X. D. Wu

Observation of two distinct components during pulsed laser deposition of high Tc superconducting films

Using Rutherford backscattering technique, we have measured the angular distribution of the composition and thickness of the Y‐Ba‐Cu oxide film deposited by firing excimer laser (30 ns, 248 nm) pulses at a stoichiometric Y1Ba2Cu3O7−x pellet. The angular distribution consisted of two distinct components: one a cosu2009θ component, a result of evaporation, and the other a highly forward directed component, a result of a secondary ejection process. The evaporated component is nonstoichiometric, as one would expect, whereas the forward‐directed component has a composition close to that of the pellet. Further, the forward‐directed stoichiometric component increases with the laser energy density in comparison with the evaporated component. These observations are discussed in the context of current models of laser‐induced material ejection at surfaces.The laser energy dependence of the deposition is of critical importance in controlling the film stoichiometry.

Fabrication of heteroepitaxial YBa2Cu3O7−x‐PrBa2Cu3O7−x‐YBa2Cu3O7−x Josephson devices grown by laser deposition

We have fabricated Josephson weak links from i n s i t u laser‐deposited four‐layer structures of YBa2Cu3O7−x /PrBa2Cu3O7−x /YBa2Cu3O7−x Au. These devices show superconductor/normal‐metal/superconductor‐like current‐voltage characteristics with good areal scaling of both the critical currentsI c and resistancesR j , with I c R j ≊3.5 mV. Constant‐voltage current steps observed in response to 84 GHz mm‐wave radiation and modulation of the dc supercurrent in a transverse magnetic field demonstrate that both the ac and dc Josephson effects occur in these devices.

Application of a near coincidence site lattice theory to the orientations of YBa2Cu3O7-x grains on (001) MgO substrates

Various orientations of YBa2Cu3O7−x grains in polycrystalline films prepared on (001)MgO substrates by in situ laser deposition were determined using electron diffraction. Eight different types of in‐plane orientations have been observed. These orientations agree well with the prediction of a simplified theory of near coincidence site lattice between YBa2Cu3O7−x and MgO. The YBa2Cu3O7−x grains were found to have a high probability of forming low angle or low Σ boundaries among themselves. These grain boundaries are of low energy and should exhibit a high connectivity of Cu‐O‐Cu chains. Therefore, YBa2Cu3O7−x thin films on MgO can attain a Tc of ∼90 K and a Jc of ∼106 A/cm2 at 77 K.

Nature of the pulsed laser process for the deposition of high Tc superconducting thin films

The pulsed laser thin‐film deposition process can enable preparation of thin films of complex composition with good control over the film stoichiometry. The film compositions are similar to that of the target pellet and as a consequence this technique appears to be an ideal method for preparing high Tc thin films on a variety of substrates.The factors which contribute to this beneficial phenomenon have been explored by a laser ionization mass spectrometry (LIMS) and a post ablation ionization (PAI) neutral velocity analysis technique in order to determine the mass and velocities of the laser ejected material. In addition, x‐ray absorption measurements on films deposited onto substrates at room temperature were performed in order to identify the presence of short‐range crystalline order in the films. Both of these studies rule out the ejection of stoichiometric clusters of material from the pellet during the laser ablation/deposition process. Instead, binary and ternary suboxides are emitted from the targe...

Epitaxial growth of ferroelectric bismuth titanate thin films by pulsed laser deposition

Epitaxial thin films of ferroelectric bismuth titanate Bi4Ti3O12 have been grown by pulsed laser deposition on single‐crystal [100]u2009SrTiO3 substrates. Bismuth titanate has a high Curie temperature (675u2009°C) and saturation polarization values of 4 and 50 μC/cm2 along the c and b axis, respectively. Its a,b lattice parameters allow thin‐film growth on substrates such as SrTiO3, LaAlO3, MgO, etc. These single crystalline films exhibit good quality as evidenced by x‐ray diffraction, Rutherford backscattering, and transmission electron microscopy. Applications for these films include memory devices and optical displays.

