Session B11: Superconductivity in Monolayer FeSe/SrTiO3

ARPES of single layer iron pnictide on STO.

Quantum systems in confined geometries have been a very rich ground for discoveries. In this talk, I will discuss recent progresses in uncovering novel physics at ultra-thin limit, with focus on mono-unit-cell (UC) superconductor FeSe grown on SrTiO3, where the Cooper pairing temperature is reported to have dramatically enhanced from its bulk value of 8K to \textasciitilde 60K. Of interest are the cause of the enhanced pairing strength, and the nature of the superconducting state. We show angle-resolved photoemission spectroscopy (ARPES) data that provide clear evidence for strong cross-interface electron-phonon coupling in single UC limit, suggesting that pairing is significantly enhanced by the strong interface mode coupling. We will also show other results on the nature of the superconducting state in this system. [References: JJ Lee et al., Nature 515, 245 (2014)].   

Enhanced superconductivity due to forward scattering in FeSe thin films on SrTiO3 substrates

We examine the consequences of an electron-phonon ($e$-$ph$) interaction that is strongly peaked in the forward scattering (${\bf q} = 0$) direction in a two-dimensional superconductor. We find that strong forward scattering results in an enhanced Tc that is linearly proportional to the strength of the dimensionless $e$-$ph$ coupling constant $\lambda_m$ in the weak coupling limit. This is in stack contrast to the exponential dependence commonly derived in conventional BCS theory. This interaction also produces distinct replica bands in the single-particle spectral function, similar to those observed in recent angle-resolved photoemission experiments on FeSe monolayers on SrTiO$_3$ and BaTiO$_3$ substrates. By comparing our model to photoemission experiments, we infer an $e$-$ph$ coupling strength that can provide a significant portion of the observed high $T_c$ in these systems. [Reference: arXiv:1507.03967]   

Electron-Phonon Couplings of the Interfacial Mode in FeSe Thin Films on SrTiO$_3$ and BaTiO$_3$

Monolayers FeSe on SrTiO$_3$ or BaTiO$_3$ substrates possess highest superconducting transition temperatures in Fe-based superconductors with $T_c\sim 70$ K measured by angle-resolved photoemission spectroscopy (ARPES) and other experiments. Furthermore, the high $T_c$'s concur with exact replica bands in ARPES spectra. A forward scattering mechanism with small momentum transfer through the electron-phonon interaction has been proposed to explain the high $T_c$'s and the replica bands.[1] We apply {\it ab initio} techniques to study such coupling in monolayer and bilayer FeSe thin films on SrTiO$_3$, BaTiO$_3$, and oxygen-vacant SrTiO$_3$ substrates. Our results confirm the forward scattering nature of electron-phonon coupling of the oxygen polar mode whose energy coincides with the off-set energy of the replica bands. [1]L.~Rademaker, et al., arXiv:1507.03967.   

Oxygen vacancy induced flat phonon mode at FeSe/STO interface.

A high-frequency optical phonon mode of SrTiO3(STO) was found to assist the high-temperature superconductivity observed recently at the interface between monolayer FeSe and STO substrate. However, the origin of this mode is not clear. Through first-principles calculations, we find that there is a novel polar phonon mode on the surface layers of the STO substrate, which does not exist in the STO crystals. The oxygen vacancies near the FeSe/STO interface drives the dispersion of this phonon mode to be flat and lowers its energy, whereas the charge transfer between STO substrate and FeSe monolayer further reduces its energy to 81 meV. This energy is in good agreement with the experimental value fitted by Lee et al. for the phonon mode responsible for the observed replica band separations and the increased superconducting gap. The oxygen-vacancy-induced flat and polar phonon mode provides clues for understanding the origin of high Tc superconductivity at the FeSe/ STO interface.   

ARPES Studies on the substrate effect on monolayer FeSe

For 2D films, interface interactions can play a critical role in determining the prevailing physics of the system. In the case of FeSe on SrTiO3, reducing the FeSe thickness to 1 monolayer (ML) from bulk leads to a significantly increased superconducting transition temperature (Tc). To fully utilize and maximize this approach to increasing Tc in FeSe and potentially apply it to other superconducting materials, the role which the substrate plays in this system must be understood. Here we present recent in-situ angle-resolved photo emission studies of the substrate effect on MBE grown 1 ML FeSe films.   

The role of the $\sqrt{13} \times \sqrt{13}-R33.7^{\circ}$ surface reconstruction on superconducting FeSe/SrTiO$_3$

The creation of specific oxide surface structures is important to nucleating epitaxial growth. Here we show that the reconstructions of the SrTiO$_3$ (STO) surface impact the properties of monolayer FeSe grown on STO. We achieve high-quality epitaxial growth of FeSe on surfaces that feature a double TiO$_2$ termination, such as the $\sqrt{13} \times \sqrt{13}-R33.7^{\circ}$ reconstructed STO surface prepared by high-temperature annealing in oxygen. Diffraction patterns characteristic of the reconstruction are observed with electron and synchrotron x-ray diffraction. The detailed structure of the FeSe/$\sqrt{13} \times \sqrt{13}-R33.7^{\circ}$ interface is determined using crystal truncation rod analysis, and the double TiO$_2$ termination observed is consistent with previous transmission electron microscopy studies. We further demonstrate the significance of this particular interface structure on epitaxial growth and its implications for the resulting electronic structure using first principles theory.   

