Showing papers in "Journal of Superconductivity in 1991"

Relative permittivity and dielectric loss tangent of substrate materials for high-Tc superconducting film

Abstract: We measure the relative permittivitye r and dielectric loss tangent tanδ of substrate materials for high-T c superconducting films at 18–300 K and at 5–10 GHz using the cavity-resonator method. The materials measured are single crystals of MgO, LaAlO3, YAlO3, LaSrGaO4, NdGaO3, sapphire, and ZrO2 ceramic. Thee r values are 10–30 and become almost constant below about 50 K. The tanδ values decrease with decreasing temperature and are below 1×10−5 at 77 K except for those of NdGaO3 and ZrO2 ceramic. This suggests that the tanδ values of MgO, LaAlO3, YAlO3, LaSrGaO4, and sapphire do not limit the quality factors of microwave passive components fabricated using high-T c superconducting films. It is also demonstrated that the tanδ of the substrate material is strongly affected by impurities.

The surface impedance of superconductors and normal conductors: The Mattis-Bardeen theory

Abstract: The contribution of the Mattis-Bardeen theory to the understanding of the surface impedance of superconductors and normal conductors is reviewed. The early theoretical and experimental studies of the surface impedance of conductors are sketched to provide the context in which the Mattis-Bardeen theory and, independently, the Abrikosov-Gor'kov-Khalatnikov theory were developed. The Mattis-Bardeen theory is described along with the methods for numerical calculation of the surface impedance from their expression for the current density. Extensions to include the effects of anisotropy and strong coupling are briefly discussed. Theory is compared with representative measurements of the surface impedance, demonstrating excellent agreement in absolute magnitude and in the dependences on frequency, temperature, and material parameters.

On the Bose-Einstein condensation of nonzero-momentum cooper pairs. A second-order phase transition

Abstract: Based on the BCS Hamiltonian, the normal-to-super phase transition is investigated, approaching the critical temperatureT c from the high-temperature side. Nonzero-momentum Cooper pairs, that is, pairs of electrons (holes) with antiparallel spins and nearly opposite momenta aboveT c in the bulk limit, are shown to move like independent bosons with the energye vs. momentump relatione=1/2vF ν, whereν F represents the Fermi velocity (1/2m*ν F 2 ≡e F≡Fermi energy). The system of free Cooper pairs undergoes a phase transition of the second order with the critical temperatureT c given byk B T c=1/2(π2ħ3 ν F 3 n/1.20257)1/3 wheren is the number density of Cooper pairs. The ratio of the jump of the heat capacity, ΔC, to the maximum heat capacity,C s, is a universal constant: ΔC/C s=0.60874; this number is close to the universal constant 0.588 obtained by the finite-temperature BCS theory. The physical significance of these results is discussed, referring to the well-known BCS theory, which treats the many-Cooper-pair ground state exactly and the thermodynamic state belowT c approximately. An explanation is proposed on the question why sodium should remain normal down to 0 K, based on the band structures with the hypothesis that the supercondensate composed of zero-momentum “electron” and “hole” Cooper pairs is electrically neutral.

The microscopic theory of superconductivity

Abstract: Key steps in the development of the microscopic understanding of superconductivity are discussed.

Proton-induced reduction ofR s, Jc, andT c in YBa2Cu3O7?? thin films

Abstract: The authors explored the effect of 2-MeV H{sup +} irradiation on the superconducting transport properties of thin films of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} ({Tc}, J{sub c} (B = 0; 77 K, 4.2 K), and R{sub s} (36 GHz; T)). The inductively measured critical temperature {Tc} changed slowly and uniformly ({approximately}2 K per 10{sup 16}/cm{sup 2}) for fluences less than {approximately}3 {times} 10{sup 16}/cm{sup 2}. Beginning at {approximately}3-4 {times} 10{sup 16}/cm{sup 2}, the superconducting transition broadened and dropped more quickly with fluence. The critical current density measured at 77 and 4.2 K changed roughly linearly with fluence. The microwave {Tc} (as defined by the sharp transition in R{sub s} as a function of temperature) resembled the low-frequency inductive {Tc} measurement at low fluences but was depressed more strongly for large fluences. The residual surface resistance ({approximately}6-10 m{Omega}) was not affected for fluences up to 5 {times} 10{sup 16}cm{sup 2}. The sudden and reproducible reduction in the microwave {Tc} transition was interpreted as a sensitive indicator of disruption in the copper-oxygen chain sublattice, and the proton-induced change was compared to that observed in oxygen gettering studies of bulk materials.

