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Electrical transport properties of random binary networks composed of high-Tc superconductor Bi2Sr2Ca2Cu3O10+x (Bi2223) microparticles and half-metal ferromagnet La2/3Sr1/3MnO3 (LSMO) nanoparticles have been investigated. The experimental current-voltage characteristics of bulk samples of nanocomposites with a volumetric content of 4:1 components are well described by the Berezinsky–Kosterlitz–Thouless (BKT) model for two-dimensional (2D) superconductors undergoing a superconducting transition. The observed 2D-like behavior of the three-dimensional transport properties of the nanocomposite is most likely associated with two different physical spatial scales involved in the formation of the properties of the nanocomposite: a significant difference between the geometric dimensions of the constituent components and the appearance of a triplet superconducting state induced by the proximity effect in semi-metallic manganite LSMO contacting Bi2223. Below the Bi2223 superconducting transition temperature, bulk resistive losses in the nanostructures are determined by the current flowing through ferromagnetic LSMO nanoparticles that cover Bi2223 microgranules. As temperature decreases, proximity-induced superconducting transition in the effectively 2D surface of LSMO nanoparticles covering Bi2223 microgranules reveals itself as the topological BKT-like superconducting transition in the bulk sample.

The results of studying the transport characteristics of the MgB2:La0.67Sr0.33MnO3 (LSMO) nanocomposite with a 3:1 volume content of components are presented. The experiments have shown that the temperature behavior of resistance of bulk MgB2:LSMO samples during the transition to the superconducting state is described by the Berezinskii–Kosterlitz–Thouless (BKT) model for 2D superconductors. The observed features of the transport properties are most likely due to two characteristic spatial scales in the system: 1) a significant difference between the geometrical dimensions of the constituent components (micrometer-sized MgB2 particles and LSMO nanoparticles), and 2) the coherence length of the triplet superconducting state of half-metal manganite LSMO contacting with MgB2. Resistive losses in such nanostructures are due to the flow of current through ferromagnetic LSMO nanogranules that cover MgB2 microgranules. As temperature decreases, the BKT transition occurs in 2D (surface) superconducting LSMO layers.

The transport characteristics of a composite consisting of microparticles of a superconductor MgB2 and nanopowder of a ferromagnetic half-metal La0.7Ca0.3MnO3 (LCMO) were investigated under hydrostatic compression. It was observed that as the concentration of the LCMO manganite is increased above the percolation threshold of MgB2, the superconducting current flows through the superconductor–ferromagnet–superconductor chains with the LCMO thickness exceeding the magnetic coherence length ξM in the half-metal (ξM ≤ 10 A) by an order of magnitude. This behavior can be attributed to the presence of triplet superconducting correlations in the half-metal, which arise due to the spin-active surface of LCMO nanograins. An increase in pressure led to an increase in the critical current of the MgB2–LCMO composite, while the critical current and the transition temperature of pure MgB2 decreased under pressure.

Unconventional high-temperature superconductivity in MgB2:La0:65Sr0:35MnO3 (MgB:LSMO) nanocomposite has been found recently [V. N. Krivoruchko and V. Yu. Tarenkov, Phys. Rev. B 86, 10502 (2012)]. In this report, the symmetry of the nanocomposite superconducting order parameter and plausible pairing mechanisms have been studied by the point-contact Andreev-reflection (PCAR) spectroscopy. To clarify the experimental results obtained, we consider a model of a ferromagnetic superconductor, which assumes a coexistence of itinerant ferromagnetism and mixed-parity superconductivity. The Balian–Werthamer state, with quasiparticle gap topology of the same form as that of the ordinary s- wave state, fits the experimental data reasonably well. Utilizing the extended Eliashberg formalizm, we calculated the contribution of MgB2 in the total composite's conductivity and estimated the magnitude of the electron–phonon effects originated from MgB2 in I–V characteristics of the composite at above-gap energies. It was found that distinctive features observed in the PC spectra of the MgB:LSMO samples and conventionally attributed to the electron–phonon interaction cannot be related to the MgB2 phonons. It is argued that the detected singularities may be a manifestation of the electron-spectrum renormalizations due to strong magnetoelastic (magnon–phonon) interaction in LSMO.

