Defa Liu

Phase diagram and electronic indication of high-temperature superconductivity at 65 K in single-layer FeSe films

Superconductivity in the cuprate superconductors and the Fe-based superconductors is realized by doping the parent compound with charge carriers, or by application of high pressure, to suppress the antiferromagnetic state. Such a rich phase diagram is important in understanding superconductivity mechanism and other physics in the Cuand Fe-based high temperature superconductors. In this paper, we report a phase diagram in the single-layer FeSe films grown on SrTiO3 substrate by an annealing procedure to tune the charge carrier concentration over a wide range. A dramatic change of the band structure and Fermi surface is observed, with two distinct phases identified that are competing during the annealing process. Superconductivity with a record high transition temperature (Tc) at 65±5 K is realized by optimizing the annealing process. The wide tunability of the system across different phases, and its high-Tc, make the single-layer FeSe film ideal not only to investigate the superconductivity physics and mechanism, but also to study novel quantum phenomena and for potential applications. 1 ar X iv :1 20 7. 68 23 v1 [ co nd -m at .s up rco n] 3 0 Ju l 2 01 2 In high temperature cuprate superconductors, superconductivity is realized by doping the parent Mott insulator with charge carriers to suppress the antiferromagnetic state[1]. In the process, the physical property experiences a dramatic change from antiferromagnetic insulator, to a superconductor and eventually to a non-superconducting normal metal. In the superconducting region, the transition temperature Tc can be tuned by the carrier concentration, initially going up with the increasing doping, reaching a maximum at an optimal doping, and then going down with further doping[1]. Such a rich evolution with doping not only provides a handle to tune the physical properties in a dramatic way, but also provides clues and constraints in understanding the origin of the high-Tc superconductivity. The same is true for the Fe-based superconductors where superconductivity is achieved by doping the parent magnetic compounds which are nevertheless metallic[2, 3]. Again, the superconducting transition temperature can be tuned over a wide doping range with an maximum Tc at the optimal doping. Understanding such a rich evolution is also a prerequisite in understanding the origin of high temperature superconductivity in the Fe-based superconductors. The latest discovery of high temperature superconductivity signature in the single-layer FeSe films[4, 5] is significant in a couple of respects. First, it may exhibit a high Tc that breaks the Tc record (∼55 K) in the Fe-based superconductors kept so far since 2008[6– 11]. Second, the discovery of such a high-Tc in the single-layer FeSe film is surprising when considering that its bulk counterpart has a Tc only at 8 K[9] although it can be enhanced to 36.7 K under high pressure[12]. Third, it provides an ideal system to investigate the origin of high temperature superconductivity. On the one hand, this system consists of a single-layer FeSe film that has a simple crystal structure and strictly two-dimensionality; its simple electronic structure may provide key insights on the high Tc superconductivity mechanism in the Fe-based compounds[5]. On the other hand, the unique properties of this system may involve the interface between the single-layer FeSe film and the SrTiO3 substrate that provides an opportunity to investigate the role of interface in generating high-Tc superconductivity[4]. Like in cuprates and other Fe-based superconductors, it is important to explore whether one can tune the single-layer FeSe system to vary its physical properties and superconductivity by changing the charge carrier concentration. In this paper, we report a wide range tunability of the electronic structure and physical properties that is realized in the single-The recent discovery of possible high-temperature superconductivity in single-layer FeSe films has generated significant experimental and theoretical interest. In both the cuprate and the iron-based high-temperature superconductors, superconductivity is induced by doping charge carriers into the parent compound to suppress the antiferromagnetic state. It is therefore important to establish whether the superconductivity observed in the single-layer sheets of FeSe--the essential building blocks of the Fe-based superconductors--is realized by undergoing a similar transition. Here we report the phase diagram for an FeSe monolayer grown on a SrTiO3 substrate, by tuning the charge carrier concentration over a wide range through an extensive annealing procedure. We identify two distinct phases that compete during the annealing process: the electronic structure of the phase at low doping (N phase) bears a clear resemblance to the antiferromagnetic parent compound of the Fe-based superconductors, whereas the superconducting phase (S phase) emerges with the increase in doping and the suppression of the N phase. By optimizing the carrier concentration, we observe strong indications of superconductivity with a transition temperature of 65±5 K. The wide tunability of the system across different phases makes the FeSe monolayer ideal for investigating not only the physics of superconductivity, but also for studying novel quantum phenomena more generally.

