Topological insulators and semimetals as well as unconventional iron-based superconductors have attracted major recent attention in condensed matter physics. Previously, however, little overlap has been identified between these two vibrant fields, even though the principal combination of topological bands and superconductivity promises exotic unprecedented avenues of superconducting states and Majorana bound states (MBSs), the central building block for topological quantum computation. Along with progressing laser-based spin-resolved and angle-resolved photoemission spectroscopy (ARPES) towards high energy and momentum resolution, we have resolved topological insulator (TI) and topological Dirac semimetal (TDS) bands near the Fermi level (E F ) in the iron-based superconductors Li(Fe,Co)As and Fe(Te,Se), respectively. The TI and TDS bands can be individually tuned to locate close to E F by carrier doping, allowing to potentially access a plethora of different superconducting topological states in the same material. Our results reveal the generic coexistence of superconductivity and multiple topological states in iron-based superconductors, rendering these materials a promising platform for high-T c topological superconductivity.High-T c iron-based superconductors feature multiple bands near E F , which enhances the difficulty in understanding the details of unconventional pairing 1-3 . It, however, also allows for a wealth of, possibly topologically non-trivial, electronic bands, of which a recent example is the TI states discovered in the ironbased superconductor Fe(Te,Se) 4 , hinting at a promising direction to realize topological superconductivity and MBSs 5-9 . In view of Fe(Te,Se), a pressing subsequent question is to which extent this marks a generic phe-nomenon in different classes of iron-based high-T c superconductors. In this work, we find that the emergence of non-trivial topological bands near the Fermi level is indeed a common feature of various iron-based superconductors. Our first-principles calculations reveal that BaFe 2 As 2 , LiFeAs and Fe(Te,Se) all exhibit band inversions along k z . To confirm these calculations, the band structures of Li(Fe,Co)As and Fe(Te,Se) were investigated by laser-based high-resolution ARPES. Firstly, we observe that TI bands reminiscent of Fe(Te,Se) exist in Li(Fe,Co)As as well, supporting the generic existence of non-trivial topology in iron-based superconductors. Secondly and more interestingly, we predict and observe TDS bands in Li(Fe,Co)As and Fe(Te,Se), which we investigate via high-resolution ARPES, spin-resolved ARPES (SARPES), and magnetoresistance (MR) measurements. Finally, we discuss the phase diagram of these topological high-T c compounds as a function of Fermi level (doping). The combination of topological states and superconductivity may produce not only surface topological superconductivity deriving from the TI edge states, but also bulk topological superconductivity from the TDS bands.Normal insulator (NI), TI, and TDS constitute topologically disti...

Atomically thin PtSe 2 films have attracted extensive research interests for potential applications in high-speed electronics, spintronics and photodetectors. Obtaining high quality, single crystalline thin films with large size is critical. Here we report the first successful layer-by-layer growth of high quality PtSe 2 films by molecular beam † The authors declare no competing financial interest. 1 arXiv:1703.04279v2 [cond-mat.mtrl-sci] 15 Mar 2017 epitaxy. Atomically thin films from 1 ML to 22 ML have been grown and characterized by low-energy electron diffraction, Raman spectroscopy and X-ray photoemission spectroscopy. Moreover, a systematic thickness dependent study of the electronic structure is revealed by angle-resolved photoemission spectroscopy (ARPES), and helical spin texture is revealed by spin-ARPES. Our work provides new opportunities for growing large size single crystalline films for investigating the physical properties and potential applications of PtSe 2 . KeywordsPtSe 2 , Molecular beam epitaxy (MBE), Raman, ARPES, Transition metal dichalcogenide (TMDC) Layered transition metal dichalcogenides (TMDCs) have attracted extensive interests for applications in electronics, optoelectronics and valleytronics due to the strong spin-orbit coupling, sizable band gap and tunability of the electronic structure by quantum confinement effect. [1][2][3][4] In the past decade, this has been witnessed by the significant efforts conducted on the atomically thin MoS 2 film. 5-7 However, its low mobility has limited applications, for inbstance, in high speed electronics. 8,9 Finding thin films of other TMDC with better properties is highly desirable. PtSe 2 has emerged as an interesting compound that belongs to TMDC.Although the bulk crystal is a semimetal, 10,11 monolayer (ML) platinum diselenide (PtSe 2 ) has been revealed to be a semiconductor with a band gap of ≈ 1.2 eV. 12 Importantly, the charge-carrier mobility of PtSe 2 has been predicted among the highest in TMDCs 9 and has been experimentally shown to be comparable to black phosphorene 13 yet with the advantage of much improved stability. 14 This makes PtSe 2 a promising candidate for high-speed electronics. Moreover, the hidden helical spin texture with spin-layer locking in monolayer PtSe 2 has been recently revealed, 15 and such spin physics induced by a local Rashba effect has great potential for electric field tunable spintronic devices. 16 In addition, remarkable performance

