S-mixing and quantum tunneling of the magnetization in molecular nanomagnets

The role of $S$-mixing in the quantum tunneling of the magnetization in nanomagnets has been investigated. We show that the effect on the tunneling frequency is huge and that the discrepancy (more than 3 orders of magnitude in the tunneling frequency) between spectroscopic and relaxation measurements in Fe$_8$ can be resolved if $S$-mixing is taken into account.Comment: REVTEX, 10 pages, 3 jpg figures, to appear in PR

Frustration driven structural distortion in VOMoO4

Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetization measurements and electronic structure calculations in VOMoO4 are presented. It is found that VOMoO4 is a frustrated two-dimensional antiferromagnet on a square lattice with competing exchange interactions along the side J1 and the diagonal J2 of the square. From magnetization measurements J1+J2 is estimated around 155 K, in satisfactory agreement with the values derived from electronic structure calculations. Around 100 K a structural distortion, possibly driven by the frustration, is evidenced. This distortion induces significant modifications in the NMR and EPR spectra which can be accounted for by valence fluctuations. The analysis of the spectra suggests that the size of the domains where the lattice is distorted progressively grows as the temperature approaches the transition to the magnetic ground state at Tc=42 K

Enhancement of rare-earth--transition-metal exchange interaction in Pr2_{2}Fe17_{17} probed by inelastic neutron scattering

The fundamental magnetic interactions of Pr$_{2}$Fe$_{17}$ are studied by inelastic neutron scattering and anisotropy field measurements. Data analysis confirms the presence of three magnetically inequivalent sites, and reveals an exceptionally large value of the exchange field. The unexpected importance of $J$-mixing effects in the description of the ground-state properties of Pr$_{2}$Fe$_{17}$ is evidenced, and possible applications of related compounds are envisaged.Comment: 4 RevTeX pages, 4 EPS figures. Accepted for publication by Appl. Phys. Lett. (will be found at http://apl.aip.org

On the Formation of Multiple Stellar Populations in Globular Clusters

Nearly all globular clusters (GCs) studied to date show evidence for multiple stellar populations, in stark contrast to the conventional view that GCs are a mono-metallic, coeval population of stars. Building on earlier work, we propose a simple physical model for the early evolution (several 10^8 yr) of GCs. We consider the effects of stellar mass-loss, type II and prompt type Ia supernovae, ram pressure, and accretion from the ambient ISM on the development of a young GC's own gas reservoir. In our model, type II SNe from a first generation of star formation clears the GC of its initial gas reservoir. Over the next several 10^8 yr, mass lost from AGB stars and matter accreted from the ambient ISM collect at the center of the GC. This material must remain quite cool (T~10^2K), but does not catastrophically cool on a crossing time because of the high Lyman-Werner flux density in young GCs. The collection of gas within the GC must compete with ram pressure from the ambient ISM. After several 10^8 yr, the Lyman-Werner photon flux density drops by more than three orders of magnitude, allowing molecular hydrogen and then stars to form. After this second generation of star formation, type II SNe from the second generation and then prompt type Ia SNe associated with the first generation maintain a gas-free GC, thereby ending the cycle of star formation events. Our model makes clear predictions for the presence or absence of multiple stellar populations within GCs as a function of GC mass and formation environment. Analyzing intermediate-age LMC clusters, we find evidence for a mass threshold of ~10^4 Msun below which LMC clusters appear to be truly coeval. This threshold mass is consistent with our predictions for the mass at which ram pressure is capable of clearing gas from clusters in the LMC at the present epoch. (ABRIDGED)Comment: 13 pages, 5 figures, ApJ in pres

Observational evidence for a different IMF in the early Galaxy

The unexpected high incidence of carbon-enhanced, s-process enriched unevolved stars amongst extremely metal-poor stars in the halo provides a significant constraint on the Initial Mass Function (IMF) in the early Galaxy. We argue that these objects are evidence for the past existence of a large population of intermediate-mass stars, and conclude that the IMF in the early Galaxy was different from the present, and shifted toward higher masses.Comment: 14 pages, 1 color figure, accepted for publication on Ap

Tuning of competing magnetic and superconducting phase volumes in LaFeAsO$_0.945F_0.055 by hydrostatic pressure

The interplay between magnetism and superconductivity in LaFeAsO_0.945F_0.055 was studied as a function of hydrostatic pressure up to p~2.4GPa by means of muon-spin rotation (\muSR) and magnetization measurements. The application of pressure leads to a substantial decrease of the magnetic ordering temperature T_N and a reduction of the magnetic phase volume and, at the same time, to a strong increase of the superconducting transition temperature T_c and the diamagnetic susceptibility. From the volume sensitive \muSR measurements it can be concluded that the superconducting and the magnetic areas which coexist in the same sample are inclined towards spatial separation and compete for phase volume as a function of pressure.Comment: 4 pages, 4 figure

Fast recovery of the stripe magnetic order by Mn/Fe substitution in F-doped LaFeAsO superconductors

$^{75}$As Nuclear Magnetic (NMR) and Quadrupolar (NQR) Resonance were used, together with M\"{o}ssbauer spectroscopy, to investigate the magnetic state induced by Mn for Fe substitutions in F-doped LaFe$_{1-x}$Mn$_{x}$AsO superconductors. The results show that $0.5$% of Mn doping is enough to suppress the superconducting transition temperature $T_c$ from 27 K to zero and to recover the magnetic structure observed in the parent undoped LaFeAsO. Also the tetragonal to orthorhombic transition of the parent compound is recovered by introducing Mn, as evidenced by a sharp drop of the NQR frequency. The NQR spectra also show that a charge localization process is at play in the system. Theoretical calculations using a realistic five-band model show that correlation-enhanced RKKY exchange interactions between nearby Mn ions stabilize the observed magnetic order, dominated by $Q_1=(\pi,0)$ and $Q_2=(0,\pi)$ ordering vectors. These results give compelling evidence that F-doped LaFeAsO is a strongly correlated electron system at the verge of an electronic instability.Comment: 5 pages, 5 figures and 4 pages of supplemental materia

Discovery of Carbon/Oxygen depleted Blue Straggler Stars in 47 Tucanae: the chemical signature of a mass-transfer formation process

We use high-resolution spectra obtained with the ESO Very Large Telescope to measure surface abundance patterns of 43 Blue Stragglers stars (BSS) in 47 Tuc. We discovered that a sub-population of BSS shows a significant depletion of Carbon and Oxygen with respect to the dominant population. This evidence would suggest the presence of CNO burning products on the BSS surface coming from a deeply peeled parent star, as expected in the case of mass-transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster.Comment: Published on 2006, August 10, in ApJ 647, L5

Suppression of Dimer Correlations in the Two-Dimensional J1J_1-J2J_2 Heisenberg Model: an Exact Diagonalization Study

We present an exact diagonalization study of the ground state of the spin-half $J_1{-}J_2$ model. Dimer correlation functions and the susceptibility associated to the breaking of the translational invariance are calculated for the $4\times 4$ and the $6\times 6$ clusters. These results -- especially when compared to the one dimensional case, where the occurrence of a dimerized phase for large enough frustration is well established -- suggest either a homogeneous spin liquid or, possibly, a dimerized state with a rather small order parameter