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In low-dimensional electron systems, charge density waves (CDW) and superconductivity are two of the most fundamental collective quantum phenomena. For all known quasi-two-dimensional superconductors, the origin and exact boundary of the electronic orderings and superconductivity are still attractive problems. Through transport and thermodynamic measurements, we report on the field-temperature phase diagram in 2H-TaS2 single crystals. We show that the superconducting transition temperature (Tc) increases by one order of magnitude from temperatures at 0.98 K up to 9.15 K at 8.7 GPa when the Tc becomes very sharp. Additionally, the effects of 8.7 GPa illustrate a suppression of the CDW ground state, with critically small Fermi surfaces. Below the Tc the lattice of magnetic flux lines melts from a solid-like state to a broad vortex liquid phase region. Our measurements indicate an unconventional s-wave-like picture with two energy gaps evidencing its multi-band nature.

The electronic structure of FeSe, the simplest iron-based superconductor (Fe-SC), conceals a potential of dramatic increase of Tc that realizes under pressure or in a single layer film. This is also the system where nematicity, the phenomenon of a keen current interest, is most easy to study since it is not accompanied by the antiferomagnetic transition like in all other Fe-SC’s. Here we overview recent experimental data on electronic structure of FeSe-based superconductors: isovalently doped crystals, intercalates, and single layer films, trying to clarify its topology and possible relation of this topology to superconductivity. We argue that the marked differences between the experimental and calculated band structures for all FeSe compounds can be described by a hoping selective renormalization model for a spin/orbital correlated state that may naturally explain both the evolution of the band structure with temperature and nematicity.

Angle resolved photoemission spectroscopy (ARPES) reveals the features of the electronic structure of quasi-two-dimensional crystals, which are crucial for the formation of spin and charge ordering and determine the mechanisms of electron-electron interaction, including the superconducting pairing. The newly discovered iron based superconductors (FeSC) promise interesting physics that stems, on one hand, from a coexistence of superconductivity and magnetism and, on the other hand, from complex multi-band electronic structure. In this review I want to give a simple introduction to the FeSC physics, and to advocate an opinion that all the complexity of FeSC properties is encapsulated in their electronic structure. For many compounds, this structure was determined in numerous ARPES experiments and agrees reasonably well with the results of band structure calculations. Nevertheless, the existing small differences may help to understand the mechanisms of the magnetic ordering and superconducting pairing in FeSC.

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?

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.

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

In many cases the standard perturbation approach appears to be too simple to describe precisely the angle resolved photoemission spectrum of strongly correlated electron system. In particular, to describe the momentum asymmetry observed in photoemission spectra of high-Tc cuprates a phenomenological approach based on extremely correlated Fermi-liquid model has been recently introduced. In this paper we analyze the general structure of the Green function of quasiparticles in strongly correlated electron systems and stress that it is defined not only by the self-energy of Hubbard quasiparticles but also by a strength operator. We show that the later leads to an additional odd momentum contribution to the spectral function and alone can explain the observed asymmetry. So, the asymmetry of the ARPES spectra can be a measure of the strength of electron correlations in materials.

The formation of superconducting junctions between MT-YBCO using TmBa2Cu3O7−δ powder as a solder has been studied. The method proposed excludes the step of a very slow coolin g( at ar ate of several degrees per hour) during seam formation. The heating and cooling rate for joining parts produced from single-domain material without visible cracks (macrocracks) can be rather high (500–1000 K h −1 )a nd aholding time at the highest temperature (1010 ◦ C) of several minutes (0.05 h) is enough to form a reliable junction. Reasonable rates of heating and cooling are however around 100 K h −1 if crack propagation is to be avoided in joined blocks used for practical application. Modelling experiments on rings and studies of the ring properties by vibrating sample magnetometer (VSM), field mapping with a Hall probe and magneto-optical microscopy have shown that superconducting properties of the junction were not lower than that of the joined material (jc of about 30 kA cm −2 was observed in zero field at 77 K) an dt hat the proposed process of joining did not adversely affect the properties of the material. The structure of the resulting junction was in good agreement with the structure of MT-YBCO.

