Official web site of Takeuchi Lab, Univ. Tokyo

ピコ秒やフェムト秒といった超高速時間領域における素励起の運動は従来，摂動的なポンププローブ法を用いて計測されるのが一般的であった。これに対して，熱揺らぎや量子揺らぎなどによる乱雑な運動はポンププローブ法と相性が悪く，多くの場合見逃されてきた。しかし過去10年で，高繰り返しレーザーの中に含まれるノイズを高精度に抽出することでこうした物性の乱雑運動を精密測定する技術が発展してきた。このような「揺らぎ」に着目した実験は，初期においては電気光学サンプリング法を用いた中赤外からテラヘルツ領域の真空電場雑音計測 [1,2]などに始まり，最近では中赤外パルス電場やコヒーレントフォノンにおけるスクイズド状態の観測 [3,4]など，非定常状態の観測へと拡張しつつある。
このような状況の中，我々は磁性体の相転移におけるスピン系の臨界現象に着目した。反強磁性体のスピン共鳴周波数はTHz帯域に達し，乱雑な実時間ダイナミクスを電気的に計測することは周波数帯域の律速のため困難である。そこで，我々は上記のような超短パルスレーザーのノイズを精密計測するというコンセプトに基づき，サブTHz帯域におけるスピン熱揺らぎのダイナミクスを自己相関として計測する手法（フェムト秒ノイズ相関分光法）を開発した [5–7]。これをオルソフェライトSm0.7Er0.3FeO3が示す室温付近のスピン再配列相転移に対して適用したところ，既知のマグノンモードである疑似強磁性マグノン振動に加えて，臨界緩和を示す未知の長寿命成分が観測された。確率的Landau-Lifshitz-Gilbert方程式シミュレーションとの比較から，この信号はスピンが磁気異方性ポテンシャルの準安定状態間を熱的に飛び移ることで生じる，定常的なスピンスイッチング（ランダムテレグラフノイズ）である，と解釈された [7]。
講演では，上述の我々の結果に留まらず，超高速時間領域におけるノイズ（揺らぎ）分光技術全般の歴史および，その展開に関する最近の傾向についてもレビューしたいと思う。

[1] C. Riek, D. V. Seletskiy, A. S. Moskalenko, J. F. Schmidt, P. Krauspe, S. Eckart, S. Eggert, G. Burkard, and A. Leitenstorfer, Direct sampling of electric-field vacuum fluctuations, Science 350, 420 (2015).
[2] I.-C. Benea-Chelmus, F. F. Settembrini, G. Scalari, and J. Faist, Electric field correlation measurements on the electromagnetic vacuum state, Nature 568, 202 (2019).
[3] C. Riek, P. Sulzer, M. Seeger, A. S. Moskalenko, G. Burkard, D. V. Seletskiy, and A. Leitenstorfer, Subcycle quantum electrodynamics, Nature 541, 7637 (2017).
[4] M. Esposito, K. Titimbo, K. Zimmermann, F. Giusti, F. Randi, D. Boschetto, F. Parmigiani, R. Floreanini, F. Benatti, and D. Fausti, Photon number statistics uncover the fluctuations in non-equilibrium lattice dynamics, Nat. Commun. 6, 10249 (2015).
[5] M. A. Weiss, F. S. Herbst, S. Eggert, M. Nakajima, A. Leitenstorfer, S. T. B. Goennenwein, and T. Kurihara, Subharmonic lock-in detection and its optimization for femtosecond noise correlation spectroscopy, Rev. Sci. Instrum. 95, 083005 (2024).
[6] M. A. Weiss, F. S. Herbst, G. Skobjin, S. Eggert, M. Nakajima, D. Reustlen, A. Leitenstorfer, S. T. B. Goennenwein, and T. Kurihara, Quantifying the Amplitudes of Ultrafast Magnetization Fluctuations in Sm$_{0.7}$Er$_{0.3}$FeO$_{3}$ Using Femtosecond Noise Correlation Spectroscopy, arXiv:2501.17531.
[7] M. A. Weiss et al., Discovery of ultrafast spontaneous spin switching in an antiferromagnet by femtosecond noise correlation spectroscopy, Nat. Commun. 14, 1 (2023).