Electronic origin of high-temperature superconductivity in single-layer FeSe superconductor

The recent discovery of high-temperature superconductivity in iron-based compounds has attracted much attention. How to further increase the superconducting transition temperature (T(c)) and how to understand the superconductivity mechanism are two prominent issues facing the current study of iron-based superconductors. The latest report of high-T(c) superconductivity in a single-layer FeSe is therefore both surprising and significant. Here we present investigations of the electronic structure and superconducting gap of the single-layer FeSe superconductor. Its Fermi surface is distinct from other iron-based superconductors, consisting only of electron-like pockets near the zone corner without indication of any Fermi surface around the zone centre. Nearly isotropic superconducting gap is observed in this strictly two-dimensional system. The temperature dependence of the superconducting gap gives a transition temperature T(c)~ 55 K. These results have established a clear case that such a simple electronic structure is compatible with high-T(c) superconductivity in iron-based superconductors.

Electronic structure and superconductivity of FeSe-related superconductors.

FeSe superconductors and their related systems have attracted much attention in the study of iron-based superconductors owing to their simple crystal structure and peculiar electronic and physical properties. The bulk FeSe superconductor has a superconducting transition temperature (Tc) of ~8 K and it can be dramatically enhanced to 37 K at high pressure. On the other hand, its cousin system, FeTe, possesses a unique antiferromagnetic ground state but is non-superconducting. Substitution of Se with Te in the FeSe superconductor results in an enhancement of Tc up to 14.5 K and superconductivity can persist over a large composition range in the Fe(Se,Te) system. Intercalation of the FeSe superconductor leads to the discovery of the AxFe2-ySe2 (A = K, Cs and Tl) system that exhibits a Tc higher than 30 K and a unique electronic structure of the superconducting phase. A recent report of possible high temperature superconductivity in single-layer FeSe/SrTiO3 films with a Tc above 65 K has generated much excitement in the community. This pioneering work opens a door for interface superconductivity to explore for high Tc superconductors. The distinct electronic structure and superconducting gap, layer-dependent behavior and insulator-superconductor transition of the FeSe/SrTiO3 films provide critical information in understanding the superconductivity mechanism of iron-based superconductors. In this paper, we present a brief review of the investigation of the electronic structure and superconductivity of the FeSe superconductor and related systems, with a particular focus on the FeSe films.

Observation of Fermi arc and its connection with bulk states in the candidate type-II Weyl semimetal WTe2

Chenlu Wang, Yan Zhang, Jianwei Huang, Simin Nie, Guodong Liu1,∗, Aiji Liang, Yuxiao Zhang, Bing Shen, Jing Liu, Cheng Hu, Ying Ding, Defa Liu, Yong Hu, Shaolong He, Lin Zhao, Li Yu, Jin Hu, Jiang Wei, Zhiqiang Mao, Youguo Shi, Xiaowen Jia, Fengfeng Zhang, Shenjin Zhang, Feng Yang, Zhimin Wang, Qinjun Peng, Hongming Weng, Xi Dai, Zhong Fang, Zuyan Xu, Chuangtian Chen and X. J. Zhou1,5,∗ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. Department of Physics and Engineering Physics, Tulane University, New Orleans, Louisiana 70118, USA Military Transportation University, Tianjin 300161, China. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Collaborative Innovation Center of Quantum Matter, Beijing 100871, China. These people contributed equally to the present work. ∗Corresponding author: gdliu arpes@iphy.ac.cn, XJZhou@iphy.ac.cn.

Common electronic origin of superconductivity in (Li,Fe)OHFeSe bulk superconductor and single-layer FeSe/SrTiO3 films

The mechanism of high-temperature superconductivity in the iron-based superconductors remains an outstanding issue in condensed matter physics. The electronic structure plays an essential role in dictating superconductivity. Recent revelation of distinct electronic structure and high-temperature superconductivity in the single-layer FeSe/SrTiO3 films provides key information on the role of Fermi surface topology and interface in inducing or enhancing superconductivity. Here we report high-resolution angle-resolved photoemission measurements on the electronic structure and superconducting gap of an FeSe-based superconductor, (Li0.84Fe0.16)OHFe0.98Se, with a Tc at 41 K. We find that this single-phase bulk superconductor shows remarkably similar electronic behaviours to that of the superconducting single-layer FeSe/SrTiO3 films in terms of Fermi surface topology, band structure and the gap symmetry. These observations provide new insights in understanding high-temperature superconductivity in the single-layer FeSe/SrTiO3 films and the mechanism of superconductivity in the bulk iron-based superconductors.