On the temperature dependence of the London penetration depth in a superconductor

Abstract: In the framework of the electron-phonon theory of superconductivity [1], it is shown that the magnetic field penetration depthδ L increases like a certain power of temperature atT≪T c due to the low-lying excitations in the phonon spectrum. For the acoustic phonons with the density of states ∼ω 2 the penetration depth increases ∼T 5. The origin of such a high power ofT is the same as that in the case of resistivity of the normal metal: the phonon corrections to the electromagnetic vertex should be taken into account, and major terms (∼T 3) cancel, the surviving ones having a higher power ofT. The possibility of linear and quadratic terms inδ L(T) is discussed in a model of electrons interacting with two-level centers [2].

Small Helical Antenna Made of High-Temperature Superconducting Thick Film

Abstract: To realize a highly efficient small antenna, high-T c superconducting thick films with zirconia cores are adopted in order to fabricate both a self-resonating small helical radiator and a quarter-wave matching circuit. The actual gain of a superconducting antenna measured at 478 MHz using a 1/70-wavelength radiator was 4.9 dB higher than that of a copper antenna.

Temperature-dependent resistivity from phonons in cuprate superconductors

Abstract: The temperature dependence of the resistivity of the cuprate superconductors arising from phonon scattering is considered within standard Bloch-Gruneisen theory allowing for several different shapes of transport spectral functionsα tr 2 F(ω). In contrast to often made comments that linear resistivity is not characteristic of electron-phonon scattering, it is shown rather that it is more difficult to get muchnonlinearity from this theory except below 50–100 K, and in this low-T regime better solutions to the Boltzmann equation might be required. The reasons for the linear behavior are clarified, and the variations to be expected from possible coupling functions are explored. Possible origins of the nonlinear resistivity seen in YBa2Cu4O8 and recently in YBa2Cu3O7 are suggested, and effects not included specifically in these calculations are discussed. It is also pointed out that phonon contributions to the energy (e) dependence of quasiparticles, which is a closely related consequence of electron-phonon coupling, will differ significantly from the simplee2 Fermi liquid behavior that is commonly assumed.

Twin-boundary pinning and flux flow in zone-melt-textured YBa2Cu3O7-δ crystals

Abstract: Using a microwave (13 GHz) technique which is sensitive to field-induced changes in the surface resistance, we have observed twin-boundary pinning in zone-melt-textured crystals of YBa2Cu3O7-δ. With the rotation of a static 1.9-T field applied in thea-b plane, the surface resistanceR s exhibits minima at 90° intervals. The twin-boundary minima are shown to be connected to the orientation of the crystalline axes of the sample. The resistance minima are observed superimposed on a flux-flow response over the temperature range ofT c to the lowest temperature attempted. Substantial flux mobility is observed to the lowest temperatures. The angular dependence of the flux-flow response at low temperatures is consistent with the “brick” model for local intergranular conductivity, which provides for an in-plane isotropic component.

On Dislocation-Induced Reentrant-Like Behavior in Deoxygenated High-Tc Superconductors

Abstract: A possible scenario for dislocation-induced intragrain weak-link generation is proposed. The theory predicts a reentrant-like behavior of the critical current density vs. magnetic field due to the sample deoxygenation. A qualitative agreement with high-T~ single crystal anomalies is also discussed.

