Correlation of Fe-Based Superconductivity and Electron-Phonon Coupling in an FeAs/Oxide Heterostructure
Researchers at KAIST have recently confirmed a novel mechanism of enhancing iron-based superconductivity; forward-scattering interfacial phonons.
Using scanning tunneling spectroscopy, they studied the correlation between superconductivity and e-ph interaction with interfacial phonons in an iron-based superconductor Sr2VO3FeAs (Tc ≈ 33 K) made of alternating FeAs and oxide layers.
The quasiparticle interference was measured over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in the VO2 layer; this supports self-consistent momentum-dependent Eliashberg calculations to provide unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.
Thus, the researchers report direct spectroscopic evidence of the enhancement of Fe-based superconductivity by interaction of the Fe electrons and the interfacial phonons in an FeAs/oxide heterostructure single crystal.
The discovery of high temperature superconductivity in monolayer iron-based superconductors (FeSC) on perovskite substrates has attracted broad attention due to the possibility of achieving Tc in the FeSCs, comparable to cuprate superconductors.
Importantly, these interfaced systems suggest an entirely new mechanism for enhancing superconductivity, applicable to a broad range of layered superconducting heterostructures: forward-scattering interfacial phonons.
This principle is evidenced by an angle-resolved photoemission spectroscopy (ARPES) study, which revealed that the increased Tc in FeSe/SrTiO3 coincides with the appearance of the replica bands as the thickness of the FeSe layers is varied.
Forward scattering by interfacial phonons is believed to produce these replicas, and this interaction has been proposed as a general means to enhance Tc in heterostructures.
However, (i) a definitive example of this principle in a bulk heterostructure, suitable for practical application, and (ii) direct real-space evidence of interfacial-phonon enhanced superconductivity are both currently lacking.
To address these issues, a bulk heterostructure made of alternating FeSC monolayers and perovskite layers has been suggested.
Interestingly, an iron-based superconductor Sr2VO3FeAs with highest Tc ≈ 33 K among all the 21311-family has a structure nearly equivalent to those values suggested; therefore it is a unique bulk candidate suitable for investigating the effect of interfacial-phonons on superconductivity.
Sr2VO3FeAs also exhibits self-doping by interlayer charge transfer between the perovskite Sr2VO3 layers and the FeAs layer, and shows a preferred and reproducible symmetric cleavage at the SrO-SrO interface.
These properties make it an ideal system for a surface spectroscopic study using a real-space probe, i.e. scanning tunneling microscopy and spectroscopy (STM/STS).
STM/STS has been used to map the spatial inhomogeneity of the superconducting gap in many unconventional superconductors; the correlations between the gap and the locations of dopant atoms, magnetic vortices or the supermodulation of lattices have been widely studied.
The researchers at KAIST have built a state-of-the-art variable temperature and magnetic field scanning probe microscope to enable research of prospective advanced materials.
In a paper recently published in Physical Review Letters, the researchers used the system to report systematic quasiparticle interference (QPI) measurements on the bulk heterostructured superconductor Sr2VO3FeAs grown using a self-flux technique, in collaboration with researchers at POSTECH (Korea).
They first observed both the filled and empty state replica bands in the QPI data, constituting direct spectroscopic evidence of e-ph coupling with forward-scattering phonons.
Furthermore, various changes in the renormalized bands were observed in regions with systematically different superconducting gaps near particular defects known to increase the e-ph strength locally.
These constitute an unprecedented direct demonstration of enhancement of superconductivity by forward-scattering phonons in a self-assembled FeAs/oxide heterostructure superconductor.
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This study was published in Physical Review Letters (September, 2017).
 S. Choi et al., Phys. Rev. Lett. 119, 107003 (2017).
* lab webpage : http://ltspm.kaist.ac.kr