Publication No.47



A. Stashans, P. Sánchez, H. Pinto. Superconductivity and JT polarons
in titanates - Workshop Vortex Dynamics HTSC, Tashkent (Uzbekistan), 
2002, p. 45. 

Abstract:
	High temperature superconductors (HTSC) cuprates and titanates have 
a number of similarities at the fundamental level. Both structures can be 
described in terms of perovskite-type lattices. Recently it was discovered 
that both type of crystals show the presence of Jahn-Teller (JT) polarons 
and their strong coupling to lattice vibrations. It is also known that both 
materials may exhibit the superconductivity phenomenon.
	In the present work we use a quantum-chemical method developed for 
crystal studies. We focus on the following two features: (i) superconductivity 
in the La-doped SrTiO3 crystal, and (ii) JT polarons in the BaTiO3 crystal. 
	In the case of SrTiO3 crystal our study [1] shows the occurrence of 
free electrons in the conduction band of the crystal and explains a number 
of the experimental results obtained for the superconducting La-doped SrTiO3 
existing under 14K. The free electrons are found to be situated on the Ti-O 
planes similar to the case of free electrons situated on Cu-O planes in the 
HTSC cuprates. The lattice might disperse the wave functions of the free 
electrons. However, due to the periodicity the dispersed waves in the 
superconducting state will be interfering constructively. Thus the gaps between 
the maximums should match exactly the distance between the superconducting 
planes in the SrTiO3 crystal allowing the electrons to travel without any 
resistance.
	Studies of the JT polarons in the BaTiO3 crystal [2] have showed 
possibility of occurrence of JT polarons in a pure BaTiO3 lattice explaining 
a number of experimental observations and matching some important results 
obtained independently by the polaron theory for the BaTiO3 crystal and the 
HTSC cuprates. In particular, the calculated JT polaron binding energy, 
-0.87 eV, practically matches the corressponding value, -0.842 eV, obtained 
by the polaron theory using the so-called Fröhlich electron-phonon 
interaction [3].
	Our studies imply that the superconductivity and the localized 
JT polarons in the HTSC cuprates and titanates could be a consequence of 
another fundamental, still undiscovered phenomenon present in these 
crystals. Depending on conditions may occur one of the two competing 
phenomena, either the superconductivity or the JT charge- and spin-ordered 
phases.

[1] P. Sánchez and A. Stashans, Phil. Mag. B, 81, 1963 (2001).
[2] H. Pinto and A. Stashans, Phys. Rev. B, submitted (2001).
[3] A. S. Alexandrov and A. M. Bratkovsky, J. Phys.: Condens. Matter 11, 
    L531 (1999).




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