Magnetic Monopoles in Spin Ice

Electrically charged particles, such as the electron, are ubiquitous. By contrast, no elementary particles with a net magnetic charge have ever been observed, despite intensive and prolonged searches. We pursue an alternative strategy, namely that of realising them not as elementary but rather as emergent particles, i.e., as manifestations of the correlations present in a strongly interacting many-body system. The most prominent examples of emergent quasiparticles are the ones with fractional electric charge e/3 in quantum Hall physics. Here we show that magnetic monopoles do emerge in a class of exotic magnets known collectively as spin ice: the dipole moment of the underlying electronic degrees of freedom fractionalises into monopoles. This enables us to account for a mysterious phase transition observed experimentally in spin ice in a magnetic field, which is a liquid-gas transition of the magnetic monopoles. These monopoles can also be detected by other means, e.g., in an experiment modelled after the celebrated Stanford magnetic monopole search.Comment: (6 pages, 6 figures) v2: fig 3 replaced with colour version. For the high-definition version of the paper click http://www-thphys.physics.ox.ac.uk/user/ClaudioCastelnovo/Publications/papersub.pd

Tunable non-equilibrium dynamics: field quenches in spin ice

We present non-equilibrium physics in spin ice as a novel setting which combines kinematic constraints, emergent topological defects, and magnetic long range Coulomb interactions. In spin ice, magnetic frustration leads to highly degenerate yet locally constrained ground states. Together, they form a highly unusual magnetic state -- a "Coulomb phase" -- whose excitations are pointlike defects -- magnetic monopoles -- in the absence of which effectively no dynamics is possible. Hence, when they are sparse at low temperature, dynamics becomes very sluggish. When quenching the system from a monopole-rich to a monopole-poor state, a wealth of dynamical phenomena occur the exposition of which is the subject of this article. Most notably, we find reaction diffusion behaviour, slow dynamics due to kinematic constraints, as well as a regime corresponding to the deposition of interacting dimers on a honeycomb lattice. We also identify new potential avenues for detecting the magnetic monopoles in a regime of slow-moving monopoles. The interest in this model system is further enhanced by its large degree of tunability, and the ease of probing it in experiment: with varying magnetic fields at different temperatures, geometric properties -- including even the effective dimensionality of the system -- can be varied. By monitoring magnetisation, spin correlations or zero-field Nuclear Magnetic Resonance, the dynamical properties of the system can be extracted in considerable detail. This establishes spin ice as a laboratory of choice for the study of tunable, slow dynamics.Comment: (16 pages, 13 figures

Thermal quenches in spin ice

We study the diffusion annihilation process which occurs when spin ice is quenched from a high temperature paramagnetic phase deep into the spin ice regime, where the excitations -- magnetic monopoles -- are sparse. We find that due to the Coulomb interaction between the monopoles, a dynamical arrest occurs, in which `non-universal' lattice-scale constraints impede the complete decay of charge fluctuations. This phenomenon is outside the reach of universal mean-field theory for a two-component Coulomb liquid. We identify the relevant timescales for the dynamical arrest and propose an experiment for detecting monopoles and their dynamics in spin ice based on this non-equilibrium phenomenon.Comment: 4 pages, 3 figures (final version

ICT for Social Inclusion and Equal Opportunities: CETI-D, an e-Governance Good Practice in Brazil

In this chapter we discuss a Knowledge Economy based approach to the inclusion of persons with disabilities (PwD) that, different from the traditional \u201cassistance\u201d model, considers PwD as active and valuable members of the present Knowledge Society, to be included in the active workforce. This approach will be discussed with reference to a specific operational case study concerning the establishment of the Center of Excellence for Technology and Innovation in Favor of Persons with Disabilities (CETI-D) conceived by Fondazione Rosselli Americas and being implemented by the State of Sao Paulo in Brazil. In the first section of the chapter we discuss the problem of the inclusion of PwD as a further aspect of the digital divide phenomenon. In the second section the principles of the United Nation Convention on the Rights of Persons with Disabilities are presented and some international best practices concerning the social inclusion of PwD are introduced. In the third section we discuss the CETI-D initiative, with the aim of showing how ICT can represent a powerful tool for social and economic inclusion. Finally, in the fourth section we discuss the conditions under which the experience of the CETI-D can be replicated in other countries, with a specific focus on less developed countries

Dynamics of single polymers under extreme confinement

We study the dynamics of a single chain polymer confined to a two dimensional cell. We introduce a kinetically constrained lattice gas model that preserves the connectivity of the chain, and we use this kinetically constrained model to study the dynamics of the polymer at varying densities through Monte Carlo simulations. Even at densities close to the fully-packed configuration, we find that the monomers comprising the chain manage to diffuse around the box with a root mean square displacement of the order of the box dimensions over time scales for which the overall geometry of the polymer is, nevertheless, largely preserved. To capture this shape persistence, we define the local tangent field and study the two-time tangent-tangent correlation function, which exhibits a glass-like behavior. In both closed and open chains, we observe reptational motion and reshaping through local fingering events which entail global monomer displacement.Comment: 22 pages, 18 figures, slightly extended version to appear in JSTA

Regulation of Schwann cells oncotransformation by changes in Nf2/merlin expression, Hippo/YAP signaling and DNA methylation

