Flow of superfluid3He through micrometer-size channels

Measurements of the critical mass currentJ _c through 0.8-µm-diameter channels in the superfluid phases of³He are reported. Experiments were made at pressures from 0 to 27.4 bar in zero external magnetic field. The pressure difference ΔP along the flow channels is immeasurable within our resolution of ±0.1 µbar for sufficiently low currents in both the A and B phases, implying small or zero dissipation. In the B phase ΔP grows rapidly with increasing current aboveJ _c. At low pressuresJ _c behaves like (1?T/T _cyl)^3/2, whereT _cyl is interpreted as the reduced superfluid transition temperature inside the flow channels;T _cyl/T _c=0.935 atP=0. If the liquid in the channels is in the A phase, the behavior of ΔP vs. the mass currentJ _s depends on the phase A or B outside the channels. During warming a drop of about30% inJ _c is found both atT _BA(cyl) and at;T _BA(cyl) is the reduced B→A transition temperature in the channels. AboveT _BA a second dissipation mechanism, with a smallerd(ΔP)/dJ _s, is observed at lower currents. These features indicate that in the A phase the ends of the channels have an important effect on the flow.     

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

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 (T _c) 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 (J _c) at 77 K has an average value of 2×10⁵ A cm^?2, with a tendency for decreasingJ _c with increasing film thickness (0.2 to 0.8μm). Variations inJ _c 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 ofJ _c. If the film composition is enriched in Y relative to Ba and Cu,T _c decreases by several degrees.     

Effect of Annealing Time and Temperature on Microstructural and Superconducting Properties of (Bi,Pb)-2212 Thin Films Produced by Magnetron Reactive Sputtering

This study deals with the role of various annealing time (1?h, 2?h, 4?h, 6?h and 8?h) and temperature (840 and 850?°C) on the microstructural and superconducting properties of thin films with the aid of scanning electron microscopy (SEM), X-Ray analysis (XRD), electron dispersive X-Ray (EDX), resistivity and transport critical current density (J _c ) measurements. The T _c , J _c , variation of transition temperatures, hole-carrier concentration, grain size, phase purity, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the films prepared are compared with each other. Critical transition temperatures (T _c ) of the samples are deduced from the dc resistivity measurement while critical current density values are estimated from the critical current and total cross-sectional area values. It is found that maximum T _c of 79.7?K and J _c of 1520?A/cm² are observed for the film annealed at 840?°C for 6?h as against 54.9?K and 30?A/cm² (minimum values), respectively, for the film annealed at 840?°C for 4?h. Moreover, SEM images indicate that the former has the best crystallinity, grain connectivity and largest grain size. Based on these results, T _c and J _c values of the samples studied are found to depend strongly on the microstructure. Additionally, EDX results show that the elements used for the preparation of all the samples are observed to distribute homogeneously. As for the XRD results, all the samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines. According to the refinement of cell parameters done by considering the structural modulation, the largest lattice parameter a and c are obtained for the film annealed at 840?°C for 8?h. To sum up, the aim of the present study is not only to investigate the changes of microstructural and superconducting properties of the samples fabricated in the varied time and temperature but to determine the best ambient for the film fabrication and show the feasibility of obtaining Bi-2212 film with tailored structure, as well.     

Improved Transport J c in MgB2 Tapes by Graphene Doping

Graphene is a special form of carbon which can effectively enhance the critical current density J _c of MgB₂. In this work, a systematic investigation on the impact of sintering conditions and doping level was carried out for graphene-doped MgB₂ tapes. It is found that an appropriate addition level, i.e., 8 at% in this work, is very critical to obtain a high J _c in graphene-doped samples. The critical field and pinning force are improved obviously due to the graphene doping. The magnetic J _c of samples sintered at 800 °C with 8 at% graphene doping reached 1.78 × 10⁴ A/cm², at 5 T, 20 K. At the same time, the transport J _c was up to 2.38 × 10⁴ A/cm² at 10 T, 4.2 K. The lattice distortion caused by C substitution and residual C at the grain boundaries were thought to be the major factors affecting the J _c of graphene-doped MgB₂ samples.     

