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We derive the radiation power from a single Josephson junction (JJ) and from layered superconductors in the flux-flow regime. For the JJ case we formulate the boundary conditions for the electric and magnetic fields at the edges of the superconducting leads using the Maxwell equations in the dielectric media and find dynamic boundary conditions for the phase difference in JJ which account for the radiation. In a finite-length JJ the power conversion efficiency is determined by the dissipation inside JJ and it tends to unity as dissipation vanishes independently of mismatch of the junction and dielectric media impedances. We formulate also the dynamic boundary conditions for the phase difference in intrinsic Josephson junctions in layered high-temperature superconductors at the boundary with free space. Using these boundary conditions, we find the dynamic phase difference in the linear regime for rectangular and triangular lattices of Josephson vortices and evaluate radiation into a free space for these simple cases., 33 pages, 9 figures. Submitted to V. Ginzburg commemorative volume, to be published in the Journal of Superconductivity and Novel Magnetism

A brief review of recent results on interlayer tunneling spectroscopy of layered high temperature superconductors (HTS) and charge density wave (CDW) materials is presented demonstrating high capability of this method for studies of both electron condensed states. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Zero bias conductance peak (ZBCP) and gap features have been studied on tunneling interlayer spectra of the micron-sized NbSe3 stacks of high quality. We show that temperature dependence and the shape of the ZBCP is self-consistently described by Bulaevskii theory of coherent interlayer tunneling of pocket carriers.

The interlayer tunneling has been studied on high quality Bi-2212 stacks of micron to the submicron lateral size. We found that low temperature and low voltage tunneling I – V characteristics can be self-consistently described by Fermi-liquid model for a d-wave superconductor with a significant contribution from coherent interlayer tunneling. For micron-sized stacks we found very clear Fraunhofer type dependence of critical current across the stack on parallel magnetic field with periodicity corresponding to one flux quantum per elementary junction. The gap and pseudogap interplay with variation of temperature and magnetic field has been extracted from the I – V characteristics. We consider also the role of charging effects for submicron stacks.

We show that application of an c-axis magnetic field decreases the effective interlayer Josephson coupling in small-area Bi2Sr2CaCu2O8+δ (Bi-2212) mesas at low temperatures. As magnetic field increases, the crossover from the classical regime of Cooper pair tunneling to the quantum regime leads to an additional drastic suppression of the Josephson coupling due to the Coulomb blockade. As result, the stack of many intrinsic junctions is nonsuperconducting at high fields. The quantum regime of superconducting tunneling may be reached in mesas with N junctions and area 0.3 μm 2 made of intercalated Bi-2212 crystals by applying c-axis magnetic fields B>4/N2 T at N≫1.

From measurements of the c-axis I-V characteristics of intrinsic Josephson junctions in Bi_2Sr_2CaCu_2O_{8+delta} (Bi-2212) mesas we obtain the field dependence (H || c) of the quasiparticle (QP) conductivity, sigma_q(H,T), and of the Josephson critical current density, J_c(H,T). The quasiparticle conductivity sigma_q(H) increases sharply with H and reaches a plateau at 0.05 T

Josephson plasma resonance has been introduced recently as a powerful tool to probe interlayer Josephson coupling in different regions of the vortex phase diagram in layered superconductors. In the liquid phase, the high temperature expansion with respect to the Josephson coupling connects the Josephson plasma frequency with the phase correlation function. This function, in turn, is directly related to the pair distribution function of the liquid. We develop a recipe to extract the phase and density correlation functions from the dependencies of the plasma resonance frequency $\omega_p({\bf B})$ and the $c$ axis conductivity $\sigma_c({\bf B}) $ on the {\it ab}-component of the magnetic field at fixed {\it c} -component. Using Langevin dynamic simulations of two-dimensional vortex arrays we calculate density and phase correlation functions at different temperatures. Calculated phase correlations describe very well the experimental angular dependence of the plasma resonance field. We also demonstrate that in the case of weak damping in the liquid phase, broadening of the JPR line is caused mainly by random Josephson coupling arising from the density fluctuations of pancake vortices. In this case the JPR line has a universal shape, which is determined only by parameters of the superconductors and temperature., Comment: 22 pages, 6 figures, to appear in Phys. Rev. B, December 1