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.

Superconducting (SC) properties of joined (soldered using TmBa 2 Cu 3 O 7− δ powder) melt-textured YBCO samples have been investigated by vibrating sample magnetometer (VSM), magneto-optical technique and field mapping (from which the map of critical current density distribution throughout the sample has been calculated and plotted). The critical current densities j c calculated from the field mapping and the values obtained from the VSM data are in good agreement and give us grounds to conclude that j c through the seam has been higher than 3.4 × 10 4 A/cm 2 in zero field at 77 K. All methods used for SC properties characterization have shown that current flows homogeneously through the seam and bulk material and made us sure that in soldering process the superconductive properties of the joined material were not deteriorated.

The discovery of high temperature superconductors (HTS) has led to understanding that, in order to explain and utilize the phenomenon, completely new physical approaches should be introduced at all scales: microscopic, mesoscopic, macroscopic. Leaving first two scales beyond the scope of the present paper we focus in the upper limit of the last one, the study of the magnetic flux dynamics in HTS bulks - the dynamics of the 'compact vortex structures'. A new direction in the experimental superconducting physics, the investigation of HTS bulks with levitation techniques, which has been elaborated during last years to effectively explore the subject, as well as the new fundamental and applied results obtained therefrom are overviewed here.

The discovery of high-temperature superconductors (HTS) has led to understanding that, in order to explain and utilize the phenomenon, completely new physical approaches should be introduced at all scales: microscopic, mesoscopic, macroscopic. Leaving first two scales beyond the scope of the present paper, we focus in the upper limit of the last scale, the study of the magnetic flux dynamics in HTS bulks—the dynamics of the ‘compact vortex structures’. A new direction in the experimental superconducting physics, the investigation of HTS bulks with levitation techniques, which has been elaborated during last years to effectively explore the subject, as well as the new fundamental and applied results obtained there from are overviewed here. Відкриття високотемпературних надпровідників (ВТНП) призвело до розуміння того, що задля пояснення та використання цього явища на всіх рівнях: мікроскопічному, мезоскопічному та макроскопічному, має бути створено принципово нові фізичні підходи. Полишаючи перші два рівні поза увагою даної роботи, ми детально розглядаємо верхню границю останнього — дослідження динаміки магнітного потоку в масивних ВТНП, динаміки «компактових вихорових структур». Дана робота — це огляд нового напрямку експериментальної фізики надпровідників — дослідження масивних ВТНП левітаційними методами — що його було вироблено останнім часом для вивчення зазначеної проблеми, а також тих нових фундаментальних та практичних результатів, що було тут отримано Открытие высокотемпературных сверхпроводников (ВТСП) привело к пониманию того, что для объяснения и использования этого явления необходимо создание принципиально новых физических подходов на всех уровнях: микроскопическом, мезоскопическом, макроскопическом. Оставляя первые два уровня за рамками данной работы, мы сфокусировали внимание на верхнем пределе последнего — исследовании динамики магнитного потока в массивных ВТСП, динамики «компактных вихревых структур». Данная работа — это обзор нового направления экспериментальной физики сверхпроводников, которое было выработано за последние годы, а также тех новых фундаментальных и прикладных результатов, которые были здесь получены.

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.

Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field of a single crystal under static magnetic fields parallel to the axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic -wave-like gap or by a two-gap model, while a d-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound, with the same Tc. We estimated the range of the large gap value 6-8 meV (depending on small variation of and its a space anisotropy of about 30%, and the small gap 1.7 meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless s-wave two gaps., 9 pages, 6 figures

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.

