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In the study of the physicochemical properties of low molecular weight polybutadiene rubber with functional hydroxyl groups (Hydroxyl Terminated Poly Butadien – HTPB), which is a raw material for solid rocket fuel (SRP), an experimental batch of rubber was made and 3 synthesis of experimental batches of low molecular weight polybutadiene rubber with functional hydroxyl groups was carried out. The current work obtains a HTPB physical and chemical characteristics that meet the requirements of regulatory documentation (ND). During this work, the physical and chemical characteristics of HTPB rubber were determined to compliance with regulatory documentation. The readiness of the equipment for conducting the synthesis of HTPB rubber (collector, dispensers, reactor, separator, rotary evaporators, vessel, igniter) was checked. The sequence of execution of technological operations for the synthesis of rubber has been worked out. According to the results of the study, the compliance of the physical and chemical parameters of the obtained raw materials for SPR with the requirements of the ND was established and a conclusion was made about the admission to production. After checking the operability of the main and auxiliary technological equipment, no comments were found. Because of the preparation of technological equipment for the synthesis of HTPB rubber, no comments were found. According to the results of the operations of loading samples into the dispensers and collector, it was found that with a decrease in the volume of butadiene in the container, it is necessary to increase the argon pressure to load it into the collector. According to the results of the operations, filling the reactor with prepared samples of raw materials, no comments were found either. The results of the study of the physicochemical properties of the obtained raw materials for the manufacture of SPR showed almost complete compliance with the requirements of ND, except for the weight loss during drying. Subsequent studies are aimed at modifying the technology for obtaining low molecular weight polybutadiene rubber and studying the physicochemical properties of raw materials for solid rocket fuel.

Magnetic topological materials are a class of compounds with the underlying interplay of nontrivial band topology and magnetic spin configuration. Extensive interests have been aroused due to their application potential involved with an array of exotic quantum states. With angle-resolved photoemission spectroscopy and first-principles calculations, here we study the electronic properties of two magnetic Weyl semimetal candidates PrAlSi and SmAlSi. Though the two compounds harbor distinct magnetic ground states (ferromagnetic and antiferromagnetic for PrAlSi and SmAlSi, respectively) and 4$f$ shell fillings, we find that they share quite analogous low-energy band structure. By the measurements across the magnetic transitions, we further reveal that there is no evident evolution of the band structure in both compounds and the experimental spectra can be well reproduced by the nonmagnetic calculations, together suggesting a negligible effect of the magnetism on their electronic structures and a possibly weak coupling between the localized 4$f$ electrons and the itinerant conduction electrons. Our results offer essential insights into the interactions between magnetism, electron correlations, and topological orders in the $R$Al$X$ ($R$ = light rare earth and $X$ = Si or Ge) family., 7 pages, 3 figures

The entanglement of charge density wave (CDW), superconductivity, and topologically nontrivial electronic structure has recently been discovered in the kagome metal AV_{3}Sb_{5} (A=K, Rb, Cs) family. With high-resolution angle-resolved photoemission spectroscopy, we study the electronic properties of CDW and superconductivity in CsV_{3}Sb_{5}. The spectra around K[over ¯] is found to exhibit a peak-dip-hump structure associated with two separate branches of dispersion, demonstrating the isotropic CDW gap opening below E_{F}. The peak-dip-hump line shape is contributed by linearly dispersive Dirac bands in the lower branch and a dispersionless flat band close to E_{F} in the upper branch. The electronic instability via Fermi surface nesting could play a role in determining these CDW-related features. The superconducting gap of ∼0.4 meV is observed on both the electron band around Γ[over ¯] and the flat band around K[over ¯], implying the multiband superconductivity. The finite density of states at E_{F} in the CDW phase is most likely in favor of the emergence of multiband superconductivity, particularly the enhanced density of states associated with the flat band. Our results not only shed light on the controversial origin of the CDW, but also offer insights into the relationship between CDW and superconductivity.

