Nori Sudhakar

Electrical resistivity study of La, B doped nanocrystalline superconducting vanadium nitride

A class of nanocrystalline superconducting vanadium nitrides has been prepared by a chemical synthesis route. This method involves the simultaneous decomposition of [VO(NH2O)2Gly]⋅H2O complex and nitridation at 973 K in ammonia atmosphere resulting pure and doped VN with controlled stoichiometry. The structure and stoichiometry of these samples were characterized by x-ray diffraction, scanning electron microscope, energy dispersive analysis of x-ray, transmission electron microscope, and chemical analysis. The VN materials crystallize in the cubic structure. The crystallite size was found to decrease with doping. We obtained samples having crystallite sizes between 8 and 32 nm. The electrical resistivity variation in these materials was carefully measured and the results are presented. The resistivity data in the range Tc<T< 30 K have been fitted to the power law ρ=ρ0+ATn and analyzed the results in the light of the existing theories. It was found that the resistivity follows a T3 law rather than the expe...

Effect of La, B doping on the electrical resistivity and magnetic susceptibility of nanocrystalline vanadium nitride

High resolution electrical resistivity and magnetic susceptibility measurements on nanocrystalline VN samples doped with 0.2 at. % B and La, and 0.5 at. % B and La (crystallite size ∼6–11 nm) were done to study their normal state and superconducting properties. We analyze the resistivity data in the light of the existing theoretical framework. The resistivity of the former sample follows a Tn behavior at low temperature with n≈4 suggesting that electron-phonon scattering play a significant role in determining the low temperature normal state resistivity. While the latter follows a more complicated behavior with a negative temperature coefficient of resistivity indicating that electron-electron scattering in addition to disorder enhanced electron interaction determine the normal state resistivity. We observe that the system is driven towards a disordered state with increasing doping levels of B and La. The superconducting transition is found in the electrical resistivity and magnetic susceptibility studies...

Enhancement of ferromagnetism and metallicity in Ru-doped layered manganite system La1.2Ca1.8Mn2−xRuxO7 (x=0,0.1,0.5,1.0)

We report here the interesting magnetic and electrical transport properties of Ru-doped two-dimensional manganite system La1.2Ca1.8Mn2−xRuxO7 (x=0,0.1,0.5,1.0). This work aims to investigate the nature of the magnetic phase especially at low temperatures and electrical transport in the light of metal insulator transition exhibited by similar systems. The Ru doping is found to affect the magnetic and electrical properties considerably. The ferromagnetic ordering temperature (Tc) increases from 264 K for x=0 to 285 K for x=0.5 sample.

Structural and electrical resistivity studies on Bi1.6Pb0.4Sr2Ca4Cu5O y composition synthesized by different routes

Since the discovery of superconductivity in the Bi-Sr-Ca-Cu-O system [1], several experimental investigators have verified that Bi-Sr-Ca-Cu-O systems synthesized by different routes possess phases with Tc of 20, 80 and 110 K. Studies of disproportionation reaction by varying the doping concentration of Pb [2-4] in this system have also been reported. In this investigation we varied the synthesis conditions in Bi2_xPbxSr2Ca4CusOy composition in order to investigate the formation of superconducting phases, their disproportionation reaction and possible textured growth. We attempted synthesis of the above composition by three different routes: matrix ceramic (MC), glass ceramic (GC) and melt texture (MT). The resultant samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAD) and resistivity [p(T)] measurements and the results are reported.

Electrical resistivity in polycrystalline La1.8Sr0.2CuO4 and YBa2Cu3O7−δ - temperature dependence and critical fluctuations

Abstract High resolution electrical resistivity data have been taken from 300 K down to zero-resistance temperatures in polycrystalline La 1.8 Sr 0.2 CuO 4 anf YBa 2 Cu 3 O 7−δ samples. Although there are abundant similar studies in YBaCuO, there is hardly any report in a relatively low T c system like LaSrCuO. The resistivity data of both systems fit better to the metallic formula than the Anderson-Zou (A-Z) relation. Using Aslamazov and Larkins theory of excess conductivity, we find strong evidence of three-dimensional critical fluctuations in both the systems. In the case of YBaCuO, it is possible to go beyond the mean-field region to the critical one through a crossover regime because of its higher T c . In log e -log e plots of excess conductivity vs reduced temperature, we observe continuous curvature for both systems. However, the local slopes found in different regimes are consistent with the theory of Lobb for 3D fluctuations. Such plots are shown to be rather sensitive to the choice of T c .

Phase separated glass and resultant superconducting glass ceramics in BiSrCaCuO (2245) composition

The crystallisation of superconducting phases in a glass of nominal composition Bi2Sr2Ca4Cu5Oy has been found to depend on phase separation within the glass. The most interesting result is the attainment of zero resistance even though the volume fraction of superconducting phase, estimated from Xac measurements, is smaller than 0.1%. These results have been discussed on the basis of exsolution of different phases, partial melting and rearrangement of ions at around grain boundaries. Presumably, a narrow superconducting path is formed along the flow line of the partially melted state.

Metallic islands in carbon cluster films: doping fullerenes with fullerenes

Abstract High-resolution scanning tunneling microscopy/spectroscopy studies of carbon cluster films revealed the existence of highly conducting islands, comprising two types of molecules: spherical C 60 (diameter ∼ 0.7 nm) packed into a fcc lattice ( a =1.4 nm ) and inclusions of a few larger (diameter ∼ 1 nm) asymmetrical molecules of higher clusters. This is supported by EPR and direct electrical conductivity measurements. The origin of metallic regions and unpaired spins in the undoped fullerene films can be accounted for by the model of charge transfer from higher carbon clusters to the neighbouring C 60 molecules. This is the first direct experimental evidence of the possibility of metallic conductivity in only-fullerene solids.