Interplay between antiferrodistortive, ferroelectric and superconducting instabilities in Sr_{1-x}Ca_{x}$TiO_{3-\delta}

TL;DR

This study investigates how antiferrodistortive, ferroelectric, and superconducting phases interact in Sr_{1-x}Ca_{x}TiO_{3- ext{delta}}, revealing calcium's role in enhancing phase transition temperatures and identifying a superconducting state in a calcium-doped sample.

Contribution

It provides the first detailed phase diagram showing the interplay of three instabilities in Sr_{1-x}Ca_{x}TiO_{3- ext{delta}}, highlighting the effects of calcium substitution and electron doping on phase transitions.

Findings

Calcium substitution increases transition temperatures for both antiferrodistortive and ferroelectric phases.

Sr_{0.9978}Ca_{0.0022}TiO_{3- ext{delta}} exhibits superconductivity with a critical temperature slightly below that of undoped SrTiO_{3- ext{delta}}.

The phase diagram shows ferroelectric and superconducting states are separated, not directly adjacent.

Abstract

SrTiO$_{3}$ undergoes a cubic-to-tetragonal phase transition at 105K. This antiferrodistortive transition is believed to be in competition with incipient ferroelectricity. Substituting strontium by isovalent calcium induces a ferroelectric order. Introducing mobile electrons to the system by chemical non-isovalent doping, on the other hand, leads to the emergence of a dilute metal with a superconducting ground state. The link between superconductivity and the other two instabilities is an open question, which gathers momentum in the context of the growing popularity of the paradigm linking unconventional superconductors and quantum critical points. We present a set of specific-heat, neutron-scattering and dielectric permittivity and polarization measurements on Sr$_{1-x}$Ca$_{x}$TiO$_{3}$ ($0<x<0.009$) and a low-temperature electric conductivity in Sr$_{0.9978}$Ca$_{0.0022}$TiO$_{3-\delta}$. Calcium substitution was found to enhance the transition temperature for both anti-ferrodistortive and ferroelectric transitions. Moreover, we find that Sr$_{0.9978}$Ca$_{0.0022}$TiO$_{3-\delta}$ has a superconducting ground state. The critical temperature in this rare case of a superconductor with a ferroelectric parent, is slightly lower than in SrTiO$_{3-\delta}$ of comparable carrier concentration. A three-dimensional phase diagram for Sr$_{1-x}$Ca$_{x}$TiO$_{3-\delta}$ tracking the three transition temperatures as a function of x and $\delta$ results from this study, in which ferroelectric and superconducting ground states are not immediate neighbours.