Ferroelectricity, Superconductivity, and SrTiO3—Passions of K.A. Müller

Abstract

SrTiO3 is an insulating material which, using chemical doping, pressure, strain or isotope substitution, can be turned into a ferroelectric material or into a superconductor. The material itself, and the two aforementioned phenomena, have been subjects of intensive research of Karl Alex Müller and have been a source of inspiration, among other things, for his Nobel prize-winning research on high temperature superconductivity. An intriguing outstanding question is whether the occurrence of ferroelectricity and superconductivity in the same material is just a coincidence, or whether a deeper connection exists. In addition there is the empirical question of how these two phenomena interact with each other. Here we show that it is possible to induce superconductivity in a two-dimensional layer at the interface of SrTiO3 and LaAlO3 when we make the SrTiO3 ferroelectric by means of 18O substitution. Our experiments indicate that the ferroelectricity is perfectly compatible with having a superconducting two-dimensional electron system at the interface. This provides a promising avenue for manipulating superconductivity in a non centrosymmetric environment.

Effects of spin-orbit interaction and electron correlations in strontium titanate

“…Introduction. Strontium titanate SrTiO 3 (STO) exhibits unique and puzzling electronic and structural properties that have motivated its extensive studies over the last 50 years [1][2][3][4]. The dielectric constant of STO is anomalously large and almost diverges at low temperature without the onset of ferroelectricity, in a manner consistent with quantum paraelectricity [5].…”

“…Hopping between the d yz orbital and the p z orbital of the oxygen on the z-axis is also prohibited by symmetry. The only symmetry-allowed matrix element involving the d yz orbital, t 2 1,3 , corresponds to hopping to the p z orbital of nearest-neighbor oxygens on the y axis, since both orbitals are described by the same dependence cos(θ y ) on the rotation angle around the y-axis. By the cubic symmetry of STO, the only finite matrix element involving hopping between the d yz orbital of Ti and the p y orbitals of oxygens is t 3 1,2 = t 2 1,3 , which corresponds to hopping along the z-axis.…”

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

“…Introduction. Strontium titanate (STO) is a wide-gap band insulator known for more than half a century for its unusual properties related to its proximity to a ferroelectric transition, see Refs [6][7][8] for recent reviews. It can be transformed to a very dilute metal by tiny doping, either by oxygen deficiency or by substitution of small portion of Ti atoms with Nb.…”

“…[9], the mechanism of electron pairing is under active debates till now (examples can be found in Refs. [6][7][8]). The key feature of superconductivity in STO (which makes it very different from most of the known superconductors) is due to its very low Fermi energy E F which is much less than Debye energy ω D ≈ 100meV .…”

“…Attractive e-e interaction defined by Eqs. (8,9) decays exponentially at r > l 0 /2 so it can be considered as nearly local in the range of electron densities n < n c1 = 1.5 • 10 18 cm −3 . Then we can employ the theory [2] for superconductivity in a Fermi-gas with local instantenous attraction.…”