Chaos and the Quantum Phase Transition in the Dicke Model

We investigate the quantum chaotic properties of the Dicke Hamiltonian; a quantum-optical model which describes a single-mode bosonic field interacting with an ensemble of $N$ two-level atoms. This model exhibits a zero-temperature quantum phase transition in the N \go \infty limit, which we describe exactly in an effective Hamiltonian approach. We then numerically investigate the system at finite $N$ and, by analysing the level statistics, we demonstrate that the system undergoes a transition from quasi-integrability to quantum chaotic, and that this transition is caused by the precursors of the quantum phase-transition. Our considerations of the wavefunction indicate that this is connected with a delocalisation of the system and the emergence of macroscopic coherence. We also derive a semi-classical Dicke model, which exhibits analogues of all the important features of the quantum model, such as the phase transition and the concurrent onset of chaos.Comment: 51 pages, 15 figures, late

Whole-genome sequencing reveals host factors underlying critical COVID-19

Critical COVID-19 is caused by immune-mediated inflammatory lung injury. Host genetic variation influences the development of illness requiring critical care1 or hospitalization2,3,4 after infection with SARS-CoV-2. The GenOMICC (Genetics of Mortality in Critical Care) study enables the comparison of genomes from individuals who are critically ill with those of population controls to find underlying disease mechanisms. Here we use whole-genome sequencing in 7,491 critically ill individuals compared with 48,400 controls to discover and replicate 23 independent variants that significantly predispose to critical COVID-19. We identify 16 new independent associations, including variants within genes that are involved in interferon signalling (IL10RB and PLSCR1), leucocyte differentiation (BCL11A) and blood-type antigen secretor status (FUT2). Using transcriptome-wide association and colocalization to infer the effect of gene expression on disease severity, we find evidence that implicates multiple genes—including reduced expression of a membrane flippase (ATP11A), and increased expression of a mucin (MUC1)—in critical disease. Mendelian randomization provides evidence in support of causal roles for myeloid cell adhesion molecules (SELE, ICAM5 and CD209) and the coagulation factor F8, all of which are potentially druggable targets. Our results are broadly consistent with a multi-component model of COVID-19 pathophysiology, in which at least two distinct mechanisms can predispose to life-threatening disease: failure to control viral replication; or an enhanced tendency towards pulmonary inflammation and intravascular coagulation. We show that comparison between cases of critical illness and population controls is highly efficient for the detection of therapeutically relevant mechanisms of disease

Ion current densities produced by energetic electrons in electrostatic precipitator geometries

A new laboratory test system for electron beam ionization in electrostatic precipitator geometries has been constructed to measure ion current densities as a function of voltage differences for clean (bare) plate conditions. The new system incorporates improved electrodes, which withstand a driving voltage of /sup + -/ 55 kV, a factor of 5 increase over the previous test system. A 3 MeV Van de Graaff accelerator produced ionizing electron beams of 1.2 and 2 MeV and currents of 10.5 and 21 ..mu..A in place of corona wire ionization. Current densities of up to 130 mA/m/sup 2/ were measured before breakdown between the plates, and no current saturation was observed. A comparison of I-V curves and sparkover voltages for various beam energies, currents, and collimation are discussed and the need for measurements with good beam geometry is reviewed

Use of energetic electrons in a particle precharger and in a sulfur dioxide reactor

Electrons energized by corona discharge to produce the necessary ionization for particle charging have been used in electrostatic precipitators for decades. This paper reports the use of an electron beam to release and energize electrons which produce copious charging currents in a bench precharger. Results have been obtained for various values of electron beam energy, beam current, electric field strength, current density, and exposure time in measurements of charging efficiency for large conducting spheres and 1 and 3 ..mu..m diameter PSL particles. After matching the beam energy and geometry in the bench precharger, particle charges greater than five times the theoretical ionic charging value were measured in the bi-electrode precharger. The increased charge can be explained by space-charge enhancement of the electric field and/or free electron charging. The use of very hgh energy electron beams for the removal of SO/sub 2/ and NO/sub x/ from flue gases has been previously established elsewhere. Since the energy regime for the electrons required for the production of oxidizing radicals is the order of 10 eV, a device which operates in a lower energy regime is attractive. A positive streamer corona device has been constructed and used to energize electrons for the purpose of producing oxidizing radicals. The performance of a pulse energized electron reactor (PEER) has been evaluated. More than 90% of the SO/sub 2/ has been removed from a test gas stream containing air, water vapor and 1666 ppM of SO/sub 2/. The power efficiency of the PEER device is greater than that for DC discharge or high energy electron beam treatment

Role of modern chemistry in sustainable arable crop protection

Organic chemistry has been, and for the foreseeable future will remain, vitally important for crop protection. Control of fungal pathogens, insect pests and weeds is crucial to enhanced food provision. As world population continues to grow, it is timely to assess the current situation, anticipate future challenges and consider how new chemistry may help meet those challenges. In future, agriculture will increasingly be expected to provide not only food and feed, but also crops for conversion into renewable fuels and chemical feedstocks. This will further increase the demand for higher crop yields per unit area, requiring chemicals used in crop production to be even more sophisticated. In order to contribute to programmes of integrated crop management, there is a requirement for chemicals to display high specificity, demonstrate benign environmental and toxicological profiles, and be biodegradable. It will also be necessary to improve production of those chemicals, because waste generated by the production process mitigates the overall benefit. Three aspects are considered in this review: advances in the discovery process for new molecules for sustainable crop protection, including tests for environmental and toxicological properties as well as biological activity; advances in synthetic chemistry that may offer efficient and environmentally benign manufacturing processes for modern crop protection chemicals; and issues related to energy use and production through agriculture