Fermi surfaces, three-dimensional (3D) abstract interfaces that define the occupied energies of electrons in a solid, are important for characterizing and predicting the thermal, electrical, magnetic, and optical properties of crystalline metals and semiconductors [1]. Angle-resolved photoemission spectroscopy (ARPES) is the only technique directly probing the Fermi surface by measuring the Fermi momenta ( kF ) from energy- and angular distribution of photoelectrons dislodged by monochromatic light [2]. Existing electron analyzers are able to determine a number of kF -vectors simultaneously, but current technical limitations prohibit a direct high-resolution 3D Fermi surface mapping. As a result, no such datasets exist, strongly limiting our knowledge about the Fermi surfaces and restricting a detailed comparison with the widely available nowadays calculated 3D Fermi surfaces. Here we show that using a simpler instrumentation, based on the Fourier electron optics combined with a retardation field of the detector, it is possible to perform 3D-mapping within a very short time interval and with very high resolution. We present the first detailed experimental 3D Fermi surface recorded in the full Brillouin zone along the kz-direction as well as other experimental results featuring multiple advantages of our technique. In combination with various light sources, including synchrotron radiation, our methodology and instrumentation offer new opportunities for high-resolution ARPES in the physical and life sciences.

We show that the cubic compound PtBi2, is a topological semimetal hosting a sixfold band touching point in close proximity to the Fermi level. Using angle-resolved photoemission spectroscopy, we map the bandstructure of the system, which is in good agreement with results from density functional theory. Further, by employing a low energy effective Hamiltonian valid close to the crossing point, we study the effect of a magnetic field on the sixfold fermion. The latter splits into a total of twenty Weyl cones for a Zeeman field oriented in the diagonal, [111] direction. Our results mark cubic PtBi2, as an ideal candidate to study the transport properties of gapless topological systems beyond Dirac and Weyl semimetals., 15 pages, 6 figures; this is the final, published version

In this work, we have investigated the wettability of Bazhenov Formation rock samples using a nuclear magnetic resonance (NMR), the methods of vapor adsorption, and wetting contact angle. We have conducted the petrographic description of rocks using ultra-thin sections and scanning electron microscopy (SEM). In addition, we used data on the organic content (TOC) obtained by the Rock-Eval method and the results of lithological typing on thin sections for detailed analysis of NMR and contact angle methods results. Thus, for target rock, the groups with a similar rock wettability were highlighted by the lithological description of thin sections. The calculation of the wetting angle provided an initial assessment of the surface wettability of the rock and made it possible to establish the relationship between the wetting angle and the TOC, which is relevant for rocks of the Abalak-Bazhenov group. The NMR method described the core plug wettability proved to be less sensitive to the rock lithotype and organic matter (OM) texture in the rock and, therefore, used for the integral characterization of core plugs. Correlations of calculated wetting angle and adsorption wettability index vs. TOC and OM texture illustrated the dependence of rock wettability behavior on both the lithological specifics and the OM properties.

The role of nematic order for the mechanism of high-temperature superconductivity is highly debated. In most iron-based superconductors (IBSs) the tetragonal symmetry is broken already in the normal state, resulting in orthorhombic lattice distortions, static stripe magnetic order, or both. Superconductivity then emerges, at least at weak doping, already from the state with broken ${C}_{4}$ rotational symmetry. One of the few stoichiometric IBSs, lithium iron arsenide superconducts below 18 K and does not display either structural or magnetic transition in the normal state. Here we demonstrate, using angle-resolved photoemission spectroscopy, that even the superconducting state in LiFeAs is also a nematic one. We observe spontaneous breaking of the rotational symmetry in the gap amplitude on all Fermi surfaces, as well as unidirectional distortion of the Fermi pockets. Remarkably, these deformations are hardly visible above superconducting ${T}_{c}$. Our results demonstrate the realization of the phenomenon of superconductivity-induced nematicity in IBSs, emphasizing the intimate relation between them. We suggest a theoretical explanation based on the emergence of a secondary instability inside the superconducting state, which leads to the nematic order and $s\ensuremath{-}d$ mixing in the gap function.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The importance of yolk antibodies (IgY) for their use in the diagnosis and treatment of human diseases is shown. The structural features of IgY compared to human immunoglobulins were studied. The disadvantages of obtaining hyperimmune sera from mammalian animals were revealed. A feature of IgY is its resistance to trypsin and chymotrypsin, which can contribute to more effective treatment of diseases. Data on optimization of the technology for obtaining yolk antibodies (IgY) from various bird species are presented. It is shown that 25-50 mg of highly purified immunoglobulin can be obtained from one bird egg. Obtaining immunoglobulins from eggs is more humane than using mammals for this purpose. Compared to ion exchange chromatography, gel chromatography, and ultrafiltration, the methods of precipitation with various chemicals (ammonium sulfate) were found to be simpler in terms of methodology. The results of electrophoresis revealed two types of proteins with a molecular weight of about 65 kDa and 30 kDa, which corresponds to the mass of the «heavy» and «light» IgY chains. Quail species Coturnix coturnix japonica as a producer of biological raw materials for the production of diagnostic immunoglobulins have great advantages over chickens (Gallus domesticus).

