Interfacial Electron-Phonon Coupling Constants Extracted from Intrinsic Replica Bands in Monolayer FeSe/SrTiO$_3$
Brendan D. Faeth, Saien Xie, Shuolong Yang, Jason K. Kawasaki, Jocienne N. Nelson, Shuyuan Zhang, Pramita Mishra, Chen Li, Christopher Jozwiak, Aaron Bostwick, … 展开作者
Brendan D. Faeth, Saien Xie, Shuolong Yang, Jason K. Kawasaki, Jocienne N. Nelson, Shuyuan Zhang, Pramita Mishra, Chen Li, Christopher Jozwiak, Aaron Bostwick, Eli Rotenberg, Darrell G. Schlom, Kyle M. Shen
The observation of replica bands by angle-resolved photoemission spectroscopy has ignited interest in the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO$_3$, where the appearance of replica bands has motivated theoretical work suggesting that the interfacial coupling of electrons in the FeSe layer to optical phonons in the SrTiO$_3$ substrate might contribute to the enhanced superconducting pairing temperature. Alternatively, it has also been recently proposed that such replica bands might instead originate from extrinsic final state losses associated with the photoemission process. Here, we perform a quantitative examination of replica bands in monolayer FeSe/SrTiO$_3$, where we are able to conclusively demonstrate that the replica bands are indeed signatures of intrinsic electron-boson coupling, and not associated with final state effects. A detailed analysis of the energy splittings between the higher-order replicas, as well as other self-energy effects, allow us to determine that the interfacial electron-phonon coupling in the system corresponds to a value of $λ= 0.19 \pm 0.02$.
Motivated by the high superconducting transition temperature of monolayer FeSe on SrTiO3, we propose a potential three-dimensional high-temperature superconductor superlattice FeSe-SrTiO3 and study its structural stability and electronic structure using density functional theory. We find that the binding energy between the FeSe and SrTiO3 layers is about 0.7 eV per (Fe2Se2) unit and that it saturates already within a single TiO2 atomic layer of SrTiO3. In addition we analyzed the dynamical stability of the superlattice and compared it to the case of bulk SrTiO3.
By the first-principles electronic structure calculations, we have studied electronic structures of FeSe monolayer and bilayer thin films on SrTiO$_3$ (001) with SrO-termination or TiO$_2$ termination. We find that both FeSe monolayer and bilayer on either termination behave like a slightly doped semiconductor and a collinear antiferromagnetic order on Fe ions. There is no substantial charge transfer between the FeSe layers and the substrate. FeSe is adhered to the SrTiO$_3$ surface by a dipole-dipole interaction. The Fermi surface is mainly the contribution of Fe-3d orbitals. A valence band c…
The effects of electron doping and phonon vibrations on the magnetic properties of monolayer and bilayer FeSe epitaxial films on SrTiO$_3$ have been studied, respectively, using first-principles calculations with van der Waals correction. For monolayer FeSe epitaxial film, the combined effect of electron doping and phonon vibrations readily leads to magnetic frustration between the collinear antiferromagnetic state and the checkerboard antiferromagnetic Néel state. For bilayer FeSe epitaxial film, such magnetic frustration is much more easily induced by electron doping in its bottom layer than…
The observation of substantially enhanced superconductivity of single-layer FeSe films on SrTiO3 has stimulated intensive research interest. At present, conclusive experimental data on the corresponding electron-boson interaction is still missing. Here we use inelastic electron scattering spectroscopy and angle resolved photoemission spectroscopy to show that the electrons in these systems are dressed by the strongly polarized lattice distortions of the SrTiO3, and the indispensable non-adiabatic nature of such a coupling leads to the formation of dynamic interfacial polarons. Furthermore, the…
The recent observation of replica bands in single-layer FeSe/SrTiO$_3$ by angle-resolved photoemission spectroscopy (ARPES) has triggered intense discussions concerning the potential influence of the FeSe electrons coupling with substrate phonons on the superconducting transition temperature. Here we provide strong evidence that the replica bands observed in the single-layer FeSe/SrTiO$_3$ system and several other cases are largely due to the energy loss processes of the escaping photoelectron, resulted from the well-known strong coupling of external propagating electrons to Fuchs-Kliewer (F-K…
