Projecte europeu per millorar la prevenció del VIH

Investigadors del CEEISCAT (Centre d'Estudis Epidemiològics sobre les ITS i Sida de Catalunya), amb participació de la UAB, coordinen l'estudi COBA-Cohort. L'objectiu és establir una cohort europea d'homes seronegatius que tenen sexe amb homes per entendre millor els patrons de realització de la prova de detecció del VIH i la seva propagació en aquest col·lectiu.Investigadores del CEEISCAT (Centro de Estudios Epidemiológicos sobre las ITS y SIDA de Catalunya), con participación de la UAB, coordinan el estudio COBA-Cohort. El objetivo es establecer una cohorte europea de hombres seronegativos que tienen sexo con hombres para entender mejor los patrones de realización de la prueba de detección del VIH y su propagación en este colectivo.Researchers from CEEISCAT (Centre for Epidemiological Studies on HIV/STI of Catalonia), with the participation of the UAB, coordinate the COBA-Cohort project. Its objective is to establish a European cohort of HIV-negative men who have sex with men to better understand HIV testing patterns and the spread of the virus among this population

From tunneling to contact: Inelastic signals in an atomic gold junction

The evolution of electron conductance in the presence of inelastic effects is studied as an atomic gold contact is formed evolving from a low-conductance regime (tunneling) to a high-conductance regime (contact). In order to characterize each regime, we perform density functional theory (DFT) calculations to study the geometric and electronic structures, together with the strength of the atomic bonds and the associated vibrational frequencies. The conductance is calculated by first evaluating the transmission of electrons through the system, and secondly by calculating the conductance change due to the excitation of vibrations. As found in previous studies [Paulsson et al., Phys. Rev. B. 72, 201101(R) (2005)] the change in conductance due to inelastic effects permits to characterize the crossover from tunneling to contact. The most notorious effect being the crossover from an increase in conductance in the tunneling regime to a decrease in conductance in the contact regime when the bias voltage matches a vibrational threshold. Our DFT-based calculations actually show that the effect of vibrational modes in electron conductance is rather complex, in particular when modes localized in the contact region are permitted to extend into the electrodes. As an example, we find that certain modes can give rise to decreases in conductance when in the tunneling regime, opposite to the above mentioned result. Whereas details in the inelastic spectrum depend on the size of the vibrational region, we show that the overall change in conductance is quantitatively well approximated by the simplest calculation where only the apex atoms are allowed to vibrate. Our study is completed by the application of a simplified model where the relevant parameters are obtained from the above DFT-based calculations.Comment: 8 pages, 5 figure

Unified description of inelastic propensity rules for electron transport through nanoscale junctions

We present a method to analyze the results of first-principles based calculations of electronic currents including inelastic electron-phonon effects. This method allows us to determine the electronic and vibrational symmeties in play, and hence to obtain the so-called propensity rules for the studied systems. We show that only a few scattering states -- namely those belonging to the most transmitting eigenchannels -- need to be considered for a complete description of the electron transport. We apply the method on first-principles calculations of four different systems and obtain the propensity rules in each case.Comment: 4 pages, 4 figures, 1 table http://link.aps.org/abstract/PRL/v100/e22660

Structural and magnetic properties of FeMnx_x (x=x=1...6) chains supported on Cu2_2N / Cu (100)

Heterogeneous atomic magnetic chains are built by atom manipulation on a Cu$_2$N/Cu (100) substrate. Their magnetic properties are studied and rationalized by a combined scanning tunneling microscopy (STM) and density functional theory (DFT) work completed by model Hamiltonian studies. The chains are built using Fe and Mn atoms ontop of the Cu atoms along the N rows of the Cu$_2$N surface. Here, we present results for FeMn$_x$ ($x$=1...6) chains emphasizing the evolution of the geometrical, electronic, and magnetic properties with chain size. By fitting our results to a Heisenberg Hamiltonian we have studied the exchange-coupling matrix elements $J$ for different chains. For the shorter chains, $x \leq 2$, we have included spin-orbit effects in the DFT calculations, extracting the magnetic anisotropy energy. Our results are also fitted to a simple anisotropic spin Hamiltonian and we have extracted values for the longitudinal-anisotropy $D$ and transversal-anisotropy $E$ constants. These parameters together with the values for $J$ allow us to compute the magnetic excitation energies of the system and to compare them with the experimental data.Comment: 10 pages 8 figure