Schwann cell (SC) express the Neurofibromin type 2 gene (Nf2), encoding the tumor suppressor protein merlin, a cytoskeleton-associated protein regulating cell proliferation and survival. Nf2/merlin inactivation causes protein loss and leads to SC transformation into a form of benign tumor called schwannoma. Moreover, Nf2/merlin is mutated in an autosomal dominant multiple syndrome, called neurofibromatosis type 2. In line with observation that physio/mechanical cues, such as environmental challenges, may be pathogenetically relevant for SC oncotransformation, we recently showed that the exposure to electromagnetic fields (EMFs) causes changes in SC Nf2/merlin expression, cell migration, chemotactic responsivity and cytoskeleton reorganization. We showed a downstream MAPK/Erk activation, involved in SC proliferation, as well as activation of Hippo/YAP signalling commonly altered during tumorigenesis. We also showed that some genes, known to be upstream or downstream mediators of Hippo (Amotl2, Dchs, Fat, Wnt1) were changed. Further studies on rat SC oncotransformation following acute EMF exposure (0.1 T, 50 Hz, 10 min) demonstrated that the number of cells in G1 phase was increased. Focus forming analysis, after repeated exposures, showed an increase in 3D SC growth. EMF affects also the SC epigenome, as total DNA methylation, de novo DNMT and HDAC were reduced. Furthermore, RT2-profile assay evidenced that genes crucial for SCs are upregulated in EMF exposed cells. Overall, we identified some mechanisms responsible of environmental-induced SC changes toward a proliferative/migrating state, which may be pathologically relevant for nerve tumor development

AB0901 PREVALENCE OF OSTEOPOROSIS IN ITALIAN POSTMENOPAUSAL WOMEN ACCORDING TO DEFRA ALGORITHM

Background:Osteoporosis is a recognized health problem and the burden of the disease is mostly associated with the occurrence of hip and vertebral fracture.Objectives:This study was aimed at evaluating the prevalence of osteoporosis in Italian postmenopausal women, defined by DeFRA calculation as a 10 years fracture risk equal or higher than 20%.Methods:This is a monocenter cohort study evaluating 1850 post-menopausal women aged 50 years and older. All the participants were evaluated as far as anthropometrics. Defra questionnaire was administered and calculated with bone mineral density (DXA) measured at lumbar spine and femoral neck.Results:The prevalence of osteoporosis as assessed by DeFRA was 29.8% in the whole population, according to literature. The frequency of a risk fracture equal or higher than 20% varied from 7.9% in the group aged 50-59 years to 35% in subjects aged >80. Among clinical risk factors for fracture, the presence of a previous fracture (spine primarily) was the most commonly observed.Conclusion:Our data showed that about one third of post-menopausal women aged 50 and older in Italy has osteoporosis on the basis of DeFRA algorithm, with a high 10 years fracture risk. A previous fracture is the most common risk factor. The data should be considered in relation to the need to increase prevention strategies and therapeutic intervention.Disclosure of Interests:None declare

Spin Dynamics at Very Low Temperature in Spin Ice Dy2_2Ti2_2O7_7

We have performed AC susceptibility and DC magnetic relaxation measurements on the spin ice system Dy$_2$Ti$_2$O$_7$ down to 0.08 K. The relaxation time of the magnetization has been estimated below 2 K down to 0.08 K. The spin dynamics of Dy$_2$Ti$_2$O$_7$ is well described by using two relaxation times ($\tau_{\rm S}$ (short time) and $\tau_{\rm L}$ (long time)). Both $\tau_{\rm S}$ and $\tau_{\rm L}$ increase on cooling. Assuming the Arrhenius law in the temperature range 0.5-1 K, we obtained an energy barrier of 9 K. Below 0.5 K, both $\tau_{\rm S}$ and $\tau_{\rm L}$ show a clear deviation from the thermal activated dynamics toward temperature independent relaxation, suggesting a quantum dynamics.Comment: 4 page

Criticality, the area law, and the computational power of PEPS

The projected entangled pair state (PEPS) representation of quantum states on two-dimensional lattices induces an entanglement based hierarchy in state space. We show that the lowest levels of this hierarchy exhibit an enormously rich structure including states with critical and topological properties as well as resonating valence bond states. We prove, in particular, that coherent versions of thermal states of any local 2D classical spin model correspond to such PEPS, which are in turn ground states of local 2D quantum Hamiltonians. This correspondence maps thermal onto quantum fluctuations, and it allows us to analytically construct critical quantum models exhibiting a strict area law scaling of the entanglement entropy in the face of power law decaying correlations. Moreover, it enables us to show that there exist PEPS within the same class as the cluster state, which can serve as computational resources for the solution of NP-hard problems

Debye-Huckel theory for spin ice at low temperature

At low temperatures, spin ice is populated by a finite density of magnetic monopoles-pointlike topological defects with a mutualmagnetic Coulomb interaction. We discuss the properties of the resulting magnetic Coulomb liquid in the framework of Debye-Huckel theory, for which we provide a detailed context-specific account. We discuss both thermodynamical and dynamical signatures and compare Debye-Huckel theory to experiment as well as numerics, including data for specific heat and AC susceptibility. We also evaluate the entropic Coulomb interaction that is present in addition to the magnetic one and show that it is quantitatively unimportant in the current compounds. Finally, we address the role of bound monopole anti-monopole pairs and derive an expression for the monopole mobility