Role of double TiO$_2$ layers at the FeSe/SrTiO$_3$ superconducting interface: A Density functional study

The recent discovery of high temperature superconductivity in monolayer FeSe on SrTiO$_3$ (STO) has drawn much attention. Since there is a strong enhancement of superconductivity compared to bulk FeSe, understanding the interfacial interactions between FeSe and STO is important. To date, density functional theory (DFT) studies have had difficulties explaining a key feature in the observed Fermi surface topology: namely the absence of a ``hole pocket'' about the $\Gamma$ point in the Brillouin zone of the heterostructure. By combining DFT and experiment, we find that the STO surface termination is not the primitive 1$\times$ 1 single-layer TiO$_2$ assumed in most works but instead is a more complex double-layered TiO$_2$ structure. We find that the double layer facilities epitaxial growth of monolayer FeSe. Our DFT calculations show that the hole pocket can be eliminated by the enhanced tendency of the double layer (compared to the single layer) termination to donate electrons to the FeSe when oxygen vacancies are present at the STO surface.   

STM investigation of FeSe/STO binding

The electronic properties of monolayer FeSe grown on a SrTiO$_3$ (STO) substrate differ dramatically from bulk FeSe, with the superconducting transition temperature (T$_c$) enhanced by an order of magnitude. This change in T$_c$ is accompanied by suppressed nematicity, electron doping, and possible coupling to substrate phonons. The first monolayer on the STO surface appears to be unique, and its electronic structure is tunable via sample preparation. We investigate the FeSe/STO binding and growth mechanism via scanning tunneling microscopy and spectroscopy, in order to understand the crucial role of the STO surface in modifying the electronic structure of FeSe.   

Defects in Thin-Film FeSe/SrTiO$_3$: STM and DFT Investigations

A single-layer of FeSe deposited on SrTiO$_3$ exhibits an order-of-magnitude enhancement of its superconducting transition temperature compared to bulk FeSe. This dramatic effect is curiously absent in a second layer of FeSe deposited on the heterostructure, leading to many questions concerning the role of the interface structure, electron doping and phonon coupling. Here, we approach these questions by using STM to characterize and compare native defects that appear in multi-layer and single-layer FeSe/SrTiO$_3$ grown by MBE under excess Se flux. We use DFT to explore candidate defect configurations, formation energies and diffusion barriers, in order to gain atomic-scale insights into the growth and structure of these film heterostructures.   

Impact of impurities on superconducting states of FeSe films on SrTiO3

Monolayer and bilayer FeSe films are grown on SrTiO$_{\mathrm{3}}$ substrates by molecular beam epitaxy, and their surface atomic structure and electronic properties are studied using scanning tunneling microscopy/spectroscopy. Tunneling spectroscopy carried out at 6K reveals a superconducting gap of 20 meV for monolayer FeSe, while bilayer FeSe is found to be mostly semiconducting. However, a gap of 16 meV is observed within 1nm of impurity sites for the bilayer FeSe, indicating a superconducting state. This observation suggests that controlled doping can significantly change the electronic property of FeSe films on SrTiO$_{\mathrm{3}}$, which can even open a large superconducting gap. This research was supported by NSF DMR-1335215.   

Controlling superconductivity at the FeSe/SrTiO3 interface by interfacial electron density

Single layer iron selenide (FeSe) on SrTiO3 substrate with a possible superconducting transition temperature (T$_{c})$ above 100K has attracted a great deal of attention recently. An important outstanding puzzle in this system is the inconsistency in T$_{c}$ as measured by different techniques. Here we systematically study the dependence of T$_{c}$ on the electron carrier density in this system and found that T$_{c}$ can be most effectively enhanced by increasing the density of electron carriers directly at the FeSe/SrTiO3 interface. We believe that our result resolves some of the puzzles in previous experiments, and open the possibility for further enhancement of T$_{c}$ in this system even when taken outside the UHV chamber.   

Charge transfer effect of FeSe thin films on SrTiO$_3$

Monolayer FeSe grown on SrTiO$_3$ substrate has shown a significant enhancement in the superconducting transition temperature (T$_c$) relative to the bulk material. Monolayers of FeSe are electron doped relative to bulk; we propose that the doping comes from work-function-mismatch driven charge transfer from SrTiO$_3$ impurity bands, modified by out-of-plane polar distortions of the SrTiO$_3$. We present a modified Schottky model combined with density functional calculations substantiating this picture for monolyaer FeSe films on Nb doped SrTiO$_3$. Physically relevant levels of Nb doping are shown to lead to doping of the FeSe compatible with observation. Adding polar fluctuations to the model leads to an electron-phonon interaction whose effect on the transition temperature is investigated.   

Observation of Dirac cone band dispersions in FeSe thin films by photoemission spectroscopy.

The search for novel materials with Dirac cone band dispersion is one of the most challenging and important works for both fundamental physics and technological applications. Here, we studied the electronic structure of FeSe thin films grown on SrTiO$_{\mathrm{3}}$ substrates by angle-resolved photoemission spectroscopy (ARPES). We reveal the existence of Dirac cone band dispersions in FeSe thin films thicker than 1 Unit Cell below the nematic transition temperature, whose apexes are located -10 meV below Fermi energy. The evolution of electronic structures for FeSe thin films as function of temperature, thickness and cobalt doping are systematically studied. The Dirac cones are found to be coexisted with the nematicity in FeSe, disappear when nematicity is suppressed. Our results provide useful guidelines for understanding the novel electronic structure, nematicity and superconductivity in FeSe system..