Using angle-resolved photoemission spectroscopy and density functional theory (DFT) we study the electronic structure of layered ${\mathrm{BaZnBi}}_{2}$. Our experimental results show no evidence of Dirac states in ${\mathrm{BaZnBi}}_{2}$ originated either from the bulk or the surface. The calculated band structure without spin-orbit interaction shows linear band dispersions at $X$ along the $X\text{\ensuremath{-}}M$ high-symmetry line. In addition, the calculations suggest a gapless band crossing point along the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}M$ high-symmetry line. However, as soon as the spin-orbit interaction is turned on, the band crossing point is significantly gapped out. These observations suggest that the Dirac fermions in ${\mathrm{BaZnBi}}_{2}$ are trivial similar to the Dirac states observed in grapheme. The experimental observations are in good agreement with the DFT calculations.

Quantum-mechanical fluctuations between competing phases at $T=0$ induce exotic finite-temperature collective excitations that are not described by the standard Landau Fermi liquid framework. These excitations exhibit anomalous temperature dependences, or non-Fermi liquid behavior, in the transport and thermodynamic properties in the vicinity of a quantum critical point, and are often intimately linked to the appearance of unconventional Cooper pairing as observed in strongly correlated systems including the high-$T_c$ cuprate and iron pnictide superconductors. The presence of superconductivity, however, precludes direct access to the quantum critical point, and makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe$_{1/3}$Co$_{1/3}$Ni$_{1/3}$)$_{2}$As$_{2}$, indicative of quantum criticality at zero temperature and zero applied magnetic field. Beyond a linear in temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find the scattering rate that obeys a universal scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the dominance of hole-like carriers close to the zero-temperature and zero-field limits, the scale invariance, isotropic field response, and lack of applied pressure sensitivity suggests a unique quantum critical system that does not drive a pairing instability., 37 pages, 15 figures

Sergey Borisenko reflects on 99 years of angle-resolved photoemission spectroscopy (ARPES), a technique widely used today to understand the electronic structure of materials.

Compound Bi14Rh3I9 consists of ionic stacks of intermetallic [(Bi4Rh)3I](2+) and insulating [Bi2I8](2-) layers and has been identified to be a weak topological insulator. Scanning tunneling microscopy revealed the robust edge states at all step edges of the cationic layer as a topological fingerprint. However, these edge states are found 0.25 eV below the Fermi level, which is an obstacle for transport experiments. Here, we address this obstacle by comparing results of density functional slab calculations with scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. We show that the n-type doping of the intermetallic layer is intrinsically caused by the polar surface and is well-screened toward the bulk. In contrast, the anionic "spacer" layer shows a gap at the Fermi level, both on the surface and in the bulk; that is, it is not surface-doped due to iodine desorption. The well-screened surface dipole implies that a buried edge state, probably already below a single spacer layer, is located at the Fermi level. Consequently, a multilayer step covered by a spacer layer could provide access to the transport properties of the topological edge states. In addition, we find a lateral electronic modulation of the topologically nontrivial surface layer, which is traced back to the coupling with the underlying zigzag chain structure of the spacer layer.

Materials with giant spin splitting are desired for spintronic applications. The fabrications of spintronic devices from half metals with one spin direction are often hampered, however, by stray magnetic fields, domain walls, short spin coherence times, scattering on magnetic atoms or magnetically active interfaces, and other characteristics that come along with the magnetism. The surfaces of topological insulators, or Dirac/Weyl semimetals, could be an alternative, but production of high-quality thin films without the presence of the bulk states at the Fermi energy remains very challenging. Here, by utilizing angle-resolved photoemission spectroscopy, a record-high Dresselhaus spin-orbit splitting of the bulk state in the nonmagnetic IrBiSe is found. The band structure calculations indicate that the splitting band is fully spin-polarized with 3D chiral spin texture. As a source of spin-polarized electrons, lightly doped IrBiSe is expected to generate electric-field-controlled spin-polarized currents, free from back scattering, and could host triplet and Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconductivity.

