- Optimal performance of generalized heat engines with finite-size baths of arbitrary multiple conserved quantities beyond i.i.d. scaling
- Speaker: Kosuke Ito (Nagoya Univ.)
- Date: 2017/6/5 (Mon.) 14:45 –
- Place: K202, Main Building, Yukawa Institute, Kyoto U.
- Abstract: In quantum thermodynamics, effects of finiteness of the baths have been lessconsidered. In particular, there is no general theory to focus on finiteness ofthe baths of arbitrary multiple conserved quantities. Thus, we focus on how theoptimal performance of generalized heat engines with multiple quantities altersin response to the size of the baths. Conventionally, the size of the baths inquantum thermodynamics has been given in terms of the number of identical copiesof a system, which does not cover even such a natural scaling as the volume. Inconsideration of the asymptotic extensivity, we deal with a generic scaling ofthe baths to naturally include the volume scaling. Based on it, we derive thebound for the performance of generalized heat engines reflecting finite-sizeeffects of the baths, which we call fine-grained generalized Carnot bound. We also construct a protocol to achieve the optimal performance of the engine givenby this bound. Finally, applying the obtained general theory, we deal withsimple examples of generalized heat engines. 1D Ising spin chain model and a toymodel of two-level particles illustrate strange behaviors of the finite-sizeeffects. The other example is a heat engine with two baths composed of an idealgas exchanging particles, where the volume scaling is applied. The resultimplies that the mass of the particle explicitly affects the performance of thisengine with finite-size baths.
- References:
- K. Ito and M. Hayashi, arXiv:1612.04047

- Prethermalization, thermalization, and recurrence in one dimensional Bosons
- Speaker: Eriko Kaminishi (Univ. of Tokyo)
- Date: 2017/5/22 (Mon.) 14:40 –
- Place: K202, Main Building, Yukawa Institute, Kyoto U.
- Abstract:

Dynamics of an isolated quantum system has attracted much interest both in experiment and theory due to their potential to testing the foundation of quantum statistical mechanics.

In contrast to classical systems, relaxation occurs even in a few-body quantum system due to quantum entanglement. A few-body system can also exhibit the recurrence phenomenon in an accessible timescale in experiment. Moreover, we found a new mechanism of prehtermalization in a few-body quantum system, which we call entanglement prethermalization. Prethermalization is a phenomenon that the system relaxes to a quasi-stationary state before reaching thermal equilibrium, which is usually associated with the presence of nearly conserved local quantities. In the entanglement prethermalization, highly nonlocal quantum entanglement between the subsystems strongly affects the local property of the (quasi) stationary state.

In this talk, we discuss those intriguing quantum dynamics in one-dimensional bosons by studying the Lieb-Liniger model. - References:

- Numerical study of granular splash and force propagation in granular bed
- Speaker: Takahiro Tanabe (Hiroshima Univ.)
- Date: 2017/5/17 (Wed.) 10:30 –
- Place: K202, Main Building, Yukawa Institute, Kyoto U.
Abstract:

Granular materials, such as powder, agricultural grains and sand, play important roles in our life. Further, their collision processes are taken as one of fundamental processes observed in natural fields. In particular, the way how sand grains are ejected from ground surface when wind-blown grains hit them is an important issue for understanding stationary sediment transport. The whole process, starting from the initial impact of a grain and ending with ejection of many grains, are shortly called “splash process”, and the transport of grains near the surface is sustained by splash process.

For this reason, splash process has been extensively studied with experiments, and previous experiments have shown the relations between the incident angle/speed of the impacted grain and ejection grains’ angle/speed [1]. We have simulated the splash process with similar system to previous experiments using the discrete element method (DEM) and obtained the qualitative agreement with these experimental results. Moreover, we found that ejection grain dynamics varies depending on their ejection timing after the initial impact, which is reflected to the drastic change of the ejection velocity distributions [2]. This result suggests that the way of impact propagation along and beneath the granular bed surface varies according to the timing after the impact. For the next step, we have investigated the relation between the grain ejection process and the force propagation process beneath the granular bed surface induced by the impact of a grain onto the granular bed surface. To elucidate this relation, we have focused on the “force path” which gives the connection from the initially impacted grain through force propagation to ejected grains by numerically analyzing binary contacts inside the bed with simple definition. We will show the relation between force propagation process and ejection process taking care of inner dynamics of granular bed.- References:
- M. Ammi, et al., Phys. Rev. E, 79, 021305, (2009)
- T. Tanabe, et al., Phys. Rev. E, 95, 022906, (2017)

