YBaCuO Thin Films Research Articles

Comparison of radio-frequency and microwave superconducting properties of YBaCuO dedicated to magnetic resonance imaging

Properties of YBaCuO thin films are evaluated in two distinct frequency ranges using different patterns made during the same process on LaAlO/sub 3/ substrate. Microwave superconducting properties in the range 1-45 GHz are determined by S-parameters measurement of a superconducting coplanar waveguide in the range 53-95 K. We obtain a surface resistance of 0.4 m/spl Omega/ at 10.8 GHz and 77 K. Radio-frequency properties are obtained by measuring the Q-factor of a superconducting resonator (YBCO multiturn transmission lines separated by a sapphire sheet) dedicated to surface magnetic resonance imaging. At 52 MHz and 77 K we measure a Q-factor of 33180. The extraction of the radio-frequency surface resistance from Q-factor measurements in the 64-95 K range takes into account external loss mechanisms and nonuniform normal current distribution and leads to a 0.0093-/spl mu//spl Omega/ surface resistance at 52 MHz and 77 K, in good agreement with the value extrapolated from microwave measurements assuming an /spl omega//sup 2/ frequency dependence. The evaluation of /spl lambda//sub 0/ is carried out by using several models for X/sub L/(t). Least squares fits to data in the microwave and radio-frequency domain are performed using the Gorter-Casimir expression for X/sub L/(t) and give the same /spl lambda//sub 0/ value for both devices.

Nanocrack junctions in unidirectionally twinned thin films of YBa2Cu3O7−δ on (001) NdGaO3

We found unidirectional nanocracks in unidirectionally twinned films of YBa2Cu3O7−δ on NdGaO3 which are normal to the twin planes. In 70–250 nm thick films, transmission electron and atomic force microscopy show that these unidirectional nanocracks are found to be 1–2 μm apart and have 2–4 nm width. Microbridges that cross these nanocracks behave as natural Josephson junctions. They show Shapiro steps under microwave irradiation and suppression of the critical current Ic in magnetic fields. A very large in-plane anisotropy of the resistivities and Ic was observed and this was correlated with the nanocracks orientation, length, and film thickness. The observed anisotropy of Ic near Tc is shown to be consistent with a d+is wave symmetry of the order parameter in YBa2Cu3O7−δ.

Paraconductivity in YBa2Cu3O7 and YBa2Cu3O7 thin film composites with cerium oxide and gold

The paraconductivity for thin films of YBa2Cu3O7 (YBCO) and composites of YBCO with CeO2 and gold was investigated and analyzed using the Aslamazov and Larkin theory. Plain YBCO thin films sputter deposited on MgO, SrTiO3, and yttrium stabilized zirconium typically showed no dimensional crossover, were described by three dimensional (3D) fluctuation conductivity, and had a critical slope just above the critical region in agreement with the prediction of theory. With few exceptions, the CeO2 and gold composite films showed a dimensional crossover and a reduced critical exponent. The change in dimensionality was typically from 3D to two dimensional (2D) fluctuation conductivity. These composite films all had reduced values for the critical exponent which we attribute to an enlarged or extended critical region. This is in agreement with the critical current response of these composites to small magnetic fields. Additionally, the composites showed a two step transition in the paraconductivity and a minimum below the critical region. The subsequent fall-off to the zero resistance state was characterized by two different slopes. Lastly, the paraconductivity was related to film degradation resulting from photoresist processing and time storage. Dimensionality changes were observed due to this degradation.

Unusual features in the nonlinear microwave surface impedance of Y-Ba-Cu-O thin films

Striking features have been found in the nonlinear microwave (8 GHz) surface impedance $Z_s=R_s + jX_s$ of high-quality YBaCuO thin films with comparable low power characteristics [$R_{res}\sim 35--60 \mu\Omega$ and $\lambda_L(15 K)\sim 130--260 nm$]. The surface resistance $R_s$ is found to increase, decrease, or remain independent of the microwave field $H_{rf}$ (up to 60 mT) at different temperatures and for different samples. However, the surface reactance $X_s$ always follows the same functional form. Mechanisms which may be responsible for the observed variations in $R_s$ and $X_s$ are briefly discussed.

