M. Abrecht

Structural phase transition in early growth of Bi2Sr2CaCu2O8+x films on SrTiO3 substrates

We used pulsed laser deposition, with a Bi2Sr2CaCu2O8+x target, to grow films ranging from (1/4) to 10 unit cells thick. We studied these films, and reference Bi2Sr2CaCu2O8+x single crystal samples, using angle-integrated photoemission, core level photoemission, and x-ray diffraction. The data indicate that all films exhibit a metallic-like Fermi edge in the photoemission data. More strikingly, a structural phase transition occurs at a nominal Bi2Sr2CaCu2O8+x thickness of approximately one unit cell, converting the precursor Bi2O2.33 highly coherent thin film into a Bi2Sr2CaCu2O8+x structure.

Structural properties of strained YBa2Cu3O6+x superconducting films grown by pulsed laser deposition

In YBa2Cu3O6+x compound the tetragonal to orthorhombic transition occurs around x equals 0.3, followed by a continuum variation of lattice parameters. Hence both, the structural and superconducting properties, depend upon the oxygen content in CuO chains. Conversely, the epitaxial stress, exerted by the substrate on YBCO films, modified the lattice parameters influencing the oxygen stability in the chains. The understanding of this mechanism is essential when growing epitaxial films for in- situ photoemission studies as well as for tunneling experiments, since the oxygen stability up to the top surface unit-cell is a central issue. We have studied this effect on c-axis oriented YBCO films grown by laser ablation on (001) STO single crystals. Accurate x-ray diffraction analysis of thick films (t GRT 500 angstrom) indicates the presence of two distinct layers, one strained and the other relaxed. Detailed analysis shows that the relaxed layer is as well oxidized as bulk samples, while the strained one is oxygen deficient. Furthermore, despite an oxygen content of about x equals 0.65, the strained layer is in the tetragonal phase (in bulk, the tetragonal phase exists for x < 0.3). We discuss these results in terms of competition between the chemical pressure induced by oxygen inclusion in the chains, and the uniaxial stress within the film.

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

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 non-cleavable 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 Tc 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.

Femtosecond optical studies of cuprates

Femtosecond optical reflectivity measurements of La2-xSrxCuO4, La2CuO4+y, Bi2Sr2CuO6+z and Bi2Sr2CaCu2O8+δ thin films and single crystal samples indicate qualitative changes with fluence. At the lowest fluencies, there is a power law divergence in the relaxation time. The divergence has an onset temperature of 55±15K, independent of whether the sample is in the superconducting or normal states. At slightly higher fluencies, still perturbative, the additional response does not exhibit this power law divergence. At quite high fluencies- no longer perturbative- the metallic samples exhibit oscillations in the reflectivity amplitude. The period of these oscillations varies with the probe wavelength but not with the pump wavelength. The oscillations exhibit a decay time as long as 10 nsec.

On the doping variation of the pseudogap in high-Tc cuprates

We briefly summarize well established and the latest experimental results concerning the doping dependence of the so-called pseudogap (T*) and emphasize that surface related techniques, like ARPES or tunneling, give somewhat higher value of T* as compared to the bulk techniques, like specific heat. In the underdoped regime most theoretical models differ in extrapolating the T*-line to zero doping. As underdoped cuprates are rather inhomogeneous and the disorder plays an important role, the complete microscopic theory will have to take into account all these subtle effects

Systematic studies of La 2-x Sr x CuO 4 in direct synchrotron light: on the role of compressive against tensile strain

We systematically study the structural and electronic properties of very thin cuprate films. Our direct angle resolved photoemission spectroscopy (ARPES) measurements on the low binding energy electronic structure of La2-xSrxCuO4 (LSCO) films confirmed that the Fermi surface evolves with doping, but changes even more significantly with growth-induced compressive strain. For a given doping, the in-plane compressive strain enhances TCs and modifies the 2-dimensional hole-like Fermi surface as to appear more electron-like. In contrast, the in-plane tensile strain reduces TC (suppressing superconductivity for huge tensile strain) and shows 3-dimensional ARPES dispersion with a corresponding 3-dimensional Fermi surface. To account for these striking changes in electronic structure and superconductivity, the out-of-plane states should be taken into account, as well as some subtle changes in the associated atomic distances.

Direct Arpes and TC Enhancement in Compressively Strained LSCO-214 Films

Direct ARPES studies on in-situ grown strained LSCO-214 films show striking strain effects on the electronic structure. The Fermi surface (FS) of LSCO-214 evolves with doping, yet changes even more significantly with strain. The compressive strain changes the Fermi surface topology from hole-like to electron-like and causes band dispersion along kx and the Fermi level crossing before the Brillouin zone boundary, in sharp contrast to the ‘usual’ flat band remaining ≈30meV below EF measured on unstrained samples. The associated reduction of the density of states does not diminish the superconductivity; rather, Tc is strongly enhanced (up to factor of two) in compressively strained thin films.