Charge transfer and electron-phonon coupling (EPC) are proposed to be two important constituents associated with enhanced superconductivity in the single unit cell FeSe films on oxide surfaces. Using high-resolution electron energy loss spectroscopy combined with first-principles calculations, we have explored the lattice dynamics of ultrathin FeSe films grown on SrTiO3. We show that, despite the significant effect from the substrate on the electronic structure and superconductivity of the system, the FeSe phonons in the films are unaffected. The energy dispersion and linewidth associated with…
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 monolay…
Monolayer FeSe on SrTiO$_3$ superconducts with reported $T_\mathrm{c}$ as high as 100 K, but the dramatic interfacial $T_\mathrm{c}$ enhancement remains poorly understood. Oxygen vacancies in SrTiO$_3$ are known to enhance the interfacial electron doping, electron-phonon coupling, and superconducting gap, but the detailed mechanism is unclear. Here we apply scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to FeSe/SrTiO$_3$ to image the diffusion of selenium into SrTiO$_3$ to an unexpected depth of several unit cells, consistent with the simultaneous…
Bulk FeSe is superconducting with a critical temperature $T_c$ $\cong$ 8 K and SrTiO$_3$ is insulating in nature, yet high-temperature superconductivity has been reported at the interface between a single-layer FeSe and SrTiO$_3$. Angle resolved photoemission spectroscopy and scanning tunneling microscopy measurements observe a gap opening at the Fermi surface below $\approx$ 60 K. Elucidating the microscopic properties and understanding the pairing mechanism of single-layer FeSe is of utmost importance as it is a basic building block of iron-based superconductors. Here, we use the low-energy …
We show that electron-phonon coupling can induce strong electron pairing in an FeSe monolayer on a SrTiO3 substrate (experimental indications for superconducting Tc are between 65 and 109K). The role of the SrTiO3 substrate in increasing the coupling is two-fold. First, the interaction of the FeSe and TiO2 terminated face of SrTiO3 prevents the FeSe monolayer from undergoing a shear-type (orthorhombic, nematic) structural phase transition. Second, the substrate allows an anti-ferromagnetic ground state of FeSe which opens electron-phonon coupling channels within the monolayer that are prevente…
The exact mechanism responsible for the tenfold enhancement of superconducting transition temperature (Tc) in a monolayer iron selenide (FeSe) on SrTiO3(STO) substrate over that of bulk FeSe, is an open issue. We present here a coordinated study of electrical transport and low temperature electron energy-loss spectroscopy (EELS) measurements on FeSe/STO films of various thicknesses. Our EELS mapping across the FeSe/STO interface shows direct evidence of band-bending caused by electrons transferred from STO to FeSe layer. The transferred electrons were found to accumulate only within the first …
The dream of room temperature superconductors has inspired intense research effort to find routes for enhancing the superconducting transition temperature (Tc). Therefore, single-layer FeSe on a SrTiO3 substrate, with its extraordinarily high Tc amongst all interfacial superconductors and iron based superconductors, is particularly interesting, but the mechanism underlying its high Tc has remained mysterious. Here we show through isotope effects that electrons in FeSe couple with the oxygen phonons in the substrate, and the superconductivity is enhanced linearly with the coupling strength atop…
Modifications of the electronic bands of thin FeSe films due to oxygen vacancies in the supporting SrTiO 3 (001) substrate - and the interplay with spin-orbit coupling, magnetism, and epitaxy - are investigated by first-principles supercell calculations. Unfolded (k-projected) bands show that the oxygen vacancies both provide electron doping to the interface FeSe layer and also have notable effects on the details of the bands around the Fermi level, including renormalizing the width of the Fe-3d band near the Fermi level by a factor of about 0.6, and causing a splitting of ~40 meV at the M poi…
