Narrow Transition Width Research Articles

YBa/sub 2/Cu/sub 3/O/sub 7/ thin films on nanocrystalline diamond films for HTSC bolometer

Superconducting YBa/sub 2/Cu/sub 3/O/sub 7-x/ films on nanocrystalline diamond thin films have been fabricated. A composite buffer layer system consisting of diamond/Si/sub 3/N/sub 4//YSZ/YBCO was explored for this purpose. The as-deposited YBCO films were superconducting with T/sub c/ of approximately 84 K and a relatively narrow transition width of approximately 8 K. Scanning electron microscope cross sections of the films showed very sharp interfaces between diamond/Si/sub 3/N/sub 4/ and between Si/sub 3/N/sub 4//YSZ. The deposited YBCO film has a surface roughness of approximately 1000 AA, which is suitable for high-temperature superconductive (HTSC) bolometer fabrication. It was also found that preannealing of the nanocrystalline diamond thin films at high temperature was very important for obtaining high-quality YBCO films.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Microstructural and magnetic studies of sinter-forged (Bi, Pb) 2Sr 2Ca 2Cu 3O x superconductor

The microstructural and magnetic properties of sinter-forged BiPbSrCaCuO superconductors have been studied. The as-sinter-forged sample consisted of significant microcracks all through the bulk and first forms into a Bi 2-2-1-2 phase ( c = 3.08 nm). However, after heat treatment at 860°C in air for 10 days, the final sample transformed to microstructurally smooth grains of Bi 2-2-2-3 phase ( c = 3.70 nm). ac magnetic susceptibility data taken at 0.1, 1 and 10 Oe evidence the onset of diamagnetic response of grains at T c = 105 K with a rather narrow transition width, ΔT c = 3 K. TEM, SEM and XRD analyses reveal remarkable microstructural changes into a smooth laminar structure during the formation of Bi 2-2-2-3 phase.

IN-SITU PREPARATION OF YBa2Cu3O7 SUPERCONDUCTING THIN FILMS BY PULSED Nd:YAG LASER DEPOSITION

High-T c superconducting thin films of YBa 2 Cu 3 O 7 were grown in-situ on (100) SrTiO 3 substrates by Nedymium:yttrium aluminum garnet [Nd:YAG] laser ablation. The effects of the substrate temperature on the transition temperature, microcrystalline structure and surface morphology of the films were discussed. Best results were obtained in the 730°–770°C range. X-ray diffraction analysis showed that these films were highly c-oriented with the c-axis perpendicular to the substrate surface. At the optimum substrate temperature, a very smooth morphology with only a few small particles were observed by scanning electron microscopy. The zero resistance temperature of these films were ≥ 90 K with a narrow transition width and the ac susceptibility measurement also gave the same result. The highest critical current density obtained at 77 K and zero magnetic field was 3.8 × 106 A/cm 2.

Superconducting phases of URu2Si2.

Thermodynamic data near the superconducting ${\mathit{T}}_{\mathit{c}}$ on several different single-crystal specimens of the heavy-fermion system ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$ are presented. Multiple-peak structure and narrow transition widths (\ensuremath{\Delta}${\mathit{T}}_{\mathit{c}}$/${\mathit{T}}_{\mathit{c}}$=0.05) are observed in the specific heat of the highest-quality samples. By carefully sectioning one of the crystals, it is shown that these peaks occur in macroscopically different parts of the crystal, and not as a result of vector superconductivity. Implications for the multiple-peak structure in ${\mathrm{UPt}}_{3}$ are discussed.

Organometallic chemical vapor deposition of superconducting YBa 2Cu 3O 7 − δ films

YBa 2Cu 3O 7 − δ thin films have been deposited In a low pressure chemical vapor deposition reactor at a total pressure of 5 Torr. Yttrium, barium and copper precursors were β — diketone chelates and argon was used as a carrier gas. The films were deposited on single crystal MgO substrates, with (100) orientation at a temperature of 825 °C. Four point probe resistivity measurements indicate zero resistance at 81 K and a narrow transition width (of the order of 2 K). Films deposited in the same conditions on SrTiO 3 (100) exhibit zero resistance at 89 K and a ΔT c of 0.8 K.

