YBa2Cu3O7-δ Thin Films Research Articles

Transient Analysis of THz-QCL Pulses Using NbN and YBCO Superconducting Detectors

We report the time-domain analysis of fast pulses emitted by a quantum cascade laser (QCL) operating at ~ 3.1 THz using superconducting THz detectors made from either NbN or YBa2Cu3O7-δ (YBCO) thin films. The ultrafast response from these detectors allows resolution of emission features occurring on a nanosecond time-scale, which is not possible with commercially available Ge or InSb bolometers owing to their much larger time constants. We demonstrate that the time-dependent emission can be strongly affected by relatively small variations in the driving pulse. The QCL output power-current relationship was determined, based on correlation of the time-dependent emission of radiation with current flow in the QCL, under different QCL bias conditions. We show that this relationship differs from that obtained using bolometric detectors that respond only to the integrated pulse energy. The linearity of the detectors, and their agreement with measurements using a Ge bolometer, was also established by studying the QCL emission as a function of bias voltage and excitation pulse length. This measurement scheme could be readily applied to the study of ultrafast modulation and mode-locking of THz-QCLs.

Surface texture and interior residual stress variation induced by thickness of YBa2Cu3O7-δ thin films

YBa2Cu3O7-δ (YBCO) thin films with various thicknesses from 80 nm to 2000 nm are prepared on single crystal SrTiO3 by means of pulsed laser deposition technique. While it is hard for the x-ray diffraction to observe the evolutions in epitaxial orientation and interior lattice structure, the atomic force microscope shows the degraded surface morphologies and coalesced particles which arise from more misoriented grains with increasing thin film thickness. A detailed Raman spectrum investigation reveals that the a-axis grains exist predominately at the top surface of the films as the total thickness of the film increases up to 2000 nm. It is also evident that the Raman peak corresponding to the O2+/O3− mode emerges with the redshift first and then the blueshift as the film thickness increases. These Raman shifts suggest that the tensile stress in the (a, b) plane arising from the lattice mismatch between the epitaxial film and substrate may release gradually with increasing the film thickness, while the additional tensile stress emerges due to the a-axis grains present at the top surface of thin film. It is believed that the thickness effect of YBCO thin films on the superconducting transition temperature (Tc) and critical current density (Jc) is attributed to the variation of interior residual stress and surface morphology associated with the lattice mismatch and grain orientation, respectively.

Influence of Field Decrements on the Relaxation Behavior of Thin High- $T_{c}$ Superconductors Measured Using a Levitation Balance

Conventional relaxation measurements on various samples (a YBa2Cu3O7-δ thin film, a YBa2Cu3O7-δ-coated conductor, and a Bi2Sr2Ca2Cu3O10+δ tape) using a levitation balance are presented that are measured after a departure from the critical state by means of a small retraction Δd of the permanent magnet, which corresponds to a small field reduction ΔHα. In the case of YBa2Cu3O7-δ, flux creep rates S are found to decrease with increasing ΔHα until a small plateau is reached, where flux creep is effectively stopped. A further increase in ΔHα before the relaxation measurement leads to a change of sign of the creep rate. For the Bi2Sr2Ca2Cu3O10+δ tape, only a continuous decrease in S is obtained. An explanation of this effect is given based on calculations of flux density profiles and critical current densities in perpendicular geometry. The differences to the Bean model (i.e., longitudinal geometry) are discussed.

Systematic Modification of Electrical and Superconducting Properties of YBCO and Nano-Patterning of High-Tc Superconducting Thin Films by Light-Ion Irradiation

Abstract Irradiation of high-temperature superconducting (HTS) YBa2Cu3O7-d (YBCO) thin films with 75 keV He + ions leads to a quasi-exponential increase of the in-plane (ρab) and the out-of-plane (ρc) resistivity in the normal state and to a non-linear decrease of the critical temperature Tc with ion dose. In situ electrical measurements at room temperature reveal an irradiation-induced reduction of resistivity anisotropy ρc/ρab and a slight relaxation of film resistivity after the ion irradiation is stopped. Ex situ measurements show a stretched-exponential relaxation of Tc and normal state resistivity that continues for several weeks after the ion irradiation. Irradiation of YBCO thin films by low-energy He + ions through stencil masks results in local modification of the electrical and superconducting properties of the HTS material. We demonstrate that masked ion-beam lithography enables to produce structures smaller than 100 nm in size that have potential for applications in future superconducting nano-electronics.