We investigate the magnetic field dependence of the plasma resonance frequency in vortex crystal state. We find that low magnetic field induces a small correction to the plasma frequency proportional to the field. The slope of this linear field dependence is directly related to the average distance between the pancake vortices in the neighboring layers, the wandering length. This length is determined by both Josephson and magnetic couplings between layers. At higher fields the plasma frequency is suppressed collectively and is determined by elastic energy of the vortex lattice. Analyzing experimental data, we find that (i) the wandering length becomes comparable with the London penetration depth near T_{c}, (ii) at small melting fields (< 20G) the wandering length does not change noticeably at the melting transition demonstrating existence of the line liquid phase in this field range, and (iii) the self consistent theory of pancake fluctuations describes very well the field dependence of the Josephson plasma resonance frequency up to the melting point., 4 pages, 2 Figers, Paper submitted to the conference proceedings of M2S-2000 Houston, TX

We report on the c -axis resistivity ϱ c ( H ) in Bi 2 Sr 2 CaCu 2 O 8+δ that exhibits a maximum in quasi-static magnetic fields up to 60 T. We show that below T c c -axis tunneling is a parallel, two-channel process: (i) σ j of Cooper pairs and (ii) σ q of quasiparticles: σ c = σ j + σ q . We demonstrate that high-field c -armaxis conductivity is linear-in-field at H H c 2 , in agreement with the field-linear dependence of quasiparticle conductivity σ q we find in mesas. Near 60 T, as temperature decreases, ϱ c saturates, confirming a gapless spectrum of quasiparticles. The linear ϱ c ( H ) behavior extends up to the maximum field in experiment (60T), beyond the applicability of the Fermi-liquid approach, which can be due to effects of the pseudo gap, not accounted in the model.

We report on the c-axis resistivity rho(c)(H) in Bi(2)Sr(2)CaCu(2)O(8+delta) that peaks in quasistatic magnetic fields up to 60 T. By suppressing the Josephson part of the two-channel (Cooper pair/quasiparticle) conductivity sigma(c)(H), we find that the negative slope of rho(c)(H) above the peak is due to quasiparticle tunneling conductivity sigma(q)(H) across the CuO2 layers below H(c2). At high fields (a) sigma(q)(H) grows linearly with H, and (b) rho(c)(T) tends to saturate ( sigma(c) not equal0) as T-->0, consistent with the scattering at the nodes of the d-wave gap. A superlinear sigma(q)(H) marks the normal state above T(c).

We consider the magnetic field dependence of the plasma resonance frequency in pristine and in irradiated Bi$_2$Sr$_2$CaCu$_2$O$_8$ crystals near $T_c$. At low magnetic fields we relate linear in field corrections to the plasma frequency to the average distance between the pancake vortices in the neighboring layers (wandering length). We calculate the wandering length in the case of thermal wiggling of vortex lines, taking into account both Josephson and magnetic interlayer coupling of pancakes. Analyzing experimental data, we found that (i) the wandering length becomes comparable with the London penetration depth near T$_{c}$ and (ii) at small melting fields ($< 20$ G) the wandering length does not change much at the melting transition. This shows existence of the line liquid phase in this field range. We also found that pinning by columnar defects affects weakly the field dependence of the plasma resonance frequency near $T_c$., RevTex, 4 pages, 2 PS figures, Submitted to Phys. Rev. B

