210 research outputs found
Negative Differential Resistance Induced by Mn Substitution at SrRuO3/Nb:SrTiO3 Schottky Interfaces
We observed a strong modulation in the current-voltage characteristics of
SrRuO/Nb:SrTiO Schottky junctions by Mn substitution in SrRuO,
which induces a metal-insulator transition in bulk. The temperature dependence
of the junction ideality factor indicates an increased spatial inhomogeneity of
the interface potential with substitution. Furthermore, negative differential
resistance was observed at low temperatures, indicating the formation of a
resonant state by Mn substitution. By spatially varying the position of the Mn
dopants across the interface with single unit cell control, we can isolate the
origin of this resonant state to the interface SrRuO layer. These results
demonstrate a conceptually different approach to controlling interface states
by utilizing the highly sensitive response of conducting perovskites to
impurities
Nanometer scale electronic reconstruction at the interface between LaVO3 and LaVO4
Electrons at interfaces, driven to minimize their free energy, are
distributed differently than in bulk. This can be dramatic at interfaces
involving heterovalent compounds. Here we profile an abrupt interface between V
3d2 LaVO3 and V 3d0 LaVO4 using electron energy loss spectroscopy. Although no
bulk phase of LaVOx with a V 3d1 configuration exists, we find a nanometer-wide
region of V 3d1 at the LaVO3/LaVO4 interface, rather than a mixture of V 3d0
and V 3d2. The two-dimensional sheet of 3d1 electrons is a prototypical
electronic reconstruction at an interface between competing ground states.Comment: 14 pages, 5 figure
Disorder Effects in the Bipolaron System TiO Studied by Photoemission Spectroscopy
We have performed a photoemission study of TiO around its two
transition temperatures so as to cover the metallic, high-temperature
insulating (bipolaron-liquid), and low-temperature insulating
(bipolaron-crystal) phases. While the spectra of the low-temperature insulating
phase show a finite gap at the Fermi level, the spectra of the high-temperature
insulating phase are gapless, which is interpreted as a soft Coulomb gap due to
dynamical disorder. We suggest that the spectra of the high-temperature
disordered phase of FeO, which exhibits a charge order-disorder
transition (Verwey transition), can be interpreted in terms of a Coulomb gap.Comment: 4 pages, 3 epsf figures embedde
Optically tuned dimensionality crossover in photocarrier-doped SrTiO: onset of weak localization
We report magnetotransport properties of photogenerated electrons in undoped
SrTiO single crystals under ultraviolet illumination down to 2 K. By tuning
the light intensity, the steady state carrier density can be controlled, while
tuning the wavelength controls the effective electronic thickness by modulating
the optical penetration depth. At short wavelengths, when the sheet conductance
is close to the two-dimensional Mott minimum conductivity we have observed
critical behavior characteristic of weak localization. Negative
magnetoresistance at low magnetic field is highly anisotropic, indicating
quasi-two-dimensional electronic transport. The high mobility of photogenerated
electrons in SrTiO allows continuous tuning of the effective electronic
dimensionality by photoexcitation.Comment: 7 pages, 7 figure
Integrable open spin chains from giant gravitons
We prove that in the presence of a maximal giant graviton state in N=4 SYM,
the states dual to open strings attached to the giant graviton give rise to an
PSU(2,2|4) open spin chain model with integrable boundary conditions in the
SO(6) sector of the spin chain to one loop order.Comment: 18 pages, 2 figures, uses JHEP
Two-Dimensional Confinement of 3d1 Electrons in LaTiO3/LaAlO3 Multilayers
We report spectroscopic ellipsometry measurements of the anisotropy of the
interband transitions parallel and perpendicular to the planes of
(LaTiO3)n(LaAlO3)5 multilayers with n = 1-3. These provide direct information
about the electronic structure of the two-dimensional (2D) 3d^1 state of the Ti
ions. In combination with LDA+U calculations, we suggest that 2D confinement in
the TiO2 slabs lifts the degeneracy of the t_{2g} states leaving only the
planar d_xy orbitals occupied. We outline that these multilayers can serve as a
model system for the study of the t_{2g} 2D Hubbard model.Comment: 7 pages, 4 figures. Accepted for publication in Phys. Rev. Let
