Interplay between antiferrodistortive, ferroelectric, and superconducting instabilities inSr1−xCaxTiO3−

Abstract

SrTiO3 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 a question gathering momentum in the context of a popular paradigm linking unconventional superconductors and quantum critical points. We present a set of specific-heat, neutronscattering,dielectric permittivity and polarization measurements on Sr1−xCaxTiO3 (0 < x < 0.009) and a study of low-temperature electric conductivity in Sr0.9978Ca0.0022TiO 3−δ . Calcium substitution was found to enhance the transition temperature for both anti-ferrodistortive and ferroelectric transitions. Moreover, we find that Sr0.9978Ca0.0022TiO 3−δ 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−δ of comparable carrier concentration. A three-dimensional phase diagram for Sr1−xCaxTiO 3−δ tracking the three transition temperatures as a function of x and δ results from this study, in which ferroelectric and superconducting ground states are not immediate neighbours.

Quantum Critical Origin of the Superconducting Dome inSrTiO3

“…Early tunneling measurements [4] and subsequent experiments [5] suggested an unusual two-band superconductivity, consistent with the closely spaced lowest conduction bands at the center of the Brillouin zone. In addition, the onset of superconductivity has been shown to occur at remarkably low carrier concentrations of 10 18 e/cm 3 [5]. Despite a long-running interest in its origin [3], a complete theoretical account of the superconducting dome remains elusive, and many aspects of superconductivity in STO remain a puzzle.…”

“…Rather than manifesting ferroelectric behavior, however, STO is a so-called quantum paraelectric, in which quantum fluctuations at zero temperature suppress the transition to the ferroelectric state [6] as is also manifested by a leveling off of the TO mode at a real frequency at low temperature [7]. The quantum paraelectric state is characterized by low energy excitations and large ferroelectric fluctuations [8], and it has been speculated that these might be relevant for the superconductivity [9,10]. Indeed, early descriptions [3,11] of the superconducting dome in STO were based on the effects of screening of the interaction between electrons and the optical phonons responsible for the large dielectric response.…”

“…The maximal value of doping at which the ferroelectric phase persists is labeled as n * . Although we have no precise calculation for n * , its value should lie in the range 10 19 < n * < 10 20 . b) Schematic illustration for the lowering of the lowest energy levels (dashed red lines) in the double well potential (black solid line) as f18 is increased.…”

A ferroelectric quantum phase transition inside the superconducting dome of Sr1−xCaxTiO3−δ

“…The nominal calcium concentration of two samples was checked using the secondary ion mass spectrometry (SIMS) analysis technique as detailed previously 31 . The oxygen content has been changed by heating the samples in vacuum (pressure 10 −6 -10 −7 mbar) to temperatures of 775-1,100 • C. To attain carrier densities above ∼ 4 × 10 18 cm −3 , a piece of titanium has been placed next to the sample during heating.…”

Kinetics of Vacancy Doping in SrTiO3 Studied by in situ Electrical Resistivity

“…This compound exhibits structural cubic to tetragonal transition at 105 K with a ferroelectric order simultaneous to antiferrodistorsive transition order 5 . The interplay between these distortions has been focus of many studies [6][7][8] .…”