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Andreev reflection at the interface between a ferromagnet and a superconductor has become a foundation of a versatile new technique of measuring the spin polarization of magnetic materials. In this paper we will briefly outline a general theory of Andreev reflection for spin-polarized systems and arbitrary Fermi surface in two limiting cases of ballistic and diffusive transport, 4 pages, 1 fig. Paper submitted to the 8th MMM-Intermag conference. Typos corrected

We report measurements of the temperature dependence of the critical current in Josephson junctions consisting of conventional superconducting banks of Nb and a weakly ferromagnetic interlayer of a Cu$_x$Ni$_{1-x}$ alloy, with $x$ around 0.5. With decreasing temperature $I_c$ generally increases, but for specific thicknesses of the ferromagnetic interlayer, a maximum is found followed by a strong decrease down to zero, after which $I_c$ rises again. Such a sharp cusp can only be explained by assuming that the junction changes from a 0-phase state at high temperatures to a $\pi$-phase state at low temperatures., Comment: 6 pages, 4 figures, submitted to PRL

We present two independent experiments, each of which suggests that the local energy gap in Ta (and Nb) has a lateral spatial variation on a scale of several μm. The first experiment is a series of current–voltage characterizations of Nb/Al/AlOx and Ta/Al/AlOx Josephson junctions, which reveals a dependence of the measured energy gap on the size of the junction. This implies a geometrical dependence of the energy gap. An extended version of the current theory of the proximity effect could explain this phenomenon when a lateral coherence length is introduced, which is of the order of the bulk coherence length of the materials. The second experiment is a series of coincidence measurements of photon absorption events in a Ta absorber between two Ta/Al junctions. There is a clear distinction in the pulse-height characteristics between events detected in the absorber and the junctions. Interestingly, there are also events indicating the presence of a transition region between the absorber and the junction. Event statistics imply that this region has a size of 6 μm, independent of photon energy, which is quite a bit larger than even the bulk coherence length in Ta. It is argued that an additional effect due to ‘smearing’ by the relaxed quasi-particle cloud must also be present. These effects are interesting and intriguing; not only from a theoretical viewpoint, but also for energy-gap engineering of superconducting materials for practical applications, e.g. in a variety of photon and particle detectors.

We investigate the properties of complex mesoscopic superconducting-normal hybrid devices, Andreev-Interferometers in the case, where the current is proped through a superconducting tunneling contact whereas the proximity effect is generated by a transparent SN-interface. We show within the quasiclassical Green's functions technique, how the fundamental SNIS-element of the such structures can be mapped onto an effective S$^\prime$IS-junction, where S$^\prime$ is the proximised material with an effective energy gap $E_g

dc SQUIDs based on double-barrier Nb/Al/AlOx/Al/AlOx/Al/Nb junctions (DBSQs) have been fabricated and tested for the first time. The current–voltage curves have been measured at temperatures down to 1.4 K. The critical current, Ic, dependence on the temperature T is partially described by the existing microscopic theory for double-barrier junctions. A peculiar initial decrease of Ic with decreasing T was observed, which can qualitatively be explained if trapping events are taken into account. We have investigated the effective resistance, Reff, determining the DBSQ dynamics. The noise properties of the DBSQ have been studied using a two-stage SQUID based system, and were compared with equivalent damped and underdamped single-barrier based SQUIDs. The best white noise level measured on a DBSQ (3.3 μΦ0/Hz1/2) is larger than the noise of an equivalent single-barrier SQUID (1.3 μΦ0/Hz1/2).

The differential resistance of NbN/two-dimensional electron gas (2DEG) contact is measured at high magnetic fields. In zero magnetic field the contact shows a pronounced resistance peak at zero bias due to a high barrier at the NbN/2DEG interface, which decreases if a magnetic field is applied in the plane of the 2DEG. For a magnetic field oriented perpendicular to the plane of the 2DEG, not only the zero-bias resistance decreases but so does the normal-state resistance which drops to vanishingly small values. A pronounced substructure due to a splitting of the resistance peak into a three-peak structure is observed at high perpendicular fields. We suggest that the appearance of this substructure can be explained by multiple Andreev reflections due to skipping orbits of electrons and holes accompanied by inelastic scattering in the 2DEG near the interface.