We investigated transport characteristics of composite that consist of the superconductor MgB2 and ferromagnetic half-metallic nanoparticles La0.7Ca0.3MnO3 (LCMO). The experiments were carried out in a wide temperature range with different volume content of the magnetic impurity. It was found that over 25% of the impurity manganite breaches percolation paths of the diboride magnesium. This leads to a considerable broadening of the MgB2 superconducting transition and the formation of the network of contacts through the ferromagnetic inclusions. This result shows the possibility of the spin - activated surface realization on the superconductor - manganite boundary. It's allowing the free transition of Cooper pairs with s wave symmetry of the order parameter in a ferromagnet with 100% spin-polarized carriers.

The transport properties of a composite consisting of the microparticles of superconductor MgB2 and nanopowder of manganite La0.7Sr0.3MnO3 (LSMO) were studied. It was found that when the amount of the ferromagnet component in the composite was above 10% by volume, a sharp increase in the resistivity of the composite occurred. This is caused by the disruption of percolation paths through the magnesium diboride microparticles, thus leading to the formation of a network of superconductor-ferromagnet-superconductor junctions. As a result, a significant broadening of the superconducting transition of the composite was observed while its ferromagnetic properties were maintained. The obtained data indicate that Cooper pairs can pass through the layer of a half-metal ferromagnet with high (about 100%) spin polarization of the charge carriers and thickness d > 10 nm.

In contacts normal metal–superconductor Bi1.6Pb0.4Sr1.8Ca2.2Cu3O10 (Bi2223), the energy gap Δ(T) of the superconductor and the excess current Iexc(T) were measured by means of the Andreev spectroscopy. It is shown that the parameter Δ(T) turns to zero at the superconducting transition temperature of the cuprate, Tc ≈ 110 K, and the dependence Δ(T) follows the BCS theory; at the same time, the excess current Iexc(T) persists up to the temperature Tpair ≈ 180 K, significantly higher than Tc. The result obtained indicates a possible existence in Bi2223 of a wide temperature range Tc

Materials with spin-polarized charge carriers are the most demanded in the spin-electronics. Particularly requested are the so-called half-metals which have the maximum attainable value of carrier spin polarization. Doped manganites are in the list of compounds with, potentially, half-metallic properties. The point-contact (PC) Andreevreflection (AR) spectroscopy is a robust and direct method to measure the degree of current spin polarization. In this report, advances in PCAR spectroscopy of ferromagnetic manganites are reviewed. The experimental results obtained on “classic” s-wave superconductor — ferromagnetic manganites PCs, as well as related theoretical models applied to deduce the actual value of charge carrier spin-polarization, are discussed. Data obtained on “proximity affected” contacts is also outlined. Systematic and repeatable nature of a number of principal experimental facts detected in the AR spectrum of proximity affected contacts suggests that some new physical phenomena have been documented here. Different models of current flow through a superconductor–half-metal ferromagnet interface, as well as possibility of unconventional superconducting proximity effect, have been discussed.

We report the analysis of pseudogap Δ* derived from resistivity experiments in FeAs-based superconductor SmFeAsO0.85, having a critical temperature Tc = 55 K. Rather specific dependence Δ*(T) with two representative temperatures followed by a minimum at about 120 K was observed. Below Ts ≈ 147 K, corresponding to the structural transition in SmFeAsO, Δ*(T) decreases linearly down to the temperature TAFM ≈ 133 K. This last peculiarity can likely be attributed to the antiferromagnetic (AFM) ordering of Fe spins. It is believed that the found behavior can be explained in terms of Machida, Nokura, and Matsubara theory developed for the AFM superconductors.

