Unconventional superconductivity, phase transitions and new polymorphs in the Sr-Ti-O system

Strontium titanate is a cubic perovskite, which is insulating in its pristine state but becomes metallic upon electron doping. It is one of the most dilute superconductors and the pairing mechanism is unknown. In addition, SrTiO3 is a quantum paraelectric, in which zero-point fluctuations prevent the structure from a ferroelectric transition. These fluctuations can be suppressed by 18O isotope substitution. Combined with the electron doping, this leads to a state in which the superconducting and the ferroelectric order coexist. The first part of this thesis describes the fabrication of samples with an aim of increasing the rate of the 18O isotope substitution in SrTiO3, and the process of the electron doping by oxygen removal, leading to a&nbsp;complete superconducting phase diagram. The second part of the thesis studies Sr2TiO4, a 2D counterpart of SrTiO3. When a growth of the tetragonal Sr2TiO4 is attempted, a high-temperature alpha-phase crystallizes first, and while cooled down a phase transition to the tetragonal phase occurs, which breaks the crystal. The second part of the thesis investigates this phase transition, and presents bulk electronic properties and crystal structure of the new alpha-polymorph, which contains titanium in an unusual tetrahedral coordination.</p

Crystal growth and structure of a high temperature polymorph of Sr2TiO4

This data base contains the data shown in the figures of "Crystal growth and structure of a high temperature polymorph of Sr2TiO4 with tetrahedral Ti-coordination, and transition to the Ruddlesden-Popper tetragonal phase", published in CrystEngComm as doi:10.1039/D2CE00366

Crystal growth and structure of a high temperature polymorph of Sr₂TiO₄ with tetrahedral Ti-coordination, and transition to the Ruddlesden–Popper tetragonal phase

We have grown single crystals of a new polymorph of Sr 2 TiO 4 . It contains titanium in an unusual tetrahedral coordination and transforms to the Ruddlesden–Popper structure with an interesting orientational relationship. </p

Isotope tuning of the superconducting dome of strontium titanate

Doped strontium titanate SrTiO3 (STO) is one of the most dilute superconductors known today. The fact that superconductivity occurs at very low carrier concentrations is one of the two reasons that the pairing mechanism is not yet understood, the other being the role played by the proximity to a ferroelectric instability. In undoped STO, ferroelectric order can in fact be stabilized by substituting 16O with its heavier isotope 18O. Here, we explore the superconducting properties of doped and isotope-substituted SrTi(18O16yO1−y)3−δ for 0≤y≤0.81 and carrier concentrations between 6×1017 and 2×1020cm−3 (δ<0.02). We show that the superconducting Tc increases when the 18O concentration is increased. For carrier concentrations around 5×1019cm−3 this Tc increase amounts to almost a factor 3, with Tc as high as 580 mK for y=0.74. When approaching SrTi18O3 the maximum Tc occurs at much smaller carrier densities than for pure SrTi16O3. Our observations agree qualitatively with a scenario where superconducting pairing is mediated by fluctuations of the ferroelectric soft mode

Open Data to the publication "Isotope tuning of the superconducting dome of strontium titanate"

AbstractDielectric constant, Raman data, resistivity data, magnetic susceptibility, Curie temperatures, superconducting Tc, quantum oscillations, mobility, of doped and isotope-substituted SrTi(18O16O1−y)3−δ for 0 ≤ y ≤ 0.81 and carrier concentrations between 6×10^17 y and 2 × 10^20 cm−3 (δ < 0.02

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

AbstractSrTiO3 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