The Josephson plasma resonance (JPR) provides a sensitive probe of vortex states in layered superconductors. We demonstrate that in the case of weak damping in the liquid phase, broadening of the JPR line is caused mainly by random Josephson coupling arising from the density fluctuations of pancake vortices. In this case the JPR line has the universal shape, which is determined only by parameters of the superconductors and temperature. This mechanism gives a natural explanation for the experimentally observed asymmetric lineshape. The tail at high frequencies arises due to mixing of the propagating plasma modes by random Josephson coupling, while the tail at small frequencies is caused by the localized plasma modes originating from a rare fluctuation suppression of the Josephson coupling in large areas., Comment: RevTeX, 4 pages, 1 PS figure, Accepted to Phys. Rev. B, Rapid Communications

We measured the angular dependence of the c-axis magnetoresistance ${\ensuremath{\rho}}_{c}(\mathbf{B})$ of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ irradiated with heavy ions. At temperatures near 69 K the scaling of ${\ensuremath{\rho}}_{c}(\mathbf{B})$ with the c-axis magnetic field component ${B}_{\ensuremath{\perp}}$ is broken and the in-plane field, ${B}_{\ensuremath{\parallel}},$ affects ${\ensuremath{\rho}}_{c}.$ At this temperature, at a specific field ${B}_{\mathrm{cr}}\ensuremath{\approx}{B}_{\ensuremath{\Phi}}/2,$ magnetoresistance becomes independent of field orientation. This crossing point allows us to estimate the correlation length L of pancake positions along the c axis. We find $L/s\ensuremath{\sim}100$ at ${B=B}_{\mathrm{cr}},$ where s is the interlayer spacing. This provides evidence of strong enhancement of pancake alignment in the vortex liquid in crystals with columnar defects.

We study the $c$-axis transport of stacked, intrinsic junctions in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystals, fabricated by the double-sided ion beam processing technique from single crystal whiskers. Measurements of the $I\ensuremath{-}V$ characteristics of these samples allow us to obtain the temperature and voltage dependence of the quasiparticle $c$-axis conductivity in the superconducting state, the Josephson critical current, and the superconducting gap. We show that the BCS $d$-wave model in the clean limit for resonant impurity scattering, with a significant contribution from coherent interlayer tunneling, describes satisfactorily the low temperature and low energy $c$-axis transport of both quasiparticles and Cooper pairs.

We have performed a microscopic calculation of the dielectric response function in highly anisotropic layered superconductors and used the developed approach to obtain the frequency-dependent London penetration length and conductivity in the case of {ital d}-wave pairing for currents perpendicular to the layers. We consider a BCS model with coherent interlayer tunneling of electrons and take into account contributions from both superconducting electrons and quasiparticles to the dielectric response. We show that quasiparticles change the low-temperature behavior of the penetration length in the intermediate frequency range where the frequency is smaller than the superconducting order parameter but larger than the inverse quasiparticle scattering time. The obtained results are used to describe the low-temperature behavior of the Josephson plasma resonance, in particular the temperature dependence of the resonance frequency and the resonance linewidth in zero external magnetic field. We compare our results with the available experimental data for Tl{sub 2}Ba{sub 2}CuO{sub 6} and Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} (Bi-2212) and show that results of a BCS model with coherent interlayer tunneling for the dc {ital c}-axis resistivity in the superconducting state are inconsistent with experimental data for underdoped and optimally doped Bi-2212 crystals. {copyright} {ital 1999} {ital The American Physical Society}

The $c$-axis resistivity in irradiated and in pristine $\mathrm{Bi}{}_{2}\mathrm{Sr}{}_{2}\mathrm{CaCu}{}_{2}\mathrm{O}{}_{8\ensuremath{-}\ensuremath{\delta}}$ crystals is measured as a function of the in-plane magnetic field component at fixed out-of-plane component ${B}_{\ensuremath{\perp}}$ in the vortex liquid phase at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}67\mathrm{K}$. From this data we extract the dependence of the phase difference correlation length inside layers on ${B}_{\ensuremath{\perp}}$ and estimate the average length of the vortex line segments confined inside columnar defects as a function of the filling factor ${f\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}B}_{\ensuremath{\perp}}/{B}_{\ensuremath{\Phi}}$. The maximum length, about 15 interlayer distances, is reached near $f\ensuremath{\approx}0.35$.