We study the low-energy electronic structure of three-dimensional Dirac semimetal, ${\mathrm{Cd}}_{3}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}$ [$x=0$ and 0.34(3)], by employing angle-resolved photoemission spectroscopy (ARPES). We observe that the bulk Dirac states in ${\mathrm{Cd}}_{3}{({\mathrm{As}}_{0.66}{\mathrm{P}}_{0.34})}_{2}$ are gapped out with an energy of 0.23 eV, contrary to the parent ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ in which the gapless Dirac states have been observed. Thus, our results confirm the earlier predicted topological phase transition in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ with perturbation. We further notice that the critical P substitution concentration, at which the two Dirac points that are spread along the $c$-axis in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ form a single Dirac point at $\mathrm{\ensuremath{\Gamma}}$, is much lower [${x}_{c}(\mathrm{P})l0.34(3)$] than the predicted value of ${x}_{c}(\mathrm{P})=0.9.$ Therefore, our results suggest that the nontrivial band topology of ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ is remarkably sensitive to the P substitution and can only survive over a narrow substitution range, i.e., $0\ensuremath{\le}x(\text{P})l0.34(3)$.

Detailed knowledge of the gap function in iron-based superconductors is a requisite to identify the mechanism of superconductivity in these materials. Here, the authors present a systematic study of the superconducting gap distribution on the three-dimensional Fermi surfaces of FeSe using synchrotron-based angle-resolved photoemission spectroscopy. The results imply a significant anisotropy of the superconducting gap on all parts of the FeSe Fermi surface not only in-plane, but also as a function of ${k}_{z}$. The observed in-plane anisotropy can be qualitatively understood in terms of orbital-selective Cooper pairing and nematicity-induced mixing of $s$- and $d$-pairing channels.

The nonmagnetic compounds showing extremely large magnetoresistance are attracting a great deal of research interest due to their potential applications in the field of spintronics. ${\mathrm{PtBi}}_{2}$ is one of such interesting compounds showing large linear magnetoresistance (MR) in both the hexagonal and pyrite crystal structure. We use angle-resolved photoelectron spectroscopy and density functional theory calculations to understand the mechanism of liner MR observed in the layered ${\mathrm{PtBi}}_{2}$. Our results uncover linear dispersive surface Dirac states at the $\overline{\mathrm{\ensuremath{\Gamma}}}$ point, crossing the Fermi level with a node at a binding energy of $\ensuremath{\approx}900$ meV, in addition to the previously reported Dirac states at the $\overline{M}$ point in the same compound. We further notice from our dichroic measurements that these surface states show an asymmetric spectral intensity when measured with left and right circularly polarized light, hinting at a substantial spin polarization of the bands. Following these observations, we suggest that the linear dispersive Dirac states at the $\overline{\mathrm{\ensuremath{\Gamma}}}$ and $\overline{M}$ points are likely to play a crucial role for the linear field dependent magnetoresistance recorded in this compound.

Nematicity plays an important role in the physics of iron-based superconductors (IBS). Its microscopic origin and in particular its importance for the mechanism of high-temperature superconductivity itself are highly debated. A crucial knowledge in this regard is the degree to which the nematic order influences the electronic structure of these materials. Earlier angle-resolved photoemission spectroscopy (ARPES) studies found that the effect is dramatic in three families of IBS including 11, 111 and 122 compounds: energy splitting reaches 70 meV and Fermi surface becomes noticeably distorted. More recent experiments, however, reported significantly lower energy scale in 11 and 111 families, thus questioning the degree and universality of the impact of nematicity on the electronic structure of IBS. Here we revisit the electronic structure of undoped parent BaFe2As2 (122 family). Our systematic ARPES study including the detailed temperature and photon energy dependencies points to the significantly smaller energy scale also in this family of materials, thus establishing the universal scale of this phenomenon in IBS. Our results form a necessary quantitative basis for theories of high-temperature superconductivity focused on the nematicity., Comment: 14 pages 4 figures

One of the most unique and robust experimental facts about iron-based superconductors is the renormalization of the electronic band dispersion by factor of 3 and more near the Fermi level. Obviously related to the electron pairing, this prominent deviation from the band theory lacks understanding. Experimentally studying the entire spectrum of the valence electrons in iron arsenides, we have found an unexpected depletion of the spectral weight in the middle of the iron-derived band, which is accompanied by a drastic increase of the scattering rate. At the same time, the measured arsenic-derived band exhibits very good agreement with theoretical calculations. We show that the low-energy Fermi velocity renormalization should be viewed as a part of the modification of the spectral function by a strong electronic interaction. Such an interaction with an energy scale of the whole $d$ band appears to be a hallmark of many families of unconventional superconductors.