As electronic devices become increasingly compact and powerful, efficient thermal management is crucial for performance, reliability, and safety. This study explores a novel approach to thermal rectification by extending the concept of the Tesla valve, originally designed for fluid flow, to solid-state heat conduction in graphite materials.

We utilized isotopically enriched graphite with 13C content reduced from 1.1% to 0.02% to create a solid-state Tesla valve structure. This material exhibits phonon hydrodynamic behavior, allowing for the formation of phonon Poiseuille flow [1]. We observed thermal rectification effects of up to 15% in the temperature range of 25-60 K. The graphite Tesla valve was fabricated as an air-bridge structure with a thickness of 90 nm and a width of 4.5 μm to ensure heat flow exclusively within the graphite. Thermal conductivity measurements were performed on forward and reverse configurations at various temperatures.

Our results demonstrate that the thermal rectification effect is most pronounced at around 45 K, where the thermal conductivity in the forward direction is 15.4% higher than in the reverse direction. This effect was observed only within the temperature range where phonons exhibit fluid-like properties [2]. This research represents a significant step towards realizing solid-state thermal rectification devices by leveraging the hydrodynamic behavior of phonons in graphite. Developing such thermal management technologies could lead to substantial advancements in the performance and efficiency of various electronic devices.

[1] X. Huang, et al., “Observation of phonon Poiseuille flow in isotopically purified graphite ribbons”, Nat. Commun. 14, 2044 (2023).
[2] X. Huang, et al., “A graphite thermal Tesla valve driven by hydrodynamic phonon transport,” Nature 634, 1086-1090 (2024)

※ Talk in Japanese

Spontaneous segregation of active matter into dense and sparse domains is ubiquitous. In systems with local interactions, it is usually well described as phase separation, and occurs not only in scalar active matter (``motility-induced phase separation'') but also in vectorial, aligning systems. This is in particular the generic situation for the simple but important case of self-propelled particles locally aligning their velocities against some noise. In such dry aligning active matter, the order-disorder transition is not direct, and the homogeneous orientationally-ordered liquid is generically separated from disorder by a coexistence phase in which dense ordered regions evolve in a remaining vapor.

We show that in collections of aligning circle swimmers with this phase separation scenario is replaced by a condensation phenomenon. The condensates, which take the form of vortices or rotating polar packets, can absorb a finite fraction of the particles in the system, and keep a finite or slowly growing size as their mass increases. Our results are obtained both at particle and continuous levels. We consider both ferromagnetic and nematic alignment, and both identical and disordered chiralities. Condensation implies synchronization, even though our systems are in 2D and bear strictly local interactions. We propose a phenomenological theory based on observed mechanisms that accounts qualitatively for our results.

Comprehending cell death is one of the central topics of biological science. Currently, the criteria for microbial cell death are purely experimental, based on PI staining and regrowth experiments. The debate on how “death” should be defined mathematically, and what mathematical properties the phenomenon of ‘death’ has, is largely untouched. In the present project, we aimed to develop a mathematical framework of cell death based on the controllability of cellular states [1].
We start by defining dead states as cellular states that are not returnable to the predefined "representative living states" regardless of the controllable parameters such as the gene expression level and external culture conditions. The definition requires a method to compute the restricted, global, and nonlinear controllability, for which no general theory exists. We have developed "The Stoichiometric Rays", a simple method to solve the controllability computation for catalytic reaction systems. This allows us to compute how the enzyme concentration should be modulated to control the metabolic state from a given state to a desired state.
Using the stoichiometric rays, we have computed the controllability and hence the dead states of a simple toy model of cellular metabolism as well as a rather realistic in silico metabolic model of E. coli [2]. We have also quantified the boundary that divides the phase space into the live and dead states, called the “Separating Alive and Non-life Zone (SANZ) hypersurface” [3].
In this talk I will present our framework for cell death, including stoichiometric rays. I will also discuss possible connections of the cell death framework to related fields such as dynamical systems, resource theory [4], and viability theory [5].

