Enhanced magnetic fluctuations in doped spin-Peierls systems: a single-chain model analysis

We analyze by means of real space Renormalization Group (RG) as well as by exact diagonalizations the properties of a single-chain model of a doped spin-Peierls system, where a major role is played by the localized moments created by the impurities. We are able to follow analytically the RG flow, which allows us to determine the relevant cross-over temperatures. In particular, we find an enhancement of magnetic correlations due to disorder, coexisting with an underlying dimerization, in an intermediate temperature range below the spin-Peierls critical temperature and above the coherence temperature of a regular array built by those localized moments (so-called soliton bandwidth). The possible relevance of these results to the doped inorganic spin-Peierls compound CuGeO$_3$ is discussed.Comment: 34 pages, 9 figures. Exact diagonalizations have been adde

Surprises in the phase diagram of an Anderson impurity model for a single C60n−_{60}^{n-} molecule

We find by Wilson numerical renormalization group and conformal field theory that a three-orbital Anderson impurity model for a C$_{60}^{n-}$ molecule has a very rich phase diagram which includes non-Fermi-liquid stable and unstable fixed points with interesting properties, most notably high sensitivity to doping $n$. We discuss the implications of our results to the conductance behavior of C$_{60}$-based single-molecule transistor devices.Comment: 4 pages, 3 figures, 2 tables. Accepted versio

A New Approach to Equations with Memory

In this work, we present a novel approach to the mathematical analysis of equations with memory based on the notion of a state, namely, the initial configuration of the system which can be unambiguously determined by the knowledge of the future dynamics. As a model, we discuss the abstract version of an equation arising from linear viscoelasticity. It is worth mentioning that our approach goes back to the heuristic derivation of the state framework, devised by L.Deseri, M.Fabrizio and M.J.Golden in "The concept of minimal state in viscoelasticity: new free energies and applications to PDEs", Arch. Ration. Mech. Anal., vol. 181 (2006) pp.43-96. Starting from their physical motivations, we develop a suitable functional formulation which, as far as we know, is completely new.Comment: 39 pages, no figur

Novel TCAD oriented definition of the off-state breakdown voltage in Schottky-gate FETs: a 4H SiC MESFET case study

Physics-based breakdown voltage optimization in Schottky-barrier power RF and microwave field-effect transistors as well as in high-speed power-switching diodes is today an important topic in technology computer-aided design (TCAD). OFF-state breakdown threshold criteria based on the magnitude of the Schottky-barrier leakage current can be directly applied to TCAD; however, the results obtained are not accurate due to the large uncertainty in the Schottky-barrier parameters and models arising above all in advanced wide-gap semiconductors and to the need of performing high-temperature simulations to improve the numerical convergence of the model. In this paper, we suggest a novel OFF-state breakdown criterion, based on monitoring the magnitude (at the drain edge of the gate) of the electric field component parallel to the current density. The new condition is shown to be consistent with more conventional definitions and to exhibit a significantly reduced sensitivity with respect to physical parameter variation

Phase diagram of doped spin-Peierls systems

The phase diagram of a model describing doped CuGeO$_3$ is derived. The model emphasizes the role of local moments released by the impurities and randomly distributed inside the gaped singlet background. The phase diagram is investigated by two methods: (i) in a mean field treatment of the interchain coupling and (ii) in a real space decimation procedure in a two dimensional model of randomly distributed moments. Both methods lead to similar results, in a qualitative agreement with experiments. In particular, a transition to an inhomogeneous N\'eel phase is obtained for arbitrary small doping. From the decimation procedure, we interpret this phase at very low doping as a {\sl Griffith antiferromagnet}. Namely, it does not have a true long range order down to zero temperature. Nonetheless, large magnetically ordered clusters appear already at relatively high temperatures. This demonstrates the role of disorder in the theoretical description of doping in CuGeO$_3$. A detailed comparison with other approaches is also given.Comment: 31 pages, 9 figure

Theory of the Metal-Paramagnetic Mott-Jahn-Teller Insulator Transition in A_4C_{60}

We study the unconventional insulating state in A_4C_{60} with a variety of approaches, including density functional calculations and dynamical mean-field theory. While the former predicts a metallic state, in disagreement with experiment, the latter yields a (paramagnetic) Mott-Jahn-Teller insulator. In that state, conduction between molecules is blocked by on-site Coulomb repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and important excitations (such as optical and spin gap) should be essentially intra-molecular. Experimental gaps of 0.5 eV and 0.1 eV respectively compare well with molecular ion values, in agreement with this picture.Comment: 4 pages, 2 postscript figure

Phase separation in quasi incompressible fluids: Cahn-Hilliard model in the Cattaneo-Maxwell framework

In this paper we propose a mathematical model of phase separation for a quasi-incompressible binary mixture where the spinodal decomposition is induced by an heat flux governed by the Cattaneo-Maxwell equation. As usual, the phase separation is considered in the framework of phase field modeling so that the transition is described by an additional field, the concentration c. The evolution of concentration is described by the Cahn-Hilliard equation and in our model is coupled with the Navier-Stokes equation. Since thermal effect are included, the whole set of evolution equations is set up for the velocity, the concentration, the temperature and the heat flux. The model is compatible with thermodynamics and a maximum theorem holds.Comment: Submitted to ZAM