Theoretical model of a rough interface in a superconductor with d-wave symmetry of the order parameter is proposed. The surface roughness is introduced by means of a surface layer with small electronic mean free path. The proximity effect between such a layer and a bulk d-wave superconductor is studied theoretically in the framework of the quasiclassical Eilenberger theory. It is shown that as a result of strong scattering in the interlayer the d-wave component of the order parameter near the interface is reduced while the s-wave component localized near the interface is generated. Angular and spatial structure of the pair potential and the electronic density of states near the interface is calculated. The interplay of the zero-energy (midgap) and finite-energy bound states leads to peculiarities in the energy dependence of the angle-averaged density of states. We argue that the model is relevant for the description of rough interfaces in high T/sub c/ superconductors. In the framework of the present approach we calculate the Josephson critical current for several types of junctions with rough interfaces.

Theoretical model of a rough surface in a d-wave superconductor is studied for the general case of arbitrary strength of electron scattering by an impurity layer covering the surface. Boundary conditions for quasiclassical Eilenberger equations are derived at the interface between the impurity layer and the d-wave superconductor. The model is applied to the selfconsisrent calculation of the surface density of states and the critical current in d-wave Josephson junctions, Comment: 4 pages, 2 postscript figures, to be published in JETP Letters

An overview of the current status of different types of non-hysteretic Josephson junctions is given with emphasis on double-barrier structures. The results of theoretical work on double-barrier SIS′IS Josephson junctions (I is a tunnel barrier, S′ is a thin film with TC′

Heat conductance and heat current across normal metal/d-wave superconductor (NID) interface are calculated in the framework of quasiclassical equations. The calculations are performed for different values of boundary transparencies and crystal axis orientation. It is shown that in contrast to N/s-wave boundary the heat conductance of transparent (D=1) NID interface is considerably larger and has a nonactivated form. Electronic heat current across NID structures is also calculated for different interface models taking into account the midgap states and the possibility of generating a gapless s-wave state in the vicinity of rough NID interface. It is shown that NID junctions cannot compete with analogous NIS devices in microrefrigerating and bolometer applications.

We generalize Abrikosov-Gor'kov solution of the problem of weakly coupled superconductor with impurities on the case of a multiband superconductor with arbitrary interband order parameter anisotropy, including interband sign reversal of the order parameter. The solution is given in terms of the effective (renormalized) coupling matrix and describes not only $T_c$ suppression but also renormalization of the superconducting gap basically at all temperatures. In many limiting cases we find analytical solutions for the critical temperature suppression. We illustrate our results by numerical calculations for two-band model systems., Comment: 18 pages (12pt) RevTeX, 4 postscript figures

We present experimental data on the transport properties of a two-dimensional electron gas in contact with a superconducting and a normal conducting electrode. A short distance ({approx}500 nm) between the electrodes is realized by means of a step geometry. At voltages below the gap voltage of the superconductor, a decrease in resistivity of about 10{percent} is observed. At low temperature and for voltages corresponding to a value smaller than the Thouless energy, an enhancement of the resistance is observed. The resistance at low energy coincides approximately with the resistance value above the gap voltage and depends sensitively on the bias current, temperature, and magnetic field. The experimental results are compared to theories of the reentrance effect. For small energies, good agreement is found. {copyright} {ital 1997} {ital The American Physical Society}

We develop a detailed microscopic analysis of electron transport in normal diffusive conductors in the presence of proximity induced superconducting correlation. We calculated the linear conductance of the system, the profile of the electric field and the densities of states. In the case of transparent metallic boundaries the temperature dependent conductance has a non-monotoneous ``reentrant'' structure. We argue that this behavior is due to nonequilibrium effects occuring in the normal metal in the presence of both superconducting correlations and the electric field there. Low transparent tunnel barriers suppress the nonequilibrium effects and destroy the reentrant behavior of the conductance. If the wire contains a loop, the conductance shows Aharonov-Bohm oscillations with the period $\Phi_0=h/2e$ as a function of the magnetic flux $\Phi$ inside the loop. The amplitude of these oscillations also demonstrates the reentrant behavior vanishing at $T=0$ and decaying as $1/T$ at relatively large temperatures. The latter behavior is due to low energy correlated electrons which penetrate deep into the normal metal and ``feel'' the effect of the magnetic flux $\Phi$. We point out that the density of states and thus the ``strengh'' of the proximity effect can be tuned by the value of the flux inside the loop. Our results are fully consistent with recent experimental findings., Comment: 16 pages RevTeX, 23 Postscript figures, submitted to Phys. Rev. B

