distinct from the overdoped non-superconducting samples. Analysis on the superconducting-state specific heat of the optimally doped crystal provides strong evidence for a two-band s-wave order parameter with gap amplitudes of Δ1(0)/kBTc=1.78and A2(0)/kBTc=3.11.4. Phase diagram and Cu doping effect in NaFe1-xCuxAs single crystalsIn this chapter, we grew a series of high-quality NaFe1-xCua;As single crystals by self-flux technique, which were systematically characterized via structural, trans-port, thermodynamic, and high pressure measurements. Both structural and magnetic transitions are suppressed by Cu doping, and bulk superconductivity is induced by Cu doping. Superconducting transition temperature (Tc) is initially enhanced from9.6to11.5K by Cu doping, and then suppressed with further doping. A phase diagram similar to NaFe1-xCoxAs is obtained except that semiconducting instead of metallic behavior is observed in the extremely overdoped samples. Tcs of underdoped, opti-mally doped, and overdoped samples are all notably enhanced by applying pressure. Although a universal maximum transition temperature (Tcmax) of about31K un-der external pressure is observed in underdoped and optimally doped NaFe1-xCoxAs, Tcmax of NaFe1-xCuxAs is monotonously suppressed by Cu doping, suggesting that impurity potential of Cu is stronger than that of Co in NaFeAs. The comparison between Cu and Co doping effect in NaFeAs indicates that Cu serves as an effective electron dopant with strong impurity potential, but part of the doped electrons are localized and do not fill the energy bands as predicted by the rigid-band model.5. Anomalous impurity effects and specific heat study of AFe2As2(A=K, Cs)In this chapter, we managed to grow K(Fe1-xCox)2As2and CsFe2As2single crys-tals by using a self-flux method. Instead of increasing superconducting transition temperature Tc through electron doping, we find that Co impurities rapidly suppress Tc down to zero at only x≈0.04. Such an effective suppression of Tc by impurities is quite different from that observed in Ba0.5K0.5Fe2As2with multiple nodless supercon-ducting gaps. Thermal conductivity measurements in zero field show that the residual linear term k0/T only changes slightly with3.4%Co doping, despite the sharp increase in the scattering rate. The implications of these anomalous impurity effects are dis-cussed. The CsFe2As2crystal shows a sharp superconducting transition at1.8K with a transition width of0.1K. The sharp superconducting transition and pronounced jump in specific heat indicate high quality of the crystals. A large ΔCP/TC accompa-nying a very low Tc, non-exponential dependence of the lowest temperature ΔCP/TC and an H1/2dependence of Δγ(H) are observed in the superconducting-state specific heat. Combined with the fitting results by the a model for superconducting-state specific heat, a pairing symmetry with nodal gaps was suggested in CsFe2As2...