YBaCuO Grain Research Articles

A broadband detector based on series YBCO grain boundary Josephson junctions.

Modeling of a broadband receiving system based on a meander series of Josephson YBaCuO grain boundary junctions integrated into a log-periodic antenna was carried out. The electromagnetic properties of the system, namely amplitude–frequency characteristic, beam pattern, and fraction of the absorbed power in each Josephson junction were investigated. Based on the obtained results, a numerical simulation of one-dimensional arrays was carried out. The dc characteristics of the detector were calculated, that is, current–voltage characteristic, responsivity, noise, and noise-equivalent power (NEP) for a 250 GHz external signal. The optimal number of junctions to obtain the minimum NEP was found. The use of a series of junctions allows one to increase the responsivity by a factor of 2.5, the NEP value by a factor of 1.5, and the power dynamic range by a factor of 5. For typical YBaCuO Josephson junctions fabricated on a ZrYO bicrystal substrate by magnetron deposition, the following parameters were obtained at a temperature of 77 K: responsivity = 9 kV/W; NEP = 3·10−13 W/Hz(1/2); power dynamic range = 1·106.

Activation energy of oxygen diffusion: A possible indicator of supercurrents through YBa2Cu3O7 grain boundaries

Oxygen diffusion is the main transportation form during the preparation process of polycrystalline superconductor YBa2Cu3O7 (YBCO) thin films. Especially the diffusion of oxygen ions in grain boundaries (GBs) can lead to the formation of oxygen vacancies, and finally have an impact on the critical current density (Jc) of YBCO conductors. To deeply understand the supercurrent-limiting mechanism of GBs, for an YBCO bicrystal (containing a single GB), in this article we intensively studied the diffusion of oxygen ions through GB region by molecular dynamics simulations. The calculated results show that the oxygen diffusion in the YBCO bicrystals can be enhanced in GBs, and specifically which is closely related to the grain boundary angle (θ). The most interesting finding is that the activation energy of oxygen diffusion shows an exponential fall with the grain boundary angle, which is exactly consistent with the relation of the critical current density versus the grain boundary angle (Jc ~ θ) in GBs. We firmly believe that the activation energy of oxygen diffusion should have an essential relevance with the Jc especially in YBCO GBs, and can be as an indicator for predicting supercurrents. For these results we further gave an in-depth discussion.

Introducing multi-source pinning centers into Y–Ba–Cu–O superconductor through addition of BiFeO3 nano-particles

Chemical doping of BiFeO3 (BFO) nano-particles in Y–Ba–Cu–O (YBCO) superconductor was performed using the powder melt and infiltration growth (PM-IG) technique. The results indicate that improved superconducting properties have been observed in 0.2wt% nano-BFO doped sample. The X-ray diffraction (XRD) result performed on the ground powder of 5wt% nano-BFO doped YBCO grain proves the decomposition of BFO phase during the PM-IG process and the formation of Y2Ba4CuBiOy (YBi-2411) phase. Consequently, the addition of BFO could introduce multi-source pinning centers, i.e. YBi-2411 nanoscale particles and Fe element substitutions, which accounts for the improvement in superconducting properties.

NanoSQUID magnetometry of individual cobalt nanoparticles grown by focused electron beam induced deposition

We demonstrate the operation of low-noise nano superconducting quantum interference devices (SQUIDs) based on the high critical field and high critical temperature superconductor YBa2Cu3O7 (YBCO) as ultra-sensitive magnetometers for single magnetic nanoparticles (MNPs). The nanoSQUIDs exploit the Josephson behavior of YBCO grain boundaries and have been patterned by focused ion beam milling. This allows us to precisely define the lateral dimensions of the SQUIDs so as to achieve large magnetic coupling between the nanoloop and individual MNPs. By means of focused electron beam induced deposition, cobalt MNPs with a typical size of several tens of nm have been grown directly on the surface of the sensors with nanometric spatial resolution. Remarkably, the nanoSQUIDs are operative over extremely broad ranges of applied magnetic field (–1 T 1 T) and temperature (0.3 K 80 K). All these features together have allowed us to perform magnetization measurements under different ambient conditions and to detect the magnetization reversal of individual Co MNPs with magnetic moments (1–30) . Depending on the dimensions and shape of the particles we have distinguished between two different magnetic states yielding different reversal mechanisms. The magnetization reversal is thermally activated over an energy barrier, which has been quantified for the (quasi) single-domain particles. Our measurements serve to show not only the high sensitivity achievable with YBCO nanoSQUIDs, but also demonstrate that these sensors are exceptional magnetometers for the investigation of the properties of individual nanomagnets.