Spatially resolved studies of chemical composition, critical temperature, and critical current density of a YBa2Cu3O7−δ thin film

Spatially resolved studies of a YBa2Cu3O7−δ thin film bridge using electron probe microanalysis (EPMA), low-temperature scanning electron microscopy (LTSEM), and magneto-optical flux visualization have been carried out. Variations in chemical composition along the bridge were measured by EPMA with 3 μm resolution. Using LTSEM the spatial distributions of the critical temperature, Tc, and of the local transition width, ΔTc, were determined with 5 μm resolution. Distributions of magnetic flux over the bridge in an applied magnetic field have been measured at 15 and 50 K by the magneto-optical technique. The critical current density jc as a function of coordinate along the bridge was extracted from the measured distributions by a new specially developed method. Significant correlations between jc, Tc, ΔTc and cation composition have been revealed. It is shown that in low magnetic fields deviation from the stoichiometric composition leads to a decrease in both Tc and jc. The profile of jc follows the Tc profile on large length scales and has an additional fine structure on short scales. The profile of jc along the bridge normalized to its value at any point is almost independent of temperature.

Microwave power handling in engineered YBa2Cu3O7−δ grain boundaries

Microwave-frequency (rf) power-dependence measurements performed on thin-film YBa2Cu3O7−δ grain boundaries engineered on sapphire bicrystal substrates with misorientation angles of θ=2°, 5°, 10°, and 24° are presented. The data are compared to measurements on films grown on single-crystal substrates. A stripline-resonator measurement technique is employed. The rf results are compared to dc measurements performed on a four-point test structure on the same substrate as the resonator. The measurements demonstrate that low-angle grain boundaries (θ⩽10°) have little effect on the rf power handling, while the high-angle grain boundaries (θ=24°) cause large nonlinear losses due to Josephson vortices created by rf currents.

Role of the oxygen plasma during in situ growth of YBa 2Cu 3O 6+ x thin films by pulsed laser deposition

The incorporation of oxygen into YBa 2Cu 3O 6+ x (YBaCuO) thin films during in situ growth by pulsed laser deposition (PLD) has been investigated as a function of the ablation conditions. A quenching technique has been used immediately after termination of growth to avoid any oxygen in or out-diffusion during the cooling down step. It is shown that superconducting YBaCuO thin films can be formed without any post-oxygenation procedure, contrary to what is expected from the ( P O 2 , T) thermodynamic diagram. Moreover, it is found that there is an optimal target–substrate distance, D, for each O 2 deposition pressure, P O 2 , that leads to the higher critical temperature (i.e., higher oxygenation) as well as to the best structural and morphological properties of quenched films. The results are discussed considering the formation of reactive oxygen in the laser-induced plasma during film growth.

Microwave electric-field imaging using a high-Tc scanning superconducting quantum interference device

We have used a 77 K thin-film YBa2Cu3O7 superconducting quantum interference device (SQUID) in a scanning SQUID microscope to image room-temperature sources of high-frequency electric field. We find that time-varying electric fields capacitively induce currents in the SQUID, which in turn are rectified by the nonlinearity of the SQUID current–voltage characteristics, leading to changes in the quasistatic voltage across the SQUID. By observing changes in the voltage modulation depth ΔV of the SQUID as a sample is scanned past the SQUID, we obtain electric-field images in the 1–15 GHz frequency range with a SQUID-to-sample separation of about 80 μm.

