Session K10: Fe-based Superconductors -- FeSe Intercalates and Interfaces

Light induced non-volatile switching of superconductivity in single layer FeSe/SrTiO3 heterostructures

The capability of using light to control the superconducting state is highly desirable for active quantum device applications. Since superconducting materials rarely exhibit strong photoresponses, and vice versa, optically sensitive materials are often not superconducting, the efficient coupling of these two characters in a single material can be a challenging task. Here we show that, in FeSe/SrTiO₃ heterostructures, the superconducting transition temperature in FeSe monolayer can be effectively raised by the interband photoexcitations in the SrTiO₃ substrate, attributed to a light induced metastable polar distortion uniquely enabled by the FeSe/SrTiO₃ interface, this effect only requires a less than 50 µW/cm² continuous-wave light field. The fast optical generation of superconducting zero resistance state is non-volatile but can be rapidly reversed by applying voltage pulses to the back of SrTiO₃ substrate. The capability of switching FeSe repeatedly and reliably between normal and superconducting states demonstrate the great potential of making energy-efficient quantum optoelectronics at designed correlated interfaces.   

Collective excitations and the superconductivity enhancement of FeSe/SrTiO3

The observation of substantially enhanced superconducting transition temperatures of single layer FeSe films on SrTiO₃ (STO) substrates has stimulated intensive research to identify the underlying microscopic mechanism. At present, the significant role of interfacial coupling has been widely recognized, but the precise nature of the superconductivity enhancement remains open. By employing high resolution electron energy loss spectroscopy, collective excitations in FeSe/STO system, including the Fuchs-Kliewer (F-K) phonons of STO and the polaronic plasmon originating from collective oscillation of polarons, were measured and studied. With FeSe growth, two dramatic contrasts were observed comparing with bare STO. First, the F-K phonons show indispensable correlation with the polaronic plasmon, indicating F-K phonons participate into the collective oscillation of polarons. Second, the linewidth broadening of the F-K phonons after FeSe growth indicates significant coupling between F-K phonons of STO and electrons in FeSe. It is evidenced that the electrons in FeSe/STO systems are dressed by the strongly polarized local lattice distortions associated with the F-K phonons across the interface. Furthermore, such an interfacial electron-phonon interaction is non-adiabatic in nature, leading to the formation of dynamic interfacial polarons that form the observed polaronic plasmon. The corresponding theoretical model shows that the interfacial polaron-polaron interaction can induce additional attraction between electrons in the systems, resulting in enhanced electron pairing strength and, accordingly, superconductivity.   

Low-Temperature Gated Raman Spectroscopy and Quantitative EELS of FeSe/STO

We characterize the phonon dynamics and interfacial charge transfer in FeTe-capped monolayer (ML) and few-layer FeSe films on SrTiO3 using low-temperature gated Raman spectroscopy and electron energy loss spectroscopy (EELS). In FeTe/ML FeSe/STO, we observe emergence of a silent STO mode at 264 cm-1 and slight hardening of STO phonons with an applied back-gate potential, consistent with the bare STO Raman response under electric field. Interestingly, we observe attenuation of the FeSe B3g peak accompanied by mild softening of both FeSe and FeTe modes, in sharp contrast to the STO dynamics. Notably, this behavior is absent in our measurement of the few-layer sample, suggesting an interplay between the STO and FeTe layer when the FeSe film is sufficiently thin. This is further corroborated by slight hardening of FeTe phonons as the FeSe film thickness is reduced, which we attribute to a non-local effect by the STO. Finally, the intrinsic charge transfer at the FeSe/STO interface is quantified through comparison of experimental and calculated EELS spectra and contrasted between the ML and few-layer samples.   