- Fracture in Different Scales: A Statistical Mechanical Approach
- Speaker: Subhadeep Roy (Univ. of Tokyo)
- Date: 2017/4/26 (Wed.) 16:00 –
- Place: K206, Main Building, Yukawa Institute, Kyoto U.
- Abstract:

Fracture is a complex phenomenon involving large span of time and length scales, starting from atomic scale or laboratory scale to geological scale like earthquake. A failure process can take place showing precursory rupture events, or catastrophically without showing such precursor. Also the rupture events might show correlation among themselves or might happen in a random manner. So the important question is:What are the physical criteria that govern the mode of failure ? In statistical mechanics, disorder systems and systems out of equilibrium are extensively studied in the last decade. So it will be interesting to study fracture in statistical mechanical point of view. The talk deals with the implementation of two main factors, disorder and stress release range, that guides failure process. A a statistical model namely Fiber Bundle Model is used to study the effect of above two parameters on the failure process, starting from laboratory scale to earthquake. - References:

Efficiency bounds on thermoelectric transport in magnetic fields: the role of inelastic processes

- Speaker: Kaoru Yamamoto (Univ. of Tokyo)
- Date: 2017/4/12 (Wed.) 16:00 –
- Place: K206, Main Building, Yukawa Institute, Kyoto U.
- Abstract:

It has been argued that breaking time-reversal symmetry which can make the Onsager matrix asymmetric is a way to achieve a high thermoelectric efficiency. Benenti et al. recently claimed [1] that in this case one can achieve the Carnot efficiency with a finite power, which appears to contradict the second law of thermodynamics. Many studies tackled this problem in individual models (see Refs. [2-4] for example) as well as in quite general cases [5,6].

In order to obtain an asymmetric Onsager matrix, we consider an Aharonov-Bohm ring threaded by a magnetic flux, incorporating electron-phonon inelastic scattering [7]. We find that breaking time-reversal symmetry in the presence of the inelastic process can significantly enhance the figure of merit for delivering electric power by supplying heat from a phonon bath [8]. The efficiency is bounded by the non-negativity of the entropy production of the original three-terminal junction [8], which is consistent with the result in [5,6]. - References:

[1] G. Benenti, K. Saito, and G. Casati, Phys. Rev. Lett. 106, 230602 (2011).

[2] K. Brandner, K. Saito, and U. Seifert, Phys. Rev. Lett. 110, 070603 (2013).

[3] K. Brandner, K. Saito, and U. Seifert, Phys. Rev. X 5, 031019 (2015).

[4] K. Proesmans and C. Van den Broeck, Phys. Rev. Lett. 115, 090601 (2015).

[5] N. Shiraishi, K. Saito and H. Tasaki, Phys. Rev. Lett. 117, 190601 (2016)

[6] N. Shiraishi and H. Tajima, arXiv:1701.01914 (2017).

[7] O. Entin-Wohlman and A. Aharony, Phys. Rev. B 85, 085401 (2012).

[8] K. Yamamoto, O. Entin-Wohlman, A. Aharony, and N. Hatano, Phys. Rev. B 94, 121402 (2016)

- Nonequilibrium Steady States in Weakly Coupled Quantum Spin Systems
- Speaker: Ayumu Sugita (Osaka City Univ.)
- Date: 2017/4/5 (Wed.) 16:00 –
- Place: K202, Main Building, Yukawa Institute, Kyoto U.
- Abstract:

熱流のある非平衡定常状態では、温度勾配に比例して熱流が流れるというフーリエ則が通常成り立っている。これは経験的には非常に普遍性の高い法則であるが、簡単な理論モデルではむしろ温度勾配を持たない定常状態が現れることが多く、フーリエ則の成立条件については完全には分かっていない。一般にはフーリエ則が成立するためには系の非可積分性（カオス性）が必要であると言われることが多いが、それが正しいとすると、温度勾配を持つノーマルな熱伝導状態をミクロな視点から解析的に調べることは非常に困難に思える。

我々は、温度勾配を持つ定常状態に対して解析的知見を得るため、弱結合量子系の非平衡定常状態を扱う摂動論を開発した。特に、スピン1/2の1次元スピン鎖において、温度勾配が存在する条件は非可積分性とは異なることを見出した。また、温度勾配を持つ系では非平衡特有の3点相関が現れ、その形は弱結合極限で相互作用に依らない普遍的な形を取ることを示した。これらの結果について解説する。

- References:

T. Ishida and A. Sugita, J. Phys. Soc. Jpn. 85, 074006(2016)