Publication No.14 P. Sánchez, B. Campuzano N., A. Stashans. Computer modelling of La-doped SrTiO3 and a new model for superconductivity in ceramics. - 14th Int. Conf. Def. Insulat. Mat., Johannesburg (South Africa), 2000, p. 67. Abstract: Due to the extensive applications in the electronic industry and its unusual properties, SrTiO3 crystal has attracted a great deal of attention. The La-doped crystal has been recently found to exhibit superconducting phenomenon below 14 K [1]. The modified for crystals quantum-chemical method and a periodic large unit cell (LUC) model was used to study this material. We have been working with LUCs of 40, 80 and 160 atoms doped with two La atoms, and considering two different system states. The singlet state corresponds to the case when two extra electrons occupy the same local energy level within the band-gap and the triplet state is when two electrons occupy two different local energy levels. In order to give a possible first approximation to the explanation of the superconductivity phenomenon for the SrTiO3 and another ceramic materials, we analyze the following aspects of these materials: (i) their similarity in structure can be important part of the theory, (ii) the crystalline structure is very hard. Those facts mean that the oscillation frequencies of the crystal at the superconductivity temperature are very separated, and the conduction electrons do not have enough energy to excite the crystal to the next oscillation level. Consequently, the electrons will not interact with the crystal moving through the material without resistance. Obviously, exists a limit for the lowest electron energy, which is does not excite the net to the next energetic level. This is the thermal energy independent of the applied electric field, which increases its value as the temperature of the crystal increases. In order to calculate the crystal oscillations and its energy levels, the atoms of the lattice were considered as quantum-mechanical oscillators in their perfect positions, including the La-atom impurity. It was considered that each atom is kept inside a potential box described by a strength matrix, and that it can undergo electrostatic Coulomb interactions within a quantum-harmonic approximation with its neighbouring atoms, not including anharmonic terms and neglecting the radiation lessening. Using this approach, the phononic oscillation frequencies are given by the eigenvalues of the strength matrix. The obtained results are discussed in terms of the available experimental data [1] and are compared with the quantum-chemical computations. [1] H. Suzuki, H. Bando, Y. Ootuka, I. H. Inoue, T. Yamamoto, K. Takahashi, and Y. Nishihara, J. Phys. Soc. Japan, 65, 1529 (1996).
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