Preparation of YBa2Cu3O7 ? δ Thin Films by the Self-Template Sputtering Method

Using the self-template technique, c-axis-oriented epitaxial YBa₂Cu₃O_{7 –} thin films have been prepared in situ on LaAlO₃ substrate by the d.c. magnetron sputtering method. The properties of thin film dependence on the deposition conditions of the two-step self-template method have been systematically investigated. By optimizing the parameters, high-quality YBCO thin films with T _c0 90 K, T _c 1 K, R _s (77 K, 10 GHz)500 were reproducibly obtained. The best sample grown under optimal conditions gave a low R _s of 330 at 77 K, 10 GHz, which can be used in a microwave field.     

Ordinary size effects and deviations from Matthiessen's rule in the resistance of fine wires

Contrary to previous statements in the literature, large deviations from Matthiessen's rule in fine wiresare to be expected on the basis of a straight-forward solution of the ordinary transport equation, assuming the relaxation-time approximation and imposing the idealized condition of diffuse scattering of electrons at the boundaries. Using Chambers' path-integral method to evaluate the current density in a wire of arbitrary cross-sectional shape, the effects of boundary scattering on the resistivity in the regimed 0.1 have been calculated for two model Fermi surface geometries. For the temperature-dependent part of the resistivity, _d (T) _d (T)– _d (0), two distinct types of behavior are found in the alternative cases: (1) for a spherical Fermi surface, _d(T) increases logarithmically with _d(0); (2) for a cylindrical Fermi surface, _d (T) increases essentially linearly with _d (0). [In each case the qualitative dependence of _d(0) on /d is, for practical purposes, linear. However, the correct value of the product in the cylindrical case is not simply given in the ordinary way by the slope of an empirical plot of _d (0) vs.d ^–1.] A comparison of theoretical results for the two simple models with the published data for indium and gallium shows that the actual temperature-dependent size effects are consistent, both qualitatively and, by a rough estimation, quantitatively, with the expected behavior.     

Investigation of the dominant point defects in tetragonal YBa2Cu3Ox at elevated temperatures

The manner in which oxygen is incorporated into YBa₂Cu₃O _x (YBCO) at 800°C for values ofx close to 6 is shown to be in the form of neutral oxygen interstitials, O _i ^x . The experimental data on which this conclusion is based are obtained from measurements of oxygen partial pressure,P(O₂), as a function of compositionx and temperatureT (5.99x 6.35, 825T1120 K). The data are obtained by a solid-state electrochemical method. Other conclusions of this study include: (a) O _i ^x are noninteracting forx 6. (b) The stoichiometric composition of YBCO isx 6.0. (c) The reaction enthalpy of oxidation is 179 kJ/mol O₂. (d) The Fermi level changes by –0.2 eV asx increases from 6.05 to 6.35.     

Theory of periodically driven,current-carrying,superconducting filaments. I. Spatially homogeneous states

Starting from the nonequilibrium theory of dirty superconductors in the Ginzburg-Landau regime, spatially homogeneous states with an applied currentI=I ₀+I ₁ cos (t) are considered. Expressions for the linear response (I₁ small) valid up to high frequencies (k _BT_c) are derived and evaluated analytically for the experimentally important case of smallI ₀ and ₀(T). Then the nonlinear response is treated for frequencies with _E1. Interesting new behavior is found for frequencies ₀ 1, where ₀ is essentially the GL relaxation time.     

On intrinsic and extrinsic effects in the surface impedance of cuprate superconductors

