Najmul Hassan

Fluctuation-induced conductivity of polycrystalline Ni doped Cu0.5Tl0.5Ba2Ca2Cu3−yNiyO10−δ (y=0, 0.5, 1.0, 1.5) superconductors

The fluctuation-induced conductivity of Ni free and Ni doped Cu0.5Tl0.5Ba2Ca2Cu3−yNiyO10−δ (y=0, 0.5, 1.0, 1.5) samples is investigated for comparison of dimensionality of fluctuations above the mean-field critical temperature. The temperature dependence of paraconductivity can be described by a power law following Aslamazov–Larkin (AL) type equations for these polycrystalline superconductors. It is observed from these studies that at higher temperatures, the fluctuations in the order parameter of the carriers follow two-dimensional (2D) AL behavior, whereas at lower temperatures (closer to transition temperature) their behavior is three-dimensional (3D) AL. From the analysis of our results, we have also evaluated the exponents of dimensionality, the coherence lengths, and the crossover temperatures. The crossover temperature from 2D to 3D have substantially been shifted to lower temperatures with increasing Ni doping, which is most likely related to the scattering of the carriers by remnant spins of Ni a...

A Density Functional Theory Study of Raman Modes of Hydrogenated Cadmium Sulphide Nanoparticles

Raman scattering investigations based on density functional theory (DFT) calculations were performed to explore the vibrational modes of wurtzite structured CdS nanoparticles (NPs). The calculations were performed to obtain the Raman spectra for the CdS containing 2, 4, 8 and 12 atoms to study the size dependence. Several vibrational modes indicating stretching and bending features related to Cd and S atoms were observed. Modifications of the frequency and intensity of different Raman modes with an increase in number of atoms in NPs are discussed in detail. It is found that the frequency of the CdS symmetric stretching mode of vibration shows a consistent red shift and that of CdS anti-symmetric stretching shows a consistent blue shift with the increase in the number of atoms. Hydrogen atoms were added in order to make the closed shell configuration and saturate the NPs as per the requisite for calculating the Raman spectra. This produced some additional modes of vibration related to hydrogen atoms. The SH stretching mode showed a consistent red shift and the CdH stretching mode showed a consistent blue shift with an increase in the number of atoms in NPs. The results generated are found to be in close agreement with the literature. The observed red shift in different modes is assigned to stimulated Raman stretching and blue shift is ascribed to the coherent anti-stokes Raman scattering.

A comparative study of structural, thermal and electrical properties of undoped and doped with dodecylbenzenesulphonic acid polypyrrole

Polypyrrole was synthesized and doped with dodecylbenzenesulphonic acid; the latter was confirmed by means of FTIR spectroscopy. The percentage of crystallinity of synthesized polymers was estimated from X-rays diffraction studies. The formation of flaky structure in doped polypyrrole was observed by means of SEM. DC conductivity was found to be influenced by dopant dodecylbenzenesulphonic acid. Temperature dependant DC shows three dimensional variable ranges hopping (3D VRH). Activation energy, density of states and hopping length were calculated and found to be influenced by adding dopant to polypyrrole. The doped polypyrrole was found to be more thermally stable as compared to that of pristine polypyrrole.

Enhanced three-dimensional excess conductivity in Be-doped Cu0.5Tl0.5Ba2Ca3−xBex Cu4O12−δ (x=0,0.5,0.75,1.0,1.25,1.5) superconductors

We report the enhanced fluctuation-induced conductivity of Be-doped Cu0.5Tl0.5Ba2Ca3−xBex Cu4O12−δ (x=0,0.5,0.75,1.0,1.25,1.5) samples. The analysis has been done with partial substitution of Be in place of Ca. In each case excess conductivity has been analyzed and we tried to make a fit with the Aslamasov–Larkin (AL) and Lawrence–Doniach equations. It is observed that our data fit well with the three-dimensional (3D) AL equation, and a crossover from two dimensions to three dimensions has been found in our samples. We have also employed this transition to estimate the Josephson coupling strength in our samples. This interlayer coupling strength J, which controls the superconducting transition, has been found to improve with increased Be content. The Ginzburg–Landau coherence lengths ξc(0) for all cases have also been calculated. The Fourier transform infrared spectroscopy (FTIR) absorption measurements also provide a clue for the enhanced 3D fluctuations. The higher electronegativity, as well as smaller ...

Interaction of point defects with twin boundaries in Au: A molecular dynamics study

The molecular dynamics simulation technique with many-body and semi-empirical potentials (based on the embedded atom method potentials) has been used to calculate the interactions of point defects with (1 1 1), (1 1 3), and (1 2 0) twin boundaries in Au at different temperatures. The interactions of single-, di-, and tri-vacancies (at on- and off-mirror sites) with the twin interfaces at 300 K are calculated. All vacancy clusters are favorable at the on-mirror arrangement near the (1 1 3) twin boundary. Single- and di-vacancies are more favorable at the on-mirror sites near the (1 1 1) twin boundary, while they are favorable at the off-mirror sites near the (1 2 0) twin boundary. Almost all vacancy clusters energetically prefer to lie in planes closest to the interface rather than away from it, except for tri-vacancies near the (1 2 0) interface at the off-mirror site and for 3.3 and 3.4 vacancy clusters at both sites near the (1 1 1) interface, which are favorable away from the interface. The interaction energy is high at high temperatures.

