Non-Makovian decoherence of a two-level system weakly coupled to a bosonic bath

Bloch-Redfield equation is a common tool for studying evolution of qubit systems weakly coupled to environment. We investigate the accuracy of the Born approximation underlying this equation. We find that the high order terms in the perturbative expansion contain accumulating divergences that make straightforward Born approximation inappropriate. We develop diagrammatic technique to formulate, and solve the improved self-consistent Born approximation. This more accurate treatment reveals an exponential time dependent prefactor in the non-Markovian contribution dominating the qubit long-time relaxation found in Phys. Rev. B 71, 035318 (2005). At the same time, the associated dephasing is not affected and is described by the Born-Markov approximation.Comment: To appear in EuroPhys. Let

Graphene Nanogap for Gate Tunable Quantum Coherent Single Molecule Electronics

We present atomistic calculations of quantum coherent electron transport through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We predict that three difficult problems in molecular electronics with single molecules may be solved by utilizing graphene contacts: (1) a back gate modulating the Fermi level in the graphene leads facilitate control of the device conductance in a transistor effect with high on/off current ratio; (2) the size mismatch between leads and molecule is avoided, in contrast to the traditional metal contacts; (3) as a consequence, distinct features in charge flow patterns throughout the device are directly detectable by scanning techniques. We show that moderate graphene edge disorder is unimportant for the transistor function.Comment: 8 pages, 6 figure

A mechanistic understanding of repellent function against mammalian herbivores

BackgroundBrowsing repellents are widely used to deter large herbivores from consuming plants of ecological, economic and aesthetic importance. Understanding how these repellents function on a behavioural mechanistic level is critical to predicting effectiveness. Here, we illustrate how these mechanisms can be tested, by exposing a model mammalian herbivore, the fallow deer, to different concentrations of a commercial chemical repellent (HaTe2) in two-choice feeding trials.ResultsThe repellent acted as a defensive chemical for the food by both reducing visitation and the amount consumed. Deer favoured the less defended feeders before ingesting any food, suggesting that the repellent altered olfactory and/or visual cues. Deer also consumed less of the more defended food when choosing between low and high repellent feeders than no and low repellent feeders, indicating that the repellent modified flavour and/or sensation. Repellent effectiveness declined with increased exposure, suggesting that consumption had no negative post-ingestive effects, and thus, deterrence was not caused by a conditioned aversion or irritation. Instead, this pattern suggests that deer learned, through repeated sampling of repellent-treated food, that there was no adverse physiological effect of ingesting it.ConclusionsThese results imply that HaTe2 repellent will not be effective over prolonged periods or in the absence of alternative untreated food. Understanding the mechanisms driving repellent function using two-choice trials could help practitioners decide whether a particular repellent is likely to be effective against mammalian herbivory in their management scenario

Efficient application of a browsing repellent: Can associational effects within and between plants be exploited?

publishedVersio

Unlocking the secrets of stellar haloes using combined star counts and surface photometry

The stellar haloes of galaxies can currently be studied either through observations of resolved halo stars or through surface photometry. Curiously, the two methods appear to give conflicting results, as a number of surface photometry measurements have revealed integrated colours that are too red to be reconciled with the halo properties inferred from the study of resolved stars. Several explanations for this anomaly have been proposed - including dust photoluminescence, extinction of extragalactic background light and a bottom-heavy stellar initial mass function. A decisive test is, however, still lacking. Here, we explain how observations of the halo of a nearby galaxy, involving a combination of both surface photometry and bright star counts, can be used to distinguish between the proposed explanations. We derive the observational requirements for this endeavour and find that star counts in filters VI and surface photometry in filters VIJ appears to be the optimal strategy. Since the required halo star counts are already available for many nearby galaxies, the most challenging part of this test is likely to be the optical surface photometry, which requires several nights of exposure time on a 4-8 m telescope, and the near-IR surface photometry, which is most readily carried out using the upcoming James Webb Space Telescope.Comment: 14 pages, 4 figures; v.2 matches published version (minor changes only

