Publication No.3



A. Stashans, P. Sánchez.
Superconductivity in La-doped SrTiO3: a computer modeling - 5th Int. Conf.
Comput. Phys., Kanazawa (Japan), 1999, p1-11.

Abstract:
The computational physics methods have shown recently to be very powerful means 
in crystal studies. We have used the periodic INDO method modified for crystal 
calculations in order to study the La doping in cubic and tetragonal crystalline 
lattices of SrTiO3. By changing the size of the large unit cell (LUC) we managed 
to take into account the effect of La-impurity concentration upon the band structure 
properties of the SrTiO3. In our computations we have considered the insertion of 
one La atom and two La atoms; in the latter case two system states, singlet and 
triplet, were investigated.
    In the 160-atom LUC doped with two La atoms, the triplet state (two electrons 
in the bandgap occupying two separate one-electron energy levels 1.5 eV above the top 
of the upper valence band) was found to be energetically more favourable as the 
corresponding single state (two electrons occupying the same energy level 2.5 eV 
above the top of the upper valence band) by 0.6 eV. When the concentration is increased 
the triplet state becomes even more favourable, 1.3 eV, which signifies that the 
spectral intensity of this peak will grow with the increase of the La concentration 
in agreement with the experimental results by Y. Aiura et al. The interesting result 
is that with the aumentation of the La concentration we observe electron transfer from 
the local energy level within the bandgap to the conduction band. This free charge-
carrier thus can be involved in the superconductivity process. It has to be noted 
that this phenomenon occurs only in the tetragonal phase of the SrTiO3 crystal while 
in the cubic lattice the electron always remains in the local energy level within the 
bandgap.
    A number of different spatial configurations were investigated by atomic 
displacements. The discussion of symmetric and asymmetric atomic relaxations around 
the La atom is given considering the impurity atom as a center of a new phase in the 
transition: cubic structure --> tetragonal structure.




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