Large and robust electrical spin injection into GaAs at zero magnetic field using an ultrathin CoFeB/MgO injector

Binary information encoded within the spin of carriers can be transferred into corresponding right-or left-handed circularly polarized photons emitted from an active semiconductor medium via carrier-photon angular momentum conversion. In order to attain maximized spin injection at out-of-plane magnetic remanence, a number of material systems have been explored as possible solid-state spin injectors. However, the circular polarization (P-C) of emitted light was still limited at 3-4% at remanence. Here, we demonstrate a sizable electroluminescence circular polarization from a III-V-based spin light-emitting diode at zero magnetic field with a perpendicular spin injector consisting of an ultrathin CoFeB ferromagnetic layer (1.2 nm) grown on a MgO tunnel barrier (2.5 nm). The maximum value of P-C measured at zero field is as large as 20% at 25 K and still 8% at 300 K. These types of ultrathin perpendicular spin injectors are of great interest (i) to realize the electrical switching of the magnetization of the injector layer owing to the advanced spin-transfer torque properties of the CoFeB layer and (ii) to be directly embedded in optical cavities for spin lasers due to their very low optical absorption loss.

Dichotomy of the electronic structure and superconductivity between single-layer and double-layer FeSe/SrTiO3 films

The latest discovery of possible high-temperature superconductivity in the single-layer FeSe film grown on a SrTiO3 substrate has generated much attention. Initial work found that, while the single-layer FeSe/SrTiO3 film exhibits a clear signature of superconductivity, the double-layer film shows an insulating behaviour. Such a marked layer-dependent difference is surprising and the underlying origin remains unclear. Here we report a comparative angle-resolved photoemission study between the single-layer and double-layer FeSe/SrTiO3 films annealed in vacuum. We find that, different from the single-layer FeSe/SrTiO3 film, the double-layer FeSe/SrTiO3 film is hard to get doped and remains in the semiconducting/insulating state under an extensive annealing condition. Such a behaviour originates from the much reduced doping efficiency in the bottom FeSe layer of the double-layer FeSe/SrTiO3 film from the FeSe-SrTiO3 interface. These observations provide key insights in understanding the doping mechanism and the origin of superconductivity in the FeSe/SrTiO3 films.

Electronic evidence of an insulator–superconductor crossover in single-layer FeSe/SrTiO3 films

Significance The doping-induced insulator-to-superconductor transition has been widely observed in cuprates, which provides important information for understanding the superconductivity mechanism. However, in the iron-based superconductors, no evidence of doping-induced insulator–superconductor transition (or crossover) has been reported so far. In this paper, to our knowledge, we report the first electronic evidence of an insulator–superconductor crossover observed in the single-layer FeSe film grown on a SrTiO3 substrate, which exhibits similar behaviors to that observed in the cuprate superconductors. The observed insulator–superconductor crossover may be associated with the two-dimensionality that enhances electron localization or correlation. The reduced dimensionality and the interfacial effect provide a new pathway in searching for new phenomena and novel superconductors with a high transition temperature. In high-temperature cuprate superconductors, it is now generally agreed that superconductivity is realized by doping an antiferromagnetic Mott (charge transfer) insulator. The doping-induced insulator-to-superconductor transition has been widely observed in cuprates, which provides important information for understanding the superconductivity mechanism. In the iron-based superconductors, however, the parent compound is mostly antiferromagnetic bad metal, raising a debate on whether an appropriate starting point should go with an itinerant picture or a localized picture. No evidence of doping-induced insulator–superconductor transition (or crossover) has been reported in the iron-based compounds so far. Here, we report an electronic evidence of an insulator–superconductor crossover observed in the single-layer FeSe film grown on a SrTiO3 substrate. By taking angle-resolved photoemission measurements on the electronic structure and energy gap, we have identified a clear evolution of an insulator to a superconductor with increasing carrier concentration. In particular, the insulator–superconductor crossover in FeSe/SrTiO3 film exhibits similar behaviors to that observed in the cuprate superconductors. Our results suggest that the observed insulator–superconductor crossover may be associated with the two-dimensionality that enhances electron localization or correlation. The reduced dimensionality and the interfacial effect provide a new pathway in searching for new phenomena and novel superconductors with a high transition temperature.

