A series of layered CeO1−xFxFeAs compounds with x=0 to 0.20 are synthesized by solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. F-doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. The very high superconducting transition temperature strongly challenges the classic BCS theory based on the electron-phonon interaction. The very closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuations play a key role in the superconducting paring mechanism. The study also reveals that the Ce 4f electrons form local moments and ordered antiferromagnetically below 4 K, which could coexist with superconductivity.PACS numbers: 74.62.Bf, 74.25.Gz The recent discovery of superconductivity with transition temperature of 26 K in LaO 1−x F x FeAs system[1] has generated tremendous interest in the scientific community. Except for a relatively high transition temperature, the system displays many interesting properties. Among others, the presence of competing ordered ground states is one of the most interesting phenomena [2]. The pure LaOFeAs itself is not superconducting but shows an anomaly near 150 K in both resistivity and dc magnetic susceptibility.[1] This anomaly was shown to be caused by the spin-density-wave (SDW) instability.[2] Electrondoping by F suppresses the SDW instability and recovers the superconductivity. Here we show that similar competing orders exist in another rear-earth transition metal oxypnictide Ce(O 1−x F x )FeAs. Most surprisingly, the superconducting transition temperature in this system could reach as high as 41 K. Except for cuprate superconductors, T c in such iron-based compounds has already become the highest.The very high superconducting transition temperature has several important implications. First, the T c value has already reached the well-accepted limit value of classic BCS theory [3,4]. Considering the small carrier density and rather week electron-phonon coupling estimated from first-principle calculations [5,6], the observation result strongly challenges the BCS theory based on the electron-phonon interaction. Second, the rare-earth Cebased compounds usually show hybridization between localized f-electrons and itinerant electrons. This often leads to a strong enhancement of carrier effective mass at low temperature. Even for 4d transition metal oxypnictide with the same type of structure, a recent report indicates that the electronic specific heat coefficient of Ce-based CeORuP (γ=77 mJ/mol K 2 ) is 20 times higher than the value of La-based LaORuP (γ=3.9 mJ/mol K 2 ) [7]. The hybridization also tends to cause various ordered states at low temperature, like ferromagnetic (FM) or antiferromagnetic (AFM) ordering. Although superconducting state could occur in Ce-based materials, the superconducti...

Mass spectrometry (Ms)-based proteomics has become the preferred tool for the analysis of protein phosphorylation. to be successful at such an endeavor, there is a requirement for an efficient enrichment of phosphopeptides. this is necessary because of the substoichiometric nature of phosphorylation at a given site and the complexity of the cell. recently, new alternative materials have emerged that allow excellent and robust enrichment of phosphopeptides. these monodisperse microsphere-based immobilized metal ion affinity chromatography (IMac) resins incorporate a flexible linker terminated with phosphonate groups that chelate either zirconium or titanium ions. the chelated zirconium or titanium ions bind specifically to phosphopeptides, with an affinity that is similar to that of other widely used metal oxide affinity chromatography materials (typically tio 2 ). Here we present a detailed protocol for the preparation of monodisperse microsphere-based ti 4 + -IMac adsorbents and the subsequent enrichment process. Furthermore, we discuss general pitfalls and crucial steps in the preparation of phosphoproteomics samples before enrichment and, just as importantly, in the subsequent mass spectrometric analysis. Key points such as lysis, preparation of the chromatographic system for analysis and the most appropriate methods for sequencing phosphopeptides are discussed. Bioinformatics analysis specifically relating to site localization is also addressed. Finally, we demonstrate how the protocols provided are appropriate for both single-protein analysis and the screening of entire phosphoproteomes. It takes ~2 weeks to complete the protocol: 1 week to prepare the ti 4 + -IMac material, 2 d for sample preparation, 3 d for Ms analysis of the enriched sample and 2 d for data analysis.

