Photoemission study of the electronic structure of CdTe

Photoemission study of electronic structure of cadmium telluride single crystal

Photoemission studies of the electronic structure of CdTe, CdSe, and CdS

Electronic structure photoemission studies of cadmium telluride, cadmium selenide, and cadmium sulfid

Super-Alfv\'enic propagation of reconnection signatures and Poynting flux during substorms

The propagation of reconnection signatures and their associated energy are examined using kinetic particle-in-cell simulations and Cluster satellite observations. It is found that the quadrupolar out-of-plane magnetic field near the separatrices is associated with a kinetic Alfv\'en wave. For magnetotail parameters, the parallel propagation of this wave is super-Alfv\'enic (V_parallel ~ 1500 - 5500 km/s) and generates substantial Poynting flux (S ~ 10^-5 - 10^-4 W/m^2) consistent with Cluster observations of magnetic reconnection. This Poynting flux substantially exceeds that due to frozen-in ion bulk outflows and is sufficient to generate white light aurora in the Earth's ionosphere.Comment: Submitted to PRL on 11/1/2010. Resubmitted on 4/5/201

On the Cause of Supra-Arcade Downflows in Solar Flares

A model of supra-arcade downflows (SADs), dark low density regions also known as tadpoles that propagate sunward during solar flares, is presented. It is argued that the regions of low density are flow channels carved by sunward-directed outflow jets from reconnection. The solar corona is stratified, so the flare site is populated by a lower density plasma than that in the underlying arcade. As the jets penetrate the arcade, they carve out regions of depleted plasma density which appear as SADs. The present interpretation differs from previous models in that reconnection is localized in space but not in time. Reconnection is continuous in time to explain why SADs are not filled in from behind as they would if they were caused by isolated descending flux tubes or the wakes behind them due to temporally bursty reconnection. Reconnection is localized in space because outflow jets in standard two-dimensional reconnection models expand in the normal (inflow) direction with distance from the reconnection site, which would not produce thin SADs as seen in observations. On the contrary, outflow jets in spatially localized three-dimensional reconnection with an out-of-plane (guide) magnetic field expand primarily in the out-of-plane direction and remain collimated in the normal direction, which is consistent with observed SADs being thin. Two-dimensional proof-of-principle simulations of reconnection with an out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion regions and the necessity of density stratification. Three-dimensional simulations confirm that localized reconnection remains collimated.Comment: 16 pages, 5 figures, accepted to Astrophysical Journal Letters in August, 2013. This version is the accepted versio

From Solar and Stellar Flares to Coronal Heating: Theory and Observations of How Magnetic Reconnection Regulates Coronal Conditions

There is currently no explanation of why the corona has the temperature and density it has. We present a model which explains how the dynamics of magnetic reconnection regulates the conditions in the corona. A bifurcation in magnetic reconnection at a critical state enforces an upper bound on the coronal temperature for a given density. We present observational evidence from 107 flares in 37 sun-like stars that stellar coronae are near this critical state. The model may be important to self-organized criticality models of the solar corona.Comment: 13 pages, 2 figures, accepted to Ap. J. Lett., February 200

Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence

Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta (ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic shear (strong guide field). Changing the simulation domain size, we find that the ion response varies greatly. For reconnecting regions with scales comparable to the ion Larmor radius, the ions do not respond to the reconnection dynamics leading to ''electron-only'' reconnection with very large quasi-steady reconnection rates. The transition to more traditional ''ion-coupled'' reconnection is gradual as the reconnection domain size increases, with the ions becoming frozen-in in the exhaust when the magnetic island width in the normal direction reaches many ion inertial lengths. During this transition, the quasi-steady reconnection rate decreases until the ions are fully coupled, ultimately reaching an asymptotic value. The scaling of the ion outflow velocity with exhaust width during this electron-only to ion-coupled transition is found to be consistent with a theoretical model of a newly reconnected field line. In order to have a fully frozen-in ion exhaust with ion flows comparable to the reconnection Alfv\'en speed, an exhaust width of at least several ion inertial lengths is needed. In turbulent systems with reconnection occurring between magnetic bubbles associated with fluctuations, using geometric arguments we estimate that fully ion-coupled reconnection requires magnetic bubble length scales of at least several tens of ion inertial lengths

Persistent Transport Barrier on the West Florida Shelf

Analysis of drifter trajectories in the Gulf of Mexico has revealed the existence of a region on the southern portion of the West Florida Shelf (WFS) that is not visited by drifters that are released outside of the region. This so-called ``forbidden zone'' (FZ) suggests the existence of a persistent cross-shelf transport barrier on the southern portion of the WFS. In this letter a year-long record of surface currents produced by a Hybrid-Coordinate Ocean Model simulation of the WFS is used to identify Lagrangian coherent structures (LCSs), which reveal the presence of a robust and persistent cross-shelf transport barrier in approximately the same location as the boundary of the FZ. The location of the cross-shelf transport barrier undergoes a seasonal oscillation, being closer to the coast in the summer than in the winter. A month-long record of surface currents inferred from high-frequency (HF) radar measurements in a roughly 60 km $\times$ 80 km region on the WFS off Tampa Bay is also used to identify LCSs, which reveal the presence of robust transient transport barriers. While the HF-radar-derived transport barriers cannot be unambiguously linked to the boundary of the FZ, this analysis does demonstrate the feasibility of monitoring transport barriers on the WFS using a HF-radar-based measurement system. The implications of a persistent cross-shelf transport barrier on the WFS for the development of harmful algal blooms on the shoreward side of the barrier are considered.Comment: Submitted to Geophysical Research Letter

Novel Pressure Induced Structural Phase Transition in AgSbTe2_{2}

We report a novel high pressure structural sequence for the functionally graded thermoelectric, narrow band gap semiconductor AgSbTe$_{2}$, using angle dispersive x-ray diffraction in a diamond anvil cell with synchrotron radiation at room temperature. The compound undergoes a B1 to B2 transition; the transition proceeds through an intermediate amorphous phase found between 17-26 GPa that is quenchable down to ambient conditions. The pressure induced structural transition observed in this compound is the first of its type reported in this ternary cubic family, and it is new for the B1-B2 transition pathway reported to date. Density Functional Theory (DFT) calculations performed for the B1 and B2 phases are in good agreement with the experimental results.Comment: 4 pages, 3 figure

New Cross-Bridged Cyclam Ligands and Their Transition Metal Complexes as CXCR4 Antagonists

CXCR4 is a co-receptor on the surface of immune cells that has been proven to facilitate the entry of HIV into the cells. Within the last 15 years the CXCR4 and CCR5 coreceptors have influenced new therapeutic approaches to the treatment of HIV via fusion inhibitor drugs that target these receptors. Our aim is to develop new antagonists for the CXCR4 coreceptor. Specifically, the goal was the synthesis of Propyl Cross-Bridged, linked analogues of the known CXCR4 antagonist AMD-3100