Quantum Nature of Edge Magnetism in Graphene

It is argued that the subtle crossover from decoherence-dominated classical magnetism to fluctuation-dominated quantum magnetism is experimentally accessible in graphene nanoribbons. We show that the width of a nanoribbon determines whether the edge magnetism is on the classical side, on the quantum side, or in between. In the classical regime, decoherence is dominant and leads to static spin polarizations at the ribbon edges, which are well described by mean-field theories. The quantum Zeno effect is identified as the basic mechanism which is responsible for the spin polarization and thereby enables the application of graphene in spintronics. On the quantum side, however, the spin polarization is destroyed by dynamical processes. The great tunability of graphene magnetism thus offers a viable route for the study of the quantum-classical crossover.Comment: 5 pages, 3 figure

Effective models for strong electronic correlations at graphene edges

We describe a method for deriving effective low-energy theories of electronic interactions at graphene edges. Our method is applicable to general edges of honeycomb lattices (zigzag, chiral, and even disordered) as long as localized low-energy states (edge states) are present. The central characteristic of the effective theories is a dramatically reduced number of degrees of freedom. As a consequence, the solution of the effective theory by exact diagonalization is feasible for reasonably large ribbon sizes. The quality of the involved approximations is critically assessed by comparing the correlation functions obtained from the effective theory with numerically exact quantum Monte-Carlo calculations. We discuss effective theories of two levels: a relatively complicated fermionic edge state theory and a further reduced Heisenberg spin model. The latter theory paves the way to an efficient description of the magnetic features in long and structurally disordered graphene edges beyond the mean-field approximation.Comment: 13 pages, 9 figure

Multiple mRNA isoforms of the transcription activator protein CREB

We have characterized cDNA clones representing mouse CREB (cyclic AMP responsive element binding protein) mRNA isoforms. These include CREBA and CREBa, of which the rat and human homologues have been previously identified. Both encode proteins with CREbinding activity and identical transactivation potential. The additional CREB mRNA isoforms potentially encode CREB related proteins. From the structural organization of the mouse CREB gene we conclude that the multiple transcripts are generated by alternative splicing. Furthermore we show that specific CREB mRNA isoforms are expressed at a high level in the adult testis. Expression of these isoforms is induced after commencement of spermatogenesis. In situ hybridization suggests that this expression occurs predominantly in the primary spermatocytes. Comparison of the CREB gene with the recently isolated CREM (cAMP responsive element modulator) cDNAs illustrates that the two genes have arisen by gene duplication and have diverged to encode transcriptional activators and repressors of the cAMP signal transduction pathway

High-Velocity Features in Type Ia Supernova Spectra

We use a sample of 58 low-redshift (z <= 0.03) Type Ia supernovae (SNe Ia) having well-sampled light curves and spectra near maximum light to examine the behaviour of high-velocity features (HVFs) in SN Ia spectra. We take advantage of the fact that Si II 6355 is free of HVFs at maximum light in all SNe Ia, allowing us to quantify the strength of HVFs by comparing the structure of these two lines. We find that the average HVF strength increases with decreasing light-curve decline rate, and rapidly declining SNe Ia (dm_15(B) >= 1.4 mag) show no HVFs in their maximum-light spectra. Comparison of HVF strength to the light-curve colour of the SNe Ia in our sample shows no evidence of correlation. We find a correlation of HVF strength with the velocity of Si II 6355 at maximum light (v_Si), such that SNe Ia with lower v_Si have stronger HVFs, while those SNe Ia firmly in the "high-velocity" (i.e., v_Si >= 12,000 km/s) subclass exhibit no HVFs in their maximum-light spectra. While v_Si and dm_15(B) show no correlation in the full sample of SNe Ia, we find a significant correlation between these quantities in the subset of SNe Ia having weak HVFs. In general, we find that slowly declining (low dm_15(B)) SNe Ia, which are more luminous and more energetic than average SNe Ia, tend to produce either high photospheric ejecta velocities (i.e., high v_Si) or strong HVFs at maximum light, but not both. Finally, we examine the evolution of HVF strength for a sample of SNe Ia having extensive pre-maximum spectroscopic coverage and find significant diversity of the pre-maximum HVF behaviour.Comment: Version accepted by MNRA

Trends in risk taking and risk reduction among German MSM: results of follow-up surveys "Gay men and AIDS" 1991-2007

"Objectives: To calculate estimates for group level trends among German men who have sex with men (MSM) regarding parameters related to HIV epidemiology: frequency of anal intercourse, consistency in condom use, risk-taking, numbers of sex partners, incident bacterial STIs, and HIV-testing behaviour. Methods: Data derived from six large cross-sectional national follow-up surveys among MSM, conducted between 1991 and 2007. Questionnaires were circulated with German magazines for gay men; in 2003/2007, online recruitment was added. 23,878 anonymously self-administered questionnaires were analyzed in multiple regression models for five subgroups. Results: With the normalization of AIDS, restriction of anal intercourse to primary partnerships has lost its relevance as a risk avoidance strategy for MSM. Other traditional risk reduction strategies among MSM in Germany showed a high degree of time stability: Between 1991 and 2007, condom use in anal intercourse has been relatively stable; numbers of sex partners have been rather constant. No positive trends in the use of Nitrite inhalants were seen, party drugs were increasingly used by a minority of gay men during the 1990s. The syphilis epidemic seen after 1999 among German gay men is well reflected in the survey data. No evidence was found for a concurrent rise in urethral gonorrhoea. The proportion of MSM with a recent HIV test has been continuously increasing. Conclusions: We found no evidence for increasing 'carelessness' or 'prevention fatigue'. However, given the increasing proportions of MSM who frequently engage in anal intercourse with casual partners, a rise in HIV incidence is likely; especially if accompanied by a syphilis epidemic. Time trend analyses of these large behavioural follow-up surveys suggest that the rise in new HIV diagnoses among MSM in Germany may partially reflect an increased uptake of HIV-testing, rather than new infections due to the erosion of condom use or increased numbers of sex partners." (author's abstract