Three-dimensional electronic instabilities in polymerized solid A1C60

The low-temperature structure of A1C60 (A=K, Rb) is an ordered array of polymerized C60 chains, with magnetic properties that suggest a non-metallic ground state. We study the paramagnetic state of this phase using first-principles electronic-structure methods, and examine the magnetic fluctuations around this state using a model Hamiltonian. The electronic and magnetic properties of even this polymerized phase remain strongly three dimensional, and the magnetic fluctuations favor an unusual three-dimensional antiferromagnetically ordered structure with a semi-metallic electronic spectrum.Comment: REVTeX 3.0, 10 pages, 4 figures available on request from [email protected]

Multi-Orbital Hubbard Model in Infinite Dimensions: Quantum Monte Carlo Calculation

Using Quantum Monte Carlo we compute thermodynamics and spectra for the orbitally degenerate Hubbard model in infinite spatial dimensions. With increasing orbital degeneracy we find in the one-particle spectra: broader Hubbard bands (consistent with increased kinetic energy), a narrowing Mott gap, and increasing quasi-particle spectral weight. In opposition, Hund's rule exchange coupling decreases the critical on-site Coulomb energy for the Mott transition. The metallic regime resistivity for two-fold degeneracy is quadratic-in-temperature at low temperatures.Comment: 4 pages, 4 figures, to be published in PR

Identification and tunable optical coherent control of transition-metal spins in silicon carbide

Color centers in wide-bandgap semiconductors are attractive systems for quantum technologies since they can combine long-coherent electronic spin and bright optical properties. Several suitable centers have been identified, most famously the nitrogen-vacancy defect in diamond. However, integration in communication technology is hindered by the fact that their optical transitions lie outside telecom wavelength bands. Several transition-metal impurities in silicon carbide do emit at and near telecom wavelengths, but knowledge about their spin and optical properties is incomplete. We present all-optical identification and coherent control of molybdenum-impurity spins in silicon carbide with transitions at near-infrared wavelengths. Our results identify spin $S=1/2$ for both the electronic ground and excited state, with highly anisotropic spin properties that we apply for implementing optical control of ground-state spin coherence. Our results show optical lifetimes of $\sim$60 ns and inhomogeneous spin dephasing times of $\sim$0.3 $\mu$s, establishing relevance for quantum spin-photon interfacing.Comment: Updated version with minor correction, full Supplementary Information include

Inter-site Coulomb interaction and Heisenberg exchange

Based on exact diagonalization results for small clusters we discuss the effect of inter-site Coulomb repulsion in Mott-Hubbard or charge transfer insulators. Whereas the exchange constant J for direct exchange is substantially enhanced by inter-site Coulomb interaction, that for superexchange is suppressed. The enhancement of J in the single-band models holds up to the critical value for the charge density wave (CDW) instability, thus opening the way for large values of J. Single-band Hubbard models with sufficiently strong inter-site repulsion to be near a CDW instability thus may provide `physical' realizations of t-J like models with the `unphysical' parameter ratio J/t=1.Comment: Revtex file, 4 PRB pages, with 5 embedded ps-files. To appear in PRB, rapid communications. Hardcopies of figures or the entire manuscript may also be obtained by e-mail request to: [email protected]

Chemical communication: does odor plume shape matter?

Many insects use chemical information to gather information about their environment. Infochemicals are spread into the environment as the wind disperses the odor molecules from the source. The structure of an odor plume around a food source is complex and time-dependent. At a large scale, it meanders as it moves with the wind. At a smaller scale, patches with odors are interspersed with regions of clean air. In this study, we compare a plume model that takes the features of a real odor plume into account, a so-called filamentous plume model, with a simplified, time-averaged model, which is commonly used in the literature, and we investigate by simulation their effect on a modeled fruit fly population. During foraging Drosophila melanogaster is attracted to food odors and its aggregation pheromone. Ample knowledge on the attraction to these infochemicals in an experimental set-up exist in the literature. The comparison of the plumes in a simulation study clearly showed that the filamentous plume attracted more fruit flies towards the source than the time-averaged plume. We discuss the results in the light of experimental findings

Detection of retained microbubbles in carotid arteries with real-time low mechanical index imaging in the setting of endothelial dysfunction

ObjectivesWe sought to determine if intravenously injected microbubbles would be retained by the carotid arteries (CAs) in the setting of endothelial dysfunction (ED) using a linear transducer equipped with a low mechanical index pulse sequence scheme (PSS).BackgroundMicrobubbles normally pass freely through large and small vessels but are retained in regions with ED. New high-frequency low mechanical index PSS can potentially be utilized to image these retained microbubbles.MethodsIntravenous albumin- and lipid-encapsulated microbubbles were administered in seven pigs while imaging the CAs before and after a 20% intralipid infusion to induce hypertriglyceridemia. The degree of microbubble retention was quantified by measuring endothelial acoustic intensity (AI) after clearance of free-flowing microbubbles. Microbubble adherence was also evaluated after selective balloon injury of the CAs. The CA diameter responses to acetylcholine were quantified.ResultsAfter induction of hypertriglyceridemia, adherence of albumin-encapsulated microbubbles was visually evident in all CAs, and endothelial AI increased significantly (p < 0.001 compared with baseline). The CA responses to acetylcholine went from vasodilation at baseline to vasoconstriction during hypertriglyceridemia. Endothelial AI also increased in the balloon-stretched vessels (p < 0.01 compared with uninjured vessels) after albumin-encapsulated microbubble injection, with a ring of microbubbles selectively adhering to the injured segment. This retention was not observed with lipid-encapsulated microbubbles. Scanning electron microscopy confirmed that albumin-coated microbubbles adhered to endothelial cells.ConclusionsRetention of intravenously injected albumin microbubbles occurs in the setting of both global and regional ED in large vessels and can be noninvasively imaged with high-frequency low mechanical index PSS

Electronic Structure of Superconducting Ba6c60

We report the results of first-principles electronic-structure calculations for superconducting Ba6C60. Unlike the A3C60 superconductors, this new compound shows strong Ba-C hybridization in the valence and conduction regions, mixed covalent/ionic bonding character, partial charge transfer, and insulating zero-gap band structure.Comment: 11 pages + 4 figures (1 appended, others on request), LaTeX with REVTE

Minimising the impact of disturbances in future highly-distributed power systems

It is expected that future power systems will require radical distributed control approaches to accommodate the significant expansion of renewable energy sources and other flexible grid devices. It is important to rapidly and efficiently respond to disturbances by, for example: utilising adaptive, wide-area protection schemes; proactive control of available grid resources (such as managing the fault level contribution from converter-interfaced generation) to optimise protection functionality; and taking post-fault action to ensure protection stability and optimal system operation. This paper analyses and highlights the protection functions which will be especially important to minimising the impact of disturbances in future power systems. These functions include: fast-acting wide-area protection methods using Phasor Measurement Units (PMUs); adaptive and “self-organising” protection under varying system conditions; protection with distributed Intelligent Electronic Devices (IEDs); enhanced fault ride-through; and pattern recognition based schemes. In particular, the paper illustrates how the increased availability of measurements and communications can enable improved protection functionality within distribution systems, which is especially important to accommodate the connection of highly-distributed generation at medium- and low-voltages