Effective dimensionality of cuprate superconductors

Abstract: A novel optical method for accurate monitoring of the carrier densityN in layered conductors is introduced. It is applied to several cuprate superconductors, and it is found thatTcdoesnot scale withN. However, if the closely spaced pairs or triplets of CuO2 planes were coupled together to act as distinct metallic slabs, the relevant variable could ben, theslab carrier density. Indeed, we found thatTcdoes scale nicely withn. This suggests that high-Tcsuperconductivity is effectively a two-dimensional phenomenon; it can occur in a cuprate slab which is only one unit cell thick. Finally, estimates are obtained for the length scales of localization of charge carriers:d ≈ 8 A andd⊥ ≲ 2 A in all the cuprates under study.

Monitoring phase formation of Pb-substituted Bi-Sr-Ca-Cu-O superconducting samples at different preparative stages using Raman spectroscopy and X-ray diffraction

Abstract: Raman spectroscopy and X-ray diffraction were used to follow structure and phase formation at various stages in order to study the effect of Pb substitution in (Bi1−xPb x Sr3Ca3Cu4O y samples. We found that major reactions involving Bi2O3 occurred at around 700°C and that reactions with Ca, Cu, and Pb started at a lower temperature. The amount of Ca2PbO4 formed increases as a function of lead concentration and annealing temperature (up to 800°C), but excess lead substitution (50%) destroys the superconductivity. The high-temperature superconducting phase (2223) is only observed in the 15% and 20% leaded samples. These two samples exhibit a higher amount of Ca2PbO4 during intermediate processing stages, which suggests that the presence of the Ca2PbO4 is important for the formation of the high-T c phase. It was found that theT c increases with lead concentration (up to 20% Pb).

Structural analysis of high- T c cuprate superconductors

Abstract: 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 YBaCu2O5-d is a superconductor.

A review of the size of the energy gap in superconducting oxides

Abstract: An attempt has been made to summarize a large body of data dealing with measurements of the energy gap in superconducting oxides like La2−x Sr x CuO4−y , YBa2Cu3O7−y , and related compounds. A plot of available data of the energy gap 2Δ as a function of the superconducting transition temperatureT c revealed a large “dispersion” of some data from 2Δ/k B T c =3.53 for the weak coupling BCS prediction. Nevertheless, we could conclude that the size of the energy gap in these exotic materials is within the weak coupling BCS prediction, in view of some controversial issues that could be responsible for the dispersion, such as the gap anisotropy as well as identification of the energy gap.

Optimization of annealing temperature for YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Abstract: Post-annealing of YBa2Cu3O7 (YBCO) thin films is usually performed at 850–900°C in atmospheric-pressure oxygen. In this study, coevaporated YBCO films on LaAlO3 were post-annealed in an oxygen partial pressure of 29 Pa at temperatures in the range 700–825°C. Zero resistance transition temperatures were 89–90 K. Both d.c. (room-temperature resistance and critical-current density) and a.c. parameters (extracted from eddy-current response measurements at 25 MHz) were monitored. The optimum temperature is close to 750°C, which is on the YBCO thermodynamic stability line at this low oxygen partial pressure.

Electronically driven short-range lattice instability: Possible role in superconductive pairing

Abstract: A superconducting pairing mechanism is suggested, mediating by collective and coherent cluster fluctuations in the materials. Our model, based on a geometrical frustration, proposes a dynamic effect driven by a special short-range electronic instability. Experimental support for this model is discussed.

“Peak effect” in the magnetic field dependence of the transport critical-current density in high-T c superconducting ceramics

Abstract: The maximum observed in the magnetic field dependence of the transport critical-current density of high-T c superconducting ceramics was analyzed. Transport critical current and magnetization measurements performed on Bi(Pb)-Sr-Ca-Cu-O bulk sintered samples allowed us to conclude that such a maximum may result from the influence of demagnetization effects at low applied field values and an increase of the intergrannular pinning at higher fields.

