It is shown that many anomalies observed in underdoped cuprates, including anomalous spectral weight transfer and a large pseudogap, appear to have a common nature due to both the cluster structure of the underdoped phase and the specific mechanism of superconducting pairing. The combined action of these factors leads to the fact that at a temperature T lying in a certain temperature range Tc

Creation of an efficient system for the frequent delivery of cryogenic fuel targets (CFT) to the focus of a powerful laser facility is one of the key directions of research in inertial confinement fusion (ICF). The paper discusses prospects for the creation of a ring magnetic system based on the contactless acceleration of a levitating CFT carrier made of high-temperature type II superconductors (HTSC), up to specified injection velocities of 200–400 m/s. For this purpose, the temperature dependence of the magnetic moment of HTSC tapes in the range Δ T = 10–92 K was studied, prototype experiments on the acceleration of HTSC carriers at T ~ 80 K due to an external action on them with a frequency of ~1 Hz were carried out, and the speed for the stall of HTSC carriers from a circular trajectory were calculated. The calculation results are in good agreement with the experiment, which makes it possible to estimate the parameters of the ring magnetic accelerator for the operating temperature of the CFT injector T ~ 17 K. It is shown that the method proposed is promising for the creation of systems for noncontact delivery of CFT based on the principles of levitation and subsequent injection of CFTs into the center of the ICF reactor chamber at the required speed. The results of planning a new series of experiments are presented: acceleration of an HTSC carrier followed by injection of a surrogate target into the chamber of the GARPUN (LPI) KrF laser.

The mechanism of formation of dome-like phase diagrams and the features of magnetic field penetration in iron pnictides with hetero- and isovalent doping are considered within the framework of the previously proposed model, which assumes the local character of doping and the cluster structure of resulted superconducting phase. It is shown that the proposed model, despite its simplicity and neglect of the features of the electronic structure, makes it possible not only to accurately calculate the positions of superconducting domes on the phase diagrams of specific HTSC compounds, but also to explain the nature and position of sharp peaks in the London penetration depth depending on the doping level.

It is shown that experimentally determined universal scaling relations which relate the critical temperature and superfluid density in cuprate HTSCs under various doping conditions, can be understood within the previously proposed model assuming self-localization of doped carriers.

We have previously shown that most of the anomalies in the superconducting characteristics of cuprates and ferropnictides observed at dopant concentrations within the superconducting dome, as well as the position of the domes in the phase diagrams, do not require knowledge of the details of their electronic structure for explanation, but can be understood and calculated with high accuracy within the framework of a simple model describing the cluster structure of the superconducting phase. This fact suggests a change in the paradigm that forms our understanding of HTSC. In this paper, we propose a unified view on the transformation of the electronic structure of cuprates and ferropnictides upon heterovalent and isovalent doping, based on the assumption of self-localization of doped carriers. In this representation, in undoped cuprates and ferropnictides, which initially have different electronic structures (Mott insulator and semimetal), local doping forms percolation clusters with the same electronic structure of a self-doped excitonic insulator where a specific mechanism of superconducting pairing is implemented, which is genetically inherent in such a system. The proposed model includes a mechanism for generating additional free carriers under heterovalent and isovalent doping and makes it possible to predict their sign, which, in the general case, does not coincide with the sign of doped carriers.

Within the framework of the previously proposed model, which assumes the local character of doping and the cluster structure of the forming superconducting phase, an explanation is given for the nature of the narrow peak on the doping dependence of London penetration depth, λ in the Ba(Fe1-xCox)2As2 high temperature superconductor. The Co concentration value corresponding to the peak position as well as the peak amplitude are in excellent agreement with the experimental results.

