Microwave absorption in high-Tc superconductors

We have investigated the 9- and 10-GHz microwave properties of bulk samples of strontium and barium substituted La₂CuO₄ and YBa₂Cu₃O₇ and one YBa₂Cu₃O₇ MBE film on SrTiO₃. Anomalous microwave absorption is seen especially for poorly compacted bulk ceramic samples. Transitions were generally broadened nearT _c and the residual losses were high. The real part of the surface impedance could be fit belowT _c to the empirical equation with an added term linear in temperature. The microwave response of the film was masked by paraelectric absorption and acoustic resonances in the substrate.     

Thermal conductivity of high-Tc superconductors

This paper reviews existing data on the thermal conductivity of high-T _c superconductors. Included are discussions of pristine polycrystalline high-T _c ceramics, single crystal specimens, and high-T _c materials structurally modified by substitution or by radiation damage. The thermal conductivity of high-T _c superconductors is compared with that of conventional superconductors, and dramatic differences are found between the two families. Mechanisms of thermal conductivity applicable to high-T _c perovskites are discussed and implications for theories of high-T _c superconductivity are noted.     

The flux-line lattice in high-Tc superconductors

The flux-line lattice in type-II superconductors has unusual nonlocal elastic properties which make it soft for short wavelengths of distortion. This softening is particularly pronounced in the highly anisotropic high-T _c superconductors (HTSC) where it leads to large thermal fluctuations and to thermally activated depinning of the Abrikosov vortices. Numerous transitions are predicted for these layered HTSC when the temperatureT, magnetic inductionB, or current densityJ are changed. In particular, the flux lines are now chains of two-dimensional (2D) pancake vortices which may evaporate by thermal fluctuations or may depin individually. At sufficiently highT, ohmic resistivity(T, B) is observed down toJ 0. This indicates that the flux lines are in a liquid state with no shear stiffness and with small depinning energy or that the 2D vortices can move independently. At lowerT, (T, B, J) is nonlinear since the pinning energy of an elastic vortex lattice or vortex glass increases with decreasingJ as predicted by theories of collective pinning and by vortex glass scaling.     

Superconductivity of 2212 phase in Bi-based high-Tc superconductors

In the Bi-based high-T _c superconductors, three superconducting transition points were observed above the liquid-N₂ temperature range. Allotropes of the 2212 phase were found. These allotropes were metastable and can interchange with the 2212 phase, and theirT _c's vary from 85 to 100 K.     

On the characteristic voltage of high-Tc superconductors

The critical currents and normal resistances of the small bridges from yttrium-based high-T _c superconducting ceramics have been measured. The characteristic voltage of these bridges was found to be approximately 20 V. This effect can be explained if, between the ceramic grains, there are contacts of an order of one crystalline cell in size.The authors are grateful to V. N. Polushkin for his help in this investigation.     

Specific heat of the high-Tc oxide superconductors

We review the specific heat measurements on La₂CuO₄, La_2–xM_xCuO₄ (M = Ca, Sr, and Ba), YBa₂Cu₃O₇, and the Bi-Ca-Sr-Cu-O and Tl-Ca-Ba-Cu-O systems. Tables of properties derived from the data are presented. Results on RBa₂Cu₃O₇ (R=rare earth elements other than Y) are summarized, as are results on YBa₂(Cu_3–xM_x)O₇ (M=Zn, Cr, Fe, or Ni). The difficulties of analyzing the specific heat data, and specifically the separation of the contributions associated with magnetic impurities, are discussed. It is tentatively concluded that the data nearT _c are consistent with BCS theory, although they show evidence of fluctuation effects. It is also concluded that the low-temperature zero-field data on a majority of the high-T _c oxide superconductors provide evidence of anintrinsic term that is proportional toT, a result that is inconsistent with a gap in the electronic density of states.     

Percolative behavior of transport properties in granular high-Tc superconductors

A new model based on percolative computation directly performed for an array system of Josephson junctions is reported. The number of percolative paths is obtained and the total critical current of the model system is estimated from 2D and 3D arrays at selected values of external magnetic field. The evolution of the critical current distribution of the junctions at increasing magnetic field is studied in view of the relevance in percolative path calculations and compared with the experimental distribution obtained fromI– V characteristics. Percolative simulation results are shown to reproduce more satisfactory experimental critical current behavior in external magnetic fields, obtained in highly porous thick YBa₂Cu₃O_7-x films electrophoretically deposited than mere statistical averages.     

Tcenhancement in superconductor-semiconductor arrays

Subsequent to our observation that the Tl- and Bi-based cuprate high-T _c superconductors are built of superconductor-semiconductor arrays (P. C. W. Fung and W. Y. Kwok,J. Superconduct., this issue), we investigate in this paper the possibility ofT _c enhancement arising from the effect of change of phonon spectrum and the effect of size quantization when one or more semiconductor blocks is attached to the basic superconductor in the unit cell.     

Theab-plane optical conductivity of high-Tc superconductors

There is anisotropy in theab-plane optical properties of the high-temperature superconductors, both in the normal state and in the superconducting state. In both states, two components appear in the optical conductivity: a free carrier part and a midinfrared component. BelowT _c , the free carriers form the superconducting condensate. In YBa₂Cu₃O_7–, the anisotropy of the penetration depth shows that the chains contribute strongly to this superfluid. In Bi₂Sr₂CaCu₂O₈, where chains are absent, there is stillab plane anisotropy. BelowT _c a finite absorption parallelb remains at frequencies as small as 20 meV. This anisotropy could be due to anisotropy either of the superconducting gap or the midinfrared component.     

