arXiv subjects

Takahiro Harada

Publications and source records attributed to Takahiro Harada.

25 records · Page 2Linked to original sources

arXiv2007

Intermittent dynamics and 1/f^beta noise in single cardiac muscle cells

Fluctuations in the spontaneous beating activity of isolated cardiac cells were studied over a timescale of six decades. The beat dynamics of single cardiac cells were heterogeneous and intermittent. The interbeat intervals (IBIs) were power-law distributed in a long-time regime. Furthermore, for long timescales up to the experimental window, the autocorrelation of IBIs exhibits a scaling behavior of 1/f^beta-noise type. These observations suggest that 1/f^beta noise is an intrinsic characteristic of spontaneous activity of single cardiac cells.

q-bio.CB↗

arXiv2007

A Reversibility Parameter for a Markovian Stepper

Recent experimental studies on the stepwize motion of biological molecular motors have revealed that the ``characteristic distance'' of a step is usually less than the actual step size. This observation implies that the detailed-balance condition for kinetic rates of steps is violated in these motors. In this letter, in order to clarify the significance of the characteristic distance, we study a Langevin model of a molecular motor with a hidden degree of freedom. We find that the ratio of the characteristic distance to the step size is equal to unity if the dominant paths in the state space are one dimensional, while it deviates from unity if the dominant paths are branched. Therefore, this parameter can be utilized to determine the reversibility of a motor even under a restricted observation.

cond-mat.stat-mech↗

arXiv2006

Fluctuations, Responses and Energetics of Molecular Motors

A novel equality relating the rate of energy dissipation to a degree of violation of the fluctuation-response relation (FRR) in nonequilibrium Langevin systems is described. The FRR is a relation between the correlation function of the fluctuations and the response function of macroscopic variables. Although it has been established that the FRR holds in equilibrium, physical significance of violation of the FRR in nonequilibrium systems has been under debate. Recently, the authors have found that an extent of the FRR violation is related in a simple equality to the rate of energy dissipation into the environment in nonequilibrium Langevin systems. In this paper, we fully explain the FRR, the FRR violation, and the new equality with regard to a Langevin model termed a Brownian motor model, which is considered as a simple model of a biological molecular motor. Furthermore, applications of our result to experimental studies of molecular motors are discussed, and, as an illustration, we predict the value of a new time constant regarding the motion of a KIF1A, which is a kind of single-headed kinesin.

cond-mat.stat-mech↗

arXiv2005

Energy dissipation and violation of the fluctuation-response relation in non-equilibrium Langevin systems

The fluctuation-response relation is a fundamental relation that is applicable to systems near equilibrium. On the other hand, when a system is driven far from equilibrium, this relation is violated in general because the detailed-balance condition is not satisfied in nonequilibrium systems. Even in this case, it has been found that for a class of Langevin equations, there exists an equality between the extent of violation of the fluctuation-response relation in the nonequilibrium steady state and the rate of energy dissipation from the system into the environment [T. Harada and S. -i. Sasa, Phys. Rev. Lett. 95, 130602 (2005)]. Since this equality involves only experimentally measurable quantities, it serves as a proposition to determine experimentally whether the system can be described by a Langevin equation. Furthermore, the contribution of each degree of freedom to the rate of energy dissipation can be determined based on this equality. In this paper, we present a comprehensive description on this equality, and provide a detailed derivation for various types of models including many-body systems, Brownian motor models, time-dependent systems, and systems with multiple heat reservoirs.

cond-mat.stat-mech↗

arXiv2005

Equality connecting energy dissipation with violation of fluctuation-response relation

In systems driven away from equilibrium, the velocity correlation function and the linear response function to a small perturbation force do not satisfy the fluctuation-response relation (FRR) due to the lack of detailed balance in contrast to equilibrium systems. In this Letter, an equality between an extent of the FRR violation and the rate of energy dissipation is proved for Langevin systems under non-equilibrium conditions. This equality enables us to calculate the rate of energy dissipation by quantifying the extent of the FRR violation, which can be measured experimentally.

cond-mat.stat-mech↗

arXiv2005

Exact transformation of a Langevin equation to a fluctuating response equation

We demonstrate that a Langevin equation that describes the motion of a Brownian particle under non-equilibrium conditions can be exactly transformed to a special equation that explicitly exhibits the response of the velocity to a time dependent perturbation. This transformation is constructed on the basis of an operator formulation originally used in nonlinear perturbation theory for differential equations by extending it to stochastic analysis. We find that the obtained expression is useful for the calculation of fundamental quantities of the system, and that it provides a physical basis for the decomposition of the forces in the Langevin description into effective driving, dissipative, and random forces in a large-scale description.

cond-mat.stat-mech↗

arXiv2003

Phenomenological framework for nonequilibrium steady states of molecular motors

An expression for the energetic efficiency of a molecular motor is presented in terms of an effective temperature, which was defined based on the ratio of the correlation function to the susceptibility of its velocity. We also present a numerical result regarding this temperature for a thermal ratchet model. Furthermore, our expression of efficiency using known values for kinesin can adequately explain the experimental results. The framework presented here has a closed form phenomenologically, and is independent of any detailed models.

cond-mat.stat-mech↗