YBa2Cu3O7 Films Research Articles

Flat coil-based tunnel diode oscillator enabling to detect the real shape of the superconductive transition curve and capable of imaging the properties of HTSC films with high spatial resolution

Owing to a pick-up coil's flat design, relatively low MHz-range operation frequency, and six orders relative resolution a flat coil-based tunnel diode oscillator has advantages, compared to all other methods. They become crucial in studies with thin high-Tc superconductivity (HTSC) materials (with small signals), especially at the start of the Cooper pairs’ formation. Due to this the superconductivity precursor ‘paramagnetic’ effect was detected recently in YBaCuO films at N/S transition. It precedes Meissner ejection and specifies details of the shape of the transition curve. We discuss the influence of the currents on this effect, and the relationship between the quality of the material and the shape of the effect. A new imaging device has also been created based on this test method (using a focused He–Ne laser beam as a probing signal), capable of imaging the properties of HTSC films with ∼3μm spatial resolution. The method is based on detection of the inductance and Q-factor value changes of a single-layer flat coil, placed at the face of the sample. This leads to frequency and/or amplitude changes of the stable oscillator. The test device enabled 2D-mapping of the grain structure of a bridge-shaped YBaCuO film. Basically, the method is capable of imaging 2D-current distribution in thin HTS with sub-μm spatial-resolution, using non-bolometric response. However, the achieved resolution ∼3μm of a bolometric nature (in a given device with 3.5mm-size coil) by no means is limited by the abilities of the method, but mainly, it depends on how narrowly it is possible to focus the probing beam, while the own resolution of a present flat coil-based technique is better than 0.1μm, and can be improved essentially by reducing the coil size.

Thickness dependence of ac losses in circular disks of YBa2Cu3O7 films in perpendicular magnetic fields

The ac losses in three disk-shaped YBa2Cu3O7 (YBCO) films, which were deposited on SrTiO3 by the pulsed-laser-deposition technique and had thicknesses, d, of 0.2, 1.0, and 3.0 μm, were measured in perpendicular applied ac magnetic fields to ∼0.14 T at 10 Hz in liquid nitrogen. The losses at low fields were found to be a strong function of the film thickness. The measured losses were compared with the theoretically calculated losses. The ac losses calculated using a field-independent critical-current density, the Bean model [J. R. Clem and A. Sanchez, Phys. Rev. B 50, 9355 (1994)], agreed very well with the 0.2-μm-thick film, while the calculated losses agreed well with the measured ones when a field-dependent critical-current density, the Kim model [D. V. Shantsev, Y. M. Galperin, and T. H. Johansen, Phys. Rev. B 61, 9699 (2000)], was used for the films of thickness 1.0 and 3.0 μm. However, a surprising discrepancy was found in the values of Bc and B0 for thinner YBCO films depending on whether they were determined by ac or dc measurements. Bc is defined as Bc=μ0Jc(0)d/2, B0 is the characteristic field in the Kim-model critical-current density JcK(Ba)=Jc(0)/(1+Ba/B0), and Jc(0) is the critical-current density at applied magnetic field Ba=0.

Mid-infrared a– b plane response of YBa 2Cu 3O 7− δ as a function of doping and temperature determined by attenuated total reflection

The mid-infrared optical response of c-axis thin films of YBa 2Cu 3O 7− δ has been studied using Otto-configuration attenuated total reflectance. The measured reflectance––angle characteristics are dominated by a strong absorption feature due to the excitation of surface plasmons, and can be modeled to determine the a– b plane dielectric function. The results show that while ε i, and therefore σ r, are temperature independent, | ε r| exhibits a moderate decrease with increasing temperature. A stronger dependence on doping is observed: the magnitudes of ε i and | ε r| are both strongly suppressed as δ increases. Critical comparison is made with the extensive body of infrared normal reflectance data. A generalized Drude analysis shows that the plasma frequency is independent of temperature, but decreases with decreasing doping. The scattering rate increases with temperature, and also increases with decreasing doping, consistent with stronger coupling in the underdoped regime. The mass-enhancement is small but increases to 30–40% at δ=0.6. Difficulties in reconciling the results with some current theories of high- T c materials are discussed. Finally, the surface plasmon propagation lengths and penetration depths are shown to vary systematically with doping.

