Quantum Phase Transitions

We give a general introduction to quantum phase transitions in strongly-correlated electron systems. These transitions which occur at zero temperature when a non-thermal parameter $g$ like pressure, chemical composition or magnetic field is tuned to a critical value are characterized by a dynamic exponent $z$ related to the energy and length scales $\Delta$ and $\xi$. Simple arguments based on an expansion to first order in the effective interaction allow to define an upper-critical dimension $D_{C}=4$ (where $D=d+z$ and $d$ is the spatial dimension) below which mean-field description is no longer valid. We emphasize the role of pertubative renormalization group (RG) approaches and self-consistent renormalized spin fluctuation (SCR-SF) theories to understand the quantum-classical crossover in the vicinity of the quantum critical point with generalization to the Kondo effect in heavy-fermion systems. Finally we quote some recent inelastic neutron scattering experiments performed on heavy-fermions which lead to unusual scaling law in $\omega /T$ for the dynamical spin susceptibility revealing critical local modes beyond the itinerant magnetism scheme and mention new attempts to describe this local quantum critical point.Comment: 13 pages, 4 figure

Supersymmetric Approach to Heavy-Fermion Systems

We present a new supersymmetric approach to the Kondo lattice model in order to describe simultaneously the quasiparticle excitations and the low-energy magnetic fluctuations in heavy-Fermion systems. This approach mixes the fermionic and the bosonic representation of the spin following the standard rules of superalgebra. Our results show the formation of a bosonic band within the hybridization gap reflecting the spin collective modes. The density of states at the Fermi level is strongly renormalized while the Fermi surface sum rule includes $n_{c}+1$ states. The dynamical susceptibility is made of a Fermi liquid superimposed on a localized magnetism contribution.Comment: 5 pages, 2 figure

Critical phenomena near the antiferromagnetic quantum critical point of Heavy-Fermions

We present a study of the critical phenomena around the quantum critical point in heavy-fermion systems. In the framework of the S=1/2 Kondo lattice model, we introduce an extended decoupling scheme of the Kondo interaction which allows one to treat the spin fluctuations and the Kondo effect on an equal footing. The calculations, developed in a self-consistent one-loop approximation, lead to the formation of a damped collective mode with a dynamic exponent z=2 in the case of an antiferromagnetic instability. The system displays a quantum-classical crossover at finite temperature depending how the energy of the mode, on the scale of the magnetic correlation length, compares to k_B T. The low temperature behavior, in the different regimes separated by the crossover temperatures, is then discussed for both 2- and 3-dimensional systems.Comment: 24 pages, 5 figures, added reference

Tessuti in laterizio e tecnologie avanzate

Un tessuto di laterizio riveste gli edifici del nuovo complesso residenziale di Locarno, con una trama aperta e ondulata che lo rende leggero e flessibile. Risultato linguistico ottenuto dalla sperimentazione avanzata di moduli preassemblati in maniera robotizzata, frutto della collaborazione dei progettisti con il mondo della ricerca e dell’industri

Versatile Electro-Dynamic Tethers Dynamics Simulator for Debris Mitigation Tools Design

As far as the space debris mitigation is concerned, the electro-dynamic tethers (EDTs) represent a valuable alternative for de-orbiting. The paper presents a high accuracy numerical simulator developed to support the design and verify the effectiveness of such systems: accurate models are exploited for the mechanical, electrodynamical and environmental representation. Results confirmed the known instabilities of EDTs; to cope with them a control strategy is here proposed, traded off among different laws. The selected control relies on varying either the load resistance or the cathodic emitter voltage drop, at the system cathode, being the current profile the controlled variable. The sensitivity analysis, run on several design parameters, is presented and the interdependencies with stability and performance are discussed