Numerical modelling of the processes of exposure and development in electron beam lithography on high temperature superconductor thin films

In the present work a numerical modelling of the processes of exposure and development of the resist during electron beam lithography (EBL) on structures incorporating YBa2Cu3O7 high temperature superconductor (HTS) thin films is performed. Monte Carlo method is used to simulate the penetration of accelerated electrons in the target and to obtain radial distributions of absorbed electron energy density in the resist, during the irradiation of structures: 125nm polymethylmethacrylate (PMMA) resist layer (PMMA is the most widely used electron resist)/YBa2Cu3O7 film of thickness d=0, 100 or 300nm (YBa2Cu3O7 is the most promising HTS material for deposition of thin films)/SrTiO3 or MgO substrate (these are the most often used substrates for YBa2Cu3O7 thin films growth) at beam energies E0=25, 50 or 75keV. These distributions are approximated by a proper analytical function (combination of double Gaussian and exponential functions), called “proximity function”, the parameters of which are calculated using an original Monte Carlo technique and are used as input data in the modelling of the process of development of the electron resist.

Electric field effects at the surface of high-temperature superconductors

The results of experiments on the influence of electrostatic field (EF) on the electrophysical parameters of high-T C superconductors are presented. It was shown that action of the negative EF leads to an increase of critical temperature for about 6K in ceramic and about 0.6 K in thin film samples. The action of EF on the fluctuation excess conductivity in YBaCuO thin film was studied. It was shown that EF decreases the amplitude of thermodynamic fluctuations of the order parameter in the domain of 3D fluctuations. The negative EF applied to the film surface has been found to increase the activation energy of flux creep in relatively low magnetic fields. The upper critical magnetic field increases in the vicinity of T C under the influence of the EF.

Transport measurements of in-plane critical fields inYBa2Cu3O7−δto 300 T

Thin-film YBa2Cu3O7−δ has been studied with in-plane magnetic fields B to 850 T, with good signal-to-noise to 300 T, using a GHz-transmission-line technique for electrical transport measurements in explosive-driven microsecond pulsed magnetic fields. At an ambient temperature of 1.6 K with B⊥c axis we find an onset of dissipation at 150 T, the highest field reported, with a saturation of resistivity at 240 T. Comparison with the Pauli limit Bp=155 T expected for this material suggests that the critical field is limited by spin paramagnetism.Received 20 February 1998DOI:https://doi.org/10.1103/PhysRevB.57.R14084©1998 American Physical Society

Submillimeter wave detection with high temperature superconducting bolometers

We have fabricated and characterized a planar antenna-coupled microbolometer working at the resistive transition edge of a HT c superconducting film for use as a submillimeter wave detector. A log-periodic antenna was used to couple the incoming radiation to a YBaCuO thin film microbridge. The performance of the antenna and the bolometer response were obtained by using a 119 μm H 2 O laser source. Both the radiation pattern directivity and the detector responsivity were clearly improved by irradiating the antenna from the dielectric substrate side by means of a hyper-hemispherical silicon lens.

Influence of the deposition parameters on the structural and transport properties of YBaCuO thin films prepared by pulsed laser deposition

YBCO superconducting films were deposited by (XeCl=308 nm) pulsed laser deposition (PLD) on (100)-oriented SrTiO 3, MgO and Si single crystal substrates. Control of the YBCO thin film epitaxial growth and microstructure quality are important factors for superconducting electronic device applications. In this aim, we studied the influence of the oxygen pressure during the growth, the distance between target–substrate and substrate structure on the degree of structural orientation and surface quality of the films. X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques were used to make comparative studies of microstructure with transport properties of the films.

Oxygen content of YBa 2Cu 3O 6+ x thin films during growth by pulsed laser deposition

The oxidation ability of the laser plasma plume during in situ formation of YBa 2Cu 3O 6+ x (YBaCuO) thin films has been studied as a function of the deposition conditions. A quenching technique has been used immediately after termination of growth to avoid any oxygen in or out-diffusion during the cooling down step. It is shown that superconducting YBaCuO thin films can be formed without any post-oxygenation procedure, contrary to that expected from the ( P O 2 , T) thermodynamic diagram. This is due to the production of oxygen activated species in the plume, which significantly increases the oxygen potential and, therefore, the oxidation state of the films during deposition. Moreover, it is demonstrated that there exits an optimal position of the substrate respecting to the visible luminous plume for each O 2 pressure, which leads to the highest T c and to the best structural and morphological properties of quenched films. It is concluded that the presence of active oxygen, which is not homogeneously distributed over space, enhances the surface reaction kinetics and, thus, plays an important role in the mechanism of growth of laser ablated YBaCuO films.

