Low-energy excitations in electron-doped metal phthalocyanine from NMR in Li0.5_{0.5}MnPc

$^7$Li and $^1$H NMR and magnetization measurements in \lpc (Pc$\equiv$C$_{32}$H$_{16}$N$_8$), recently proposed as a strongly correlated metal, are presented. Two different low-frequency dynamics are evidenced. The first one, probed by $^1$H nuclei gives rise to a slowly relaxing magnetization at low temperature and is associated with the freezing of MnPc $S=3/2$ spins. This dynamic is similar to the one observed in pristine $\beta$-MnPc and originates from Li depleted chain segments. The second one, evidenced by $^7$Li spin-lattice relaxation rate, is associated with the hopping of the electrons along Li-rich chains. The characteristic correlation times for the two dynamics are derived and the role of disorder is briefly discussed.Comment: 7 two-columns pages, 11 figure

Detailed abundances of a large sample of giant stars in M 54 and in the Sagittarius nucleus

Homogeneous abundances of light elements, alpha and Fe-group elements from high-resolution FLAMES spectra are presented for 76 red giant stars in M54, a massive globular cluster (GC) lying in the nucleus of the Sagittarius dwarf galaxy. We also derived detailed abundances for 27 red giants belonging to the Sgr nucleus. Our abundances assess the intrinsic metallicity dispersion (~0.19 dex, rms scatter) of M54, with the bulk of stars peaking at [Fe/H]~-1.6 and a long tail extending to higher metallicities, similar to omega Cen. The spread in these probable nuclear star clusters exceeds those of most GCs: these massive clusters are located in a region intermediate between normal GCs and dwarf galaxies. M54 shows the Na-O anticorrelation, typical signature of GCs, which is instead absent in the Sgr nucleus. The light elements (Mg, Al, Si) participating to the high temperature Mg-Al cycle show that the pattern of (anti)correlations produced by proton-capture reactions in H-burning is clearly different between the most metal-rich and most metal-poor components in the two most massive GCs in the Galaxy, confirming early result based on the Na-O anticorrelation. As in omega Cen, stars affected by most extreme processing, i.e. showing the signature of more massive polluters, are those of the metal-rich component. This can be understood if the burst of star formation giving birth to the metal-rich component was delayed by as much as 10-30 Myr with respect to the metal-poor one. The evolution of these massive GCs can be reconciled in the general scenario for the formation of GCs sketched in Carretta et al.(2010a) taking into account that omega Cen could have already incorporated the surrounding nucleus of its progenitor and lost the rest of the hosting galaxy while the two are still observable as distinct components in M54 and the surrounding field.Comment: 22 pages (3 pages of appendix), 25 figures. Tables 2, 3, 5, 6, and 7 are only available in electronic form at the CDS Accepted for publication on Astronomy and Astrophysic

Low-energy excitations in the S=(1/2) molecular nanomagnet K6[V₁₅^IVAs6O42(H2O)]·8H2O from proton NMR and µSR

Zero- and longitudinal-field muon-spin-rotation (µSR) and 1H NMR measurements on the S=(1/2) molecular nanomagnet K6[V15IVAs6O42(H2O)]·8H2O are presented. In LF experiments, the muon asymmetry P(t) was fitted by the sum of three different exponential components with fixed weights. The different muon relaxation rates lambdai (i=1,2,3) and the low-field H=0.23 T 1H NMR spin-lattice relaxation rate 1/T1 show a similar behavior for T>50 K: starting from room temperature they increase as the temperature is decreased. The increase of lambdai and 1/T1 can be attributed to the "condensation" of the system toward the lowest-lying energy levels. The gap Delta~550 K between the first and second S=(3/2) excited states was determined experimentally. For T<2 K, the muon relaxation rates lambdai stay constant, independently of the field value H<=0.15 T. The behavior for T<2 K strongly suggests that, at low T, the spin fluctuations are not thermally driven but rather originate from quasielastic intramolecular or intermolecular magnetic interactions. We suggest that the very-low-temperature relaxation rates could be driven by energy exchanges between two almost degenerate S=(1/2) ground states and/or by quantum effects

The poisoning effect of Mn in LaFe(1-x)Mn(x)AsO(0.89)F(0.11): unveiling a quantum critical point in the phase diagram of iron-based superconductors

