YBa2Cu3O7-δ Thin Films Research Articles

Epitaxial Growth of Textured YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> Films on Silver

YBa2Cu3O7-δ (YBCO) films were deposited on (100), (110) and (111) oriented silver single crystals and {100}<100>, {110}<211> , {110}<100>+{110}<011>， {110}<011> and {012}<100> textured Ag substrates using pulsed laser deposition. The relationship between the epitaxial growth YBCO film and silver substrate has been determined. It is shown that among polycrystalline Ag substrates, {110}<011> textured tape is suitable for the deposition of YBCO thin films having strong texture.

Investigation of Terahertz Emission Phenomena of High-Tc Superconductors

The c-axis transmission spectra of underdoped Bi2Sr2CaCu2O8+δ (BSCCO) and YBa2Cu3O7-δ (YBCO) thin films were measured in the frequency range from 0.1 to 1.5 THz to investigate the mechanism of terahertz (THz)-pulse emission from high-TC superconductors (HTSs). The transmission spectra showed different properties depending on both the kind and hole-doping level of HTS thin film. A relatively strong shielding effect was observed for THz waves below the characteristic frequency f0, which almost corresponds to the cutoff frequency fc in the THz-wave radiation spectrum. The observed THz-pulse transmission and emission properties are explained by the intrinsic anisotropic characteristic peculiar to each HTS sample. Particularly, the magnitude of the superconducting order parameter that is induced in the insulating barrier layer between CuO2 double layers plays an important role in determining the cutoff frequency fc.

Fabrication and characterization of YBa2Cu3O7－δ/SrNb0.01Ti0.99O3 p-n junctions

A pn junction is fabricated by using the pulsed laser deposition (PLD) technique for a ptype YBa2Cu3O7-δ (YBCO) film on the ntype SrNb001Ti099O3(SNTO) single crystal substrate. The measured results reveal that the YBCO thin film is highly corientated structure and shows very good superconducting properties. The YBCO/SNTO pn junction has good rectification property and stability under variation of temperature and magneticfield.

Detwinning YBa2Cu3O7−δ thin films

Single crystals of YBa2Cu3O7-δ (YBCO) have been detwinned under the simultaneous application of heat and uniaxial stress. This approach, however, has proved futile in thin films as they become heavily twinned due to strain from the underlying substrate as well as the orthorhombic unit cell. This lack of control over twinning has made it difficult to determine the role that twins play in determining the transport properties of films. Here, we present a technique for detwinning YBCO thin films. The detwinning in YBCO thin film is achieved by suspending a portion of the YBCO film above the underlying substrate. The suspended material is no longer influenced by the confining effects of the substrate and this allows it to detwin under the application of a uniaxial stress brought about by an anneal. With this technique, we are able to produce high-quality detwinned regions that will help aid the understanding of transport mechanisms in YBCO.

Deposition and Evaluation of YBCO Thin Films by the Visible Light Pulsed Laser Ablation Method.

Thin film of YBa2Cu3O7-δ (YBCO) was deposited on a MgO substrate by the visible light (532nm) pulsed laser ablation method using a Nd:YAG laser. The purpose of this study is to find out the optimum composition of target and to improve the quality of thin films by introducing oxygen radicals. Thin film deposited in O2 gas at substrate temperature of 600°C and thermally annealed at 925°C in the air using a target with mol ratio of 1:2:4 of Y, Ba, and Cu (1:2:4 target) changed to the super conducting state at 69K, which was the highest among those of thin films deposited using targets with different compositions. Thin film deposited in O2 gas including oxygen radicals produced by the oxygen plasma using a 1:2:4 target changed to the superconducting state at 75K. Resistance decreased rapidly with decreasing temperature near superconducting transition temperature. Thin film deposited in O2 gas including oxygen radicals at 730°C using a 1:2:4 target without thermal annealing changed to the superconducting state at 50K.

Evaluation of YBa2Cu3O7-δ thin films ablated from melt-textured-growth YBa2Cu3O7-δ target by electron microscopy

YBa2Cu3O7-δ (YBCO) thin films ablated from melt-textured-growth (MTG) YBCO target (containing-Y211 phases) by pulsed laser deposition were studied by transmission electron microscopy. The films grow epitaxially on the SrTiO3 substrates in a c-axis orientation with very few large particles on the film surface: instead rectangular Y2O3 inclusions were embedded in the films. The crystallographic relationship was (100)Y2O3//(001)YBCO, [011]Y2O3//[010]YBCO. The inclusions do not degrade the superconducting property of the YBCO films. MTG YBCO targets result in films with lower level of large impurity particles.

