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An essential step toward elucidating the mechanism of superconductivity is to determine the sign or phase of the superconducting order parameter, as it is closely related to the pairing interaction. In conventional superconductors, the electron-phonon interaction induces attraction between electrons near the Fermi energy and results in a sign-preserved s-wave pairing. For high-temperature superconductors, including cuprates and iron-based superconductors, prevalent weak coupling theories suggest that the electron pairing is mediated by spin fluctuations which lead to repulsive interactions, and therefore that a sign-reversed pairing with an s_{±} or d-wave symmetry is favored. Here, by using magnetic neutron scattering, a phase sensitive probe of the superconducting gap, we report the observation of a transition from the sign-reversed to sign-preserved Cooper-pairing symmetry with insignificant changes in T_{c} in the S-doped iron selenide superconductors K_{x}Fe_{2-y}(Se_{1-z}S_{z})_{2}. We show that a rather sharp magnetic resonant mode well below the superconducting gap (2Δ) in the undoped sample (z=0) is replaced by a broad hump structure above 2Δ under 50% S doping. These results cannot be readily explained by simple spin fluctuation-exchange pairing theories and, therefore, multiple pairing channels are required to describe superconductivity in this system. Our findings may also yield a simple explanation for the sometimes contradictory data on the sign of the superconducting order parameter in iron-based materials.

All superconductors in a magnetic field are characterized by three critical magnetic fields: lower critical H c 1 , upper critical H c 2 and thermodynamic critical field H c . Only two sets of inequalities H c 2 > H c > H c 1 or H c 1 > H c > H c 2 are possible in a single-component superconductor. Here, we report our study of the critical fields in multicomponent superconductors with two superconducting components in the framework of the Ginzburg-Landau functional. We derive the relationship between the phases of the components of the superconducting complex order parameter from the charge conservation law in explicit form and insert it into the Ginzburg-Landau functional. Using the modified Ginzburg-Landau equation, we acquire the single vortex state including the analytical expression for asymptotics. Also, we obtain the analytical form for the state in the upper critical field. We find that in some cases an unusual sequence of critical fields H c 1 , H c 2 > H c can be realized in multicomponent superconductors., (© The Author(s) 2023.)

Despite decades of intensive experimental and theoretical efforts, the physics of cuprate high-temperature superconductors in general, and, in particular, their normal state, is still under debate. Here, we report our investigation of low-energy charge excitations in the normal state. We find that the peculiarities of the electronic band structure at low energies have a profound impact on the nature of the intraband collective modes. It gives rise to a new kind of mode with huge intensity and non-Lorentzian spectral function in addition to well-known collective excitations like conventional plasmons and spin fluctuation. We predict two such modes with maximal spectral weight in the nodal and antinodal directions. Additionally, we found a long-living quasi-one-dimensional plasmon becoming an intense soft mode over an extended momentum range along the antinodal direction. These modes might explain some of the resonant inelastic X-ray scattering spectroscopy data.

Layered van der Waals materials of the family TaTMTe 4 (TM = Ir, Rh, Ru) are showing interesting electronic properties. We report the growth and characterization of TaIrTe 4 , TaRhTe 4 , TaIr 1- x Rh x Te 4 ( x = 0.06, 0.14, 0.78, 0.92), Ta 1+ x Ru 1- x Te 4 single crystals. X-ray powder diffraction confirms that TaRhTe 4 is isostructural to TaIrTe 4 . All these compounds are metallic with diamagnetic behavior. Below T ≈ 4 K we observed signatures of the superconductivity in the TaIr 1- x Rh x Te 4 compounds for x = 0.92. All samples show weak quadratic-in-field magnetoresistance (MR). However, for TaIr 1- x Rh x Te 4 with x ≈ 0.78, the MR has a linear term dominating in low fields that indicates the presence of Dirac cones in the vicinity of the Fermi energy. For TaRhTe 4 series the MR is almost isotropic. Electronic structure calculations for TaIrTe 4 and TaRhTe 4 reveal appearance of the Rh band close to the Fermi level.