Superconducting Bi1.5Pb0.5Sr2Ca2Cu3Ox ceramics by rapid melt quenching and glass crystallization

A glass of nominal Bi1.5Pb0.5Sr2Ca2Cu3Ox composition, prepared by rapid quenching of the melt, showed a glass transition temperature of 383 °C, crystallization temperature of 446 °C, melting temperature of 855 °C, and bulk density of 5.69 g/cm3 in air. The activation energy for crystallization of the glass has been estimated to be 292 kJ/mol from nonisothermal differential scanning calorimetry. On heating in oxygen, the glass showed a slow and continuous weight gain starting at 530 °C which reached a plateau at 820 °C. The weight gained during heating was retained on cooling to ambient conditions indicating an irreversible oxidation step. The influence of annealing conditions on the formation of various phases in the glass has been investigated. The Bi2Sr2Ca0Cu1O6 phase crystallized out first followed by formation of other phases at higher temperatures. The high-Tc phase, isostructural with Bi2Sr2Ca2Cu3O10, was not detected below 840 °C, but its fraction increased with the annealing time at 840 °C. A sample annealed at 840 °C for 243 h in air and furnace cooled showed the highest Tc(R=0) of 107.2 K and a narrow transition width, ΔTc (10%–90%), of 2 K. The high-Tc phase does not seem to crystallize out directly from the glass but is rather produced at high temperature by reaction between the phases formed at lower temperatures. The kinetics of 110-K phase formation was sluggish. It appears that the presence of lead helps in the formation and/or stabilization of the 110-K phase.

Narrow transition width two-dimensional digital fan filters with sum of powers-of-two coefficients

General expressions are presented for the transfer functions of quadrantal digital fan filters and 90° digital fan filters. Through a study of the coefficients of the transfer functions, a method for the design of multiplierless fan filters with sum of power-of-two lpar;SOPOT) coefficients using these formats is introduced and a table of SOPOT coefficients is given for filter order M ranging from 4 to 10, A number of examples have been tried.

Improvements of sintered YBCO samples by citrate pyrolysis and ozone annealing

YBCO sintered samples have been fabricated by the citrate pyrolysis method. The annealing, with the usual thermal processes, but using a mixture of oxygen and ozone, generates large microcrystals. Samples fabricated with this process have been characterized by measuring the a.c. susceptibility. Low field measures show a very narrow transition width, while high field measures performed both at 50 and 100 Gauss denote the higher strength of the intergranular junctions in the ozone made samples.

Partial Reduction/Re-Oxidation Processing of Y-Ba-Cu-O Sputtered Thin Films

AbstractThin films of Y‐Ba‐Cu‐O were produced by RF sputtering of YBa2Cu3O7‐x ceramic targets, using a variety of plasma compositions, RF power levels, and substrate temperatures. Post annealing of these films in oxygen produced superconducting films with Tc values between 40‐60 K, broad transition widths and semiconductor‐like electrical behavior above Tc. Subsequent annealing at 850°C in an inert gas with a residual oxygen partial pressure of ≤10 ppm followed by an oxygen anneal produced high quality thin films: Tc&gt; 85 K with narrow transition widths. The structure and morphology of these films during reduction‐oxidation processing were monitored using X‐ray diffraction and electron microscopy.

