[Paper Review] Superconductivity above 28 K in single unit cell FeSe films interfaced with GaO$_{2-\delta}$ layer on NdGaO$_{3}$(110)

Haohao Yang, Guanyu Zhou+13 more|arXiv (Cornell University)|Jan 18, 2019

Iron-based superconductors researchMaterials Science40 references3 citations

TL;DR

This study demonstrates interface-enhanced superconductivity in single-unit-cell FeSe films grown on GaO2-δ-terminated NdGaO3(110) substrates, achieving a superconducting transition temperature (Tc) onset of 28 K—significantly higher than bulk FeSe’s 8 K. The enhancement arises from interfacial electron doping via charge transfer, confirmed by scanning transmission electron microscopy and electron energy loss spectroscopy, establishing FeSe/GaO2-δ as a new platform for studying high-temperature superconductivity at oxide interfaces.

ABSTRACT

We prepared single unit cell FeSe films on GaO$_{2-\delta}$ terminated perovskite NdGaO$_{3}$(110) substrates and performed ex situ transport and scanning transmission electron microscopy measurements on the FeTe protected films. Our experimental measurements showed that the single unit cell FeSe films interfaced with GaO$_{2-{\delta}}$ layer are electron doped via interface charge transfer. Most importantly, this type of heterostructure can host interface enhanced superconductivity with an onset temperature of about 28 K, which is much higher than the value of 8 K in bulk FeSe. Our work indicates that FeSe/GaO$_{2-{\delta}}$can be a comparative platform with the FeSe/TiO$_{2-{\delta}}$ family for understanding the mechanism of interface enhanced high temperature superconductivity.

Motivation & Objective

• To investigate whether GaO2-δ-terminated NdGaO3(110) substrates can induce high-temperature superconductivity in single-unit-cell FeSe films.
• To explore the role of interfacial charge transfer and electronic structure in enhancing superconducting transition temperature (Tc) beyond bulk FeSe.
• To compare FeSe/GaO2-δ with established FeSe/TiO2-δ systems in terms of superconducting properties and interface mechanisms.
• To establish GaO2-δ as a viable alternative oxide capping layer for engineering high-Tc superconducting heterostructures.

Proposed method

• Epitaxial growth of single-unit-cell FeSe films on GaO2-δ-terminated NdGaO3(110) substrates using molecular beam epitaxy (MBE) under ultra-high vacuum conditions.
• Preparation of GaO2-δ-terminated surfaces via wet chemical etching in 10% HCl followed by thermal annealing at 930 °C under O2 flux.
• Use of 10-unit-cell FeTe capping layers to protect FeSe films from ambient oxidation during ex situ measurements.
• Employment of scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to probe interfacial atomic structure and charge transfer.
• Ex situ transport measurements under magnetic fields to determine upper critical field (Hc2), coherence length (ξ), and London penetration depth (Λ).
• Application of the Ginzburg-Landau theory and Arrhenius plots of resistance to extract activation energy and estimate London penetration depth.

Experimental results

Research questions

• RQ1 Can GaO2-δ-terminated NdGaO3(110) substrates induce superconductivity in single-unit-cell FeSe films with Tc higher than bulk FeSe?
• RQ2 What is the nature of interfacial charge transfer in FeSe/GaO2-δ heterostructures, and how does it influence superconducting properties?
• RQ3 How does the superconducting behavior of FeSe on NdGaO3(110) compare to that on TiO2-δ-terminated substrates in terms of Tc, coherence length, and upper critical field?
• RQ4 To what extent does the interface structure and dielectric screening affect the enhancement of superconductivity in FeSe-based heterostructures?

Key findings

• The FeSe/GaO2-δ heterostructure exhibits a superconducting transition temperature (Tc) onset of approximately 28 K, significantly higher than bulk FeSe’s 8 K.
• Interfacial electron doping is confirmed via electron energy loss spectroscopy (EELS), indicating charge transfer from the GaO2-δ layer to FeSe.
• The upper critical field (Hc2) at zero temperature is measured as 22.5 T for single-unit-cell FeSe, with an in-plane coherence length of 3.59 nm.
• The London penetration depth is estimated at 84 nm for single-unit-cell FeSe, indicating a type-II superconductor with a Ginzburg-Landau parameter (κ) greater than 20.
• The thickness dependence of Tc shows that only the first unit cell of FeSe is superconducting, consistent with other FeSe/oxide systems.
• The system exhibits a linear dependence of vortex activation energy on ln(μ0H), supporting collective vortex creep and validating the extraction of penetration depth.