We propose a generalized model of electronic structure modification in HTSC cuprates and ferropnictides under doping. In this model, the role of doping consists in only a local change in the electronic structures of the parent phases of cuprates and ferropnictides due to the formation of trion complexes comprising a doped carrier localized in unit cell and charge transfer (CT) excitons around it. These CT excitons emerge in CuO4 or AsFe4 plaquettes in the CuO2 or FeAs basal planes (CT plaquettes) under the influence of the doped carrier, restricting its itinerancy. As the dopant concentration is increased, CT plaquettes combine into clusters of the so-called CT phase. It is this CT phase that is related in the model to the HTSC phase. In support of this assumption, we determined the ranges of dopant concentrations conforming to the existence of percolation clusters of the CT phase; these ranges were shown to coincide with the positions of the superconducting domes on the phase diagrams of these compounds. The model also perfectly describes subtle features of the phase diagrams of various cuprates and ferropnictides including the “1/8” anomaly, narrow peaks in the dependences of the London penetration depth on the concentration of the dopant, and other specific features. The mechanism of the generation of free carriers in the CT phase, provided by intrinsic self-doping, was considered. The mechanism is not directly related to external doping, but is due to the interaction of band electrons with so-called Heitler–London (HL) centres inherently existing in the percolation cluster of the CT phase and representing pairs of adjacent CuO4 or AsFe4 CT plaquettes in the CuO2 or FeAs basal planes. Material in the CT phase was shown to represent a medium, in which the mechanism of excitonic superconductivity, specified by the interaction of band electrons with HL centres, can be realized.

Earlier we have proposed a new approach to the analysis of superconducting phase diagrams for cuprates and pnictides and have shown that the positions of superconducting domes on the diagrams can be predicted with high accuracy proceeding from only the crystal structure of a particular compound. The proposed approach uses the concept of the self-localization of doped carriers due to their formation of trion complexes that represent a bound state of the doped carrier and charge transfer excitons emerging under its influence. Here, as exemplified by cuprates, we show that the use of the proposed approach to the analysis of the transformation of an electronic structure with doping enables an explanation to a range of their anomalies: Fermi arcs, large and small pseudogaps etc. The basic conclusion is that the role of the Fermi surface in cuprates is played by an isoenergetic contour that emerges at the sectioning of the surface of a band dispersion by a dispersionless biexciton pair level. This level additionally plays the role of an acceptor to lead to the emergence of hole carriers on the isoenergetic contour and to a jump of the chemical potential. Based on the conducted consideration, we propose a possible mechanism of superconducting pairing genetically inherent in such a system.

Earlier we have proposed a new approach to the analysis of superconducting phase diagrams for cuprates and pnictides and have shown that the positions of superconducting domes on the diagrams can be predicted with high accuracy proceeding from only the crystal structure of a particular compound. The proposed approach uses the concept of the self-localization of doped carriers due to their formation of trion complexes that represent a bound state of the doped carrier and charge transfer excitons emerging under its influence. Here, as exemplified by cuprates, we show that the use of the proposed approach to the analysis of the transformation of an electronic structure with doping enables an explanation to a range of their anomalies: Fermi arcs, large and small pseudogaps etc. The basic conclusion is that the role of the Fermi surface in cuprates is played by an isoenergetic contour that emerges at the sectioning of the surface of a band dispersion by a dispersionless biexciton pair level. This level additionally plays the role of an acceptor to lead to the emergence of hole carriers on the isoenergetic contour and to a jump of the chemical potential. Based on the conducted consideration, we propose a possible mechanism of superconducting pairing genetically inherent in such a system.

In this paper, we present a descriptive "electrotechnical" model for calculating the distribution of induction currents in a superconducting film near which a measuring coil inducing a local alternating low-frequency magnetic field is placed. The change in the coil inductance, caused by a superconductor with a set London penetration depth of a magnetic field is calculated within the model. The possibility of determining the penetration depth from experimental data is shown; the sensitivity of the proposed approach for films of various thicknesses and various problem configurations is considered.

This paper reviews experimental phase diagrams of cuprates and pnictides to demonstrate that specific features of the superconducting phase diagrams in both HTSC families can be understood within the framework of the proposed approach, which assumes the formation, under heterovalent doping, of localized trion complexes consisting of a doped carrier and charge transfer (CT) excitons. The geometry of such cells containing CT excitons (CT plaquettes) in the basal plane of the crystal is determined by its crystal structure and the type of dopant, so that the dopant concentration range corresponding to the existence of a percolation cluster of CT plaquettes can be readily determined for each particular compound. These dopant concentration ranges coincide with good accuracy with the experimental ranges of superconducting domes in the phase diagrams of the HTSC compounds considered. The generation of free carriers and the mechanism of superconducting pairing in this pattern is related to biexciton complexes (Heitler-London centers) emerging in neighboring CT plaquettes.