Star Formation in QSO Host Galaxies

Many of the conditions that are necessary for starbursts appear to be important in the triggering of QSOs. However, it is still debatable whether starbursts are ubiquitously present in galaxies harboring QSOs. In this paper we review our current knowledge from observations of the role of starbursts in different types of QSOs. Post-starburst stellar populations are potentially present in the majority of QSO hosts. QSOs with far-infrared colors similar to those of ultraluminous infrared galaxies invariably reside in merging galaxies that have interaction-induced starbursts of a few hundred Myr or less. Similar, but dramatically more luminous post-starburst populations are found in the recently discovered class of QSOs known as post-starburst QSOs, or Q+A's. Both of these classes, however, comprise only a small fraction (10-15%) of the total QSO population. The so-called "red" QSOs generally suffer from strong extinction at optical wavelengths, making them ideal candidates for the study of hosts. Their stellar populations typically show a post-starburst component as well, though with a larger range of ages. Finally, optical "classical" QSO hosts show traces of major star formation episodes (typically involving >10% of the mass of the stellar component) in the more distant past (1-2 Gyr). These starbursts appear to be linked to past merger events. It remains to be determined whether these mergers were also responsible for triggering the QSO activity that we observe today.Comment: 8 pages, 5 figures, invited review for "QSO Host Galaxies: Evolution and Environment", held at the Lorentz Center, Universiteit Leiden, August, 200

A Comparison of the Intrinsic Shapes of Two Different Types of Dwarf Galaxies: Blues Compact Dwarfs and Dwarf Ellipticals

We measure the apparent shapes for a sample of 62 blue compact dwarf galaxies (BCDs), and compare them with the apparent shapes for a sample of 80 dwarf elliptical galaxies (dEs). The BCDs are flatter, on average, than the dEs, but the difference is only marginally significant. We then use both non-parametric and parametric techniques to determine possible distributions of intrinsic shapes for the BCDs. The hypothesis that BCDs are oblate spheroids can be ruled out with a high confidence level ($> 99$), but the hypothesis that they are prolate spheroids cannot be excluded. The apparent shapes of BCDs are totally consistent with the hypothesis that they are triaxial ellipsoids. If the intrinsic axis ratios, $\beta$ and $\gamma$, are distributed according to a Gaussian with means $\beta_0$ and $\gamma_0$ and standard deviation $\sigma$, we find the best-fitting distribution for BCDs has $(\beta_0,\gamma_0,\sigma)= (0.66,0.55,0.16)$, while that for dEs has $(\beta_0,\gamma_0,\sigma)= (0.85,0.64,0.24)$. Our results are consistent with the hypothesis that BCDs have a close evolutionary relation with dEs.Comment: total 23 pages, 9 figures, and 1 Table, submitted to ApJ on Sep 19 1997. Email addresses: [email protected], [email protected], [email protected], [email protected], [email protected]

Cheminformatics-aided pharmacovigilance: application to Stevens-Johnson Syndrome

Objective Quantitative Structure-Activity Relationship (QSAR) models can predict adverse drug reactions (ADRs), and thus provide early warnings of potential hazards. Timely identification of potential safety concerns could protect patients and aid early diagnosis of ADRs among the exposed. Our objective was to determine whether global spontaneous reporting patterns might allow chemical substructures associated with Stevens-Johnson Syndrome (SJS) to be identified and utilized for ADR prediction by QSAR models

Helical Strands in the Jet-like Narrow Line Region of ESO 428-G14

We present HST/WFPC2 images of the narrow line region (NLR) of the Seyfert 2 galaxy ESO 428-G14 (0714-2914, M4-1). The NLR consists of many individual, thin strands, which are very closely related to the radio jet and produce a highly complex yet ordered structure. We find that the jet is two-sided with a double-helix of emission-line gas apparently wrapped around the NW side. To the SE, the jet seems to be deflected at a ridge of highly excited gas. The strands to the SE may also wrap around the radio jet, but here complete helices are not seen. The overall structure is reminiscent of the jet seen in NGC 4258. Faint symmetric features aligned with the nucleus could indicate the presence of a highly collimated beam of photons or plasma from the center.Comment: ApJ Letters, accepted for publication, 10 pages, 2 PS Figures, AASTeX, also available at http://www.astro.umd.edu/~hfalcke/publications.html#eso42