Preparation, structure and properties of the high T c Bi-based and Y-based cuprates

The structure and some properties of the new bismuth-based high T c superconductor are discussed and compared to those of the 40K and 90K materials. While the superconducting behavior of the 90K is very sensitive to the oxygen composition and therefore to the annealing procedure, the Bi-based phase is dramatically less affected. But it is more sensitive to the solid state reaction temperature and starting composition. Thus a better control of the chemistry using a solution technique should enhance the performances of this material. The Y-based 90K superconducting oxide has been synthesized using a novel solution technique (Sol-Gel process). Physical properties, including resistivity and critical current, are compared to those obtained by solid state reaction in which the starting materials include carbonates, acetates and nitrates. The solution technique produces YBa 2Cu 3O 7−y powders at lower reaction temperatures and shorter times. The smaller particle size obtained by this method (1μm) yields denser compacts upon sintering. The solution derived materials are made from inexpensive reagent-grade (98% pure) starting materials and they superconduct at 91K with narrow transition widths (0.6K). Although the material is homogeneous, the critical currents do not significantly exceed those reported for solid-state reacted materials and we attribute this to the presence of carbon at the grain boundaries. Finally, the ability to produce stable viscous gels allows us to prepare large-area thick films on various substrates. Best results are obtained on SrTiO 3 or MgO substrates, and problems encountered in making thick films on substrates of technological interest are discussed.

Effect of slow cooling rates on the superconducting characteristics of YBa2Cu3O7−x

We describe an oxygen annealing procedure to obtain bulk YBa2Cu3O7−x samples with zero electrical resistance states at 105 K, a narrow transition width of ∼1 K, and Jc values in excess of 200 A cm−2 at 77 K. It is suggested that enhanced superconducting characteristics result from incorporating very slow cooling rates (0.1 K/min) during oxygen annealing which possibly maximizes the occupancy of 0, 1/2, 0 oxygen sites and leads to a near-perfect ordering of vacancies at 1/2, 0, 0 sites in the orthorhombic unit cell of YBa2Cu3O7−x.

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La1−xSrx)2CuO4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature Tc ≲ 38 K and narrow transition width ΔTc. For the sample with the maximum Tc among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔVs(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏωop/kB ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high Tc of this material.

Detailed study of the Brunhes/Matuyama reversal boundary on the east Pacific rise at 19°30′ S: Implications for crustal emplacement processes at an ultra fast spreading center

We have conducted the first detailed survey of the recording of a geomagnetic reversal at an ultra-fast spreading center. The survey straddles the Brunhes/Matuyama reversal boundary at 19°30′ S on the east flank of the East Pacific Rise (EPR), which spreads at the half rate of 82 mm yr-1. In the vicinity of the reversal boundary, we performed a three-dimensional inversion of the surface magnetic field and two-dimensional inversions of several near-bottom profiles including the effects of bathymetry. The surface inversion solution shows that the polarity transition is sharp and linear, and less than 3–4 km wide. These values constitute an upper bound because the interpretation of marine magnetic anomalies observed at the sea surface is limited to wavelengths greater than 3–4 km. The polarity transition width, which represents the distance over which 90% of the change in polarity occurs, is narrow (1.5–2.1 km) as measured on individual 2-D inversion profiles of near-bottom data. This suggests a crustal zone of accretion only 3.0–4.2 km wide. Our method offers little control on accretionary processes below layer 2B because the pillow and the dike layers in young oceanic crust are by far the most significant contributors to the generation of marine magnetic anomalies. The Deep-Tow instrument package was used to determine in situ the polarity of individual volcanoes and fault scarps in the same area. We were able to make 96 in situ polarity determinations which allowed us to locate the scafloor transition boundary which separates positively and negatively magnetized lava flows. The shift between the inversion transition boundary and the seafloor transition boundary can be used to obtain an estimate of the width of the neovolcanic zone of 4–10 km. This width is significantly larger than the present width of the neovolcanic zone at 19°30′ S as documented from near-bottom bathymetric and photographic data (Bicknell et al., 1987), and also larger than the width of the neovolcanic zone at 21° N on the EPR as inferred by the three-dimensional inversion of near-bottom magnetic data (Macdonald et al., 1983). The eruption of positively magnetized lava flows over negatively magnetized crust from the numerous volcanoes present in the survey area and episodic flooding of the flanks of the ridge axis by extensive outpourings of lava erupting from a particularly robust magma chamber may result in a widened neovolcanic zone. We studied the relationship between spreading rate and polarity transition widths obtained from 2-D inversions of the near-bottom magnetic field over various spreading centers. The mean transition width corrected for the time necessary for the reversal to occur decreases with increasing spreading rate but our data set is still too sparse to draw firm conclusions from these observations. Perhaps more interesting is the fact that the range of the measured transition widths also decreases with spreading rate. In the light of these results, we propose a new model for the spreading rate dependency of polarity transition widths. At slow spreading centers, the zone of dike injection is narrow but the locus of crustal accretion is prone to small lateral shifts depending on the availability of magmatic sources, and the resulting polarity transition widths can be narrow or wide. At intermediate spreading centers, the zone of crustal accretion is narrow and does not shift laterally, which leads to narrower transition widths on the average than at slow spreading centers. An intermediate, or even a slow spreading center, may behave like a fast or hot-spot dominated ridge for short periods of time when its magmatic budget is increased due to melting events in the upper mantle. At fast spreading centers, the zone of dike injection is narrow, but the large magmatic budget of fast spreading centers may result in occasional extensive flows less than a few tens of meters thick from the axis and off-axis volcanic cones. These thin flows will not significantly contribute to the polarity transition widths, which remain narrow, but they may greatly increase the width of the neovolcanic zone. Finally the gabbro layer in the lower section of oceanic crust may also contribute to the observed polarity transition widths but this contribution will only become significant in older oceanic crust (≈50–100 m.y.).

