Metallicity and Superconductivity in Doped Strontium Titanate

Collignon, Clément; Lin, Xin; Rischau, Carl Willem; Fauqué, Benoît; Behnia, Kamran

doi:10.1146/annurev-conmatphys-031218-013144

Cited by 116 publications

(136 citation statements)

References 138 publications

(253 reference statements)

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“…17, are for the same films studied in this work. The phase diagram shows the well-known superconducting "dome" [1] in close proximity to the quantum critical point of the spontaneous ferroelectric order. In this region, the superconducting critical temperature is enhanced by up to a factor of two in comparison with identically doped strainfree films grown on SrTiO 3 substrates [17], highlighting the essential role played by the spontaneous ferroelectricity.…”

Section: Interplay Between Ferroelectricity and Superconductivitymentioning

confidence: 88%

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Ferroelectric enhancement of superconductivity in compressively strained SrTiO3 films

Russell

Ratcliff

Ahadi

et al. 2019

Phys. Rev. Materials

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SrTiO3 is an incipient ferroelectric on the verge of a polar instability, which is avoided at low temperatures by quantum fluctuations. Within this unusual quantum paraelectric phase, superconductivity persists despite extremely dilute carrier densities. Ferroelectric fluctuations have been suspected to play a role in the origin of superconductivity by contributing to electron pairing. To investigate this possibility, we used optical second harmonic generation to measure the doping and temperature dependence of the ferroelectric order parameter in compressively strained SrTiO3 thin films. At low temperatures, we uncover a spontaneous out-of-plane ferroelectric polarization with an onset that correlates perfectly with normal-state electrical resistivity anomalies. These anomalies have previously been associated with an enhancement of the superconducting critical temperature in doped SrTiO3 films, directly linking the ferroelectric and superconducting phases. We develop a long-range mean-field Ising model of the ferroelectric phase transition to interpret the data and extract the relevant energy scales in the system. Our results support a long-suspected connection between ferroelectricity and superconductivity in SrTiO3, but call into question the role played by ferroelectric fluctuations.

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Section: Interplay Between Ferroelectricity and Superconductivitymentioning

confidence: 88%

“…Despite its long history and contemporary interest, the superconducting phase of SrTiO 3 is still not well understood [1]. A central open question is how the superconductivity emerges from an exceptionally dilute Fermi gas.…”

Section: Introductionmentioning

confidence: 99%

Ferroelectric enhancement of superconductivity in compressively strained SrTiO3 films

Russell

Ratcliff

Ahadi

et al. 2019

Phys. Rev. Materials

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“…Moreover, this quantum paraelectric can become a dilute metal (with a carrier concentration as small as ≈ 10 16 cm −3 ) upon oxygen reduction [16,17]. The dilute metal undergoes a superconducting transition below 0.3 K [18][19][20]. Rischau et al have recently found that a superconducting phase coexists with a FE-like instability in ndoped Sr 1−x Ca x TiO 3−δ and superconductivity and ferroelectricity (FE) may be intimately linked [21].…”

Section: Introductionmentioning

confidence: 99%

Charge transport in a polar metal

et al. 2019

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The fate of electric dipoles inside a Fermi sea is an old issue, yet poorly-explored. Sr1−xCaxTiO3 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 Sr1−xCaxTiO 3−δ , 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 7.9±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. At higher carrier concentrations, our data resolves slight upturns in low-temperature resistivity in both Ca-free and Ca-substituted samples, reminiscent of Kondo effect and most probably due to oxygen vacancies.arXiv:1909.04278v1 [cond-mat.supr-con]

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“…For non-local potentials satisfying Eqs. (3), no such ad-hoc regularization is needed: the integral in Eq. (4) is mathematically well-defined [21].…”

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confidence: 99%

Ubiquity of Superconducting Domes in the Bardeen-Cooper-Schrieffer Theory with Finite-Range Potentials

2019

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Based on recent progress in mathematical physics, we present a reliable method to analytically solve the linearized BCS gap equation for a large class of finite-range interaction potentials leading to s-wave superconductivity. With this analysis, we demonstrate that the monotonic growth of the superconducting critical temperature Tc with the carrier density, n, predicted by standard BCS theory, is an artifact of the simplifying assumption that the interaction is quasi-local. In contrast, we show that any well-defined non-local potential leads to a "superconducting dome", i.e. a non-monotonic Tc(n) exhibiting a maximum value at finite doping and going to zero for large n. This proves that, contrary to conventional wisdom, the presence of a superconducting dome is not necessarily an indication of competing orders, nor of exotic superconductivity.

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Metallicity and Superconductivity in Doped Strontium Titanate

Cited by 116 publications

References 138 publications

Ferroelectric enhancement of superconductivity in compressively strained SrTiO3 films

Ferroelectric enhancement of superconductivity in compressively strained SrTiO3 films

Charge transport in a polar metal

Ubiquity of Superconducting Domes in the Bardeen-Cooper-Schrieffer Theory with Finite-Range Potentials

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