We discuss various mechanisms that can lead to interband sign reversal of the order parameter in a multiband superconductor. In particular, we generalize Abrikosov-Gor'kov solution of the problem of weakly coupled superconductor with magnetic and nonmagnetic impurities on the case of arbitary order parameter anisotropy, including extreme cases as $d-$pairing or interband sign reversal of the order parameter, and show that interband scattering by magnetic impurities can stabilize an interband sign-reversal state. We discuss a possibility of such state in YBa$_2$Cu$_3$O$_7$ in the context of various experiments: Josephson tunneling, neutron scattering, isotope effect measurements.

We show that in a superconductor where two or more bands cross the Fermi level it is possible, in the framework of the conventional (s-wave) BCS theory, that the sign of the superconducting gap is different on the different sheets of the Fermi surface. At least one of two conditions has to be satisfied: (1) The interband pairing interaction is weaker than the Coulomb pseudopotential, while the intraband one is stronger, or (2) there is strong interband scattering by magnetic impurities. In the case of YBa$_2$% Cu$_3$O$_7$ we shall argue that the first condition is possibly satisfied, and the second one very likely satisfied. In many aspects such a sign-reversal $s-$wave superconductor is similar to a d-wave superconductor, and thus demands revising recent experiments aimed to distinguish between the $s-$ and $d-$wave superconductivity in this compound., Comment: RevTex; Physica C, in press

Using STM we imaged Abrikosov vortex lattices in 2H-NbSe 2. At a reduced temperature of T/T c = 0.6 we found a distinct decrease of the vortex-core radius with increasing magnetic field. In order to interpret the experimental results the microscopic equations of the superconducting state were solved self-consistently. A good quantitative agreement with the experimental data was obtained without any variational free parameters. Since the demonstrated kind of tunneling spectroscopy is restricted to superconductors with ideal surfaces, we additionally investigated the applicability of magnetically sensitive scanning probe techniques to vortex imaging.

In the version of this Letter originally published, in Fig. 2b, the label on the y axis should have been 'mΩ'. On page 1, the end of the second paragraph should have read “Now the existence of the topological surface states has been established it is time to study the interaction with other materials. Efforts have been made to contact a topological insulator to a superconductor in the search for the Majorana fermion and supercurrents have been reported22, 23. In this Letter, we report the observation of the two hallmarks of the Josephson effect: a Fraunhofer-like magnetic field modulation of the critical current and the appearance of Shapiro steps on microwave irradiation, resulting from the d.c. and a.c. Josephson effects, respectively. We demonstrate the existence of a Josephson supercurrent in the surface state of the topological insulator Bi2Te3.” These errors have been corrected in the HTML and PDF versions.

On the basis of a microscopic model of the proximity effect in an SS'-sandwich we have calculated effective trapping, excitation, and tunneling rates of the reduced gap region in an SS′IS″S junction as a function of temperature, voltage over the junction and strength of the proximity effect. We developed a simple model to describe the effect of magnetic fields on the gap of the superconductor, and calculated the trapping rate as function of the field. The experimentally determined gap reduction and quasiparticle loss times as function of the field were described in terms of this model.

Effective quasi-particle relaxation rates in reduced gap regions of a dirty superconductor (S) at low temperatures are calculated from microscopic theory.

Regions with reduced energy gap induced by the proximity effect give rise to quasi-particle loss in Josephson-junction X-ray detectors, but may also be used advantageously for quasi-particle collection. The influence of the thickness of the Al proximity layers in Nb/Al/sub 1/, AlO/sub x/, Al/sub 2//Nb Josephson tunnel junctions on the electrical characteristics has been investigated theoretically and experimentally. Theoretically it is found that the strength of the proximity effect is mainly determined by the proximity effect is mainly determined by the proximity parameters gamma /sub M1/ ( gamma /sub M2/) of the electrodes. Good fits of the measured I-V curves with theory were obtained for junctions with thicknesses d/sub A11/ ranging from 4 to 25 nm and d/sub A12/=3 nm, with gamma /sub M2/ approximately=0.12 and gamma /sub M1// gamma /sub M2/=d/sub A11//d/sub A12/. For all junctions the proximity knee remains more pronounced than predicted. >

We have used low-temperature scanning electron microscopy for the investigation of the interaction of fluxons (Josephson vortices) with a superconducting (Abrikosov) vortex trapped in one of the electrodes of a long quasi-one-dimensional Josephson junction. The Abrikosov vortex penetrated a top superconducting film and spread out in the tunnel barrier area between the films. In addition, we were able to move the Abrikosov vortex across the junction by the electron beam and to measure at the same time the value of the critical current of the junction. A model explaining a critical-current dependence on the external magnetic field and on the Abrikosov vortex position is proposed. We have also observed the bound states of a fluxon trapped by the Abrikosov vortex in the Josephson junction.

