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We present a microscopic investigation of frequently observed impurity-induced states in stoichiometric LiFeAs using low temperature scanning tunneling microscopy and spectroscopy (STM/STS). Our data reveal seven distinct well defined defects which are discernible in topographic measurements. Depending on their local topographic symmetry, we are able to assign five defect types to specific lattice sites at the Li, Fe and As positions. The most prominent result is that two different defect types have a remarkably different impact on the superconducting state. A specific and quite abundant Fe-defect with $D_2$-symmetry generates significant impurity-induced additional states primarily at positive bias voltage with pronounced peaks in the on-site local density of states (LDOS) at about 4~mV and 12~mV. On the other hand, a $D_4$-symmetric As-defect causes a significantly enhanced LDOS at both positive and negative bias voltages. We expect that these findings provide fresh input for further experimental and theoretical studies on elucidating the nature of superconductivity in LiFeAs.

In this paper we report a detailed $\mu^{+}$SR and {}$^{19}$F-NMR study of the La$_{0.7}$Y$_{0.3}$FeAsO$_{1-x}$F$_{x}$ class of materials. Here, the diamagnetic La$_{1-y}$Y$_{y}$ substitution increases chemical pressure and, accordingly, sizeably enhances the optimal superconducting transition temperature. We investigate the magnetic-superconducting phase transition by keeping the Y content constant ($y = 0.3$) and by varying the F content in the range $0.025 \leq x \leq 0.15$. Our results show how magnetism and superconductivity coexist for $x = 0.065$. Such coexistence is due to segregation of the two phases in macroscopic regions, resembling what was observed in LaFeAsO$_{1-x}$F$_{x}$ materials under applied hydrostatic pressure. This scenario is qualitatively different from the nanoscopic coexistence of the two order parameters observed when La is fully substituted by magnetic rare-earth ions like Sm or Ce.

D. Haberer*,1, L. Petaccia2, Y. Wang3, H. Quian3, M. Farjam4, S. A. Jafari4,5, H. Sachdev6,7, A. V. Federov8, D. Usachov8, D. V. Vyalikh8,9, X. Liu10, O. Vilkov8,9, V. K. Adamchuk8, S. Irle3, M. Knupfer1, B. Buchner1, and A. Gruneis1,10 1 IFW Dresden, P.O. Box 270116, 01171 Dresden, Germany 2 Elettra Synchrotron Light Laboratory, Sincrotrone Trieste SCpA, S.S. 14 Km 163.5, 34149 Trieste, Italy 3 Department of Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan 4 Institute for Research in Fundamental Sciences (IPM), P.O. Box 19395-5531, Tehran, Iran 5 Department of Physics, Sharif University of Technology, Tehran 11155-9161, Iran 6 Anorganische Chemie 8.11, Universitat des Saarlandes, 66041 Saarbrucken, Germany 7 Max-Planck-Institut fur Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany 8 St. Petersburg State University, St. Petersburg 198504, Russia 9 Institut fur Festkorperphysik, TU Dresden, Mommsenstrasse 13, 01069 Dresden, Germany 10 Faculty of Physics, University of Vienna, Strudlhofgasse 4, 1090 Wien, Austria

Here, we report the growth of homogenously horizontally aligned single wall carbon nanotubes on stable temperature cut single crystal quartz using chemical vapor deposition with controllable yield and length from binary metallic mixtures as well as fullerene derivatives. We manage the yield and length of the as-grown tubes on stable temperature cut single crystal quartz by controlling the surface roughness of the support substrates by thermal treatment in air. Carbon caps derived from pre-treated fullerenes are also explored for their potential to nucleate growth of single wall carbon nanotubes without the need of a catalyst particle. Exohedrally functionalized fullerenes are apparently better nucleators than C 60 fullerenes.

Phone: þ49 351 4659 759, Fax: þ49 351 4659 313The potential for ceramics as catalysts for carbon nanotubes(CNTs) formation exceeds that of metal catalysts in thatceramics can serve not only as a catalyst particle for CNTnucleation/growth as found with metal catalysts, but they canalso serve as a template for the synthesis of carbon nano-structures. They can also be purified more readily. Here wepresent studies on the growth of CNTs from ceramics in laserpyrolysis and chemical vapor deposition (CVD) routes. Weshow CNT growth from both nanowires and ceramic particles.In addition, doping of the CNTs can also be achieved throughthe use of ceramic catalysts. Since ceramic materials are easilyremoved from the as-produced samples as compared to metalcatalysts, they are attractive for further application, e.g., incarbon-based electronics.

