YBa2Cu3O7 Films Research Articles

Coherent Raman spectroscopy of YBa_2Cu_3O_7

We report preliminary results on the optical driving and spectroscopic detection of Raman-active features in thin films of the high-temperature superconductor YBa(2)Cu(3)O(7) in its normal phase by means of coherent Stokes and anti-Stokes Raman scattering, in a reflection geometry. Reference measurements on germanium are also reported. We observe phonon resonances which interfere coherently with a broad electronic resonance centered at zero frequency, giving rise to characteristic asymmetric spectral features. Our measurements provide a first step towards applying nonlinear optical wave-mixing spectroscopy to correlated electron systems.

Quality of YBCO thin films grown on LAO substrates exposed to the film deposition - film removal processes

The characteristics are investigated of high temperature superconducting YBa2Cu3O7 (YBCO) films grown on LaAlO3 (LAO) substrates being exposed a different number of times to YBCO film deposition and acid-solution-based cleaning procedures. Possible mechanisms of degradation of the substrate surface quality reflecting on the growing YBCO film parameters are discussed and analyzed.

Q factor of megahertz LC circuits based on thin films of YBaCuO high-temperature superconductor

High-frequency properties of resonant structures based on thin films of YBa2Cu3O7–δ high-temperature superconductor are studied experimentally in the frequency range 30–100 MHz. The structures planar induction coils with a self-capacitance fabricated on neodymium gallate and lanthanum aluminate substrates. The unloaded Q factor of the circuits exceeds 2 × 105 at 77 K and 40 MHz. Possible loss mechanisms that determine the Q factor of the superconducting resonant structures in the megahertz range are considered.

Effect of cyclic annealing on the static and high-frequency characteristics of YBCO-film-based resonant structures

We have studied correlations between the static parameters and high-frequency Q values of megahertz-range resonant LC circuits representing planar coils with self-capacitance manufactured on the basis of thin films of YBa2Cu3O7 − δ (YBCO) high-temperature superconductor. It is established that the static characteristics of resonant circuits reversibly vary in the course of cyclic annealing, while the Q value does not exhibit such changes.

Epitaxial (La, Sr)TiO3 on textured Ni–W as a conductive buffer architecture for high temperature superconducting coated conductor

The formation of epitaxial (La,Sr)TiO3 on biaxially-textured Ni–W metal alloy tape was examined as a conductive buffer layer for YBa2Cu3O7 film growth. A TiN epitaxial nucleation layer served as an effective template layer for subsequent (La,Sr)TiO3 film growth on the Ni–W tape. The TiN/(La,Sr)TiO3 multilayer was epitaxial on the Ni–W substrate as confirmed via four-circle X-ray diffraction. A high temperature superconducting YBa2Cu3O7 film was deposited epitaxially on the (La,Sr)TiO3/TiN/Ni–W tape. The YBa2Cu3O7 film exhibited a superconducting transition temperature of 86K and critical current density of 0.42×106A/cm2 at 77K.

Highly heteroepitaxial thin films of YBa 2Cu 3O y grown on silicon with Eu 2CuO 4/Y-ZrO 2 bi-layer buffer

Highly epitaxial thin films of YBa 2Cu 3O y (YBCO) have been grown on silicon with a double buffer of Eu 2CuO 4(ECO)/Y-ZrO 2(YSZ). The utilization of the ECO layer, which possesses a very stable 214-T' structure and excellent compatibility with YBCO, greatly improves the epitaxy and surface morphology of the grown YBCO film, whereas the YSZ layer efficiently blocks the interaction between silicon substrate and YBCO. The grown films were characterized by grazing incidence X-ray reflection, rocking scans of the X-ray diffraction peaks, scanning electron microscopy, cross-sectional transmission electron microscopy, and surface profilometry. The results showed an enhanced crystallinity and very clear interfaces between layers, demonstrating the advantages of such an ECO/YSZ double buffer. It was also found that this bi-layer buffer structure could greatly improve the surface roughness of the YBCO films.