This paper presents the results of numerical calculations of two-phase flow taking into account the coagulation of a polydisperse condensate in the combustion chamber of a solid rocket motor. The effect of fluctuations in the particle velocity on the evolution of their mean size is analyzed.

The specific features of the phonon spectrum of the MgB2 compound (Tc = 38 K) are investigated by tunneling spectroscopy. It is demonstrated that both the position and the energy width of the fundamental optical mode E2g in the phonon spectrum are in good agreement with inelastic X-ray spectroscopy data but differ substantially from Raman spectroscopy results. Among possible factors responsible for this discrepancy, the anharmonic and nonadiabatic effects that are characteristic of the MgB2 system are discussed.

Problems concerning the dependence of classes of functions of two variables of bounded variation and of generalized bounded variation on the choice of a coordinate system are considered. A description of all Waterman classes containing a function of two variables of bounded variation after rotation of a coordinate system by an angle not divisible by π/2 is obtained. A similar problem for alternative Waterman classes is considered.

Earlier we introduced a continuous scale of monotony for sequences (classes M α, α ≥ 0), where, for example, M 0 is the set of all nonnegative vanishing sequences, M 1 is the class of all nonincreasing sequences, tending to zero, etc. In addition, we extended several results obtained for trigonometric series with monotone convex coefficients onto more general classes. The main result of this paper is a generalization of the well-known Hardy—Littlewood theorem for trigonometric series, whose coefficients belong to classes M α, where α ∈ (\( \tfrac{1} {2} \), 1). Namely, the following assertion is true.

The structure and thermooxidative stability of highly crosslinked polymethylsilsesquioxanes in the temperature range 400–700°C were compared. Using nonlinear multifactor regression analysis, the statistically effective equations were obtained allowing the comparative estimation of the structure of basic fragments of polymethylsilsesquioxanes, the determination of the composition of solid products of thermooxidation of these polymers, and prediction of their thermooxidative stability. It was found that stability characteristics, such as the degree of degradation by carbon, mass loss, and the amount of OH groups in the products of thermooxidation of polymethylsilsesquioxanes, are determined by the interchain distances and the entropy of the polymer network structure. It was shown that the oxidation stability of methyl substituents and the degree of conversion of OH groups upon thermooxidative transformations of polymethylsilsesquioxanes correlate with the position and intensity of the bands of bending vibrations of CH3 groups and asymmetric stretching vibrations of the siloxane bond.

The possibility of high-field Josephson magnetic resonance (HFJMR) in high-T c superconductors (HTSCs) is theoretically demonstrated. The HFJMR is caused by the penetration of Abrikosov vortices into the grains in fields within H c1 ≤Hx (where H c1 is the first critical field and Hx is the field corresponding to vortex separation from pinning centers in the grains). The signals from microwave absorption is dependent on the ratio of grain sizes that produces junction, the anisotropy and the intergrain coupling strength. These properties can be used for investigations of HTSC macrostructures and a vortex matter.

The excess current Iexc in In/MgB2 and Pb/MgB2 contacts was investigated as a function of the energy gaps Δσ and Δπ in magnesium diboride MgB2. It was shown that, even in the “dirty” limit, electrons from the σ and π bands do not undergo mixing; i.e., they remain confined to their bands over a long period of time. On this basis, the measurements of the excess current Iexc for contacts prepared in different crystallographic directions of the MgB2 crystals made it possible to determine the energy gaps Δπ ≈ 3.0 meV and Δσ = 7.5 meV.

Andreev reflection in contacts between a low-temperature superconductor (Pb or MgB2) and the manganite La0.65Ca0.35MnO3 (LCMO) is investigated. In the majority of cases behavior typical of superconductor/half-metallic ferromagnet contacts is observed: an excess voltage Vexc and suppression of the contact conductance G(V)=dI(V)∕dV in the region of voltages e∣V∣≤ΔS, where ΔS is the energy gap of the injector. However, some of the contacts demonstrate an increase of the conductance and an excess current Iexc on the current–voltage characteristic of the contact at e∣V∣≤ΔS. The character of the dI(V)∕dV curves observed for those contacts corresponds to Andreev spectrum of a superconductor with an energy gap much larger than the injector gap ΔS. It is assumed that in contacts of the latter type, specific conditions prevail whereby the penetration of Cooper pairs into the manganite from an electrode with a singlet order parameter induces in the surface region of the manganite a superconducting state with a tripl...

