NASA Langley Research Center HBCU/OMU program: 1990 student support survey

The results of a survey of students who are receiving support through the Historically Black Colleges and Universities and Other Minority Universities are given. Information is given on the race, sex, ethnic distribution, grade point average distribution, and target degree distribution

Classical Electron Model with Negative Energy Density in Einstein-Cartan Theory of Gravitation

Experimental result regarding the maximum limit of the radius of the electron \sim 10^{-16} cm and a few of the theoretical works suggest that the gravitational mass which is a priori a positive quantity in Newtonian mechanics may become negative in general theory of relativity. It is argued that such a negative gravitational mass and hence negative energy density also can be obtained with a better physical interpretation in the framework of Einstein-Cartan theory.Comment: 12 Latex pages, added refs and conclusion

Magnetoconductance signatures of chiral domain-wall bound states in magnetic topological insulators

Recent magnetoconductance measurements performed on magnetic topological insulator candidates have revealed butterfly-shaped hysteresis. This hysteresis has been attributed to the formation of gapless chiral domain-wall bound states during a magnetic field sweep. We treat this phenomenon theoretically, providing a link between microscopic magnetization dynamics and butterfly hysteresis in magnetoconductance. Further, we illustrate how a spatially resolved conductance measurement can probe the most striking feature of the domain-wall bound states: their chirality. This work establishes a regime where a definitive link between butterfly hysteresis in longitudinal magneto-conductance and domain-wall bound states can be made. This analysis provides an important tool for the identification of magnetic topological insulators.Comment: v2: Final published version; 7 pages, 3 figure

Missing Shapiro steps and the 8π8\pi-periodic Josephson effect in interacting helical electron systems

Two-particle backscattering in time-reversal invariant interacting helical electron systems can lead to the formation of quasiparticles with charge $e/2$. We propose a way to detect such states by means of the Josephson effect in the presence of proximity-induced superconductivity. In this case, the existence of $e/2$ charges leads to an $8 \pi$-periodic component of the Josephson current which can be identified through measurement of Shapiro steps in Josephson junctions. In particular, we show that even when there is weak explicit time-reversal symmetry breaking, which causes the two-particle backscattering to be a sub-leading effect at low energies, its presence can still be detected in driven, current-biased Shapiro step measurements. The disappearance of some of these steps as a function of the drive frequency is directly related to the existence of non-Abelian zero-energy states. We suggest that this effect can be measured in current state-of-the-art Rashba wires.Comment: 9 pages, 5 figures. A new submission extending and expanding our analysis in arXiv:1507.08881. (v2) References adde

Dynamic response functions and helical gaps in interacting Rashba nanowires with and without magnetic fields

A partially gapped spectrum due to the application of a magnetic field is one of the main probes of Rashba spin-orbit coupling in nanowires. Such a "helical gap" manifests itself in the linear conductance, as well as in dynamic response functions such as the spectral function, the structure factor, or the tunnelling density of states. In this paper, we investigate theoretically the signature of the helical gap in these observables with a particular focus on the interplay between Rashba spin-orbit coupling and electron-electron interactions. We show that in a quasi-one-dimensional wire, interactions can open a helical gap even without magnetic field. We calculate the dynamic response functions using bosonization, a renormalization group analysis, and the exact form factors of the emerging sine-Gordon model. For special interaction strengths, we verify our results by refermionization. We show how the two types of helical gaps, caused by magnetic fields or interactions, can be distinguished in experiments.Comment: 15 pages, 7 figures, v2 refs adde

Acid-adaption by a medic microsymbiont: new insights from the genome of Sinorhizobium medicae WSM419

The poor availability of nitrogen is one of the principal factors limiting global biomass. Legumes are vital components of agricultural systems because of their ability to associate symbiotically with root nodule bacteria (RNB) and subsequently fix atmospheric nitrogen to a form that can be utilised by the plant partner. Furthermore, this symbiotic relationship provides available soil nitrogen for subsequent non-leguminous crops. This RNB-legume interaction is affected by a number of environmental factors. Progressive acidification of agricultural soils is one of the big challenges in agriculture as soil acidity negatively impacts legume productivity. One genus of RNB, Sinorhizobium, is particularly acid-sensitive causing a major reduction in Medicago productivity in acidic soils. Due to the importance of Medic pasture production, alternative strains have been captured, and are still being captured, from the genetic pool that display superior acid tolerance characteristics. This presentation will focus on the acid-tolerant species S. medicae (previously known as S. meliloti) and in particular on the previously used commercial inoculant WSM419

Detecting nonlocal Cooper pair entanglement by optical Bell inequality violation

Based on the Bardeen Cooper Schrieffer (BCS) theory of superconductivity, the coherent splitting of Cooper pairs from a superconductor to two spatially separated quantum dots has been predicted to generate nonlocal pairs of entangled electrons. In order to test this hypothesis, we propose a scheme to transfer the spin state of a split Cooper pair onto the polarization state of a pair of optical photons. We show that the produced photon pairs can be used to violate a Bell inequality, unambiguously demonstrating the entanglement of the split Cooper pairs.Comment: 11 pages, 9 figures, v3 with added reference