Recent research into the channelling of few layer graphene via catalytic hydrogenation using metallic catalyst nanoparticles shows that this technique is potentially a key engineering route for graphene nanoribbon fabrication with atomic precision. In this contribution we utilized gas phase prepared cobalt nanoparticles to create etch trenches in multi-layer graphene. Aberration-corrected low voltage high resolution transmission electron microscopy allowed us to directly determine the etch direction of various tracks. The etch channels are shown to be commensurate with the graphite crystal lattice. More importantly, preferred etching along armchair edges was observed, indicating that tailored fabrication of graphene nanostructures with well-defined armchair edges may be possible applying ultrafine catalyst particles.

Oxides are rapidly emerging as an exciting alternative to metal catalysts for carbon nanotubes (CNT) formation because they are more easily integrated in Si technology, are usually easily removed in purification steps and are cheap. In this contribution we highlight the potential of oxides to synthesise single walled carbon nanotubes (SWCNT) (via laser evaporation), multi-walled carbon nanotubes and few layer graphene (via thermal CVD). Schematic illustration of graphitic carbon grown from oxide step site to form a CNT (left image) or graphene layer (right image).

We performed angle resolved electron energy-loss spectroscopy on vertically aligned single wall carbon nanotubes. We observe two distinct loss-peaks in the energy : range from 4.6 eV to 5.1 eV. This doublet of loss-peaks is attributed to the on-axis and crossed plasma oscillations of the π electrons in sp 2 carbon. The unique identification and assignment of these two oriented excitations is derived from their dispersion relations. The available momenta are aligned on the nanotube axis and reflect the confinement of a particle on a wire. The observed angle resolved loss-peaks are a gapless extension oi'polarded optical ipectroscopy into finite momenta.

In this contribution we present various data from CNT grown from supported catalysts. The observed data are in agreement with the catalyst volume to surface area model developed for floating catalysts and which predicts a direct CNT diameter ; and number of wall dependence on the catalyst size. Further, our studies point to the root of a CNT lying at the support rather than the catalyst. Additional supporting studies point to . oxygen playing a critical role in the growth of CNT, viz. the CNT can grow from an oxide support rather than the catalyst. In this scenario the catalysts role is twofold; to precipitate carbon from the catalyst-carbon eutectic upon supersaturation and thus form the embryonic caps and to form circular defect sites at its circumference on the oxide support. These circular defect sites enable continued CNT growth directly from the support. Our findings point to catalytic processes very reminiscent of many other heterogeneous catalytic processes. Illustration of the catalyst volume to surface area dependencies between floating catalysts and supported catalysts. With floating catalysts excess carbon at nucleation leads to encapsulation (preventing SWCNT growth) whilst for supported catalysts excess carbon forms new caps within the previous cap since encapsulation is prevented by the catalyst support interaction.

The preparation of nanoparticles in the gas phase through inert gas condensation is a very versatile method to fabricate catalyst particles that act as catalytic templates for carbon nanotube synthesis since the method allows for easy control of the catalysts elemental composition and their morphology. In this contribution we report on the synthesis of carbon nanotubes via two distinct chemical vapour deposition routes, thermal and plasma enhanced chemical vapour deposition. using such gas phase prepared elemental Fe catalyst particles as well as binary FePt nanoparticles.

We report on a systematic approach to exploit the potential of filled carbon nanotubes (CNT) to act as sensors that might provide non-invasive temperature control in biological systems on a cellular level. In this case the temperature is detected by measuring NMR parameters on the filling materials. The beneficial feature of a carbon nanotube is to provide protection of both (i) a human body against toxic adverse effects from the filling material and (ii) a filling material against chemical and biochemical exposure. The feasibility of this concept has been demonstrated on the example of the temperature-dependent NMR frequency and relaxation time measured on the CNT filled with the appropriate sensor materials.

Carbon nanotubes are archetypical one dimensional systems, with peculiar anisotropic electronic properties. Only recently μm thick films of vertically aligned SWNT became available. The vertical alignment of the nanotube mats allows the realization of scattering geometries promoting specific dipole transitions parallel and normal to the axis of the SWNT. We find well expressed mosaic spreads for the π* and σ* resonances. Full three dimensional modelling of the dipole transitions as well as of the alignment demonstrate polarization dependent X-ray absorption to be an accurate, versatile non destructive and reliable tool to quantify the alignment of SWNT on a bulk scale.