Angle resolved photoemission spectroscopy (ARPES) enables direct observation of the Fermi surface and underlying electronic structure of crystals---the basic concepts to describe all the electronic properties of solids and to understand the key electronic interactions involved. The method is the most effective to study quasi-2D metals, to which the subjects of almost all hot problems in modern condensed matter physics have happened to belong. This has forced incredibly the development of the ARPES method which we face now. The aim of this paper is to introduce to the reader the state-of-the-art ARPES, reviewing the results of its application to such topical problems as high temperature superconductivity in cuprates and iron based superconductors, and electronic ordering in the transition metal dichalcogenides and manganites., Comment: Invited Review to the Special Issue of Low Temp. Phys., for more details see http://www.imp.kiev.ua/~kord

The Tm-Ba-Cu-O solder can be successfully used to produce a superconductive joint between MT-YBCO parts. The peculiarities of solidification, phase formation, structure transformations and electromagnetic properties of MT-YBCO soldered with TmBa2Cu3O7-d are discussed., Comment: PS of 6 pages text and 5 figures, presented at ICMC'2000, Brasil

A method of reconstruction of critical current profiles from measurements of a hysteretic AC loss is proposed. As an example, we have applied this approach to study the degradation in the bulk melt-textured HTS sample with a resonance oscillations technique developed earlier for granular HTS samples and now modified to study HTS bulks. The critical current density profile was determined after the degradation in the thin undersurface layer of about 60 μm.

In this study, we investigate the electronic and magnetic properties of Rb${}_{0.77}$Fe${}_{1.61}$Se${}_{2}$ (${T}_{\mathrm{c}}\phantom{\rule{0.16em}{0ex}}$ $=$ 32.6 K) in normal and superconducting states by means of photoemission and $\ensuremath{\mu}$SR spectroscopies as well as band-structure calculations. We demonstrate that the unusual behavior of these materials is the result of separation into metallic ($\ensuremath{\sim}$$12%$) and insulating ($\ensuremath{\sim}$$88%$) phases. Only the former becomes superconducting and has a usual electronic structure of electron-doped FeSe slabs. Our results thus imply that the antiferromagnetic insulating phase is just a by-product of Rb intercalation and its magnetic properties have no direct relation to the superconductivity. Instead, we find that also in this class of iron-based compounds, the key ingredient for superconductivity is a certain proximity of a Van Hove singularity to the Fermi level.

The iron based superconductors (FeSC) promise interesting physics and applications, and, while the interplay of superconductivity and magnetism, as well as their mechanisms remain the issues of active debates and studies, one thing in FeSC puzzle is clear, namely that it is the complex multi-band electronic structure of these compounds that determines their rich and puzzling properties [1, 2]. What is important and fascinating is that this complexity seems to play a positive role in the struggle for understanding the FeSC physics and also for search of the materials with higher Tc's. This is because the multiple electronic bands and resulting complex Fermiology offer exceptionally rich playground for establishing useful empirical correlations. This is also because this electronic structure is well understood-the band structure calculations well reproduce its complexity: all the bands and their symmetry. The role of the experiment, in this case, is just to define the exact position and renormalization for each band. This piece of experimental knowledge, however, appears to be vitally important for understanding of the electronic properties of these new compounds.

Iron-based high-temperature superconductors form an in-creasingly growing subject for investigation. Unlike othertypes of high-temperature superconductors, iron-based com-pounds can be synthesized in the form of various crystals, ex-hibiting a large variety of electronic band structures, magneticproperties, superconducting order parameters, and electronicproperties in general.

superconductor. Our results show a maximum gap of � = 3.3 ± 0.3 meV on hole pockets and of 2.9 ± 0.3 meV on electron pockets. From temperature dependent ARPES measurements we show that the SC gap closes above the transition temperature (Tc), suggesting an absence of pseudogaps in these compounds. Our results clearly demonstrate that the gap evolution with temperature follows the BCS gap function near the � , Z, and M high symmetry points. From photon energy dependent measurements we also observe an almost isotropic superconductivity along the kz direction in the momentum space.

