Sensitivity to Dispersion Forces in First-Principles Modeling of Disordered Chalcogenides

The inclusion of dispersion (van der Waals, vdW) forces in first-principles modeling of disordered chalcogenides is analyzed and critically discussed in view of their impact on the atomic structure. To this purpose we considered the case of glassy GeTe4. We selected a vdW correction (termed vdW(G) hereafter) introduced by Grimme (2006) and, as an alternative, the approach (termed vdW(W) hereafter) based on the Wannier functions formalism (Silvestrelli, 2008). It appears that a strategy based on the update of the vdW interactions due to changes in the electronic structure during the dynamical evolution (i.e., the vdW(W) one) provides results clearly different from those obtained in the vdWG framework. By keeping in mind that the nature of the present results is preliminary and reflect a trend to be confirmed, we draw attention to the different levels of agreement with experimental data obtained with the two vdW schemes employed.

Properties of La2-xSrxCuO4 under epitaxial strain

Dominique Cloëtta

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.

EPFL2005

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

First principles investigation of defects at interfaces between silicon and amorphous high-kappa oxides

Peter Broqvist, Alfredo Pasquarello

Experimental characterization is facing severe difficulties in elucidating the nature of defects at interfaces between silicon and amorphous high-kappa oxides. This motivates recourse to first principle simulation approaches based on density functional theory. A major issue for these approaches is the choice of the model structure. indeed, the most trivial choice of parent crystalline structures may fail in describing the amorphous nature of the oxide or the specific bonding arrangements occurring at the interface. A second critical issue relates to the determination of energy levels with respect to band edges of the semiconductor and the oxide, which often represent the only contact with experimental data. An improved description of the electronic structure is required going beyond standard density functional approaches that severely underestimate electronic band gaps. Here, these issues are illustrated by a comparative study of the oxygen vacancy in monoclinic HfO2, in amorphous HfO2, and at the Si-HfO2 interface.