The elucidation of protein post-translational modifications, such as phosphorylation, remains a challenging analytical task for proteomic studies. Since many of the proteins targeted for phosphorylation are low in abundance and phosphorylation is typically substoichiometric, a prerequisite for their identification is the specific enrichment of phosphopeptide prior to mass spectrometric analysis. Here, we presented a new method termed as immobilized titanium ion affinity chromatography (Ti4+-IMAC) for enriching phosphopeptides. A phosphate polymer, which was prepared by direct polymerization of monomers containing phosphate groups, was applied to immobilize Ti4+ through the chelating interaction between phosphate groups on the polymer and Ti4+. The resulting Ti4+-IMAC resin specifically isolates phosphopeptides from a digest mixture of standard phosphoproteins and nonphosphoprotein (BSA) in a ratio as low as 1:500. Ti4+-IMAC was further applied for phosphoproteome analysis of mouse liver. We also compared Ti4+-IMAC to other enrichment methods including Fe3+-IMAC, Zr4+-IMAC, TiO2 and ZrO2, and demonstrate superior selectivity and efficiency of Ti4+-IMAC for the isolation and enrichment of phosphopeptides. The high specificity and efficiency of phosphopeptide enrichment by Ti4+-IMAC mainly resulted from the flexibility of immobilized titanium ion with spacer arm linked to polymer beads as well as the specific interaction between immobilized titanium ion and phosphate group on phosphopeptides.

We performed optical spectroscopy measurement on single crystals of BaFe2As2 and SrFe2As2, the parent compounds of FeAs-based superconductors. Both are found to be quite metallic with fairly large plasma frequencies at high temperature. Upon entering the spin-density-wave state, the formation of partial energy gaps was clearly observed with the surprising presence of two different energy scales. A large part of the Drude component was removed by the gapping of Fermi surfaces. Meanwhile, the carrier scattering rate was even more dramatically reduced. We elaborate that the spin-density-wave instability is more likely to be driven by the Fermi surface nesting of itinerant electrons than a local-exchange mechanism.

We performed a high-resolution angle-resolved photoemission spectroscopy study of the Tl0.63K0.37Fe1.78Se2 superconductor (Tc = 29 K). We show the existence of two electronlike bands at the M(π, 0) point which cross the Fermi level at similar Fermi wave vectors to form nearly circular electronlike Fermi surface pockets. We observe a nearly isotropic ∼ 8.5 meV superconducting gap (∆/kBTc ∼ 7) on these Fermi surfaces. Our analysis of the band structure around the Brillouin zone centre reveals two additional electronlike Fermi surfaces: a very small one and a larger one with kF comparable to the FS pockets at M. Interestingly, a SC gap with a magnitude of ∼ 8 meV also develops along the latter FS. Our observations are consistent with the s-wave strong coupling scenario.PACS numbers: 74.25.Jb, 74.70.Xa, The amplitude and symmetry of the superconducting (SC) gap of a material are determined by its band structure, its Fermi surface (FS) topology and the pairing mechanism itself. The experimental observation of enhanced gap amplitude on holelike and electronlike FS pockets quasi-nested by the antiferromagnetic (AF) wave vector in iron-based superconductors [1][2][3][4][5] has been widely considered as suggestive of the importance of AF interband scattering in these materials. In particular, the quasi-nesting model is consistent with the strong suppression of superconductivity in heavily hole-doped [6] and heavily electron-doped [7] BaFe 2 As 2 compounds, for which the FS quasi-nesting conditions vanish. Recently, this model faced a serious challenge with the discovery of superconductivity above 30 K in heavily electron-doped K 0.8 Fe 2−x Se 2 and (Tl,K)Fe 2−x Se 2 [8,9]. Indeed, previous angle-resolved photoemission spectroscopy (ARPES) measurements revealed only electronlike FS pockets [10,11].In this letter, we report high-energy resolution ARPES measurements on the Tl 0.63 K 0.37 Fe 1.78 Se 2 superconductor (T c = 29 K). We observed two electronlike M(π, 0)centred FS pockets that develop a nearly isotropic SC gap below T c with a magnitude of ∼ 8.5 meV, leading to a 2∆/k B T c of ∼ 7. In addition, a weak electronlike FS pocket with a similar size and a tiny electronlike pocket are also observed at the Γ(0, 0) point. The former one also exhibits a SC gap size of about 8 meV. In addition, a high-energy (∼ 0.8 eV) incoherent peak undergoes a significant energy shift of ∼ 100 meV through the metalnonmetal crossover around 70K, while the low-energy valence band shows little change. We discuss the possible implications of the SC gap symmetry and the FS topology for the SC pairing mechanism in this unusual iron- * Electronic address: dingh@iphy.ac.cn based superconductor .Single crystals of Tl 0.63 K 0.37 Fe 1.78 Se 2 (T onset c = 29.1 K; T mid c = 28.6 K; T zero c = 27.5 K) were grown by the Bridgeman method [9]. The precise composition was determined using an Energy Dispersive X-ray Spectrometer (EDXS). The lattice parameters a = 3.85Å and c = 14.05Å were obtained by fitting XRD data. We performed ARPES measurements at th...