Using the angle-resolved photoemission spectroscopy (ARPES) data accumulated over the whole Brillouin zone (BZ) in LiFeAs we analyze the itinerant component of the dynamic spin susceptibility in this system in the normal and superconducting state. We identify the origin of the incommensurate magnetic inelastic neutron scattering (INS) intensity as scattering between the electron pockets, centered around the $(\pi,\pi)$ point of the BZ and the large two-dimensional hole pocket, centered around the $\Gamma$-point of the BZ. As the magnitude of the superconducting gap within the large hole pocket is relatively small and angle dependent, we interpret the INS data in the superconducting state as a renormalization of the particle-hole continuum rather than a true spin exciton. Our comparison indicates that the INS data can be reasonably well described by both the sign changing symmetry of the superconducting gap between electron and hole pockets as well as sign preserving gap, depending on the assumptions made for the fermionic damping., Comment: 7 pages, 5 figures

The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (ω) and momentum (k) and the probability to find them in a particular state with given ω and k is described by the spectral function A(k, ω). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample. In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He(3) cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr2RuO4 and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.

Single crystals of Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ with $x$ = 0, 0.25, 0.35, 0.4 were grown using a self-flux high temperature solution growth technique. The superconducting and normal state properties were studied by temperature dependent magnetic susceptibility, electrical resistivity and specific heat revealing that the magnetic and structural transition is rapidly suppressed upon Na-substitution at the Ba-site in BaFe$_2$As$_2$, giving rise to superconductivity. A superconducting transition as high as 34 K is reached for a Na-content of $x$=0.4. The positive Hall coefficient confirms that the substitution of Ba by Na results in hole-doping similarly to the substitution of Ba by K. Angle resolved photoemission spectroscopy was performed on all Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ crystals. The Fermi surface of hole-doped Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ is to high extent the same as the Fermi surface found for the K-doped sister compounds, suggesting a similar impact of the substitution of Ba by either K or Na on the electronic band dispersion at the Fermi level

Line-shape analysis of the photoemission intensity from the surface states of Bi2Se3 reveals two unusual features: spectral line asymmetry and anomalously enhanced photoemission from the Dirac point. The former can be described by the one-particle spectral function assuming that the self-energy has an energy-momentumdependent contribution to the imaginary part, which changes sign when crossing both the dispersion curves and the energy of the Dirac point. The anomalous enhancement can hardly be understood as a self-energy effect, while a final-state interference seems to be a more plausible explanation.

This work was supported by the DFG priority program SPP1458, Grants No. KN393/4, BO1912/2-1, and No. BO3537/1-1 (D.S.I. and J.T.P.); by the Slovak Research and Development Agency under Contract No. VVCE-0058-07, Slovak VEGA Grants No. 0148/10 and No. 1/0138/10, and the 7th FP MNT-ERA.Net II. ESO (T.S., P.S., J.G.R, and P.S.); by the Spanish MEC under projects Consolider Ingenio Molecular Nanoscience CSD2007-00010 and FIS2008-00454 (J.G.R.); and by the Korean government (MEST) Grants No. R15-2008-006-01002-0 and 2011-0028736 (N.H.S., B.K.C.)

The electronic structure of SrPd2Ge2 single crystals is studied by angle-resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS), and band structure calculations within the local-density approximation (LDA). The STS measurements show a single s-wave superconducting energy gap (0) = 0.5 meV. The photon-energy dependence of the observed Fermi surface reveals a strongly three-dimensional character of the corresponding electronic bands. By comparing the experimentally measured and calculated Fermi velocities a renormalization factor of 0.95 is obtained, which is much smaller than typical values reported in Fe-based superconductors. We ascribe such an unusually low band renormalization to the different orbital character of the conduction electrons and, using ARPES and STS data, argue that SrPd2Ge2 is likely to be a conventional superconductor, which makes it clearly distinct from isostructural iron pnictide superconductors of the “122” family, This work was supported by the DFG priority program SPP1458, Grants No. KN393/4, BO1912/2-1, and No. BO3537/1-1 (D.S.I. and J.T.P.); by the Slovak Research and Development Agency under Contract No. VVCE-0058-07, Slovak VEGA Grants No. 0148/10 and No. 1/0138/10, and the 7th FP MNT-ERA.Net II. ESO (T.S., P.S., J.G.R, and P.S.); by the Spanish MEC under projects Consolider Ingenio Molecular Nanoscience CSD2007-00010 and FIS2008-00454 (J.G.R.); and by the Korean government (MEST) Grants No. R15-2008-006-01002-0 and 2011-0028736 (N.H.S., B.K.C.)