MOCVD-SrTiO3 thin film microwave coplanar tunable devices: modelling of varactors

SrTiO3 (STO) ferroelectric (FE) thin films associated with High Temperature Superconductor (HTSC) is a good compromise to realize electronically tunable microwave devices combining tunable dielectric properties of FE films with low loss microwave conductivity in HTSC. STO, which exhibits a perovskite structure, is suitable for epitaxial growth of YBaCuO films and has been widely studied to realize tunable components around 77 K. Up to now, STO thin films were essentially deposited by sputtering and pulsed laser deposition. In the framework of this study, we have explored the feasibility of microwave devices made from STO thin films prepared by metalorganic chemical vapour deposition (MOCVD). We have characterized Au/STO(250 nm thick)/MgO coplanar waveguide transmission lines and microwave variable capacitors (varactors) from 45 MHz up to 40 GHz, in the 300 K to 60 K temperature range. Dielectric characteristics of the STO films were extracted from measurements and studied as a function of frequency, electric field and temperature. The geometry of the interdigital capacitors (IDC) was chosen after evaluating their capacitances using an analytical theoretical model. We have used a de-embedding method to extract, from electromagnetic simulations using Sonnet® Software, the capacitance of the IDC alone.

On the theory of strong pinning in high-temperature superconducting films

Several pinning mechanisms proposed to explain extremely high critical current densities jc in c-oriented epitaxial YBaCuO films are analysed. It is concluded that only strong pin-breaking mechanisms, such as pinning by cores of edge dislocations threading the whole film and forming low-angle boundaries between single-crystal domains (at low and moderate fields) and the pinning by sparse large point defects, such as nanoscale inclusions of non-superconducting phases (at higher fields) are relevant. It is shown that available expressions for jc for the last mechanism have certain limits of applicability, such as unphysical divergence of jc with increasing anisotropy Γ leading to an essential overestimate even for YBaCuO with the moderate anisotropy Γ ≈ 7. A nature of such inconsistency and the ways of its overcoming are elucidated. Some aspects of the strong pinning theory are discussed. In the case of thin films it is shown that for the accurate description both tilt and shear elastic moduli should be taken into account as competing factors. The obtained area of vortex trapping on finite-length pinning centre describes crossover from 0D to 1D case and allows a unification of strong pinning theory for different types of defects.

Microstructure and the grain boundaries evolution in sequential epitaxial buffer layers on RABiTS-Substrates

Epitaxial buffer layers of CeO2 and Yttria-stabilized ZrO2 (YSZ) have been deposited on biaxially textured nickel substrates using reel to reel thermal reactive evaporation, and rf sputtering. The degree of texture of the deposited buffer layers were analysed by X-ray pole figure, Out-of-plane (w-scan) and in-plane (Φ-scan) texture. The Microstructures of oxide buffer layer was determined by electron backscattering diffraction (EBSD) partially and SEM in selected marked area, providing information on the propagation of the grain boundary network from the substrate to the buffer layers in the same expected area. The oxide layer sequence, YBCO/CeO2/YSZ/CeO2/NiW was the same in each sample, but different deposition techniques were used to deposit the YBa2Cu3O7 films: MOCVD, PLD, and DC-sputtering. Transport measurements on the coated conductor samples were measured 1.2 MA/cm2, and 1.0 MA/sm2, 1.8 MA/cm2 at 77 K (0 T) respectively.

Smectic Vortex Phase in Optimally DopedYBa2Cu3O7Thin Films

Angular dependent resistivity measurements of optimally doped YBa2Cu3O7 films in fields H pulsed to 50 T are presented. Up to the highest H, the vortex melting field Hm increases and vortex motion is reduced for H aligned with the correlated pinning centers along the main crystalline axes, otherwise 3D anisotropic scaling describes the vortex dynamics. For H parallel ab, the rapid increase in Hm at low temperatures and a critical exponent analysis near Hm confirm the presence of the liquid-crystalline smectic phase predicted for layered superconductors.

