Theory of superconductivity due to Ngai's mechanism in lightly doped SrTiO3

Abstract

We develop a theory of superconducting pairing in low-density strontium titanate due to the quadratic coupling of electron density to soft transverse optical phonons first proposed by Ngai [Phys. Rev. Lett. 32, 215 (1974)]. This leads to a static attractive potential between electrons with decay length leff that scales inversely with soft optical gap ωT. For low electron densities n≤1018 cm−3, attraction between electrons is static and local in space; thus the transition temperature Tc was found using known results for low-density electron gas. The Tc(n) dependence for low doping was calculated and found to be in agreement with experimental data. In addition, we show that suppression of Tc by hydrostatic pressure and strong increase in Tc due to isotope substitution O16→18O observed experimentally also can be explained within our theory.

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