Séminaires du jeudi 12 avril

Séminaire du LPTMS: Corrado Rainone *** séminaire exceptionnel ***


Mechanical Failure in Amorphous Solids: Scale Free Spinodal Criticality

Corrado Rainone (Dpt of Chemical and biological physics, Weizmann Institute of Science, Israël)

The mechanical failure of amorphous media is a ubiquitous phenomenon from material engineering to geology. It has been noticed for a long time that the phenomenon is "scale-free", indicating some type of criticality. In spite of attempts to invoke "Self-Organized Criticality", the physical origin of this criticality, and also its universal nature, being quite insensitive to the nature of microscopic interactions, remained elusive. Recently we proposed that the precise nature of this critical behavior is manifested by a spinodal point of a thermodynamic phase transition. Moreover, at the spinodal point there exists a divergent correlation length which is associated with the system-spanning instabilities (known also as shear bands) which are typical to the mechanical yield. Demonstrating this requires the introduction of an "order parameter" that is suitable for distinguishing between disordered amorphous systems, and an associated correlation function, suitable for picking up the growing correlation length. The theory, the order parameter, and the correlation functions used are universal in nature and can be applied to any amorphous solid that undergoes mechanical yield. Critical exponents for the correlation length and the system size dependence are estimated. We conclude with some perspectives and modelling ideas on the subject. Réf:
  • Itamar Procaccia, Corrado Rainone, and Murari Singh, Mechanical failure in amorphous solids: Scale-free spinodal criticality, Phys. Rev. E 96, 032907 (2017)

Séminaire du LPTMS: Grégoire Ithier *** séminaire exceptionnel ***


Typicality and unconventional equilibrium states of an embedded quantum system.

Grégoire Ithier (Royal Holloway, London, UK)

In recent years, the progress in quantum engineering has provided new tools for simulating the dynamics of truly isolated quantum systems. These systems, made of trapped ions or cold atoms[1], can be prepared in a global pure state and their level of isolation is such that they evolve unitarily according to the Schrödinger equation. Surprisingly, despite being at all times in a pure quantum state, they display signatures of a local equilibration which can be in strong disagreement with the predictions of statistical physics [2]. These experimental facts are clearly questioning what kind of statistical description is relevant for isolated many body quantum systems.
In this talk, I will present our recent results on a theoretical model dedicated to this problem. This model considers a quantum system coupled to a large quantum environment, and introduces  some randomness at the level of the interaction Hamiltonian. We then demonstrate that the system has a typical dynamics for several classes of random interactions and most importantly for arbitrary system, environment, and global initial state [3]. In other words, the microscopic structure of interaction Hamiltonians does not matter and reduced density matrices have a self-averaging property. 
These results have two important consequences: first they can explain the absence of sensitivity to microscopic details of processes like e.g. thermalization. Second they provide the rigorous ground for an averaging procedure over random interactions which can be used for analytical non perturbative calculations performed with full generality i.e. for arbitrary system, environment, and initial state.
We apply this technique to calculate analytically the stationary state at long times of the system and find a new thermodynamical ensemble more general than the microcanonical one [4].