In recent years, well-defined and nearly perfect single crystal surfaces of oxide perovskites have become increasingly important. A single terminated surface is a prerequisite for reproducible thin film growth and fundamental growth studies. In this work, atomic and lateral force microscopy have been used to display different terminations of SrTiO3. We observe hydroxylation of the topmost SrO layer after immersion of SrTiO3 in water, which is used to enhance the etch-selectivity of SrO relative to TiO2 in a buffered HF solution. We reproducibly obtain perfect and single terminated surfaces, irrespective of the initial state of polished surfaces and the pH value of the HF solution. This approach to the problem might be used for a variety of multi-component oxide single crystals. True two-dimensional reflection high-energy electron diffraction intensity oscillations are observed during homo epitaxial growth using pulsed laser deposition on these surfaces.

A theoretical model for quasiparticle and Josephson tunneling in multiband superconductors is developed and applied to MgB2-based junctions. The gap functions in different bands in MgB2 are obtained from an extended Eliashberg formalism, using the results of band structure calculations. The temperature and angle dependencies of MgB2 tunneling spectra and the Josephson critical current are calculated. The conditions for observing one or two gaps are given. We argue that the model may help to settle the current debate concerning two-band superconductivity in MgB2. PACS numbers: 74.50.+r, 74.70.Ad, 74.80.Fp, 85.25.Cp Soon after the discovery of superconductivity in MgB 2 , 1 first principle calculations were performed to determine the electronic structure of this material. It was found that the Fermi surface consists of two three-dimensional sheets, from the π bonding and antibonding bands, and two nearly cylindrical sheets from the two-dimensional σ bands.2,3,4,5 The multiband picture has given rise to the concept that two superconducting energy gaps can coexist 6,7 in MgB 2 . Two-band superconductivity is a phenomenon that has been observed in Nb doped SrTiO 3 .8 Recent experimental STM and point-contact spectroscopy, 9,10,11 high-resolution photoemission spectroscopy, 12 Raman spectroscopy, 13 specific heat measurements 14 and muon-spin-relaxation studies of the magnetic penetration depth 15 support the concept of a double gap in MgB 2 (see Ref.16 for a review of experiments). However, there is an ambiguity in the interpretation of point-contact data concerning the existence of two gaps. 9,10,11 Moreover, some tunneling measurements 17 show only one gap with a magnitude smaller than the BCS value of ∆ = 1.76 k B T c .In order to resolve this discrepancy, we address the question, how multiband superconductivity will manifest itself in tunneling. We present the theoretical model for quasiparticle and Josephson tunneling in MgB 2 -based junctions. Using the results of band-structure calculations, we apply an extended Eliashberg formalism to obtain the gap functions in different bands, taking strong coupling effects into account. Tunneling from a normal metal (N) into MgB 2 is considered in an extended Blonder-Tinkham-Klapwijk (BTK) model. 18 The temperature dependencies and absolute values of the I c R N product (I c is the critical current and R N is the normal state resistance) are calculated in MgB 2 -based SIS tunnel junctions, where S denotes a superconductor and I an insulator. Tunneling in the direction of the a-b plane, in the c-axis direction and under arbitrary angle is considered. Furthermore, the Josephson supercurrent between a single-gap superconductor and MgB 2 is calculated.According to the labeling of Liu et al., 6 the four Fermi surface sheets in MgB 2 are grouped into quasi-two-dimensional σ bands and three-dimensional π bands. Hence, normal and superconducting properties of MgB 2 can be described by an effective two-band model. Within this model, Liu et al.6 estimated the coupling constants a...

The phase of the macroscopic electron-pair wavefunction in a superconductor can vary only by multiples of 2pi when going around a closed contour. This results in quantization of magnetic flux, one of the most striking demonstrations of quantum phase coherence in superconductors. By using superconductors with unconventional pairing symmetry, or by incorporating pi-Josephson junctions, a phase shift of pi can be introduced in such loops. Under appropriate conditions, this phase shift results in doubly degenerate time-reversed ground states, which are characterized by the spontaneous generation of half quanta of magnetic flux, with magnitude 1/2 Phi(0)(Phi(0) = h/2e = 2.07 x 10(-15) Wb) (ref. 7). Until now, it has only been possible to generate individual half flux quanta. Here we report the realization of large-scale coupled pi-loop arrays based on YBa2Cu3O7-Au-Nb Josephson contacts. Scanning SQUID (superconducting quantum interference device) microscopy has been used to study the ordering of half flux quanta in these structures. The possibility of manipulating the polarities of individual half flux quanta is also demonstrated. These pi-loop arrays are of interest as model systems for studying magnetic phenomena--including frustration effects--in Ising antiferromagnets. Furthermore, studies of coupled pi-loops can be useful for designing quantum computers based on flux-qubits with viable quantum error correction capabilities.

A suitable in situ monitoring technique for growth of thin films is reflection high energy electron diffraction (RHEED). Deposition techniques, like pulsed laser deposition (PLD) and sputter deposition, used for fabrication of complex oxide thin films use relatively high oxygen pressures (up to 100 Pa) and are, therefore, not compatible with ultrahigh vacuum RHEED equipment. We have developed a RHEED system which can be used for growth monitoring during the deposition of complex oxides at standard PLD conditions. We are able to increase the deposition pressure up to 50 Pa using a two-stage differential pumping system. Clear RHEED patterns are observable at these high pressures. The applicability of this system is demonstrated with the study of homoepitaxial growth of SrTiO3 as well as the heteroepitaxial growth of YBa2Cu3O7−δ on SrTiO3. Intensity oscillations of the RHEED reflections, indicating two-dimensional growth, are observed up to several tens of nanometers film thickness in both cases.

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