A wide variety of complex phases in quantum materials are driven by electron-electron interactions, which are enhanced through density of states peaks. A well-known example occurs at van Hove singularities where the Fermi surface undergoes a topological transition. Here we show that higher order singularities, where multiple disconnected leaves of Fermi surface touch all at once, naturally occur at points of high symmetry in the Brillouin zone. Such multicritical singularities can lead to stronger divergences in the density of states than canonical van Hove singularities, and critically boost the formation of complex quantum phases via interactions. As a concrete example of the power of these Fermi surface topological transitions, we demonstrate how they can be used in the analysis of experimental data on Sr_{3}Ru_{2}O_{7}. Understanding the related mechanisms opens up new avenues in material design of complex quantum phases.

The article presents the results of expert appraisal of corporation approaches to protection of health of workers, role of social partnership and measures of incentives to implementation of programs of development of healthy life-style at enterprises. The results of study of modern approaches of Russian companies to protection of health of workers permits to establish availability among employers of significant potential for amelioration of health of working population. It is most expressed among large companies and is expressed in development on the following main directions: evaluation of health of workers, evaluation of industrial risks, organization of medical care, organization of sanitary anti-epidemiological activities, formation of healthy life-style. To stimulate employers to implementing corporation programs combining priorities of business and state, development of national standard or model intersectoral program considering best practices of companies-leaders. The experts emphasized importance of development of standard of medical specialist of management of health protection of workers. Their competences are to bypass the framework of occupational safety and include skills in personnel management, economic efficiency calculation, development of complex programs of protection and promotion of health of workers, including healthy life-style.

We study momentum and energy dependencies of the quasiparticle interference (QPI) response function in multiband superconductors in the framework of the strong-coupling Eliashberg approach. Within an effective two-band model we study the s ± and s ++ symmetry cases, corresponding to opposite or equal signs of the order parameters in the bands. We demonstrate that the momentum dependence of the QPI function is strikingly different for s ± and s ++ symmetries of the order parameter at energies close to the small gap. At the same time, the QPI response becomes indistinguishable for both symmetries at higher energies around the large gap. This result may guide future experiments on probing pairing symmetry in iron pnictides as well as in other unconventional superconductors.

Strong interplay of spin and charge/orbital degrees of freedom is the fundamental characteristic of the iron-based superconductors (FeSCs), which leads to the emergence of a nematic state as a rule in the vicinity of the antiferromagnetic state. Despite intense debate for many years, however, whether nematicity is driven by spin or orbital fluctuations remains unsettled. Here, by use of transport, magnetization, and 75 As nuclear magnetic resonance (NMR) measurements, we show a striking transformation of the relationship between nematicity and spin fluctuations (SFs) in Na 1-x Li x FeAs; For x ≤ 0.02, the nematic transition promotes SFs. In contrast, for x ≥ 0.03, the system undergoes a non-magnetic phase transition at a temperature T 0 into a distinct nematic state that suppresses SFs. Such a drastic change of the spin fluctuation spectrum associated with nematicity by small doping is highly unusual, and provides insights into the origin and nature of nematicity in FeSCs.

A quantum critical point (QCP) is currently being conjectured for the BaFe 2 (As 1-x P x ) 2 system at the critical value x c  ≈ 0.3. In the proximity of a QCP, all thermodynamic and transport properties are expected to scale with a single characteristic energy, given by the quantum fluctuations. Such a universal behavior has not, however, been found in the superconducting upper critical field H c2 . Here we report H c2  data for epitaxial thin films extracted from the electrical resistance measured in very high magnetic fields up to 67 Tesla. Using a multi-band analysis we find that H c2 is sensitive to the QCP, implying a significant charge carrier effective mass enhancement at the doping-induced QCP that is essentially band-dependent. Our results point to two qualitatively different groups of electrons in BaFe 2 (As 1-x P x ) 2 . The first one (possibly associated to hot spots or whole Fermi sheets) has a strong mass enhancement at the QCP, and the second one is insensitive to the QCP. The observed duality could also be present in many other quantum critical systems.

