Charge Transport in a Polar Metal

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 ($0.002<x<0.02$) 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, $n^{\ast }(x)$, 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 $7.9\pm 0.6$ 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 $n<n^{\ast }(x)$ 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 $n\ge n^{\ast }(x)$, 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.

arXiv comment8 pages, 5 figures

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• Heavy non-degenerate electrons in doped strontium titanate
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• Superconductivity enhancement in polar metal regions of Sr${}_{0.95}$Ba${}_{0.05}$TiO${}_3$ and Sr${}_{0.985}$Ca${}_{0.015}$TiO${}_3$ revealed by the systematic Nb doping
• Huge permittivity and premature metallicity in Bi${}_2$O${}_2$Se single crystals
• Quasi-isotropic orbital magnetoresistance in lightly doped SrTiO${}_3$
• Spin-Phonon Resonances in Nearly Polar Metals with Spin-Orbit Coupling
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• Emergent superconductivity in doped ferroelectric hafnia
• Majorana-Weyl cones in ferroelectric superconductors
• Phonon-Induced Collective Modes in Spin-Orbit Coupled Polar Metals
• Quantum fluctuation of ferroelectric order in polar metals
• Beyond being free: glassy dynamics of SrTiO${}_3$-based two-dimensional electron gas
• Clues to potential dipolar-Kondo and RKKY interactions in a polar metal
• A large modulation of electron-phonon coupling and an emergent superconducting dome in doped strong ferroelectrics
• Third-order Electrical Conductivity of the Charge-ordered Organic Salt $\alpha$-(BEDT-TTF)${}_2$I${}_3$
• Electron-electron and electron-phonon collision cross sections in CsV3Sb5