We have analyzed the possible temperature variation of the penetration depth λ(T) in the (d+s)-wave superconductivity model of hybrid pairing, which accounts for the high Tc cuprates with an orthorhombic distortion. The slope of the Δλ(T)∝T for TTc and its dependence on the Δs/Δd admixture in the gap function is analyzed quantitatively. The impurity scattering is taken into account. Recent measurements of the penetration depth in YBa2Cu3O6.95 single crystals are discussed in terms of this model.

Critical supercurrents, Ic in Nb_Cu1−xNix_Nb Josephson SFS junctions with F-layers prepared from ferromagnetic Cu1−xNix alloys have been studied. For value x=0.52 and particular F-layer thickness we have observed Ic(T) oscillations with Ic vanishing for some values of T. We associate this reentrant superconducting behavior with a crossover of the SFS junction from ‘0’- to ‘π’-state that is related to temperature dependence of spatial oscillation period of induced superconducting order parameter in the weak ferromagnet. We argue this is the first experimental evidence of the π-behavior of a Josephson junction, that is the special feature of superconducting pair flow through a ferromagnet predicted for SFS junctions by Bulaevskii, Buzdin et al [1].

In a superconductor with magnetic impurities, Kondo scattering results in the formation of localized states inside the superconducting gap. We show that inelastic electronic transitions involving quasiparticle scattering into and out of the localized states may result in significant changes in the non-equilibrium properties of the superconductor. Using the model of Muller-Hartmann and Zittartz for the extreme dilute limit, and including both deformation potential and spin-lattice coupling, we have calculated the rates of such inelastic transitions between continuum and discrete states, and shown that they may greatly modify quasiparticle interactions. The individual processes are: quasiparticle trapping into discrete states, enhanced recombination with localized quasiparticles, and pair breaking and de-trapping of localized quasiparticles by sub-gap phonons. We find that all these processes give rise to clearly distinguishable temperature dependences of the kinetic parameters.

We reconsider the long-standing problem of the effect of spin fluctuations on the critical temperature and the isotope effect in a phonon-mediated superconductor. We discuss the general physics of the interplay between phonons and paramagnons, and show that the previously used approximate formulas fail to describe the correct behavior of Tc for general phonon and paramagnon spectra. Using a controllable approximation, we derive an analytical formula for Tc which agrees well with exact numerical solutions of the Eliashberg equations for a broad range of parameters. On the basis of both numerical and analytical results, we predict a strong enhancement of the isotope effect when the frequencies of spin fluctuation and phonons are of the same order. We discuss application to near-magnetic superconductors such as MgCNi3.

We study the induced odd-frequency pairing states in ballistic normal metal/superconductor (N/S) junctions where a superconductor has even-frequency symmetry in the bulk and a normal metal layer has an arbitrary length. Using the quasiclassical Green's function formalism, we demonstrate that, quite generally, the pair amplitude in the junction has an admixture of an odd-frequency component due to the breakdown of translational invariance near the N/S interface where the pair potential acquires spatial dependence. If a superconductor has even-parity pair potential (spin-singlet s-wave state), the odd-frequency pairing component with odd-parity is induced near the N/S interface, while in the case of odd-parity pair potential (spin-triplet $p_{x}$-wave or spin-singlet $d_{xy}$-wave) the odd-frequency component with even-parity is generated. We show that in conventional s-wave junctions, the amplitude of the odd-frequency pairing state is enhanced at energies corresponding to the peaks in the local density of states (LDOS). In $p_x$- and $d_{xy}$-wave junctions, the amplitude of the odd-frequency component on the S side of the N/S interface is enhanced at zero energy where the midgap Andreev resonant state (MARS) appears due to the sign change of the pair potential. The odd-frequency component extends into the N region and exceeds the even-frequency component at energies corresponding to the LDOS peak positions, including the MARS., Comment: 27 pages, 12 figures