Surface impedance measurements in the normal and superconducting state are an excellent method to study conduction electron dynamics and extended defects. Electron dynamics show up most clearly in the relaxation range, i.e., for distances traveled in one rf periods= _F/ ( _F Fermi velocity) being smaller or of the order of the penetration depth and mean free pathl. For materials with _F10⁷ cm/sec the relaxation range is easily accessible forf0.1 THz. Then, in the normal state, relaxation defines the surface impedance with an intrinsic penetration depth _I approaching the London penetration depth _L andR _{I 0 L}/ 2 as surface resistance, allowing measurement of _L and relaxation time(T, ). In the superconducting state the photon interaction scales with _L/ _L=1/( _F dimension of Cooper pairs forl) and causes at low frequencies an absorption rate growing with, which is decreasing with _F/l. The rate increase proportional to turns to a decrease above 0.1 THz, which is accompanied by a decrease ofA with frequency which is stronger for large and small _F/l. These characteristic dependences allow measurement of material parameters, anisotropy, and dynamics of electrons, especially the relaxation rate. But presently, the rf surface impedanceZ is still shrinking with material improvements, which shows, clearly, that theZ=Z _I+Z _res is still dominated by extrinsic properties summarized inZ _res. We present evidence thatZ _res is due to the large leakage currentj _bl and the smallj _cJ of weak links where the latter destroys the intrinsic shielding from a _I-thin seam _J deep into the bulk. This causes rf residual lossesR _res( ₀)² _J ³ _bl/2.R _res stays finite atT-0 due to _bl(T0) _bl(j _bl) being amplified by ( _J/ _I)³>10³ as a weighting factor. The appropriate measure of weak links are the grain-boundary resistanceR _bn((0)) enhancing _JR _bn andR _resR _bn ² . Thus,Z _res is minimal for minimal extrapolated resistivity(T0).To identify the weak links as a new entity, the H-field dependence is most helpful, because at very low fieldsH _c 1J1/ _J Josephson fluxons penetrate into the weak links. These Josephson fluxons show negligible flux flow or flux creep, and enhanceZ _res by _J(H, T) . The measuredj _cJ(H, T) andj _bl values explainZ _res quantitatively as well as in temperature (a+T ⁿ ) (n1,T<T _c /2) and in field (b+H ⁿ ) (n1,H>H _c1J) dependence. The strength of the field dependencedZ _res/dHZ _res(H _c1J )/H _c2J(T) is not only a measure ofZ _res andH _c2J(T) but is crucial for nonlinear effects and (fluxon) noise also, which limit the performance of rf devices.     

Temperature and randomness dependence of the pair-breaking parameter in superconducting aluminum films

Magnetoconductance and excess conductance due to superconducting fluctuations in aluminum films are measured in order to study the temperature dependence of the pair-breaking parameter at temperatures nearT _c . The parameter _M is estimated from the relation =/8k _B T_in, where _in is the inelastic scattering time deduced from the analysis of the magnetoconductance. The parameter _F is determined by fitting theories to data on the excess conductance at zero magnetic field. It is shown that: (1) For films with a wide range of the sheet resistanceR , 12R 200 /, the temperature dependence of _M nearT _c agrees well with the theory of Brenig et al. (2) For clean films withR 100 /, the value of _F analyzed with theories including the correction term to the Maki-Thompson contribution shows almost the same temperature dependence as _M . In a film withR 200 /, however, a discrepancy between _M and _F remains.On leave from College of General Education, Kyushu University, Ropponmatsu, Fukuoka, Japan.     

Simultaneous Double-Sided Deposition of Large Size HTSC Thin Films

An apparatus for simultaneous deposition of large size double-sided YBCO thin films by sputtering from single inverted cyclindrical target was constructed. Double-sided YBCO thin films on LaAlO₃ (100) substrate up to 30 mm in diameter were prepared to test the performance of the apparatus. The microwave surface resistance, R _s (77 K, 10 GHz, 0 T), of the YBCO thin films on both sides of the wafer ranged from 500 to 800 . Values of R _s (75 K, 145 GHz, 0 T) below 60 m were reached over the area of 20 mm × 15 mm on the wafer. The majority of the wafer area given in percent has R _s (75 K, 145 GHz, 0 T) values in the range of 21.5– 45.5 m. Lateral homogeneity of R _s values in the whole wafer was good enough to well meet the microwave application.     

γ ? α2 Phase transformation in fractured high temperature stress rupture Ti-48Al-2Nb(at.%)

The ₂ phase transformation in fractured high temperature stress rupture Ti-48Al-2Nb(at.%) alloy has been studied by analytical electron microscopy. ₂ and phases were found at grain boundaries. ₂ layers that suspended in layers and interfacial ledge higher than 2d ₍₁₁₁₎ at /₂ interfaces were observed in the lamellar grains. These facts indicated that ₂ phase transformation and dynamic recrystallization have occurred during high temperature stress rupture deformation. It can be concluded that deformation induced ₂ phase transformation and dynamic recrystallization resulted in the presence of particles at grain boundaries. A structural and compositional transition area between deformation-induced ₂(or ) and its adjacently original (or ₂) phases was found by HREM and EDS and is suggested as a way to transform between and ₂ phase during high temperature stress rupture deformation. The transition area was formed by slide of partial dislocations on close-packed planes and diffusion of atoms.     