Single-layer FeSe films with extremely expanded in-plane lattice constant of 3.99A are fabricated by epitaxially growing FeSe/Nb:SrTiO3/KTaO3 heterostructures, and studied by in situ angle-resolved photoemission spectroscopy. Two elliptical electron pockets at the Brillion zone corner are resolved with negligible hybridization between them, indicating the symmetry of the low energy electronic structure remains intact as a free-standing single-layer FeSe, although it is on a substrate. The superconducting gap closes at a record high temperature of 70K for the iron based superconductors. Intrigu…
The sensitive dependence of monolayer materials on their environment often gives rise to unexpected properties. It was recently demonstrated that monolayer FeSe on a SrTiO$_3$ substrate exhibits a much higher superconducting critical temperature T$_C$ than the bulk material. Here, we examine the interfacial structure of FeSe / SrTiO$_3$ and the effect of an interfacial Ti$_{1+x}$O$_2$ layer on the increased T$_C$ using a combination of scanning transmission electron microscopy and density functional theory. We find Ti$_{1+x}$O$_2$ forms its own quasi-two-dimensional layer, bonding to both the …
We present theoretically the electronic structure of antiferromagnetic (AFM) FeSe monolayer on TiO$_{2}$ terminated SrTiO$_{3}$(001) surface. It is revealed that the striking disappearance of the Fermi surface around the Brillouin zone (BZ) center can be well explained by the antiferromatnetic (AFM) phase. We show that the system has a considerable charge transfer from SrTiO$_{3}$(001) substrate to FeSe monolayer, and so has a self-constructed electric field. The FeSe monolayer band structure near the BZ center is sensitive to charge doping, and the spin-resolved energy bands at BZ corner are …
In many unconventional superconductors, the presence of a pseudogap - a suppression in the electronic density of states extending above the critical temperature - has been a long-standing mystery. Here, we employ combined \textit{in situ} electrical transport and angle-resolved photoemission spectroscopy (ARPES) measurements to reveal an unprecedentedly large pseudogap regime in single-layer FeSe/SrTiO$_3$, an interfacial superconductor where incoherent Cooper pairs are initially formed above $T_Δ$ $\approx$ 60 K, but where a zero resistance state is only achieved below $T_{0}$ $<$ 30 K. We sh…
The significantly enhanced superconducting transition temperature ($T_c$) of an FeSe monolayer on SrTiO$_3$(001) substrate has attracted extensive attention in recent years. Here, based on first-principles electronic structure calculations, we propose another candidate substrate LaO(001) for the epitaxial growth of FeSe monolayer to realize superconductivity. Our calculations show that for the optimal adsorption structure of FeSe monolayer on LaO(001), the stripe antiferromagnetic state and the dimer antiferromagnetic state are almost energetically degenerate, indicating the existence of stron…
We theoretically study, through combining the density functional theory and an unfolding technique, the electronic band structure and the charge doping effects for the deposition of potassium (K) on multilayer FeSe films grown on SrTiO3 (001) surface. These results form a theoretical base line for further detailed studies of low-temperature electronic properties and their multiway quantum engineering of FeSe thin films. We explain the Fermi surface topology observed in experiment and formulate the amount of doped electrons as a function of atomic K coverage. We show that the atomic K depositio…
Compared to experiment, the calculated bulk lattice constant of FeSe is too small in LDA, PBEsol and PBE type exchange-correlation (xc) functionals even though the mismatch decreases from LDA to PBE. In bulk SrTiO3 (STO) on the other hand LDA over-binds, PBE under-binds and PBEsol agrees best with experiment. With the errors in opposite directions, the strain in the FeSe monolayer on STO depends on the xc functional and, especially in the non-magnetic state, is strongly overestimated. In this work we investigate the influence of the xc functional and magnetism in density functional calculation…
Monolayer FeSe thin film grown on SrTiO_{3}(001) (STO) shows the sign of T_{c}> 77 K, which is higher than the T_{c}-record of 56 K for the bulk FeAs-based superconductors. However, little is known about the magnetic ground state of FeSe, which should be closely related to its unusual superconductivity. Previous studies presume the collinear stripe antiferromagnetic (AFM) state as the ground state of FeSe, same to that in FeAs superconductors. Here we find a novel magnetic order named "pair-checkboard AFM" as the magnetic ground state of tetragonal FeSe. The novel pair-checkboard order results…