We present c-axis magnetoresistance and I-V measurements in longitudinal magnetic field in single crystals of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} irradiated with heavy ions that display distinct anomalies near fractional filling of the columnar defects created by the irradiation. The anomalous behavior extends over a limited range of temperature and, in many crystals, is manifested by a minimum in {rho}{sub c}(H). The region of temperature-field where these anomalies appear coincides with that where two Josephson-plasma resonance lines have been recently observed. Our measurements directly show that a reduced pancake and phase slip mobility is associated with columnar defect filling in the vortex liquid state. {copyright} {ital 1998} {ital The American Physical Society}

We consider the Josephson plasma mode in the mixed state of a single Josephson junction and of a layered superconductor. We focus on the frequencies and optical weights of plasma modes which can be excited by an external spatially homogeneous ac electric field in the mixed state. Our consideration is based on the sum rules for the dielectric function which describe the response of a Josephson coupled system to an ac electric field. We show that for a single Josephson junction in a parallel field the plasma frequency increases with a magnetic field applied parallel to the junction. For layered superconductors we discuss qualitatively the angular dependence of the plasma frequency for a tilted magnetic field using an interpolation between results known for perpendicular and parallel fields. We predict that two plasma peaks may be observed in magnetic fields of the order H{sub 0}={Phi}{sub 0}/{gamma}s{sup 2}, orientated close to the ab plane, and that the plasma frequency increases with increasing magnetic field B{le}H{sub 0} applied parallel to the layers. {copyright} {ital 1997} {ital The American Physical Society}

The c axis resistivity ϱ c of Bi-2:2:1:2 single crystals is measured in magnetic fields up to 26.4 T. In the fluctuation regime, B > H c2 ( T ), the increase of ϱ c as B approaches H c2 from above at given T is described by the theory based on the BCS model for Josephson coupled layered superconductors. In contrast, the increase of ϱ c as T approaches T c ( B ) is much stronger than this theory predicts. This indicates evidence for a temperature dependent ϱ c in the normal state.

We have performed transport, resistivity and critical current measurements on Bi-2223/Ag tapes with current directions both parallel and perpendicular to the tape plane in magnetic fields up to 7 T and 50 >

We discuss the interaction of a vortex array in a superconductor with sound oscillations, taking into account pinning forces between vortices and crystal displacements. The generation of ultrasound by an ac electromagnetic field acting on the vortex array is considered. We find that at low temperatures and in the linear regime the amplitude of the induced sound wave is proportional to the applied dc magnetic field and independent of pinning strength. We show how the subsequent multiple echo amplitudes allow one to obtain information on the Magnus force and viscosity of the vortex array.

Flux pinning in high-temperature superconductors such as YBa2Cu3O7−x (YBCO) in the past has been accomplished by pinning the vortex cores. We demonstrate magnetic-domain-induced flux pinning of the magnetic flux of vortices in a ferromagnet-superconductor bilayer consisting of CoPt grown on YBCO, where the ferromagnet has uniaxial perpendicular magnetic anisotropy and a random domain structure. We observe an improvement of the critical current due to magnetic pinning at temperatures close to the transition temperature.

The vortex crossing rate in thin current-biased superconducting strips, calculated within the London approach employing the concept of a vortex as a particle, is very sensitive to the cutoff at the vortex core size. To account properly for the vortex core, one needs to use microscopic theory.

