Structural and Electrical Transport Properties of Doped Nd-123 Superconductors

It is generally believed that one of the key parameterscontrolling the normal state and superconducting properties ofhigh temperature superconductors is the charge carrierconcentrationpin the CuO2planes.By changing the non-isovalent dopingconcentration on the RE site as well as the oxygen content in(RE)Ba2Cu3O7−δ, an excellent tool is obtained tovary the hole concentration over a wide range from theunderdoped up to the overdoped regime.In the present thesis thefocus is on the doping effects on the structural and normalstate electrical properties in Nd-123 doped with Ca, La, Pr,Ca-Pr, and Ca-Th.T he effects of doping have been investigatedby X-ray and neutron powder diffraction, and by measurements ofthe resistivity, thermoelectric powerS, and Hall coefficient RH.T he thermoelectric power is a powerful tool forstudies of high temperature superconductivity and is highlysensitive to details of the electronic band structure.Sas a function of temperature has been analyzed in twodifferent two band models.The parameters of these models arerelated to charactristic features of the electron bands and asemiempirical physical description of the doping dependence ofSis obtained.So me important results are following: (i)The valence of Pr in the RE-123 family.Results from thestructural investigations, the critical temperature Tc, and thethermoelectric power indicated a valence +4 at low dopingconcentration, which is in agreement with results of chargeneutral doping in the RE-123 family.(ii)Hole localization. The results of bond valence sum (BVS)calculations from neutron diffraction data showed that holelocalization on the Pr+4site was the main reason for the decrease of thehole concentration p.Differ ent types of localization wereinferred by S measurements for Ca-Th and Ca-Pr dopings.(iii)Competition between added charge and disorder. Theresults of RH measurements indicated that Ca doping introduceddisorder in the CuO2planes in addition to added charge.This could bethe main reason for the observed small decrease of thebandwidth of the density of states in the description of aphenomenological narrow band model.(iv) Empirical parabolic relation between γ and p.S data were analyzed and well described by a two-band modelwith an additional linear T term, γT.An empiricalparabolic relation for γ as a function of holeconcentration has been found. Key words:high temperature superconductors, criticaltemperature, resistivity, thermoelectric power, Hallcoefficient, X-ray diffraction, Neutron diffraction, NdBa2Cu3O7−δ, hole concentration,substitution.NR 2014080

The charge transport mechanisms in conducting polymer polypyrrole films and fibers

Smart materials based on inherently conducting polymers are of practical interest and extend the advantages of electromaterials from the nanoscale to macroscale applications. However understanding mechanisms of the their electrical properties are still remained challanging. Herein, we report the transport properties of chemically and electrochemically preapred conducting polymer polypyrrole films and fibers. Conduction mechanisms of the as-prepared polypyrrole (PPy) were investigated using their electrical properties as a function of temperature (7-300 K). The results of the PPy resistivity were evaluated using a linear combination of Mott\u27s and Efros-Shklovskii\u27s Variable- Range Hopping (Mott-VRH and ES-VRH) models. It was found that the localization length was about 10 and 15 monomer units for the PPy-fiber and PPy-film, respectively. The contributions of Mott-and ES-VRH conductivities versus temperature exhibited that electrical conductivity of the PPy-film is two-dimensional Mott-VRH at temperatures of lower than∼100 K. It was indicated that the contribution of ES-VRH are larger than the contribution of Mott-VRH at temperatures of higher than ∼185 K. It was also found that ES-VRH law predominates at room temperature showed that average size of crystallites in the PPy-film was about three times larger than PPy-fiber

In-field Conductivity Fluctuations in Ba0.72K0.28Fe2As2 Single Crystals

2017 Springer Science+Business Media, LLC, part of Springer Nature conductivity of Ba 0.72 K 0.28 Fe 2 As 2 single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki−Thompson and Aslamakov−Larkin (MT-AL) two- to three-dimensional (2D-3D) crossover is found on the excess conductivity (Δσ) curves as the temperature approaches the superconducting critical temperature, T c . 3D fluctuations in superconductivity are realized near T c that are well fitted to experimental data by the 3D Aslamazov-Larkin theory. The Maki-Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition, T 0 , which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(ε 0 ), as functions of magnetic field were used to determine the coherence length and the lifetime, τ φ , of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba 0.72 K 0.28 Fe 2 As 2 single crystal gives a value of 3.76 x 10 − 12 s for the τ φ in the absence of magnetic field and it decreases to 2.4 x 10 − 12 s in magnetic field of 13 T

