Session L10: Fe-based Superconductors -- FeSe Intercalates and Interfaces II

Spectroscopic evidence of pair-mediated bosonic modes in superconductor FeSe/SrTiO3(100) film

Single layer FeSe on SrTiO₃(100) is atypical but noticed system in superconductivity. This has unique properties due to the substrate phonon. Unlike other bulk systems, the presence of the interface allows the substrate phonons to affect the superconducting layer. We have investigated substrate phonon effects on superconducting FeSe layer by using scanning tunneling spectroscopy and Eliashberg theory. We were able to measure acoustic, optical and substrate phonons in d²I/dV² spectroscopy. We found these phonon modes attribute to the paring of electrons in this superconducting layer. These results are analyzed by Eliashberg model and we will discuss the coupling strength of these bosonic features. We have found that the substrate phonon has major contribution to increase the transition temperature of this system.
  

Replica bands in FeSe monolayers on STO

The recent observation of replica bands in FeSe monolayers on STO in angle-resolved photoemission spectroscopy (ARPES) [1] has triggered intense discussions concerning the influence of FeSe electron coupling with STO phonon on the enhanced superconductivity. To obtain narrow replica bands tracing closely the dispersion of main bands rather than broad shake-off features or kinks, one has to require the coupling to be strongly peaked at q=0. However, whether the uniform electric field generated by q=0 STO phonon can strengthen the Cooper pairing in FeSe monolayer is still in discussion. We follow up with a new interpretation of the replica bands demonstrating they are largely due to the energy loss process of the escaping photoelectron, resulted from the well-known strong coupling of external propagating electrons to q=0 Fuchs-Kliewer (FK) surface phonons in STO [2]. Photoelectron energy loss on FeSe monolayer is calculated using the demonstrated successful semiclassical dielectric theory in describing low energy high-resolution electron energy loss spectroscopy (HREELS) [2]. We reduce the loss probability in HREELS by a factor 2 since the electron traveling path in ARPES is 1/2 of the HREELS one. Our calculation turns out to be able to reproduce the replica intensity and other experimental features in detail very well without the need for any fitting parameter [3]. This strongly suggests that the observed replica bands are mostly due to extrinsic photoelectron energy loss and not a result of the electron-phonon interaction of the Fe d electrons with the STO phonons. Therefore, the mechanism of the enhanced superconductivity in these monolayers remains an open question although other phonons than the FK types may still contribute.

[1] J. Lee et al., Nature (London) 515, 245 (2014).
[2] H. Ibach and D. L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (1982).
[3] F. Li and G. A. Sawatzky, Phys. Rev. Lett. 120, 237001 (2018).   

Molecular Beam Epitaxy Growth of FeSe/SrTiO3 Heterostructures

Monolayer of FeSe grown on SrTiO₃ surface has shown to exhibit a large increase in superconducting transition temperature, from ~8K in bulk FeSe to above 100K (Ge J, et al. Nature Materials 14, 285-289(2015) ). Extensive studies performed on this system to date suggested that both electronic and phononic mechanism might be responsible for this enhancement of superconductivity. Therefore, a route towards increasing the Tc even further might involve using oxide substrates with carefully engineered chemical composition. We use molecular beam epitaxy to grow thin film SrTiO₃ on commercial substrates with a precise control of the surface termination (SrO vs. TiO₂). Furthermore, we discuss our preliminary results on comparing the structural and electronic properties of FeSe deposited on SrTiO₃ thin films and bulk commercial substrates.   

Suppression of superconductivity and the magnetotransport behaviour in ultra-thin flakes of FeSe

The discovery of high temperature superconductivity in a monolayer of FeSe on SrTiO3 has ignited significant interest in understanding its two-dimensional superconductivity and the interfacial phenomena that determine its properties. To address these aspects, we examine the superconducting and electronic properties of exfoliated thin flakes of FeSe as a function of decreasing thickness, from bulk down towards 10 nm. We present a magnetotransport study in magnetic fields up to 38 T, which allows us to assess the changes in the multi-band electronic properties as a function of thickness. By reducing the thickness, the superconductivity of FeSe flakes is suppressed and the superconducting phase diagrams show significant changes in anisotropy.   

Coexistence of Magnetism and Superconductivity in Separate Layers of Iron-Based Superconductor Li1-xFex(OH)Fe1-ySe

The family of compounds of general formula Li_1-xFe_x(OH)Fe_1-ySe, consisting of alternating Fe_1-ySe and Li_1-xFe_x(OH) layers, have been found to superconduct with T_c’s of ≈ 40 K. The hydroxide layer, containing roughly 20 % substitution of Li for Fe, has been shown to display magnetism well below the superconducting transition in the region of 10 K. This has been alternately reported as an ordering of the hydroxide layer Fe moments ferromagnetically and antiferromagnetically. Our study elucidates this magnetism by taking advantage of the flexible synthesis of these compounds allowing investigation of superconducting and non-superconducting variants in an effort to disentangle magnetism form superconductivity. Utilizing static and dynamic magnetometry, measurement of heat capacity and muon spin relaxation we propose the magnetism of the hydroxide layer to be glass-like and coexist with superconductivity.

