Electronic phase diagram of Cr-doped VO2 epitaxial films studied by in situ photoemission spectroscopy

Through in situ photoemission spectroscopy (PES), we investigated the changes in the electronic structure of Cr-doped VO2 films coherently grown on TiO2 (001) substrates. The electronic phase diagram of CrxV1-xO2 is drawn by a combination of electric and spectroscopic measurements. The phase diagram is similar to that of bulk CrxV1-xO2, while the temperature of metal-insulator transition (TMIT) is significantly suppressed by the epitaxial strain effect. In the range of x = 0-0.04, where TMIT remains unchanged as a function of x, the PES spectra show dramatic change across TMIT, demonstrating the characteristic spectral changes associated with the Peierls phenomenon. In contrast, for x > 0.04, the TMIT linearly increases, and the metal-insulator transition (MIT) may disappear at x = 0.08-0.12. The PES spectra at x = 0.08 exhibit pseudogap behavior near the Fermi level, whereas the characteristic temperature-induced change remains almost intact, suggesting the existence of local V-V dimerization. The suppression of V-V dimerization with increasing x was confirmed by polarization-dependent x-ray absorption spectroscopy. These spectroscopic investigations reveal that the energy gap and V 3d states are essentially unchanged with 0 $\le$ x $\le$ 0.08 despite the suppression of V-V dimerization. The invariance of the energy gap with respect to x suggests that the MIT in CrxV1-xO2 arises primarily from the strong electron correlations, namely the Peierls-assisted Mott transition. Meanwhile, the pseudogap at x = 0.08 eventually evolves to a full gap (Mott gap) at x = 0.12, which is consistent with the disappearance of the temperature-dependent MIT in the electronic phase diagram. These results demonstrate that a Mott insulating phase without V-V dimerization is stabilized at x > 0.08 as a result of the superiority of Mott instability over the Peierls one.Comment: 27 pages, 3 main figures, 5 supplementary figures. arXiv admin note: text overlap with arXiv:2005.0030

Built-in and induced polarization across LaAlO3_3/SrTiO3_3 heterojunctions

Ionic crystals terminated at oppositely charged polar surfaces are inherently unstable and expected to undergo surface reconstructions to maintain electrostatic stability. Essentially, an electric field that arises between oppositely charged atomic planes gives rise to a built-in potential that diverges with thickness. In ultra thin film form however the polar crystals are expected to remain stable without necessitating surface reconstructions, yet the built-in potential has eluded observation. Here we present evidence of a built-in potential across polar \lao ~thin films grown on \sto ~substrates, a system well known for the electron gas that forms at the interface. By performing electron tunneling measurements between the electron gas and a metallic gate on \lao ~we measure a built-in electric field across \lao ~of 93 meV/\AA. Additionally, capacitance measurements reveal the presence of an induced dipole moment near the interface in \sto, illuminating a unique property of \sto ~substrates. We forsee use of the ionic built-in potential as an additional tuning parameter in both existing and novel device architectures, especially as atomic control of oxide interfaces gains widespread momentum.Comment: 6 pages, 4 figures. Submitted to Nature physics on May 1st, 201

Presynaptic partner selection during retinal circuit reassembly varies with timing of neuronal regeneration in vivo

Whether neurons can restore their original connectivity patterns during circuit repair is unclear. Taking advantage of the regenerative capacity of zebrafish retina, we show here the remarkable specificity by which surviving neurons reassemble their connectivity upon regeneration of their major input. H3 horizontal cells (HCs) normally avoid red and green cones, and prefer ultraviolet over blue cones. Upon ablation of the major (ultraviolet) input, H3 HCs do not immediately increase connectivity with other cone types. Instead, H3 dendrites retract and re-extend to contact new ultraviolet cones. But, if regeneration is delayed or absent, blue-cone synaptogenesis increases and ectopic synapses are made with red and green cones. Thus, cues directing synapse specificity can be maintained following input loss, but only within a limited time period. Further, we postulate that signals from the major input that shape the H3 HC's wiring pattern during development persist to restrict miswiring after damage

Control of electronic conduction at an oxide heterointerface using surface polar adsorbates

The transfer of electrons between a solid surface and adsorbed atomic or molecular species is fundamental in natural and synthetic processes, being at the heart of most catalytic reactions and many sensors. In special cases, metallic conduction can be induced at the surface of, for example, Si-terminated SiC1, or mixed-terminated ZnO2, in the presence of a hydrogen adlayer. Generally, only the surface atoms are significantly affected by adsorbates. However, remotely changing electronic states far from the adsorbed layer is possible if these states are electrostatically coupled to the surface. Here we show that the surface adsorption of common solvents such as acetone, ethanol, and water can induce a large change (factor of three) in the conductivity at the buried interface between SrTiO3 substrates and LaAlO3 thin films3-8. This phenomenon is observed only for polar solvents. Our result provides experimental evidence that adsorbates at the LaAlO3 surface induce accumulation of electrons at the LaAlO3/SrTiO3 interface, suggesting a general polarization-facilitated electronic transfer mechanism, which can be used for sensor applications.Comment: 14 pages, 4 figure

Intracerebroventricular Administration of Neuropeptide Y Induces Hepatic Insulin Resistance via Sympathetic Innervation

OBJECTIVE—We recently showed that intracerebroventricular infusion of neuropeptide Y (NPY) hampers inhibition of endogenous glucose production (EGP) by insulin in mice. The downstream mechanisms responsible for these effects of NPY remain to be elucidated. Therefore, the aim of this study was to establish whether intracerebroventricular NPY administration modulates the suppressive action of insulin on EGP via hepatic sympathetic or parasympathetic innervation