Direct evidence of interaction-induced Dirac cones in a monolayer silicene/Ag(111) system.

Significance Silicene is a one-atom-thick 2D crystal of silicon. The low-energy Dirac cone states in silicene lie at the heart of all novel quantum phenomena and potential applications. However, so far, the evidence of Dirac cones in silicene is highly controversial. We report a direct observation of Dirac cones in monolayer silicene grown on a Ag(111) substrate. We further show that this unusual Dirac cone structure originates from the interaction of silicene with the Ag(111) substrate, establishing the case of a unique type of Dirac fermion generated through the interaction of two different constituents. The observation of Dirac cones in silicene(3 × 3)/Ag(111) opens a unique materials platform for investigating unusual quantum phenomena and for applications based on 2D silicon systems. Silicene, analogous to graphene, is a one-atom-thick 2D crystal of silicon, which is expected to share many of the remarkable properties of graphene. The buckled honeycomb structure of silicene, along with enhanced spin-orbit coupling, endows silicene with considerable advantages over graphene in that the spin-split states in silicene are tunable with external fields. Although the low-energy Dirac cone states lie at the heart of all novel quantum phenomena in a pristine sheet of silicene, a hotly debated question is whether these key states can survive when silicene is grown or supported on a substrate. Here we report our direct observation of Dirac cones in monolayer silicene grown on a Ag(111) substrate. By performing angle-resolved photoemission measurements on silicene(3 × 3)/Ag(111), we reveal the presence of six pairs of Dirac cones located on the edges of the first Brillouin zone of Ag(111), which is in sharp contrast to the expected six Dirac cones centered at the K points of the primary silicene(1 × 1) Brillouin zone. Our analysis shows clearly that the unusual Dirac cone structure we have observed is not tied to pristine silicene alone but originates from the combined effects of silicene(3 × 3) and the Ag(111) substrate. Our study thus identifies the case of a unique type of Dirac cone generated through the interaction of two different constituents. The observation of Dirac cones in silicene/Ag(111) opens a unique materials platform for investigating unusual quantum phenomena and for applications based on 2D silicon systems.

Electronic structure, Dirac points and Fermi arc surface states in three-dimensional Dirac semimetal Na3Bi from angle-resolved photoemission spectroscopy*

The three-dimensional (3D) Dirac semimetals have linearly dispersive 3D Dirac nodes where the conduction band and valence band are connected. They have isolated 3D Dirac nodes in the whole Brillouin zone and can be viewed as a 3D counterpart of graphene. Recent theoretical calculations and experimental results indicate that the 3D Dirac semimetal state can be realized in a simple stoichiometric compound A 3Bi (A = Na, K, Rb). Here we report comprehensive high-resolution angle-resolved photoemission (ARPES) measurements on the two cleaved surfaces, (001) and (100), of Na3Bi. On the (001) surface, by comparison with theoretical calculations, we provide a proper assignment of the observed bands, and in particular, pinpoint the band that is responsible for the formation of the three-dimensional Dirac cones. We observe clear evidence of 3D Dirac cones in the three-dimensional momentum space by directly measuring on the k x –k y plane and by varying the photon energy to get access to different out-of-plane k z s. In addition, we reveal new features around the Brillouin zone corners that may be related with surface reconstruction. On the (100) surface, our ARPES measurements over a large momentum space raise an issue on the selection of the basic Brillouin zone in the (100) plane. We directly observe two isolated 3D Dirac nodes on the (100) surface. We observe the signature of the Fermi-arc surface states connecting the two 3D Dirac nodes that extend to a binding energy of ~150 meV before merging into the bulk band. Our observations constitute strong evidence on the existence of the Dirac semimetal state in Na3Bi that are consistent with previous theoretical and experimental work. In addition, our results provide new information to clarify on the nature of the band that forms the 3D Dirac cones, on the possible formation of surface reconstruction of the (001) surface, and on the issue of basic Brillouin zone selection for the (100) surface.