We argue that photon counts in a superconducting nanowire single-photon detector (SNSPD) are caused by the transition from a current-biased metastable superconducting state to the normal state. Such a transition is triggered by vortices crossing the thin and narrow superconducting strip from one edge to another due to the Lorentz force. Detector counts in SNSPDs may be caused by three processes: (a) a single incident photon with sufficient energy to break enough Cooper pairs to create a normal-state belt across the entire width of the strip (direct photon count), (b) thermally induced single-vortex crossing in the absence of photons (dark count), which at high-bias currents releases the energy sufficient to trigger the transition to the normal state in a belt across the whole width of the strip, and (c) a single incident photon of insufficient energy to create a normal-state belt but initiating a subsequent single-vortex crossing, which provides the rest of the energy needed to create the normal-state belt (vortex-assisted single-photon count). We derive the current dependence of the rate of vortex-assisted photon counts. The resulting photon count rate has a plateau at high currents close to the critical current and drops as a power law with high exponent at lower currents. While the magnetic field perpendicular to the film plane does not affect the formation of hot spots by photons, it causes the rate of vortex crossings (with or without photons) to increase. We show that by applying a magnetic field one may characterize the energy barrier for vortex crossings and identify the origin of dark counts and vortex-assisted photon counts.

We have measured the high-magnetic-field dependences (up to 18 T) of the in-plane and out-of-plane transport properties of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ (Bi-2:2:1:2) single crystals. From I-V curves of c-axis transport we extract the magnetic field and temperature dependences of the interlayer critical current and define a decoupling line ${\mathit{H}}_{\mathit{D}}$(T) above which ${\mathit{J}}_{\mathit{c},}$c vanishes. The temperature dependence of ${\mathit{J}}_{\mathit{c},}$c supports a first-order decoupling transition. The decoupling line lies close to the irreversibility line in the field interval 0.5--5 T. The field dependence of the in-plane vortex activation energy is \ensuremath{\sim}${\mathit{B}}^{\mathrm{\ensuremath{-}}0.5\ifmmode\pm\else\textpm\fi{}0.1}$.

The field ([ital B]) and temperature dependence of reversible magnetization [ital M] in Bi (2:2:1:2) single crystals is measured for the orientation of the field along the [ital c] axis. The slope of the curve [ital M] vs ln[ital B] decreases remarkably with field below 70 K. In the mean-field approach this slope should be almost [ital B] independent. We explain this change of slope by the quantum fluctuations of vortices. We show that for magnetization quantum fluctuations are important at all temperatures except in a narrow region near [ital T][sub [ital c]].

A vortex crossing a thin-film superconducting strip from one edge to the other, perpendicular to the bias current, is the dominant mechanism of dissipation for films of thickness d on the order of the coherence length XI; and of width w much narrower than the Pearl length LAMBDA >> w >> XI. At high bias currents, I* < I < Ic, the heat released by the crossing of a single vortex suffices to create a belt-like normal-state region across the strip, resulting in a detectable voltage pulse. Here Ic is the critical current at which the energy barrier vanishes for a single vortex crossing. The belt forms along the vortex path and causes a transition of the entire strip into the normal state. We estimate I* to be roughly Ic/3. Further, we argue that such "hot" vortex crossings are the origin of dark counts in photon detectors, which operate in the regime of metastable superconductivity at currents between I* and Ic. We estimate the rate of vortex crossings and compare it with recent experimental data for dark counts. For currents below I*, i.e., in the stable superconducting but resistive regime, we estimate the amplitude and duration of voltage pulses induced by a single vortex crossing., 9 pages, 3 figures

We report a detailed study of the temperature and magnetic-field dependence of free energy, magnetization, and specific heat of superconducting ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ in the critical fluctuation region near ${\mathit{H}}_{\mathit{c}2}$(T) line. Our approach is to apply the recently developed nonperturbative scaling theory (Te\ifmmode \check{s}\else \v{s}\fi{}anovi\ifmmode \acute{c}\else \'{c}\fi{} et al.) in high fields to describe the reversible magnetization data taken on a c-axis-oriented superconducting ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ thin tape. We find good agreement on some of the superconducting parameters obtained in the present study with those derived by applying the Ginzburg-Landau theory to our reversible magnetization data taken outside the critical fluctuation region.

