Tomorrow David will talk at group meeting. See below for his title and abstract.
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Title: Effective working fluid-bath decoupling at the strong coupling regime
Abstract: Heat machines in the strong coupling regime have been proposed to study fundamental thermodynamics questions, and to improve upon the low power produced by the weak coupling counterpart. At weak coupling, the power is proportional to the square of the coupling strength. A fair hypothesis may be that an increase of the coupling strength will produce an increase on the machine output. However, studies of a wide range of different heat machines predict an effective decoupling between the working fluid and strongly coupled baths, reducing the machine output to levels comparable to the weak coupling regime. This effect has been predicted for fermonic and bosonic baths and for a wide range of working fluids that includes single and multiple spins or harmonic oscillator and molecular systems. A diverse variety of theoretical methods has been applied to these models, including the polaron transformation, Holstein Primakoff transformation, non-interacting blip approximation (NIBA), surrogate Hamiltonian approach and reaction coordinate mapping. The wide range of systems and methods shows that the effective decoupling is not an artifact of some approximation, which in turn may indicate that the effective decoupling is a universal property of the strong coupling regime, regardless of the character of the bath, the working fluid or the interaction between them.
We continue the exploration of this universality question by studying different heat machines composed of a spin working fluid that interacts with spin baths. In order to better understand strong coupling effects, we first analyze a toy model of a small spin bath and use numerical calculations to extend our results to larger spin baths. Besides addressing the question of universality, and the underlying physical mechanism that produces the working fluid-bath decoupling, we study methods to avoid the effective decoupling by adding an external decoherence source to the spin baths.