We investigate the interplay between the magnetic and the superconducting degrees of freedom in unconventional multi-band superconductors such as iron pnictides. For this purpose a dynamical mode-mode coupling theory is developed based on the coupled Bethe-Salpeter equations. In order to investigate the region of the phase diagram not too far from the tetracritical point where the magnetic spin density wave, (SDW) and superconducting (SC) transition temperatures coincide, we also construct a Ginzburg-Landau functional including both SC and SDW fluctuations in a critical region above the transition temperatures. The fluctuation corrections tend to suppress the magnetic transition, but in the superconducting channel the intraband and interband contribution of the fluctuations nearly compensate each other.

The Hall effect is a powerful tool for investigating carrier type and density. For single-band materials, the Hall coefficient is traditionally expressed simply by , where e is the charge of the carrier, and n is the concentration. However, it is well known that in the critical region near a quantum phase transition, as it was demonstrated for cuprates and heavy fermions, the Hall coefficient exhibits strong temperature and doping dependencies, which can not be described by such a simple expression, and the interpretation of the Hall coefficient for Fe-based superconductors is also problematic. Here, we investigate thin films of Ba(Fe1-xCox)2As2 with compressive and tensile in-plane strain in a wide range of Co doping. Such in-plane strain changes the band structure of the compounds, resulting in various shifts of the whole phase diagram as a function of Co doping. We show that the resultant phase diagrams for different strain states can be mapped onto a single phase diagram with the Hall number. This universal plot is attributed to the critical fluctuations in multiband systems near the antiferromagnetic transition, which may suggest a direct link between magnetic and superconducting properties in the BaFe2As2 system.

A fundamental and unconventional characteristic of superconductivity in iron-based materials is that it occurs in the vicinity of two other instabilities. In addition to a tendency towards magnetic order, these Fe-based systems have a propensity for nematic ordering: a lowering of the rotational symmetry while time-reversal invariance is preserved. Setting the stage for superconductivity, it is heavily debated whether the nematic symmetry breaking is driven by lattice, orbital or spin degrees of freedom. Here, we report a very clear splitting of NMR resonance lines in FeSe at Tnem = 91 K, far above the superconducting Tc of 9.3 K. The splitting occurs for magnetic fields perpendicular to the Fe planes and has the temperature dependence of a Landau-type order parameter. Spin-lattice relaxation rates are not affected at Tnem, which unequivocally establishes orbital degrees of freedom as driving the nematic order. We demonstrate that superconductivity competes with the emerging nematicity.

Bulk TiSe2 is an intrinsically layered transition metal dichalcogenide hosting both superconducting and charge-density-wave ordering. Motivated by the recent progress in preparing two-dimensional transition metal dichalcogenides, we study these frustrated orderings in a single trilayer of TiSe2. Using a renormalization group approach, we find that electronic correlations can give rise to charge-density-wave order and two kinds of superconductivity. One possible superconducting state corresponds to unconventional s(+-) pairing. The other is particularly exciting as it is chiral, breaking time-reversal symmetry. Its stability depends on the precise strength and screening of the electron-electron interactions in two-dimensional TiSe2.

We discuss the form of the damping of magnetic excitations in a metal near a ferromagnetic instability. The paramagnon theory predicts that the damping term should have the form γ(q,Ω)∝Ω/Γ(q), with Γ(q)∝q (the Landau damping). However, the experiments on uranium metallic compounds UGe2 and UCoGe showed that Γ(q) is essentially independent of q. A nonzero γ(q=0,Ω) is impossible in systems with one type of carrier (either localized or itinerant) because it would violate the spin conservation. It has been conjectured recently that a near-constant Γ(q) in UGe2 and UCoGe may be due to the presence of both localized and itinerant electrons in these materials, with ferromagnetism involving predominantly localized spins. We present the microscopic analysis of the damping of near-critical localized excitations due to interaction with itinerant carriers. We show explicitly how the presence of two types of electrons breaks the cancellation between the contributions to Γ(0) from the self-energy and vertex correction insertions into the spin polarization bubble. We compare our theory with the available experimental data.