Using the angle-resolved photoemission spectroscopy data accumulated over the whole Brillouin zone (BZ) in LiFeAs, we analyze the itinerant component of the dynamic spin susceptibility in this system in the normal and superconducting state. We identify the origin of the incommensurate magnetic inelastic neutron scattering (INS) intensity as scattering between the electron pockets, centered around the (π,π) point of the BZ, and the large two-dimensional hole pocket, centered around the Γ point of the BZ. As the magnitude of the superconducting gap within the large hole pocket is relatively small and angle dependent, we interpret the INS data in the superconducting state as a renormalization of the particle-hole continuum rather than a true spin exciton. Our comparison indicates that the INS data can be reasonably well described by both the sign-changing symmetry of the superconducting gap between electron and hole pockets and the sign-preserving gap, depending on the assumptions made for the fermionic damping. © 2012 American Physical Society.

This report proposes an approximate approach to setting normalized standards for determining the effect of industrial enterprises on the environment. It includes emissions and discharges of toxic and radioactive substances, separately or combined. The proposal is based on the requirement that the normal state of a region, with respect to man, flora, and fauna, must be preserved. Several values and algorithms for determining values of normal environmental states are suggested. 19 refs.

Superconductive junctions between superconductive melt-textured YBa 2 Cu 3 O 7-δ blocks (MT-YBCO), that are practically invisible in a polarized light, have been made using a TmBa 2 Cu 3 O 7-δ powder, which is close to materials being joint in superconductive and mechanical properties (microhardness and bending strength). By SEM study we have found that in the best seams Tm has been present in a thin (about 40-50 μm) layer along the seam.

The onset of superconductivity at the transition temperature is marked by the onset of order, which is characterized by an energy gap. Most models of the iron-based superconductors find a sign-changing (s\pm) order parameter [1-6], with the physical implication that pairing is driven by spin fluctuations. Recent work, however, has indicated that LiFeAs has a simple isotropic order parameter [7-9] and spin fluctuations are not necessary [7,10], contrary to the models [1-6]. The strength of the spin fluctuations has been controversial [11,12], meaning that the mechanism of superconductivity cannot as yet be determined. Here we report the momentum dependence of the superconducting energy gap, where we find an anisotropy that rules out coupling through spin fluctuations and the sign change. The results instead suggest that orbital fluctuations assisted by phonons [13,14] are the best explanation for superconductivity., SI added, references re-arranged, 14 pages, PDF format, Related work: http://www.ifw-dresden.de/institutes/iff/research/SC/arpes

Single crystals of the new unconventional superconductor LiFe${}_{1\ensuremath{-}x}$Co${}_{x}$As with $x$$=$ 0, 0.025, 0.05 were grown by a new approach using the self-flux technique. The superconducting transition temperature was found to decrease upon Co doping at the Fe site. Apparently, in LiFeAs this doping scheme suppresses superconductivity, in contrast to the effects of Co doping in other Fe-As compounds, where it suppresses the spin-density wave and establishes superconductivity. Angle-resolved photoemission spectroscopy shows that the bottom of electron-like bands sinks by about 17 meV upon $5%$ Co doping, which indicates that the chemical substitution of Co for Fe in LiFeAs results in charge doping.