• Dinesh Varshney

      Articles written in Bulletin of Materials Science

    • Composition dependence of transition temperature in new superconductors: La2 −x(Ba, Sr)xCuO4

      R K Singh A C Sharma N K Gaur Dinesh Varshney

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      We have investigated the variation of transition-temperature and coupling parameter with the composition concentration in La2 −x(Ba, Sr)xCuO4 superconductors using a formulation developed on the basis of an idea of pairing of charge carriers by exchange of both acoustic plasmons and phonons. Reasonably good agreement is found between the recent experimental results and our theoretical results on the superconducting transition temperature.

    • Composition dependence of transition temperature in some ceramic superconductors

      R K Singh Dinesh Varshney N K Gaur A C Sharma

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      The composition dependence of transition temperature in some ceramic superconductors (La2−x(Ba, Sr)xCuO4) was studied by modifying our earlier approach and developing a Fourier-transformed effective potential which involves the effect of two-dimensional (2D) acoustic plasmons. This potential was used to obtain the pairing (electron-electron attraction) parameter (λ), the averaged Coulomb repulsive parameter (μ*) and the cut-off 2D acoustic plasmon frequency (ωc) required to compute the superconducting transition temperature (Tc) from the strong coupling theory. The variations ofTc with compositions (x) obtained for La2−x(Ba, Sr)xCuO4 show reasonably good agreement with experimental data.

    • Study of doping effects on transition temperature of La2 −x (Ca, Na, K)xCuO4 superconductors

      R K Singh Dinesh Varshney N K Gaur

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      We have investigated doping effects on the transition temperature (Tc) of La2 −xMxCuO4 (M=Ca, Na and K) by incorporating the effects of two-dimensional (2D) acoustic plasmons in the framework of the strong coupling theory. The contributions from 2D acoustic plasmon mechanism toTc have been obtained from a Fourier-transformed effective potential, which has been earlier found to be successful in predicting the composition dependence ofTc in La2 −x(Ba, Sr)xCuO4. The results obtained by us on the variation of transition temperature with composition (x) in La2−xMxCuO4 superconductors are in reasonably good agreement with the available experimental data. This success has led to the conclusion that 2D acoustic plasmons are adequate to explain the pairing mechanism and the variation ofTc with composition (x) in cuprate superconductors.

    • Effect of doping concentration on normal state resistivity of La2 −x (Ba, Sr)x CuO4 superconductors

      Dinesh Varshney R K Singh

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      Based on free electron layered electron gas model of quasi two dimensional CuO2 layers in La(Ba/Sr)CuO superconductors a model potentialV(q) is developed earlier with the electron-electron and electron-phonon interactions. The model approach facilitates the dielectric functions and the dispersion relations of 2D acoustic phonon and plasmon modes. We have then worked out the coupling strength (γ) linking electrons to the 2D acoustic phonon mode (ħω_) from the residue at the pole ofV(q). Furthermore, the scattering time (τe−ph) during electron-phonon interaction (EPI) for this simplified system is also estimated. The contribution to the normal state in plane resistivity due to EPI is then evaluated. Finally, the variations ofτ andρ is studied with the doping concentration (x) and temperature (T) and the results obtained by us show reasonably good agreement with the available experimental data.

    • Effect of oxygen deficiency (δ) on transition temperature of yttrium cuprate superconductors

      R K Singh Dinesh Varshney A K Khaskalam

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      The nature of pairing mechanism as well as transition temperature of yttrium cuprates is discussed using the strong coupling theory. An interaction potential has been developed for the layered structure with two conducting CuO2(a–b) layers in a unit cell. The interaction potential properly takes care of electron-electron, electron-phonon and electron-plasmon interactions. Furthermore, the electron-phonon coupling parameter (λ), the modified Coulomb repulsive parameter (μ*) and the 2D acoustic phonon (plasmon) energy as a function of oxygen deficiency is worked out. Finally, the superconducting transition temperature (Tc) is then evaluated by using these coupling parameters and obtainedTc = 95(92)K for Y(Yb)Ba2Cu3O7−δ superconductors withδ = 0·0. The model parameters estimated from the layered structure approach are consistent with the strong coupling theory. The result deduced on the variation ofTc withδ are in fair agreement with the earlier reported data on yttrium cuprates. The analysis of the above results are discussed.