We present ARPES data taken from the structurally simplest representative of iron-based superconductors, FeSe, in a wide temperature range. Apart from the variations related to the nematic transition, we detect very pronounced shifts of the dispersions on the scale of hundreds of kelvins. Remarkably, upon warming the sample up, the band structure has a tendency to relax to the one predicted by conventional band structure calculations, right opposite to what is intuitively expected. Our findings shed light on the origin of the dominant interaction shaping the electronic states responsible for high-temperature superconductivity in iron-based materials., 8 pages, 4 figures, PDF only

a b s t r a c t We derive an effective quasiparticle tight-binding model which is able to describe with high accuracy the low-energy electronic structure of Sr2RuO4 obtained by means of low temperature angle resolved photo- emission spectroscopy. Such an approach is applied to determine the momentum and orbital dependent effective masses and velocities of the electron quasiparticles close to the Fermi level. We demonstrate that the model can provide, among the various computable physical quantities, a very good agreement with the experimentally measured specific heat coefficient and compares well with the plasma frequency estimated from local density calculations. Its use is underlined as a realistic input in the analysis of the possible electronic mechanisms related to the superconducting state of Sr2RuO4.

Iron-based superconductors have been found to exhibit an intimate interplay of orbital, spin, and lattice degrees of freedom, dramatically affecting their low-energy electronic properties, including superconductivity. Albeit the precise pairing mechanism remains unidentified, several candidate interactions have been suggested to mediate the superconducting pairing, both in the orbital and in the spin channel. Here, we employ optical spectroscopy (OS), angle-resolved photoemission spectroscopy (ARPES), ab initio band-structure, and Eliashberg calculations to show that nearly optimally doped NaFe$_{0.978}$Co$_{0.022}$As exhibits some of the strongest orbitally selective electronic correlations in the family of iron pnictides. Unexpectedly, we find that the mass enhancement of itinerant charge carriers in the strongly correlated band is dramatically reduced near the $\Gamma$ point and attribute this effect to orbital mixing induced by pronounced spin-orbit coupling. Embracing the true band structure allows us to describe all low-energy electronic properties obtained in our experiments with remarkable consistency and demonstrate that superconductivity in this material is rather weak and mediated by spin fluctuations., Comment: Open access article available online at http://www.nature.com/articles/srep18620

Recent breakthrough in search for the analogs of fundamental particles in condensed matter systems lead to experimental realizations of 3D Dirac and Weyl semimetals. Weyl state can be hosted either by non-centrosymmetric or magnetic materials and can be of the first or the second type. Several non-centrosymmetric materials have been proposed to be type-II Weyl semimetals, but in all of them the Fermi arcs between projections of multiple Weyl points either have not been observed directly or they were hardly distinguishable from the trivial surface states which significantly hinders the practical application of these materials. Here we present experimental evidence for type-II non-centrosymmetric Weyl state in TaIrTe$_4$ where it has been predicted theoretically. We find direct correspondence between ARPES spectra and calculated electronic structure both in the bulk and the surface and clearly observe the exotic surface states which support the quasi-1D Fermi arcs connecting only four Weyl points. Remarkably, these electronic states are spin-polarized in the direction along the arcs, thus highlighting TaIrTe$_4$ as a novel material with promising application potential., Comment: 9 pages, 4 figures

The growing significance of the precise spectroscopic tools in condensed matter physics is to a very considerable degree indebted to the recent advances in instrumentation. Angle-resolved photoemission has strongly benefitted from the introduction of angle-multiplexing analyzers at the end of last century [1], which virtually turned the technique from a band-mapping tool into a sophisticated many-body spectroscopy. It was soon realized that in order to deliver detailed and undistorted information about the spectral function – the ultimate output of ARPES – one needs to thoroughly explore the parameter space of the experiment. Assuming that all the photoelectrons are detected, these parameters are basically the photon energy and angles of incidence of the linearly polarized light. It is the combinations of these quantities which can strongly suppress the photocurrent from a given sample via the matrix element effects. For a modern ARPES experiment it is therefore desired to have a full control of photon en...