Within the framework of the brick-wall'' model, we review in detail how various physical mechanisms could act to limit the critical current in high-[ital T][sub [ital c]] superconducting tapes. The brick-wall model attempts to mimic in a simplified manner the complicated tape microstructure. We generalized it to take into account a distribution of grains sizes and possible different strengths of weak links between the grains and the existence of weak links associated with intergrain boundaries perpendicular to the plane of the tape. We discuss what experiments can distinguish between a situation where weak links limit the critical current, and another plausible scenario where intragranular pinning is the limiting factor. Our conclusion is that according to the available experimental data only a small, well-connected fraction of the high-[ital J][sub [ital c]] tapes is effective in providing high critical current densities. For this fraction of the tape, pinning turns out to be the limiting mechanism. Finally, several possibilities for increasing the critical current of the high-[ital T][sub [ital c]] superconducting tapes are discussed.

We calculate the magnetic field and temperature dependence of the critical current density [ital j][sub [ital c]] along the [ital c] axis in Josephson-coupled layered systems when the applied magnetic field is parallel to the [ital c] axis. [ital j][sub [ital c]] decreases with field and temperature because of the thermal fluctuations of the vortices which induce a phase difference across the junctions. The anisotropy ratio [gamma][proportional to]1/ [radical][ital j][sub [ital c]] acquires a field and temperature dependence. The decoupling phase transition line above which the superconducting current along the [ital c] axis vanishes is determined.

We show that shunt capacitor stabilizes synchronized oscillations in intrinsic Josephson junction stacks biased by DC current. This synchronization mechanism has an effect similar to the previously discussed radiative coupling between junctions, however, it is not defined by the geometry of the stack. It is particularly important in crystals with smaller number of junctions, where radiation coupling is week, and is comparable with the effect of strong super-radiation in crystal with many junctions. The shunt also helps to enter the phase-locked regime in the beginning of oscillations, after switching on the bias current. Shunt may be used to tune radiation power, which drops as shunt capacitance increases., Comment: 9 pages, 1 fig

In type-II superconductors that contain a lattice of magnetic moments, vortices polarize the magnetic system inducing additional contributions to the vortex mass, vortex viscosity, and vortex-vortex interaction. Extra magnetic viscosity is caused by radiation of spin waves by a moving vortex. Like in the case of Cherenkov radiation, this effect has a characteristic threshold behavior and the resulting vortex viscosity may be comparable to the well-known Bardeen-Stephen contribution. The threshold behavior leads to an anomaly in the current-voltage characteristics, and a drop in dissipation for a current interval that is determined by the magnetic excitation spectrum., Comment: 4 pages, 1 figure

The free energy functional for a single vortex in a system of Josephson-coupled superconducting layers is derived in terms of coordinates of two-dimensional vortices, «pancakes,» which make the vortex «line.» The energy consists of equally important two-, three-, and four-pancake interactions

The temperature ${\mathit{T}}_{\mathit{s}}$ of spontaneous creation of vortex lines in Josephson coupled layered superconductors is obtained by taking into account the entropy contribution to the free energy due to thermal distortions. For Bi- and Ti-based high-${\mathit{T}}_{\mathit{c}}$ superconductors and superlattices, the superconducting critical temperature ${\mathit{T}}_{\mathit{s}}$ lies noticeably below the mean-field transition temperature ${\mathit{T}}_{\mathit{c}0}$(${\mathit{T}}_{\mathit{c}0}$-${\mathit{T}}_{\mathit{s}}$\ensuremath{\approxeq}4 K or more). The contribution of thermal distortions to the free enery causes significant changes in the temperature and field dependences of magnetization below ${\mathit{T}}_{\mathit{s}}$ seen in Bi compounds.

A grain-structure model is used to obtain the low-temperature transport properties of polycrystalline Bi-based high-temperature superconducting tapes, including the magnetic-field-dependent critical current. The grain structure is regarded as resembling a brick wall, such that the net horizontal supercurrent passes from brick to brick chiefly through the horizontal junctions between bricks. A high-field critical-current plateau is predicted, assuming inhomogeneous Josephson junctions between highly anisotropic superconducting grains.