Unusual Pseudogap-like Features Observed in Iron Oxypnictide Superconductors
We have performed a temperature-dependent angle-integrated laser
photoemission study of iron oxypnictide superconductors LaFeAsO:F and LaFePO:F
exhibiting critical transition temperatures (Tc's) of 26 K and 5 K,
respectively. We find that high-Tc LaFeAsO:F exhibits a temperature-dependent
pseudogap-like feature extending over ~0.1 eV about the Fermi level at 250 K,
whereas such a feature is absent in low-Tc LaFePO:F. We also find ~20-meV
pseudogap-like features and signatures of superconducting gaps both in
LaFeAsO:F and LaFePO:F. We discuss the possible origins of the unusual
pseudogap-like features through comparison with the high-Tc cuprates
Metal-insulator Crossover Behavior at the Surface of NiS_2
We have performed a detailed high-resolution electron spectroscopic
investigation of NiS and related Se-substituted compounds
NiSSe, which are known to be gapped insulators in the bulk at all
temperatures. A large spectral weight at the Fermi energy of the room
temperature spectrum, in conjunction with the extreme surface sensitivity of
the experimental probe, however, suggests that the surface layer is metallic at
300 K. Interestingly, the evolution of the spectral function with decreasing
temperature is characterized by a continuous depletion of the single-particle
spectral weight at the Fermi energy and the development of a gap-like structure
below a characteristic temperature, providing evidence for a metal-insulator
crossover behavior at the surfaces of NiS and of related compounds. These
results provide a consistent description of the unusual transport properties
observed in these systems.Comment: 12 pages, 3 figure
Serial optical coherence microscopy for label-free volumetric histopathology
The observation of histopathology using optical microscope is an essential procedure for examination of tissue biopsies or surgically excised specimens in biological and clinical laboratories. However, slide-based microscopic pathology is not suitable for visualizing the large-scale tissue and native 3D organ structure due to its sampling limitation and shallow imaging depth. Here, we demonstrate serial optical coherence microscopy (SOCM) technique that offers label-free, high-throughput, and large-volume imaging of ex vivo mouse organs. A 3D histopathology of whole mouse brain and kidney including blood vessel structure is reconstructed by deep tissue optical imaging in serial sectioning techniques. Our results demonstrate that SOCM has unique advantages as it can visualize both native 3D structures and quantitative regional volume without introduction of any contrast agents
Voxel-wise comparisons of cellular microstructure and diffusion-MRI in mouse hippocampus using 3D Bridging of Optically-clear histology with Neuroimaging Data (3D-BOND)
A key challenge in medical imaging is determining a precise correspondence between image properties and tissue microstructure. This comparison is hindered by disparate scales and resolutions between medical imaging and histology. We present a new technique, 3D Bridging of Optically-clear histology with Neuroimaging Data (3D-BOND), for registering medical images with 3D histology to overcome these limitations. Ex vivo 120 × 120 × 200 μm resolution diffusion-MRI (dMRI) data was acquired at 7 T from adult C57Bl/6 mouse hippocampus. Tissue was then optically cleared using CLARITY and stained with cellular markers and confocal microscopy used to produce high-resolution images of the 3D-tissue microstructure. For each sample, a dense array of hippocampal landmarks was used to drive registration between upsampled dMRI data and the corresponding confocal images. The cell population in each MRI voxel was determined within hippocampal subregions and compared to MRI-derived metrics. 3D-BOND provided robust voxel-wise, cellular correlates of dMRI data. CA1 pyramidal and dentate gyrus granular layers had significantly different mean diffusivity (p > 0.001), which was related to microstructural features. Overall, mean and radial diffusivity correlated with cell and axon density and fractional anisotropy with astrocyte density, while apparent fibre density correlated negatively with axon density. Astrocytes, axons and blood vessels correlated to tensor orientation
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