Large and high quality single crystals of the new unconventional superconductor LiFeAs were grown by a new approach using the self-flux technique. Both energy dispersive X-ray spectroscopy and inductively coupled plasma mass spectroscopy revealed a stoichiometric Li/Fe/As composition. Measurements of the magnetic susceptibility reveal the superconducting transition at TC = 17 K with a very sharp ΔTC and a 100% shielding fraction and, thus, bulk superconductivity. This sharp transition is also found by measurements of the specific heat and by measurements of the temperature dependence of the resistivity. Nuclear quadrupole resonance (NQR) spectroscopy reveals a very sharp resonance line, with a much smaller line width than reported for all other FeAs superconductors, confirming the high ordering of the LiFeAs single crystals also on a local scale.

Angle resolved photoemission spectroscopy (ARPES) has been playing a crucial role in understanding of physics behind high-temperature superconductivity. Our ARPES investigation of superconducting cuprates, performed over a decade and accomplished by very recent results, suggests a consistent view of electronic interactions in cuprates which provides natural explanation of both the origin of the pseudogap state and the mechanism for high-temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. In view of these results and their projection to numerous other materials, two general questions are arising: is the normal state in 2D metals ever stable and how does this intrinsic instability interplay with superconductivity?

We report a comprehensive study of the tridimensional nature and orbital character of the low-energy electronic structure in 50% Cobalt doped Ba(Fe1−xCox)2As2 (d6.5), by using polarization- and photon energy-dependent angle-resolved photoemission spectroscopy. An extra electron-like Fermi surface is observed around the Brillouin zone boundary compared with isoelectronic KyFe2−xSe2 (d6.5). The bands near the Fermi level (EF) are mainly derived from Fe/Co 3d t2g orbitals, revealing visible dispersions along the kz direction. In combination with the local density approximation and the dynamical mean-field theory calculations, we find that the As 4p bands are non-renormalized and the whole 3d band needs to be renormalized by a “single” factor of ∼1.6, indicating moderate electronic correlation effects. The “single” factor description of the correlation strength among the different 3d orbitals is also in sharp contrast to orbital-dependent correlation effects in BaFe2As2. Our findings indicate a remarkable reduction of correlation effects with little difference among 3d orbitals in BaFeCoAs2, due to the increased filling of the electronic 3d shell in the presence of significant Hund's coupling. The results support that the electronic correlation effects and multiple orbital physics play an important role in the superconductivity of the 122 system and in other ferropnictides.

Here we apply high resolution angle-resolved photoemission spectroscopy (ARPES) using a wide excitation energy range to probe the electronic structure and the Fermi surface topology of the Ba 1− x K x Fe 2 As 2 ( T c = 32 K) superconductor. We find significant deviations in the low energy band structure from that predicted in calculations. A set of Fermi surface sheets with unexpected topology is detected at the Brillouin zone boundary. At the X -symmetry point the Fermi surface is formed by a shallow electron-like pocket surrounded by four hole-like pockets elongated in Γ − X and Γ − Y directions.

In the family of the iron-based superconductors, the $RE$FeAsO-type compounds (with $RE$ being a rare-earth metal) exhibit the highest bulk superconducting transition temperatures ($T_{\mathrm{c}}$) up to $55\ \textrm{K}$ and thus hold the key to the elusive pairing mechanism. Recently, it has been demonstrated that the intrinsic electronic structure of SmFe$_{0.92}$Co$_{0.08}$AsO ($T_{\mathrm{c}}=18\ \textrm{K}$) is highly nontrivial and consists of multiple band-edge singularities in close proximity to the Fermi level. However, it remains unclear whether these singularities are generic to the $RE$FeAsO-type materials and if so, whether their exact topology is responsible for the aforementioned record $T_{\mathrm{c}}$. In this work, we use angle-resolved photoemission spectroscopy (ARPES) to investigate the inherent electronic structure of the NdFeAsO$_{0.6}$F$_{0.4}$ compound with a twice higher $T_{\mathrm{c}}=38\ \textrm{K}$. We find a similarly singular Fermi surface and further demonstrate that the dramatic enhancement of superconductivity in this compound correlates closely with the fine-tuning of one of the band-edge singularities to within a fraction of the superconducting energy gap $\Delta$ below the Fermi level. Our results provide compelling evidence that the band-structure singularities near the Fermi level in the iron-based superconductors must be explicitly accounted for in any attempt to understand the mechanism of superconducting pairing in these materials., Comment: Open access article available online at http://www.nature.com/articles/srep18273