Modification of a YBa2Cu3O7-δ ThinFilm Using an Atomic Force Microscope

A YBa2Cu3O7-δ thin film is modified by a probe electricfield of an atomic force microscope to form a ridge with the width of only a graincell. The modification varies with the operation parameters of the biasvoltage, the moving velocity of the probe and the ambient humidity. Energydispersive spectroscopy analysis shows only oxygen deficiency in the modifiedYBCO thin film. As a result, the suppressed superconductivity was found inthe junction crossing the ridge.

Direct injection of spin-polarized carriers across YBa2Cu3O7-δ-La0.3Ca0.7MnO3 interface at 77 K

We report here injection of spin-polarized carriers from a half-metallic La0.3Ca0.7MnO3 (LCMO) colossal magnetoresistive (CMR) thin film into a high-temperature superconducting YBa2Cu3O7-δ (YBCO) thin film studied using a micro-bridge. The LCMO and YBCO films were grown on 〈100〉 LaAlO3 (LAO) substrate sequentially using pulsed laser deposition (PLD). I-V measurements carried out at 77 K show that while normal critical current, Icn, of the micro-bridge is 80 mA, the critical current, Icp, through the micro-bridge when injected from the CMR layer is 38 mA. This clearly shows that spin-polarized quasiparticles injected from the the CMR layer into the YBCO layer suppress the critical current of the superconductor via the pair-breaking phenomena.

Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La0.7Ca0.3MnO3 (LCMO) and YBa2Cu3O7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO3 (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 µm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J c of ∼ 2 × 106 A/cm2, the LCMO side showed about half the value for the same thickness (1800 Å). The difference in J c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J c at two different thicknesses (1800 Å and 1500 Å) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 Å. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO3 deposited by PLD on YBCO did not show any evidence of self-injection.

Sensitivity of surface resistance measurement of HTS thin films by cavity resonator, dielectric resonator and microstrip line resonator

Microwave surface resistance (R s) of silver-doped YBa2Cu3O7-δ (YBCO) thin film, deposited by laser ablation technique on 10 mm × 10 mm LaAlO3 substrate, has been measured by resonant techniques in the frequency range from 5 GHz to 20 GHz. The geometrical factor of the sample and the resonator has been determined theoretically by the knowledge of the electromagnetic field distribution in the resonators. The microwave surface resistance of the superconducting sample is then extracted from the measured Q value as a function of temperature. The sensitivity of the R s measurement, that is, the relative change in the Q value with the change in the R s value is determined for each resonator.

Peak effect at microwave frequencies in swift heavy ion irradiated YBa2Cu3O7-δ thin films

The vortex dynamics at microwave frequencies in YBa2Cu3O7-δ (YBCO) films have been studied. We observe a peak in the microwave (4.88 and 9.55 GHz) surface resistance in some films in magnetic fields up to 0.8 T. This is associated with the ‘peak-effect’ phenomenon and reflects the order-disorder transformation of the flux line lattice near the transition temperature. Introduction of artificial pinning centers like columnar defects created as a result of irradiation with 200 MeV Ag ion (at a fluence of 4×1010 ions/cm2) leads to the suppression of the peak in films previously exhibiting ‘peak effect’.

Optical magnetic flux generation in superconductor

The generation of the magnetic flux quanta inside the superconductors is studied as a new effect to destroy superconductivity using femtosecond (fs) laser. The vortices are successfully generated in the YBa2Cu3O7-δ thin film striplines by the fs laser. It is revealed that the vortex distribution in the strip reflects the fs laser beam profile.

Ion-beam nanotexturing of buffer layers for near-single-crystal thin-film deposition: Application to YBa2Cu3O7-δ superconducting films

A method of producing biaxially textured template layers for near-single-crystal-quality film growth on substrates that do not provide a template for oriented crystalline growth is described and compared to existing methods. This technique, ion-beam nanotexturing (ITEX), produces a biaxially textured layer by oblique ion irradiation of an amorphous film surface. Using in situ reflection high-energy electron diffraction and ex situ x-ray diffraction, an yttria-stabilized zirconia (YSZ) template layer fabricated by ITEX is shown to have the appropriate surface texture for YBa2Cu3O7-δ coated conductor fabrication. A YBa2Cu3O7-δ thin film deposited on an ITEX YSZ layer has a critical current of 2.5×105 A/cm2 (77 K, 1 μV/cm). ITEX produces texture rapidly and should be ideally suited for future low-cost manufacturing.

Terahertz Radiation from Strongly Correlated Electron Systems and Related Phenomena.

We review the study on terahertz (THz) beam generation from strongly correlated electron systems; Pr0.7Ca0.3MnO3(PCMO), Tl2Ba2CaCu2O8+δ (Tl2212), and Bi2Sr2CaCu2O8+δ (Bi2212) thin films. Remarkable difference in thier emission properties and those from semiconductors or YBa2Cu3O7-δ (YBCO) is observed. The ultrafast carrier dynamics in the optically excited YBCO thin films is also discussed on the basis of its THz emission properties. Furhtermore, we report new results on the THz time domain spectroscopic studies of PCMO, La0.7Ca0.3MnO3, Bi2212, and SrBi2Ta2O9 thin films.