Fabrication and Surface Morphology of YBCO Superconducting Thin films on STO Buffered Si Substrates

ABSTRACTPulsed Electron Deposition (PED) is an attractive alternative to Pulsed Laser Deposition (PLD) for growing high temperature superconductor thin films because of its relatively low cost. In this study, YBa2Cu3O7-δ(YBCO) thin film has been fabricated on silicon substrates by Pulsed Electron Deposition technique. SrTiO3 (STO) as a buffer layer has been grown between Si substrate and YBCO superconducting layer. The crystalline structures of STO/Si and YBCO/STO/Si films have been investigated by x-ray diffraction (XRD). The surface morphology and microstructure of YBCO/STO/Si thin film have been characterized with atomic force microscope (AFM) and scanning electron microscope (SEM). From the θ-2θ XRD analysis of YBCO thin films, (00l) diffraction peaks are obtained indicating they have a poor c-axis oriented structure. SEM analysis shows that the surfaces of films are crack-free, but they have some particulates. On AFM images, the droplets are clearly observed leading to a roughly surface.

Persistent critical current of YBa2Cu3O7-δ nanowires

C-axis oriented YBa2Cu3O7-δ (YBCO) nanowires have been fabricated so as to connect two c-axis oriented semi-ring-shaped YBCO thin films along the c-axis direction. This allows the characterization of high-Tc nanowires with the electrodes being of the same material. Four different lengths of the nanowire arrays were characterized. When the YBCO nanowires were narrower than 100 nm, their persistent critical current revealed an anomalous temperature dependence. The narrow YBCO nanowires behave like superconductor/semiconductor/superconductor junctions with a universal critical current density Jc(T)∝(Tc-T)3/2 at low temperatures. Above a certain temperature T*, the flow of the supercurrent is controlled by thermally activated phase-slip events, with Jc(T) following a power-law dependence Jc∝(Tc-T)α>3/2.

Micro-Raman spectroscopy of laser processed YBa2Cu3O7-δ thin films

The oxygen content and the local oxygen arrangement of laser processed YBa2Cu3O7-δ (YBCO) thin films grown on LaAlO3 substrates have been probed by micro-Raman spectroscopy On these films, channels for easy vortex motion have been laser written (LW) into the bridge constrictions by heat-induced, partial depletion of oxygen from the laser-illuminated areas of these films. Raman microscopy has been used to investigate and characterize the oxygen content and structure of the laser written channels. Direct evidence for local micro-structural changes in the film during the LW process is presented and the heat affected zone, surrounding the channel borders, has been mapped.

Correlation Between Flux Pinning Properties and Interfacial Defects in ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}/{\rm CeO}_{2}$ Multilayer Thin Films

YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> (YBCO) thin films doped with either secondary nanoparticles or nanolayers have demonstrated enhanced flux pinning properties in applied magnetic field. One possible reason for the enhanced flux pinning properties is the interfacial defects generated at the heterogeneous interfaces between YBCO and nanoparticles/nanolayers. In this work, we conducted a systematic study to correlate the pinning properties of YBCO thin films with interfacial defect density by introducing CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> multilayered structures. Multilayered YBCO thin films with 1-, 2-, and 4- CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interlayers and a pure YBCO reference thin film were prepared by pulsed laser deposition through alternating YBCO and CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> targets. A detailed microstructure and superconducting property analysis was conducted by X-ray diffraction (XRD), high resolution cross-sectional transmission electron microscopy (TEM), and physical properties measurement system (PPMS) with vibrating sample magnetometer (VSM). The result showed that introducing CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanolayers can effectively increase the interfacial defects without degrading the epitaxy quality of YBCO films. We found that an optimum density of interfacial defects in YBCO matrix is needed for the enhanced self-field and in-field performance.

Flux Pinning Properties in YBCO Thin Films With Self-Aligned Magnetic Nanoparticles

In this work, we conducted a systematic investigation on both self-field and in-field performance of YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> (YBCO) thin films with self-aligned (ordered) Fe <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> magnetic nanoparticles and compared with a pure YBCO reference sample. The doping architectures include YBCO films with ordered Fe <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> nanoparticles on the top, Fe <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> :CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanocomposite on the top, with self-aligned Fe <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> nanoparticles at the bottom and with multilayered nanoparticles in the matrix. All samples were prepared by pulse laser deposition (PLD). We explored the self-field and in-field performance ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Jcsf</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Jcin</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">field</i> (H// <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> )) of the YBCO films with the vibrating sample magnetometer (VSM) in a physical property measurement system (PPMS) at various temperatures and correlated with their microstructural characteristics. This work suggests that the self-aligned Fe <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> nanoparticles can form in YBCO matrix without severe deterioration of YBCO superconducting properties. Instead, they work as effective pinning centers at all temperatures measured and the multilayer sample and the sample with Fe <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> : CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanocomposite show the best pinning properties among all the samples examined.