Creating pseudo Kondo-resonances by field-induced diffusion of atomic hydrogen

In low temperature scanning tunneling microscopy (STM) experiments a cerium adatom on Ag(100) possesses two discrete states with significantly different apparent heights. These atomic switches also exhibit a Kondo-like feature in spectroscopy experiments. By extensive theoretical simulations we find that this behavior is due to diffusion of hydrogen from the surface onto the Ce adatom in the presence of the STM tip field. The cerium adatom possesses vibrational modes of very low energy (3-4meV) and very high efficiency (> 20%), which are due to the large changes of Ce-states in the presence of hydrogen. The atomic vibrations lead to a Kondo-like feature at very low bias voltages. We predict that the same low-frequency/high-efficiency modes can also be observed at lanthanum adatoms.Comment: five pages and four figure

Colloquium: Atomic spin chains on surfaces

In the present Colloquium, we focus on the properties of 1-D magnetic systems on solid surfaces. From the emulation of 1-D quantum phases to the potential realization of Majorana edge states, spin chains are unique systems to study. The advent of scanning tunnelling microscope (STM) based techniques has permitted us to engineer spin chains in an atom-by-atom fashion via atom manipulation and to access their spin states on the ultimate atomic scale. Here, we present the current state of research on spin correlations and dynamics of atomic spin chains as studied by the STM. After a brief review of the main properties of spin chains on solid surfaces, we classify spin chains according to the coupling of their magnetic moments with the holding substrate. This classification scheme takes into account that the nature and lifetimes of the spin-chain excitation intrinsically depend on the holding substrate. We first show the interest of using insulating layers on metals, which generally results in an increase in the spin state's lifetimes such that their quantized nature gets evident and they are individually accessible. Next, we show that the use of semiconductor substrates promises additional control through the tunable electron density via doping. When the coupling to the substrate is increased for spin chains on metals, the substrate conduction electron mediated interactions can lead to emergent exotic phases of the coupled spin chain-substrate conduction electron system. A particularly interesting example is furnished by superconductors. Magnetic impurities induce states in the superconducting gap. Due to the extended nature of the spin chain, the in-gap states develop into bands that can lead to the emergence of 1-D topological superconductivity and, consequently to the appearance of Majorana edge states

Understanding the Interlayer Coupling in 1T/1H-NbSe2_2 Hetero-Bilayers

The properties of 2D materials are strongly influenced by their substrate, leading to a variety of "proximity effects" like screening, charge transfer, and hybridization. Surprisingly, there is a dearth of theoretical studies on these effects. Particularly, previous theoretical research on the Star of David (SOD) structure in 1T-NbSe$_2$ has focused on single-layer configurations or stacking with the same 1T phase without any real substrate. Here, we depart from these approaches and explore how these proximity effects shape the electronic and magnetic properties of the 1T-NbSe$_2$ phase when it is grown on the metallic 1H-NbSe$_2$ substrate. Using Density Functional Calculations, we establish a common framework to define the key characteristics of both free-standning 1T-NbSe$_2$ and 1H-NbSe$_2$. We then identify the optimal stacking arrangement for these two layers, revealing a transfer from the 1T to the 1H phase and a reorganization of charge within each layer. Our findings indicate that the magnetic moment of the SOD structure is still robust; however, is diminished due to a reduction in the on-site Coulomb interaction of the Hubbard bands. Additionally, the interlayer coupling induces metallicity in the 1T phase and increases the decoupling of the lower Hubbard band from the valence band

Influence of Internalised Homonegativity on Sexual Risk Behaviour of Men Who Have Sex with Men in Spain

In a sample of men who have sex with men (MSM) (N = 3436) in Spain who bear intrinsic HIV risk, we investigated how internalised homonegativity (IH) is associated with the number of non-steady male partners with condomless intercourse (as a proxy of sexual risk behaviour). Using structural equation modelling (SEM), we examined the relationship between IH and sexual risk behaviour, and mediating effects of HIV/PrEP knowledge and substance use during sex on this relationship. We found no direct association between IH and sexual risk behaviour, nor did IH influence substance use during sex. In line with our hypothesis, association between IH and sexual risk behaviour was significant when mediated by HIV/PrEP knowledge. We found that as IH increased, sexual risk behaviour decreased, because higher IH was associated with lower HIV/PrEP knowledge while higher HIV/PrEP knowledge was associated with increased non-condom use with non-steady partners. Substance use during sex was significantly associated with sexual risk behaviour. Our results emphasize the continuing importance of prevention strategies focused on behavioural changes and community level interventions, especially targeting substance use