Absence of quasiparticles in the photoemission spectra of quasi-one-dimensional Bechgaard salts

We measured typical quasi-one-dimensional organic metals (Bechgaard salts), using high resolution angle-resolved photoemission spectroscopy. We observe neither quasiparticle features near the chemical potential nor traces of a Fermi surface. The spectra, indicating a deep pseudogap around the Fermi energy, are incompatible with a Fermi liquid picture. None of the alternative interpretations we explored can fully explain the experimental data.

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

Spectroscopic Studies on Semiconducting Interfaces with Giant Spin Splitting

Emmanouil Frantzeskakis

The application of an external magnetic field can lift the spin degeneracy of electronic states through its interaction with the electronic magnetic moment. A closely-related phenomenon is the Rashba-Bychkov (RB) effect where symmetry breaking at surfaces or interfaces gives rise to an electric field which is in turn seen as an effective magnetic field in the electrons' rest frame. The resulting k-dependent energy splitting of spin-polarized electronic states has been observed on various metal surfaces but the effect is much larger in artificially-grown surface alloys; such as the BiAg2 grown at the surface of Ag(111). The spin splitting magnitude observed in these systems might be very useful in spintronics applications since it could decrease the spin precession time in a spin transistor and distinguish between the extrinsic and intrinsic spin Hall effects. Nevertheless, their metallic character poses serious obstacles in the exploitation of the RB effect due to the presence of spin-degenerate electronic states at the Fermi level which would dominate transport experiments. We have used angle-resolved photoelectron spectroscopy (ARPES) to explore the RB effects on various artificially grown structures, formed on semiconducting substrates. The interplay of quantum confinement and giant RB splitting on a trilayer Si(111)-Ag-BiAg2 system reveals the formation of a complex spin-dependent structure, which can be externally tuned by varying the Ag layer thickness. This provides a means to tailor the electronic structure and spin polarization near the Fermi level, with potential applications on Si-compatible spintronic devices. Moreover, we have discovered a giant spin splitting in a true semiconducting system, namely the Si(111)-Bi trimer phase. The size of the RB parameters is comparable to those of metallic surface alloys. Using theoretical models we have identified the peculiar band topology as the origin of the giant spin splitting on the Bi/Si(111) system. All our findings are supported by relativistic first-principles calculations. Finally, a chapter of this thesis manuscript is devoted to the description of phenomeno-logical theoretical simulation, which can capture the experimental results related to the RB effect on low-dimensional systems. A parallel experimental project is discussed in a separate chapter. It has been focused on the band topology of the novel p(2 × 2) reconstruction of the Pt(111)-Ag-Bi trilayer. We investigated the symmetry properties of the interface states by varying the amount of Ag. ARPES results present the electronic signature of a strain-related structural transition.

EPFL2010

Local spectroscopy of the Kondo lattice YbAl3: Seeing beyond the surface with scanning tunneling microscopy and spectroscopy

Klaus Kern, Peter Wahl

We report on an atomic-scale study of the Kondo lattice compound YbAl3 using scanning tunneling microscopy (STM) and spectroscopy (STS). An analysis of the surface orientation and structure is performed based on STM images containing more than one crystal facet. We compare tunneling spectra acquired on different facets and discuss their relation with the temperature scales observed in measurements of bulk quantities and the states observed in photoemission spectra. On specific facets, we observe strong additional resonances close to the Fermi energy which are not consistent with the characteristic energy scales found in bulk measurements, and which we interpret in terms of a modified Kondo state of the near-surface Ytterbium atoms.