A.K. Rastogi

Low-temperature resistance minimum in non-superconducting 3R-Nb1+xS2 and 3R-GaxNbS2

We report the structural and electron transport properties of 3R-Nb1+xS2 (x≥0.07) and 3R-GaxNbS2 (0.1≤x≤0.33) prepared as polycrystalline pellets as well as single crystals grown by vapour transport. We observe a resistance minimum in these compounds between 20 and 60 K, with Tmin proportional to x. The resistance scales as ρ/ρmin(T/Tmin) in the range 0.2<T/Tmin<2 for different phases with x≤0.25 whose resistivity differs by an order of magnitude. Powder x-ray diffraction also shows progressively increasing intensity of superlattice lines with cation concentration. The thermopower changes sign around the resistance minimum. The explanation of the resistance minimum and the simultaneous rapid suppression of superconductivity is sought in electron-electron scattering effects in the presence of cation disorder in these narrow-band anisotropic materials.

AC and non-linear conduction in V4 cluster compounds GaV4S8 and Ga0.98Zn0.02V4S8: Evidence for a Wigner glass

Abstract The conduction in pure and 2% Zn substituted GaV4S8 has been studied by means of moderate as well as high DC fields and AC fields upto 1 MHz. σ(ω, T) shows a typical temperature scaling, characteristic of hopping conduction in disordered solids. At low temperatures, these compounds show non-linear I–V characteristics and a negative differential resistance in rather moderate fields. The results are analysed using Motts variable range hopping relations and are found to give consistent parameters like localisation length, density of states (DOS) at the Fermi level and conduction bandwidth except for an unreasonably large value of the phonon assisted transition rates. The result that the conductivity exhibits all the elements of a variable range hopping is interpreted to support the thesis that a Wigner glass state exists among the electrons localised on the V4 clusters.

Peak effect studies in 2H-NbS2

Abstract We have studied the peak effect (PE) phenomenon in a single crystal of anisotropic, weakly pinned, low T c (T c =6.2 K ) superconductor 2H-NbS2. We present the vortex phase diagrams of 2H-NbS2 for H||c and H||ab, obtained from the magnetization data and compare the PE phenomenon in it (which does not show charge density wave ordering) with that in 2H-NbSe2 (which shows charge density wave ordering).

Transport properties of V4 and Mo4-cluster compounds: An electron glass?

Abstract DC and AC conductivity, thermopower and non-ohmic conduction properties have been reported for GaV 4 S 8 and GaMo 4 Se 4 Te 4 . The transport properties indicate the formation of electron glass state at low temperature. In a simplified model the band spread of localised states, DOS and localisation length are estimated. Coulomb gap effects in variable range hopping (VHR) and AC conductivity σ ac ( ω ) are found in the more ordered Mo 4 compound. Both the compounds show negative resistance behaviour at high current densities that may indicate the melting of the electron glass.

Weak localization effects in Ga-intercalated layered compound NbS2

Abstract Ga x NbS 2 ( x = 0 − 0.33) was prepared in two batches-≈ 1000°C (LT) and ≈ 1125°C (HT). X-ray diffraction shows random stacking of 2H-NbS 2 layers in the HT phase. Resistivity and thermopower measurements from 15 to 300 K show metallic behaviour and anomalies aroung 45 K. The disordered phases indicate weak localisation effects in conductivity above 45 K and a log T variation in the x = 0.25 compound indicating 2-D conduction. The thermopower is unaffected by the disorder. The 45 K transition suppresses this localisation.