Transition to Turbulence and Critical Velocity in Superfluid Solution of 3He in 4He

By the method of oscillating tuning fork, we carried out researches of the transition to turbulence in superfluid solution of 5% ³He in ⁴He at temperatures of 100 mK–300 mK. The critical velocity υ _c of the turbulence appearance is determined through measuring the volt-ampere characteristics. It is established that in the mixture the temperature dependence of the critical velocity is non-monotonous and differs strongly from that in pure ⁴He. Unlike ⁴He, the step-like anomalies on resonance curves were observed which, presumably, is connected with instability of the vortex system under the conditions where the core of the vortex is filled by the atoms of ³He. It is shown that such anomalies appear at the temperatures below 0.9 K, at the same time at temperatures below ～0.5 K they appear even at υ<υ _c .     

Properties of MgB2 Thin Films Deposited on Different Substrates Prepared by Ex-Situ Annealing Process

MgB₂ thin films were deposited on MgO (100) substrate and r-plane Al₂O₃ $(1\bar{1}02)$ substrate by ex-situ annealing of boron film in magnesium vapor. The thickness of ex-situ annealed MgB₂ films is approximately 600 nm according to data observation by ellipsometer. The magnetic properties of samples were determined using a vibrating sample magnetometer. The magnetic field dependence of the critical current density J _c was calculated from M–H loops and also the magnetic field dependence of F _p was compared for the different temperature ranges from 5 to 25 K. The critical current density J _c was found to be around 1.0×10⁶ A/cm² and 1.7×10⁶ A/cm² in zero field at 5 K for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates, respectively. It was found that the critical current density of the film deposited on MgO became stronger than that of r-plane Al₂O₃ in the magnetic field. The superconducting transition temperature was determined by ac susceptibility measurement using physical properties measurement system. ac susceptibility measurements for MgB₂ films deposited on MgO and r-plane Al₂O₃ substrates were performed as a function of temperatures at constant frequency and ac field amplitude in the absence of dc bias field. The critical current densities as a function of temperature were estimated from the ac susceptibility data.     

NMR Study of Superfluid Phases of 3He Confined in 97.5% Porous Silica Aerogel

We have studied the scattering effect from aerogel strands on superfluid phases of ³He by a cw NMR method at 920 kHz. Liquid ³He at a pressure of 13 bar was confined in 97.5% porous aerogel from the same batch as that of a recent 4th sound study. The NMR experiment was performed in a magnetic field of 28.4 mT down to 0.3 mK. As temperature decreased, the NMR resonant frequency increased below 0.76 mK. The temperature of 0.76 mK agrees with the superfluid transition temperature T ^aerogel _c observed in the 4th sound study at the same pressure. Below T ^aerogel _c the behavior of thefrequency shift as a function of temperature indicates that there is no phasetransition to the other superfluid phase down to about 0.4 T ^aerogel _c . Owing to a very large surface solid ³He magnetization, we could not determine the superfluid phase of ³He in the aerogel in the magnetization measurement.     

Fabrication of BSCCO Films on SrTiO3 Substrates Using Ultrasonic Spray Pyrolysis (USP) Technique

In this study, the BiSrCaCuO (Bi-2212) films on SrTiO₃ substrates were fabricated using an ultrasonic spray pyrolysis technique (USP). Structural, electrical, magnetic, and critical current density, J _c, properties of the films fabricated were investigated under different heat treatment conditions. XRD analysis showed that the films mainly consisted of the Bi-2212 phase, but the Bi-2223 phase was also detected. T _c values of the films were found between 81 K and 88 K, depending on the heat treatment conditions. J _c values of the films were calculated using the Beans’ equation. Highest J _c value was found to be 2.93×10⁵ A?cm^?2 at 5 K and 0 T for Film C. The results obviously showed that USP method is a very effective technique for fabrication of the HT _c films having high J _c values as well as its simplicity, low cost, and easily coating.     

Texture and Vortex of Rotating Superfluid 3He-A in Parallel-plate Geometry

Unique types of textures and vortices of superfluid ³He in restricted geometries have been discussed. We investigated cw-NMR in rotating ³He A-phase in parallel plate geometries with gaps of 12.5 μm at 3.05 MPa. We observed a very sharp spectrum at rest which had shifted negatively as f=f _L?0.93f _A(T), where f _A(T) is the full transverse dipole shift in A-phase. The large negative shift of 0.93f _A(T) indicates that l ⊥ d. When we rotated the sample with rotation speed Ω faster than the critical Ω _Fr, a new satellite signal appeared nearly at f _L. The satellite signal intensity increased with increasing Ω, reached a maximum at Ω _c and slowly decreased up to the maximum Ω of 2π rad/s. When Ω decreased from the maximum speed, the satellite signal rapidly decreased and disappeared at 5.5 rad/s down to zero rad/s. We measured the temperature dependence of the satellite signal intensity, Ω _Fr and Ω _c. We propose a model for the satellite signal, which is attributed to spin wave in Fréedericksz transition region induced by the counter flow. We also compare the satellite signal with Kee and Maki’s bound pair of HQV.     