    • Oxygen deficiency dependence of transition temperature in (Sm, Er) Ba2Cu3O7−δ superconductors

      R K Singh Dinesh Varshney Amit K Khaskalam

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      We have investigated the effects of oxygen deficiency (δ) on the transition temperature (Tc) of (Sm, Er)Ba2Cu3O7−δ superconductors by incorporating the effects of the two dimensional (2D) acoustic phonons and plasmons in the framework of strong coupling theory. The proposed approach for yttrium cuprates properly takes care of the double CuO2 plane in a unit cell and has been found earlier to be successful in describing the pairing mechanism as well as the variation ofTc withδ in Y Ba2Cu3O7−δ system. The coupling strength (λ), the screening parameter (μ*) and the two dimensional acoustic phonon (plasmon) energyħω+) as a function of oxygen deficiency is worked out. Finally, the transition temperature is evaluated and is found to be consistent with the earlier experimental data on yttrium cuprates. Thus, coupled phonon-plasmon mechanism is adequate to understand the nature of pairing mechanism and oxygen deficiency dependence of transition temperature in 90 K (Sm, Er)Ba2Cu3O7−δ superconductors.

    • Two-component model for optical conductivity in Y-Ba-CuO superconductors

      Dinesh Varshney Sanjay Shah R K Singh

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      The optical conductivity of optimized doped YBa2Cu3O7-δ (δ=0·0,Tc=92 K) superconductors, which are frequency dependent, has been theoretically investigated based on two-component (Drude and mid infrared terms) approach within the Fermi liquid description. Our approach incorporates the Drude contribution as well as hopping of charge carriers in the model dielectric function along with the structure factor. It explains the anomalies observed in the optical measurements for the normal state as the frequency dependence of optical conductivity using the Drude term which gives a sharp peak at zero frequency, and a long tail at higher frequencies, i.e. in the infrared region. The extra term (hopping carriers) gives a peak value in the optical conductivity centred in the mid infrared region. The two species of charge carriers contribution to the conduction in the CuO chain layer as well as CuO2 layer will account for the optical conductivity in the mid infrared as well as infrared frequency regions. The analysis reveals an interesting relation$$\sigma _{CuO_2 layer} \approx 3\sigma _{chain layer} $$, and the nature for optical conduction with energy is similar qualitatively, the only difference is quantitatively. It is shown that the analysis is consistent with the published data on optical conductivity in optimized-doped YBa2Cu3O7-δ superconductors.

    • Effect of oxygen deficiency (δ) on normal state resistivity of YBa2Cu3O7-δ superconductors

      Dinesh Varshney R K Singh Amit K Khaskalam

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      We have investigated theoretically the effect of oxygen deficiency (δ) on normal state resistivity (ρ) as well as its temperature dependence in YBa2Cu3O7-δ superconductors. This has been based on a potential which incorporates the structure factors and various interactions for double two-dimensional (2-D) conducting CuO2 plane. Using the Coulomb and electron-phonon terms of the interaction potential, we have then worked out the coupling strength (γ) for neighbouring electrons linked via 2-D acoustic phonons (_). Furthermore, the scattering time (τe-ph) due to electron-phonon interaction is deduced. The variations inτe-ph andρe-ph are studied with oxygen deficiency (δ) which is in the range of 0·0≤δ≤1·0, and the results thus obtained are found to be consistent with the earlier reported data. The residual resistivityρ0 obtained by extrapolation from experimental data together withρe-ph will predict the nearly-linear behaviour of normal state resistivity at temperatures (T) [90≤T≤300 K] in YBa2Cu3O7-δ superconductors.