Correlation between Superconducting Gap and Pseudogap in High-T c Cuprates

We studied the low-temperature energy gap 2 ₀ on Bi ₂ Sr ₂ CaCu ₂ o ₈₊ (Bi2212) and La _2–x Sr _x CuCO ₄ (La214) systematically over a wide range of doping level p using STS, break junction tunneling spectroscopy, Raman scattering and low-T electronic specific heat data. We have also studied the electronic specific heat of La214 in the normal state at T > T _c , and confirmed that pseudogap behavior appears at around T*, below which the in-plane resistivity and magnetic susceptibility tend to be slightly suppressed. Similar suppression appears in and of Bi2212 below the onset temperature of pseudogap T*. It is pointed out in the present study that 2 ₀ is closely related to T* in both Bi2212 and La214 systems; T* 2 ₀ /4.3k _B . It is also pointed out that 2 ₀ is in almost linear proportion to k _B T _max ( T*), where T _max is the temperature exhibiting a broad peak in –T curves and k _B T _max can be considered to give a measure of the effective antiferromagnetic exchange energy J _eff. The factors in 2 ₀ k _B T _max J _eff are 1 for La214 and 2 for Bi2212, respectively. We also report that in both Bi22l2 and La214 systems T _c roughly scales with p ₀ except in highly doped samples, where T _c 2 ₀ .     

Superfluid density near the lambda point of4He under pressure

The superfluid density in ⁴ He was determined near T from the second-sound velocity as a function of T–T and pressure. The critical exponent of the superfluid density was found to depend, even slightly, on the pressure. Furthermore, the fundamental length ₀ in the coherence length = ₀ [1–(T/T)]^–' seemed to be proportional to the mean interatomic distance. The implications of the results are also discussed.This work was partly supported by The Ito Science Foundation and by The Nishina Memorial Foundation.     

Soft magnetic properties of NiFeTa thin films for biasing film applications in magnetoresistive elements

The effects of Ta addition on the magnetic properties of permalloy thin films have been investigated. The alloy compositions on a weight basis are (Ni₈₁Fe₁₉)_1-x Ta _x with 0 x 0.105, and the films are sputtered onto a glass substrate at between room temperature and 300 C. The saturation magnetization and anisotropic magnetic field decrease with increasing Ta content. The saturation magnetization is 0.75 T at 5 wt % Ta. The coercivity remains constant at 125 Am^-1. The electrical resistivity increases linearly with increasing Ta content, then saturates at approximately 7.5 wt% Ta. The saturation resistivity is approximately 1.00 m. The magnetoresistivity ratio (/) decreases with increasing Ta content, mainly due to increased electrical resistivity (). The magnetostriction changes from negative to positive with increasing Ta content and reaches nearly zero at 2 wt% Ta. The NiFeTa films containing 5–6 wt% Ta have potential for use as the soft-biasing film in magnetoresistive elements.     

Silver doping: A potential means to grow high quality YBa2Cu3O7?x thin films on bare sapphire

High qualityAg-doped YBa₂Cu₃O_7–x (YBCO) thin films have been grown by laser ablation on ¯1012 bare sapphire. This work demonstrates thatAg-doping can be used as very convenient means to realize good quality YBCO films on highly coveted sapphire substrates for microwave applications of highT _c thin films.     

Microstructure and mechanical properties of rapidly quenched L20 and L20+L12 alloys in Ni-Al-Fe and Ni-Al-Co systems

Ductile L2₀-type wires and+L1₂-type duplex wires with high strengths and large elongation in the Ni-Al-Fe and Ni-Al-Co ternary systems have been manufactured directly from the liquid state by an in-rotating-water spinning method. The wire diameter was in the range 80 to 180m and the average grain size was 2 to 4m for the wires and 0.2 to 1.0m for the+ wires. _y, _f and _p of the wires were found to be about 360 to 760 MPa, 560 to 960 MPa, and 0.2 to 5.5%, respectively, for the Ni-Al-Fe system, those of the+ wires were about 395 to 660 MPa, 670 to 1285 MPa, and 3.5 to 17%, respectively, for the Ni-Al-Fe system, and about 260 to 365 MPa, 600 to 870 MPa, and 4.0 to 7.0%, respectively, for the Ni-Al-Co system. Cold-drawing caused a significant increase in _y and _f and the values attained were about 1850 and 2500 MPa, respectively, for Ni-20Al-30Fe and Ni-25Al-30Co wires drawn to about 90% reduction in area. The high strengths, large elongation and good cold-workability of the melt-quenched and+ compound wires have been inferred to be due to the structural change into a low-degree ordered state containing a high density of phase boundaries, suppression of grain-boundary segregation and refinement of grain size.     

Two-gap structure in Yb(Y)Ba2Cu3O7?x single crystals

Tunneling effects in YbBa₂Cu₃O_7–x (YbBCO) and YbBa₂Cu₃O_7–x (YBCO) single crystals have been investigated using an improved technique of break junction preparation. A two-gap structure with BCS-like density of states has been observed. The value of the large gap is in the range of 24–28 meV for YBCO (T _c =85–90 K) and 29–30 meV for YbBCO (T _c =87–90 K). A variation of the value of the small gap between 0.5 and 11 meV has been detected, even for the same crystal. As a possible explanation, two-band superconductivity, surface properties, or strong gap anisotropy will be discussed.     