Josephson and quasiparticle currents in SNcNS contacts are calculated for the regimes of ballistic constriction “c”. The relation between the current and electronic densities of states in NS proximity bilayer is found. The effect of the proximity N layer on the Andreev and normal reflections of electrons at the constriction is studied. Excess current at large voltages and subharmonic structure at low bias are analyzed as a function of material parameters of N and S and transparency of the NS interface. The appearance of new series of subharmonic peaks due to two-gap structure in the density of states of proximity NS electrodes is demonstrated. The relation between excess and critical currents is analyzed. The model is relevant for SNS junctions of constricted geometry which is realized in S-2DEG-S and step-edge highTc SNS contacts.

The core structure of the Abrikosov vortex perpendicular to the layers of a proximity effect SN multilayer is discussed. We assume that the superconducting correlations are induced in N via the proximity effect and the dirty limit conditions are fulfilled. The approach for calculation of the local densities of states in N and S layers is developed in the framework of the quasiclassical Usadel equations. For the practically interesting case of SN bilayer, relevant to the STM spectroscopy, the densities of states and apparent STM vortex-core radius, related to the energy-dependent coherence length in N, are calculated.

We have studied the proximity effect in superconductor/ferromagnet multilayers by systematically varying the magnetic moment, which influences the superconductivity via the coherence length in the ferromagnetic layer. We show that also the interface transparency has to be taken into account in such proximity systems. This interface transparency is found to depend on the magnetic moment.

We have measured the temperature dependence of the surface impedance of high quality YBa2Cu3O7−δ (YBCO) films at 87 GHz. The data cannot be described by a single-gap BCS temperature dependence, but are consistent with a weak exponential behaviour corresponding to 2Δ kT c = 1−2 at low temperatures and a strong-coupling behaviour corresponding to 2Δ kT c = 4−5 at temperatures. The λ(T results agree with calculations of a two-band model based on s-wave BCS theory.

We have studied theoretically a two-band superconductor in which superconductivity is induced by a strong retarded pairing interaction in one of the bands and has s-wave symmetry. This picture results in strongly anisotropic order parameter. Some consequencies of the model are discussed.

Using a scanning tunneling microscope operated in a spectroscopic mode we imaged flux-line lattices in niobium diselenide at various external magnetic fields. From the evaluation of a large number of tunneling-current profiles taken across the individual vortices we deduced the dependence of the vortex-code radius on the applied magnetic field. It was found that the core radius shows a pronounced decrease with increasing field, even for H/Hc2<

The structure of the vortex and the anisotropy of the penetration depth, λ, in a superlattice compound of nonequivalent superconducting layers ( S i ) and isolating layers ( I i ) has been studied theoretically. Our particular interest concerned the case when one of the layers is a normal metal ( N ) whose superconducting properties are induced via the proximity effect with S -layers. It was shown that the anisotropy, α = λ z.snfc; / λ ⊥ , for S/ N multilayers is temperature dependent and may either increase or decrease with decreasing temperature.

The new thermomagnetic effect in the long Josephson SNS junction (L ≫ λJ) Ta/Cu/Ta has been observed. This effect is caused by the thermal diffusion of coreless Josephson vortices. The model based on the sine-Gordon equation has been proposed. From the experimental results the transport entropy has been obtained. Its value is Sϕ ≈ 10−12 J/Km in agreement with the theoretical estimation.

We have investigated both experimentally and theoretically the normal state resistance and Josephson critical current of ramp-type Josephson junctions having YBCO (DyBCO) electrodes and 8–30 nm thick Ga-doped barriers PrBa2Cu3−xGaxO7−δ with Ga content x = 0, 0.05 and 0.1. Analysis of the data shows that the behavior of the junctions can be well described by the model assuming transport through a finite number of localized states in the barrier.

