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1. INTRODUCTION The full semiconducting state is modelled well by this combination σ ∝ 1/R = Aexp(−B/T(1/4)) + Cexp(−D/T). (1.16) A, B, C, D being constants. Vassiliou gave several possible interpretations of the obtained data for NdNiO3. One of them considers NdNiO3 to be a Mott-Hubbard insulator. The RT half-filled metallic d-band is at the antiferromagnetic transition split into two band - one fully filled and the other one empty. The band gap emerges from this splitting. No antiferromagnetic transition is visible in susceptibility data as the paramagnetic (PM) signal of f-electrons in Nd3+ ions is strong enough to mask it. Second explanation involves considering polaron formation below the MIT temperature. Room temperature conductivity of NdNiO3 is above the minimal value for metals of 103(Ω · cm)−1. A value still so low that the mean free path of electrons is of the order of interatomic distances. With the electrons moving in a diffusive way in the narrow d-bands, the conductivity of the compound is also diffusive. The width of the d-band W would be given by uncertainty principle W = h/ts, where ts is the relaxation time. Below 130 K the localization of the electrons occur due to polaron formation, which changes the metal to semiconductor. In this state, the charge transfer is realized by the phonon-assisted hoping of the electron from site to site, and thus the relaxation time is limited by this process. A possible Jahn-Teller distortion accompanies the 3d electron localization. The hysteresis of resistivity may be an effect of hysteresis of lattice distortion. The ab initio calculations revealed how DOS of the NNO thin films is influ- enced when strain is applied or when the film is confined along z direction(97). While compressive or tensile strain is applied by the substrate the 3d nickel or- bitals become delocalized in contrast to bulk NNO. In bulk NNO the band gap forms between occupied Ni2 t2g states and unoccupied Ni2 eg states but also eg states of both Ni1 spins. In case of confined film, there is a van Hove singularity that creates dips in Ni ions DOS and this increases the band gap. There is also a non-zero spin-polarization of Ni1 sites. 1.4 Influence of the reduced dimension on the physical properties The crystal structure defines most of the properties of thin film materials. In NNO thin films deposited on various substrates show different structure and orientation of crystallites which involves the variation of electronic behaviour of the material 22PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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