66 research outputs found
Double-Layer Buffer Template to Grow Commensurate Epitaxial BaBiO3 Thin Films
We propose a BaCeO3/BaZrO3 double-layer buffer template, grown on a SrTiO3 substrate, for epitaxial growth of a target oxide film with large lattice constants of over 4.1 . Lattice mismatch from the substrate was mostly accommodated for by a BaZrO3 arbitrating layer. Having an ideal in-plane lattice structure, BaCeO3 served as the main-buffer to grow the target material. We demonstrated commensurate epitaxy of BaBiO3 (BBO,a = 4.371 ) utilizing the new buffer template. Our results can be applied to heteroepitaxy and strain engineering of novel oxide materials of sizable lattice constants. © Author(s) 20161421sciescopu
Controlled manipulation of oxygen vacancies using nanoscale flexoelectricity
Oxygen vacancies, especially their distribution, are directly coupled to the
electromagnetic properties of oxides and related emergent functionalities that
have implication in device applications. Here using a homoepitaxial strontium
titanate thin film, we demonstrate a controlled manipulation of the oxygen
vacancy distribution using the mechanical force from a scanning probe
microscope tip. By combining Kelvin probe force microscopy imaging and
phase-field simulations, we show that oxygen vacancies can move under a
stress-gradient-induced depolarisation field. When tailored, this nanoscale
flexoelectric effect enables a controlled spatial modulation. In motion, the
scanning probe tip thereby deterministically reconfigures the spatial
distribution of vacancies. The ability to locally manipulate oxygen vacancies
on-demand provides a tool for the exploration of mesoscale quantum phenomena,
and engineering multifunctional oxide devices.Comment: 35 pages, Main text and the supplementary information combine
Universal bound to the amplitude of the vortex Nernst signal in superconductors
A liquid of superconducting vortices generates a transverse thermoelectric
response. This Nernst signal has a tail deep in the normal state due to
superconducting fluctuations. Here, we present a study of the Nernst effect in
two-dimensional hetero-structures of Nb-doped strontium titanate (STO) and in
amorphous MoGe. The Nernst signal generated by ephemeral Cooper pairs above the
critical temperature has the magnitude expected by theory in STO. On the other
hand, the peak amplitude of the vortex Nernst signal below is comparable
in both and in numerous other superconductors despite the large distribution of
the critical temperature and the critical magnetic fields. In four
superconductors belonging to different families, the maximum Nernst signal
corresponds to an entropy per vortex per layer of k.Comment: Accepted for publication in Phys. Rev. Let
Variation in superconducting transition temperature due to tetragonal domains in two-dimensionally doped SrTiO<sub>3</sub>
Strontium titanate is a low-temperature, non-Bardeen-Cooper-Schrieffer
superconductor that superconducts to carrier concentrations lower than in any
other system and exhibits avoided ferroelectricity at low temperatures. Neither
the mechanism of superconductivity in strontium titanate nor the importance of
the structure and dielectric properties for the superconductivity are well
understood. We studied the effects of twin structure on superconductivity in a
5.5-nm-thick layer of niobium-doped SrTiO embedded in undoped
SrTiO. We used a scanning superconducting quantum interference device
susceptometer to image the local diamagnetic response of the sample as a
function of temperature. We observed regions that exhibited a superconducting
transition temperature 10% higher than the temperature at
which the sample was fully superconducting. The pattern of these regions varied
spatially in a manner characteristic of structural twin domains. Our results
emphasize that the anisotropic dielectric properties of SrTiO are
important for its superconductivity, and need to be considered in any theory of
the mechanism of the superconductivity.Comment: 14 pages, 11 figures, Supplemental Information available at
http://stanford.edu/group/moler/papers/Noad_STOsuperconductivity_SI.pd
Superconductivity in (Ba,K)SbO3
Funding: This research was carried out in part due to funding from the Max Planck-UBC-UTokyo Centre for Quantum Materials. Part of the research described in this paper was performed at the Canadian Light Source, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation, the Natural Sciences and Engineering Research Council, the National Research Council, the Canadian Institutes of Health Research, the Government of Saskatchewan and the University of Saskatchewan. We thank the Science and Technology Facilities Council ISIS facility for the provision of beamtime.(Ba,K)BiO3 constitute an interesting class of superconductors, where the remarkably high superconducting transition temperature Tc of 30 K arises in proximity to charge density wave order. However, the precise mechanism behind these phases remains unclear. Here, enabled by high-pressure synthesis, we report superconductivity in (Ba,K)SbO3 with a positive oxygen–metal charge transfer energy in contrast to (Ba,K)BiO3. The parent compound BaSbO3−δ shows a larger charge density wave gap compared to BaBiO3. As the charge density wave order is suppressed via potassium substitution up to 65%, superconductivity emerges, rising up to Tc = 15 K. This value is lower than the maximum Tc of (Ba,K)BiO3, but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced charge density wave gap and superconductivity in (Ba,K)SbO3 indicates that strong oxygen–metal covalency may be more essential than the sign of the charge transfer energy in the main-group perovskite superconductors.Publisher PDFPeer reviewe
Quantum longitudinal and Hall transport at the LaAlO3/SrTiO3 interface at low electron densities
We examined the magneto-transport behavior of electrons confined at the
conducting LaAlO3/SrTiO3 interface in the low sheet carrier density regime. We
observed well resolved Shubnikov-de Haas quantum oscillations in the
longitudinal resistance, and a plateau-like structure in the Hall conductivity.