Nanograins with 90° grain boundaries in high transition temperature superconducting films

A theoretical model is suggested which describes a non-conventional relaxation mechanism instrained high transition temperature superconducting films, namely the formation of nanograins with90° grain boundaries. It is theoretically revealed here that misfit stresses insuperconducting thin films at early stages of their growth are effectively relaxedthrough the formation of nanograins with their crystal lattice misoriented by90° relative to the crystal lattice of the film matrix. With increasing film thickness, the formation of acontinuous layer resulting from the convergence of nanograins becomes energetically favourable.The results of the model account for experimental data on the observation of nanograins with90° grain boundaries in YBaCuO films, reported in the literature.

Third-order intermodulation measurements of 2 GHz microstrip bandpass filters based on high-temperature superconductors

To elucidate the key parameters for improving the power-handling capability of rf filters of high-temperature superconductors with microstrip structures, we synthesize bandpass filters with different layouts using several kinds of thin film high-temperature superconductors, and perform third-order intermodulation measurements on them. The third-order intercept (IP3) at 70 K is found to increase from +53 dBm to +65 dBm as the film thickness increases from 0.62 µm to 0.80 µm for the filters of YBa2Cu3O7 (YBCO). We find that filters made from films of NdBa2Cu3O7 (NBCO) grown by molecular-beam epitaxy yield higher IP3 values than those made from commercially obtained YBCO films. These experimental results indicate that increasing the film thickness and utilizing MBE-grown NBCO films are effective in obtaining high-power-handling microstrip filters.

High-Tc dc-SQUID gradiometers in flip-chip configuration

We describe a new design of a gradiometric flip-chip antenna, which is inductively coupled to a dc-SQUID gradiometer. Both components are patterned out of thin films of the high-Tc superconductor YBa2Cu3O7−x (YBCO). For the flip-chip antenna, a 40 mm × 10 mm SrTiO3 single crystalline substrate is used, while the gradiometer sensors are prepared on 10 mm × 10 mm SrTiO3 bicrystal substrates. Special attention is paid to the inductive coupling between the flip-chip antenna and the read-out gradiometer antenna. We investigate different designs of coupling loops in order to optimize the coupling inductance between both components of the sensor. With optimized coupling the sensor achieves a field-gradient resolution of 12 fT cm−1 Hz−1/2 in the white noise region and of 310 fT cm−1 Hz−1/2 at 1 Hz in the unshielded laboratory environment.

Features of the angular dependence of the critical current in thin epitaxial films of HTSC YBa2Cu3O7−δ in a magnetic field

The dependence of the critical current density jc on the magnitude and direction of the magnetic field H is investigated in thin epitaxial films of YBa2Cu3O7−δ having a high value of jc in the absence of field (∼106 A/cm2 at 77 K) and a thickness d less than twice the magnetic field penetration depth λ. It is found that the jc(H) curves have a low-field plateau both for fields perpendicular and parallel to the film. In a magnetic field perpendicular to the film, the “effective pinning” plateau extends to a field corresponding to a density of Abrikosov vortices threading the film at which it is no longer favorable for them all to be pinned at edge dislocations in the interblock walls, and a fraction of them become unpinned. In contrast, in a field parallel to the film the end of the plateau is unrelated to depinning of threading vortices parallel to the film; instead, jc(H) decreases after the plateau region because the magnetic field parallel to the film weakens the pinning of vortices perpendicular to the film. The low-field plateau on jc(H) for an in-plane is wider than that for the case of normal magnetization. Therefore, the dependence of jc on the angle between H and the normal to the film has a single maximum at which the field is lying in the film plane. In films obtained by laser or electron-beam evaporation of YBa2Cu3O7−δ or its constituents, the measurements of jc on which were made by an inductive method, the decrease of jc with increasing field in the film plane begins at fields lower than the first critical field for penetration of the vortices into the film plane. For the magnetron-deposited film, where jc was measured by a transport method, this decrease of jc begins in a field much higher than that critical field. The possible relationships between the parameters of the jc(H) curves and the first critical field for penetration of the vortices into the film plane are discussed, including some which lead to different angular dependences of the critical current.