A negative transition width in perpendicular recording?

In perpendicular recording, the measured density response curve corrected with gap, spacing, and thickness losses may show a positive slope. This corresponds to an unphysical negative transition width in an arctangent transition. Analysis has been done to understand this phenomenon. We found that for a perpendicular media with Hc much less than 4πMs, an isolated transition is partially demagnetized in the tail region and consists of more high frequency component than an arctangent transition. By linear superposition, the high frequency readback amplitude can be boosted by this enhanced high frequency spectrum in the isolated pulse. The readback resolution of a thick perpendicular media in fact is equivalent to an infinitesimally thin longitudinal media with extremely narrow transition width. However, the readback amplitude is higher due to the thicker magnetic layer. As to a perpendicular media with Hc greater than 4πMs, the so-called ‘‘negative transition width’’ does not occur and the resolution of the readback signal becomes relatively poor.

Studies of the perpendicular magnetization mode in Co-Cr sputtered films

The extremely high resolution properties in a perpendicular magnetic recording system were studied with a recently developed Co-Cr film medium. Bitter technique was used to observe magnetization distribution in the medium, and the transition of magnetization was directly observed on the top and the bottom surfaces of the film. It was proved that an ideal perpendicular magnetization mode, having a very narrow transition width, takes place even in a high recording density over 40 kBPI. From the angular variation of H c and the micro-structure of the Co-Cr film, it was concluded that the high recording resolution of the film results from the fine columnar particles which show the rotational mechanism of magnetization reversal accompanied by a perpendicular magnetocrystalline anisotropy. The Co-Cr film was also found to be superior in perpendicular anisotropy to any other Co-M films studied here in.

SUPERCONDUCTIVE FIXED POINTS FOR TEMPERATURES ABOVE 0.5 K

A temperature fixed point device incorporating samples of the elements Cd, Zn, Al, In and Pb has been available from the NBS for several years. This paper describes current efforts to reduce the temperature uncertainties of the NBS device below the present one milli-kelvin level. It also notes that efforts to produce narrow transition widths in samples of Nb, V3Ga and Nb3Sn have resulted in widths of 9 mK, 40 mK and 17 mK, respectively.