Copper oxycarbonates and mercury-based cuprates: Promising high-Tc superconductors

A short overview of two recently discovered series of high-T _c superconductors is given. The first one deals with copper oxycarbonates which exhibit _c 's up to 82 K. Their structural principle is based on the fact that rows of CO₃ groups can assure the connection between either octahedral or pyramidal copper layers forming new intergrowths withT _c 's superior to those of the mother structures. The second family concerns the mercury-based cuprates, characterized byT _c 's ranging from 27 to 132 K. The structure of the mercury compounds is closely related to those of the thallium cuprates TlBa₂Ca_m–1Cu _m O_2m+3 which exhibit thallium monolayers; they differ from the Tl cuprates by a high oxygen deficiency at the level of the mercury layers due to the preference of this cation for two-fold coordination.     

Transport Ic measurement technique with non-soldered contacts for Ag-sheathed high Tc superconductors

A transport current measurement technique suitable for the technological development of hightemperature superconducting tapes has been designed. The main advantage of the technique presented is in the application of non-soldered current and voltage contacts which allows measurement of the same element of superconducting tape at different technological states (roll-sinter or press-sinter step). Voltage-current characteristics can be measured in self field as well as in external magnetic field at 77 K or 4.2 K and at different field orientations. The technique capabilities are demonstrated by presenting I_c data obtained from Bi(2223)/Ag tape.     

Structural analysis of high-Tccuprate superconductors

We have shown in this paper that members of the Tl-Ba-Ca-Cu-O and Bi-Sr-Ca-Cu-O superconducting families fabricated so far are built up of superconductor-semiconductor arrays. Four general homologous series are written down for these two families. The basic fundamental block in each series is itself a superconductor. We suggest that the Y-Ba-Cu-O family has a similar characteristic if YBaCu₂O_5-d is a superconductor.     

Intergranular fluxons in high-Tcsuperconductors

The granular high-T _c superconductors can carry very low transport current, in comparison to that found in the bulk of the material. Magnetization and critical current measurements at very low field indicate that this low transport current behaves as expected from a critical state model. The presence of weak links between the grains in granular aggregates is firmly established, together with the Josephson character of such links. The existence of some kind of magnetic particles, and of a mechanism of pinning for them, is required to explain the critical state regime. In this paper we examine the flux structures which can be present in the granular systems, which can be described by an array of SQUIDs, and we show their similarities to fluxons in a continuous medium and to fluxons in uniform Josephson junctions. A simple model, based on a two-dimensional network of pointlike Josepson junctions, is adequate to demonstrate the existance of the IF (intergranular fluxon). Its characteristics depend on a single parameter, which gives the coupling strength between grains. The discreteness of the system is the cause of an intrinsic pinning of the intergranular fluxons.     

Optical study of charge dynamics along thec-axis in high-Tc superconductors

Comparing the optical spectra withEc for typical high-T _c superconducting cuprates, we discuss the charge dynamics along thec-axis. The plasma energy or the mass anisotropic factorm _* ^c //m _* ^b is one of the key parameters determining the spectrum forEc. In Bi₂Sr₂CaCu₂O₈ and La_2–x Sr _x CuO₄ with a largem _* ^c /m _* ^b , the plasma energy is smaller than the superconducting gap energy, and thus the supercurrent along thec-axis is a kind of Josephson current flowing through insulating layers such as the BiO layers. On the other hand, in YBa₂Cu₃O₇ with a smallm _* ^c /m _* ^a , it seems that the coherent supercurrent flows along thec-axis. Although the spectrum forEc strongly depends on samples, presumably due to the difference in the hole concentration, a clear anisotropy between theab- and thec-directions is observed in the characteristics energy scales such as the plasma energy as well as the reflectivity knee energy.     

Magnetic hysteresis of the zero-resistance critical temperature of high-Tc granular superconductors

We have found a notorious hysteretic behavior in the dependence of the zero-resistance critical temperature obtained through resistivity () versus temperature (T) measurements with applied field (H _e ) in High-T _c granular superconductors. This behavior is explained semi-quantitatively based on the analogy between the present observation and a similar hysteresis found in the field dependence of the transport critical current in these materials.     

Measurements of the surface resistance of high-Tc superconductors at high RF fields

We report measurements of the r.f. surface resistance of a wide variety of high-T _c superconductors in bulk form and as films on silver substrates. Several apparatuses have been constructed and used for measurements at frequencies from 0.15 to 40 GHz and r.f. surface magnetic fields as high as 640 G. In every case in which the field dependence of the surface resistance was measured, the surface resistance increased monotonically with field amplitude through a transition region characterized by a strong field dependence. It then saturated at high field at a value of a few percent of the normal-state surface resistance just aboveT _c . In the presence of this field dependence, the frequency dependence of the surface resistance changed from quadratic to less than linear.     

High-Tc superconductor in an a.c. field

The impedance of the layered high-T _c oxide is evaluated. The response of the S-N proximity system is studied. The mechanism of intrinsic losses due to the low-lying collective mode is proposed.     

A possible explanation for the isotope effect in high-Tc and Rb3C60 superconductors

The role of pair breaking effects on the isotope effect coefficient is investigated using the generalized Abrikosov and Gorkov theory of superconductors. It is found that the pair breaking effects enhance the isotope effect coefficient. This provides a plausible explanation for the enhancement of the isotope effect coefficient as a function of impurity concentration in yttrium-, europium-, and lanthanum-based high-T _c superconductors. It may also explain the very large value of the isotope effect coefficient in Rb₃C₆₀ superconductors provided some intrinsic disorder is assumed.     

Positron-annihilation lifetime in the high-Tc Oxides

The superconducting transition affects the positron-annihilation lifetime in the high-T_c oxides. This effect is due to the unusual properties of the oxides in the normal state and, in particular, to the small Fermi energy. The value of the shift also depends on the interlayer distance.