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Register a new userEffect of high pressure annealing on the normal state transport of LaO0.5F0.5BiS2
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We study normal state electrical, thermoelectrical and thermal transport in polycrystalline BiS2-based compounds, which become superconducting by F doping on the O site. In particular we explore undoped LaOBiS2 and doped LaO0.5F0.5BiS2 samples, prepared either with or without high pressure annealing, in order to evidence the roles of doping and preparation conditions. The high pressure annealed sample exhibits room temperature values of resistivity ro around 5 mohmcm, Seebeck coefficient S around -20 microV/K and thermal conductivity k around 1.5 W/Km, while the Hall resistance RH is negative at all temperatures and its value is -10-8 m3/C at low temperature. The sample prepared at ambient pressure exhibits RH positive in sign and five times larger in magnitude, and S negative in sign and slightly smaller in magnitude. These results reveal a complex multiband evolution brought about by high pressure annealing. In particular, the sign inversion and magnitude suppression of RH, indicating increased electron-type carrier density in the high pressure sample, may be closely related to previous findings about change in lattice parameters and enhancement of superconducting Tc by high pressure annealing. As for the undoped sample, it exhibits the 10 times larger resistivity, 10 times larger |S| and 10 times larger |RH| than its doped counterpart, consistently with its insulating nature. Our results point out the dramatic effect of preparation conditions in affecting charge carrier density as well as structural, band and electronic parameters in these systems.
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We examined Lead (Pb) Substitution effect on a single crystal of a layered superconductor LaO0.5F0.5BiS2. Pb concentration dependence of the lattice constant showed slight anomaly at about 8% and 9% substitution of Pb for Bi. These samples showed the enhancement of the superconducting transition temperature and the superconducting volume fraction. Furthermore, these samples showed the anomaly in the temperature dependence of the resistivity at about 150K. These results were not observed in Pb substituted NdO0.7F0.3BiS2. Therefore, the enhancement of the superconducting properties by Pb substitution is related to the structural instability for the pale perturbation in LaO0.5F0.5BiS2.
The influence of room-temperature annealing on the parameters of the basal-plane electrical resistance of underdoped YBa$_2$Cu$_3$O$_{7-delta}$ and HoBa$_2$Cu$_3$O$_{7-delta}$ single crystals in the normal and superconducting states is investigated. The form of the derivatives $drho(T)/dT$ makes it possible to determine the onset temperature of the fluctuation conductivity and indicates a nonuniform distribution of the labile oxygen. Annealing has been revealed to lead to a monotonic decrease in the oxygen deficiency, that primarily manifests itself as a decrease of the residual resistance, an increase of $T_c$, and a decrease of the Debye temperature.
High-quality polycrystalline samples of LaO0.5F0.5BiS2 were obtained using high-pressure synthesis technique. The LaO0.5F0.5BiS2 sample prepared by heating at 700 C under 2 GPa showed superconductivity with superconducting transition temperatures (Tc) of Tconset = 11.1 and Tczero = 8.5 K in the electrical resistivity measurements and Tconset = 11.5 and Tcirr = 9.4 K in the magnetic susceptibility measurements, which are obviously higher than those of the LaO0.5F0.5BiS2 polycrystalline samples obtained using conventional solid-state reaction. It was found that the high-Tc phase can be stabilized under high pressure and relatively-low annealing temperature. X-ray diffraction analysis revealed that the high-Tc phase possessed a small ratio of lattice constants of a and c, c/a.
In this article, I review progress towards an understanding of the normal state (in-plane) transport properties of high-$T_c$ cuprates in the light of recent developments in both spectroscopic and transport measurement techniques. Against a backdrop of mounting evidence for anisotropic single-particle lifetimes in cuprate superconductors, new results have emerged that advocate similar momentum dependence in the transport decay rate $Gamma$({bf k}). In addition, enhancement of the energy scale (up to the bare bandwidth) over which spectroscopic information on the quasiparticle response can be obtained has led to the discovery of new, unforeseen features that surprisingly, may have a significant bearing on the transport properties at the dc limit. With these two key developments in mind, I consider here whether all the ingredients necessary for a complete phenomenological description of the anomalous normal state transport properties of high-$T_c$ cuprates are now in place.
Following the discovery of superconductivity in epsilon-iron, subsequent experiments hinted at non-Fermi liquid behaviour of the normal phase and sensitive dependence of the superconducting state on disorder, both signatures of unconventional pairing. We report further resistive measurements under pressure of samples of iron from multiple sources. The normal state resistivity of epsilon-iron varied as rho_0+AT^{5/3} at low temperature over the entire superconducting pressure domain. The superconductivity could be destroyed by mechanical work, and was restored by annealing, demonstrating sensitivity to the residual resistivity rho_0. There is a strong correlation between the rho_0 and A coefficients and the superconducting critical temperature T_c. Within the partial resistive transition there was a significant current dependence, with V(I)=a(I-I_0)+bI^2, with a >> b, possibly indicating flux-flow resistivity, even in the absence of an externally applied magnetic field.
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