Combination of Ag Substrate Decoration with Introduction of?BaZrO3 Nano-Inclusions for?Enhancing Critical Current Density of?YBa2Cu3O7 Films

The combination of two methods: Ag substrate decoration and introduction of BZO nano-inclusions has been used in a pulsed laser deposition (PLD) method to increase the critical density (J _c ) of YBCO films. The films were deposited on single crystal SrTiO₃ (STO) substrates decorated with various architecture of Ag nano-dots. We have studied the diameter and density of Ag nano-dots and their influence on J _c of BZO-added YBCO films. We found that 15 laser pulses on the Ag target gives an optimum result in increasing J _c in comparison with BZO-doped YBCO films of the same thickness in self-field and low applied magnetic fields. A higher number of laser pulses on the Ag target led to increasing critical current density in high applied magnetic fields only (above 2 T). We have studied films of the thickness from 0.4 ??m to 3.8 ??m and found that the highest J _c at all applied fields investigated is achieved for a 1.2 ??m thick film. The transmission electron microscopy clearly shows BZO nano-rods that provide strong c-axis pinning centres in the films.     

Synthesis of BSCCO Films on MgO (100) Substrate by Ultrasonic Spray Pyrolysis Technique

In this study, the HT _c Bi-2212 films were fabricated using the ultrasonic spray pyrolysis technique (USP). Structural/microstructural, electrical, magnetic, and critical current density, J _c , properties of the films fabricated were studied depending on heat treatment conditions in detail. XRD analysis revealed that a pure Bi-2212 phase formed in all the films, which is confirmed by SEM-EDX analysis. T _c values of the films were found near 75?K. J _c values of the films were calculated using Beans?? equation. Highest J _c value was found to be 3.36×10⁵?A?cm^?2 at 5?K and 0?T for film?A. The results obviously showed that the USP method is a very effective technique for fabrication of the HT _c films having high J _c values as well as its simplicity, low cost, easily coating.     

The Damping of a Quartz Tuning Fork in Superfluid 3He-B at Low Temperatures

We have measured the damping on a quartz tuning fork in the B-phase of superfluid ³He at low temperatures, below 0.3T _c. We present extensive measurements of the velocity dependence and temperature dependence of the damping force. At the lowest temperatures the damping is dominated by intrinsic dissipation at low velocities. Above some critical velocity an extra temperature independent damping mechanism quickly dominates. At higher temperatures there is additional damping from thermal quasiparticle excitations. The thermal damping mechanism is found to be the same as that for a vibrating wire resonator; Andreev scattering of thermal quasiparticles from the superfluid back-flow leads to a very large damping force. At low velocities the thermal damping force varies linearly with velocity, but tends towards a constant at higher velocities. The thermal damping fits very well to a simple model developed for vibrating wire resonators. This is somewhat surprising, since the quasiparticle trajectories through the superfluid flow around the fork prongs are more complicated due to the relatively high frequency of motion. We also discuss the damping mechanism above the critical velocity and compare the behaviour with other vibrating structures in superfluid ³He-B and in superfluid ⁴He at low temperatures. In superfluid ⁴He the high velocity response is usually dominated by vortex production (quantum turbulence), however in superfluid ³He the response may either be dominated by pair-breaking or by vortex production. In both cases the critical velocity in superfluid ³He-B is much smaller and the high velocity drag coefficient is much larger, compared to equivalent measurements in superfluid ⁴He.     