We report the chemical vapor deposition of Fe-filled carbon nanotubes (CNTs) from metallorganic precursors (ferrocene, cyclopentadienyliron dicarbonyl dimer) on oxidized silicon substrates precoated with single-layered or multi-layered catalyst films (Fe, Au/Fe or Au/AVFe). The performed experiments result in thin multi-walled carbon nanotubes (

The synthesis of high quality isotope engineered SWCNT by means of laser ablation and the use of Pt-Rh-Re catalyst mixtures has been established. Optical absorption and Raman spectroscopy as well as transmission electron microscopy are utilized to characterize the obtained SWCNTs with regard to purity and yield. The absence of any ferromagnetic materials, as well as the remarkably low abundance of amorphous carbon renders this material ideal for magnetic studies. The controlled augmentation of 13 C is conveniently confirmed by phonon softening and broadening observed in Raman spectroscopy. Isotope labelling at constant sample quality was achieved in the whole range from 1% 13 C up to 98% 13 C.

We report on the doping dependence of the electronic structure of the valence band of mats of C 60 -filled single wall carbon nanotubes (C 60 peapods) studied by high resolution photoemission spectroscopy. In the pristine state, the bundles consist of approximately one third metallic tubes which are Tomonaga-Luttinger liquids (TLL) and thus show a power law scaling of the density of states near the Fermi level. The power law scaling factor of the pristine peapods is hardly increased compared to that of single-walled carbon nanotubes (SWCNT). Upon doping we find a complex doping behavior and charge distribution between the electronic states of the SWCNT and the encaged C 60 chains. Close to the Fermi level this has consequences on the doping induced crossover from a TLL to a conventional Fermi liquid which is found at a higher charge transfer as compared to intercalated SWCNT bundles.

A photoemission study of potassium intercalated trimetal-nitride fullerene Dy 3 N@C 80 is reported. A continuous electron doping to the carbon cage with increasing potassium intercalation levels is observed as shape changes in the carbon-sensitive ultra-violet photoemission spectrum. X-ray photoemission shows that the valence electronic state of the trivalent Dy ions is not affected by the additional charge in the fullerene cage.

Nanotube growth by chemical vapour deposition carbon is carried out for two different catalyst preparation methods: a wet chemistry catalyst is compared to a thin metal film multilayer catalyst. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs are recorded to investigate the nanotube alignment and the number of walls. Resonance Raman spectroscopy is used for analysis of diameters and the defect concentration.

In this article, we review our angle- and time-resolved photoemission studies (ARPES and trARPES) on various ferropnictides. In the ARPES studies, we focus first on the band structure as a function of control parameters. We find near optimally doped compounds a Lifshitz transition of hole/electron pocket vanishing type. Second, we investigated the inelastic scattering rates as a function of the control parameter. In contrast to the heavily discussed quantum critical scenario, we find no enhancement of the scattering rate near optimally doping. Correlation effects which show up by the non-Fermi-liquid behavior of the scattering rates, together with the Lifshitz transition offer a new explanation for the strange normal state properties and suggests an interpolating superconducting state between BCS and BE condensation. Adding femtosecond time resolution to ARPES provides complementary information on electron and lattice dynamics. We report on the response of the chemical potential by a collective periodic variation coupled to coherent optical phonons in combination with incoherent electron and phonon dynamics described by a three temperature heat bath model. We quantify electron phonon coupling in terms of 2 and show that the analysis of the electron excess energy relaxation is a robust approach. The spin density wave ordering leads to a pronounced momentum dependent relaxation dynamics. In the vicinity of kF, hot electrons dissipate their energy by electron-phonon coupling with a characteristic time constant of 200fs. Electrons at the center of the hole pocket exhibit a four time slower relaxation which is explained by spin-dependent dynamics with its smaller relaxation phase space. This finding has implications beyond the material class of Fe-pnictides because it establishes experimental access to spin-dependent dynamics in materials with spin density waves. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Spin–orbit coupling (SOC) is a fundamental interaction in solids that can induce a broad spectrum of unusual physical properties from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors (IBS) its role has so far been considered insignificant with the models based on spin- or orbital fluctuations pairing being the most advanced in the field. In this review we discuss the recent direct observation of SOC by angle-resolved photoemission spectroscopy (ARPES). Electron-like Fermi surface of LiFeAs split by SOC.