Andreev reflection spectroscopy has been used to probe mutual influence of superconductivity and ferromagnetism in contacts of a low-temperature superconductor (S), Pb, and a half-metallic ferromagnet, La0.65Ca0.35MnO3 (LCMO). In the contacts, which we distinguish as proximity-affected ones, a few unusual effects have been detected: i) a spectacular drop in the resistance of the contacts at the onset of the Pb superconductivity; ii) excess current and doubling of the normal-state conductance; iii) subharmonic gap structure; and iv) the ratio of the single-particle gap to the Pb superconducting transition temperature is anomalously large. Microscopically, our results can most reasonably be explained assuming a conversion from spin singlet pairs to spin triplets at the S/LCMO interface and a long-range proximity effect.

Spin polarization of charge carriers in La0.65Ca0.35MnO3 (LCMO) is studied using point-contact Andreev spectroscopy. Pb and MgB2 are used to make superconducting electrodes. In all cases, the transport spin polarization obtained from the conductivity of LCMO/superconductor point contacts does not exceed 80–85%. Different models of the current flow through the superconductor-ferromagnetic metal contact and possible reasons for noncomplete spin polarization of a current in manganites are explored. The level of spin polarization observed in Sharvin contacts (contact area ∼104 A2) is most naturally explained in terms of a model that suggests separation of the crystal into nanosized magnetic phases, only one of which is a ferromagnetic metal with full spin polarization of charge carriers.

Weakly-transmitting junctions usable for tunneling spectroscopy experiments were fabricated from polycrystalline bulk samples of magnesium diboride with a niobium counter-electrode. A strong nonlinear conductance-versus-voltage behavior for voltages above the values corresponding to the energy gaps of the electrodes was interpreted as a fingerprint of the electron-phonon interaction in superconducting MgB2. A partial Eliashberg spectral function α2(ω)F(ω) was extracted from the data by the numeric inversion of the standard strong-coupling Eliashberg equations. In a full agreement with recent theoretical simulations for magnesium diboride [1], we have observed a dominant contribution from the π-band of this two-band superconductor. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have measured the temperature dependence of gap features revealed by Andreev reflection Delta_s and by tunneling Delta in the ab-plane of optimal and slightly overdoped microcrystals of (BiPb)2Sr2Ca2Cu3O10 (Bi2223) with critical temperature Tc=110-115 K, and Bi2Sr2CaCu2O8 (Bi2212) with Tc=80-84 K. The tunneling conductance of Bi2223-Insulator-Bi2223 junction shows peaks at the 2Delta gap voltage, as well as dips and broad humps at other voltages. In Bi2223, similarly to the well known Bi2212 spectra, the energies corresponding to 2Delta, to the dip, and to the hump structure are in the ratio of 2:3:4. This confirms that the dip and hump features are generic to the high temperature superconductors, irrespective of the number of CuO2 layers or the BiO superstructure. On the other hand, in both compounds Delta(T) and Delta_s(T) dependences are completely different, and we conclude that the two entities have different nature., LaTeX 2e, 17 pages, 7 figures in .eps format

Magnetization and magnetostriction loops were calculated in terms of the Kim–Anderson model incorporating the flux jump instability criterion and taking into account magnetic field and temperature dependencies of the heat capacity of YBaCuO. Regions in the H – T diagram where flux jumps occur were determined. The stabilizing role of the specific heat for the critical state of the material was observed.