[1]. Himeoka et al., (2024), arXiv., https://arxiv.org/abs/2403.02169.
[2]. Boecker et al., (2021), Mol. Syst. Biol., 17 (12): e10504.
[3]. The Sanzu hypersurface is derived from a mythical river in the Japanese Buddhist tradition, the Sanzu River that represents the boundary between the world of the living and the afterlife.
[4]. Sagawa, (2022), “Entropy, Divergence, and Majorization in Classical and Quantum Thermodynamics”, Springer
[5]. Aubin et al., (2011), “Viability Theory: New Directions”, Springer

When a single fluid flows through porous media like soils or geological reservoirs, the transport of contaminants, nutrients, microorganisms, and chemicals is relatively well understood. However, when multiple fluids flow together, these transport phenomena have largely not been considered despite their significance in natural systems. Forces between the flowing fluids and the solid boundaries may create large variations in the local flow rates and form time-varying flow pathways, which can in turn accelerate solute spreading. In this study, we employ extensive computer simulations to propose a new theory on solute spread in systems with two fluids flowing through porous media, offering insights that could enhance our understanding and control of transport properties in natural environments.

The concept of odd elasticity is useful in characterizing non-reciprocality in active systems such as micromachines and microswimmers. As an example, we first introduce a model for a thermally driven microswimmer in which three spheres are connected by two springs with odd elasticity. Using Onsager’s variational principle, we derive dynamical equations for a nonequilibrium active system with odd elasticity. We further investigate the emergence of odd elasticity in an elastic microswimmer model using a reinforcement learning method. If time allows, I will discuss a new type of information microswimmer.

Understanding the spatiotemporal development of microbial communities is crucial for biomedical and ecological studies. However, our knowledge of how biological and physical processes shape these structures is limited by the lack of simultaneous gene expression and behavior measurements. In Bacillus subtilis, swarming on soft-agar media forms structured colonies with distinct motility behaviors over time. In this seminar, I will present research that combined spatiotemporal transcriptome data with live-cell microscopic data to map B. subtilis swarm development, revealing subpopulations with unique metabolic states and a cross-feeding mechanism that drives swarm expansion.

Many physical and biological systems are constituted by dense, disordered arrangements of individual units. A broad class of these systems exhibit hyperuniform behavior, whereby density fluctuations are suppressed at large spatial scales.  Here, we find that the arrangement of chromatophores on squid skin behaves in an opposite manner, such that density fluctuations grow with spatial scale, akin to a critical system. We term this behavior `hyperdisordered'. We combine experiments and theory to reveal how this unexpected scaling is due to the interplay between growth and volume exclusion. The ubiquity of these two features implies that the simple mechanism we describe may apply to a broad class of growing systems.

The presence of interfaces makes active matter systems different from their bulk behavior as in other soft matter systems. Also, interfaces act as soft confinement for active particles. Here we study the system of a single light-driven Janus particle confined in a very thin oil droplet at an air-water interface. Activation by a laser leads to the particle's horizontal fast motion of 1mm/s-1cm/s while it rests at the center of the droplet without activation. The particle shows periodic or intermittent motions, which can be related to the three-fluid contact angle. The particle is driven by a local thermal Marangoni flow; however, it always couples with the droplet thickness profile, which results in a time-varying propulsion speed. This might result from complex couplings among heat Marangoni flow, the particle wettability change by heating, hydrodynamic flow, and the capillary force. This new type of coupled dynamics will shed light on the hydrodynamic pressure due to the motion of active particles and provide insight into developing activity-controlled materials.

We will discuss new methods to, in principle, obtain all cumulants of von Neumann entropy over different models of random states. The new methods uncover the structures of cumulants in terms of lower-order joint cumulants involving families of ancillary linear statistics. Importantly, the new methods avoid the tedious tasks of simplifying nested summations that prevent existing methods in the literature to obtain higher-order cumulants. This talk is based on an ongoing joint work with Youyi Huang.