Abstract
The fate of electric dipoles inside a Fermi sea is an old issue, yet poorly explored. Sr{}_{1-x}Ca{}_{x}TiO{}_{3} hosts a robust but dilute ferroelectricity in a narrow (0.0018 < x < 0.02) window of substitution. This insulator becomes metallic by removal of a tiny fraction of its oxygen atoms. Here, we present a detailed study of low-temperature charge transport in Sr{}_{1-x}Ca{}_{x}TiO{}_{3-delta }, documenting the evolution of resistivity with increasing carrier concentration (n). Below a threshold carrier concentration, {n}^{* }(x), the polar structural-phase transition has a clear signature in resistivity and Ca substitution significantly reduces the 2 K mobility at a given carrier density. For three different Ca concentrations, we find that the phase transition fades away when one mobile electron is introduced for about 7.9pm 0.6 dipoles. This threshold corresponds to the expected peak in anti-ferroelectric coupling mediated by a diplolar counterpart of Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction. Our results imply that the transition is driven by dipole–dipole interaction, even in presence of a dilute Fermi sea. Charge transport for n < {n}^{* }(x) shows a non-monotonic temperature dependence, most probably caused by scattering off the transverse optical phonon mode. A quantitative explanation of charge transport in this polar metal remains a challenge to theory. For nge {n}^{* }(x), resistivity follows a T-square behavior together with slight upturns (in both Ca-free and Ca-substituted samples). The latter are reminiscent of Kondo effect and most probably due to oxygen vacancies.
Highlights
The concept of a polar or “ferroelectric” metal was first proposed by Anderson and Blount in 1960s.1 They considered a continuous structural-phase transition breaking the inversion symmetry and leading to the appearance of a polar axis in a metal
The polar structural transition, which manifests itself as a kink in resistivity of this metal, is analogue to what occurs in its insulating ferroelectric cousins LiNbO3 and LiTaO3.2 Puggioni and Rondinelli[3] have argued that such polar metals can be found when there is an unusually weak coupling between mobile electrons and transverse optical phonons, which drive ferroelectricity
We present a study of low-temperature electrical resistivity in dozens of Sr1ÀxCaxTiO3Àδ single-crystals with x 1⁄4 0, 0:22%, 0:45%, 0:9% documenting in detail the evolution of charge transport with increasing concentrations of electric dipoles and charge carriers, a question, which was not addressed in depth by a previous study.[23]
Summary
The concept of a polar or “ferroelectric” metal was first proposed by Anderson and Blount in 1960s.1 They considered a continuous structural-phase transition breaking the inversion symmetry and leading to the appearance of a polar axis in a metal. Raman scattering found that the hardening of the FE soft mode in the dilute metal is indistinguishably similar to what is seen in the insulator.[23] The anomaly in resistivity was found to terminate at a threshold carrier density (nÃ), near which the superconducting transition temperature was enhanced[23] providing evidence for a link between superconducting pairing and ferroelectricity, a subject of present attention.[24,25,26,27,28,29,30,31,32,33,34]
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