The transport properties of textured films and tunneling junctions of La0.6Sr0.4MnO3, defined by the surface state of the granule, are studied in low magnetic fields (below 100 Oe) and at pressures of up to 10 kbar. Tunneling junctions of two types are investigated, namely, mechanical break junctions and La0.6Sr0.4MnO3-insulator-superconductor junctions. Although only one electrode represents the magnetic material in the latter case, all samples exhibit a low-field magnetoresistive effect. Hydrostatic compression suppresses the magnetoresistive effect to considerably change the transport properties of ceramic and tunnel samples. The reasons for such behavior are discussed in connection with the model of spin-polarized inelastic tunneling of charge carriers through a potential barrier formed both by the intergranular region and by the surface of contacting granules. Reasons are given for the fact that it is most probable that the magnetic state of the barrier and its height vary under the effect of pressure because of the transition of the surface of granules to the metallic state.

Complete magnetization and magnetostriction loops for type II superconductors were calculated in terms of the following pinned critical state models which incorporate the flux jump instability criterion: the Kim–Anderson model, the exponential model, the linear model, and the model with peak effect. These results were used for constructing magnetic field–temperature (H–T) diagrams of instability of these critical state models. Magnetothermal instabilities of superconductors with peak effect were investigated experimentally and theoretically. Regions in the H–T diagram where flux jumps occur were built and compared with experimental M(H) hysteresis loops. We have explained the phenomenon of “island” jumps. The influence of flux trapping on the flux jump regions of magnetic fields was studied.

The influence of hydrostatic pressure up to 10 kbar on the superconducting transition temperature in doped single crystal of (Sr,Ca) 10 Cu 17 O 29 has been studied. A strong decrease of the normal state electrical resistivity and the pressure effect d T c /d P as high as 0.4 K/kbar were obtained. It is suggested that the superconductivity in the doped (Sr,Ca) 10 Cu 17 O 29 system is related to the inhomogeneous charge carrier distribution. The existence of quasi-one-dimensional (quasi-1D) metallic and spin-gapped regions in close electrical contact can explain the observed magnetic properties of (Sr,Ca) 10 Cu 17 O 29 and the effect of pressure on its transport properties.

Tunneling experiments were performed on ceramic samples with the composition La0.6Sr0.4MnO3, manifesting negative magnetoresistance. Two types of contacts were studied: symmetric (break junction type) and asymmetric ceramic-insulator-metal contact. A high magnetic-field sensitivity of the conductivity σ(H) of the contacts was observed even when only one of the electrodes was magnetic. The effect was explained by the existence of spin-polarized localized states in the tunneling barrier. Their appearance was attributed to the formation of an oxygen-depleted, magnetically two-phase state of localized ferromagnetic nanoregions in an anti-ferromagnetic dielectric matrix in the near-contact region.

Spectroscopic studies of the silver-optimum-doped Bi2223 contacts show that the temperature dependence of the parameter Δ follows the BCS curve. However, the tunnel measurements performed for the same series of specimens did not reveal any temperature dependence of the energy gap Δ. The feature observed in the tunnel density of states was retained at temperatures T>T c , manifesting the presence of the temperature-independent pseudogap E p . The difference between the data obtained with tunnel spectroscopy and Andreev reflection spectroscopy is explained by the fact that the latter measures the true superconducting energy gap Δs(T), whereas the peaks of the tunneling conductivity are related to the total energy gap Δ of cuprates, which includes both the parameter Δs and the pseudogap \(E_p :\Delta \approx \sqrt {\Delta _s^2 + E_p^2 } \).

We performed tunneling experiments on single crystals of hole-doped two-leg ladders $(\mathrm{SrCa}{)}_{10}{\mathrm{Cu}}_{17}{\mathrm{O}}_{29}$ with ${T}_{c}\ensuremath{\cong}75$ K and, for reference purposes, on textured bulk samples of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ (Bi2212). Point-contact and break-junction (Bi2212 only) junction configurations were used. The point-contact junctions, both superconductor--insulator--normal-metal and superconductor--normal-metal, were using Ag counterelectrodes. In all experiments the tunneling direction was along ${\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ planes (for the ladders) and ${\mathrm{CuO}}_{2}$ planes (for Bi2212). The recorded tunneling conductance spectra were reproducible with sharp, well-defined energy-gap structures. All junctions exhibited a pronounced zero-bias conductance peak disappearing at the critical temperature. Experimental results are consistent with the formation of zero-energy Andreev bound states at the surface of the superconductors and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave symmetry of the order parameter $\ensuremath{\Delta}$ in both compounds.

