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A pump is an alternating current-driven device that generates a direct current (DC). In the simplest configuration a pump has two leads connected to two reservoirs. In such open geometry the pump takes particles from one reservoir and emits them into the other. Accordingly, a current is produced even if the reservoirs have the same temperature and chemical potential. Stirring is the operation of inducing a circulating current with a non-vanishing DC component in a closed system. The simplest geometry is obtained by integrating a pump in a closed circuit. More generally one can consider any type of stirring mechanism such as moving a spoon in a cup of coffee. ==Main observations== Pumping and stirring effects in quantum physics have counterparts in purely classical stochastic and dissipative processes. The studies of quantum pumping〔M. Buttiker, H. Thomas and A Pretre, Z. Phys. B Condens. Mat. 94, 133 (1994). P. W. Brouwer, Phys. Rev. B 58, R10135 (1998). B. L. Altshuler, L. I. Glazman, Science 283, 1864 (1999). J. A. Avron, A. Elgart, G. M. Graf and L. sadun, Phys. Rev. B 62, R10618 (2000). D. Cohen, Phys. Rev. B 68, 201303(R) (2003). M. Moskalets and M. Büttiker, Phys. Rev. B 68, 161311 (2003). 〕〔M. Switkes, C. M. Marcus, K. Campman, A. C. Gossard, Science 283, 1905 (1999).〕 and of quantum stirring〔D. Cohen, arXiv:cond-mat/0208233 (2002). D. Cohen, Phys. Rev. B 68, 155303 (2003). M. Aunola and J. J. Toppari, Phys. Rev. B 68, 020502 (2003). D. Cohen, T. Kottos and H. Schanz, Phys. Rev. E 71, 035202(R) (2005). G. Rosenberg and D. Cohen, J. Phys. A 39, 2287 (2006). I. Sela and D. Cohen, J. Phys. A 39, 3575 (2006). M. Hiller, T. Kottos and D. Cohen, Europhysics Letters 82, 40006 (2008); Phys. Rev. A 78, 013602 (2008). I. Sela and D. Cohen, Phys. Rev. B 77, 245440 (2008); Phys. Rev. B 78, 155404 (2008). 〕 emphasize the role of quantum interference in the analysis of the induced current. A major objective is to calculate the amount of transported particles per a driving cycle. There are circumstances in which is an integer number due to the topology of parameter space.〔 More generally is affected by inter-particle interactions, disorder, chaos, noise and dissipation. Electric stirring explicitly breaks time-reversal symmetry. This property can be used to induce spin polarization in conventional semiconductors by purely electric means.〔.〕 Strictly speaking stirring is a non-linear effect, because in linear response theory (LRT) an AC driving induces an AC current with the same frequency. Still an adaptation of the LRT Kubo formalism allows the analysis of stirring. The quantum pumping problem (where we have an open geometry) can be regarded as a special limit of the quantum stirring problem (where we have a closed geometry). Optionally the latter can be analyzed within the framework of scattering theory. Pumping and Stirring devices are close relatives of ratchet systems.〔P. Reimann Phys. Rep. 361 (2002) 57 H. Schanz, M. F. Otto, R. Ketzmerick and T. Dittrich Phys. Rev. Lett. 87 (2001) 070601 H. Schanz, T. Dittrich and R. Ketzmerick Phys. Rev. E 71 (2005) 026228 T. Dittrich, M. Gutiérrez and G. Sinuco Physica A 327 (2003) 145 H. Linke et al., Appl. Phys. A 75 (2002) 237-246. 〕 The latter are defined in this context as AC driven spatially periodic arrays, where DC current is induced. It is possible to induce a DC current by applying a bias, or if the particles are charged then by applying an electro-motive-force. In contrast to that a quantum pumping mechanism produces a DC current in response to a cyclic deformation of the confining potential. In order to have a DC current from an AC driving, time reversal symmetry (TRS) should be broken. In the absence of magnetic field and dissipation it is the driving itself that can break TRS. Accordingly, an adiabatic pump operation is based on varying more than one parameter, while for non-adiabatic pumps 〔.〕 modulation of a single parameter may suffice for DC current generation. The best known example is the peristaltic mechanism that combines a cyclic squeezing operation with on/off switching of entrance/exit valves. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Quantum stirring, ratchets, and pumping」の詳細全文を読む スポンサード リンク
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