Effect of Sn substitution on the para-conductivity of polycrystalline Cu0.5Tl0.5Ba2Ca2Cu3−ySnyO10−δ superconductors

High quality electrical resistivity ρ(T) versus temperature data of as-prepared and O2-annealed Cu0.5Tl0.5Ba2Ca2Cu3−ySnyO10−δ (y=0, 0.25, 0.5, 0.75, 1.0, 1.25, and 1.5) superconductors has been studied for fluctuation-induced phenomena setting in at temperatures well above the critical temperature [Tc(R=0)]. The analysis of the data is done by using Aslamazov–Larkin (AL) and Lawrence–Doniach models for the excess conductivity. We have estimated several physical parameters, including coherence length, interplane coupling strength, exponents, and dimensionality of the fluctuations. The as-prepared and oxygen postannealed samples have shown a cross-over temperature associated with two distinct exponents and the excess conductivity data fits well with the two-dimensional and three-dimensional AL equations. The coherence length along the c-axis [ξc(0)] and the interlayer coupling strength (J) are found to decrease with increased Sn doping. These values are increased after annealing the samples in oxygen atmosp...

Annealing effect on the excess conductivity of Cu0.5Tl0.25M0.25Ba2Ca2Cu3O10−δ (M = K, Na, Li, Tl) superconductors

The effect of oxygen annealing on the excess conductivity of Cu0.5Tl0.25M0.25Ba2Ca2Cu3O10−δ (M = K, Na, Li, Tl) samples is investigated. From the analysis of results, we have evaluated the exponents, the coherence length, the crossover temperature, and interlayer coupling strength (J) of the samples. These studies show that the K-doped post-annealed sample has shown narrow transition width with improvement in 2D and 3D conductivities. Oxygen annealing has also enhanced the coherence length and interlayer coupling strength of the material. Na- and Li-doped post-annealed samples have shown increased transition width, poor 2D and 3D conductivities within narrow temperature windows. After annealing, reduction in coherence length and the interlayer coupling strength is also observed. While in Tl-doped sample, oxygen annealing has not caused much deviation in fluctuation induced conductivity (FIC) parameters. The underlying cause of improved behavior of post-annealed K-doped sample is also discussed.

A density functional theory study of Raman modes of cadmium hexathiohypodiphosphate (CdPS3)

Abstract Raman scattering investigations based on density functional theory (DFT) calculations were performed to explore the vibrational modes of a cadmium hexathiohypodiphosphate CdPS3 single crystal. The calculations were performed to obtain the Raman spectra for the cadmium hexathiohypodiphosphate atoms to study the size dependence. Several vibrational modes indicating stretching and bending features related to Cd, S and P atoms were observed. Modifications of the frequency and intensity of different Raman modes with an increase in the number of atoms in CdPS3 were discussed in detail. Hydrogen atoms were added in order to make the closed shell configuration and saturate the CdPS3 as per the requisite for calculating the Raman spectra. This produced some additional modes of vibration related to hydrogen atoms. Band gap and formation energy were also calculated. The results generated are found to be in close agreement with the experimental values.

Fabrication and Para-Conductivity Analysis of Oxygen Annealed TlBa_2(Ca_(3-y)Mg_y)Cu_4(O_(12-δ) Superconductors

Oxygen-annealed TlBa_2(Ca_(3-y)Mg_y)Cu_4O_(12-δ) (y = 0; 0:5; 1:0; 1:5; 2) superconductors are synthesized at normal pressure, and the influence of doped Mg-atoms on the superconducting parameters at the microscopic level is investigated by carrying out a para-conductivity analysis. The samples have shown a tetragonal structure, and the unit cell volume decreases with increased Mg-doping. The onset temperature of superconductivity [T_c(onset)] and zero resistivity critical temperature [T_c(R=0)] decreases with Mg doping. The maximum magnitude of diamagnetism is observed for the samples with a Mg concentration of y = 2. The apical oxygen mode of the type Tl-O_A-Cu(2) and CuO_2 planar oxygen modes are softened, as observed in the FTIR (Fourier transform infrared spectroscopy) absorption measurements. The fluctuation induced conductivity (FIC) analysis of conductivity data has shown the enhancement of the inter-plane coupling and coherence length along the c-axis. These studies have shown that there is less suppression of the order parameter of the carriers from its value |ψ|^2 = 1 due to the proximity effect in the CuO_2 planes in the Mg-doped samples.

Structural and electrical properties of doped polypyrrole and its composite with montmorillonite clay

Polypyrrole is synthesized and doped with Dodecylbenzenesulphonic acid. The doped Polypyrrole with Dodecylbenzenesulphonic acid is intercalated into the layers of Montmorillonite clay successfully by in situ polymerization. The structural properties of synthesized doped Polypyrrole and intercalated doped Polypyrrole were studied by XRD analysis. The crystallinity of intercalated doped Polypyrrole into the layers of Montmorillonite clay is confirmed by means of X-rays diffraction studies, which is more than the doped Polypyrrole. Enhanced d-spacing of Montmorillonite confirmed that doped Polypyrrole is interclated into the layers of Montmorillonite clay at nanoscale. The scanning electron micrographs also confirm the formation of dual phase of platelet as well as of flaky structure of intercalated doped Polypyrrole. Temperature dependant conductivity showed three dimensional variable ranges hopping model. Activation energy, density of states and hopping length are calculated and found to be influenced by intercalating Doped Polypyrole into the layers of Montmorillonite clay.