• K K Choudhary

      Articles written in Bulletin of Materials Science

    • 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.

    • Interpretation of anomalous normal state optical conductivity of K3C60 fullerides

      K K Choudhary

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      The observed frequency dependent optical response of alkali–metal-doped fulleride superconductors (𝑇c ≈ 19 K) has been theoretically analysed. The calculations of the optical conductivity, 𝜎(𝜔), have been made within the two-component schemes: one is the coherent Drude carriers (electrons) responsible for superconductivity and the other is incoherent motion of carriers from one atom to other atom of C60 molecule to a pairing between Drude carriers. The approach accounts for the anomalies reported (frequency dependence of optical conductivity) in the optical measurements for the normal state. The model has only one free parameter, the relaxation rate. The frequency dependent relaxation rates are expressed in terms of memory functions. The coherent Drude carriers form a sharp peak at zero frequency and a long tail at higher frequencies, i.e. in the infrared region. However, the hopping of carriers from one atom to the other (incoherent motion of doped electrons) yields a peak value in the optical conductivity centred at mid-infrared region. It is found that both the Drude and hopping carriers will contribute to the optical process of conduction in the K3C60 and shows similar results on optical conductivity in the mid-infrared as well as infrared frequency regions as those revealed from experiments.

    • Spin fluctuation and small polaron conduction dominated electrical resistivity in La0.875Sr0.125MnO3 manganite nanostructures

      K K Choudhary N Gupta N Kaurav S Katiyal S K Ghosh

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      The anomalous temperature-dependent electrical resistivity ρ(𝑇) of La0.875Sr0.125MnO3 manganite nanoparticles (particle size 18 nm) is theoretically analysed. ρ(𝑇) exhibits semiconducting phase in lowtemperature regime (20 𝐾 < 𝑇 < 53 K), shows a minima near 53 K and increases with 𝑇 at high temperatures (53 𝐾 < 𝑇 < 170 K). The resistivity in metallic phase (𝑇 > 53 K) is theoretically analysed by considering the strong spin-fluctuation effect, which is modelled using Drude–Lorentz type function. In addition to the spin fluctuation-induced contribution, the electron–phonon and electron–electron $ ρe–e(𝑇) = 𝐵𝑇2 contributions are also incorporated for complete understanding of experimental data. The contributions to the resistivity by inherent acoustic phonons ( ρac) as well as high-frequency optical phonons ( ρop) were estimated using Bloch–Gruneisen (BG) model of resistivity. It is observed that the resistivity contribution due to electron–electron interaction shows typical quadratic temperature dependence. Spin fluctuation-induced resistivity is dominant over electron–electron and electron–phonon contributions in overall temperature range in the manganite nanoparticles. Resistivity in the semiconducting phase is discussed with small polaron conduction (SPC) model. SPC model consistently retraces the low-temperature resistivity behaviour (𝑇 < 53 K). Finally, the theoretically calculated resistivity compared with experimental data is found to be consistent in wide range of temperature.

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