The temperature dependences of the fluctuation conductivity and pseudogap in SmFeAsO1−x are determined from resistivity measurements. It is shown that the temperature dependence of the pseudogap contains a number of features that substantially distinguish it from that of HTSC-cuprates.

The current-voltage characteristics of Ag–I–LaCaMnO tunneling junctions have shown an anomalously sharp increase of current near voltages V≈±1V, irrespective of the polarity of the applied potential and the resistance of the sample. The anomalous growth of current in the IVCs of the metal-insulator-manganite contacts is attributed to the presence of resonance levels in the barrier of the tunneling contact and a redistribution of oxygen vacancies under the action of a current of “hot” electrons, resulting in partial metallization of the barrier surface.

A softening of the high-frequency phonon modes in Bi2223 ( T c =110 K) at high pressure is found. These modes correspond to the oxygen breathing mode and also the Cu–O bending modes. Qualitatively, the result obtained explains the observed rise in R =2 Δ / kT c ( P ) ratio in the framework of the strong electron–phonon interaction model. However, the analysis of the R -ratio in the framework of the s+d model has revealed a possibility of explanation of the observed rise in R ( P ) also for the d-wave pairing.

Andreev reflection effect has allowed one to determine the anisotropy of energy gap Δ ( ϕ ) in high- T c superconductors and its pressure response. The values of Δ ( ϕ ) in the ab plane are found to be Δ max =42 mV, Δ min =19.5 mV ((BiPb)2223 phase, T c =110 K, d T c /d P =0.16 K/kbar); Δ max =44 mV, Δ min =12.5 mV, d T c /d P =0.28 K/kbar (HgPb)2223 phase, T c =133 K). A rise in r =2 Δ max / kT c with pressure is found: for Bi2223 d r /d P ≈0.03 kbar −1 , and for Hg2223 d r /d P ≈0.06 kbar −1 .

The pinning induced magnetostriction in an isotropic superconductor was calculated for different sample shapes. We analyzed some special shapes for which the solution can be found analytically. The magnetostriction of a finite slab was considered. In order to determine the influence of demagnetization effects, the pinning induced magnetostriction of an infinitely long and thin strip was calculated. A simple, approximate formula can be used in particular cases to analyze the magnetostriction induced by pinning forces. In this formula the magnetostriction of a sample is connected directly with its magnetization. We also present some experimental results on high temperature superconductors which are analyzed in frames of the developed theory.

We have studied magnetothermal instabilities, giant flux jumps, both theoretically and experimentally. Magnetostriction and magnetization hysteresis loops with flux jumps were calculated over a wide range of experimental parameters employing two critical-state models: the Kim-Anderson model and the exponential model. The influence of the magnetic history on the flux jumps' magnetostriction and magnetization were investigated for the LaSrCuO single crystal. The shape of the unstable region of the critical state in the temperature-magnetic field plane was constructed from calculations and experimental results. In the Kim-Anderson model H-T diagrams of flux instabilities agree well with experimental results on HTSC single-crystal LaSrCuO and experimental results on LTSC niobium.

Current transport properties of YBaCuO–Ag composites with different silver contents have been studied. The pressure response of the resistance of a single Ag/YBCO junction has been determined. The experiments reveal the absence of a dielectric layer in the junction. It has been found that the proximity effect does not contribute significantly to the conductivity of the Ag–YBCO composite for the different ways of introduction of the metallic inclusions into the YBCO. For the strip-like samples of Ag–YBCO composite with the Ag content of 15% by volume the resistivity ρ was found to be ρ =4.7×10 −9 Ω cm at the density of measuring current of J =3×10 4 A/cm 2 and magnetic field B =1 T.

