Galactic cosmic rays (GCRs) are affected by solar modulation while they propagate through the heliosphere. The study of the time variation of GCR spectra observed at Earth can shed light on the ...underlying physical processes, specifically diffusion and particle drifts. Recently, the AMS-02 experiment measured with very high accuracy the time variation of the cosmic ray proton and helium flux between May 2011 and May 2017 in the rigidity range from 1 to 60 GV. In this work, a comprehensive three-dimensional (3D) steady-state numerical model is used to solve Parker's transport equation and is used to reproduce the monthly proton fluxes observed by AMS-02. We find that the rigidity slope of the perpendicular mean free path above 4 GV remains constant, while below 4 GV it increases during solar maximum. Assuming the same mean free paths for helium and protons, the models are able to reproduce the time behavior of the p/He ratio observed by AMS-02. The dependence of the diffusion tensor on the particle mass-to-charge ratio, A/Z, is found to be the main cause of the time dependence of p/He below 3 GV.
Electronic flat bands in momentum space, arising from strong localization of electrons in real space, are an ideal stage to realize strongly-correlated phenomena. Theoretically, the flat bands can ...naturally arise in certain geometrically frustrated lattices, often with nontrivial topology if combined with spin-orbit coupling. Here, we report the observation of topological flat bands in frustrated kagome metal CoSn, using angle-resolved photoemission spectroscopy and band structure calculations. Throughout the entire Brillouin zone, the bandwidth of the flat band is suppressed by an order of magnitude compared to the Dirac bands originating from the same orbitals. The frustration-driven nature of the flat band is directly confirmed by the chiral d-orbital texture of the corresponding real-space Wannier functions. Spin-orbit coupling opens a large gap of 80 meV at the quadratic touching point between the Dirac and flat bands, endowing a nonzero Z
invariant to the flat band. These findings demonstrate that kagome-derived flat bands are a promising platform for novel emergent phases of matter at the confluence of strong correlation and topology.
A kagome lattice of 3d transition metal ions is a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic ground states. However, the ...paradigmatic states of the idealized two-dimensional kagome lattice-Dirac fermions and flat bands-have not been simultaneously observed. Here, we use angle-resolved photoemission spectroscopy and de Haas-van Alphen quantum oscillations to reveal coexisting surface and bulk Dirac fermions as well as flat bands in the antiferromagnetic kagome metal FeSn, which has spatially decoupled kagome planes. Our band structure calculations and matrix element simulations demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals, and evidence that the coexisting Dirac surface state realizes a rare example of fully spin-polarized two-dimensional Dirac fermions due to spin-layer locking in FeSn. The prospect to harness these prototypical excitations in a kagome lattice is a frontier of great promise at the confluence of topology, magnetism and strongly correlated physics.
TRPV1 channels are an important class of membrane proteins that play an integral role in the regulation of intracellular cations such as calcium in many different tissue types. The anionic ...phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) is a known positive modulator of TRPV1 channels and the negatively charged phosphate groups interact with several basic amino acid residues in the proximal C-terminal TRP domain of the TRPV1 channel. We and other groups have shown that physiological sub-micromolar levels of long-chain acyl CoAs (LC-CoAs), another ubiquitous anionic lipid, can also act as positive modulators of ion channels and exchangers. Therefore, we investigated whether TRPV1 channel activity is similarly regulated by LC-CoAs. Our results show that LC-CoAs are potent activators of the TRPV1 channel and interact with the same PIP2-binding residues in TRPV1. In contrast to PIP2, LC-CoA modulation of TRPV1 is independent of Ca2+i, acting in an acyl side-chain saturation and chain-length dependent manner. Elevation of LC-CoAs in intact Jurkat T-cells leads to significant increases in agonist-induced Ca2+i levels. Our novel findings indicate that LC-CoAs represent a new fundamental mechanism for regulation of TRPV1 channel activity that may play a role in diverse cell types under physiological and pathophysiological conditions that alter fatty acid transport and metabolism such as obesity and diabetes.
We investigate the effects of external dielectric screening on the electronic dispersion and the band gap in the atomically thin, quasi-two-dimensional (2D) semiconductor WS_{2} using angle-resolved ...photoemission and optical spectroscopies, along with first-principles calculations. We find the main effect of increased external dielectric screening to be a reduction of the quasiparticle band gap, with rigid shifts to the bands themselves. Specifically, the band gap of monolayer WS_{2} is decreased by about 140 meV on a graphite substrate as compared to a hexagonal boron nitride substrate, while the electronic dispersion of WS_{2} remains unchanged within our experimental precision of 17 meV. These essentially rigid shifts of the valence and conduction bands result from the special spatial structure of the changes in the Coulomb potential induced by the dielectric environment of the monolayer.