Study for AC Loss Reduction in Bi2223 Tapes by Introducing Oxide Barriers

This paper presents our recent activities for the development of low-loss Bi2223 tapes with interfilamentary oxide barriers. In order to suppress the side effect on Bi2223 phase formation during sintering process, SrZrO₃ was selected as barrier materials. Moreover, small amount of Bi2212 was mixed with SrZrO₃ to improve their ductility for cold working. By controlling coating thickness of oxide barriers before stacking, reducing a tape width below 3 mm and careful twisting of the filaments with its length below 5 mm, coupling frequency f _c exceeded 250 Hz even in an AC perpendicular magnetic field. Critical current densities J _c of tightly twisted barrier tapes were ranged in 12?C14 kA/cm² at 77 K and self-field, which was 25% lower than the nontwisted one (=18 kA/cm²). To our knowledge, this is the first result to achieve both J _c>12 kA/cm² and f _c>250 Hz simultaneously for Bi2223 tapes in an isolated state. These twisted barrier tapes showed 60?C70% lower perpendicular field losses than a conventional 4 mm-width tape with fully coupled filaments at 50 mT and 50 Hz.     

Superfluid Phases of 3He in a Periodic Confined Geometry

Predictions and discoveries of new phases of superfluid ³He in confined geometries, as well as novel topological excitations confined to surfaces and edges of near a bounding surface of ³He, are driving the fields of superfluid ³He infused into porous media, as well as the fabrication of sub-micron to nano-scale devices for controlled studies of quantum fluids. In this report we consider superfluid ³He confined in a periodic geometry, specifically a two-dimensional lattice of square, sub-micron-scale boundaries (“posts”) with translational invariance in the third dimension. The equilibrium phase(s) are inhomogeneous and depend on the microscopic boundary conditions imposed by a periodic array of posts. We present results for the order parameter and phase diagram based on strong pair breaking at the boundaries. The ordered phases are obtained by numerically minimizing the Ginzburg-Landau free energy functional. We report results for the weak-coupling limit, appropriate at ambient pressure, as a function of temperature T, lattice spacing L, and post edge dimension, d. For all d in which a superfluid transition occurs, we find a transition from the normal state to a periodic, inhomogeneous “polar” phase with $T_{c_{1}} < T_{c}$ for bulk superfluid ³He. For fixed lattice spacing, L, there is a critical post dimension, d _c , above which only the periodic polar phase is stable. For d<d _c we find a second, low-temperature phase onsetting at $T_{c_{2}} < T_{c_{1}}$ from the polar phase to a periodic “B-like” phase. The low temperature phase is inhomogeneous, anisotropic and preserves time-reversal symmetry, but unlike the bulk B-phase has only $\mathtt{D}_{\text{4h}}^{\text{L}+\text{S}}$ point symmetry.     

Superfluid Transition of a 4He Thin Film Pressurized by Bulk Liquid 3He

We measured transverse acoustic impedance Z of normal fluid ³He at 46.6 MHz on a surface coated with a thin ⁴He film. The real component of the impedance, Z′, in the coated samples deviates from Z′ in the pure ³He in the low temperature region. Z′ on the coated samples is almost identical with Z′ in the pure sample at high temperature and gradually deviates below a particular temperature T _onset . T _onset  is possibly the superfluid onset temperature of the ⁴He film pressurized by the bulk liquid ³He. The gradual decrease in Z′ means that the superfluid component in ⁴He film increases gradually, which is expected from the dynamic KT transition at high frequency. The thicker is the film, the higher is the T _onset . The range of T _onset we observed was between 40 and 160 mK. This is much lower than that at the saturated vapor pressure. Suppression of T _onset achieved by the applied pressure from bulk liquid ³He was presumably caused by the dissolved ³He in the film, thickening of the inert layers and/or by the strong correlation effect. The result shows that the specularity of ³He quasiparticle scattering is strongly affected by superfluidity of the ⁴He film.     

YBa2SnO5·5, a novel ceramic substrate for YBCO and BiSCCO superconductors

YBa₂SnO_5·5 has been synthesized and sintered as single phase material for its use as substrate for both YBCO and BiSCCO superconductors. YBa₂SnO_5·5 has a complex cubic perovskite (A₂BB’O₆) structure with the lattice constanta = 8·430 Å. The dielectric constant and loss factor of YBa₂SnO_5·5 are in a range suitable for its use as substrate for microwave applications. YBa₂SnO_5·5 is found to be chemically compatible with both YBCO and BiSCCO superconductors. The thick film of YBCO screen printed on polycrystalline YBa₂SnO_5·5 substrate gave aT _c(0) of 92 K and a critical current density (J _c) of 4 × 10⁴ A/cm² at 77 K. A screen printed BiSCCO thick film on YBa₂SnO_5·5 substrate gaveT _c(0) = 110 K and current density 3 × 10³ A/cm² at 77 K.     