Observation of discrete CdGM states inthe vortex core of single layer FeSe/SrTiO3

For conventional type-II superconductors, confined quasi-particles in the vortex cores will give rise to Caroli-de Gennes-Matricon (CdGM) bound states with energies of E=μΔ²/E_F (where μ is a half integer, ±1/2, ±3/2…). However, discrete CdGM states were seldom identified due to the small value of Δ²/E_F, which is usually in the micro-eV range. Here we report a clear observation of multiple discrete CdGM states in single layer FeSe film on SrTiO₃, through high energy resolved tunneling spectrum. We found that the energy of these CdGM states can be well described by E=μΔ²/E_F with μ equal to half integers (no zero-bias conductance peak observed). Therefore our results support a simple s-wave pairing scenario for this high Tc system. Besides, we studied magnetic impurity (Fe) induced in-gap state with high energy resolution (T=0.4K). We found that all the in-gap state are off zero bias and split at high field, despite their intensity and location may vary for different impurities. Our results would help to further unveil the mechanism underlying the high Tc single layer FeSe/SrTiO₃ superconductor.   

Cavity-enhanced electron-phonon coupling in monolayer FeSe/SrTiO3

We propose a quantum-electrodynamical setting to address the coupling of a low-dimensional quantum material to quantized electromagnetic fields in quantum cavities. Using a protoypical model system describing FeSe/SrTiO with electron-phonon long-range forward scattering, we study how the formation of phonon polaritons at the 2D interface of the material modifies effective couplings and superconducting properties in a Migdal-Eliashberg simulation. We find that through highly polarizable dipolar phonons, large cavity-enhanced electron-phonon couplings are possible but superconductivity is not enhanced for the forward-scattering pairing mechanism due to the interplay between coupling enhancement and mode softening. Our results demonstrate that quantum cavities enable the engineering of fundamental couplings in solids paving the way to unprecedented control of material properties.

arXiv:1802.09437, to appear in Science Advances   

On the Tc enhancement mechanism at the FeSe/SrTiO3 interface

At the interface between monolayer FeSe films and STO substrate the superconducting transition temperature (Tc) is unexpectedly high. The mechanism for the Tc enhancement has been the central question, as it may present a new strategy for seeking out higher Tc materials.
To reveal the mechanism, by combining advances in high quality interface growth, ¹⁶O-¹⁸O isotope substitution, and extensive data from angle resolved photoemission spectroscopy, we provide striking evidence that the high Tc in FeSe/STO is the cooperative effect of the intrinsic pairing mechanism in the FeSe and interactions between the FeSe electrons and STO phonons.
Furthermore, our results point to the promising prospect that similar cooperation between different Cooper pairing channels may be a general framework to understand and design HTSCs.   

Effect of Mn substitution on superconductivity in iron selenide (Li, Fe)OHFeSe single crystals

We synthesize a series of Mn substituted (Li, Fe)OHFeSe superconductor single crystals via a modified ion-exchange method, with the Mn dopant concentration z (the atomic ratio of Mn:Se) ranging from zero to 0.07. Interestingly, we find that the superconducting transition temperature T_c and unit cell parameter c of the Mn-doped (Li, Fe)OHFeSe samples display similar V-shaped evolutions with the dopant concentration z. The Mn dopant may be accommodated in the tetrahedral sites of both the (Li, Fe)OH- and FeSe-layers depending on the doping level, leading to a reduced or enhanced interlayer separation of (Li, Fe)OHFeSe. The observed positive correlation between the T_c and lattice parameter c, regardless of the Mn doping level z, indicates that a larger interlayer separation, or a weaker interlayer coupling, is essential for the high-T_c superconductivity in (Li, Fe)OHFeSe.
Corresponding authors: dong@iphy.ac.cn, zhxzhao@iphy.ac.cn
References
1. Huaxue Zhou, et al., Chin. Phys. B 26, 057402 (2017)
2. Yiyuan Mao, et al., Chin. Phys. B 27, 077405 (2018)   

Substrate Engineering of Monolayer FeSe Films

Monolayer FeSe films will exhibit various properties on different substrates: the superconducting transition temperature (T_c) is suppressed if we choose graphene as the substrate, while SrTiO₃ abd TiO₂ substrates can boost the T_c to ~60 K. In this talk, we present the systematic tuning of the substrate structure, and the resulting change in the properties of FeSe, using a combined in-situ system of oxide MBE, chalcogenide MBE, STM and ARPES.   