X-Ray Diffraction Measurements on c-Axis Oriented YBaCuO Thin Films Deposited by Metalorganic Vapour Deposition

X-Ray Diffraction Measurements on c-Axis Oriented YBaCuO Thin Films Deposited by Metalorganic Vapour Deposition

Microwave microstrip low-pass filters using high- Tc superconducting YBaCuO thin films on LaAlO 3

We have characterized a 5-pole Tchebychev microstrip low-pass filter using YBaCuO thin film with a 5.3 GHz cutoff frequency. The YBaCuO thin film is grown on a LaAlO 3 substrate by means of magnetron sputtering. A matrix modeling of the filter coincides quite well with experimental measurement. Performances of the YBaCuO filter are compared with a gold filter made in the same conditions and with the same pattern. The average insertion loss and the rejection slope are appreciably better with the superconductor filter. Finally, the performances of the superconducting filter are studied at various temperatures and they remain approximately constant up to the transition temperature.

Penetration depth from far-infrared transmission in YBa2Cu3O7 thin films

We have deduced the penetration depth λ(T) on YBa2Cu3O7 thin films from transmission measurement in the range 100–550 GHz. Our data yield simultaneously the absolute value and the temperature dependence of λ(T). In lower-quality films, we report a usualT2 dependence, associated to a large value of λ(0) compared to the standard 1400A value reported in the literature. In better-quality films λ exhibits a linear temperature dependence associated to a value of λ(0) compatible with the standard one. This suggests that theT2 variation is of extrinsic origin. Thin films of sufficiently high quality exhibit the same linear behavior as single crystals (with a slope of about 4A/K). We also observe a dependence with the frequency of the penetration depth which is the signature of the rising inductive effect of the quasiparticles. This gives us direct access to the quasiparticle scattering time at low temperature, and stresses the potential interest of using far infrared synchrotron radiation to extend our frequency range.

Microstructure imaging of the YBCO thin film/MgO substrate interface: HRTEM and Fourier analysis of the Moiré fringe pattern

Detailed microstructural aspects of the interface between YBaCuO thin films and MgO substrate are studied by means of a Fourier analysis of Moiré fringe pattern obtained from HRTEM investigations of plan view samples. The main features of the observations are large, well oriented crystallographic domains surrounded by wide boundaries. HRTEM investigations together with the Fourier analysis show evidence of both orthorhombic and pseudo-tetragonal structure in the YBaCuO film. An accommodation mechanism is suggested from the Fourier analysis of the Moiré fringe pattern.

Tetragonal-orthorhombic phase transition in YBaCuO thin films observed by perturbed angular correlation spectroscopy

Oxygen-deficient, tetragonal thin films of YBa2Cu3O6+x with x ≈ 0.25, quenched from the deposition temperature, change to the oxygenated, orthorhombic phase with x ≈ 1, between 200 °C and 400 °C in flowing oxygen. The transition is not reversible in flowing oxygen, and cannot be completely reversed by cooling in flowing argon. We do not observe a transition of the orthorhombic films to the tetragonal phase up to 800 °C in flowing oxygen. We observe that the major impurity phases to appear under nonoptimal annealing conditions are oriented phases of YcuO2 and BaCu2O2, with Y2BaCu2O5 and Y2Cu2O5 conspicuously absent. These conclusions have been drawn from a study that uses perturbed angular correlation spectroscopy to probe the local microstructure of the films.

Evolution of surface structure, microstructure, and superconducting properties during YBa2Cu3O7a-axis thin film growth

The evolution of surface structure, grain microstructure, and superconducting transition temperature ofa-axis YBa2Cu3O7thin films has been studied as a function of film thickness. The grain size increases and develops a bimodal distribution of grain size during film growth, concurrent with an improvement inTc. The surface structure does not represent the grain size, but rather the development of the bimodal grain structure follows the formation of the surface structure. The results are discussed in terms of thin film growth modes.