In animals, adverse early experience alters oxytocinergic and glucocorticoid activity and maternal behavior in adulthood. This preliminary study explored associations among childhood trauma (loss of ...a parent or sexual abuse in childhood), maternal self-efficacy, and leukocyte gene expression (mRNA) of oxytocin and glucocorticoid receptors (OXTR and NR3C1) in mothers of infants.
62 mothers (20 with early life trauma) with healthy 3-month old infants reported maternal self-efficacy, depression, infant temperament, and overall social support; the effects of early trauma on these measures were assessed. Of these, 35 mothers (14 with early trauma) underwent blood draws after 2 infant feeding times; their OXTR and NR3C1 mRNA was compared to a control group of 25 no-infant women without early trauma, and also was examined for associations with self-efficacy.
OXTR mRNA was increased in mothers of infants versus no-infant controls (p < 0.0003), and mothers with greatest prior maternal experience had higher OXTR than those with less experience (0–2 vs. 3+ older children, p < 0.033). Mothers with early trauma and less maternal experience had lower OXTR mRNA than no-trauma mothers (p < 0.029) and lower NR3C1 mRNA than controls (p < 0.004). Mothers with depression also had lower NR3C1 than other mothers (p < 0.003) but did not differ in OXTR. Mothers with early trauma also reported their support network to be less helpful and more upsetting and unpredictable than other mothers (p < 0.035–p < 0.005). Regarding maternal behavior, in mothers with early trauma, helpful support networks increased self-reported nurturing self-efficacy when babies were not fussy but decreased it with fussy babies (p < 0.05). Support was unrelated to self-efficacy in no-trauma mothers. Similarly, among mothers with low OXTR or NR3C1 (−1SD, most having early trauma and lower maternal experience), greater support was associated with lower self-efficacy (p < 0.05), while mothers with high OXTR or NR3C1 (+1SD) tended to have higher self-efficacy with greater support.
These preliminary findings need confirmation in a larger sample but suggest that childhood trauma influences maternal behavior and both OXTR and NR3C1 pathways in mothers of infants, and that both depression and prior maternal experience may be other important factors. Effects on maternal behavior appear to require more complex modeling.
•OXTR mRNA was increased in mothers of infants vs. no-infant controls.•Mother who experienced early life trauma had lower OXTR mRNA than mothers with no early trauma and lower NR3C1 mRNA than no-infant controls.•Mothers with depression had lower NR3C1 mRNA than non-depressed mothers.•Mothers who experienced early life trauma evaluated their support networks more negatively than mothers without early life trauma.
Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and ...integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, single-crystal 2D gallium, indium and tin that are stabilized at the interface of epitaxial graphene and silicon carbide. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to the graphene overlayer; that is, they are 'half van der Waals' metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals offer compelling opportunities for superconducting devices, topological phenomena and advanced optoelectronic properties. For example, the reported 2D Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly free-electron Fermi surface that closely approaches the Dirac points of the graphene overlayer.
We demonstrate a facile method to improve upconversion quantum yields in Yb,Er-based nanoparticles via emission dye-sensitization. Using the commercially available dye ATTO 542, chosen for its high ...radiative rate and significant spectral overlap with the green emission of Er3+, we decorate the surfaces of sub-25 nm hexagonal-phase Na(Y/Gd/Lu)0.8F4:Yb0.18Er0.02 upconverting nanoparticles with varying dye concentrations. Upconversion photoluminescence and absorption spectroscopy provide experimental confirmation of energy transfer to and emission from the dye molecules. Upconversion quantum yield is observed to increase with dye sensitization, with the highest enhancement measured for the smallest particles investigated (10.9 nm in diameter); specifically, these dye-decorated particles are more than 2× brighter than are unmodified, organic-soluble nanoparticles and more than 10× brighter than are water-soluble nanoparticles. We also observe 3× lifetime reductions with dye adsorption, confirming the quantum yield enhancement to result from the high radiative rate of the dye. The approach detailed in this work is widely implementable, renders the nanoparticles water-soluble, and most significantly improves sub-15 nm nanoparticles, making our method especially attractive for biological imaging applications.
Abstract
Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials. In situations where charge ...carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get “dressed", which leads to the formation of polaronic quasiparticles. The exploration of polaronic effects on low-energy excitations is in its infancy in two-dimensional materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of single-layer WS
2
. By using micro-focused angle-resolved photoemission spectroscopy during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the single-layer WS
2
conduction band minimum. Our results are explained by an effective many-body model in terms of a coupling between single-layer WS
2
conduction electrons and an interlayer plasmon mode. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides.
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