Harada Takahiro, Yoshikawa Kenichi
Department of Physics, Graduate School of Science, Kyoto University & CREST, Kyoto 606-8502, Japan.
Phys Rev E Stat Nonlin Soft Matter Phys. 2004 Mar;69(3 Pt 1):031113. doi: 10.1103/PhysRevE.69.031113. Epub 2004 Mar 31.
The relationship between the positional fluctuation and mechanical response for a thermal ratchet at a nonequilibrium steady state was investigated experimentally and considered theoretically. We constructed a ratchet system using a scanning optical trapping system and microbeads (0.20 microm diameter), which is interpreted as a realization of a rocking ratchet [M. O. Magnasco, Phys. Rev. Lett. 71, 1477 (1993)]. In the experiment, an asymmetric periodic potential (2.5 microm period) was generated by optical trap scanning, which traps a bead. When the potential profile was rocked sinusoidally, diffusion of the bead was rectified in one direction. We confirmed that both the diffusion coefficient and the mobility increased with potential rocking with a positive correlation. To obtain better insight, we performed numerical and theoretical analyses of the corresponding Langevin system. Although there is a positive relationship between the diffusion coefficient and mobility, the diffusion coefficient is greater than the value given in the Einstein relation. This result means that the effective temperature of the thermal ratchet at a nonequilibrium steady state becomes greater than that of the environment. We propose that this elevation of the effective temperature causes a decrease in the energetic efficiency of the thermal ratchet through irreversible dissipation to the heat bath.
研究了非平衡稳态下热棘轮的位置涨落与力学响应之间的关系，并进行了理论分析。我们使用扫描光学捕获系统和微珠（直径0.20微米）构建了一个棘轮系统，该系统可被解释为摇摆棘轮的一种实现方式[M. O. 马尼亚斯科，《物理评论快报》71, 1477 (1993)]。在实验中，通过光学阱扫描产生一个不对称周期势（周期为2.5微米）来捕获微珠。当势分布以正弦形式摆动时，微珠的扩散在一个方向上被整流。我们证实，扩散系数和迁移率都随着势的摆动而增加，且呈正相关。为了更深入了解，我们对相应的朗之万系统进行了数值和理论分析。尽管扩散系数和迁移率之间存在正相关关系，但扩散系数大于爱因斯坦关系给出的值。这一结果意味着非平衡稳态下热棘轮的有效温度高于环境温度。我们认为，有效温度的升高会通过向热库的不可逆耗散导致热棘轮的能量效率降低。
