Strong electronic correlations in Lix_xZnPc organic metals

Nuclear magnetic resonance, electron paramagnetic resonance and magnetization measurements show that bulk Li$_x$ZnPc are strongly correlated one-dimensional metals. The temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ and of the static uniform susceptibility $\chi_S$ on approaching room temperature are characteristic of a Fermi liquid. Moreover, while for $x\simeq 2$ the electrons are delocalized down to low temperature, for $x\to 4$ a tendency towards localization is noticed upon cooling, yielding an increase both in $1/T_1$ and $\chi_s$. The $x$-dependence of the effective density of states at the Fermi level $D(E_F)$ displays a sharp enhancement for $x\simeq 2$, at the half filling of the ZnPc lowest unoccupied molecular orbitals. This suggests that Li$_x$ZnPc is on the edge of a metal-insulator transition where enhanced superconducting fluctuations could develop.Comment: 5 pages, 4 figure

Disentangling multipole resonances through a full x-ray polarization analysis

Complete polarization analysis applied to resonant x-ray scattering at the Cr K-edge in K2CrO4 shows that incident linearly polarized x-rays can be converted into circularly polarized x-rays by diffraction at the Cr pre-edge (E = 5994 eV). The physical mechanism behind this phenomenon is a subtle interference effect between purely dipole (E1-E1) and purely quadrupole (E2-E2) transitions, leading to a phase shift between the respective scattering amplitudes. This effect may be exploited to disentangle two close-lying resonances that appear as a single peak in a conventional energy scan, in this way allowing to single out and identify the different multipole order parameters involved.Comment: 6 pages, 6 figure

An optimum time-stepping scheme for N-body simulations

We present a new time-stepping criterion for N-body simulations that is based on the true dynamical time of a particle. This allows us to follow the orbits of particles correctly in all environments since it has better adaptivity than previous time-stepping criteria used in N-body simulations. Furthermore, it requires far fewer force evaluations in low density regions of the simulation and has no dependence on artificial parameters such as, for example, the softening length. This can be orders of magnitude faster than conventional ad-hoc methods that employ combinations of acceleration and softening and is ideally suited for hard problems, such as obtaining the correct dynamics in the very central regions of dark matter haloes. We also derive an eccentricity correction for a general leapfrog integration scheme that can follow gravitational scattering events for orbits with eccentricity e -> 1 with high precision. These new approaches allow us to study a range of problems in collisionless and collisional dynamics from few body problems to cosmological structure formation. We present tests of the time-stepping scheme in N-body simulations of 2-body orbits with eccentricity e -> 1 (elliptic and hyperbolic), equilibrium haloes and a hierarchical cosmological structure formation run.Comment: 15 pages, 10 figures, replaced with version that matches published versio

Intracluster stellar population properties from N-body cosmological simulations -- I. Constraints at z=0z=0

We use a high resolution collisionless simulation of a Virgo--like cluster in a $\Lambda$CDM cosmology to determine the velocity and clustering properties of the diffuse stellar component in the intracluster region at the present epoch. The simulated cluster builds up hierarchically and tidal interactions between member galaxies and the cluster potential produce a diffuse stellar component free-flying in the intracluster medium. Here we adopt an empirical scheme to identify tracers of the stellar component in the simulation and hence study its properties. We find that at $z=0$ the intracluster stellar light is mostly unrelaxed in velocity space and clustered in structures whose typical clustering radii are about 50 kpc at R=400--500 kpc from the cluster center, and predict the radial velocity distribution expected in spectroscopic follow-up surveys. Finally, we compare the spatial clustering in the simulation with the properties of the Virgo intracluster stellar population, as traced by ongoing intracluster planetary nebulae surveys in Virgo. The preliminary results indicate a substantial agreement with the observed clustering properties of the diffuse stellar population in Virgo.Comment: 39 pages, 10 figures, 8 tables, in press on ApJ. Bad image quality for some figures because resizing is neede

The descendents of Lyman Break Galaxies in galaxy clusters: spatial distribution and orbital properties

