Molecular beam epitaxy technique has enabled synthesis of atomically smooth thin films, multilayers, and
superlattices of cuprates and other complex oxides. Such heterostructures show high temperature superconductivity and
enable novel experiments that probe the basic physics of this phenomenon. For example, it was established that high
temperature superconductivity and anti-ferromagnetic phases separate on Ångström scale, while the pseudo-gap state
apparently mixes with high temperature superconductivity over an anomalously large length scale (the "Giant Proximity
Effect"). We review some recent experiments on such films and superlattices, including X-ray diffraction, atomic force
microscopy, angle-resolved time of flight ion scattering and recoil spectroscopy, transport measurements, highresolution
transmission electron microscopy, resonant X-ray scattering, low-energy muon spin resonance, and ultrafast
photo-induced reflection high energy electron diffraction. The results include an unambiguous demonstration of strong
coupling of in-plane charge excitations to out-of-plane lattice vibrations, a discovery of interface high temperature
superconductivity that occurs in a single CuO2 plane, evidence for local pairs, and establishing tight limits on the
temperature range of superconducting fluctuations.
We systematically study the structural and electronic properties of very thin cuprate films. Our direct angle resolved photoemission spectroscopy (ARPES) measurements on the low binding energy electronic structure of La2-xSrxCuO4 (LSCO) films confirmed that the Fermi surface evolves with doping, but changes even more significantly with growth-induced compressive strain. For a given doping, the in-plane compressive strain enhances TC's and modifies the 2-dimensional hole-like Fermi surface as to appear more electron-like. In contrast, the in-plane tensile strain reduces TC (suppressing superconductivity for huge tensile strain) and shows 3-dimensional ARPES dispersion with a corresponding 3-dimensional Fermi surface. To account for these striking changes in electronic structure and superconductivity, the out-of-plane states should be taken into account, as well as some subtle changes in the associated atomic distances.
We use a pulsed laser deposition (PLD) setup to grow ultra-thin films of high temperature superconductors (HTSC) and transfer them in-situ into a photoemission chamber. Photoemission measurements on such films allow us to study non-cleavable materials, but can also give insights into aspects never measured before, like the influence of strain on the low energy electronic structure. Systematic studies of many different materials grown as films showed that Bi2Sr2CaCu2O8+x, Bi2Sr2Cu1O6+x, Bi2Sr2Ca2Cu3O10+x and La2-xSrxCuO4 films exhibit a conductor-like Fermi edge, but materials containing chains (such as YBa2Cu3O7-x) are prone to very rapid surface degradation, possibly related to critical oxygen loss at the surface. Among HTSC materials, La2-xSrxCuO4 is extremely interesting because of its rather simple structure and the fact that its critical temperature Tc can be enhanced by epitaxial strain. Here we present our first high resolution angular resolved photoemission spectroscopy (ARPES) results on 8 unit-cell thin La2-xSrxCuO4 films on SrLaAlO4 [001] substrates. Due to the lattice mismatch, such films are compressed in the copper oxygen planes and expanded in the c-axis direction. Results show a surprisingly modified Fermi surface compared to the one of non-strained samples.
Femtosecond optical reflectivity measurements of La2-xSrxCuO4, La2CuO4+y, Bi2Sr2CuO6+z and Bi2Sr2CaCu2O8+δ thin films and single crystal samples indicate qualitative changes with fluence. At the lowest fluencies, there is a power law divergence in the relaxation time. The divergence has an onset temperature of 55±15K, independent of whether the sample is in the superconducting or normal states. At slightly higher fluencies, still perturbative, the additional response does not exhibit this power law divergence. At quite high fluencies- no longer perturbative- the metallic samples exhibit oscillations in the reflectivity amplitude. The period of these oscillations varies with the probe wavelength but not with the pump wavelength. The oscillations exhibit a decay time as long as 10 nsec.
We report on ab-plane resistivity ((rho) ) and angle-resolved photoemission (ARPES) spectra for Bi2Sr2CaCu2O8+x single crystals irradiated with neutrons or electron-beam irradiation. Both the normal and superconducting states were measured with angle-resolved photoemission. Electron-beam irradiation leads to an increase in the residual resistivity, and a decrease in the superconducting transition temperature (Tc). The resistivity data does not indicate any pseudogap; the resistivity is linear from Tc to 300 K for all levels of disorder, and the slope (d(rho) /dT) is the same for all levels of disorder. The superconducting state ARPES data exhibits no change in the binding energy of the 'peak' for Brillouin zone locations near the (O,(pi) ) point. The peak spectral intensity decreases with increasing disorder, the gap fills in, but the peak neither shifts nor broadens. The normal state exhibits a pseudogap developing with disorder; the size of the pseudogap increases as the residual resistivity increases. The pseudogap is anisotropic, largest near the (O,(pi) ) point and zero in the <(pi) ,(pi) > direction. Neutron-beam irradiation causes an increase in the residual resistivity. The resistivity data exhibit a change of slope and indications of a pseudogap for neutron irradiation. For normal state ARPES data of neutron-beam irradiated samples, there is also an anisotropic pseudogap; it is also zero in the <(pi) ,(pi) > direction and large near the (O,(pi) ) point. We discuss implications of these data.