Microstructure and superconducting properties of Y-Ba-Cu-O films prepared by chemical spray pyrolysis

Abstract: Y-Ba-Cu oxide superconducting films were prepared by spray pyrolysis on (100) MgO substrates and alumina coated with a silver buffer layer. Acetate, nitrate, and oxide precursors were used as starting materials. The optimum starting compounds have been assessed and the effectiveness of rapid thermal processing as a fabrication tool for annealing of highTc superconducting films has been established. The superconducting behavior was found to be strongly dependent on the type of the precursor used, on the method of heat treatment, and on the substrate material. The films show preferred orientation and good adherence to the substrate. Transport measurements indicated that the films exhibited a superconducting transition with an onset temperature between 83–93 K and ending between 52–74 K. The critical current density was found to be strongly dependent on film processing.

John Bardeen and the theory of superconductivity

Abstract: Superconductivity was discovered in 1911, when John Bardeen was three years old. Who might have thought that over half a century would be required to develop an understanding of the mechanism of superconductivity and that Bardeen in particular would play the key role in this development?

Analysis of specific heat anomalies of high-Tc superconductors based on a two-carrier high-Tc superconductor model

Abstract: We investigate the specific heat properties of a high-Tc superconductor model, the “two-carrier model” which includes an electron-phonon interaction driven by lattice defects such as oxygen deficiencies in the otherwise conventional BCS superconductor. The normal state is identified as a two-carrier model system which is composed of two groups of charge carriers: the normal and the renormalized Bloch carriers. An enhanced normal-state electronic specific heat is obtained, and there is good agreement between theoretical predictions about the specific heat anomalies at and aboveTc and experimental data.

Effects of Sr/Ca ratio and CuO content of the superconducting phases in the Bi-Pb-Sr-Ca-Cu-O system

Abstract: The Bi-Pb-Sr-Ca-Cu-O superconductor was prepared by the three-step reaction process; the effects of Sr/Ca ratio and CuO content on the formation of the superconducting phases were studied. The Sr-short and Cu-rich nominal compositions are not advantageous for the formation of the 2223 phase but we are favored for the formation of the 2201 phase. The differences among the previous reports on the effect of composition on the formation of the superconducting phase is discussed. The best J c value of the Ag-sheathed tape filled with the kind of powder is 13,000 A/cm2 (77 K, OT).

Reproducibility of transition temperature and critical current density of thin films of YBa2Cu3O7 on (100) LaAlO3

Abstract: Submicrometer epitaxial films of YBa2Cu3O7(YBCO) on (100) LaAlO3 were made by coevaporation and furnace annealing. Samples from more than a dozen runs are used in this study. The zero resistance transition temperature (Tc) is high (89 or 90 K) if the film composition is phase pure (Ba/Y=2, Cu/Y=3) or if it is enriched in Ba and Cu. For these compositions the critical current density (Jc) at 77 K has an average value of 2×105 A cm−2, with a tendency for decreasingJc with increasing film thickness (0.2 to 0.8μm). Variations inJc are not correlated with deviations from ideal stoichiometry. Steeper slopes of the resistance-temperature curves above 100 K and lower values of the room-temperature resistivity are associated with high values ofJc. If the film composition is enriched in Y relative to Ba and Cu,Tc decreases by several degrees.

Oxygen content determination of Bi2Sr2Ca2Cu4O11+x superconductor by thermogravimetric analysis

Abstract: Thermogravimetric analysis of the individual, binary, ternary, and bismuth-based superconductor mixtures have been carried out to elucidate the excess oxygen content of the Bi2Sr2Ca2Cu4O11+x. Our systematic approach eliminates the need to assume initial phase present, original oxygen content, and degree of reduction as in other TG studies. The excess oxygen content of the bismuth superconductor increases fromx=0.38 tox=0.64 after three heating cycles in oxygen atmosphere. Most of the excess oxygen is associated with the highly oxidized copper (Cu3+) in the superconducting phase/phases.