We study the energy spectrum of Bi2223 (Bi1.6Pb0.4Sr1.8Ca2.2Cu3Ox) at high hydrostatic pressures by Andreev-and tunneling-spectroscopy methods. We determine the gap anisotropy in the basal ab plane and find the following values for the parameters Δ(ϕ): Δmax=42 mV, and Δmin=19.5 mV (T c =110 K and dT c /dP=0.16 K/kbar). We detect an increase in the ratio R=2Δmax/kT c with pressure P; for Bi2223 cuprate, dR/dP≈0.017 kbar−1. In the phonon-frequency region we detect a “softening,” due to pressure, of the high-frequency part of the phonon spectrum corresponding to “breathing” modes of oxygen, as well as other optical modes of Cu-O. The characteristic frequencies of the spectrum for ℏΩ>60 mV are found to decrease, with increasing pressure, at a rate d ln(ℏΩ)/dP≈−6.5±0.5×10−3 kbar−1. This result explains the observed increase in the ratio 2Δ/kT c (P) in the model of strong electron-phonon interaction.

The possibility of high-field Josephson magnetic resonance (HFJMR) in high-temperature super-conductors (HTSCs) is theoretically demonstrated. The HFJMR mechanism is related to the Josephson oscillations in intergranular junctions of a polycrystalline HTSC, which are caused by the penetration of Abrikosov vortices into the grains. In fields within H c1 ≤ H < H x (where H c1 is the first critical field and H x is the field corresponding to vortex separation from pinning centers in the grains), measurements of the microwave absorption spectra of a sample can be used for investigations of the HTSC macrostructure and the properties of vortex matter. In the latter aspect, the HFJMR method is analogous to the SQUID technique.

Models describing the appearance of an abnormal hysteresis in the critical current of granular superconductors are analyzed. It has been revealed that the most suitable for High T c Superconductors ( H T c S ) is a model based on an irreversibility of the grain surface current that determines the critical current of the Josephson weak links or depairing current of narrow superconducting part in ( H T c S ) oxides. This leads to the irreversibility phenomena in ( H T c S ) at the high magnetic fields H≫ H c1 .

The interplay between superconductivity and magnetism has been a long-standing fascinating problem [1], and relation between the spin-density wave (SDW) and superconducting (SC) order is a central topic in the current research on the FeAs-based high-Tc superconductors. However, the clear nature of the complex interplay between magnetism and superconductivity in FeAs-based HTS's is still rather controversial [2].

The anisotropy of the development of instabilities in the metl-textured YBaCuO samples with remanent magnetization of 600 emu/g has been studied in a magnetic field up to 12 T. Large 500 emu/g jumps of the magnetic moment of the sample due to the development of the magnetothermal instability have been observed. A giant (10 −4 ) magnetostriction caused by the magnetic flux pinning has been revealed. Stabilization of the critical state of the sample by reversal of magnetization has been found. However, when the sample is heated to temperatures higher than 100 K and subsequently cooled the magnetic jumps restore completely. A new reversible mechanism of instability generation based on the phenomenon reversibility of plastic deformations of high-temperature superconductors (HTS) is proposed to explain the regeneration of critical state instability.

The high-temperature phase Hg0.8Pb0.2Ba2Ca2Cu3O8+δ (HBCCO) with the onset transition temperature Tc ≈ 133 ± 0.5 K has been investigated under hydrostatic pressure up to 12 kbar. We found a positive pressure effect on Tc with a rate 0.28 ± 0.04 K/kbar and a positive pressure effect on the transport critical current Jc, dln(Jc)/dP = 0.2 ± 0.02 kbar−1. The observation of a strong hysteresis effect in the Jc versus magnetic field H dependences shows the possibility of surface barriers to vortex motion in the high-Tc HBCCO system.