Improved Superconducting Properties of Thick YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Film by Adding Amino-Rich Polyethyleneimine in Precursor Solution

Polyethyleneimine (PEI) with an amount of –NH₂ groups used in precursor solution could effectively reduce Cu²⁺ volatilization during the pyrolysis process. Thermogravimetric analysis shows that the temperature window of low-temperature pyrolysis for precursor solution with PEI (PEI-YBCO) is widened significantly. The slower pyrolysis process can enrich Cu²⁺ and improve critical current density (J_c) of PEI-YBCO films. The highest J_c is 3.03 MA/cm² at 77 K when the amount of PEI is 0.5 g/10 mL and the film thickness is 400 nm. Then the thickness increases from 0.4 to 2.0 μm by changing the coating times. The J_c values of PEI-YBCO films decrease gradually with the thickness increase. However, the critical current (I_c) can be up to 197 A/cm (at 77 K, self-field) and J_c can still keep 1.68 MA/cm² at 1.2 μm.     

Relationship Between Annealing Time and Magnetic Properties in Bi-2212 Textured Composites

In this study, we report the physical and magnetic properties of Bi₂Sr₂CaCu₂O _x textured materials prepared by a LFZ melting technique and annealed for different times (60, 72, 96 and 120 h). SEM images of the annealed samples for 96 and 120 hours indicate very good alignment with the longitudinal rod axis. In all cases, X-ray diffraction patterns show that the Bi-2212 phase is the major one. The magnetization measurements have been carried out as a function of the magnetic field up to 9 kOe. J _c values of the samples were calculated by using the Bean model. The results indicate that the different annealing time has no significant effects on the T _c values but, significant change on the critical current values of samples, J _c, has been obtained for sample annealed at 96 hours. We also found that the maximum critical density of J _c is 5.5×10⁵ A/cm² at 10 K for the 96 hours annealed sample.     

High-Quality YBa2Cu3O7−x Films with CeO2/YSZ Buffer Layers on 2-Inch Si Wafers Deposited by Pulsed Laser

The pulsed laser deposition (PLD) process is shown for in situ reproducibly fabricating YBa₂Cu₃O_7?x (YBCO) superconducting films with yttrium-stabilized zirconia (YSZ) and CeO₂ buffer layers, nonsuperconducting crystalline YBa₂Cu₃O_7?x (YBCO*) passivation layer, and silver contact film on 2-inch silicon wafers. Variations of less than ±7% in film thickness have been obtained for this multilayer growth over the whole wafer. The YBCO films on 2-inch silicon wafers have homogeneous superconducting properties with zero resistance temperature T _c0 from 88.4 K to 88.9 K. and critical current density J _c at 77 K and zero field from 2.5 × 10⁶ to 7× 10⁶ A/cm². The YSZ, CeO₂ and YBCO layers grow epitaxially on silicon wafers. Full widths at half maximum (FWHMs) of (113) reflections of 40 nm thick YBCO layer from φ-scan patterns are only 1.71° and 1.85° corresponding to the center and edge of the wafer, respectively. These results are very promising for developing high-quality high-T _c superconducting devices on large-area silicon wafers.     

Preparation of High-Performance SmBa2Cu3O7−x Superconducting Films by Chemical Solution Deposition Approach

Pure and Co³⁺-doped SmBa₂Cu₃O_7?z (SmBCO) superconducting films were prepared on (00l) LaAlO₃ single crystal substrate by self-developed fluorine-free chemical solution deposition approach. According to the X-ray diffraction and SEM observation, SmBCO films with biaxial texture possess dense, smooth, and microcrack-free surface microstructures. However, critical transition temperature (T _c) of a Co³⁺-doped SmBCO film is lower than that of pure SmBCO film, which may be attributed to the Co³⁺ doping decreasing the concentration of hole carriers for doped film. In addition, the Co³⁺-doped film has higher normalized critical current densities (J _c) in the whole magnetic fields, indicating better magnetic flux pinning properties. These results show that Co³⁺ doping by this chemical method is one of the promising ways to prepare high-performance SmBCO films.