Spin-lattice coupling in frustrated antiferromagnets

We review the mechanism of spin-lattice coupling in relieving the geometrical frustration of pyrochlore antiferromagnets, in particular spinel oxides. The tetrahedral unit, which is the building block of the pyrochlore lattice, undergoes a spin-driven Jahn-Teller instability when lattice degrees of freedom are coupled to the antiferromagnetism. By restricting our considerations to distortions which preserve the translational symmetries of the lattice, we present a general theory of the collective spin-Jahn-Teller effect in the pyrochlore lattice. One of the predicted lattice distortions breaks the inversion symmetry and gives rise to a chiral pyrochlore lattice, in which frustrated bonds form helices with a definite handedness. The chirality is transferred to the spin system through spin-orbit coupling, resulting in a long-period spiral state, as observed in spinel CdCr2O4. We discuss explicit models of spin-lattice coupling using local phonon modes, and their applications in other frustrated magnets.Comment: 23 pages, 6 figures. Lecture notes for Trieste Summer School, August 2007. To appear as a chapter in "Highly Frustrated Magnetism", Eds. C. Lacroix, P. Mendels, F. Mil

Implementation of the Hierarchical Reference Theory for simple one-component fluids

Combining renormalization group theoretical ideas with the integral equation approach to fluid structure and thermodynamics, the Hierarchical Reference Theory is known to be successful even in the vicinity of the critical point and for sub-critical temperatures. We here present a software package independent of earlier programs for the application of this theory to simple fluids composed of particles interacting via spherically symmetrical pair potentials, restricting ourselves to hard sphere reference systems. Using the hard-core Yukawa potential with z=1.8/sigma for illustration, we discuss our implementation and the results it yields, paying special attention to the core condition and emphasizing the decoupling assumption's role.Comment: RevTeX, 16 pages, 2 figures. Minor changes, published versio

Recent developments of the Hierarchical Reference Theory of Fluids and its relation to the Renormalization Group

The Hierarchical Reference Theory (HRT) of fluids is a general framework for the description of phase transitions in microscopic models of classical and quantum statistical physics. The foundations of HRT are briefly reviewed in a self-consistent formulation which includes both the original sharp cut-off procedure and the smooth cut-off implementation, which has been recently investigated. The critical properties of HRT are summarized, together with the behavior of the theory at first order phase transitions. However, the emphasis of this presentation is on the close relationship between HRT and non perturbative renormalization group methods, as well as on recent generalizations of HRT to microscopic models of interest in soft matter and quantum many body physics.Comment: 17 pages, 5 figures. Review paper to appear in Molecular Physic

Dynamical arrest and replica symmetry breaking in attractive colloids

Within the Replica Symmetry Breaking (RSB) framework developed by M.Mezard and G.Parisi we investigate the occurrence of structural glass transitions in a model of fluid characterized by hard sphere repulsion together with short range attraction. This model is appropriate for the description of a class of colloidal suspensions. The transition line in the density-temperature plane displays a reentrant behavior, in agreement with Mode Coupling Theory (MCT), a dynamical approach based on the Mori-Zwanzig formalism. Quantitative differences are however found, together with the absence of the predicted glass-glass transition at high density. We also perform a systematic study of the pure hard sphere fluid in order to ascertain the accuracy of the adopted method and the convergence of the numerical procedure.Comment: 7 pages, 6 figure

Phase transitions in simple and not so simple binary fluids

Compared to pure fluids, binary mixtures display a very diverse phase behavior, which depends sensitively on the parameters of the microscopic potential. Here we investigate the phase diagrams of simple model mixtures by use of a microscopic implementation of the renormalization group technique. First, we consider a symmetric mixture with attractive interactions, possibly relevant for describing fluids of molecules with internal degrees of freedom. Despite the simplicity of the model, slightly tuning the strength of the interactions between unlike species drastically changes the topology of the phase boundary, forcing or inhibiting demixing, and brings about several interesting features such as double critical points, tricritical points, and coexistence domains enclosing `islands' of homogeneous, mixed fluid. Homogeneous phase separation in mixtures can be driven also by purely repulsive interactions. As an example, we consider a model of soft particles which has been adopted to describe binary polymer solutions. This is shown to display demixing (fluid-fluid) transition at sufficiently high density. The nature and the physical properties of the corresponding phase transition are investigated.Comment: 6 pages + 3 figures, presented at the 5th EPS Liquid Matter Conference, Konstanz, 14-18 September 200