We have employed a new route to synthesize single phase F-doped LaOFeAs compound and confirmed the superconductivity above 20 K in this Fe-based system. We show that the new superconductor has a rather high upper critical field of over 50 T. A clear signature of superconducting gap opening below T(c) was observed in the far-infrared reflectance spectra, with 2Delta/kT(c) approximately 3.5-4.2. Furthermore, we show that the new superconductor has electron-type conducting carriers with a rather low-carrier density.

A 150 microm internal diameter capillary monolithic column with a strong cation-exchange stationary phase was prepared by direct in situ polymerization of ethylene glycol methacrylate phosphate and bisacrylamide in a trinary porogenic solvent consisting dimethylsulfoxide, dodecanol, and N,N'-dimethylformamide. This phosphate monolithic column exhibits higher dynamic binding capacity, faster kinetic adsorption of peptides, and more than 10 times higher permeability than the column packed with commercially available strong cation-exchange particles. It was applied as a trap column in a nanoflow liquid chromatography-tandem mass spectrometry system for automated sample injection and online multidimensional separation. It was observed that the sample could be loaded at a flow rate as high as 40 microL/min with a back pressure of approximately 1300 psi and without compromising the separation efficiency. Because of its good orthogonality to the reversed phase separation mechanism, the phosphate monolithic trap column was coupled with a reversed-phase column for online multidimensional separation of 19 microg of the tryptic digest of yeast proteins. A total of 1522 distinct proteins were identified from 5608 unique peptides (total of 54,780 peptides) at the false positive rate only 0.46%.

We have successfully grown high-quality single crystals of SrFe 2 As 2 and A 0.6 K 0.4 Fe 2 As 2 ͑A = Sr, Ba͒ using flux method. The resistivity, specific heat, and Hall coefficient have been measured. For parent compound SrFe 2 As 2 , an anisotropic resistivity with c / ab as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivities due to the spin-density-wave ͑SDW͒ instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows twofold symmetry with field rotating within ab plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K-doped A 0.6 K 0.4 Fe 2 As 2 ͑A = Sr, Ba͒, the SDW instability is suppressed and the superconductivity appears with T c above 35 K. The rather low anisotropy in upper critical field between H ʈ ab and H ʈ c indicates that interplane coupling plays an important role in hole-doped Fe-based superconductors.The recent discovery of superconductivity with transition temperature T c ϳ 26 K in LaFeAsO 1−x F x has generated tremendous interest in scientific community. 1 Shortly after this discovery, the T c was raised to 41-55 K by replacing La by rare-earth Ce, Sm, Pr, Nd, etc., making those systems with T c exceeding 50 K. 2-5 The undoped quaternary compounds crystallize in a tetragonal ZrCuSiAs-type structure, which consists of alternate stacking of edge-sharing Fe 2 As 2 tetrahedral layers and La 2 O 2 tetrahedral layers along c axis. Very recently, superconductivity with T c of up to 38 K was discovered in AFe 2 As 2 ͑A = Ba, Sr, Ca͒ upon K or Na doping. 6-10 AFe 2 As 2 compounds crystallize in a tetragonal ThCr 2 Si 2 -type structure with identical Fe 2 As 2 tetrahedral layers as in LaFeAsO, but separated by single elemental A layers. These compounds contain no oxygen in A layers. The simpler structure of AFe 2 As 2 system makes it more suitable for research of intrinsic physical properties of Fe-based compounds.Except for a relatively high transition temperature, the system displays many interesting properties. The existence of a spin-density-wave ͑SDW͒ instability in parent LaFeAsO ͑Ref. 11͒ was indicated by specific heat, optical measurements, and first-principles calculations, and subsequently confirmed by neutron-scattering, 12 NMR, 13 sR, 14 and Mössbauer 15 spectroscopic measurements. The superconductivity only appears when SDW instability was suppressed by doping carriers or applying pressure. The competition between superconductivity and SDW instability was identified in other rare-earth substituted systems. 2,16,17 Besides the SDW instability, structural distortions from tetragonal to monoclinic were also observed for both ReFeAsO ͑Re = rare earth͒ and AFe 2 As 2 ͑A = Ba, Sr, Ca͒. 18-23 The structural transition temperatures were found to occur at slightly higher than SDW transition temperature in LaFeAsO, 12 but the two transitions occur simultaneously in AFe 2 As 2 ͑A = Ba, Sr, Ca͒. 18,19,24 The band-structure calculation and neutronscattering exp...