Bernardo Spagnolo — Università di Palermo#
Nonlinear Relaxation Phenomena in Metastable Condensed Matter Systems.#
Nonlinear relaxation phenomena in three different systems of  
condensed matter are investigated. (i) First, the phase dynamics in  
Josephson junctions is analyzed. Specifically, a  
superconductor-graphene-superconductor (SGS) system exhibits quantum  
metastable states, and the average escape time from these metastable  
states in the presence of Gaussian and correlated fluctuations is  
calculated, varying the noise source intensity and the bias frequency.  
Moreover the transient dynamics of a long-overlap Josephson junction  
(JJ) subject to thermal fluctuations and non-Gaussian noise sources,  
Levy type, is investigated. Noise induced phenomena are observed, such  
as the noise enhanced stability and the resonant activation. The  
analysis of the time evolution of the order parameter highlights the  
influence of the noise induced solitons on the mean switching time  
behavior and the observation of breathers. Effect of non-Gaussian  
noise sources on the distributions of switching currents in graphene  
Josephson junctions  will be presented [1]. (ii) Second, the electron  
spin relaxation process in n-type GaAs crystals driven by a  
fluctuating electric field and two different noise sources is  
investigated. Monte Carlo numerical simulations show, in both cases,  
an enhancement of the spin relaxation time by increasing the amplitude  
of the external noise [2]. (iii) Finally, the stabilization of quantum  
metastable states by dissipation is presented. Normally, quantum  
fluctuations enhance the escape from metastable states in the presence  
of dissipation. We show that dissipation can enhance the stability of  
a quantum metastable system, consisting of a particle moving in a  
strongly asymmetric double well potential, interacting with a thermal  
bath. We find that the escape time from the metastable region has a  
nonmonotonic behavior versus the system-bath coupling and the  
temperature, producing a stabilizing effect [3].
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References
1. Claudio Guarcello, Davide Valenti, and Bernardo Spagnolo, Phase  dynamics in graphene-based Josephson junctions in the presence of  thermal and correlated fluctuations, Phys. Rev. B 92, 174519 (2015).
2. B. Spagnolo, D. Valenti, C. Guarcello, A. Carollo, D. Persano  Adorno, S. Spezia, N. Pizzolato, B. Di Paola, Noise-induced effects in  nonlinear relaxation of condensed matter systems, Chaos Solitons and  Fractals 81, 412-424 (2015).
3. D. Valenti, L. Magazzu', P. Caldara, and B. Spagnolo, Stabilization  of quantum metastable states by dissipation, Physical Review B 91,  235412 (7) (2015).