The critical temperature T_c of a superconductor/ferromagnet (SF) bilayer can exhibit nonmonotonic dependence on the thickness d_f of the F layer. SF systems have been studied for a long time; according to the experimental situation, the "dirty" limit is often considered which implies that the mean free path in the layers is the second smallest spatial scale after the Fermi wavelength. However, all calculations reported for the dirty limit were done with some additional assumptions, which can be violated in actual experiments. Therefore, we develop a general method (to be exact, two independent methods) for investigating T_c as a function of the bilayer's parameters in the dirty case. Comparing our theory with experiment, we obtain good agreement. In the general case, we observe three characteristic types of T_c(d_f) behavior: 1) nonmonotonic decay of T_c to a finite value exhibiting a minimum at particular d_f, 2) reentrant behavior, characterized by vanishing of T_c in a certain interval of d_f and finite values otherwise, 3) monotonic decay of T_c and vanishing at finite d_f. Qualitatively, the nonmonotonic behavior of T_c(d_f) is explained by the interference of quasiparticles in the F layer, which can be either constructive or destructive depending on the value of d_f., Comment: 13 pages (including 7 EPS figures), REVTeX 4. Version 2: minor corrections

The Eliashberg theory gives the basis to study the electron-phonon system. The authors note that any kind of excitations (excitons, magnons and so on) may play the same role as phonons so long as their energy is smaller than Fermi energy. In the following they will use the termin phonons for such type of excitations also. The limiting case of weak electron-phonon coupling (/lambda/ >1 was not studied properly up to now. It is this case which seems to be interesting from the point of view of high T/sub c/ oxides and where the authors hope to find the remarkable difference from the BCS model.

The system of electrons interacting with phonons and excitons is studied. In the standard phonon model modes with low and high frequencies are considered, the latter being interact strongly with electrons. The combined model takes into account low frequency modes and excitons interacting weakly with electrons.

Transport mechanisms in HTS Josephson Junctions are discussed theoretically. Two types of conductive channels are considered: geometrical constrictions and localized electronic states (LS). Temperature dependencies of excess and critical currents in ScS, SNcNS and SIS junctions are calculated in the framework of the models assuming transport via geometrical constrictions or resonant tunnelling via LS. The consequences of d-wave pairing symmetry are discussed qualitatively. We argue that both regimes of large and small widths of a channel in the momentum space are relevant to different types of HTS interfaces. The first regime is possibly realized at grain boundaries and HTS/noble metal interfaces, while the other — in semiconducting barriers. We discuss an applicability of these models to description of the data for critical currents in YBCO/PBCO ramp junctions and HTS SNS junctions.

We study the suppression of the critical current in a multi-terminal superconductor/two-dimensional electron gas/superconductor Josephson junction by means of hot carrier injection. As a superconductor Nb is used, while the two-dimensional electron gas is located in a strained InGaAs/InP heterostructure. Two different modes of injection are employed. First, in the three-terminal injection mode, where the injection current flows from an injector contact to one of the superconducting electrodes, only a partial suppression is obtained. Second, in the four-terminal mode, where the injection current flows between two opposite injector contacts, a complete suppression is achieved. A theoretical model for the critical current suppression in a short junction is presented, which takes the two-dimensional character of the junction into account. Qualitatively, the experimental data agree well with the theoretical predictions. The injection voltage required in the experiment to suppress the supercurrent is lower than theoretically predicted. This is explained by the fact that the width of the normal region of the junction is slightly too large to be in the short-junction limit.

We study superconductor/ferromagnet/superconductor junctions with CuNi, PtNi, or Ni interlayers. Remarkably, we observe that supercurrents through Ni can be significantly larger than through diluted alloys. The phenomenon is attributed to the dirtiness of disordered alloys leading to a short coherence length despite a small exchange energy. To the contrary, pure Ni is clean resulting in a coherence length as long as in a normal metal. Analysis of temperature dependencies of critical currents reveals a crossover from short (dirty) to long (clean) range proximity effects in ${\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}$ with increasing Ni concentration. Our results point out that structural properties of a ferromagnet play a crucial role for the proximity effect and indicate that conventional strong-but-clean ferromagnets can be advantageously used in superconducting spintronic devices.

The system of electrons interacting with phonons and excitons is studied. In the standard phonon model modes with low and high frequencies are considered, the latter being interact strongly with electrons. The combined model takes into account low frequency modes and excitons interacting weakly with electrons. Both models cannot explain all essential features of copper oxide superconductors.

Microwave absorption by long Josephson junction in external magnetic field is calculated to explain the surface magnetic impedance of dense ceramic samples of high T c superconductors. The model describes qualitatively the main features of the experimental picture of the microwave absorption in dense high T c ceramics.