We argue that superconductor/ferromagnet multilayers of nanoscale period should exhibit strong pinning of vortices by the magnetic domain structure in magnetic fields below the coercive field when ferromagnetic layers exhibit strong perpendicular magnetic anisotropy. The estimated maximum magnetic pinning energy for single vortex in such a system is about 100 times larger than the pinning energy by columnar defects. This pinning energy may provide critical currents as high as 106−107 A/cm2 at high temperatures (but not very close to Tc) at least in magnetic fields below 0.1 T.

The Lawrence-Doniach model with inequivalent conducting layers (isotropic CuO{sub 2} planes and CuO planes with chains) gives the positive curvature in the temperature dependence of the perpendicular upper critical field, if the weakest coupling of layers is between CuO{sub 2} and CuO planes or if the bare critical temperatures of the CuO{sub 2} and CuO layers are essentially different. In the former case {ital H}{sub {ital c}1} and {ital H}{sub {ital c}2} are anisotropic in the ({ital a},{ital b}) plane near {ital T}{sub {ital c}}, while in the latter case such anisotropy is absent.

Single crystals of layered high-temperature superconductors intrinsically behave as stacks of Josephson junctions. We analyze response of current-biased stack of intrinsic junctions to irradiation by the external electromagnetic (em) wave. In addition to well-known Shapiro steps in the current-voltage characteristics, irradiation promotes stimulated radiation which adds with spontaneous Josephson radiation from the crystal. Such enhancement of radiation from current-biased crystal may be used for amplification of em waves. Irradiation also facilitates synchronization of Josephson oscillations in all intrinsic Josephson junctions of a single crystal as well as oscillations in intrinsic junctions of different crystals.

Strongly correlated electron insulators are one of the most important classes of materials in modern solid-state physics. We demonstrate that certain classes of Mott insulators, that usually are geometrically frustrated, exhibit unexpected charge effects: for certain spin textures, spontaneous circular electric currents or nonuniform charge distribution exist in the ground state of Mott insulators. In addition, low-energy "magnetic" states contribute comparably to the dielectric and magnetic response functions, epsilon(ik)(omega) and mu(ik)(omega), leading to interesting phenomena such as dipole-active ESR transitions, rotation of the electric-field polarization, and resonances which may be common to both functions producing a negative refraction index in a window of frequencies.

The anisotropy of the gap in layered compounds is studied in the framework of the BCS model, taking into account the interlayer and different intralayer couplings in the system. The compounds with alternating normal ({ital N}) and superconducting ({ital S}) layers are studied in detail, the {ital N} layers being assumed to have zero pairing intralayer interaction. In {ital S}-{ital N} systems with Josephson coupling of layers, there are two quite different gaps and also different temperature scales of the superconducting properties of the {ital S} and {ital N} layers. The results are used to discuss the behavior of high-{Tc} superconductors and possible artificial superlattices based on these compounds.

We consider electromagnetic emission from a Josephson junction (JJ) in a resistive state in an external magnetic field and derive the radiation power from the dielectric layer inside a JJ directly into outside dielectric media. Matching the electric and magnetic fields at the JJ edges, we find dynamic boundary conditions for the phase difference in JJ. We find that the fraction of the power transformed into radiation is determined by the dissipation inside the JJ. It tends to unity as dissipation vanishes independently of the mismatch of the junction and dielectric media impedances.

We have studied the microwave properties of 0.4 μm thick YBa2Cu3O7−x (YBCO) films on polycrystalline substrates with ion-beam-assisted-deposited yttria-stabilized zirconia buffer layers using a parallel-plate resonator technique at 10 GHz. The YBCO films with similar in-plane texture grown on both forsterite and Ni-based alloy substrates show similar microwave properties. We measure low-power surface resistance Rs values of about 0.5 mΩ at 76 K and 0.15 mΩ at 4 K for films with an in-plane mosaic spread of about 7°. Single-tone power-dependence measurements show that the surface resistance and the surface reactance increase linearly and by the same amount with increasing microwave field level. At intermediate power levels, the intermodulation measurements show odd-order intermodulation products that increase quadratically with two-tone input power. These results indicate a hysteretic vortex penetration mechanism in the weak links as the most plausible source of the observed nonlinearities in these films.

