Latest Papers in Condensed Matter Physics

Statistical Mechanics

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Probing Out-of-Time-Order Correlators (1807.09731v5)

Soumyadeep Chaudhuri, R. Loganayagam

2018-07-25

We present a method to probe the Out-of-Time-Order Correlators (OTOCs) of a general system by coupling it to a harmonic oscillator probe. When the system's degrees of freedom are traced out, the OTOCs imprint themselves on the generalized influence functional of the oscillator. This generalized influence functional leads to a local effective action for the probe whose couplings encode OTOCs of the system. We study the structural features of this effective action and the constraints on the couplings from microscopic unitarity. We comment on how the OTOCs of the system appear in the OTOCs of the probe.

The many avatars of Curzon-Ahlborn efficiency (1903.04381v1)

Ramandeep S. Johal, Arun M. Jayannavar

2019-03-11

Efficiency at maximum power output of irreversible heat engines has attracted a lot of interest in recent years. We discuss the occurance of a particularly simple and elegant formula for this efficiency in various different models. The so-called Curzon-Ahlborn efficiency is given by the square-root formula: , where and are the cold and hot reservoir temperatures.

Large fluctuations of the first detected quantum return time (1903.03394v2)

Ruoyu Yin, Klaus Ziegler, Felix Thiel, Eli Barkai

2019-03-08

How long does it take a quantum particle to return to its origin? Under repeated projective measurements aimed to detect the return, the closed cycle yields a geometrical phase which shows that the average first detected return time is equal to the winding number of the underlying generating function in units where the sampling time is set to unity. Here we focus on the blow-up of the fluctuations of return times close to critical sampling times or special choices of control parameters. Gr"unbaum, Vel'azquez, Werner and Werner, Comm. Math. Phys., 320 543 (2013), showed how to map this problem onto a classical charge theory in two dimensions with charges located on the unit circle. Using this we find the critical divergence of the fluctuations showing how a picture based on a single weak charge or closely situated pairs or triplets of charges, yields formulas for the diverging variance with physical insights which differ in each case.

Physically consistent numerical solver for time-dependent Fokker-Planck equations (1804.01285v4)

Viktor Holubec, Klaus Kroy, Stefano Steffenoni

2018-04-04

We present a simple thermodynamically consistent method for solving time-dependent Fokker--Planck equations (FPE) for over-damped stochastic processes, also known as Smoluchowski equations. It yields both transition and steady-state behavior and allows for computations of moment-generating and large-deviation functions of observables defined along stochastic trajectories, such as the fluctuating particle current, heat and work. The key strategy is to approximate the FPE by a Master equation with transition rates in configuration space that obey a local detailed balance condition for arbitrary discretization. Its time-dependent solution is obtained by a direct computation of the time-ordered exponential, representing the propagator of the FPE, by summing over all possible paths in the discretized space. The method thus not only preserves positivity and normalization of the solutions, but also yields a physically reasonable total entropy production, regardless of the discretization. To demonstrate the validity of the method and to exemplify its potential for applications, we compare it against Brownian-dynamics simulations of a heat engine based on an active Brownian particle trapped in a time-dependent quartic potential.

Poisson-Boltzmann theory with non-linear ion correlations (1711.04949v5)

Mao Su, Zhijie Xu, Yanting Wang

2017-11-14

The Poisson-Boltzmann (PB) theory is widely used to depict ionic systems. As a mean-field theory, the PB theory neglects the correlation effect in the ionic atmosphere and leads to deviations from experimental results as the concentration or charge valance increases. A modified PB theory including ion correlation effect while retaining its simplicity is critical for many important applications in which ion correlation effect can be significant. In this paper, we present a new model to incorporate ion correlations into the original PB equation by utilizing the Green's function with a non-linear form of the self-energy, which is different from the linear self-energy equation obtained by the Field-Theoretic (FT) approach. Both equations are solved numerically and compared with our molecular dynamics (MD) simulation. The co-ion distribution calculated by the FT approach deviates significantly from the MD simulation, while our results for both counter-ion and co-ion distributions are justified by the MD simulation.

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