Electron-phonon Coupling and the Superconducting Phase Diagram of the LaAlO3-SrTiO3 Interface.

Hans Boschker,Christof W Schneider,Jochen Mannhart,Evangelos Fillis-Tsirakis,Christoph Richter

doi:10.1038/srep12309

Hans Boschker, Christof W Schneider + Show 3 more

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https://doi.org/10.1038/srep12309

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Abstract

The superconductor at the LaAlO3—SrTiO3 interface provides a model system for the study of two-dimensional superconductivity in the dilute carrier density limit. Here we experimentally address the pairing mechanism in this superconductor. We extract the electron—phonon spectral function from tunneling spectra and conclude, without ruling out contributions of further pairing channels, that electron—phonon mediated pairing is strong enough to account for the superconducting critical temperatures. Furthermore, we discuss the electron—phonon coupling in relation to the superconducting phase diagram. The electron—phonon spectral function is independent of the carrier density, except for a small part of the phase diagram in the underdoped region. The tunneling measurements reveal that the increase of the chemical potential with increasing carrier density levels off and is zero in the overdoped region of the phase diagram. This indicates that the additionally induced carriers do not populate the band that hosts the superconducting state and that the superconducting order parameter therefore is weakened by the presence of charge carriers in another band.

Highlights

The superconductor at the LaAlO3—SrTiO3 interface provides a model system for the study of twodimensional superconductivity in the dilute carrier density limit
A prominent example is the superconductor at the LaAlO3—SrTiO3 interface[2,3,4]
We recently mapped out the superconducting gap across the phase diagram and obtained a picture qualitatively similar to the phase diagram of the high-Tc-cuprate superconductors: in the underdoped region the gap increases with charge carrier depletion[15]

Summary

Introduction

The superconductor at the LaAlO3—SrTiO3 interface provides a model system for the study of twodimensional superconductivity in the dilute carrier density limit. The tunneling measurements reveal that the increase of the chemical potential with increasing carrier density levels off and is zero in the overdoped region of the phase diagram. We recently mapped out the superconducting gap across the phase diagram and obtained a picture qualitatively similar to the phase diagram of the high-Tc-cuprate superconductors: in the underdoped region the gap increases with charge carrier depletion[15] This similarity between the doping dependence of the superconducting gap of LaAlO3—SrTiO3 and the high-Tc-cuprates makes understanding the pairing mechanism in LaAlO3—SrTiO3 even more relevant. At n > 5·1019 cm−3, the plasma edge energy exceeds 100 meV, so that screening becomes more effective and the electron—phonon coupling is reduced This mechanism has been proposed to explain the reduction of Tc of doped SrTiO3 in the overdoped regime[20]. The additional carriers result in Coulomb scattering of the electrons in the superconducting band, thereby reducing the superconducting gap

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