Comparison of Mechanical and Superconducting Properties of YBaCuO and GdBaCuO Single Grains Prepared by Top-Seeded Melt Growth

It is known that mechanical properties of a high-temperature superconductor (HTS) can affect the superconducting behaviour of the material. In this study, the mechanical and basic superconducting properties of single-domain YBaCuO and GdBaCuO were determined and compared. The bulk single-grain superconductor samples were prepared by top-seeded melt growth process. Distribution of trapped fields in the samples was mapped using the Hall probe technique. Levitation force was assessed using a dynamometer. Phase composition of the samples, was characterised by X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) measurements. Microstructure was studied using scanning electron microscopy (SEM) and optical microscopy. Microhardness and comprehensive strength of the prepared samples were measured at a room temperature. The GdBaCuO superconductor has shown higher average hardness than the YBCO sample; higher fragility was also observed by comprehensive strength measurement of both types. Maximum trapped magnetic field was 0.65 T at 77 K in the case of a Gd-based material. In comparison with YBaCuO, the values of the trapped magnetic field and levitation force of GdBaCuO were higher. The results obtained can be useful for rare earth element (RE) superconductor device engineering as well as for the research in the field of superconducting ceramics.

Effect of Y-211 particle size on the growth of single grain Y–Ba–Cu–O bulk superconductors

The engineering of fine Y2Ba1Cu1O5 (Y-211) inclusions of average particle size 1−2μm within the continuous, superconducting YBa2Cu3O7−δ (Y-123) phase matrix of single-grain, bulk high temperature Y–Ba–Cu–O (YBCO) superconductors is fundamental to achieving high critical current density in these materials. However, bulk samples fabricated using fine Y-211 precursor powders generally suffer from incomplete growth of the Y-123 phase compared to samples fabricated with coarser Y-211 particles of average particle size >5μm in the precursor powder. In this study, the effects of Y-211 particle size on processing parameters during growth of large single YBCO grains are reported. Chemical analysis using EDX of cross-sections of single YBCO grains indicates that the loss of liquid phase during melt processing is greater when finer Y-211 precursor powder is employed, which accounts for the observed growth limitations and low sample yield. Specific modifications to the single grain growth process have been made based on the results of this investigation to reduce liquid loss and to enable the use of fine Y-211 precursor powder without compromising complete single grain growth or yield.

Role of the Organic Matrix in the Biopolymer-Mediated Synthesis of Platelike YBCO

The biomimetic method of achieving dense platelike grain growth of the high temperature superconducting ceramic YBa2Cu3O7 (YBCO) has been studied. By using various organic matrices (dextran and chitosan) having different molecular structures, the final size and arrangement of the platelike YBCO grains exhibit significant differences, possible reasons for which are discussed. The different organic matrices used also indirectly affect the superconducting properties of the YBCO formed. By elucidating the role of the organic matrix in the biomimetic synthesis process in this way, the whole process could be controlled more precisely, leading to structural and electrical properties of the final product more desireable for specific applications.

Terahertz emission and detection both based on high-Tc superconductors: Towards an integrated receiver

We have combined a stand-alone Bi2Sr2CaCu2O8 intrinsic Josephson junction stack, emitting terahertz radiation, with a YBa2Cu3O7 grain boundary Josephson junction acting as detector. The detector is mounted on a lens, positioned 1.2 cm away from the emitter on a similar lens. With the emitter radiating at 0.5 THz, we observed up to 7 Shapiro steps on the current-voltage characteristic of the detector. The ac current induced in this junction was 0.9 mA, and the dissipated power was 1.8 μW. The setup, although far from being optimized, may be considered as a first step towards an integrated high-Tc receiver.