We consider the Josephson vortex flow in layered superconductors in the presence of the transport DC current perpendicular to the layers and high magnetic field parallel to the layers. Moving vortex lattice induces the radiation from the edge of the crystal into the dielectric. We derive radiation power for moving rectangular and triangular lattice. We estimate corresponding radiation power for Bi-based cuprate superconductor when the magnetic field of order of one tesla is applied and the radiation frequency is in THz interval. We show qualitatively that radiation power for lattice disordered along the c-axis is weaker than that for regular lattice. We estimate the heat flow which in cuprate crystals is determined mainly by the nodal quasiparticles.

In magnetic superconductors a moving vortex lattice is accompanied by an ac magnetic field which leads to the generation of spin waves. At resonance conditions the dynamics of vortices in magnetic superconductors changes drastically, resulting in strong peaks in the dc I-V characteristics at voltages at which the washboard frequency of vortex lattice matches the spin wave frequency $\omega_s({\bf g})$, where ${\bf g}$ are the reciprocal vortex lattice vectors. We show that if washboard frequency lies above the magnetic gap, peaks in the I-V characteristics in borocarbides and cuprate layered magnetic superconductors are strong enough to be observed over the background determined by the quasiparticles., Comment: 4 pages, 1 figure

We show that the flux-flow transport of the Josephson vortex lattice (JVL) in layered high-temperature superconductors provides a convenient probe for both components of quasiparticle conductivity, ${\ensuremath{\sigma}}_{c}$ and ${\ensuremath{\sigma}}_{\mathrm{ab}}.$ We found that the JVL flux-flow resistivity, ${\ensuremath{\rho}}_{\mathrm{ff}},$ in a wide range of magnetic fields is mainly determined by the in-plane dissipation. In the dense lattice regime $(Bg1$ T) the ${\ensuremath{\rho}}_{\mathrm{ff}}(B)$ dependence is well fitted by the theoretical formula for that limit. That allows us to independently extract from the experimental data the values of ${\ensuremath{\sigma}}_{c}$ and of the ratio ${\ensuremath{\sigma}}_{\mathrm{ab}}/({\ensuremath{\sigma}}_{c}{\ensuremath{\gamma}}^{4}).$ The extracted temperature dependence ${\ensuremath{\sigma}}_{\mathrm{ab}}(T)$ is consistent with microwave data. The shape of the current-voltage characteristics is also sensitive to the frequency dependence of ${\ensuremath{\sigma}}_{\mathrm{ab}},$ which allows us to estimate the quasiparticle relaxation time and relate it to the impurity bandwidth using data obtained for the same crystal.

International audience; Using small staked junctions of high quality we have measured interlayer tunnelling spectra of the layered quasi-one-dimensional material NbSe3. We identified a number of new features in the spectra: zero bias conductance peak (ZBCP), charge density wave (CDW) gap structure for lower and upper CDW, the sub-gap structure inside the CDW gap. The ZBCP dominates in low-temperature spectra. We discuss its origin as being related to the interlayer coherent tunnelling of the carriers uncondensed into CDW. We found that ZBCP is sensitive to magnetic field and its orientation. The field perpendicular to the layers broadens ZBCP while the parallel field narrows it. We consider the sub-gap structure as associated with self-localized states inside the gap.

We have measured the anisotropic transport properties of Bi2223/Ag tapes with and without the silver sheath. The anisotropy ratio of the resistivity along the rolling plane to the resistivity perpendicular to the rolling plane (∼4–10) is orders of magnitude smaller than single‐crystal anisotropy ratios. Results are consistent with current flow along crystallographic ab planes even for transport normal to the tape plane.