Optical response of nongranular high‐Tc Y1Ba2Cu3O7−x superconducting thin films

We have investigated the optical response of c‐axis oriented crystalline Y1Ba2Cu3O7−x thin films (bridge and meander devices) on strontium titanate and MgO substrates. cw optical response to a He‐Ne laser radiation (wavelength of 0.63 μm) was primarily bolometric. The pulsed optical response was studied with Q‐switched and mode‐locked Q‐switched short pulses from a Nd:YAG laser at the wavelength of 1.06 μm. We identify two distinct components contributing to the pulsed optical response: a nonbolometric (as fast as 1 ns) and a bolometric component (several ns). The bolometric component is strong at temperatures in the vicinity of the transition region to the normal state. The nonbolometric component is dominant at temperatures below the transition region showing weak temperature dependence and a linear dependence on the bias current. These results are discussed using the flux motion model and also electron‐phonon scattering relaxation dynamics of nonequilibrium superconductors based on the theory of Bardee...

Microstructure of in situ epitaxially grown superconducting Y‐Ba‐Cu‐O thin films

The microstructure of in situ epitaxially grown Y‐Ba‐Cu‐O thin films on (001)u2009SrTiO3 substrates was studied using cross‐sectional transmission electron microscopy. The films, prepared by pulsed laser deposition at substrate holder temperature of 650u2009°C without post‐annealing, exhibit zero resistivity above 90 K and critical currents exceeding 106 A/cm2 at 77 K. The films are of heavily faulted single crystalline structure with the c axis approximately perpendicular to the substrate (001) surface. We suggest that, due to the fast quenching and low substrate temperature, crystalline defects and chemical fluctuations are locked into a faulted structure after each laser pulse. Despite their rather imperfect microstructure, the films are free from macroscopic grain boundaries and secondary phases and possess superb superconducting properties.

Observation of fast nonbolometric optical response of nongranular high Tc Y1Ba2Cu3O7−x superconducting thin films

We report fast optical response measurements in predominantly c‐axis oriented crystalline Y1Ba2Cu3O7−x superconducting thin films at the wavelength of 1.06 μm. We identify two distinct components contributing to the optical response: a nonbolometric (as fast as 1 ns) and a bolometric component (several ns). The bolometric component is stronger at temperatures in the vicinity of the transition region to the normal state. The nonbolometric component is dominant at temperatures below the transition region showing weak temperature dependence and a linear dependence on the bias current. This nonbolometric component may be evidence for nonequilibrium effects (e.g., breaking of Cooper pairs and generation of quasiparticles or other mechanisms). These results suggest that with proper optimization of device parameters (its geometry, critical current density, etc.) high‐speed detectors with reasonable sensitivity covering a broad electromagnetic spectrum (visible and infrared) may be developed.

Direct observation of structural defects in laser-deposited superconducting y-ba-cu-o thin films.

The defect structure of in situ pulsed, laser-deposited, thin films of the high—transition temperature superconductor Y-Ba-Cu-O has been observed directly by atomic resolution electron microscopy. In a thin film with the nominal composition YBa2Cu3O7 (123), stacking defects corresponding to the cationic stoichiometry of the 248, 247, and 224 compounds have been observed. Other defects observed include edge dislocations and antiphase boundaries. These defects, which are related to the nonequilibrium processing conditions, are likely to be responsible for the higher critical currents observed in these films as compared to single crystals.

Wet chemical etching of high-temperature superconducting Y-Ba-Cu-O films in ethylenediaminetetraacetic acid

A new wet chemical etchant for high‐temperature superconducting films is reported, which leaves transition temperature unaffected within experimental accuracy (1 K) and does not require reoxygenation. The solution consists of ethylenediaminetetraacetic acid (EDTA) in water, and is suitable for micropatterning using standard photolithography. We have fabricated 3–50 μm patterns on laser‐deposited Y‐Ba‐Cu‐O films. The bulk of the films etches at 0.14 μm/min in a saturated solution at room temperature. Porous surface layers are removed three times faster than the dense portions of the films. Etch rate depends linearly on the solution concentration and exponentially on the solution temperature. These rates are reduced if etching is interrupted and the samples are exposed to atmosphere.