Some remarks on 'superradiant' phase transitions in light-matter systems

In this paper we analyze properties of the phase transition that appears in a set of quantum optical models; Dicke, Tavis-Cummings, quantum Rabi, and finally the Jaynes-Cummings model. As the light-matter coupling is increased into the deep strong coupling regime, the ground state turns from vacuum to become a superradiant state characterized by both atomic and photonic excitations. It is pointed out that all four transitions are of the mean-field type, that quantum fluctuations are negligible, and hence these fluctuations cannot be responsible for the corresponding vacuum instability. In this respect, these are not quantum phase transitions. In the case of the Tavis-Cummings and Jaynes-Cummings models, the continuous symmetry of these models implies that quantum fluctuations are not only negligible, but strictly zero. However, all models possess a non-analyticity in the ground state in agreement with a continuous quantum phase transition. As such, it is a matter of taste whether the transitions should be termed quantum or not. In addition, we also consider the modifications of the transitions when photon losses are present. For the Dicke and Rabi models these non-equilibrium steady states remain critical, while the criticality for the open Tavis-Cummings and Jaynes-Cummings models is completely lost, i.e. in realistic settings one cannot expect a true critical behaviour for the two last models.Comment: 25 pages (single column), 6 figure

The assessment of long-term orbital debris models

Existing long-term orbital debris models are assessed as a first step in the Air Force's effort to develop an Air Force long-term orbital debris model which can perform the following functions: (1) operate with the necessary accuracy at the relevant altitudes and orbital parameters; (2) benefit from new Air Force and non-Air Force debris measurements; and (3) accommodate current and future Air Force space scenarios. Model assessment results are shown for the NASA engineering model. The status of the NASA EVOLVE model assessment is discussed

Some local--global phenomena in locally finite graphs

In this paper we present some results for a connected infinite graph $G$ with finite degrees where the properties of balls of small radii guarantee the existence of some Hamiltonian and connectivity properties of $G$. (For a vertex $w$ of a graph $G$ the ball of radius $r$ centered at $w$ is the subgraph of $G$ induced by the set $M_r(w)$ of vertices whose distance from $w$ does not exceed $r$). In particular, we prove that if every ball of radius 2 in $G$ is 2-connected and $G$ satisfies the condition $d_G(u)+d_G(v)\geq |M_2(w)|-1$ for each path $uwv$ in $G$, where $u$ and $v$ are non-adjacent vertices, then $G$ has a Hamiltonian curve, introduced by K\"undgen, Li and Thomassen (2017). Furthermore, we prove that if every ball of radius 1 in $G$ satisfies Ore's condition (1960) then all balls of any radius in $G$ are Hamiltonian.Comment: 18 pages, 6 figures; journal accepted versio

Could any black holes be produced at the LHC?

We introduce analytical quantum gravity modifications of the production cross section for terascale black holes by employing an effective ultraviolet cut off $l$. We find the new cross sections approach the usual "black disk" form at high energy, while they differ significantly near the fundamental scale from the standard increase with respect to $s$. We show that the heretofore discontinuous step function used to represent the cross section threshold can realistically be modeled by two functions representing the incoming and final parton states in a high energy collision. The growth of the cross section with collision energy is thus a unique signature of $l$ and number of spatial dimensions $d$. Contrary to the classical black disk result, our cross section is able to explain why black holes might not be observable in LHC experiments while they could be still at the reach of ultra-high energy cosmic ray events.Comment: v1: 10 pages, 1 figure; v2: 11 pages, 1 figure, additional references, minor modifications; v3: new title, expanded discussion with updated production estimates, additional references, 4 figures, 19 pages, version accepted for publication on Physical Review

Hydrogen peroxide-induced cell damage: The role of free radicals and iron complexes

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis presents data exploiting one of the important chemico-biological characteristics of the transition metal iron: its ability to exist in two oxidation states. Manipulation of the reactivity of iron by ligands has been reflected by the oxygen utilisation or the amount of ferrozine-detectable Fe2+. Ligand-dependent cell associated iron has been demonstrated by prussian blue staining and ferrozinedetectable iron. Iron/8-hydroxyquinoline enters the cell by diffusion, whereas iron dextran is endocytosed as is evident from extensive vesicular staining. The involvement of iron in the cytotoxicity of H202 has been examined (a) indirectly by introduction of reducing agents which restore the cytotoxicity of H202 at 4°C (an effect which is abolished by desferrioxamine) and (b) directly by extracellularly added iron, which enhances the cytotoxicity if the iron is internalised but protects the cell if the iron remains extracellular. Cytotoxicity has been estimated by plating efficiency and by a modified tritiated thymidine incorporation assay permitting a 24 hour delay before exposure to the label. Direct free radical interaction with a ligand itself is exemplified by the production of the nitroxide free radical in desferrioxamine. This causes extensive damage to yeast alcohol dehydrogenase but can be protected by ascorbate, methionine and iron/EDTA but not iron dextran. The findings lend further support for the suggestion that free radical generation by the Fenton reaction enhances the potential cytotoxic effect of H202 provided the reactions occur at critical sites within the cell.Financial support obtaind from Fisons Pharmaceuticals Plc