A superconducting-to-magnetic transition is reported for LaFe$_{1-x}$Mn$_x$AsO$_{0.89}$F$_{0.11}$ where a per thousand amount of Mn impurities is dispersed. By employing local spectroscopic techniques like muon spin rotation (muSR) and nuclear quadrupole resonance (NQR) on compounds with Mn contents ranging from x=0.025% to x=0.75%, we find that the electronic properties are extremely sensitive to the Mn impurities. In fact, a small amount of Mn as low as 0.2% suppresses superconductivity completely. Static magnetism, involving the FeAs planes, is observed to arise for x > 0.1% and becomes further enhanced upon increasing Mn substitution. Also a progressive increase of low energy spin fluctuations, leading to an enhancement of the NQR spin-lattice relaxation rate 1/T1, is observed upon Mn substitution. The analysis of 1/T1 for the sample closest to the the crossover between superconductivity and magnetism (x = 0.2%) points towards the presence of an antiferromagnetic quantum critical point around that doping level.Comment: 11 pages, 10 figure

The normal chemistry of multiple stellar populations in the dense globular cluster NGC 6093 (M 80)

We present the abundance analysis of 82 red giant branch stars in the dense, metal-poor globular cluster NGC 6093 (M 80), the largest sample of stars analyzed in this way for this cluster. From high resolution UVES spectra of 14 stars and intermediate resolution GIRAFFE spectra for the other stars we derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu. On our UVES metallicity scale the mean metal abundance of M 80 is [Fe/H]=-1.791+/-0.006+/-0.076 (+/-statistical +/-systematic error) with rms=0.023 (14 stars). M 80 shows star to star variations in proton-capture elements, and the extension of the Na-O anticorrelation perfectly fit the relations with (i) total cluster mass, (ii) horizontal branch morphology, and (iii) cluster concentration previously found by our group. The chemistry of multiple stellar populations in M 80 does not look extreme. The cluster is also a typical representative of halo globular clusters for what concerns the pattern of alpha-capture and Fe-group elements. However we found that a significant contribution from the s-process is required to account for the distribution of neutron-capture elements. A minority of stars in M 80 seem to exhibit slightly enhanced abundances of s-process species, compatible with those observed in M 22 and NGC 1851, although further confirmation from larger samples is required.Comment: 18 pages, 21 figures, 10 tables; accepted for publication on Astronomy and Astrophysic

The Na-O anticorrelation in horizontal branch stars. III. 47 Tuc and M 5

To check the impact of the multiple population scenario for globular clusters on their HB, we present an analysis of the composition of 110 red HB (RHB) stars in 47 Tucanae and of 61 blue HB (BHB) and 30 RHB stars in M5. In 47 Tuc we found tight relations between the colours of the stars and their abundances of p-capture elements. This strongly supports the idea that the He content - which is expected to be closely correlated with the abundances of p-capture elements - is the third parameter (after overall metallicity and age) that determines the colour of HB stars. However, the range in He abundance must be small (Delta Y<0.03) in 47 Tuc to reproduce our observations; this agrees with previous analyses. There is possibly a correlation between the abundances of p- and n-capture elements in 47 Tuc. If confirmed, this might suggest that AGB stars of moderate mass contributed to the gas from which second-generation stars formed. Considering the selection effects in our sample (which does not include stars warmer than 11000 K and RR Lyrae variables) is important to understand our results for M5. In this case, we find that, as expected, RHB stars are Na-poor and O-rich, and likely belong to the primordial population. There is a clear correlation of the [Na/O] ratio and N abundance with colour along the BHB. A derivation of the He abundance for these stars yields a low value of Y=0.22\pm 0.03. This is expected because HB stars of a putative He-rich population in this cluster should be warmer than 11000 K, and would accordingly not have been sampled by our analysis. However, we need some additional source of scatter in the total mass loss of stars climbing up the RGB to reproduce our results for M5. Finally, we found a C-star on the HB of 47 Tuc and a Ba-rich, fast-rotating, likely binary star on the HB of M5. These stars are among the brightest and coolest HB stars.Comment: Accepted on Astronomy and Astrophysics. 16 pages, 19 figure

The onset of magnetism peaked around x=1/4 in optimally electron-doped LnFe(1-x)Ru(x)AsO(1-y)F(y) (Ln = La, Nd or Sm) superconductors

The appearance of static magnetism, nanoscopically coexisting with superconductivity, is shown to be a general feature of optimally electron-doped LnFe(1-x)Ru(x)AsO(1-y)F(y) superconductor (Ln - lanthanide ion) upon isovalent substitution of Fe by Ru. The magnetic ordering temperature T_N and the magnitude of the internal field display a dome-like dependence on x, peaked around x=1/4, with higher T_N values for those materials characterized by a larger z cell coordinate of As. Remarkably, the latter are also those with the highest superconducting transition temperature (T_c) for x=0. The reduction of T_c(x) is found to be significant in the x region of the phase diagram where the static magnetism develops. Upon increasing the Ru content superconductivity eventually disappears, but only at x=0.6.Comment: accepted for publication in PR

The extreme chemistry of multiple stellar populations in the metal-poor globular cluster NGC 4833

