Zuyan Xu

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.

Distinct Fermi Surface Topology and Nodeless Superconducting Gap in a (Tl0.58Rb0.42)Fe1.72Se2 Superconductor

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the (Tl0.58Rb0.42)Fe1.72Se2 superconductor with a T(c) = 32  K. The Fermi surface topology consists of two electronlike Fermi surface sheets around the Γ point which is distinct from that in all other iron-based superconductors reported so far. The Fermi surface around the M point shows a nearly isotropic superconducting gap of ∼12  meV. The large Fermi surface near the Γ point also shows a nearly isotropic superconducting gap of ∼15  meV, while no superconducting gap opening is clearly observed for the inner tiny Fermi surface. Our observed new Fermi surface topology and its associated superconducting gap will provide key insights and constraints into the understanding of the superconductivity mechanism in iron-based superconductors.

Deep UV nonlinear optical crystal: RbBe 2 "BO 3 …F 2

Sizeable crystals of RbBe2(BO3)F2 (RBBF) were obtained by the flux method. The crystal structure was determined by x-ray data and the space group was proven to be R32, belonging to the uniaxial class. The linear and nonlinear optical parameters, including the cutoff wavelength, refractive indices, phase-matching angles, and effective nonlinear optical coefficients were determined for the first time to our knowledge, and then the Sellmeier equations were also constructed. By using an RBBF prism coupling device (PCD), tunable fourth-harmonic output from a Ti:sapphire laser and the sixth harmonic of an Nd-based laser were also obtained with relatively high power.

Coexistence of Fermi arcs and Fermi pockets in a high-T(c) copper oxide superconductor.

In the pseudogap state of the high-transition-temperature (high-Tc) copper oxide superconductors, angle-resolved photoemission (ARPES) measurements have seen Fermi arcs—that is, open-ended gapless sections in the large Fermi surface—rather than a closed loop expected of an ordinary metal. This is all the more puzzling because Fermi pockets (small closed Fermi surface features) have been suggested by recent quantum oscillation measurements. The Fermi arcs cannot be understood in terms of existing theories, although there is a solution in the form of conventional Fermi surface pockets associated with competing order, but with a back side that is for detailed reasons invisible to photoemission probes. Here we report ARPES measurements of Bi2Sr2-xLaxCuO6+δ (La-Bi2201) that reveal Fermi pockets. The charge carriers in the pockets are holes, and the pockets show an unusual dependence on doping: they exist in underdoped but not overdoped samples. A surprise is that these Fermi pockets appear to coexist with the Fermi arcs. This coexistence has not been expected theoretically.

Development of a vacuum ultraviolet laser-based angle-resolved photoemission system with a superhigh energy resolution better than 1 meV

The design and performance of the first vacuum ultraviolet (VUV) laser-based angle-resolved photoemission (ARPES) system are described. The VUV laser with a photon energy of 6.994 eV and bandwidth of 0.26 meV is achieved from the second harmonic generation using a novel nonlinear optical crystal KBe2BO3F2. The new VUV laser-based ARPES system exhibits superior performance, including superhigh energy resolution better than 1 meV, high momentum resolution, superhigh photon flux, and much enhanced bulk sensitivity, which are demonstrated from measurements on a typical Bi2Sr2CaCu2O8 high temperature superconductor. Issues and further development related to the VUV laser-based photoemission technique are discussed.

Second-harmonic generation from a KBe(2) BO(3)F(2) crystal in the deep ultraviolet.

By use of KBe(2)BO(3)F(2) (KBBF) crystal with a size of 10 mmx10 mm x1.2 mm and a special prism-coupling technique (PCT), fourth-harmonic generation of Ti:sapphire laser systems from 200 to 179.4 nm has been achieved. Moreover, with a Ti:sapphire laser with a 50-fs pulse duration and a 1-kHz repetition rate, conversion efficiency as high as 13% from 400 to 200 nm without any surface-loss correction has also been obtained. The data show that with the PCT a KBBF crystal can produce deep-UV coherent light with measurable power output.

Robustness of topological order and formation of quantum well states in topological insulators exposed to ambient environment

The physical property investigation (like transport measurements) and ultimate application of the topological insulators usually involve surfaces that are exposed to ambient environment (1 atm and room temperature). One critical issue is how the topological surface state will behave under such ambient conditions. We report high resolution angle-resolved photoemission measurements to directly probe the surface state of the prototypical topological insulators, Bi2Se3 and Bi2Te3, upon exposing to various environments. We find that the topological order is robust even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the exposed surface of the topological insulators. These findings provide key information in understanding the surface properties of the topological insulators under ambient environment and in engineering the topological surface state for applications.

Highly efficient ultraviolet generation at 355 nm in LiB 3 O 5

Highly efficient generation of UV radiation at 355 nm has been achieved in a LiB(3)O(5) crystal through the frequency mixing of the fundamental and second-harmonic radiation of a Nd:YAG laser. An energy conversion efficiency of 60% has been obtained under the experimental conditions described here. Thus the usefulness of LiB(3)O(5) for frequency-conversion applications is demonstrated.

Generation of vacuum-ultraviolet light by an optically contacted, prism-coupled KBe2BO3F2 crystal

We have demonstrated the second harmonic of a frequency-tripled Nd:YVO4 laser with 2.5-mW quasi-cw output by using an optically contacted, prism-coupled KBe2BO3F2 crystal. We also achieved the second harmonic with a frequency-doubled single-mode Ti:sapphire laser at 172.5 nm and sum-frequency mixing with a dual-wavelength Ti:sapphire laser at 163.3 nm. These wavelengths are to our knowledge the shortest obtained by use of nonlinear crystals for second-harmonic generation and sum-frequency mixing, respectively.