Single crystals of ${\mathrm{Na}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{T}}_{x}\mathrm{As}$ with T = Co, Rh have been grown using a self-flux technique. The crystals were thoroughly characterized by powder x-ray diffraction, magnetic susceptibility, and electronic transport with particular focus on the Rh-doped samples. Measurements of the specific heat and ARPES were conducted exemplarily for the optimally doped compositions. The spin-density wave transition (SDW) observed for samples with low Rh concentration $(0\ensuremath{\le}x\ensuremath{\le}0.013)$ is fully suppressed in the optimally doped sample. The superconducting transition temperature $({T}_{c})$ is enhanced from 10 K in ${\mathrm{Na}}_{1\ensuremath{-}\ensuremath{\delta}}\mathrm{FeAs}$ to 21 K in the optimally doped sample $(x=0.019)$ of the ${\mathrm{Na}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}\mathrm{As}$ series and decreases for the overdoped compounds, revealing a typical shape for the superconducting part of the electronic phase diagram. Remarkably, the phase diagram is almost identical to that of Co-doped ${\mathrm{Na}}_{1\ensuremath{-}\ensuremath{\delta}}\mathrm{FeAs}$, suggesting a generic phase diagram for both dopants.

In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc ≈ 55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-based compounds and has grown to be generally accepted in the field. In this work we show that a prototypical compound of the 1111-type, SmFe0.92Co0.08AsO , is at odds with this description and possesses a distinctly different Fermi surface, which consists of two singular constructs formed by the edges of several bands, pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. Such singularities dramatically affect the low-energy electronic properties of the material, including superconductivity. We further argue that occurrence of these singularities correlates with the maximum superconducting transition temperature attainable in each material class over the entire family of iron-based superconductors., Scientific Reports, 5, ISSN:2045-2322

Angle-resolved photoemission spectroscopy (ARPES) is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictides systems. Our ARPES results show linear-in-energy dependent scattering rates which are constant in a wide range of control parameter and which depend on the orbital character of the bands. We demonstrate that the linear energy dependence gives rise to weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. In the superconducting phase the related small effective Fermi energy favors a Bardeen-Cooper-Schrieffer (BCS)\,\cite{Bardeen1957}-Bose-Einstein (BE)\,\cite{Bose1924} crossover state., Comment: 5 pages, 4 figures Supplement 4 pages, 6 figures

While the pronounced doping dependence of the quasiparticle spectral weight in the antinodal region of the superconducting cuprates, as seen by ARPES, unambiguously points to the magnetic origin of the strong electron-boson coupling there, the nature of the electron scattering in the nodal direction remained unclear. Here we present a short review of our recent detailed investigations of the nodal direction of Bi-2212. Our findings prove the existence of well defined quasiparticles even in the pseudogap state and show that the essential part of the quasiparticle scattering rate, which appears on top of Auger-like electron-electron interaction, also implies a magnetic origin., 6 revtex pages, short review, presented at SNS 2004

We report a systematic investigation of the photon energy (hm =1 8� 29.5 eV) dependence of the near-EF photoemission intensity in Pb-doped and pristine Bi2Sr2CaCu2O8+d high-temperature superconductors. We observe variations in the (p, 0)-spectras line shape which are in good agreement with previous data and can be explained in terms of the different excitation probability (matrix elements) for each of the bilayer split bands. Moreover, we show that the photon energy dependence in the momentum distribution of the photocurrent at the Fermi level supports this interpretation. Similar trends in the momentum distribution maps of the pristine compound strongly suggest that the bilayer splitting is present also in near-optimally doped samples. A quantitative analysis of the (p, 0)-spectra in the superconducting state reveals a maximum of the bonding/antibonding ratio at hm =2 0� 22 eV. The photon energy dependence of the (p, p)spectra, which are good representatives of the incoherent background, is also discussed.

Using the angle-resolved photoemission spectroscopy and band structure calculations we study the electronic structure of KFeCoAs2, which is isoelectronic to the parent material of 122 series of iron-based superconductors BaFe2As2. Although band structure calculations predict nearly identical dispersions of the electronic states in both compounds, experiment reveals drastic differences in both the global renormalization and Fermi surfaces. On the basis of the comparison of electronic structures of these two isoelectronic compounds, we demonstrate local magnetic correlations as a vital role for the peculiar low-energy electron dynamics of iron-based superconductors.

We use angle-resolved photoemission spectroscopy to study bandwidth renormalization in layered superconductor 2H-NbSe2. Renormalization of the conduction band in comparison with the very similar 2H-TaSe2 is of the factor of two. We discuss the possibility that the Van Hove singularity could be responsible for this effect not only in dichalcogenides but also in pnictides. (C) 2010 Elsevier Ltd. All rights reserved.