The temperature dependence of critical current in YBa2Cu3O7<FONT FACE=Symbol>-</FONT>d thin films deposited on MgO by an eclipse PLD

YBa2Cu3O7-d thin films were deposited on MgO single crystals by means of an eclipse pulsed laser deposition (PLD) method. A deposited lm exhibited the critical temperature of 74 K. The dependence of critical current on temperature was well explained by the Ginzburg-Landau theory. Although the theory is described only near critical temperature, experimental results are well fitted in a wide range between 25 K and 74 K. It stems from the strongly Josephson-coupling regime.

Double-sided YBa2Cu3O7-δ thin films made by off-axispulsed laser deposition

Double-sided YBa2Cu3O7-δ (YBCO) thin films were prepared on LaAlO3substrates 50 mm in diameter by an off-axis pulsed laser depositionmethod. The surface of the YBCO films appeared to have low a boulder densitywith a surface roughness of about 3 nm. Spiral growth was observed at bothsides of the thin films. X-ray diffraction data showed that the YBCO thin filmshadgrown epitaxially on LaAlO3 with c-axis orientation and a full width at halfmaximum for the (005) rocking curve of 0.15°. The transition temperature wasin the range of 86-88 K and the critical current density at 77 K was2.4-3.7×106 A cm-2 at various positions on both sides of the film. The surface resistanceof the YBCO films was estimated to be as low as ~1.0 mΩ at 19.57 GHz and 77 Kon both sides of the YBCO films, which corresponds to a value of ~250 µΩ at10 GHz and 77 K.

Post-deposition cooling in oxygen is critical for YBa2Cu3O7<FONT FACE=Symbol>-</FONT>d films deposited by eclipse pulsed laser deposition method

YBa2Cu3O7-d thin films were deposited on MgO single crystals by means of an eclipse pulsed laser deposition method. Deposited films are cooled down in situ under an oxygen atmosphere at a given oxygen pressure. The relationship between critical temperature and oxygen deficiency was investigated by means of electrical resistance R(T) and x-ray diffraction measurements. Post-deposition cooling is critical and the high pressure of oxygen during cooling is favorable.

Terahertz Radiation from Superconducting YBa2Cu3O7-δ Thin Films with 1.55 µm Excitation

We observed terahertz (THz) radiation from superconducting YBa2Cu3O7-δ (YBCO) thin films excited by femtosecond laser pulses with a center wavelength around 1.55 µm. The THz radiation mechanism is revealed to be a one photon excitation process corresponding to an intraband transition in the in-gap state. The property of the THz radiation excited by a 1.55 µm laser is similar to that excited by an 800 nm laser.

Origin of a-axis outgrowth in pulsed-laser-depositedYBa2Cu3O7-δ thin films from modified melt-textured growntargets

YBa2Cu3O7-δ (YBCO) thin films were deposited usingpulsed laser deposition (PLD) from modified melt-textured grown targets. Asthe laser energy density was increased, the surface of the films wascovered with enhanced a-axis outgrowths. In order to determine the origin ofthese formations, the microstructures of films deposited at 2 and4 J cm-2 were investigated using x-ray diffraction, transmission electronmicroscopy and high-resolution electron microscopy. It was found that asignificant number of Y2O3 inclusions were formed during the growth of c-axis-oriented films at 4 J cm-2. These inclusions formed nucleationsites for the a-axis outgrowths. The formation of Y2O3 inclusions wasattributed to the Y-rich targets. In addition, the non-equilibriumcharacteristics of PLD and the enhanced ablation of Y-rich phases attributedto the increase of laser energy density led to these Y2O3 inclusions. Asfor the a-axis outgrowth nucleation, it is considered that, due to theunstable growth conditions with a high flux density of incident vapour speciesand the strain induced by the surrounding c-axis films, the Y2O3inclusions would prefer the nucleation of a-axis grains.

An anomalous weak link model for microwave surface impedance ofYBa2Cu3O7-δ films in a weak dc magnetic field

By incorporating an anomalous weak link (AWL), the coupled-grainmodel is modified to describe the anomalous decrease of microwave surfaceimpedance (Zs) of YBa2Cu3O7-δ thin films in aweak dc magnetic field (Bdc). The critical current density of the AWL hasa minimum at zero Bdc, a linear dependence on Bdc for low Bdcand a saturation at high Bdc, unlike a conventional weak link. Using thismodel, the field dependences of surface resistance are calculated at differentrf input powers and resonant modes. The dimensionless parameters rH,defined as the ratio of the change of surface resistance and reactance, arealso computed. The simulation results agree with the measurementqualitatively. We believe that the anomalous Zs behaviour may originatefrom the AWL.