YBaCuO/PrBaCuO/YBaCuO trilayer junctions with the polished YBaCuO base electrodes

Superconducting wiring layer is indispensable to actualize digital circuits constructed with trilayer junctions. However, it is complicated to fabricate the junctions with the superconducting wiring layer. Thus, we have formed Josephson junctions on mechanical polished surface of YBaCuO films for introducing the simple device structure, such as a Pb junction, to the YBaCuO junction. The surface of YBaCuO film sputtered on vicinal SrTiO 3(0 0 1) substrate was polished by 12 nm depth. Peak–valley roughness of the surface decreased from 8 to 2 nm. After that YBaCuO film with 2 nm thickness for re-growth, PrBaCuO film with 13 nm thickness for barrier layer, and YBaCuO film for counter electrodes were consecutively deposited on the polished surface of the YBaCuO base electrodes. The junctions with the size of 10 μm × 10 μm showed resistively shunted junction like I– V curves with no excess current. The spreads of I c were 12% for 16 junctions on a chip. Applying mechanical polishing may improve the uniformity of the barrier thickness.

Ac losses in circular disks of thin YBa2Cu3O7 films in perpendicular magnetic fields

The ac losses at 20 and 30 Hz were measured for two disk-shaped YBa2Cu3O7 films in perpendicular peak applied magnetic fields up to ∼0.2 T in liquid nitrogen. One of the films had a significantly higher critical-current density than the other as determined from the loss measurements. Also, it exhibited a more-uniform flux penetration around the circumference of the disk than the other as observed by magneto-optical images of these films in perpendicular dc fields. The results from this film were compared with theoretical predictions of ac losses for disks of thin superconducting films in perpendicular magnetic fields using the Bean model [J. R. Clem and A. Sanchez, Phys. Rev. B 55, 9355 (1994)], and the Kim critical-current model [D. V. Shantsev et al., Phys. Rev. B 61, 9699 (2000)]. The asymptotic Bean model predictions for low and high fields were in reasonably good agreement with the data. The numerical calculation of the losses following Shantsev et al. was found to give extremely good agreement with the loss data throughout the entire field range of the measurement when the Kim model for the critical-current density was used.

Complicated shape of the superconductive transition curve revealed by a sensitive OFC-magnetometer

Because of pick-up coil's flat design, relatively low operation frequency (/spl sim/ 23 MHz) and high relative resolution (10/sup -6/) a single-layer Open-Flat Coil-based tunnel diode oscillator (OFC-magnetometer) has advantages, compared to other test methods. They become crucial at nondestructive studies with thin flat HTS materials (with small signals), especially near T/sub c/ (at start of the 'Cooper' pairs' formation). Due to these a new 'Para-Magnetic' peculiarity of the normal-to-superconductive (N/S) transition was detected recently in YBaCuO films. It precedes the 'Meissner' ejection and specifies details of an actually complicated shape of the N/S transition curve. We discuss influence of the currents and magnetic field on the PM effect and relationship between the quality of HTS material and amplitude of the effect. Differences between PM effect and externally close 'paramagnetic Meissner' and resistive 'peak' effects are discussed too.

Fabrication of all YBaCuO trilayer josephson junctions with YBaCuO wiring layer

We report a fabrication process of c-axis oriented all YBaCuO trilayer Josephson junctions with YBaCuO wiring layers. The trilayer junctions consisted of YBaCuO (170nm)/PrBaCuO (28 nm)/YBaCuO (170 nm) structures. CeO/sub 2/ films (520nm) and YBaCuO films (1.2 /spl mu/m) were used as insulating layers and wiring layers, respectively. A junction with a dimension of 7 /spl mu/m/spl times/7 /spl mu/m showed a critical current I/sub c/ of 1.6 mA and a junction resistance R/sub n/ of 1.2 /spl Omega/ at 4.2 K. The values of the critical current density and the I/sub c/R/sub n/ product were estimated to be 3.3kA/cm/sup 2/ and 1.9 mV, respectively. Note that the junction exhibited I/sub c/R/sub n/ product of 1.0 mV even at 30 K. From these results, the junction is promising candidate for satisfying the requirements for HTS integrated circuits.

