B. Dutta

High‐temperature superconductivity in ultrathin films of Y1Ba2Cu3O7−x

We have grown ultrathin films of Y1Ba2Cu3O7−x in situ on (001) SrTiO3 by pulsed laser deposition. The zero resistance transition temperature (Tc0) is >90 K for films >300 A thick. The critical current density (Jc at 77 K) is 0.8×106 A/cm2 for a 300 A film and 4–5×106 A/cm2 for a 1000 A film. The Tc0 and Jc deteriorate rapidly below 300 A, reaching values of 82 K and 300 A/cm2 at 77 K, respectively, for a 100 A film. Films only 50 A thick exhibit metallic behavior and possible evidence of superconductivity without showing zero resistance to 10 K. These results are understood on the basis of the defects formed at the film‐substrate interface, the density of which rapidly decreases over a thickness of 100 A. We have studied these defects by ion channeling measurements and cross‐section transmission electron microscopy. Our results suggest that the superconducting transport in these films is likely to be two dimensional in nature, consistent with the short coherence length along the c axis of the crystals.

Optical spectroscopy: An in situ diagnostic for pulsed laser deposition of high Tc superconducting thin films

High oxygen partial pressure has been found to be an important parameter for the pulsed laser deposition of as‐deposited superconducting thin films with high Tc and Jc. The optical emissions from both elemental and oxide species ejected from the target of YBa2Cu3O7−x during the pulsed laser deposition process increase with the oxygen pressure, with the oxide emission showing a stronger pressure dependence than the elemental emission. The dynamics of the interaction between the oxygen atoms and the species in the laser‐produced plume were studied by a wavelength and time‐resolved measurement. The results are qualitatively explained using a simple model based on optical emission arising from inelastic and recombination collision between the elemental species and electronically excited oxygen atoms. The formation of oxides in the plume is shown to be essential for the production of higher quality superconducting films, indicating the value of optical spectroscopy as a diagnostic tool.

Epitaxy of Y‐Ba‐Cu‐O thin films grown on single‐crystal MgO

The epitaxy of a thin‐film Y‐Ba‐Cu‐O (YBCO) superconductor deposited on a single‐crystal [001] MgO substrate was examined by transmission electron microscopy. The large lattice mismatch (8–10%) in the basal plane of YBCO and MgO is accommodated mainly by the formation of a polycrystalline, mosaic structure. The grain boundaries correspond to unique crystallographic interfaces, determined by the crystal symmetry of the substrate and the thin film.

Epitaxial Y1Ba2Cu3O7−y/Y1−xPrxBa2Cu3O7−y heterostructures

For a variety of device applications, junction devices in particular, we have demonstrated a heterostructure system of Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}}/Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}} which maintains epitaxy over the entire Pr composition range {ital x}=0--1. We have grown both trilayer and multiperiod superlattices which show nearly single crystalline helium ion backscattering minimum yields of {lt}6% in the topmost layer. X-ray diffraction measurements indicate {ital c}-axis orientation by a transverse scan across (005) line with a full width at half maximum of 0.6{degree} and 0.4{degree} on MgO and SrTiO{sub 3} substrates, respectively. Scanning Auger electron depth profiles and cross-sectional transmission electron micrographs indicate abrupt Pr/Y interfaces within one unit cell and virtually no disruption of the layered structure at the interface. These results indicate the potential for the growth of excellent heterostructures and superlattices of the high-temperature superconductors.For a variety of device applications, junction devices in particular, we have demonstrated a heterostructure system of Y1Ba2Cu3O7−y/Y1−xPrxBa2Cu3O7−y which maintains epitaxy over the entire Pr composition range x=0–1. We have grown both trilayer and multiperiod superlattices which show nearly single crystalline helium ion backscattering minimum yields of <6% in the topmost layer. X‐ray diffraction measurements indicate c‐axis orientation by a transverse scan across (005) line with a full width at half maximum of 0.6° and 0.4° on MgO and SrTiO3 substrates, respectively. Scanning Auger electron depth profiles and cross‐sectional transmission electron micrographs indicate abrupt Pr/Y interfaces within one unit cell and virtually no disruption of the layered structure at the interface. These results indicate the potential for the growth of excellent heterostructures and superlattices of the high‐temperature superconductors.

Surface layers on superconducting Y-Ba-Cu-O films studied with x-ray photoelectron spectroscopy

The chemical compositions of surface and bulk regions of pulsed laser deposited YBa2Cu3O7−x films on SrTiO3(001) substrates were examined. From the relative intensities of the surface and bulk components of the Ba 3d and Ba 4d spectra taken at different take‐off angles and different escape depths [using Al Kα (1486.6 eV) and Mg Kα (1253.6 eV) excitations], we have concluded that there is a nonsuperconducting surface layer of 1 nm thickness with a composition of BaCuO2. This layer contains oxygen with a photoelectron binding energy of 531 eV, and Cu2+ ions. A superconducting film which was only 8 nm thick also had a layer of 1 nm thickness. By detecting the substrate Ti signal through this film, and ruling out a high density of pinholes, we provide evidence that the x‐ray photoelectron spectroscopy data contain information from the superconducting phase. A polycrystalline pellet scraped in vacuum had a similar surface layer, which was only 0.4 nm thick.

Infrared response of YBa2Cu3O7-delta films to pulsed broadband synchrotron radiation

We report studies of a thin high-Ta film operating as a fast bolometric detector of infrared radiation. The film has a response of several mV when exposed to a 1 W, 1 ns duration broadband infrared pulse. The decay after the pulse was about 4 ns. The temperature dependence of the response accurately tracked dR/dT. A thermal model, in which the films temperature varies relative to the substrate, provides a good description of the response. We find no evidence for other (non-bolometric) response mechanisms for temperatures near or well below T.

Infrared spectra and the energy gap in thin film YBa2Cu3O7-delta

Interpretations of IR and millimeter wave measurements in superconductors are generally carried out in terms of the Mattis-Bardeen calculations, which apply either to the anomalous skin effect regime or to the dirty limit regime. In high temperature superconductors neither limit applies. Reflectance measurements on high quality, epitaxially-grown, laser-deposited films indicate that these samples are in the clean-limit, normal skin effect regime. Features that have been previously identified as the gap appear in both the superconducting and the normal-state spectra, although obscured by the free carrier absorption above Tc. Below Tc these features become more evident as the free carrier contribution condenses into a delta function at zero frequency.

Infrared studies of laser-deposited high- T c films

Two important issues for the infrared properties of high-T c materials are the superconducting energy gap and the nature of the strong midinfrared absorption. In this work we discuss the infrared reflectance of epitaxially-grown laser-deposited films of YBa2Cu3O7−δ, PrBa2Cu307−δ, and Y1−xPrxBa2Cu3O7−δ. (x = 0.2, and 0.4).