The Landau indices of the plateaus in the Hall conductivity data show spacing
close to 4, in units of the quantum of conductance. These experimental features
can be explained by a magnetic breakdown transition, which quantitatively
explains the area, structure, and degeneracy of the measured Fermi surface.Comment: 16 pages, 4 figures and 1 tabl
Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface
LaAlO3 and SrTiO3 are insulating, nonmagnetic oxides, yet the interface
between them exhibits a two-dimensional electron system with high electron
mobility,1 superconductivity at low temperatures,2-6 and electric-field-tuned
metal-insulator and superconductorinsulator phase transitions.3,6-8 Bulk
magnetization and magnetoresistance measurements also suggest some form of
magnetism depending on preparation conditions5,9-11 and suggest a tendency
towards nanoscale electronic phase separation.10 Here we use local imaging of
the magnetization and magnetic susceptibility to directly observe a landscape
of ferromagnetism, paramagnetism, and superconductivity. We find submicron
patches of ferromagnetism in a uniform background of paramagnetism, with a
nonuniform, weak diamagnetic superconducting susceptibility at low temperature.
These results demonstrate the existence of nanoscale phase separation as
suggested by theoretical predictions based on nearly degenerate interface
sub-bands associated with the Ti orbitals.12,13 The magnitude and temperature
dependence of the paramagnetic response suggests that the vast majority of the
electrons at the interface are localized, and do not contribute to transport
measurements.3,6,7 In addition to the implications for magnetism, the existence
of a 2D superconductor at an interface with highly broken inversion symmetry
and a ferromagnetic landscape in the background suggests the potential for
exotic superconducting phenomena.Comment: Edited version to appear in Nature Physic
An international collaborative evaluation of central serous chorioretinopathy: different therapeutic approaches and review of literature. The European Vitreoretinal Society central serous chorioretinopathy study
Purpose: To study and compare the efficacy of different therapeutic options for the treatment of central serous chorioretinopathy (CSCR). Methods: This is a nonrandomized, international multicentre study on 1719 patients (1861 eyes) diagnosed with CSCR, from 63 centres (24 countries). Reported data included different methods of treatment and both results of diagnostic examinations [fluorescein angiography and/or optical coherent tomography (OCT)] and best-corrected visual acuity (BCVA) before and after therapy. The duration of observation had a mean of 11 months but was extended in a minority of cases up to 7 years. The aim of this study is to evaluate the efficacy of the different therapeutic options of CSCR in terms of both visual (BCVA) and anatomic (OCT) improvement. Results: One thousand seven hundred nineteen patients (1861 eyes) diagnosed with CSCR were included. Treatments performed were nonsteroidal anti-inflammatory eye drops, laser photocoagulation, micropulse diode laser photocoagulation, photodynamic therapy (PDT; Standard PDT, Reduced-dose PDT, Reduced-fluence PDT), intravitreal (IVT) antivascular endothelial growth factor injection (VEGF), observation and other treatments. The list of the OTHERS included both combinations of the main proposed treatments or a variety of other treatments such as eplerenone, spironolactone, acetazolamide, beta-blockers, anti-anxiety drugs, aspirin, folic acid, methotrexate, statins, vitis vinifera extract medication and pars plana vitrectomy. The majority of the patients were men with a prevalence of 77%. The odds ratio (OR) showed a partial or complete resolution of fluid on OCT with any treatment as compared with observation. In univariate analysis, the anatomical result (improvement in subretinal fluid using OCT at 1 month) was favoured by age <60 years (p < 0.005), no previous observation (p < 0.0002), duration less than 3 months (p < 0.0001), absence of CSCR in the fellow eye (p = 0.04), leakage outside of the arcade (p = 0.05) and fluid height >500 \u3bcm (p = 0.03). The OR for obtaining partial or complete resolution showed that anti-VEGF and eyedrops were not statistically significant; whereas PDT (8.5), thermal laser (11.3) and micropulse laser (8.9) lead to better anatomical results with less variability. In univariate analysis, the functional result at 1 month was favoured by first episode (p = 0.04), height of subretinal fluid >500 \u3bcm (p < 0.0001) and short duration of observation (p = 0.02). Finally, there was no statistically significant difference among the treatments at 12 months. Conclusion: Spontaneous resolution has been described in a high percentage of patients. Laser (micropulse and thermal) and PDT seem to lead to significant early anatomical improvement; however, there is little change beyond the first month of treatment. The real visual benefit needs further clarification
Low-Dimensional Electronic Properties of Oxide Heterostructure
報告番号: ; 学位授与年月日: 2010-03-24 ; 学位の種別: 修士 ; 学位の種類: 修士(科学) ; 学位記番号: 修創域第3191号 ; 研究科・専攻: 新領域創成科学研究科物質系専
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