4He very nearT λ heated from above

We discuss the feasibility and likely results of measurements of the thermal conductivity (Q,t) of⁴He very nearT T (Q = 0) as a function of the heat currentQ and the reduced temperature t [T —T]/T by heating a sample from above and cooling it from below. Although the expansion coefficient is negative, the experiment should be possible without inducing convection in the HeI layer provided the sample length does not significantly exceed one cm. Fort 10^–7(Q 0.2 erg/s cm²), a state of self-organized criticality can be attained. For these conditions, the thermal gradient cancels the gradient inT induced by gravity, thus permitting measurements extremely close to the transition even in an Earthbound laboratory. However, the data will be only for a unique path in the Q—t plane. For 0.2 Q 0.5 erg/s cm² (10^–7t 10^–8) they will be in the range of linear response and give (0,t); for Q between about 0.5 and 10 erg/s cm² and over a temperature range of about 20 nK, the experimental path samples the regime where the conductivity is expected to be influenced by nonlinear finite-current effects. The small currents and narrow temperature range of the experiment imply that ultra-high resolution thermometry as well as very careful control of stray heat currents will be required. ForQ 10 erg/s cm² and up to very largeQ, the method can be used to determine the onset temperatureT _c (Q) of thermal resistance.     

High-field measurements on YBCO

We have performed resistance measurements on thin films of the high-temperature superconductor YBa₂Cu₃O₇ (YBCO) in applied magnetic fields to above 200 T (2 MegaOersted) at temperatures as low as 2.5 K. The fields are produced by an explosively driven flux-compression system. We can see a particularly clear onset, without replotting the data, of the hydrodynamic flow of vortices probably because of the very fast increasing field. The low-temperature critical field for the field parallel to the c-axis of the sample is 135 T. Our data in the other direction are still preliminary. We discuss possible interpretation of our results.     

Preparation of YBCO Superconducting Thick Films on MgO Substrates by Modified Melt Growth Process

Superconducting thick films (200–300 m) of YBCO have been fabricated successfully on MgO substrates by a new approach. The precursor powders (YBa₂Cu₃O_7– (Y123) + 0.1 mol Y₂BaCuO₅(Y211)) are placed between two MgO (10 mm×10 mm) slices to form sandwich structure. The YBCO thick films have been obtained from the precursors by modified melt growth process. Resistance measurements of YBCO thick films show T _con of 87.3 K and T of 4.5 K. Estimates using hysteresis loops and the Bean model give a value of 2.78×10³ A/cm² (77 K, 0T) for the critical current density. The observations of scanning electron microscopy (SEM) and X-ray diffraction (XRD) patterns show the supercoducting Y123 phase matrix containing discrete Y211 inclusions.     

Mössbauer studies of α″-Fe16N2 and α′-Fe8N films

Conversion-electron Mössbauer spectra of epitaxial -Fe₁₆N₂ and -Fe₈N films have been studied and their differences are discussed in detail. The Mössbauer spectrum of -Fe₁₆N₂ can be decomposed into three subspectra, which correspond to the 4d, 8h and 4c sites. The Mössbauer spectrum of -Fe₈N can be fitted using four spectra based on a nitrogen-atom-random-distribution model. The average hyperfine field is larger (3%) for -Fe₁₆N₂ than for -Fe₈N, which is approximately consistent with a 4.1% enhancement of the magnetic moments for -Fe₁₆N₂. The iron moments tend to locate in the film plane for -Fe₁₆N₂ and to arrange perpendicularly to the film plane for -Fe₈N.     

The microstructure of sintered Si-Al-O-N ceramics

Fine-grained ceramics, based on the substituted -Si₃N₄ crystal structure, have been prepared with near theoretical densities by pressureless sintering of compacts containing a large Y₂O₃ additive concentration. The microstructure contains two phases, and an Y/Al rich silicate glassy matrix, in the as-sintered condition. The matrix may be crystallized on heat treatment to a Si-substituted yttrium-aluminium garnet phase. The phase compositions and transformation mechanisms have been analysed by X-ray microanalysis, electron diffraction and imaging of thin sections.A preliminary survey of mechanical properties of these sintered ceramics shows that they are comparable with commercial hot-pressed silicon nitrides, particularly at high temperatures. The possibility of development of a range of ceramic alloys, dependent on their chemical environment and temperature of application, is discussed.