In YBa2Cu3O6+x compound the tetragonal to orthorhombic transition occurs around x equals 0.3, followed by a continuum variation of lattice parameters. Hence both, the structural and superconducting properties, depend upon the oxygen content in CuO chains. Conversely, the epitaxial stress, exerted by the substrate on YBCO films, modified the lattice parameters influencing the oxygen stability in the chains. The understanding of this mechanism is essential when growing epitaxial films for in- situ photoemission studies as well as for tunneling experiments, since the oxygen stability up to the top surface unit-cell is a central issue. We have studied this effect on c-axis oriented YBCO films grown by laser ablation on (001) STO single crystals. Accurate x-ray diffraction analysis of thick films (t GRT 500 angstrom) indicates the presence of two distinct layers, one strained and the other relaxed. Detailed analysis shows that the relaxed layer is as well oxidized as bulk samples, while the strained one is oxygen deficient. Furthermore, despite an oxygen content of about x equals 0.65, the strained layer is in the tetragonal phase (in bulk, the tetragonal phase exists for x < 0.3). We discuss these results in terms of competition between the chemical pressure induced by oxygen inclusion in the chains, and the uniaxial stress within the film.
We describe a new pled laser deposition (PLD) system that is linked to an angle-resolved photoemission (ARPES) chamber at the Synchrotron Radiation Center (SRC) in Wisconsin, USA. We also discuss our first results on epitaxially grown YBa2Cu3O7-(delta ) (YBCO) films. The core level photoemission data indicate that a Ba-oxide layer is the dominant surface layer. We were not able to reproducibly detect a sharp fermi edge in the photoemission spectra and thus conclude that the surface layer is non-metallic, probably due to oxygen loss at the surface. The absence of screening of the Y and Ba core levels is a further argument for this conclusion. Further experiments with ozone treated film surfaces are currently under way.
We report a combined analysis of resistivity and x-ray diffraction rocking curve measurements on c-axis oriented YBA2Cu3O6.9 films epitaxially grown on (100) SrTiO3 and LaAlO3 by ion-beam sputtering. We find that the growth-induced reduction of long-range lattice order in the films begins to depress superconductivity and normal conductivity at a critical value of lattice coherence length of approximately equals 10 and 5 nm for the two above types of substrates respectively. Evidence for disorder-induced localization is given by a deviation from linearity of the temperature-dependence of the resistivity which scales as the reduction of superconducting critical temperature. Similar nonlinear dependence observed in slightly reduced or lightly Co-doped samples suggests that the disorder in our films significantly affects the CuO chains. Our analysis of the paraconductivity term in the films gives evidence for the enhancement of the superconducting fluctuations by the disorder.
We present an overview of potential applications of high-Tc superconductors (HTSC) in the context of hybrid optoelectronic technology. We briefly summarize the main requirements for in-situ growth of epitaxial YBa2Cu3O7-δ (YBCO) films on SrTiO3, and discuss the properties of YBCO layers grown on Si and GaAs substrates with intermediate, conducting Indium Tin Oxide buffer layers. We compare the performance of the micro-bridge and the meander type of HTSC bolometer and conclude with a discussion of several novel concepts and results that may become relevant for future hybrid optoelectronic technology.
Superconducting YBa2Cu3O7-delta (YBCO) thin films have been grown in situ by ion beam sputtering on Si and GaAs substrates with intermediate, conducting Indium Tin Oxide (ITO) buffer layers. Uniform, textured YBCO films on ITO exhibit Tc onset at 92K and Tc0 at 68K and 60K on Si and GaAs substrates respectively, the latter value is the highest Tc reported on GaAs. YBCO/ITO films exhibit metallic resistivity behavior. In situ YBCO films on SrTiO3 show Tc onset = 92K and Tc0 = 90.5K, transition widths are less than 1K. A simple optical bolometer has been constructed from YBCO films on SrTiO3. Tunnelling measurements have also been carried out using the first YBCO-Pb window-type tunnel junctions.
This short overview briefly summarizes the most important parameters for successful preparation and associated properties of thin films of YBa2Cu3O(7-delta) (YBCO) superconductors. The principles are illustrated by using the example of monotarget ion beam sputtering technique: YBCO films grown in situ on SrTiO3 show Tc(onset)
Superconducting YBa2C u 3 0 7 (YBCO) thin films were grown on Si with
transparent, conducting Indium Tin Oxide (ITO) buffer layers The onset
temperature at 92K and zero resistance at 68K were measured. Both, ITO and
YBCO films were deposited by ion beam co-deposition. The YBCO/ITO films
exhibit metallic resistivity with positive slopes (r0.055 1K). The YBCO is
uniform, textured and polycrystalline. The relevance for hybrid opto-electronic
device structures is briefly discussed.
Superconducting YBa2Cu3O 7 (YBCO) thin films were grown on Si with
transparent, conducting Indium Tin Oxide (ITO) buffer layers The onset
temperature at 92K and zero resistance at 68K were measured. Both, ITO and
YBCO films were deposited by ion beam co-deposition. The YBCO/ITO films
exhibit metallic resistivity with positive slopes (''O.O55 □/K). The YBCO is
uniform, textured and polycrystalline. The relevance for hybrid opto-electronic
device structures is briefly discussed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.