Breakdown of the perturbative renormalization group for S >= 1 random antiferromagnetic spin chains

We investigate the application of a perturbative renormalization group (RG) method to random antiferromagnetic Heisenberg chains with arbitrary spin size. At zero temperature we observe that initial arbitrary probability distributions develop a singularity at J=0, for all values of spin S. When the RG method is extended to finite temperatures, without any additional assumptions, we find anomalous results for S >= 1. These results lead us to conclude that the perturbative scheme is not adequate to study random chains with S >= 1. Therefore a random singlet phase in its more restrictive definition is only assured for spin-1/2 chains.Comment: 5 pages, 3 figures. To appear in Physical Review

Single and Double Photoionization and Photodissociation of Toluene by Soft X-rays in Circumstellar Environment

The formation of polycyclic aromatic hydrocarbons (PAHs) and their methyl derivatives occurs mainly in the dust shells of asymptotic giant branch (AGB) stars. The bands at 3.3 and 3.4 $\mu$m, observed in infrared emission spectra of several objects, are attributed C-H vibrational modes in aromatic and aliphatic structures, respectively. In general, the feature at 3.3 $\mu$m is more intense than the 3.4 $\mu$m. Photoionization and photodissociation processes of toluene, the precursor of methylated PAHs, were studied using synchrotron radiation at soft X-ray energies around the carbon K edge with time-of-flight mass spectrometry. Partial ion yields of a large number of ionic fragments were extracted from single and 2D-spectra, where electron-ion coincidences have revealed the doubly charged parent-molecule and several doubly charged fragments containing seven carbon atoms with considerable abundance. \textit{Ab initio} calculations based on density functional theory were performed to elucidate the chemical structure of these stable dicationic species. The survival of the dications subjected to hard inner shell ionization suggests that they could be observed in the interstellar medium, especially in regions where PAHs are detected. The ionization and destruction of toluene induced by X-rays were examined in the T Dra conditions, a carbon-rich AGB star. In this context, a minimum photodissociation radius and the half-life of toluene subjected to the incidence of the soft X-ray flux emitted from a companion white dwarf star were determined.Comment: 11 pages, 4 figures, accept for publication in Ap

Dissociation of the benzene molecule by UV and soft X-rays in circumstellar environment

Benzene molecules, present in the proto-planetary nebula CRL 618, are ionized and dissociated by UV and X-ray photons originated from the hot central star and by its fast wind. Ionic species and free radicals produced by these processes can lead to the formation of new organic molecules. The aim of this work is to study the photoionization and photodissociation processes of the benzene molecule, using synchrotron radiation and time of flight mass spectrometry. Mass spectra were recorded at different energies corresponding to the vacuum ultraviolet (21.21 eV) and soft X-ray (282-310 eV) spectral regions. The production of ions from the benzene dissociative photoionization is here quantified, indicating that C6H6 is more efficiently fragmented by soft X-ray than UV radiation, where 50% of the ionized benzene molecules survive to UV dissociation while only about 4% resist to X-rays. Partial ion yields of H+ and small hydrocarbons such as C2H2+, C3H3+ and C4H2+ are determined as a function of photon energy. Absolute photoionization and dissociative photoionization cross sections have also been determined. From these values, half-life of benzene molecule due to UV and X-ray photon fluxes in CRL 618 were obtained.Comment: The paper contains 8 pages, 9 figures and 4 tables. Accepted to be published on MNRAS on 2008 November 2

Breakdown of the perturbative renormalization group for S \u3e= 1 random antiferromagnetic spin chains

We investigate the application of a perturbative renormalization group (RG) method to random antiferromagnetic Heisenberg chains with arbitrary spin size. At zero temperature we observe that initial arbitrary probability distributions develop a singularity at J = 0, for all values of spin S. When the RG method is extended to finite temperatures, without any additional assumptions, we find anomalous results for S \u3e= 1. These results lead us to conclude that the perturbative scheme is not adequate to study random chains with S \u3e= 1. Therefore a random singlet phase in its more restrictive definition is only assured for spin-1/2 chains

Phase diagram of the random Heisenberg antiferromagnetic spin-1 chain

We present a new perturbative real space renormalization group (RG) to study random quantum spin chains and other one-dimensional disordered quantum systems. The method overcomes problems of the original approach which fails for quantum random chains with spins larger than S=1/2. Since it works even for weak disorder we are able to obtain the zero temperature phase diagram of the random antiferromagnetic Heisenberg spin-1 chain as a function of disorder. We find a random singlet phase for strong disorder and as disorder decreases, the system shows a crossover from a Griffiths to a disordered Haldane phase.Comment: 4 pages, 10 figure

Ising Spin Glass in a Transverse Magnetic Field

We study the three-dimensional quantum Ising spin glass in a transverse magnetic field following the evolution of the bond probability distribution under Renormalisation Group transformations. The phase diagram (critical temperature $T_c$ {\em vs} transverse field $\Gamma$) we obtain shows a finite slope near $T=0$, in contrast with the infinite slope for the pure case. Our results compare very well with the experimental data recently obtained for the dipolar Ising spin glass LiHo$_{0.167}$Y$_{0.833}$F$_4$, in a transverse field. This indicates that this system is more apropriately described by a model with short range interactions than by an equivalent Sherrington-Kirkpatrick model in a transverse field.Comment: 7 pages, RevTeX3, Nota Cientifica PUC-Rio 23/9

Metabolic interactions in asthma

Metabolomics can be used to explore altered metabolic pathways in asthma, giving insights into its pathophysiology. We aimed to review how metabolomics has been used to understand asthma by describing metabolic pathways under research and discussing clinical implications. The search was performed in PubMed, and studies published since 2000 using a metabolomics approach, were included. A total of 32 studies were analysed. Pathways related with cellular energy homeostasis, lipid metabolism and oxidative stress, immune and inflammatory processes and others were altered. Initial studies focused on biomarker discovery. But, metabolomics can be used to evaluate drug effects on specific pathways, to highlight pathways that can further develop in new targeted treatments and to identify differences according to asthma severity and phenotypes.The authors acknowledge funding support by FCT - Fundação para Ciência e Tecnologia, for the project ExPOSE - Establishing protocols to assess occupational exposure to microbiota in clinical settings (02/SAICT/2016, Project no. 23222)

Percolation Transition in the random antiferromagnetic spin-1 chain

We give a physical description in terms of percolation theory of the phase transition that occurs when the disorder increases in the random antiferromagnetic spin-1 chain between a gapless phase with topological order and a random singlet phase. We study the statistical properties of the percolation clusters by numerical simulations, and we compute exact exponents characterizing the transition by a real-space renormalization group calculation.Comment: 9 pages, 4 encapsulated Postscript figures, REVTeX 3.