We present the G$_0$W$_0$ band structure, core levels, and deformation potential of monolayer FeSe in the paramagnetic phase based on a starting mean field of the Kohn Sham density functional theory (DFT) with the PBE functional. We find the GW correction increases the bandwidth of the states forming the $M$ pocket near the Fermi energy, while leaving the $Γ$ pocket roughly unchanged. We then compare the G$_0$W$_0$ quasiparticle band energies with the band structure from a simple empirical +A approach, which was recently proposed to capture the renormalization of the electron-phonon interaction going beyond DFT in FeSe, when used as a starting point in density functional perturbation theory (DFPT). We show that this empirical correction succeeds in approximating the GW non-local and dynamical self energy in monolayer FeSe and reproduces the GW band structure near the Fermi surface, the core energy levels, and the deformation potential (electron-phonon coupling).
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 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…
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 …
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 interest in Fe-chalcogenide unconventional superconductors is intense after the critical temperature of FeSe was reported enhanced by more than one order of magnitude in the monolayer limit at the interface to an insulating oxide substrate. In heterostructures comprising interfaces of FeSe with topological insulators, additional interesting physical phenomena is predicted to arise e.g. in form of {\it topological superconductivity}. So far superconductive properties of Fe-chalcogenide monolayers were mostly studied by local scanning tunneling spectroscopy experiments, which can detect pseu…
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…
By means of the first-principles calculations and magnetic topological quantum chemistry, we demonstrate that the low energy physics in the checkerboard antiferromagnetic (AFM) monolayer FeSe, very close to an AFM topological insulator that hosts robust edge states, can be well captured by a double-degenerate fragile topologically flat band just below the Fermi level. The Wilson loop calculations identify that such fragile topology is protected by the $S_{4z}$ symmetry, which gives rise to an AFM higher-order topological insulator that support the bound state with fractional charge $e/2$ at th…
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 ass…
Sublattice Dichotomy in Monolayer FeSe Superconductor
Tsinghua University（清华大学）；Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area（粤港澳大湾区量子科学中心）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Frontier Science Center for Quantum Information（量子信息前沿科学中心）；Southern University of Science and Technology（南方科技大学）；Shanghai Jiao Tong University（上海交通大学）；New Cornerstone Science Laboratory（新基石科学实验室）
The electronic structural properties in the presence of constrained magnetization and a charged background are studied for a monolayer of FeSe in non-magnetic, checkerboard-, and striped-antiferromagnetic (AFM) spin configurations. First principles techniques based on the pseudopotential density functional approach and the local spin density approximation are utilized. Our findings show that the experimentally observed shape of the Fermi surface is best described by the checkerboard AFM spin pattern. To explore the underlying pairing mechanism, we study the evolution of the non-magnetic to the…
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…
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…
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 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…
A new iron-oxychalcogenide (CaO)(FeSe) was obtained which crystallizes in the orthorhombic space group Pnma (No. 62) with a = 5.9180(12) Å, b = 3.8802(8) Å, c = 13.193(3) Å. The unique structure of (CaO)(FeSe) is built up of a quasi-two-dimensional network of corrugated infinite layers of corner-shared FeSe2O2 tetrahedra that extend in the ab-plane. The corrugated layers composed of corner-shared FeSe2O2 tetrahedra stack along the c-axis with Ca2+ cations sandwiched between the layers. Optical spectroscopy and resistivity measurements reveal semiconducting behavior with an indirect optical ban…
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…
The magnetic properties of monolayer FeSe films are investigated via first-principles spin-spiral calculations. Although the (pi,pi) collinear antiferromagnetic (CL-AFM) mode is lowest in energy, the spin-wave energy E(q) - which exhibits intrinsic non-Heisenberg behavior - is found to be extremely very flat over a large region of the two-dimensional Brillouin zone centered at the checkerboard antiferromagnetic (CB-AFM) q=0 configuration, giving rise to a sharp peak in the spin density of states. Considering the paramagnetic state as an incoherent average over spin-spiral states, we find that …