Macroscopic quantum tunneling and retrapping processes in moderately damped YBaCuO Josephson junctions

The moderately damped regime in Josephson junctions (JJ) is quite common in devices characterized by low critical currents and, therefore, by low Josephson energies. Measurements of the switching current distribution (SCD) are a direct way of determining the phase dynamics in the nontrivial case of moderate damping, which is becoming more common with advances in nanopatterning superconductors and in materials science for fabricating hybrid systems. We report measurements of SCDs, in both the thermal and quantum regimes for moderately damped YBaCuO grain boundary biepitaxial JJs. A direct transition from a phase diffusion regime to macroscopic quantum tunnelling occurs at about 130 mK. The crossover to the quantum regime is determined by the magnetic field and the phase dynamics can be described by a fully consistent set of junction parameters obtained by numerical simulation.

Resonant behavior of the barrier of YBa2Cu3O7 grain boundary Josephson junctions fabricated on bicrystalline substrates with different geometries

We have analyzed a resonant behavior in the dielectric constant associated to the barrier of YBa2Cu3O7 (YBCO) grain boundary Josephson junctions (GBJJs) fabricated on a wide variety of bicrystalline substrates: 12° [001] tilt asymmetric, 24° [001] tilt asymmetric, 24° [001] tilt symmetric, 24° [100] tilt asymmetric, 45° [100] tilt asymmetric and 24° [001] tilt symmetric +45° [100] tilt asymmetric bicrystals. The resonance analysis allows us to estimate a more appropriate value of the relative dielectric constant, and so a more adequate value for the length L of the normal N region assuming a SNINS model for the barrier. In this work, the L dependence on the critical current density Jc has been investigated. This analysis makes possible a single representation for all the substrate geometries independently on around which axes the rotation is produced to generate the grain boundary. On the other hand, no clear evidences exist on the origin of the resonance. The resonance frequency is in the order of 1011Hz, pointing to a phonon dynamic influence on the resonance mechanism. Besides, its position is affected by the oxygen content of the barrier: a shift at low frequencies is observed when the misorientation angle increases.

Energy scales in YBaCuO grain boundary biepitaxial Josephson junctions

Self-assembled nanoscale channels may naturally arise in the growth process of grain boundaries (GBs) in high critical temperature superconductor (HTS) systems, and deeply influence the transport properties of the GB Josephson junctions (JJs). By isolating nano-channels in YBCO biepitaxial JJs and studying their properties, we sort out specific fingerprints of the mesoscopic nature of the contacts. The size of the channels combined to the characteristic properties of HTS favors a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current–voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Thouless energy emerges as a characteristic energy of these types of Josephson junctions. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed, along with elements to take into account when designing HTS nanostructures.

Microstructure and Superconducting Properties of Single Grains of Y-Ba-Cu-O Containing Y-2411(M) and ${\rm Y}_{2}{\rm O}_{3}$

Y-Ba-Cu-O (YBCO) single grains have the potential to generate large trapped magnetic fields for engineering applications, and research on the processing and properties of this material has attracted interest world-wide over the past 20 years. In particular, the introduction of flux pinning centers to the large grain microstructure to improve its current density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Jc</i> , and hence trapped field, has been investigated extensively. Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> CuMO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> [Y-2411(M)], where M=Nb , Ta, Mo, W, Ru, Zr, Bi and Ag, has been discovered recently to form very effective flux pinning centers due primarily to its ability to form nano-size inclusions in the superconducting phase matrix. However, the addition of the Y-2411(M) phase to the precursor composition complicates the melt-processing of single grains. The addition of Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> to the precursor composition, however, broadens the growth window of single YBCO grains containing Y-2411 (M). We report an investigation of the microstructures and superconducting properties of single grains of this composition grown by top seeded melt growth (TSMG).