Detection of Critical Current Distribution of YBCO-Coated Conductors Using Permanent Magnet Method

We developed a non-destructive and contactless system for measuring the critical current (Ic) in YBa2Cu3O7-δ (YBCO)-coated conductors by using a permanent magnet (Sm2Co17). This Ic measurement method is based on the repulsive force (Fr) between the magnet and the shielding current in high-temperature superconductor-coated conductors. We measured Fr using a high-resolution load sensor and found that accurate Fr could be determined without the effect of thick copper film on the YBCO thin film and Hastelloy tape of the substrate. We can determine Ic from Fr0, which is the maximum repulsive force determined from an extrapolated value of the Fr vs. L curve for L = 0 mm, described in our previous paper. We show that the permanent magnet method can be used to determine the longitudinal Ic distribution and large-scale defects in YBCO-coated conductors. In addition, it turned out from this experiment that the permanent magnet method was effective to rapidly measure the longitudinal Ic distribution of long-scale coated conductors.

Electron Phase Microscopy of Magnetic Fields in Ferromagnets and Superconductors

ABSTRACTHighly sensitive electron phase microscopy based on the Aharonov-Bohm (AB) effect principle has been used to observe microscopic distributions of magnetic fields in ferromagnets and superconductors. The observation examples include the unconventional behaviors of interlayer Josephson vortices in anisotropic layered high-Tc superconducting YBa2Cu3O7-δ (YBCO) thin films, which are produced when the applied magnetic field is greatly tilted to the layer plane, and the magnetic-field distributions of tiny magnetic heads for perpendicular recording and of colossal magnetoresistance (CMR) materials.

Addition of ferromagnetic CoFe2O4 to YBCO thin films for enhanced flux pinning

An attempt has been made to prepare a YBa2Cu3O7-δ (YBCO) thin film doped with ferromagnetic CoFe2O4. Transmission electron microscopy of the resultant samples shows, however, that Y(Fe,Co)O3 forms as a nanoparticulate dispersion throughout the film in preference to CoFe2O4, leaving the YBCO yttrium deficient. As a consequence, the superconducting properties of the sample are poor, with a self-field critical current density of just 0.25MAcm−2. Magnetic measurements indicate however that the Y(Fe,Co)O3 content, together with any other residual phases, is also ferromagnetic, and some interesting features are present in the in-field critical current behaviour, including a reduced dependence on applied field and a strong c-axis peak in the angular dependence. The work points the way towards future attempts utilising YFeO3 as an effective ferromagnetic pinning additive for YBCO.

Heating and high frequency nonlinearities in the surface impedance of high Tc superconductors

Using the dielectric resonator method, we have investigated nonlinearities in the surface impedance Zs = Rs + jXs of YBa2Cu3O7-δ thin films at 10 GHz as a function of the incident microwave power level and temperature. The use of a rutile dielectric resonator allows us to measure the precise temperature of the films. We conclusively show that the usually observed increase of the surface resistance of YBa2Cu3O7-δ thin film as function of microwave power is due to local heating.

Thermal effects in the flux-creep regime of YBa2Cu3O7-δ thin film microbridges under high current densities in self field

currents well above the critical current density Jc but below the quenching current density J* (the current inducing the full transition to the normal state), the electric field vs current density (E-J) curves may be deeply affected by self-heating effects. Until now the influence of these thermal effects on the E-J curves was mainly studied around J*. Here we will first present measurements of the E-J curves in several YBa2Cu3O7-δ thin film microbridges in the whole range between Jc and J*. Our results confirm that whereas Jc is almost independent on the microbridge width (w), J* and E* strongly increase when the width decreases. It is also shown that the width dependence on the E-J curves manifests itself already for current densities well below J*(w). Then by using a finite-element method (FEM), it is shown that the measured E-J curves may be explained, from the dissipation onset (Jc) up to the jump itself (J*), in terms of a conventional (smooth) flux-creep plus self-heating effects. An important aspect of our results is that the same flux-creep behaviour in the low dissipation regime allows us to explain the whole E-J curves in film microbridges with different widths.

Strain effect on microwave surface resistance of YBa2Cu3O7-δ/LaAlO3 and Tl2Ba2CaCu2O8/LaAlO3 high temperature superconducting thin films

YBa2Cu3O7-δ (YBCO) thin films were prepared by thermal coevaporation on (LaAlO3) LAO substrates,and Tl2Ba2CaCu2O8 (TBCCO) thin films were synthesized by magnetron sputtering method on LAO substrates. We obtained the strain from the results of XRD of the films,which had the values of ΔCY=48483×10-3 and ΔCT=85272×10-5. We found that the strain in YBCO was larger than that in TBCCO. In addition, the curve of surface resistance of the films as a funttion of temperature was obtained by coplanar resonator technique, which showed the resistance of YBCO was larger than that of TBCCO at the same reduced temperature. In the paper, we also analyzed the strain effect on the surface resistance of the films.