Recent experiments have reported that inserting metal atoms or small molecules in between the FeSe layers of $β$-FeSe can significantly enhance the superconducting transition temperature. Here, based on first-principles electronic structure calculations, we propose a stable compound SnOFeSe by alternatively stacking the SnO and $β$-FeSe layers. The predicted SnOFeSe has the same tetragonal structure as the well-known FeAs-based compound LaOFeAs, meanwhile their electronic structures in the nonmagnetic state are quite similar. The magnetic ground state of SnOFeSe is predicted to be the dimer an…
Donostia International Physics Center（圣塞巴斯蒂安国际物理中心）；University of the Basque Country (UPV/EHU)（巴斯克大学）；Goethe-Universität Frankfurt（法兰克福歌德大学）；Université de Sherbrooke（舍布鲁克大学）；RIKEN iTHEMS（理化学研究所 iTHEMS）；Center for Emergent Matter Science, RIKEN（理化学研究所 emergent matter science 中心）；Regroupement Québécois sur les Matériaux de Pointe (RQMP)（魁北克前沿材料研究联盟）
Recently, two-dimensional ferromagnetic semiconductors have been an important class of materials for many potential applications in spintronic devices. Based on density functional theory, we systematically explore the magnetic and electronic properties of CrGeS$_3$ with the monolayer structures. The comparison of total energy between different magnetic states ensures the ferromagnetic ground state of monolayer CrGeS$_3$. It is also shown that ferromagnetic and semiconducting properties are exhibited in monolayer CrGeS$_3$ with the magnetic moment of 3 $μ_{B}$ for each Cr atom, donated mainly b…
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…
MoTe2 belongs to the semiconducting transition metal dichalcogenide family with some properties differing from the other well-studied members (Mo,W)(S,Se)2. The optical band gap is in the near infrared region and both monolayers and bilayers may have a direct optical band gap. We first simulate the band structure of both monolayer and bilayer MoTe2 with DFT-GW calculations. We find a direct (indirect) electronic band gap for the monolayer (bilayer). By solving the Bethe-Salpeter equation, we calculate similar energies for the direct excitonic states in monolayer and bilayer. We then study the …
The two-dimensional boron monolayers were reported to be metallic both in previous theoretical predictions and experimental observations, however, we have firstly found a family of boron monolayers with the novel semiconducting property as confirmed by the first-principles calculations with the quasi-particle G0W0 approach. We demonstrate that the vanished metallicity characterized by the pz-derived bands cross the Fermi level is attributed to the motif of a triple-hexagonal-vacancy, with which various semiconducting boron monolayers are designed to realize the band-gap engineering for the pot…
Moiré superlattices of two-dimensional van der Waals materials have emerged as a powerful platform for designing electronic band structures and discovering emergent physical phenomena. A key concept involves the creation of long-wavelength periodic potential and moiré bands in a crystal through interlayer hybridization when two materials are overlaid. Here we demonstrate a new approach based on spatially periodic dielectric screening to create moiré bands in a monolayer semiconductor. It relies on reduced dielectric screening of the Coulomb interactions in monolayer semiconductors and their en…
The interfacial charge transfer from the substrate may influence the electronic structure of the epitaxial van der Waals (vdW) monolayers and thus their further technological applications. For instance, the freestanding Sb monolayer in puckered honeycomb phase (α-antimonene), the structural analog of black phosphorene, was predicted to be a semiconductor, but the epitaxial one behaves as a gapless semimetal when grown on the Td-WTe2 substrate. Here, we demonstrate that interface engineering can be applied to tune the interfacial charge transfer and thus the electron band of epitaxial monolayer…
This paper reports that ~10-nm-thick FeSe thin films exhibit insulator-like behavior in terms of the temperature dependence of their electrical resistivity even though bulk FeSe has a metallic electronic structure that has been confirmed by photoemission spectroscopy and first-principles calculations. This apparent contradiction is explained by potential barriers formed in the conduction band. Very thin FeSe epitaxial films with various [Fe]/[Se] were fabricated by molecular beam epitaxy and classified into two groups with respect to lattice strain and electrical properties. Lattice parameter …