    • Specific heat studies in Ho-Ba-CuO superconductors: Fermionic and bosonic contributions

      Dinesh Varshney Sanjay Shah R K Singh

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      The specific heats of superconducting HoBa2Cu3O7-δ (Tc≅ 92 K) have been theoretically investigated in the temperature domain 70 ≤T ≤110 K. The bosonic (phonons) contribution to the specific heat is estimated from Debye model in the harmonic approximation for high temperature expansion (T > θD/2π) using the moments of the phonon density of states. The fermionic constituent as the electronic specific heat is deduced using a suitable trial function above and belowTc. As a next step the contribution of specific heat by charge oscillations (plasmons) are obtained. The theoretical results from bosonic and fermionic terms are then compared with the experimental results. We find that the specific heats from electronic as well as plasmon term are only a fraction of lattice specific heat and in particular, plasmons do not influence the thermal conduction significantly. The implications of the above analysis are discussed.

    • Superconductivity and electrical resistivity in alkali metal doped fullerides: Phonon mechanism

      Dinesh Varshney A Dube K K Choudhary R K Singh

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      We consider a two-peak model for the phonon density of states to investigate the nature of electron pairing mechanism for superconducting state in fullerides. We first study the intercage interactions between the adjacent C60 cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to estimate phonon frequencies from the dynamical matrix for the intermolecular alkali-C60 phonons. Electronic parameter as repulsive parameter and the attractive coupling strength are obtained within the random phase approximation. Transition temperature, 𝑇c, is obtained in a situation when the free electrons in lowest molecular orbital are coupled with alkali-C60 phonons as 5 K, which is much lower as compared to reported 𝑇c (≈ 20 K). The superconducting pairing is mainly driven by the high frequency intramolecular phonons and their effects enhance it to 22 K. To illustrate the usefulness of the above approach, the carbon isotope exponent and the pressure effect are also estimated. Temperature dependence of electrical resistivity is then analysed within the same model phonon spectrum. It is inferred from the two-peak model for phonon density of states that high frequency intramolecular phonon modes play a major role in pairing mechanism with possibly some contribution from alkali-C60 phonon to describe most of the superconducting and normal state properties of doped fullerides.

    • Pressure dependence of elastic properties of ZnX (X = Se, S and Te): Role of charge transfer

      Dinesh Varshney P Sharma N Kaurav R K Singh

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      An effective interaction potential (EIOP) is developed to invoke the pressure induced phase transition from zinc blende (𝐵3) to rocksalt (𝐵1) structure and anharmonic properties in ZnX (X = Se, S, Te) semiconductors. The effective interaction potential incorporates the long range Coulomb interaction, van der Waals interaction and short-range repulsive interaction up to second neighbour ions within the Hafemeister and Flygare approach as well as the charge transfer effects caused by the electron-shell deformation of the overlapping ions. The van der Waals coefficients are computed by the Slater Kirkwood variation method as a first step. Later on, we evaluate volume collapse, second order and third order elastic constants with pressure pointing to the systematic trends in all compounds of zinc blende structure and their thermal properties such as force constant, Gruneisen parameter, compressibility, Debye temperature etc. The vast volume discontinuity in pressure–volume (PV) phase diagram identifies the structural phase transition from zinc blende (𝐵3) to rock salt (𝐵1) structure and is consistent with those revealed from earlier reports.

    • Phonon, magnon and electron contributions to low temperature specific heat in metallic state of La0.85Sr0.15MnO3 and Er0.8Y0.2MnO3 manganites

      Dinesh Varshney Irfan Mansuri E Khan

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      The reported specific heat 𝐶\ (𝑇) data of the perovskite manganites, La0.85Sr0.15MnO3 and Er0.8Y0.2MnO3, is theoretically investigated in the temperature domain 3 ≤ 𝑇 ≤ 50 K. Calculations of 𝐶\ (𝑇) have been made within the three-component scheme: one is the fermion and the others are boson (phonon and magnon) contributions.Lattice specific heat is well estimated fromthe Debye temperature for La0.85Sr0.15MnO3 and Er0.8Y0.2MnO3 manganites. Fermion component as the electronic specific heat coefficient is deduced using the band structure calculations. Later on, following double-exchange mechanism the role of magnon is assessed towards specific heat and found that at much low temperature, specific heat shows almost T3/2 dependence on the temperature. The present investigation allows us to believe that electron correlations are essential to enhance the density of states over simple Fermi-liquid approximation in the metallic phase of both the manganite systems. The present numerical analysis of specific heat shows similar results as those revealed from experiments.

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