Properties of La2-xSrxCuO4 under epitaxial strain

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.

The growth by laser ablation and electronic properties of thin cuprate films

Claudia Cancellieri

In my thesis work I have concentrated on the growth and the in-depth analysis of high temperature superconducting thin films with the central aim to elucidate their electronic properties, predominantly by in-situ angle resolved photoemission spectroscopy (ARPES). I have used two somewhat complementary approaches and two laser ablation set-ups. The first one, developed previously in Wisconsin, was used mainly for studies of strained La2-xSrxCuO4 (LSCO) with a transfer to the Scienta analyzer via an appropriate suitcase. The second one, at the EPFL, where I have built a new pulsed laser deposition (PLD) system, was used to optimize the growth of Bi2Sr2-xLaxCuO4 (Bi-2201) and study in-situ ARPES. In-situ ARPES is the most direct tool to probe the electronic structure. We performed it at the Synchrotron Radiation Center (SRC, University of Wisconsin), where we used the aforementioned experimental set-up consisting in a dedicated PLD system coupled with the SCIENTA beamline. The sample transfer procedure assures that the surface quality is preserved on the way to the SCIENTA analyzer. There we studied in detail the effect of strain in LSCO thin films. In a previous work the in-plane compressive strain was studied and the main result was that the Fermi surface (FS) topology changed from hole-like to electron-like. The tensile strained films showed completely different results. ARPES analysis show evidence for a 3-dimensional (3D) electronic dispersion relation in contrast to the strictly 2-dimensional (2D) dispersion observed in all other studied LSCO films. In this thesis this result has been confirmed mapping the FS at different photon energies. We found that the strain related to the thickness of the films, is playing an important role in inducing a 3D dispersion. Furthermore, the 3D parameters, evolve according to the level of strain. Moreover, we observe a staircase structure for different photon energies, revealing both the 3D nature of the electronic dispersion and the quantization of the electron wave vector along the direction normal to the film surface. Taking advantage of the wavevector quantization we were able to determine directly the band parameters and map the FS without using the nearly-free-electron approximation (NFEA). Moreover, introducing an effective anisotropic photoelectron effective mass, related to the local structure of the excited band, improves the use of the NFEA for single photon energy measurements. In parallel, I have built an improved PLD system at the EPFL which can be connected to the SCIENTA analyzer, and which enables us to perform in-situ ARPES measurements at any time rather than only during allocated beamtimes at the synchrotron. We also produced our own targets for the laser ablation and all the films were fully characterized at the EPFL performing X-ray diffraction (XRD), resistivity and magnetic measurements. I analyzed in detail the growth mechanism of Bi-2201 and I investigated the presence of random intergrowths. We developed a model to explain the presence of these polytypes and studied their presence as a function of the deposition parameters and the annealing treatment. The model predicts a very particular spatial distribution of defects: a Markovian-like sequence of displacements along the grow direction, as well as a two-component in-plane correlation function, characteristic of self-organized intercalates. We varied the growth conditions in order to study the presence of intergrowths and to produce single-phase samples. Subsequently, we performed in-situ photoemission experiments on thin films of Bi-2201 films free from intergrowths and we analyzed their FS. This method is successful and can be extended to other related oxide films.

EPFL2008

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

Direct photoemission spectroscopy and electronic properties of in situ grown, strained high-T-c and related oxide films

Mike Abrecht, Dominique Cloëtta, Giorgio Margaritondo, Davor Pavuna, Dusan Vobornik

We have optimized laser ablation thin film deposition and transfer procedures within synchrotron vault, specifically to perform angle-integrated and angle-resolved photoemission spectroscopy (ARPES) on high-T-c and related films without cleaving the samples. However, the chain-containing phases like YBCO-123 easily loose surface oxygen and do not exhibit stable Fermi edge, hence are not suitable for ARPES studies. Direct in situ ARPES studies on strained LSCO-214 films show striking strain effects on the electronic structure. The Fermi surface (FS) of LSCO evolves with doping, yet changes even more significantly with strain. The strain changes the FS topology from hole-like to electron-like, and causes band dispersion along k(x) and the Fermi level crossing before the Brillouin zone boundary, in sharp contrast to the 'usual' flat band remaining approximate to 30 meV below E-F measured on unstrained samples. The associated reduction of the density of states does not diminish the superconductivity; T-c is enhanced in all our strained samples. Implications for the evolving high-T-c theory and studies of nano-engineered film heterostructures are briefly discussed. (C) 2004 Elsevier Ltd. All rights reserved.

2004