• A K Rastogi

      Articles written in Pramana – Journal of Physics

    • Charge density wave and superconductivity in 2H- and 4H-NbSe2: A revisit

      I Naik A K Rastogi

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      Good-quality hexagonal NbSe2 single crystals were prepared. In 2H-NbSe2, superconducting and charge density wave (CDW) transitions were found at $T_s = 7.4$ K and $T_c = 35$ K respectively as reported previously. We have noticed that these two transitions are changed to $T_c = 42$ K and $T_s = 6.5$ K, in 4H-NbSe2. Thermopower has shown clear anomaly at CDW transitions. The anisotropic upper critical field was calculated as $\sim 3$ and 6.3 for 2H- and 4H-single crystals around $t = 0.81$, where $t = T/T_s$, from resistivity and explained in terms of coherence length. From the relation, $H_{c2}(T) = H_{c2}(0)[1 − t^2]$, $H^l_{c2}(0)$ was calculated as $\sim 8.15$ T and 16.98 T at $t = 0.84$ in 2H-NbSe2 and 4H-NbSe2 respectively. However, $H^t_{c2}(0) = 2.68$ T for both single crystals.

    • Electronic properties of GaV4S8: A percolation approach

      I Naik S Hansda A K Rastogi

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      Two polycrystalline V4-cluster compounds of GaV4S8 were prepared at different annealing temperatures (GaV4S8-1 sintered at 800°C and GaV4S8-2 sintered at 500°C). Their temperature-dependent resistivity and structural phase transformation temperature (45 K for GaV4S8-1 and 43K for GaV4S8-2) are found to be very sensitive to the annealing condition. Above 320 K, activation energy 𝜀3 is calculated to be ∼0.23 eV which decreases to ∼0.18 eV around 300 K in GaV4S8-1 and GaV4S8-2 on cooling. According to percolation theory, the gradual decrease in 𝜀3 below 300 K is expected due to the increase in separation between V4-clusters are significantly different in GaV4S8-1 and GaV4S8-2. This statement is strongly supported by the calculated bandwidth 𝛤 per cluster in GaV4S8 (∼0.342 eV in GaV4S8-1 and ∼0.374 eV in GaV4S8-2). A negative magnetoresistance (MR) is also found around 43 K in GaV4S8-2 at 6.0 T magnetic field associated with structural transition.

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