Nonmonotonic anisotropy in charge conduction induced by antiferrodistortive transition in metallicSrTiO3

Abstract

Cubic SrTiO3 becomes tetragonal below 105 K. The antiferrodistortive (AFD) distortion leads to clockwise and counter-clockwise rotation of adjacent TiO6 octahedra. This insulator becomes a metal upon the introduction of extremely low concentration of n-type dopants. However, signatures of the structural phase transition in charge conduction have remained elusive. Employing the Montgomery technique, we succeed in resolving the anisotropy of charge conductivity induced by the AFD transition, in the presence of different types of dopants. We find that the slight lattice distortion (< 6 × 10 −4 ) gives rise to a twenty percent anisotropy in charge conductivity, in agreement with the expectations of band calculations. Application of uniaxial strain amplifies the detectable anisotropy by disfavoring one of the three possible tetragonal domains. In contrast with all other known anisotropic Fermi liquids, the anisotropy has opposite signs for elastic and inelastic scattering. Increasing the concentration of dopants leads to a drastic shift in the temperature of the AFD transition either upward or downward. The latter result puts strong constraints on any hypothetical role played by the AFD soft mode in the formation of Cooper pairs and the emergence of superconductivity in SrTiO3.

Metallicity without quasi-particles in room-temperature strontium titanate

“…Above 50K, it starts a significant shift upward and exceeds 3 around 150K, before steadily decreasing afterwards. The antiferrodistortive transition [40] is the source of the small anomaly at 105K. resistivity prefactor and their electronic specific heat.…”

Variation in superconducting transition temperature due to tetragonal domains in two-dimensionally doped SrTiO3

“…Structurally driven anisotropy in the Fermi surface could potentially alter T c . First-principles calculations of the Fermi surface in bulk electron-doped STO showed that the tetragonal anisotropy produces a considerable distortion in the Fermi surface, compressing it along the c direction by as much as 35% [51]. In our δ-doped material, the Fermi surface is dramatically altered from the bulk by confinement in the vertical direction [29], yet despite the change in dimensionality, the overall scale of Tc in our N D = 1 at.% Nb δ-doped material is within the range of peak T c s observed for material doped in the bulk [1,[12][13][14].…”

Metallicity and Superconductivity in Doped Strontium Titanate

“…The AFD transition is extremely sensitive to the presence of a tiny amount of extrinsic atoms. Introducing oxygen vacancies reduces T AF D [56,57]. However, substituting Ti with Nb shifts it upward.…”

“…However, substituting Ti with Nb shifts it upward. Substitution of Sr with La or Ca is beneficial for T AF D [57,58], but it is detrimental with Ba and Pb [59]. McCalla et al [60] have recently examined in detail the complex response of T AF D to substitution and proposed a unified picture to explain it that is based on a combination of impurityinduced change in the Goldschmidt tolerance factor and the ionic valence mismatch between the host and the impurity.…”