The electronic structure of FeSe thin films grown on SrTiO3 substrate is studied 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 apex are located -10 meV below Fermi energy. The evolution of Dirac cone electronic structure for FeSe thin films as function of temperature, thickness and cobalt doping is systematically studied. The Dirac cones are found to be coexisted with the nematicity in FeSe, disappear when nematicity is suppressed. Our resul…
Hexagonal FeSe thin films were grown on SrTiO3 substrates and the temperature and thickness dependence of their electronic structures were studied. The hexagonal FeSe is found to be metallic and electron doped, whose Fermi surface consists of six elliptical electron pockets. With decreased temperature, parts of the bands shift downward to high binding energy while some bands shift upwards to EF. The shifts of these bands begin around 300 K and saturate at low temperature, indicating a magnetic phase transition temperature of about 300 K. With increased film thickness, the Fermi surface topolog…
It is well known that superconductivity in Fe-based materials is favoured under tetragonal symmetry, whereas competing orders such as spin-density-wave (SDW) and nematic orders emerge or are reinforced upon breaking the fourfold (C4) symmetry. Accordingly, suppression of orthorhombicity below the superconducting transition temperature (Tc) is found in underdoped compounds. Epitaxial film growth on selected substrates allows the design of crystal specific lattice distortions. Here we show that despite the breakdown of the C4 symmetry induced by a 5% difference in the lattice parameters, monolay…
A two dimensional topological insulator exhibits helical edge states topologically protected against single particle backscattering. Such protection breaks down, however, when electron electron interactions are significant or when edge reconstruction occurs, leading to suppressed density of states (DOS) at Fermi level that follows universal scaling with temperature and energy, characteristic of Tomonaga Luttinger liquid (TLL). Here, we grow multilayer FeSe on SrTiO3 by molecular beam epitaxy, and observe robust edge states at both {100}Se and {110}Se steps using scanning tunneling microscopy/s…
Superconductivity arises from two distinct quantum phenomena: electron pairing and long-range phase coherence. In conventional superconductors, the two quantum phenomena generally take place simultaneously, while the electron pairing occurs at higher temperature than the long-range phase coherence in the underdoped high-Tc cuprate superconductors. Recently, whether electron pairing is also prior to long-range phase coherence in single-layer FeSe film on SrTiO3 substrate is under debate. Here, by measuring Knight shift and nuclear spin-lattice relaxation rate, we unambiguously reveal a pseudoga…
Recently, monolayer CoSb/SrTiO$_3$ has been proposed as a candidate harboring interfacial superconductivity in analogy with monolayer FeSe/SrTiO$_3$. Experimentally, while the CoSb-based compounds manifesting as nanowires and thin films have been realized on SrTiO$_3$ substrates, serving as a rich playground, their electronic structures are still unknown and yet to be resolved. Here, we have fabricated CoSb$_{1-x}$ nanoribbons with quasi-one-dimensional stripes on SrTiO$_3$(001) substrates using molecular beam epitaxy, and investigated the electronic structure by in situ angle-resolved photoem…
We study the electron and phonon spectral functions as well as superconductivity for a two dimensional electron gas couped to dipolar phonons via small-momentum transfer scattering. The results reported here are obtained through sign-problem-free quantum Monte-Carlo simulation. Hence aside from modeling there is no further approximations. Our results can be relevant to the high-temperature superconductor at the interface between a unit-cell-thick FeSe and SrTiO3.
Electron-phonon interaction plays an important role in metals and can lead to superconductivity and other instabilities. Previous theoretical studies on superconductivity are largely based on the Migdal-Eliashberg theory, which neglects all the vertex corrections to electron-phonon coupling and breaks down in many unconventional superconductors. Here, we go beyond the Migdal-Eliashberg approximation and develop a nonperturbative Dyson-Schwinger equation approach to deal with the superconducting transition. Remarkably, we take into account all the vertex corrections by solving two coupled Ward-…