We combine semi-analytical methods with a ultra-high resolution simulation of a galaxy cluster (of mass 2.3 10^14h-1Msolar, and 4 10^6 particles within its virial radius) formed in a standard CDM universe to study the spatial distribution and orbital properties of the present-day descendents of Lyman Break Galaxies (LBGs). At the present time only five (out of 12) of halos containing LBGs survive as separate entities inside the cluster virial radius. Their circular velocities are in the range 200 - 550 km/sec. Seven halos merged together to form the central object at the very center of the cluster. Using semi-analytical modeling of galaxy evolution we show that descendents of halos containing LBGs now host giant elliptical galaxies. Galaxy orbits are radial, with a pericenter to apocenter ratio of about 1:5. The orbital eccentricities of LBGs descendents are statistically indistinguishable from those of the average galaxy population inside the cluster, suggesting that the orbits of these galaxies are not significantly affected by dynamical friction decay after the formation of the cluster's main body. In this cluster, possibly due to its early formation time, the descendents of LBGs are contained within the central 60% of the cluster virial radius and have an orbital velocity dispersion lower than the global galaxy population, originating a mild luminosity segregation for the brightest cluster members. Mass estimates based only on LBGs descendents (especially including the central cD) reflect this bias in space and velocity and underestimate the total mass of this well virialized cluster by up to a factor of two compared to estimates using at least 20 cluster members.Comment: 6 Pages, 2 Postscript figures. Submitted to Ap

Density profiles and substructure of dark matter halos: converging results at ultra-high numerical resolution

Can N-body simulations reliably determine the structural properties of dark matter halos? Focussing on a Virgo-sized galaxy cluster, we increase the resolution of current ``high resolution simulations'' by almost an order of magnitude to examine the convergence of the important physical quantities. We have 4 million particles within the cluster and force resolution 0.5 kpc/h (0.05% of the virial radius). The central density profile has a logarithmic slope of -1.5, as found in lower resolution studies of the same halo, indicating that the profile has converged to the ``physical'' limit down to scales of a few kpc. Also the abundance of substructure is consistent with that derived from lower resolution runs; on the scales explored, the mass and circular velocity functions are close to power laws of exponents ~ -1.9 and -4. Overmerging appears to be globally unimportant for suhalos with circular velocities > 100 km/s. We can trace most of the cluster progenitors from z=3 to the present; the central object (the dark matter analog of a cD galaxy)is assembled between z=3 and 1 from the merging of a dozen halos with v_circ \sim 300 km/s. The mean circular velocity of the subhalos decreases by ~ 20% over 5 billion years, due to tidal mass loss. The velocity dispersions of halos and dark matter globally agree within 10%, but the halos are spatially anti-biased, and, in the very central region of the cluster, they show positive velocity bias; however, this effect appears to depend on numerical resolution.Comment: 19 pages, 13 figures, ApJ, in press. Text significantly clarifie

Perturbative signature of substructures in strong gravitational lenses

In the perturbative approach, substructures in the lens can be reduced to their effect on the two perturbative fields $f_1$ and $\frac{d f_0}{d\theta}$. A simple generic model of elliptical lens with a substructure situated near the critical radius is investigated in details. Analytical expressions are derived for each perturbative field, and basic properties are analyzed. The power spectrum of the fields is well approximated by a power-law, resulting in significant tails at high frequencies. Another feature of the perturbation by a substructure is that the ratio of the power spectrum at order $n$ of the 2 fields $R_n$ is nearly 1. The ratio $R_n \simeq 1$ is specific to substructures, for instance an higher order distortion ($n>2$) but with auto-similar isophotes will result in $R_n \propto \frac{1}{n^2}$. Finally, the problem of reconstructing the perturbative field is investigated. Local field model are implemented and fitted to maximize image similarity in the source plane. The non-linear optimization is greatly facilitated, since in the perturbative approach the circular source solution is always known. Examples of images distortions in the subcritical regime due to substructures are presented, and analyzed for different source shapes. Provided enough images and signal is available, the substructure field can be identified confidently. These results suggests that the perturbative method is an efficient tool to estimate the contribution of substructures to the mass distribution of lenses.Comment: 18 pages, 11 figure

The Missing Piece: The Structure of the Ti3C2TxMXene and Its Behavior as Negative Electrode in Sodium Ion Batteries

The most common MXene composition Ti3C2Tx (T = F, O) shows outstanding stability as anode for sodium ion batteries (100% of capacity retention after 530 cycles with charge efficiency >99.7%). However, the reversibility of the intercalation/deintercalation process is strongly affected by the synthesis parameters determining, in turn, significant differences in the material structure. This study proposes a new approach to identify the crystal features influencing the performances, using a structural model built with a multitechnique approach that allows exploring the short-range order of the lamella. The model is then used to determine the long-range order by inserting defective elements into the structure. With this strategy it is possible to fit the MXene diffraction patterns, obtain the structural parameters including the stoichiometric composition of the terminations (neutron data), and quantify the structural disorder which can be used to discriminate the phases with the best electrochemical properties