We present our results on utilization of the quantum levitation effect for HTSC samples (superconducting ceramics based on YBa2Cu3O7−x and SuperOx J-PI-12-20Ag-20Cu superconducting tapes) in magnetic fields of different configurations with respect to developing special carriers for hybrid systems of noncontact transport of cryogenic targets in ICF experiments. We implement the obtained results for developing and engineering of “HTSC-MAGLEV” delivery system to minimize the risk for damage of the fuel layer at the target acceleration and during target injection into the center of the ICF reaction chamber.

An approach for the assessment of London penetration depth of superconducting films is proposed. This approach is based on the analysis of linear response of the sample to a local low-frequency alternating magnetic field generated by the measuring coil disposed near the film surface. A visual "electrical engineering" model of induced currents distribution in the superconductor taking into account the kinetic inductance was developed for a description of this response. The possibility of determining of the penetration depth from changing the inductance of the system "coil-sample" is shown in the framework of this model. The sensitivity of the proposed method for the films with different thicknesses is considered.

We report an experimental investigation of the hot-spot evolution in superconducting single-photon detectors made of disordered superconducting materials with different diffusivity and energy downconversion time values, i.e., 33-nm-thick NbN and 23-nm-thick NbC films. We have demonstrated that, in NbC film, only 405-nm photons produce sufficiently large hot spot to trigger a single-photon response. The dependence of detection efficiency on bias current for 405-nm photons in NbC is similar to that for 3400-nm photons in NbN. In NbC, large diffusivity and downconversion time result in 1-D critical current suppression profile compared with the usual 2-D profile in NbN.

Based on low-temperature resistivity, heat capacity, and magnetization investigations, we show that the unusually strong suppression of superconductivity in LuxZr1-xB12 (x < 8%) BCS-type superconductors is caused by the emergence of static spin polarization in the vicinity of nonmagnetic lutetium impurities. The analysis of the obtained results points to a formation of static magnetic moments with mu(eff) approximate to 6 mu(B) per Lu3+ ion (S-1(0) ground state, 4f(14) configuration) incorporated in the superconducting ZrB12 matrix. The size of these spin-polarized nanodomains was estimated to be about 5 angstrom.

The influence of 200 keV He+ ion irradiation on superconducting and magnetotransport properties of Ba(Fe0.94Co0.06As)2 films has been studied. It was shown that in the film under the corresponding irradiation conditions, mainly nonmagnetic defects are generated. It was found that suppression of superconductivity at increasing concentration of nonmagnetic defects is noticeably slower than expected from a simple theory assuming s ± symmetry of the superconducting order parameter. The influence of defects on the magnetotransport properties has been analyzed. It was shown that the results of Hall measurements can be explained on the assumption that carriers are localized in the vicinity of radiation defect. The conclusion was made that the complete suppression of superconductivity occurs at a critical disorder in the system, which implies s ++ symmetry of the order parameter. The results are explained basing on the assumption about electron pairing in real space on definite centers of pairing.

It is proposed to use the HTSC quantum levitation phenomenon in magnetic fields of various configurations to develop the systems of contact-free positioning and transport of cryogenic fuel targets (CFTs) to the focus of a high-power laser installation or the IFE reactor. The results are presented of a large cycle of experimental studies using YBa2Cu3O7−x superconducting ceramics and permanent magnet guideways based on various combinations of permanentmagnets to develop “CFT-MAGLEV” delivery systems.

Determining the symmetry of the superconducting order parameter is the most important but also the most complicated step in elucidating the mechanism of superconductivity. Here we present an experimental approach to investigate the order parameter symmetry of unconventional multiband superconductors, which is based on a disorder-induced change from sign-reversed (s(+/-)) to sign-preserved (s(++)) symmetry. Therefore, we investigated a Ba(Fe0.9Co0.1)(2)As-2 thin film by THz spectroscopy and stepwise proton irradiation. In our experiments, the low-energy superconducting gap first vanishes but recovers at higher irradiation doses. At the same time, the decrease of the superfluid density with disorder comes to a halt. Thus, we confirm with the method presented here that the superconducting order parameter in the pristine sample possesses s(+/-) symmetry.