ARPES and MBE studies on the role of the interface in Monolayer FeSe on SrTiO3

Monolayer FeSe on SrTiO3 (STO) has been heavily studied in recent years due to the discovery of a large enhancement in its superconducting Tc compared to bulk FeSe, which is attributed to interfacial coupling between the film and substrate. To better understand this interface and how it leads to such a Tc enhancement, we reexamine this system by using MBE grown FeSe and STO coupled with in-situ synchrotron based angle-resolved photoemission spectroscopy (ARPES). Here we present the influence of different STO surface terminations and photon energy dependent ARPES results which help to clarify the role of the substrate in the monolayer FeSe system.   

Electronic structure of high-Tc topological superconductor monolayer FeTe1-xSex/SrTiO3(001)

Topological superconductors are promising platforms for finding Majorana bound states, which are expected to be used in robust quantum computation. However, transition temperature of conventional superconductors is relatively low and single materials combined with high-temperature superconductivity (T_c) and topological nontrivial states are scarce in reality. Here, we performed angle-resolved photoemission spectroscopy studies on a series of FeTe_1-xSe_x monolayer films grown on SrTiO₃(STO). The superconductivity of the films is robust and rather insensitive to the variations of the band position. However, the band gap between the electron- and hole-like bands at the Brillouin zone center decreases towards band inversion and parity exchange, which drive the system to a nontrivial topological state predicted by theoretical calculations. Our results provide a clear experimental indication that the FeTe_1-xSe_x monolayer materials are high-temperature connate topological superconductors in which band topology and superconductivity are integrated intrinsically.   

BCS-BEC Crossover of the FeSe/SrTiO3 Interface

Superconductivity can be described either in the Bardeen-Cooper-Schrieffer (BCS) limit or in the Bose-Einstein condensation (BEC) limit. Unlike the superconductivity in the BCS limit where the involved electron-boson interaction (EBI) is adiabatic, in the BEC limit, the low superfluid density and short coherence length are expected in the non-adiabatic EBI picture. Our previous studies have shown that the EBI in FeSe/SrTiO₃ systems exhibits an indispensable non-adiabatic nature, which leads to the superconductivity beyond the BCS limit. Here, through the analysis of BCS-BEC phase diagram, we find the superconductivity of single-layer FeSe on SrTiO₃ substrate locates close to the BCS-BEC crossover unitary. Moreover, the superconductivity can be tuned toward to the BEC limit by surface hole doping. Indeed the potential pseudogap was observed in our scanning tunneling spectroscopy study. These findings provide a rare platform to explore the many-body interactions in superconductors.   

Decoupling Contributions to the Superconducting Enhancement Mechanism in Epitaxial FeSe Monolayers

The discovery of greatly increased superconducting Tc in FeSe/SrTiO3 monolayer films (>60 K vs 8 K in bulk) has drawn enormous interest to the prospect of interfacial high-Tc materials. The presence of the STO substrate has been shown to induce heavy electron doping and lattice strain on the adjacent FeSe layer, and is widely proposed to contribute additional enhancement via interfacial phonon coupling. In order to effectively decouple the contributions of these effects to the resultant high-Tc state, we systematically explore the evolution of superconductivity as measured unambiguously by in situ electrical resistivity under varied conditions of strain, surface doping concentration, and substrate interface condition. We show that the potassium surface dosing produces a heavily electron doped superconducting layer constrained to the film-vacuum interface, analogous to the monolayer FeSe/STO interface but lacking any STO phonon contribution. In contrast to observations from spectroscopic probes, we observe only modest discrepancies in the zero-resistance Tc for surface doped layers in comparison to monolayer films. We discuss possible explanations for this discrepancy and the implications of these results on the broader understanding of the FeSe/STO enhancement phenomenology.