Intermodulation distortion and third-harmonic generation in YBCO films of varying oxygen content

We have measured the nonlinear surface impedance, intermodulation distortion (IMD), and third-harmonic generation (THG) in a series of identically prepared YBaCuO films that have been carefully annealed to produce a controlled oxygen stoichiometry. These are also compared with an unannealed film. The measurements were performed using a stripline-resonator technique as a function of temperature at a fundamental frequency of 2.3 GHz; the IMD tone separation was 10 kHz. We have found that overdoping films with oxygen substantially lowers the IMD relative to optimally and underdoped films. We have also observed differences in the slopes of the THG and IMD, with IMD slope of close to 2:1 while the same film shows THG slope of 3:1. A possible explanation of the differences lies in the different time scales to which IMD and THG are sensitive.

Quench behavior of YBaCuO films for fault current limiters under magnetic field

The resistive superconducting fault current limiters (SFCLs) are very attractive devices for the electric power network. But they have some serious problems when the YBCO thin films were used for the current limiting materials due to the inhomogeneities caused by manufacturing process. When the YBCO films have some inhomogeneities, simultaneous quenches are difficult to achieve when the fault current limiting units are connected in series for increasing operating voltage ratings. In order to solve these problems, vertical magnetic fields varied from 0 to 130 mT were applied to the YBCO elements. Then, extensive electric field-current (E-I) and quench characteristics were investigated for all elements by using both electrical measuring method and observations of bubble behaviors. The experimental results were compared with the quench properties of YBCO elements, which were connected in series. From the experiment works, it was revealed that applied magnetic fields generated by surrounding coils could induce uniform quench distribution for all strips and simultaneous quenches were realized in all YBCO elements. Finally, by applying vertical magnetic fields perpendicular to the limiting devices, 1.2 kV/sub rms/ rated resistive fault current limiter were realized using five YBCO films in series.

Mechanisms of limitation and nature of field dependence of critical current in HTS epitaxial YBaCUO films

Magnetic field and temperature dependencies of the critical current density, J/sub c/(H/spl par/c, T) were measured by SQUID-magnetometry, ac magnetic susceptibility, and dc transport current techniques in the single-crystalline epitaxially-grown by off-axis dc magnetron sputtering YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films with J/sub c/(H/spl par/c, 77 K) /spl ges/ 2 /spl middot/ 10/sup 6/ A/cm/sup 2/. The mechanism of vortex depinning from growth-induced linear defects, i.e., out-of-plane edge dislocations in low-angle tilt domain boundaries, is shown to describe quantitatively measured J/sub c/(H/spl par/c, T). The developed model takes into account a statistical distribution of the dislocation domain boundaries ordered in a network as well as the interdislocation spacing within boundaries. Actual structural features of YBCO film known from HREM data turn out to be extracted from J/sub c/(H/spl par/c, T)-curves by a fitting procedure within the proposed model.

Force measurements of a superconducting-film actuator for a cryogenic interferometric gravitational-wave detector

We measured forces applied by an actuator with a YBa2Cu3O7 (YBCO) film at near 77 K for the Large-scale Cryogenic Gravitational-wave Telescope (LCGT) project. An actuator consisting of both a YBCO film of 1.6 μm thickness and 0.81 cm2 area and a solenoid coil exerted a force of up to 0.2 mN on a test mass. The presented actuator system can be used to displace the mirror of LCGT for fringe lock of the interferometer.

Nondestructive determination of current–voltage characteristics of superconducting films by inductive critical current density measurements as a function of frequency

A nondestructive inductive method was developed to determine the current–voltage characteristics of superconducting films by measuring the critical current density Jc as a function of the frequency of ac magnetic fields. In well-known inductive methods that measure the Jc distribution of large-area films, Jc is measured at a frequency and the electric field E induced in the superconductor is proportional to the frequency; therefore, the E versus current density relation can be determined by measuring Jc at various frequencies. Current–voltage characteristics of YBa2Cu3O7 films measured by this method agree well with those determined by measuring magnetic-field-sweep rate dependence of magnetization.