The discovery of superconductivity with a transition temperature, Tc, up to 65 K in single-layer FeSe (bulk Tc=8 K) films grown on SrTiO3 substrates has attracted special attention to Fe-based thin films. The high Tc is a consequence of the combined effect of electron transfer from the oxygen-vacant substrate to the FeSe thin film and lattice tensile strain. Here we demonstrate the realization of superconductivity in the parent compound BaFe2As2 (no bulk Tc) just by tensile lattice strain without charge doping. We investigate the interplay between strain and superconductivity in epitaxial BaFe2As2 thin films on Fe-buffered MgAl2O4 single crystalline substrates. The strong interfacial bonding between Fe and the FeAs sublattice increases the Fe-Fe distance due to the lattice misfit, which leads to a suppression of the antiferromagnetic spin density wave and induces superconductivity with bulk Tc≈10 K. These results highlight the role of structural changes in controlling the phase diagram of Fe-based superconductors.

The article discusses the actions recommended by WHO and applied in foreign countries to overcome the issues concerning public health financing. The emphasis is made upon enhancing the effectiveness of implementation of available resources and struggle with corruption and fraud. The corresponding measures applied in the Russian Federation are listed.

The paper demonstrates the importance of healthy lifestyle in the prevention and control of diseases. The description is given of the main methods and organizational technologies used in developed countries to ensure the effective performance of general practitioners on the introduction of healthy lifestyle. The concrete examples from WHO documents, USA and Australian organizations and also some recommendations for the healthcare system of Russia are provided.

The development of healthy life-style among population is accepted as one of the major directions of activities of physician of medical primary care services. However, most of the research studies related to this problem deals with the investigation of inter-sectoral approach and functioning of medical prevention centers. There is not enough studies on the exploration of healthy life-style implementation techniques as applied to patients, on the determination of criteria to assess the quality and effectiveness of medical establishments and medical professionals functioning in the field of healthy life-style development. In view of this, it seems actual to generalize the approaches to the resolving of these problems as proposed by WHO and to assess the possibilities to apply them in Russian Federation. Furthermore, to develop on the basis of the implemented analysis corresponding suggestions related to the enhancement of quality and effectiveness of activities of physicians of primary medical care services related to the development of healthy life-style.

We propose an explanation of the peculiar linear temperature dependence of the uniform spin susceptibility chi(T) in ferropnictides. We argue that the linear in T term appears to be due to the nonanalytic temperature dependence of chi(T) in a two-dimensional Fermi liquid. We show that the prefactor of the T term is expressed via the square of the spin-density-wave (SDW) amplitude connecting nested hole and electron pockets. Because of an incipient SDW instability, this amplitude is large, which, along with a small value of the Fermi energy, makes the T dependence of chi(T) strong. We demonstrate that this mechanism is in quantitative agreement with the experiment.

Transition metal oxides with a perovskite-type structure constitute a large group of compounds with interesting properties. Among them are materials such as the prototypical ferroelectric system BaTiO(3), colossal magnetoresistance manganites and the high-T(c) superconductors. Hundreds of these compounds are magnetic, and hundreds of others are ferroelectric, but these properties very seldom coexist. Compounds with an interdependence of magnetism and ferroelectricity could be very useful: they would open up a plethora of new applications, such as switching of magnetic memory elements by electric fields. Here, we report on a possible way to avoid this incompatibility, and show that in charge-ordered and orbitally ordered perovskites it is possible to make use of the coupling between magnetic and charge ordering to obtain ferroelectric magnets. In particular, in manganites that are less than half doped there is a type of charge ordering that is intermediate between site-centred and bond-centred. Such a state breaks inversion symmetry and is predicted to be magnetic and ferroelectric.