Nature of the Bad Metallic Behavior of Fe1.06Te Inferred from Its Evolution in the Magnetic State

We investigate with angle-resolved photoelectron spectroscopy the changes of the Fermi surface and the main bands from the paramagnetic state to the antiferromagnetic (AFM) state occurring below 72 K in Fe1.06Te. The evolution is completely different from that observed in Fe pnictides, as nesting is absent. The AFM state is a rather good metal, in agreement with our magnetic band structure calculation. On the other hand, the paramagnetic state is very anomalous with a large pseudogap of similar to 65 meV on the electron pocket that closes in the AFM state. We discuss this behavior in connection with spin fluctuations existing above the magnetic transition and the correlations predicted in the spin-freezing regime of the incoherent metallic state.

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

Kinks in the angle resolved photoemission and inelastic x-ray spectra of high temperature superconductors

Jeff Graf

Though the high superconducting transition temperature (Tc) is the most interesting technological aspect of high temperature superconductors, the complex way in which the spin, lattice and electronic degrees of freedom interplay, makes them of the highest scientific interest. This is illustrated by their rich phase diagram characterized by a variety of exotic groundstate including; Mott insulating, pseudogap and high temperature superconductivity. One of the most serious barriers that has prevented our understanding of the mechanism of high temperature superconductors thus far is the lack of information on how low energy Landau quasiparticles result from an organization of real particles. This organization, often colloquially referred to as "dressing", is a fundamental general concept in physics that explains a variety of physical phenomena, such as exotic particle formations and phase transitions. The role of the lattice in this dressing is particularly controversial. Phonons are quanta of lattice vibration energy, and play a crucial role in conventional superconductivity. They provide an attractive interaction allowing the electrons to condensate in superconducting Cooper pairs. However, high temperature superconductivity in the cuprates in achieved through hole doping in an antiferromagnetic Mott insulator. In this case, the antiferromagnetic background, the strong coulomb repulsion and the anisotropic superconducting gap all suggest a marginal role of the phonons. In order to assess experimentally the exact role of the phonon, angle resolved photoemission spectroscopy (ARPES) is the weapon of choice given its unique ability to probe the electronic structure of solids in an energy- and momentum-resolved manner. In this thesis, I will use ARPES to characterize the various form of dressing of the quasiparticles in the high temperature superconductor from the Fermi level all the way to the valence band complex. I'll show that when combined with isotope substitution and inelastic x-rays scattering, the strong electron-phonon interaction can be probed in vivid details. This thesis presents three fundamental results. 1) Using the isotope effect, the important and unusual role of the phonon is demonstrated, as well as the strong interplay between the magnetic and phonic degrees of freedom. 2) Using inelastic scattering, the intriguing interplay between the Fermi surface and the bond stretching phonon is exposed. And 3) By exploring the ARPES data up to high energy, two new energy scales at high binding energy as well as the spectral waterfalls phenomenon are revealed. When all these new elements are considered together, they clearly show the need to consider simultaneously the spin, lattice, Fermi surface topology and electronic degrees of freedom. The spectacular progress in ARPES techniques over the past two decades was critical for these results, and I will present in this thesis our current effort in pushing the techniques even further. In particular I'll present my efforts in building a laser based ARPES setup with a hemispherical analyzer and a spin resolved time of flight analyzer.

EPFL2008

Optimization of Embedded Atom Method Interatomic Potentials to Simulate Defect Structures and Magnetism in [alpha]-Fe

Samuele Chiesa

Magnetism is largely responsible for the body centered cubic to face centered cubic structural phase transition occurring in iron at 1185 K and to many anomalies in the vicinity of the ferromagnetic to paramagnetic phase transition at 1043 K, as for instance an anomalous softening of the tetrahedral shear modulus. Current atomistic models including magnetism are either limited to the treatment of perfect lattice models or to zero temperatures, while research and development of candidate materials for future fission and fusion power plants requires the modeling of irradiation induced defects in ferritic/martensitic steels at high temperatures. An attempt to fill this gap is the Dudarev-Derlet potential, which includes zero temperature magnetism in an embedded atom method formalism, together with a more recent extension of the method to the inclusion of spin rotations at non zero temperature with nearly half the computational speed of an embedded atom method potential. In this work, we report on the optimization of the Dudarev-Derlet potential to the zero temperature bulk properties of the non-magnetic and ferromagnetic bcc and fcc phases, including the third order elastic constants of the ferromagnetic bcc phase, the point defects formation and migration energies and the core structure of the screw dislocation with Burgers vector 1/2[111], either from experiments or from density functional theory calculations, where we develop a method to fit the core structure of the screw dislocation based on the Suzuki-Takeuchi model. Three representative fits from the optimization of the Dudarev-Derlet potential are compared with recent semi empirical potentials for iron, with density functional theory and experiments. The migration energies of the self-interstitial range from 0.31 eV to 0.42 eV, compared to a density functional theory value close to 0.35 eV and an experimental value close to 0.3 eV, and the vacancy migration energies range from 0.85 eV to 0.94 eV, compared to a density functional theory value close to 0.65 eV. Clusters composed of parallel self-interstitials are oriented along ‹110› if the number of interstitials composing the cluster is smaller or equal than 3, while for bigger clusters the ‹111› orientation is more stable, in qualitative agreement with density functional theory. Depending on which one of the three representative fits is chosen, the formation entropy of one ‹110› dumbbell calculated by the thermodynamical integration method in the range from 300 K to 600 K varies from 0.28 kB to 4.02 kB. The diffusion coefficient of the ‹110› dumbbell at 600 K ranges from 1×10-6 cm2/s to 10×10-6 cm2/s, while at room temperatures the scatters extends over three orders of magnitude. The main difficulties, common to all the semi empirical potentials considered in the work, are related to the description of the fcc phase and the migration mechanism of the screw dislocation. The semi empirical potentials are unable to distinguish the anti-ferromagnetic fcc from the low spin ferromagnetic fcc or the high spin ferromagnetic fcc. Considering the equilibrium volume and the bulk magnetic moment, the high spin phase is the one which most resembles the ferromagnetic fcc phase of the Dudarev-Derlet potentials. Finally, for those fits with a non-degenerate core structure, we investigate some fundamental aspects of the migration mechanism of the screw dislocation with Burgers vector 1/2[111] at zero temperature and at zero applied stress, by calculating the Peierls potential in the [211] direction between two structurally equivalent soft cores. This confirms the existence of a stable core structure in the middle of the migration path not observed in density functional theory, which is actually found to be energetically degenerate with the soft core. The consequences of this are discussed in terms of formation energies of double kinks in the [211] direction.

EPFL2010