Formation of YBa2Cu3O6 + x clusters in laser-generated plasma

We have studied the influence of YBa2Cu3O6 + x clusters formed in the plasma generated by laser ablation of a YBa2Cu3O7 − δ target on the optical transmission spectra of amorphous YBaCuO films deposited on glass substrates arranged along the direction of predominant plasma expansion in the laser plume. It is established that intense cluster formation begins in the region of rapid decrease in the film thickness, where the temperature of plasma decreases to a level at which stable atomic complexes characteristic of the target composition can form (under the experimental conditions studied, this was observed at as distance of L > 6 cm from the target). As the amount of clusters in the deposit increases, the magnitude of the interference fringes, which are characteristic of optically homogeneous media, gradually decreases and eventually almost vanishes. At the same time, features typical of the electron structure of YBa2Cu3O7 − δ appear and grow in the optical transmission spectra of the YBaCuO films, including the absorption due to free charge carriers at ℏω < 1.2 eV (characteristic of “metallic” clusters) and the minima at ℏω = 1.4 and 1.75 eV (characteristic of a dielectric state).

EPITAXIAL THIN FILM YBCO MEMBRANE STRUCTURE FOR THERMAL DETECTORS

ABSTRACT YBa2Cu3O7(YBCO) films have been used at room temperature as conducting oxide electrodes, as well as an excellent template layer for epitaxial growth PZT thin films. This research addresses the fabrication of epitaxial thin film membrane structures based on high temperature superconductor (HTSC) microbolometers for highly sensitive thermal detectors that can be miniaturized for affordable passive millimeter-wave (MMW) imaging. A completely dry etch process is described for fabricating transition-edge microbolometer airbridges based on YBCO film. Pulse Laser Deposition (PLD) grown YBCO film on silicon substrate have anomalies on the surface that must be removed prior to the release process.

Correlation between cation disorder and flux pinning in the YBa2Cu3O7 coated conductor

A correlation has been observed between the through-thickness cation disorder level in YBa2Cu3O7 (YBCO) coated conductor films and the commonly observed falloff in layer-by-layer through-thickness critical current density (Jc) of YBCO films produced by pulsed laser deposition (PLD) on single crystal substrates. The amount of cation disorder with respect to the aggregate, c-axis-textured YBCO content was measured by Raman spectroscopy methods combined with step milling of 1.5–1.7μm thick YBCO films. For PLD YBCO films on single crystal strontium titanate, the Raman results indicate a sharp drop off in cation disorder from the substrate surface outward toward the top of the YBCO film. This drop off appears to level out in the 0.4–0.6μm range, after which the cation disorder level remains relatively constant. This trend appears to be very much like the one reported by others for the through-thickness Jc dependence of PLD YBCO on single crystal specimens. Conversely, metal-organic-deposition-produced YBCO films on rolling assisted biaxially textured substrates do not exhibit a strong through-thickness Jc dependence or a measurable change in the relative amount of cation disorder, again through thickness. The results of this study suggest that cation disorder in the YBCO lattice leads to increased flux pinning strength and that the cation disorder domains most probably occur on a size scale in the range from one to perhaps a few YBCO unit cell dimensions.

Crossover from 2D to 3D behavior in the superfluid density of thin YBa 2Cu 3O 7− δ films

Ultrathin underdoped films of YBa 2Cu 3O 7− δ (YBCO) exhibit a prominent and abrupt downturn in ab-plane superfluid density n S( T) at a temperature consistent with a 2D vortex-pair unbinding transition. In this paper, we show that this characteristic feature of 2D superconductors diminishes with increasing film thickness consistent with theory, provided one uses the full film thickness rather than a copper-oxide bilayer thickness to calculate T C. In thick films, there is no evidence for 3D-XY fluctuations preceding the 2D downturn, as would be expected in a quasi-2D superconductor.

Growth of stacked heterostructures of SIS-type with S = YBCO orBSCCO and I = (Ba,Ca)CuO2 or (Sr,Ca)CuO2