Near-bottom magnetic anomalies, asymmetric spreading, oblique spreading, and tectonics of the Mid-Atlantic Ridge near lat 37°N

A detailed study of the magnetic anomalies of the Mid-Atlantic Ridge crest near lat 37 °N (FAMOUS) was conducted using a deeply towed instrument package. The most recent expression of the accreting plate boundary in rift valley 2 is an alternating series of linear central volcanoes and depressions that are marked by a sharp maximum in crustal magnetization only 2 to 3 km wide. Spreading in the FAMOUS area is highly asymmetric, with rates of 13.4 mm/yr to the east and 7.0 mm/yr to the west. At 1.7 m.y. B.P., the sense of asymmetry reverses in direction, with spreading faster to the west; this results in a gross symmetry when averaged through time. The change in spreading asymmetry occurred in less than 0.15 m.y. Spreading in the FAMOUS area is 17° oblique. Even on a fine scale there is no indication of readjustment to an orthogonal plate-boundary system. Spreading has been stably oblique for at least 8 m.y., even through a change in spreading direction. The presence of negative polarity crust within the Brunhes normal epoch in the inner floor has been observed and may be due to old crust left behind or to recording of a geomagnetic field event. Crustal magnetization decays to 1/e its initial value in less than 0.6 m.y. The rapid decay may be facilitated by very intense crustal fracturing observed in the inner floor. Crustal magnetic sources may be approximated (mathematically) by a uniformly magnetized layer 700 m thick. Magnetic studies indicate that over 90 percent of the extrusive volcanism occurs within the rift inner floor and is extremely rare in the rift mountains. Magnetic anomaly transition widths vary from 1 to 8 km with time and appear to reflect a time-varying median-valley structure. The valley has either a wide inner floor and narrow terraces, in which case the volcanic zone is wide and magnetic anomalies are poorly recorded (wide transition widths), or it has a narrow inner floor and wide terraces — the volcanic zone is then narrow and anomalies are clearly recorded (narrow transition widths). The median valley of any ridge segment varies between these two structures with time. At present rift valley 2 has a narrow inner floor and volcanic zone (1 to 3 km), whereas rift valley 3 is at the opposite end of the cycle with a wide inner floor and volcanic zone (9 to 11 km).

Synthesis of Linear Phase Sharp Transition FIR Digital Filter

This paper proposes a new technique for synthesis of a sharp transition, equiripple passband, low arithmetic complexity, linear phase lowpass FIR filter. The frequency response of the filter with narrow transition width is modeled using trigonometric functions of frequency and its transfer function is evolved in frequency and time domain. The synthesized filter proves to be a good alternative to filters of the same class reported in the literature with added advantage of simplicity of design and ease of computation of the impulse response.

The phase diagram and phase transition of the V 2O 3-V 2O 5 system

The insulator-metal transition (IMT) in vanadium dioxide (VO2) which occurs above room temperature (67 °C) is highly sensitive to atomic defects caused by oxygen stoichiometry. The strained growth and the degree of oxygen deficiency in VO2 epitaxial films result in lowering of transition temperature below room temperature as well as the broadening of transition parameters such as transition width and hysteresis width, which limit its application potential. Here we demonstrate the growth of highly oriented strain-relaxed VO2 thin films on (001)-oriented TiO2 substrates at various oxygen partial pressures, exhibiting the narrow transition and hysteresis width. The cross-sectional transmission electron microscopy and x-ray diffraction analyses of the films reveal the highly oriented growth of insulating monoclinic VO2. The IMT parameters associated with temperature-dependent phase transition vary with the oxygen partial pressure used during the deposition. The presence of multiple and mixed valence states of vanadium in the films was confirmed by Raman and XPS analyses. We have achieved a narrow transition width (2.3 °C) and hysteresis width (1.2 °C) through controlling the oxygen stoichiometry during the growth of VO2/TiO2 films.

Magnetoresistive amplifiers with superconductive elements

The construction and properties of certain superconductive amplifiers in which the resistance of a gate element is controlled by the electromagnetic field of a signal coil are described in this paper. Vacuum-deposited films of tin, which have narrow resistive transition widths, have been used as the gate elements, and the field has been applied perpendicularly to the film surface. The parameters of small-signal analysis are determined, and the influence of temperature, bias magnetic field, and zero-signal gate current upon power gains are shown. The limitations and merits of the device in its present form are discussed.