Resistively shunted YBa2Cu3O7 grain boundary junctions and low-noise SQUIDs patterned by a focused ion beam down to80 nm linewidth

YBa2Cu3O7 24° (30°) bicrystal grain boundary junctions (GBJs), shunted with 60 nm (20 nm)thick Au, were fabricated by focused ion beam milling with widths80 nm ≤ w ≤ 7.8 µm. At 4.2 K we find criticalcurrent densities jc inthe 105 A cm − 2 range (without aclear dependence on w) and an increase in resistance times junction areaρn with an approximate scaling . For the narrowest GBJs jcρn = IcRn≈100 µV (with critical current Ic and junction resistance Rn), which is promising for the realization of sensitive nanoSQUIDs for thedetection of small spin systems. We demonstrate that our fabrication processallows the realization of sensitive nanoscale dc SQUIDs; for a SQUID withw≈100 nm wide GBJs we find an rms magnetic flux noise spectral density ofSΦ1/2≈4 μΦ0 Hz − 1/2 in the white noise limit. We also derive an expression for the spin sensitivitySμ1/2, whichdepends on SΦ1/2, on the location and orientation of the magnetic moment of a magnetic particle to be detected bythe SQUID, and on the SQUID geometry. For the unoptimized SQUIDs presented here, we estimateSμ1/2 = 390 μB Hz − 1/2, which could be further improved by at least an order of magnitude.

Nucleation and growth rate influence on microstructure and critical currents of TFA-YBa2Cu3O7 under low-pressure conditions

The effects of variable conversion parameters on the microstructure and critical currents of TFA-derived YBa2Cu3O7 (YBCO) films annealed under low-pressure conditions were investigated, accompanied by the analysis of their relationship with the nucleation process and the growth rate. It is found that non-c-axis oriented YBCO grains are formed under high supersaturation conditions, i.e., by increasing oxygen pressure, water pressure, or temperature. The optimal PH2O–PO2 window for preparation of completely c-axis oriented YBCO films expands as the total pressure rises from 50 to 100 mbar due to the decrease of supersaturation at enhanced total pressure; the corresponding maximum growth rate is only slightly increased up to 0.6 nm/s. Additionally, it is shown that the gas flow needs to be high enough to avoid random nucleation of YBCO grains. A single gas-flow–water-pressure diagram, showing simultaneously the film-growth rate, allows visualizing the cross-linked influence of processing parameters to achieve c-axis oriented YBCO films with Jc above 1 MA/cm2 in one single growth step.

Growth rate of YBCO single grains containing Y-2411(M)

Y-Ba-Cu-O (YBCO) single grains have the potential to generate large trapped magnetic fields for a variety of engineering applications, and research on the processing and properties of this material has attracted world-wide interest. In particular, the introduction of flux pinning centres to the large grain microstructure to improve its current density, Jc, and hence trapped field, has been investigated extensively over the past decade. Y2Ba4CuMOx [Y-2411(M)], where M = Nb, Ta, Mo, W, Ru, Zr, Bi and Ag, has been reported to form particularly effective flux pinning centres in YBCO due primarily to its ability to exist as nano-size inclusions in the superconducting phase matrix. However, the addition of the Y-2411(M) phase to the precursor composition complicates the melt-processing of single grains. We report an investigation of the growth rate of single YBCO grains containing Y-2411(Bi) phase inclusions and Y2O3. The superconducting properties of these large single grains have been measured specifically to investigate the effect of Y2O3 on broadening the growth window of these materials.

Fiske steps and hysteresis in YBa2Cu3O7 grain boundary Josephson junctions: Structural information of the barrier by means of a nondestructive approach

A deep analysis of the current-voltage (I-V) characteristics of YBa2Cu3O7 grain boundary Josephson junctions (GBJJs) allows us to go much farther than the usual calculus of the transport parameters. It is possible to construct a structural image of the barrier by an exhaustive and complementary analysis of both transport and electromagnetic parameters obtained from I-V curves. For such an approach, we have chosen the following three representative bicrystalline geometries: 24° [001] asymmetric, 45° [100] asymmetric, and 24° [001] symmetric +45° [100] asymmetric. The dependence of the product ICRN on the junction normal resistance is of the ICRN−1 type pointing to a SNINS model (S denotes superconductor, I denotes insulator, and N denotes normal metal) for all our GBJJs. A satisfactory explanation of the discrepancy of the capacitance of the barrier estimated from Fiske resonance positions and hysteresis in the I-V curves needs of such a model. Moreover an estimation of the length of the normal regions adjacent to the crystallographic barrier can be made. This comparative analysis is presented in order to extract interesting information about the particular transport mechanisms involved in these GBJJs.