Several hole-doped BaFe2As2 compounds were recently shown to exhibit a second magnetic phase transition in the concentration range close to the full suppression of antiferromagnetic (AFM) order. At this additional transition ordered magnetic moments reorient from in-plane to out-of-plane alignment associated with a suppression of the orthorhombic distortion. We have studied the magnetic properties of such a representative hole-doped system, Ba1–xNaxFe2As2 with 0.25 ≤ x ≤ 0.40, by neutron diffraction on large single crystals. With increasing Na substitution (0.25 ≤ x ≤ 0.39) the AFM transition temperature sharply decreases, while the spin-reorientation transition temperature is rather constant, until both magnetic phases are completely suppressed at x = 0.40. For all studied Na concentrations the additional transition is related to the spin reorientation, which, however, is complete only in the middle of the concentration range of the out-of-plane phase. In the superconducting state, the intensities of magnetic Bragg reflections become heavily suppressed; this effect seems to increase for larger Tc and reaches ∼50% in Ba0.61Na0.39Fe2As2. In samples with coexisting in-plane and out-of-plane ordering, this superconductivity induced suppression of ordered moments is significantly stronger for the out-of-plane components indicating that this phase more strongly competes with superconductivity.

We present properties of single crystals of the iron pnictide superconductor RbFe2As2 grown using a self-flux method. A bulk superconducting transition at Tc ∼2.6 K was consistently observed in resistivity, magnetic susceptibility, and specific heat measurements. While the resistivity behaves like a conventional metal at low T, the magnetic susceptibility shows an unusual broad maximum around 80 K in the normal state. A large Sommerfeld coefficient, γ0=127mJ/mol K2, was observed seemingly a common feature of heavily hole-doped “122” compounds. In the superconducting state, we analyzed our specific heat data based on the s- and d-wave two-gap models. The presence of the small gap with 2Δ1(0)/kBTc

The generic structure of most high Tc superconductors is a stacking sequence of superconducting planes separated by so-called charge reservoir layers. It is well known that carrier doping of these materials is achieved either by substitution of atoms or by nonstoichiometry in the charge reservoir layer. The alternating type of stacking causes yet another two important consequences. First, the transport anisotropy of these materials can be so high, that in the superconducting state the c-axis transport is governed by the intrinsic Josephson effect. Whereas the anisotropy of our investigated LaO1–xFxFeAs samples was not sufficient, optimum-doped Ca10(FeAs)10Pt4As8 showed hysteretic c-axis current–voltage characteristics. Together with the Ambegaokar–Baratoff like temperature dependence of the critical current, this is a strong indication of intrinsic Josephson effects. Second, it is possible to deposit in the charge reservoir layers a substantial amount of charge only by injecting large c-axis currents. By charge compensation, this decreases the concentration of mobile electrons in the conducting layers of electron-doped materials. We were able to verify this in all details by c-axis transport measurements of LaO1−xFxFeAs single crystals and pure and Pt doped (CaFe1–xPtxAs)10Pt4As8 single crystals. After current injection, we observed a decrease of Tc for doping levels at or below the Tc maximum, and a Tc increase for doping levels beyond the maximum. In all cases, the resistivity of the samples increased significantly. In both material classes, heavily overdoped samples showed a spectacular Tc increase by more than 10 K only accomplished by carrier injection. Generic stacking sequence of layered superconductors and crystal structure of the pnictide LaOFeAs.

authoren Orbital ordering has recently emerged as another important state in iron-based superconductors, and its role for superconductivity as well as its connection to magnetic order and orthorhombic lattice distortion are heavily debated. In order to search for signatures of this so-called nematic phase in oxypnictides, we revisit the normal state properties of the pnictide superconductor LaFeAsO1−x Fx with a focus on resistivity, Nernst effect, thermal expansion, and As nuclear magnetic resonance (NMR) data. The transport properties at the underdoped level exhibit pronounced anomalies at about the same temperature where undoped LaFeAsO develops long-range nematic ordering, i.e., at about 160 K. Furthermore, the As-NMR spin-lattice relaxation rate reveals a progressive slowing down of spin fluctuations. Yet, long-range magnetic order and also a detectable orthorhombic lattice distortion are absent. Thus, we conclude from the data that short-range orbital-nematic ordering or a slowly fluctuating form of it sets in near 160 K. Remarkably, all anomalies in the transport and also the indications of slow spin fluctuations disappear close to optimal doping which suggests that in LaFeAsO1−x Fx the nematic phase actually competes with superconductivity. Schematic electronic phase diagram of LaFeAsO1−x Fx.