We investigated MOCVD growth ofc-axis two-layer and three-layer stacked heterostructures on(100)SrTiO3 of SI, IS or SIS type. S denotes high temperature superconductor thin films ofYBa2Cu3O7 (Y-123),Bi2Sr2Ca2Cu3O10 (Bi-2223)or Bi2Sr2CaCu2O8 (Bi-2212), while I stands for insulating thin films of(Ba,Ca)CuO2 or(Sr,Ca)CuO2. X-ray diffraction, atomic force microscopy and measurements of resistivity versustemperature were performed after the deposition of each layer. The best results,suitable for further device fabrication, were obtained for the SIS structure(100)SrTiO3/Bi- 2223/(Sr,Ca)CuO2/Bi- 2212. Similarstructures with I = (Ba,Ca)CuO2 have shown higher impurity phases and lower superconducting quality of the topsuperconducting layer. For two-layer or three-layer structures, Bi-2223 as a top layercould not be obtained and, instead, a phase with Bi-2212 formed. Structures with(100)SrTiO3/Y- 123/(Sr,Ca)CuO2/Y- 123 or(100)SrTiO3/Y- 123/(Ba,Ca)CuO2/Y- 123 were also promising, but a current leak often occurred due to greater roughness. Structureswith Y-123 are less sensitive to growth temperature than those with BSCCO.

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.

A study on the microwave responses of YBCO and TBCCO thin films by coplanar resonator technique

YBa2Cu3O7(YBCO) thin films have been prepared by thermal coevaporation on LaAlO3(LAO) substrates, and Tl2Ba2CaCu2O8(TBCCO) thin films are synthesized by magnetron sputtering method on LAO substrates. The transition temperature Tc is90 K for YBCO/LAO and 104 K for TBCCO/LAO. Microwave responses of the films are studied systematically by coplanar resonator technique. Energy gaps of the films obtained are Δ0 = 1.04kBTc for YBCO films and Δ0 = 0.84kBTc for TBCCO films by analysing the temperature dependence of resonant frequencies of coplanar resonator. Penetration depth at 0 K λ0 = 198 nm for YBCO films and λ0 = 200 nm for TBCCO films could also be obtained by using the weak coupling theory and two fluid theory. Results of penetration depth and energy gap confirm the weak coupling properties of the films. In addition, microwave surface resistances Rs of YBCO/LAO and TBCCO/LAO are also investigated by analysing the quality factor and insert loss of the coplanar resonator. Surface resistance of TBCCO/LAO is less than that of YBCO/LAO, so that TBCCO/LAO films may have more potential applications.

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.

Atomically flat MOD La0.7Sr0.3MnO3 bufferlayers for high critical current YBa2Cu3O7 TFA films

Superconducting YBa2Cu3O7 (YBCO) thin films were grown by the trifluoroacetates route onto metallorganic depositedLa0.7Sr0.3MnO3 (LSMO) buffer layers. The growth conditions of the LSMO films have been studied in detail andfully strained epitaxial layers with atomically flat surfaces have been obtained with local roughness∼1 nm, as determined by atomic force microscopy. We prove that this surface morphology is a keyparameter for achieving YBCO/LSMO multilayers with high critical current densities in self-field,Jcsf (77 K) = 1.8 MA cm−2. Angular pinning studies demonstrate that the defect structure is modifiedwith respect to standard TFA–YBCO films as a pronounced peak inJc is observed for at high temperatures. This indicates the presence of anisotropic defects playing animportant role in vortex pinning even at the irreversibility line stressing the suitability ofthese all-chemical TFA–YBCO/LSMO multilayers as conductive coated conductorarchitectures.

Vortex pinning landscape in YBa2Cu3O7 films grown by hybrid liquid phase epitaxy

The influence of film thickness and growth rate on the vortex pinning in hybridliquid phase epitaxy (HLPE) films was explored. Film growth rates as high as12 nm s−1 (0.7 µm min−1) producedhigh Jc films. Weak or no thickness dependence was found in films of thickness ranging from 0.4 to3 µm. Field and angular measurements of the critical current density(Jc) and the power-lawexponent (N) of the current–voltage curves were used to determine the nature of pinning. Films thinner than0.6 µm showed a higher density of correlated defects parallel to theab plane than thicker films. Using HLPE, it was possible to achieve very strong pinning in films∼3 µm thick, yielding criticalcurrents over 300 A cm−1 width atself-field, and as high as 35 A cm−1 width at μ0H = 3 T at T = 75.5 K. Decreasing the deposition rate allowed improving the high field performance,opening up the possibility to engineer the pinning landscape of the HLPE films.