Strong influence of the YBa2Cu3O7 grain size on critical current densities of thickYBa2Cu3O7 layers made by a metal–organic deposition process

We correlate the critical current performance of thick film second-generationYBa2Cu3O7 (YBCO) coated conductors, made by metal–organic deposition,with specimen grain size. The transport critical current densities,JcT, of these conductors were observed to be inversely proportional to YBCO grain size in the5–40 µm size range. Analysis of magnetic hysteresis curves of the conductors and transmissionelectron microscopy data from a large YBCO grain revealed that the reducedJcT in tapes with large YBCO grain size is due to the accumulation of secondary phaseprecipitates within the grains near the periphery of, and in, the grain boundaries.Grain size is determined to be one of the important factors in determiningJcT of YBCO coated conductors made by the metal–organic processes.

Application of low-angle polishing for rapid assessment of the texture and morphology of thick film Y1Ba2Cu3O7 superconducting tapes

Metallographic polishing was used for the rapid assessment of the texture quality of 2μm thick YBa2Cu3O7 (YBCO) layers deposited on oxide buffered RABiTS™ metal substrates. The YBCO films were grown using the barium fluoride ex situ process. The method is based on partial removal of the YBCO layer by mechanical polishing, resulting in the formation of a 200–300μm wide wedge of YBCO material with an approximately linear thickness profile. Due to the linearity of the thickness profile of the wedge, the depth below the film surface, of any visible structural detail in the wedge area, can be determined by simply measuring the lateral distance from the detail of interest to the film surface/wedge intersection. In this study the wedge area was inspected by optical microscopy for changes in morphology, twin plane pattern, and YBCO grain size. It is demonstrated how X-ray diffraction, combined with low-angle polishing can help establish the origins of YBCO texture degradation in the sub-surface layers of thick YBCO films. It is concluded that homogeneous nucleation is responsible for the appearance of random texture in sub-surface YBCO layers.

Critical current densities and the structural quality of 3- and 4-μm-thick superconducting YBa2Cu3O7 layers synthesized using the ex situ process

In this work, we used quantitative x-ray diffraction measurements and optical metallography to investigate the relationship between structural quality and critical current densities Jc for 3- and 4-μm-thick YBa2Cu3O7 (YBCO) films grown on CeO2-buffered Ni–W substrates by the BaF2 process. The Jc of the films was shown to be approximately: (1) proportional to the intensity of the YBCO (006) diffraction line, and (2) inversely proportional to the average grain size of the c-axis oriented YBCO as determined from optical micrographs of polished surface of the films. We conclude that to achieve Jcs well above 106 A/cm2 in self-field and at 77 K, it is critical to suppress the formation of randomly oriented YBCO grains while maintaining high crystallographic quality of the c-axis oriented part of the film. The quality of the c-axis oriented YBCO was found to be strongly dependent on the YBCO grains size—e.g., the grains, which are smaller than 10 μm, are required for high Jc films. The fine-grain high Jc films can be synthesized under processing conditions that promote a high rate of nucleation of c-axis oriented YBCO.

Strongly Coupled Artificial Bulk HTS Grain Boundaries With High Critical Current Densities

A multi-seeding process has been developed to fabricate single Y-Ba-Cu-O (YBCO) grains containing strong artificial grain boundaries. Multi-seeding of heterogeneous YBCO grains with controlled orientation was achieved using large Sm-Ba-Cu-O (SmBCO) single crystal seeds of rod-like geometry with slots of various widths (up to 13 mm) cut into their bottom surface (i.e. parallel to the c-axis of the seed) to produce a bridge-like structure. Several YBCO grains with artificial grain boundaries were fabricated from these seed crystals and used to investigate the effect of varying the distance between the individual grain nucleation sites and the grain orientation (in-plane and out of plane) on the nature of grain boundaries. The measured local magnetic critical current density (Jc) and the magnitude of the trapped field across these artificial grain boundaries indicate that seed alignment is a key parameter in achieving strongly-coupled grain boundaries in multi-seeded grains.