Magnetoreresistance of carbon nanotube-polypyrrole composite yarns

Three types of samples, carbon nanotube yarn and carbon nanotube-polypyrrole composite yarns had been investigated by measurement of the electrical conductivity as a function of temperature and magnetic field. The conductivity was well explained by 3D Mott variable range hopping (VRH) law at T \u3c 100 K. Both positive and negative magnetoresistance (MR) were observed by increasing magnetic field. The MR data were analyzed based a theoretical model. A quadratic positive and negative MR was observed for three samples. It was found that the localization length decreases with applied magnetic field while the density of states increases. The increasing of the density of states induces increasing the number of available energy states for hopping. Thus the electron hopping probability increases in between sites with the shorter distance that results to small the average hopping length

The second peak effect and vortex pinning mechanisms in Ba(Fe,Ni)2As2 superconductors

Vortex pinning mechanisms have been studied systematically in BaFe 1.9 Ni 0.1 As 2 single crystal as a function of temperature and magnetic field. The obtained shielding current density, J s , showed a second peak in the intermediate magnetic field range at high temperatures. The temperature dependence of the shielding current density, J s (T), was analysed within the collective pinning model at different magnetic fields. It was found that the second peak reflects the coexistence of both ¿l pinning, reflecting spatial variation in the mean free path (l), and ¿T c pinning, reflecting spatial variation in the superconducting critical temperature (T c ) at low temperature and low magnetic fields in BaFe 1.9 Ni 0.1 As 2 single crystal. The results clearly show that pinning mechanism effects are strongly temperature and magnetic field dependent, and the second peak effect is more powerful at higher temperatures and magnetic fields. It was also found that the magnetic field mainly controls the pinning mechanism effect

Excess conductivity in nano-carbon doped MgB 2 superconductor

In this research, the excess conductivity in 5 wt% nano-carbon doped MgB 2 superconductor was systematically studied as a function of magnetic fieldby measurements of the resistivity. The mean field temperature was calculated using two different methods. Two- and three-dimensional (2D and 3D) models were used to scale the excess conductivity caused by fluctuations. The mean field of the coherence length for the sample was obtained by using the Aslamazo-Lockerian model in the region of critical temperature. A transition from the 2D to the 3D region was observed in different fields at a crossover temperature as the temperature increased. The crossover temperatures were obtained by using the Maki-Thompson-Lawrence-Doniach (MT-LD) model. The results show that the crossover temperature decreases as the field increases. The phase-relaxation time of the fluctuation pairs was obtained by using the crossover temperature. The fluctuation pair lifetime, τ ϕ , and the coherence length were obtained by using the transition temperature and the reduced temperature crossover values as functions of magnetic field. The phase-relaxation time decreases with increasing field. It was found that the excess conductivity has 2D dimensionality behavior due to the Cooper pairs. Graphical abstract: [Figure not available: see fulltext.]

Excess conductivity in nano-carbon doped MgB

In this research, the excess conductivity in 5 wt% nano-carbon doped MgB2 superconductor was systematically studied as a function of magnetic fieldby measurements of the resistivity. The mean field temperature was calculated using two different methods. Two- and three-dimensional (2D and 3D) models were used to scale the excess conductivity caused by fluctuations. The mean field of the coherence length for the sample was obtained by using the Aslamazo–Lockerian model in the region of critical temperature. A transition from the 2D to the 3D region was observed in different fields at a crossover temperature as the temperature increased. The crossover temperatures were obtained by using the Maki-Thompson–Lawrence-Doniach (MT–LD) model. The results show that the crossover temperature decreases as the field increases. The phase-relaxation time of the fluctuation pairs was obtained by using the crossover temperature. The fluctuation pair lifetime, τϕ, and the coherence length were obtained by using the transition temperature and the reduced temperature crossover values as functions of magnetic field. The phase-relaxation time decreases with increasing field. It was found that the excess conductivity has 2D dimensionality behavior due to the Cooper pairs

Hydrostatic pressure induced transition from δtC to δℓ pinning mechanism in MgB2

The impact of hydrostatic pressure up to 1.2 GPa on the critical current density (Jc) and the nature of the pinning mechanism in MgB2 have been investigated within the framework of the collective theory. We found that the hydrostatic pressure can induce a transition from the regime where pinning is controlled by spatial variation in the critical transition temperature (δTc ) to the regime controlled by spatial variation in the mean free path (δℓ). Furthermore, critical temperature (Tc) and low field Jc are slightly reduced, although the Jc drops more quickly at high fields than at ambient pressure. We found that the pressure raises the anisotropy and reduces the coherence length, resulting in weak interaction of the vortex cores with the pinning centres. Moreover, the hydrostatic pressure can reduce the density of states [Ns(E)], which, in turn, leads to a reduction in the Tc from 39.7 K at P = 0 GPa to 37.7 K at P = 1.2 GPa