Completely Positive Maps on Some C*-Algebras

Completely positive maps on the C*-algebra of the canonical anticommutation relations induced by contractions on the underlying Hilbert space are investigated and a Stinespring decomposition exhibited. These maps are used to construct examples of dynamical semi-groups; their explicit Stinespring decompositions yield unitary dilations. The relation of this work with earlier results on the C*-algebra of the canonical commutation relations is discussed

Imperfect identity

Questions of identity over time are often hard to answer. A long tradition has it that such questions are somehow soft: they have no unique, determinate answer, and disagreements about them are merely verbal. I argue that this claim is not the truism it is taken to be. Depending on how it is understood, it turns out either to be false or to presuppose a highly contentious metaphysical claim

Impacts of bromine and iodine chemistry on tropospheric OH and HO2 : Comparing observations with box and global model perspectives

The chemistry of the halogen species bromine and iodine has a range of impacts on tropospheric composition, and can affect oxidising capacity in a number of ways. However, recent studies disagree on the overall sign of the impacts of halogens on the oxidising capacity of the troposphere. We present simulations of OH and HO2 radicals for comparison with observations made in the remote tropical ocean boundary layer during the Seasonal Oxidant Study at the Cape Verde Atmospheric Observatory in 2009. We use both a constrained box model, using detailed chemistry derived from the Master Chemical Mechanism (v3.2), and the three-dimensional global chemistry transport model GEOS-Chem. Both model approaches reproduce the diurnal trends in OH and HO2. Absolute observed concentrations are well reproduced by the box model but are overpredicted by the global model, potentially owing to incomplete consideration of oceanic sourced radical sinks. The two models, however, differ in the impacts of halogen chemistry. In the box model, halogen chemistry acts to increase OH concentrations (by 9.8% at midday at the Cape Verde Atmospheric Observatory), while the global model exhibits a small increase in OH at the Cape Verde Atmospheric Observatory (by 0.6% at midday) but overall shows a decrease in the global annual mass-weighted mean OH of 4.5%. These differences reflect the variety of timescales through which the halogens impact the chemical system. On short timescales, photolysis of HOBr and HOI, produced by reactions of HO2 with BrO and IO, respectively, increases the OH concentration. On longer timescales, halogen-catalysed ozone destruction cycles lead to lower primary production of OH radicals through ozone photolysis, and thus to lower OH concentrations. The global model includes more of the longer timescale responses than the constrained box model, and overall the global impact of the longer timescale response (reduced primary production due to lower O3 concentrations) overwhelms the shorter timescale response (enhanced cycling from HO2 to OH), and thus the global OH concentration decreases. The Earth system contains many such responses on a large range of timescales. This work highlights the care that needs to be taken to understand the full impact of any one process on the system as a whole

Synthesis and characterisation of halide, separated ion pair, and hydride cyclopentadienyl iron bis(diphenylphosphino)ethane derivatives

Treatment of anhydrous FeX₂ (X = Cl, Br, I) with one equivalent of bis(diphenylphosphino)ethane (dppe) in refluxing THF afforded analytically pure white (X = Cl), light green (X = Br), and yellow (X = I) [FeX₂(dppe)]n (X = Cl, I; Br, II; I, III). Complexes I–III are excellent synthons from which to prepare a range of cyclopentadienyl derivatives. Specifically, treatment of I–III with alkali metal salts of C₅H₅ (Cp, series 1), C₅Me₅ (Cp*, series 2), C₅H₄SiMe₃ (Cp′, series 3), C₅H₃(SiMe₃)₂ (Cp′′, series 4), and C₅H₃(But)₂ (Cptt, series 5) afforded [Fe(Cp†)(Cl)(dppe)] 1Cl–5Cl, [Fe(Cp†)(Br)(dppe)] 1Br–5Br, and [Fe(Cp†)(I)(dppe)] 1I–5I (Cp† = Cp, Cp*, Cp′, Cp′′, or Cptt). Dissolution of 1I–5I in acetonitrile, or treatment of 1Cl–5Cl with Me₃SiI in acetonitrile (no halide exchange reactions were observed in other solvents) afforded the separated ion pair complexes [Fe(Cp†)(NCMe)(dppe)][I] 1SIP–5SIP. Attempts to reduce 1Cl–5Cl, 1Br–5Br, and 1I–5I with a variety of reductants (Li-Cs, KC₈, Na/Hg) were unsuccessful. Treatment of 1Cl–5Cl with LiAlH₄ gave the hydride derivatives [Fe(Cp†)(H)(dppe)] 1H–5H. This report provides a systematic account of reliable methods of preparing these complexes which may find utility in molecular wire and metal–metal bond chemistries. The complexes reported herein have been characterised by X-ray diffraction, NMR, IR, UV/Vis, and Mössbauer spectroscopies, cyclic voltammetry, density functional theory calculations, and elemental analyses, which have enabled us to elucidate the electronic structure of the complexes and probe the variation of iron redox properties as a function of varying the cyclopentadienyl or halide ligand

HST PanCET program: A Cloudy Atmosphere for the promising JWST target WASP-101b

We present results from the first observations of the Hubble Space Telescope (HST) Panchromatic Comparative Exoplanet Treasury (PanCET) program for WASP-101b, a highly inflated hot Jupiter and one of the community targets proposed for the James Webb Space Telescope (JWST) Early Release Science (ERS) program. From a single HST Wide Field Camera 3 (WFC3) observation, we find that the near-infrared transmission spectrum of WASP-101b contains no significant H$_2$O absorption features and we rule out a clear atmosphere at 13{\sigma}. Therefore, WASP-101b is not an optimum target for a JWST ERS program aimed at observing strong molecular transmission features. We compare WASP-101b to the well studied and nearly identical hot Jupiter WASP-31b. These twin planets show similar temperature-pressure profiles and atmospheric features in the near-infrared. We suggest exoplanets in the same parameter space as WASP-101b and WASP-31b will also exhibit cloudy transmission spectral features. For future HST exoplanet studies, our analysis also suggests that a lower count limit needs to be exceeded per pixel on the detector in order to avoid unwanted instrumental systematics.Comment: 7 pages, 4 figures, 1 table, Accepted to ApJ

On non-completely positive quantum dynamical maps on spin chains

The new arguments based on Majorana fermions indicating that non-completely positive maps can describe open quantum evolution are presented.Comment: published; small change

Thinking through illusion

Perception of a property (e.g. a colour, a shape, a size) can enable thought about the property, while at the same time misleading the subject as to what the property is like. This long-overlooked claim parallels a more familiar observation concerning perception-based thought about objects, namely that perception can enable a subject to think about an object while at the same time misleading her as to what the object is like. I defend the overlooked claim, and then use it to generate a challenge for a standard way of thinking about the relationship between visual experience and rational belief formation. Put informally, that view holds that just as we can mislead others by saying something false, illusory experience misleads by misrepresenting how things stand in the world. I argue that we ought to abandon this view in favour of some radical alternative account of the relationship between visual experience and rational belief formation