Theory of Superconductivity in Polar Semiconductors and Its Application to N-Type Semiconducting SrTiO<sub>3</sub>

Takada, Yogo

doi:10.1143/jpsj.49.1267

Cited by 120 publications

(118 citation statements)

References 30 publications

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“…More recently, a sharp Fermi surface and a superconducting ground state have been found to persist down to a carrier concentration of 10 17 cm −3 in SrTiO 3−δ (refs 5,6). In this range of carrier concentration, exceptionally low compared to any other known superconductor, the Fermi temperature is an order of magnitude lower than the Debye temperature and the ability of phonons to form Cooper pairs is questionable 7,8 . Recent theories invoke fluctuations of the ferroelectric mode 9 , longitudinal optical phonons 10 or plasmons 8 as bosons responsible for the generation of Cooper pairs.…”

mentioning

confidence: 90%

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

Rischau¹,

Lin²,

Grams³

et al. 2017

Nature Phys

202

229

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SrTiO 3 , a quantum paraelectric 1 , becomes a metal with a superconducting instability after removal of an extremely small number of oxygen atoms 2 . It turns into a ferroelectric upon substitution of a tiny fraction of strontium atoms with calcium 3 . The two orders may be accidental neighbours or intimately connected, as in the picture of quantum critical ferroelectricity 4 . Here, we show that in Sr 1−x Ca x TiO 3−δ (0.002 < x < 0.009, δ < 0.001) the ferroelectric order coexists with dilute metallicity and its superconducting instability in a finite window of doping. At a critical carrier density, which scales with the Ca content, a quantum phase transition destroys the ferroelectric order. We detect an upturn in the normal-state scattering and a significant modification of the superconducting dome in the vicinity of this quantum phase transition. The enhancement of the superconducting transition temperature with calcium substitution documents the role played by ferroelectric vicinity in the precocious emergence of superconductivity in this system, restricting possible theoretical scenarios for pairing.A perovskite of the ABO 3 family, SrTiO 3 is a quantum paraelectric whose dielectric constant rises to ∼20,000 at low temperature 1 , but avoids long-range ferroelectric order. It becomes a metal by substituting Sr with La, Ti with Nb, or by removing O. It has been known for half a century that this metal is a superconductor at low temperatures 2 . More recently, a sharp Fermi surface and a superconducting ground state have been found to persist down to a carrier concentration of 10 17 cm −3 in SrTiO 3−δ (refs 5,6 However, mobile electrons screen polarization and therefore only insulating solids are expected to host a ferroelectric order. Hitherto, as a paradigm, ferroelectric quantum criticality, in contrast to its magnetic counterpart, was deprived of an experimental phase diagram in which a superconducting phase and a ferroelectric order share a common boundary.Here, we produce such a phase diagram in the case of Sr 1−x Ca x TiO 3−δ . The main new observations are the following: metallic Sr 1−x Ca x TiO 3−δ hosts a phase transition structurally indistinguishable from the ferroelectric phase transition in insulating Sr 1−x Ca x TiO 3 ; the coexistence between this ferroelectric-like order and superconductivity ends beyond a threshold carrier concentration; and, in the vicinity of this quantum phase transition, calcium substitution enhances the superconducting critical temperature and induces an upturn in the normal-state resistivity.Figure 1 summarizes what we know about the emergence of ferroelectricity, metallicity and superconductivity in this system. When a small fraction of Sr atoms (x > 0.002) is replaced with isovalent Ca, Sr 1−x Ca x TiO 3 becomes ferroelectric 3 , with a Curie temperature steadily increasing with Ca content in the dilute limit 0.002 < x < 0.02 (refs 3,13,14). Macroscopic polarization below the Curie temperature has been observed in dielectric and linear birefringence measurements, a...

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mentioning

confidence: 90%

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

Rischau¹,

Lin²,

Grams³

et al. 2017

Nature Phys

202

229

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“…The adequate theoretical method for superconductivity in the non-adiabatic electron -LO-phonon system is the dielectric function approach [17,21,22]. We apply the dielectric function approach as a unique method to treat superconductivity both at the SrTiO 3 -LaAlO 3 interface [18] and in bulk doped strontium titanate.…”

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confidence: 99%

Multiband Superconductivity Due to the Electron–LO–Phonon Interaction in Strontium Titanate and on a SrTiO3/LaAlO3 Interface

Klimin

Tempere

Devreese

et al. 2016

J Supercond Nov Magn

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In strontium titanate, the Fröhlich electron -LO-phonon interaction dominates the electron response and can also provide superconductivity. Because of high LO-phonon frequencies in SrTiO 3 , the superconducting system is non-adiabatic. We demonstrate that the dielectric function approach is an adequate theoretical method for superconductivity in SrTiO 3 and on the SrTiO 3 -LaAlO 3 interface. The critical temperatures are calculated using realistic material parameters. The obtained critical temperatures are in line with experimental data both for bulk and interface superconductivity. The present method explains the observed multidome shape of the critical temperature in SrTiO 3 as a function of the electron concentration due to multiband superconductivity.

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“…It is generally accepted that this behavior can be explained by the plasmon and polar optic phonon model. 18 However, because the crystal quality in general gets worse as the carrier concentration increases ͑at high chemical doping͒, one could argue that the crystal quality has an influence on the T c behavior. From this view point, field effect ''doping,'' which does not affect the crystal quality, is also of interest.…”

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Electrostatic modulation of the electronic properties of Nb-doped SrTiO3 superconducting films

Takahashi

Matthey

Jaccard

et al. 2004

Applied Physics Letters

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We have performed ferroelectric field effect experiments using an epitaxial heterostructure composed of ferroelectric Pb(Zr0.2Ti0.8)O3 and superconducting Nb-doped SrTiO3. The films were prepared on (001) SrTiO3 substrates by off-axis radio-frequency magnetron sputtering and pulsed-laser deposition. By switching the polarization field of the 500-Å-thick Pb(Zr0.2Ti0.8)O3 layer, a large change of about 30% in resistivity and a 20% shift of Tc (ΔTc∼0.05 K) were induced in the 400-Å-thick epitaxial Nb-doped SrTiO3 layer. The relationship between Tc and the electrostatically modulated average carrier concentration can be mapped onto the phase diagram of chemically doped SrTiO3.

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Theory of Superconductivity in Polar Semiconductors and Its Application to N-Type Semiconducting SrTiO₃

Cited by 120 publications

References 30 publications

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

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

Multiband Superconductivity Due to the Electron–LO–Phonon Interaction in Strontium Titanate and on a SrTiO3/LaAlO3 Interface

Electrostatic modulation of the electronic properties of Nb-doped SrTiO3 superconducting films

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Theory of Superconductivity in Polar Semiconductors and Its Application to N-Type Semiconducting SrTiO3

Cited by 120 publications

References 30 publications

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

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

Multiband Superconductivity Due to the Electron–LO–Phonon Interaction in Strontium Titanate and on a SrTiO3/LaAlO3 Interface

Electrostatic modulation of the electronic properties of Nb-doped SrTiO3 superconducting films

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Theory of Superconductivity in Polar Semiconductors and Its Application to N-Type Semiconducting SrTiO₃