We present an ARPES study of the surface states of Ru$_2$Sn$_3$, a new type of a strong 3D topological insulator (TI). In contrast to currently known 3D TIs, which display two-dimensional Dirac cones with linear isotropic dispersions crossing through one point in the surface Brillouin Zone (SBZ), the surface states on Ru$_2$Sn$_3$ are highly anisotropic, displaying an almost flat dispersion along certain high-symmetry directions. This results in quasi-one dimensional (1D) Dirac electronic states throughout the SBZ that we argue are inherited from features in the bulk electronic structure of Ru$_2$Sn$_3$, where the bulk conduction bands are highly anisotropic. Unlike previous experimentally characterized TIs, the topological surface states of Ru$_2$Sn$_3$ are the result of a d-p band inversion rather than an s-p band inversion. The observed surface states are the topological equivalent to a single 2D Dirac cone at the surface Brillouin zone, 4 figures, includes supplementary information

Using the angle-resolved photoemission spectroscopy (ARPES) with resolution of all three components of electron momentum and electronic states symmetry, we explicate the electronic structure of hole-doped BaFe2As2, and show that widely discussed nesting and dimensionality of Fermi surface (FS) sheets have no immediate relation to the superconducting pairing in iron-based superconductors. Alternatively a clear correlation between the orbital character of the electronic states and their propensity to superconductivity is observed: The magnitude of the superconducting gap maximizes at 10 meV exclusively for iron 3d(xz, yz) orbitals, while for others drops to 3 meV. Presented results imply that the relation between superconducting and magnetostructural transitions goes beyond simple competition for FS, and demonstrate importance of orbital physics in iron superconductors.

Spin-orbit coupling (SOC) is a fundamental interaction in solids which 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. Using angle-resolved photoemission spectroscopy we directly observe a sizeable spin-orbit splitting in all main families of IBS. We demonstrate that its impact on the low-energy electronic structure and details of the Fermi surface topology is much stronger than that of possible nematic ordering. Intriguingly, the largest pairing gap is always supported exactly by SOC-induced Fermi surfaces., Comment: Significant spin-orbit coupling is observed in all families of IBS. Figures 3 and 4 are new. Supplementary information is extended

We analyze the structure of the pairing interaction and superconducting gap in LiFeAs by decomposing the pairing interaction for various kz cuts into s- and d-wave components and by studying the leading superconducting instabilities. We use the ten orbital tight-binding model, derived from ab-initio LDA calculations with hopping parameters extracted from the fit to ARPES experiments. We find that the pairing interaction almost decouples between two subsets, one consists of the outer hole pocket and two electron pockets, which are quasi-2D and are made largely out of dxy orbital, and the other consists of the two inner hole pockets, which are quasi-3D and are made mostly out of dxz and dyz orbitals. Furthermore, the bare inter-pocket and intra-pocket interactions within each subset are nearly equal. In this situation, small changes in the intra-pocket and inter-pocket interactions due to renormalizations by high-energy fermions give rise to a variety of different gap structures. We find four different configurations of the s-wave gap immediately below Tc: the one in which superconducting gap changes sign between two inner hole pockets and between the outer hole pocket and two electron pockets, the one in which the gap changes sign between two electron pockets and three hole pockets, the one in which the gap on the outer hole pocket differs in sign from the gaps on the other four pockets, and the one in which the gaps on two inner hole pockets have one sign, and the gaps on the outer hole pockets and on electron pockets have different sign. Different s-wave gap configurations emerge depending on whether the renormalized interactions increase attraction within each subset or increase the coupling between particular components of the two subsets. We argue that the state with opposite sign of the gaps on the two inner hole pockets has the best overlap with ARPES data., Comment: 23 pages, 15 figures

We report studies of the electronic structure and elementary excitations of doped and undoped cuprate chains, ladders and planes. Using high energy spectroscopies such as x-ray absorption, core level photoemission and angle resolved photoemission spectroscopy, important information regarding the charge distribution and hole dynamics can be obtained. The comparison of the experimental data with suitable theoretical models sets constraints on the parameters entering into the model calculations, and offers insight into the important physical quantities governing the electronic structure of these materials. Recurring themes include the importance of the dimensionality of the Cu-O network (1D->2D) and the crucial role played by the spin-background in determining the dynamics of low-lying excitations in these strongly correlated systems., J. Electron Spec. Relat. Phenom.: special issue on electron correlation, in press