Study of the oxygen incorporation and structure in the low pressure sputtered YBCO films

Epitaxial thin films of YBa 2Cu 3O 7– δ (YBCO) have successfully been grown by reactive RF-magnetron sputtering technique at a low pressure (0.95 mTorr) with T C ( R=0)=85.3 K and J C mag (4.2 K)≈2×10 7 Å/cm 2. The incorporation of oxygen in the as-grown films has been controlled by using different ambient—oxygen, air and argon—during in situ cooling. The superconducting behavior of the resulting films was studied using resistance–temperature and low field a.c.-susceptibility measurements and correlated with their structure. All the films exhibited metallic conduction in the normal state. The oxygen- and air-cooled films were superconducting, and exhibited usual orthorhombic structure, but the argon-cooled films were non-superconducting and possessed tetragonal structure. This implied that the structure of the film during deposition is tetragonal, and transforms to either of the oxygen rich orthorhombic-I or -II phases depending upon the oxygen/air ambient. The ‘ δ’ values of 0.14, 0.32 and 0.70 and higher ‘ c’-parameters of 1.1785, 1.180 and 1.183 nm have been obtained for oxygen-, air- and argon-cooled films, respectively.

Orientation and direct current field dependent dielectric properties of bulk single crystal SrTiO3 at microwave frequencies

The dielectric properties of bulk single crystal SrTiO3 (STO) are studied in the temperature range 30–300 K, under dc field 0–10 kV/cm at 1.0 kHz and in the frequency range 0.3–2.0 GHz. Two sets of circular disk shaped parallel plate resonators have been fabricated and tested experimentally. In the first set thin YBa2Cu3O7 films have been epitaxially grown on both surfaces of [100] STO disks. In the second set, three crystal orientations of SrTiO3 [100], [110], and [111] have been used to make parallel-plate resonators with evaporated Cu/Ti electrodes. No peculiarities in field dependent dielectric permittivity are detected for STO with smaller densities of impurities. For samples with larger impurity densities double loop hysteresis is observed in the dc field dependent permittivity. For all orientations the losses are minimum at temperatures about 50–55 K. The losses at microwave frequencies increase with the applied dc field regardless of the orientation of the STO crystal. At relatively high dc fields (depending on the temperature) the losses start decreasing slightly.

Anomalous behavior of gold nanoislands on top of SrTiO 3(0 0 1) during their overgrowth by thin YBaCuO films

Gold nanoparticles are deposited on top of atomically flat SrTiO 3(0 0 1) substrates either by exploiting the self-organization of Au-loaded inverse diblock-copolymer micelles or by dewettting thin (3 nm) Au films at 700 °C. The resulting nanoparticles were overgrown by YBaCuO films prepared by off-axis cylindrical magnetron sputtering at 730 °C, the standard conditions to grow c-axis-oriented epitaxial films. In the present case, no influence of the Au particles on the structural (orientation, lattice parameter, rocking angle) or superconducting properties could be observed. Applying high resolution transmission electron microscopy, it turned out that the Au nanoparticles under the above conditions were “lifted” by the YBaCuO film, i.e. one obtains the surprising result that the deposition of a YBaCuO film on top of Au nanoparticles on SrTiO 3(0 0 1) leads to Au nanoparticles on top of the YBaCuO film immediately explaining why the overall structural and superconducting properties remain unaffected.

Surface resistance measurements of surface and interface sides of YBa2Cu3O7 films on sapphire and LaAlO3

We have measured the surface resistance of YBCO superconducting thin films deposited on sapphire and lanthanum aluminate substrates at the film side and at the interface between the substrate and the film. The measured difference in the Rs values of the interface and the film sides was between 30% and 60% for the YBCO films on LAO in the temperature range 15–80 K. For the Ag-doped films on sapphire the difference in Rs monotonically varied from 10% to 65% when temperature increased from 30 K to 80 K.