Electronic properties of high temperature superconducting thin films grown by pulsed laser deposition

Mike Abrecht, Dominique Cloëtta, Giorgio Margaritondo, Davor Pavuna
Spie-Int Society Optical Engineering, 2002
Chapitre de livre

Résumé

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 noncleavable 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 T, 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.

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https://infoscience.epfl.ch/record/90410?ln=fr

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Photoemission studies of thin films grown by pulsed laser deposition

Mike Abrecht

In this thesis I report on the unique novel possibilities offered by combining pulsed laser deposition (PLD) film growth of high temperature superconductors with angle integrated and angle resolved photoemission spectroscopy (PES and ARPES). The originality of the procedure rests on the in situ preparation and transfer of samples, hence by-passing the usual cumbersome cleaving or scraping practices to obtain high quality surfaces suitable for photoemission studies. Photoemission spectroscopy is one of the major experimental techniques to analyze the electronic structure that holds the key to understanding the properties of the high Tc cuprates (copper oxides) for which a complete microscopic theory still needs to materialize. In chapter 1, I introduce these complex copper oxide materials. The vast majority of ARPES measurements on the cuprate superconductors have so far been limited to the easily cleavable Bi2Sr2CaCu2O8+x (BSCCO-2212) single crystals. In chapter 2, I present the principle of an ARPES measurement, and show as an example (obtained in the "traditional way" of cleaving single crystals) a crossover in the electronic response at a special temperature T+>Tc in overdoped samples. In chapter 3, our unique laser ablation set-up, built on site at the Wisconsin Synchrotron Radiation Center (SRC), is introduced. This is followed by reports on film characterization such as transport, crystal and surface properties which enables the optimization of growth parameters. From here on to the end of the thesis, we show that instead of being a competing technique to cleaving samples, in situ growth of films appears to be a complementary one. In particular, our results show that preparing cuprates which contain oxygen chains, like Re-Ba2Cu3O6+x (RBCO-123; R=Y, Nd), for the surface sensitive ARPES measurements is very difficult due to a material intrinsic loss of oxygen near the surface. This, in turn, curbs the photoemission signal near the chemical potential due to the loss of superficial conductivity (chapter 3). Among the cuprates studied, La2-xSrxCuO4 (LSCO) and Bi2Sr2CaCu2O8+x films, however, both exhibit a sharp Fermi step in angle integrated photoemission spectra, indicating a conducting behavior at the surface. Moreover, thanks to using film samples, we were able to study ultra thin cuprates, down to about 1 unit cell (UC), and we were able to induce epitaxial strain in the samples which can remarkably enhance superconductivity. In chapter 4, combining core level, angle integrated valence band photoemission, and x-ray diffraction, we demonstrate that a structural phase transition occurs in the early growth of BSCCO-2212 films on SrTiO3 (STO) substrates, transforming the precursor Bi6O7 natural reactive buffer layer into the BSCCO-2212 structure. This is qualitatively different from the initial growth of YBCO-123 characterized by nucleation of well separated islands. Chapter 5 then presents the main success of our method: the first ever ARPES data on the low energy electronic structure of any high Tc superconductor under strain. In particular, we confirm that the Fermi surface (FS) of LSCO evolves with doping, but changes even more significantly with strain. More specifically, the FS becomes electron-like for in-plane compressed samples (for all doping values x studied, 0.1

EPFL2003

Chargement

The subject of this thesis is the growth and analysis of high temperature superconductor (HTSC) films and the study of their electronic structure and properties. In particular, the effect of epitaxial strain is investigated, predominantly by means of in-situ angle resolved photoemission spectroscopy (ARPES), as well as X-ray diffraction, resistivity and susceptibility measurements. In order to achieve that goal we have developed a unique experimental set-up at the Synchrotron Radiation Center (University of Wisconsin), consisting in a dedicated pulsed laser deposition system, which enables us to grow thin films with excellent surface quality. Our sample transfer procedures assure that this surface quality is not affected on the way to the ARPES analyzer, which is connected to a beamline. We have built a similar film growth system at the EPFL with the aim to connect it to the SCIENTA analyzer at the IPN and to an analyzer on a beamline at the SLS in Villigen. For the first time we were able to perform ARPES measurements on in-situ grown films of HTSC. Previously to our work all the ARPES measurements were carried out on cleaved or scraped samples, predominantly single crystals of Bi2Sr2CaCu2O8 compounds. Thin films offer the possibility to study the effect of epitaxial strain induced through lattice mismatch between the film and its substrate. Compressive strain in the CuO2 plane has been known to enhance the critical temperature (TC) up to 50%, therefore we expected to see a signature of strain in the electronic dispersion. The Fermi surface of unstrained La2-xSrxCuO4 evolves with doping as reported for scraped single crystals, but also changes strongly with strain. Our studies show, that the in-plane compressive strain changes the Fermi surface topology from hole-like to electron-like. It enhances band dispersion and the Fermi level is crossed before the Brillouin zone boundary, in sharp contrast to the "usual" saddle point remaining ~30 meV below the Fermi level measured along the direction of the Cu-O bonds on unstrained samples. The associated reduction of the density of states near the Fermi level does not diminish the superconductivity; TC is enhanced in all our compressively strained samples. This result is rather surprising since such a reduction of the density of states, according to many mean field models, does not favor the increase of TC measured in our films. By comparing the ARPES measurements on our films with measurements on bulk crystals, we could also show that the results from our relaxed films are equivalent to those on bulk crystals, therefore excluding an explanation through finite size effects other than strain. Our latest results on films under huge tensile strain (1% change in c-axis) are significantly different: ARPES shows evidence for a 3-dimensional dispersion, in contrast with the strictly 2-dimensional dispersion observed on compressively strained films. Already the conduction band of relaxed La2-xSrxCuO4 is atypical: It has considerable apical-oxygen pz and Cu3dz2-r2 out-of-plane character, while for the rest of the cuprate HTSC, those orbitals hybridize far less with the conduction band. We relate the observed z-axis dispersion with the significant displacement of the apical-oxygen towards the CuO2 plane, induced by the epitaxial strain. Resistivity measurements show an insulating behavior of films under extreme tensile strain and no TC. Films with weaker tensile strain still exhibit superconductivity, but diminished as compared to the relaxed films. In summary, while the in-plane compressive strain tends to push the apical oxygen far away from the CuO2 plane, enhances the 2-dimensional character of the dispersion and increases TC, the tensile strain seems to act exactly in the opposite direction and the resulting dispersion is 3-dimensional. We have established the shape of the Fermi surface for both cases, yet further experiments are required to clarify fine details.

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On detection of the Fermi edge in in situ grown thin films of high-T-c oxides

Mike Abrecht, Giorgio Margaritondo, Davor Pavuna

We discuss our systematic series of experiments on the photoelectric detection of the Fermi edge using a cylindrical mirror analyser on films of high-T-c oxides. grown in situ by pulsed laser ablation. The Fermi edge (comparable to the edge of the reference Ag) is very easily observed even in the two-phase BSCCO-2212 film that exhibits onsets of superconducting transitions, at 85 and 45 K. In contrast, the Fermi edge is weaker and more difficult to observe even in the state-of-the-art. highly epitaxial, monophase YBa2Cu3O7-y (YBCO) and NdBa2Cu3O7-y. (NBCO-123) films (both with T-c = 92 K). So far we could not detect the Fermi edge in the films of the double-'chain' YBCO-124. (C) 2001 Elsevier Science B.V. All rights reserved.

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Photoemission studies of thin films grown by pulsed laser deposition

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