Microwave properties of highly oriented YBa2Cu3O7-? superconducting thin films

Abstract: We have performed millimeter-wave frequency (94 GHz) measurements on high-quality YBa2Cu3O7-δ superconducting films on yttrium-stabilized (100) ZrO2 and MgO substrates. The ∼0.2μm thin films fabricated by magnetron sputteringin situ with the YBa2Cu3O7-δ powders as target exhibit superconducting transition temperatures up to 88 K. The critical current density of 6×105 A/cm2 at 77 K and the X-ray diffraction spectrum as well as scanning electron microscope photographs indicate these thin films are fullyc-axis oriented, extremely high in density, and universally homogeneous. Millimeter-wave surface resistances have been measured on a hemisphere open resonator in the temperature range of 20 K toT c and beyond. The surface resistance at 94 GHz and 77 K for these films is found to be about 30 mΩ, nearly 1/4 that for copper, and a drop of two orders in the surface resistance within 4 K is observed, which indicates that these films are good materials for applications in the millimeter-wave range, especially for fabricating microwave devices. We observed such low surface resistance in these thin films due to the near absence of grain and phase boundaries coupled with a high degree of crystalline orientation.

A qualitative explanation of tunneling characteristics of high-Tc junctions in terms of the McMillan proximity model

Abstract: In this paper the influence of proximity effects on the transport properties of high-Tc superconductors is considered. On the basis of the experimental results on tunneling spectroscopy reported in the current literature, we discuss some of the main features ofdI/dV vs.V characteristics. More particularly, attention is focused on the depression of gap structure, the presence of conductance peaks due to the counterelectrode gap, and “zero-bias anomalies.” A possible explanation of those features in terms of the McMillan proximity model is given along with some examples of application of the model to experimental data.

Tc enhancement in superconductor-semiconductor arrays

Abstract: Subsequent to our observation that the Tl- and Bi-based cuprate high-Tcsuperconductors 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 ofTcenhancement 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.

Quasiparticle spectrum in copper oxides

Abstract: The results of electronic structure calculations of the CuO2 layer, performed by the generalized tight-binding method, are presented. The strong electron correlations on both the Cu and O ions are included explicitly. The resulting quasiparticle band structure contains an insulator gap for undoped compounds like La2CuO4. The valence band consists of a large number of narrow Hubbard quasiparticle bands. The dispersion law of quasiparticles at the top of the valence band has a minimum at the Γ point, indicating the existence of the multivalley Fermi surface in the doped La2-xSr x CuO4. The origin of the Fermi liquid behavior in the narrow quasiparticle bands is discussed.

Charge exchange and the “Plateau” effect in YBa2Cu3O7−y

Abstract: The superconducting properties of YBa2Cu3O7-y, (YBCO) depend strongly on the oxygen content, whereas the transition temperatureTcremains relatively constant for oxygen content between 6.8 and 7.0 (the “Plateau” effect) this effect has attracted considerable attention [see, e.g., D. de Fontaineet al., Nature (London)343, 544 (1990); H. Paulsenet al., Nature (London)349, 594 (1991); J. Jorgensen,Phys. Today44, No. 6, 34 (1991)]. In this letter we introduce a microscopic theory of this phenomenon.

On the characteristic voltage of high-T c superconductors

Abstract: The critical currents and normal resistances of the small bridges from yttrium-based high-Tc superconducting ceramics have been measured. The characteristic voltage of these bridges was found to be approximately 20 μV. This effect can be explained if, between the ceramic grains, there are contacts of an order of one crystalline cell in size.

High-temperature ceramic SQUIDs

Abstract: Many fields of physics, and of science and technology in general, have radically changed over the past two decades due to the development of macroscopic quantum devices: lasers, masers, quantum frequency standards, and, of course, superconducting quantum interference detectors-SQUIDs. The SQUIDs today are the most sensitive and simple devices for registration of weak magnetic fields and electric and nonelectric quantities transformed into magnetic flux. There have naturally been many reviews and articles on SQUID operation and applications [1–3]. In this paper we will not touch on the well-known problems and will focus our attention mainly on high-temperature SQUIDs based on the results obtained at the JINR, Dubna.