Gertjan Koster

Misfit strain accommodation in epitaxial ABO3 perovskites: Lattice rotations and lattice modulations

We present a study of the lattice response to the compressive and tensile biaxial stress in La0.67Sr0.33MnO3 (LSMO) and SrRuO3 (SRO) thinfilms grown on a variety of single-crystal substrates: SrTiO3,DyScO3, NdGaO3, and (La,Sr)(Al,Ta)O3. The results show that, in thin films under misfit strain, both SRO and LSMO lattices, which in bulk form have orthorhombic (SRO) and rhombohedral (LSMO) structures, assume unit cells that are monoclinic under compressive stress and tetragonal under tensile stress. The applied stress effectively modifies the BO6 octahedra rotations, whose degree and direction can be controlled by the magnitude and sign of the misfit strain. Such lattice distortions change the B-O-B bond angles and therefore are expected to affect magnetic and electronic properties of the ABO3 perovskites.

Dependence of the electronic structure of SrRuO3 and its degree of correlation on cation off-stoichiometry

We have grown and studied high quality SrRuO{sub 3} films grown by MBE as well as PLD. By changing the oxygen activity during deposition we were able to make SrRuO{sub 3} samples that were stoichiometric (low oxygen activity) or with ruthenium vacancies (high oxygen activity). Samples with strontium vacancies were found impossible to produce since the ruthenium would precipitate out as RuO{sub 2}. The volume of the unit cell of SrRuO{sub 3} becomes larger as more ruthenium vacancies are introduced. The residual resistivity ratio (RRR) and room temperature resistivity were found to systematically depend on the volume of the unit cell and therefore on the amount of ruthenium vacancies. The RRR varied from {approx}30 for stoichiometric samples to less than two for samples that were very ruthenium poor. The room temperature resistivity varied from 190 {micro}{Omega} cm for stoichoimetric samples to over 300 {micro}{Omega} cm for very ruthenium poor samples. UPS spectra show a shift of weight from the coherent peak to the incoherent peak around the Fermi level when samples have more ruthenium vacancies. Core level XPS spectra of the ruthenium 3d lines show a strong screened part in the case of stoichiometric samples. This screened part disappears whenmorexa0» ruthenium vacancies are introduced. Both the UPS and the XPS results are consistent with the view that correlation increases as the amount of ruthenium vacancies increase.«xa0less

Experimental investigation of electronic properties of buried heterointerfaces of LaAlO3 on SrTiO3

We have made very thin films of LaAlO3 on TiO2 terminated SrTiO3 and have measured the properties of the resulting interface in various ways. Transport measurements show a maximum sheet carrier density of 1016 cm-2 and a mobility around 104 cm2 V-1 s-1. In situ ultraviolet photoelectron spectroscopy (UPS) indicates that for these samples a finite density of states exists at the Fermi level. From the oxygen pressure dependence measured in both transport as well as the UPS, we detail, as reported previously by us, that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process used to make the heterointerfaces. Under the conditions studied the effect of LaAlO3 on the carrier density is found to be minimal.

Use of real-time Fourier transform infrared reflectivity as an in situ monitor of YBa2Cu3O7 film deposition and processing

Fourier transform infrared reflectivity (FTIR) has been utilized as an in situ, real-time diagnostic during high-rate, e-beam deposition of YBa2Cu3O7 (YBCO). The results demonstrate the great utility of FTIR as an in situ monitor of YBCO deposition and postdeposition processing. They also point to a completely different picture of the nature and associated kinetic and thermodynamic pathways of the high-rate, e-beam process, both during deposition and as a function of subsequent trajectories through temperature and oxygen pressure space.

Preparation and properties of amorphousMgB2∕MgOsuperstructures: Model disordered superconductor

In this paper, we introduce a unique method for fabricating MgB2/MgO multilayers and demonstrate the potential for using them as a new model for disordered superconductors. In this approach, we control the annealing of the MgB2 to yield an interesting new class of disordered (amorphous) superconductors with relatively high transition temperatures. The multilayers appear to exhibit quasi-two-dimensional superconductivity with controlled anisotropy. We discuss the properties of the multilayers as the thickness of the components of the bilayers vary.

Discovering superconductors: A path to new science and higher Tc's

The search for superconductivity in new and unexpected structures has been ongoing since the initial discovery in Leiden over 9 decades ago. Though the successes are few the rewards are great. Our meeting here today is a direct result of Bednorz and Muellers discovery of cuprate superconductivity [1]. The questions which have arisen as a result of this single discovery have uncovered inadequacies of theory and stimulated new ways of thinking. Understanding the mechanism(s) of high temperature superconductivity is among the foremost challenges of theoretical and experimental research today [2]. Searching for new superconductors has always been a fruitful research enterprise, and as we see, there are new opportunities for doing so today. For more than 4 decades after the initial discovery there was no microscopic theory (the most outstanding theorists from Heisenberg down tried and failed to come up with a satisfactory theory) and the experimental basis for understanding the underlying mechanisms was inadequate. It must have been a surprise for Kamerlingh Onnes, after taking care to use the purest Hg he could obtain in the investigation that led to the discovery of superconductivity, to find that ordinary solder was also superconducting. In 1932 Meissner discovered barely metallic copper sulfide was superconducting, while high conductivity copper itself was not superconducting. These puzzles and others like it suggested that a comprehensive search for new superconductors might reveal a pattern of occurrence that would reveal clues, and prompted John Hulm and Bernd Matthias, with encouragement from Enrico Fermi [3] in 1951 to undertake a full-scale effort to find new superconductors. This was a propitious time for such an undertaking for a number of reasons. Today parallel reasons exist.