Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films

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Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films ( investigation-metal-insulator-transition-magnetron-sputtered )

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1. INTRODUCTION However, when the become closer due to decrease of the interatomic distances - they for once start to feel strong repulsion from other electrons, due to Pauli prin- ciple, and stop feeling the electrostatic force from the positively charged lattice site as the force is screened by the proximity of the other electrons. As estimated by Mott, the critical density N at which the transition occurs is approximately equal to N1/3 ≈ 0.2. (1.7) aB The formal treatment of the Mott transition was first given by Hubbard. He introduced a Hamiltonian which takes into consideration the basic tight bind- ing approximation of band theory together with the onsite Coulomb interaction between electrons. The two terms in second-quantized version give in total: H=−t 􏰤(c†iσcjσ+c†jσci,σ)+U􏰤ni↑ni↓=Ht+HU (1.8) ,σ i The model, although considering still only electron in a single band, gives a good description of some materials. In the Mott-Hubbard model the hopping of electron occurs only between the metal sites. In transition metal oxides it involves a transport of electron between d electron sites dndn → dn−1dn+1 between i and j transition metal sites. However ijij some researchers indicate that the fluctuations are very high in energy as they require energy about 7-10 eV (for late TMs (99)), so highly improbable in some compounds. The transition occurs due to thermal fluctations. At the transition the gap between lower (LHB) and upper (UHB) Hubbard bands is closed. The Mott-Hubbard picture explains the insulating state well but can not de- scribe the quasi-particle peak that appears at the Fermi level in the metallic state. This state is addressed in Brinkman-Rice model assisted by Gutzwiller wave func- tions. This results in electron effective mass, electric susceptibility and specific heat enhancement (divergence) close to the MI transition which is according to experiments (47). The insulating state occurs for strong electron correlations with a half filled band. The occurrence of both Hubbard bands are not explained within the theory. A different situation is considered in so called charge-transfer insulators where electron moves inside the unit cell from the ligand (anion) to the metal site. It is described by dn → dn+1L. The energy involved is denoted as δ and is related to ii the electronegativity of anion and Madelung potential. The above pictures do not take into account the magnetic properties of elec- tron, they even do not regard spin as a factor in MIT. In Slater theory of MIT the 16

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