Our FLAMES survey of Na-O anticorrelation in globular clusters (GCs) is extended to NGC 4833, a metal-poor GC with a long blue tail on the horizontal branch (HB). We present the abundance analysis for a large sample of 78 red giants based on UVES and GIRAFFE spectra acquired at the ESO-VLT. We derived abundances of Na, O, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ba, La, Nd. This is the first extensive study of this cluster from high resolution spectroscopy. On the scale of our survey, the metallicity of NGC 4833 is [Fe/H]=-2.015+/-0.004+/-0.084 dex (rms=0.014 dex) from 12 stars observed with UVES, where the first error is from statistics and the second one refers to the systematic effects. The iron abundance in NGC 4833 is homogeneous at better than 6%. On the other hand, the light elements involved in proton-capture reactions at high temperature show the large star-to-star variations observed in almost all GCs studied so far. The Na-O anticorrelation in NGC 4833 is quite extended, as expected from the high temperatures reached by stars on the HB, and NGC 4833 contains a conspicuous fraction of stars with extreme [O/Na] ratios. More striking is the finding that large star-to-star variations are seen also for Mg, which spans a range of more than 0.5 dex in this GC. Depletions in Mg are correlated to the abundances of O and anti-correlated with Na, Al, and Si abundances. This pattern suggests the action of nuclear processing at unusually high temperatures, producing the extreme chemistry observed in the stellar generations of NGC 4833. This extreme changes are also seen in giants of the much more massive GCs M 54 and omega Cen, and our conclusion is that NGC 4833 has probably lost a conpicuous fraction of its original mass due to bulge shocking, as also indicated by its orbit.Comment: 18 pages, 16 figures, 11 tables; accepted for publication on Astronomy and Astrophysic

NGC 362: another globular cluster with a split red giant branch

We obtained FLAMES GIRAFFE+UVES spectra for both first and second-generation red giant branch (RGB) stars in the globular cluster (GC) NGC 362 and used them to derive abundances of 21 atomic species for a sample of 92 stars. The surveyed elements include proton-capture (O, Na, Mg, Al, Si), alpha-capture (Ca, Ti), Fe-peak (Sc, V, Mn, Co, Ni, Cu), and neutron-capture elements (Y, Zr, Ba, La, Ce, Nd, Eu, Dy). The analysis is fully consistent with that presented for twenty GCs in previous papers of this series. Stars in NGC 362 seem to be clustered into two discrete groups along the Na-O anti-correlation, with a gap at [O/Na] 0 dex. Na-rich, second generation stars show a trend to be more centrally concentrated, although the level of confidence is not very high. When compared to the classical second-parameter twin NGC 288, with similar metallicity, but different horizontal branch type and much lower total mass, the proton-capture processing in stars of NGC 362 seems to be more extreme, confirming previous analysis. We discovered the presence of a secondary RGB sequence, redder than the bulk of the RGB: a preliminary estimate shows that this sequence comprises about 6% of RGB stars. Our spectroscopic data and literature photometry indicate that this sequence is populated almost exclusively by giants rich in Ba, and probably rich in all s-process elements, as found in other clusters. In this regards, NGC 362 joins previously studied GCs like NGC 1851, NGC 6656 (M 22), and NGC 7089 (M 2).Comment: 16 pages, 23 figures, 11 tables, accepted for publication on Astronomy and Astrophysic

The Na-O anticorrelation in horizontal branch stars. IV. M22

We obtained high-resolution spectra for 94 candidate stars belonging to the HB of M22 with FLAMES. The HB stars we observed span a restricted temperature range (7,800<Teff<11,000 K), where about 60% of the HB stars of M22 are. Within our sample, we can distinguish three groups of stars segregated (though contiguous) in colours: Group 1 (49 stars) is metal-poor, N-normal, Na-poor and O-rich with abundances that match those determined for the primordial group of RGB stars from previous studies. Group 2 (23 stars) is still metal-poor, but it is N- and Na-rich, though only very mildly depleted in O. We can identify this intermediate group as the progeny of the metal-poor RGB stars that occupy an intermediate location along the Na-O anti-correlation. The third group (20 stars) is metal-rich, Na-rich, and O-rich and likely corresponds to the most O-rich component of the previously found metal-rich RGB population. We did not observe any severely O-depleted stars and we think that the progeny of these stars falls on the hotter part of the HB. The metal-rich population is also over-abundant in Sr, in agreement with results for corresponding RGB and SGB stars. However, we do not find any significant variation in the ratio between the sum of N and O abundances to Fe. There is some evidence of an enhancement of He content for Groups 2 and 3 stars (Y=0.338\pm 0.014\pm 0.05). Our results agree with the proposition that chemical composition drives the location of stars along the HB of a GC. Furthermore, we found a number of fast rotators. They are concentrated in a restricted temperature range along the HB of M22.Comment: Accepted for publication on Astronomy and Astrophysics. 23 pages, 21 figure