PRB, 95, 134419 (2017)   

Sign reversal of order parameter in single layer FeSe/SrTiO3 film

Single layer FeSe film grown on SrTiO₃ substrate has drawn much interest for its novel interfacial effects, which have led to the highest superconducting temperature (T_C) to date amongst all Fe-based superconductors. While several paring symmetries, such as s₊₊-wave, nodeless d-wave, as well as s_±-wave have been suggested, the experimental determination of its superconducting paring symmetry remains elusive. Here we investigate the intrinsic impurity-induced in-gap bound states and quasiparticle interference (QPI) patterns in single layer FeSe/SrTiO₃ by scanning tunneling microscopy/spectroscopy. We observed bound states induced by nonmagnetic impurities, which strongly suggests a sign-changing order parameter. Through detailed analysis of the phase-sensitive QPI patterns, we further confirm that the order parameter indeed changes sign within the electron pockets. This identification of a sign change pairing symmetry in single layer FeSe/SrTiO₃ presents an important step towards the understanding of the mechanism of its high T_c superconductivity.   

Evidence for nodeless d-wave superconductivity in single layer FeSe/SrTiO3 by quasiparticle scattering off step edges

The de Gennes extrapolation length is a direction-dependent measure of the pairing gap near the boundary of a superconductor, and thus provides a viable means to probe its pairing symmetry. The extrapolation length is infinite and isotropic for plain s-wave pairing, and finite and anisotropic for d-wave pairing. Here, we synthesize single layer FeSe films on SrTiO₃(001) substrates by molecular beam epitaxy, and determine the de Gennes extrapolation length by scanning tunneling microscopy/spectroscopy. We find a 40% reduction of the superconducting gap near specular [110]_Fe edges, yielding an extrapolation length of 8.0 nm. Near specular [100]_Fe edges, however, the extrapolation length is nearly infinite. These findings are consistent with a phase changing pairing with 2-fold symmetry, indicating nodeless d-wave superconductivity. This work provides direct experimental evidence for nodeless d-wave superconductivity in single layer FeSe/SrTiO₃, and demonstrates that quasiparticle scattering at boundaries can be a viable phase sensitive probe of pairing symmetry in Fe-based superconductors.<audio controls="controls" style="display: none;"> </audio>   

Evidence of Two-dimensional Superconducting Behavior in Atomically Thin Fe(Te0.7Se0.3) Flakes

We report detailed thickness-dependent transport studies on strain-free Fe(Te_0.7Se_0.3) thin flakes. Notably, we present evidence of two-dimensional superconducting behavior in flakes <10nm. For >10nm flakes we find a systematic suppression of the superconductivity and broadening of the superconducting phase transition. Due to non-uniform Te/Se spacial distribution, we find that R(T) behavior in flakes <10nm can be explained and confirmed using the BKT transition for inhomogeneity model, as well as finite state effects(FSE). This inhomogeneous model is supported by the observed thickness dependence of the superconducting transition. We propose a 2D network of superconducting paths connecting superconducting islands within Fe(Te_0.7Se_0.3) thin flakes to describe the behavior of this natural percolating system.   

Hydrothermal design of magnetic heterolayer iron-based superconductors

Recently, a new iron-based superconductor, (Li_1-xFe_xOH)FeSe (critical temperature, T_c = 43 K), was reported. Pachmayr et al. and Lu et al. reported observation of ferromagnetism and antiferromagnetism, respectively, in this 43 K superconductor, but no magnetic peak(s) have been observed for this system in several neutron diffraction studies. Thus, our current work aimed at controlling this magnetism through selective doping of other transition metals onto the Li site within the (Li_1-xFe_xOH)^δ+ spacer layers. We have synthesized powder and single crystals (Li_1-x-yFe_xM_yOH)FeSe samples (M = Cr, Mn, Co, Ni, Cu, Zn) through a hydrothermal ion-exchange reaction of KFe_2-2yM_2ySe₂ precursors. These samples have been characterized by x-ray diffraction, electrical resistivity, and magnetic susceptibility to confirm their crystal structure, chemical composition and the influence of transition metal dopants on their superconductivity and magnetism. Neutron diffraction revealed correlation between magnetism and the crystal structure tuned by transition metal dopants in the spacer layers which coexists with superconductivity. Our work demonstrates coexistence of superconductivity and magnetism can be designed through chemical manipulation of heterolayer transition metal chalcogenides.   

Engineering nanoscale superconductivity through the competition between ferromagnetism and superconductivity

As electronic devices continue to decrease in size, it become increasingly more important to understand how to engineer nanoscale systems. Reduced dimensionality in superconductors allows fluctuations to play a greater role in the behavior of the system. For example, bulk iron selenide (FeSe) has a superconducting critical temperature of 8K, but the single layer FeSe grown on SrTiO₃ exhibits a significantly higher critical temperature. Motivated by these results, we have performed density functional theory (DFT) calculations for various systems of SrTiO₃ with different surface reconstructions for oxygen vacancies. We find that a robust surface ferromagnetism can emerge due to the periodic pattern of oxygen vacancies in the presence of Hubbard on-site interaction. Using a single-band model for the superconductor-ferromagnet heterostructure, we demonstrate that for nanoscale systems the superconducting properties are highly susceptible to modulation caused by the ferromagnetism, which can lead to the decrease of the pairing and critical temperature.