Topological nodal line semimetals, a novel quantum state of materials, possess topologically nontrivial valence and conduction bands that touch at a line near the Fermi level. The exotic band structure can lead to various novel properties, such as long-range Coulomb interaction and flat Landau levels. Recently, topological nodal lines have been observed in several bulk materials, such as PtSn4, ZrSiS, TlTaSe2 and PbTaSe2. However, in two-dimensional materials, experimental research on nodal line fermions is still lacking. Here, we report the discovery of two-dimensional Dirac nodal line fermions in monolayer Cu2Si based on combined theoretical calculations and angle-resolved photoemission spectroscopy measurements. The Dirac nodal lines in Cu2Si form two concentric loops centred around the Γ point and are protected by mirror reflection symmetry. Our results establish Cu2Si as a platform to study the novel physical properties in two-dimensional Dirac materials and provide opportunities to realize high-speed low-dissipation devices.

Two-dimensional (2D) materials have attracted great attention and spurred rapid development in both fundamental research and device applications. The search for exotic physical properties, such as magnetic and topological order, in 2D materials could enable the realization of novel quantum devices and is therefore at the forefront of materials science. Here, we report the discovery of two-fold degenerate Weyl nodal lines in a 2D ferromagnetic material, a single-layer gadoliniumsilver compound, based on combined angle-resolved photoemission spectroscopy measurements and theoretical calculations. These Weyl nodal lines are symmetry protected and thus robust against external perturbations. The coexistence of magnetic and topological order in a 2D material is likely to inform ongoing efforts to devise and realize novel nanospintronic devices.Spintronics is an emerging technique that uses electron spin as a medium for data storage and transfer [1,2]. Ferromagnets are widely used as spintronic materials because they possess electronic band structures that are spin split by the exchange interaction. Recently, the desire to miniaturize future quantum devices has stimulated great research interest in low-dimensional materials. For example, various two-dimensional (2D) materials have been realized, such as graphene [6,7], phosphorene [8,9], and borophene [10,11], providing the possibility to realize novel quantum devices at the atomic scale. The search for 2D ferromagnetic materials is thus a promising route towards future nanospintronics. However, the long-range ferromagnetic order in 2D systems is vulnerable to low-energy spinwave excitations, making it difficult to realize 2D ferromagnetism; such low-energy excitations can be gapped out by magnetic anisotropy [12]. Intrinsic 2D ferromagnetism has been experimentally observed only recently in several van der Waals crystals, including Cr 2 Ge 2 Te 6 , CrI 3 , and VSe 2 [13-15].The transport properties of a material, which are crucial for its use in spintronic devices, largely depend on the band structure near the Fermi level. Therefore, exploration of ferromagnetic materials with exotic band structures provides great opportunities to realize novel spintronic devices. Recently, topological band structures including the Dirac cone, Weyl cone, and Dirac/Weyl nodal line have attracted great attention because of their potential device applications [16][17][18][19]. To date, most materials that host topological band structures are non-magnetic and three dimensional, strongly limiting their applicability to spintronic devices. Therefore, it remains important to search for novel 2D materials with both magnetic order and topological band structure.Here, we study a novel 2D ferromagnet, single-layer GdAg 2 , which can be synthesized using a bottom-up approach. Our ab initio molecular dynamic (AIMD) simulation results show that freestanding single-layer GdAg 2 is stable at room temperature.Moreover, all the atoms of GdAg 2 are coplanar; therefore, its thickness reaches the atomic li...

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