Compton-thick AGN in the NuSTAR era II: A deep NuSTAR and XMM-Newton view of the candidate Compton thick AGN in NGC 1358

We present the combined NuSTATR and XMM-Newton 0.6-79 keV spectral analysis of a Seyfert 2 galaxy, NGC 1358, which we selected as a candidate Compton thick (CT-) active galactic nucleus (AGN) on the basis of previous Swift/BAT and Chandra studies. According to our analysis, NGC 1358 is confirmed to be a CT-AGN using physical motivated models, at >3 $\sigma$ confidence level. Our best-fit shows that the column density along the 'line-of-sight' of the obscuring material surrounding the accreting super-massive black hole is N$\rm _H$ = [1.96--2.80] $\times$ 10$^{24}$ cm$^{-2}$. The high-quality data from NuSTAR gives the best constraints on the spectral shape above $\sim$10 keV to date on NGC 1358. Moreover, by combining NuSTAR and XMM-Newton data, we find that the obscuring torus has a low covering factor ($f_c$ <0.17), and the obscuring material is distributed in clumps, rather than uniformly. We also derive an estimate of NGC 1358's Eddington ratio, finding it to be $\lambda_{\rm Edd}$ $\sim$$4.7_{-0.3}^{+0.3}$ $\times$ 10$^{-2}$, which is in acceptable agreement with previous measurements. Finally, we find no evidence of short-term variability, over a $\sim$100 ks time-span, in terms of both 'line-of-sight' column density and flux.Comment: 12 pages, 6 figure

Providing Telco-oriented Network Services with eBPF: The Case for a 5G Mobile Gateway

Although several technologies exist for high-speed data plane processing, such as DPDK, the above technologies require a rigid partitioning of the resources of the system, such as dedicated CPU cores and network interfaces. Unfortunately, this is not always possible when running at the edge of the network, in which a few servers are available in each cluster and a mixture of data and control plane services must coexist on the same hardware. In this respect, eBPF can become a better alternative thanks to its integration in the vanilla Linux kernel, which enables contemporary support for data and control plane services, hence enabling a more efficient usage of the (scarce) computing resources. This paper proposes the first proof-of-concept open-source implementation of a 5G Mobile Gateway based on eBPF/XDP, highlighting the possible challenges (e.g., to create traffic policers, as buffering is not available in eBPF) and the resulting architecture. The result is characterized in terms of performance and scalability and compared with alternative technologies, showing that it outperforms other in-kernel solutions (e.g., Open vSwitch) and is comparable with DPDK-based platforms

A proof-of-concept 5G mobile gateway with eBPF

In this poster we propose the first proof-of-concept open-source implementation of a 5G Mobile Gateway based on eBPF/XDP and present benchmarks that compare its performance with alternative technologies. We show how it outperforms other in-kernel solutions (e.g., OvS) and is comparable with DPDK-based platforms

The Swift-BAT survey reveals the orbital period of three high-mass X-ray binaries

A growing number of previously hidden Galactic X-ray sources are now detected with recent surveys performed by the Integral and Swift satellites. Most of these new sources eluded past surveys due to their large local X-ray extinction and consequent low soft X-ray flux. The Swift-BAT performs daily monitoring of the sky in an energy band (15-150 keV) which is only marginally affected by X-ray extinction, thus allowing for the search of long periodicities in the light curve and identification of the nature of the X-ray sources. We performed a period search using the folding technique in the Swift-BAT light curves of three Integral sources: IGR J05007-7047, IGR J13186-6257 and IGR J17354-3255. Their periodograms show significant peaks at 30.77$\pm$0.01 d, 19.994$\pm$0.01 d and 8.448$\pm$0.002 d, respectively. We estimate the significance of these features from the chi squared distribution of all the trials, finding a probability less than 1.5$\times10^{-4}$ that the detections occurred due to chance. We complement our analysis with the study of their broadband X-ray emission. We identify the periodicities with the orbital periods of the sources. The periods are typical for the wind accretors X-ray binaries and we support this identification showing that also their energy spectra are compatible with an X-ray spectral emission characteristic of high-mass X-ray binaries. The spectrum of IGR J05007-704 that resides in the Large Magellanic Cloud, does not show any intrinsic local absorption, whereas the spectra of the Galactic sources IGR J17354-3255 and IGR J13186-6257 may be affected by a local absorber. The folded light curve for IGR J13186-6257 suggests a possible Be companion star.Comment: 10 pages, 14 figures. Accepted for publication in A&