In-situ XRD study on the peritectic reaction of YBCO thin film on MgO substrate

Abstract In order to study the superheating phenomenon during the melting process of YBa2Cu3O7-δ(YBCO) thin films, in-situ XRD measurements were explored around the peritectic temperature (Tp) of YBCO with a high temperature sample stage. As the temperature heating up from 960 to 1080 °C, several phase transformations take place in the YBCO thin-film on an MgO substrate. XRD analysis shows that Ba2Cu3O5.9 phase was formed at the temperature 1050 °C, while YBCO remained in the film even at this temperature evidently higher than its Tp (1010 °C). In addition, Ba2Cu3O5.9 has a certain oriented relationship with its parent phase YBCO.

Atomic Force Microscope Based Lithography of YBa2Cu3O7-δ Thin Films

We demonstrate the patterning of controlled, arbitrary structures into the surface of thin film YBa2Cu3O7-δ samples by application of a bias voltage between the sample and the tip of an atomic force microscope operated in contact mode, and programmed to trace out the desired structure. The modified volume, as observed above the film surface, is shown to increase linearly with the applied bias voltage, beyond some threshold value, suggesting an electrochemical anodisation process, while increasing the scan speed results in a decrease in the modified volume, but an apparent improvement in homogeneity. Energy dispersive X-ray measurements support the scenario of oxidation as the means of modification, while measurements of current flow into the modified region indicate that it is insulating, offering potential application of the technique to the fabrication of superconducting device elements.

Influence of the microbridge width on the transition to the normal state induced by high current densities ramps in YBa2Cu3O7-δ thin films

We report current voltage curves (CVC) on c-axis oriented YBa2Cu3O7-δ superconducting thin films patterned in several microbridges of different width and the same width/length ratio (1/10). The microbridge widths range from 5 µm to 100 µm. Our present measurements extend previous experiments performed in our research group to microbridges with different geometries. Measurements of CVC spanned the whole range: from the critical current Jc (when the dissipation starts) to the supercritical current J* (when an abrupt jump to a highly dissipative state occurs). We have found that Jc values do not change for all the microbridges of the same film whereas J* decreases as the microbridge width increases. These findings indicate that near Jc the current is almost uniformly distributed over all the microbridge volume. Moreover, although our results do not exclude an intrinsic origin for the transition to the normal state induced by high current densities, they suggest that J* could be deeply affected by the thermal interchange between the films and their substrates.

AFM study of SrTiO3 and YBa2Cu3O7-δ multilayer surface treated with chemical mechanical polishing process

We have studied the surface morphology of SrTiO3 (STO) and YBa2Cu3O7-δ (YBCO) thin films fabricated by pulsed laser deposition technique in a multilayer structure. Study was focused on the surface morphology of an STO thin film deposited on an YBCO layer that received chemical and mechanical polishing (CMP) treatment. The CMP process could remove the presence of the outgrowth on the STO thin film, improve its smoothness and facilitate a good epitaxial growth of the next deposited YBCO film. As a result, we could fabricate a high quality YBCO thin film on the YBCO/STO structure with Tc = 91 K and Jc = 4 × 106 A/cm2.

LPE growth of PZNT film using superheating YBCO thin film as seed layer

A superheating YBa2Cu3O7-δ (YBCO or Y123) thin film was applied as a seed layer to grow Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 (PZNT) films by a liquid phase epitaxy (LPE) process. In the present work, the YBCO thin film underwent a growth temperature of 1050 °C, which was about 40 °C higher than its peritectic temperature. It is very interesting that the superheated YBCO seed film could be used to grow not only Nd1+xBa2-xCu3O7-δ (NBCO) films with similar compositions but also PZNT films with completely different compositions. The XRD analysis confirmed that the PZNT film grown on the YBCO seeded MgO substrate had a good epitaxial relationship of [100](001)PZNT//[100](001)YBCO//[100] (001)MgO. Compared with the PZNT films directly grown on MgO substrates, the LPE PZNT film on YBCO/MgO presented a better surface morphology. It was found that the superheating YBCO seed film plays a crucial role for the LPE growth of PZNT in the process. Furthermore, the superheating mechanism was discussed in terms of thermodynamic theories as well.