Changes of Tc under Epitaxial Strain: Implications for the Mechanism of Superconductivity

Recent results showed that a relationship exists between a large modulation of the critical temperature and small variations in the lattice parameters for strontium-doped La(2)CuO(4) "214'' thin
films grown by molecular beam epitaxy under compressive or tensile strain. In this paper we search for the subtle deviation of the atom positions responsible for the observed behavior. We compare our findings regarding charge transfer, anisotropy, in-plane resistivity, and superfluid density with trends observed among the family of the one-CuO(2)-layer compounds. This systematics leads us to the following conclusions: (i) increasing the distance between the charge reservoir and the CuO(2) plane is essential to increase Tc, (ii) the electrical coupling from one CuO(2) plane to the next
CuO(2) plane may not be essential for the mechanism, (iii) a sufficient density of carriers is present in the underdoped state but the appearance of superconductivity is hindered by a large scattering
rate, and (iv) this scattering rate is determined by the two-dimensional confinement of the carriers in the CuO(2) planes.