Tailoring the nature and strength of electron–phonon interactions in the SrTiO3(001) 2D electron liquid

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Abstract

Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO3(001) surface to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO3/SrTiO3 interface, our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO3-based 2DELs.

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Interfacial mode coupling as the origin of the enhancement of T(c) in FeSe films on SrTiO3.

J J-W Lee, F. T. Schmitt, R. G. Moore … (2014)

Films of iron selenide (FeSe) one unit cell thick grown on strontium titanate (SrTiO3 or STO) substrates have recently shown superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 kelvin), which is a record for the iron-based superconductors. The gap opening temperature usually sets the superconducting transition temperature Tc, as the gap signals the formation of Cooper pairs, the bound electron states responsible for superconductivity. To understand why Cooper pairs form at such high temperatures, we examine the role of the SrTiO3 substrate. Here we report high-resolution angle-resolved photoemission spectroscopy results that reveal an unexpected characteristic of the single-unit-cell FeSe/SrTiO3 system: shake-off bands suggesting the presence of bosonic modes, most probably oxygen optical phonons in SrTiO3 (refs 5, 6, 7), which couple to the FeSe electrons with only a small momentum transfer. Such interfacial coupling assists superconductivity in most channels, including those mediated by spin fluctuations. Our calculations suggest that this coupling is responsible for raising the superconducting gap opening temperature in single-unit-cell FeSe/SrTiO3.

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Electric field control of the LaAlO3/SrTiO3 interface ground state.

A Caviglia, S Gariglio, N. Reyren … (2008)

Interfaces between complex oxides are emerging as one of the most interesting systems in condensed matter physics. In this special setting, in which translational symmetry is artificially broken, a variety of new and unusual electronic phases can be promoted. Theoretical studies predict complex phase diagrams and suggest the key role of the charge carrier density in determining the systems' ground states. A particularly fascinating system is the conducting interface between the band insulators LaAlO(3) and SrTiO(3) (ref. 3). Recently two possible ground states have been experimentally identified: a magnetic state and a two-dimensional superconducting condensate. Here we use the electric field effect to explore the phase diagram of the system. The electrostatic tuning of the carrier density allows an on/off switching of superconductivity and drives a quantum phase transition between a two-dimensional superconducting state and an insulating state. Analyses of the magnetotransport properties in the insulating state are consistent with weak localization and do not provide evidence for magnetism. The electric field control of superconductivity demonstrated here opens the way to the development of new mesoscopic superconducting circuits.