統計力学セミナー予定表 2013年度


連絡先: 島田尚(shimadaあっと ap.t.u-tokyo.ac.jp)、 森貴司(moriあっと spin.phys.s.u-tokyo.ac.jp)
宮下研究室伊藤研究室 (セミナー)| 羽田野研究室 (セミナー)

これからのセミナー

日時 場所 講演者(敬称略) 講演題目

これまでのセミナー

日時 講演者(敬称略) 講演題目
1月14日15時 森田悟史 (東京大学物性研究所) Many-variable variational Monte Carlo calculations of the J1-J2 Heisenberg model
12月17日15時 西川功(東京大学生産技術研究所) Nonstandard scaling law of fluctuations in globally coupled oscillator systems and its application to estimation of a critical exponent
11月26日15時 勝本信吾(東京大学物性研究所教授) Quantum decoherence in a quantum dot - Aharonov-Bohm interferometer hybrid system due to entanglement between orbital and spin
11月12日15時 大越孝洋(東京大学大学院工学系研究科物理工学専攻今田研究室) Recent development on a quantum Monte Carlo method for strongly-correlated fermionic systems
11月5日15時 辻直人(東大理青木研究室) Nonthermal fixed point in the Hubbard model
10月29日15時 伊與田英輝(東京大学大学院総合文化研究科沙川研究室) Purity of quantum single-particle generated from a quantum dot
10月22日15時 羽田野直道 Arrow of time in open quantum systems
10月15日15時 森貴司 Nonadditivity in quasi-equilibrium states in a local model
7月9日15時 Pierre-Marie Billangeon (Macroscopic Quantum Coherence Team - RIKEN, Smart Energy Research Laboratories - NEC Corporation) Dicke superradiance in circuit QED
7月5日15時 鷲尾巧(東京大学大学院 新領域創成科学研究科 特任准教授) 筋収縮の数理モデルとその心臓シミュレーションへの応用について
7月2日15時 荒畑恵美子 Two-particle current from Superfluid Fermi Gases in the BCS-BEC Crossover
6月25日15時 島田尚 On the "state" of the market and its dynamics
6月20日15時 Per Arne Rikvold (Department of Physics, Florida State University) Phase separation in a hyperbolic plane
6月18日15時 Cristian Enachescu (Faculty of Physics and CARPATH Center, "Al. I. Cuza" University of Iasi, Romania) First Order Reversal Curves Method for Hysteresis: Application on Spin Crossover Compounds
6月4日15時 羽田野直道 Complete and bi-orthogonal basis for resonant states
5月28日15時 池田達彦(東大理上田研究室) The second law of pure state thermodynamics
5月21日15時 伊藤伸泰 Molecular dynamics simulation for gas-liquid transition
5月7日16時30分 宮下精二 多体型の定常状態
4月23日15時 Eric Vincent (Service de Physique de l'Etat Condense (SPEC), CEA-Saclay and Triangle de la Physique, FCS Campus Paris Saclay, France) Superspin glass state in interacting magnetic nanoparticle systems
4月16日15時 新M1 研究紹介
所属が書かれていない講演者は、宮下研、伊藤研、羽田野研の所属です。
過去のセミナー: 2012年度2011年度2010年度2009年度2008年度2007年度2006年度2005年度2004年度2003年度2002年度2001年度2000年度1999年度
->

第20回

日時:1月14日15時より

場所:理1号館447号室

講演者:森田悟史 (東京大学物性研究所)

講演タイトル:Many-variable variational Monte Carlo calculations of the J1-J2 Heisenberg model

講演要旨:
In the presence of strong geometrical frustration and quantum fluctuations, the quantum spin liquid states, insulators without any long range order, may appear even at zero temperature. One of the simplest models to realize the quantum spin liquid state is a spin-1/2 antiferromagnetic J1-J2 Heisenberg model on the square lattice. To investigate the nature of the quantum spin liquid state, we perform many-variable variational Monte Carlo (mVMC) simulations combining with quantum-number projection technique [1]. Our variational wave function is able to describe the exact ground states of the 4×4 system and provides higher accuracy than the conventional VMC method in 6×6 systems. [2] We can also obtain excited states with quantum numbers different from the ground state with the help of the quantum-number projections. Our direct calculations up to 16×16 sites support that the phase between the staggered and striped antiferromagnetic phases is characterized as a spin-gapped spin liquid without any long-range order. We discuss the relation to a recent result obtained by density matrix renormalization group method [3].

[1] D. Tahara and M. Imada: J. Phys. Soc. Jpn. 77 (2008) 114701.
[2] L. Capriotti, et al.: Phys. Rev. Lett. 87 (2001) 097201.
[3] H.-C. Jiang, H. Yao, and L. Balents: Phys. Rev. B 86 (2012) 024424.

第19回

日時:12月17日15時より

場所:理1号館447号室

講演者:西川功(東京大学生産技術研究所)

講演タイトル:Nonstandard scaling law of fluctuations in globally coupled oscillator systems and its application to estimation

講演要旨:
Universal scaling laws form one of the central issues in physics. A nonstandard scaling law or a breakdown of a standard scaling law, on the other hand, often suggests the existence of a new universality class in physical systems. I show that a statistical quantity related to fluctuations follows a nonstandard scaling law with respect to the system size in a synchronized state of globally coupled oscillators, by numerical simulations of several different models. The conditions required for the unusual scling law are also discussed [1]. Finally, I briefly report a way to estimate a critical exponent in the globally coupled oscillator system by using the statistical quantity described above and finite-size scaling [2].

[1] I. Nishikawa, G. Tanaka, and K. Aihara, Phys. Rev. E 88, 024102 (2013).
[2] I. Nishikawa, K. Iwayama, G. Tanaka, T. Horita, and K. Aihara, arXiv:1211.4364.

第18回

日時:11月26日15時より

場所:理1号館447号室

講演者:勝本信吾(東京大学物性研究所教授)

講演タイトル:Quantum decoherence in a quantum dot - Aharonov-Bohm interferometer hybrid system due to entanglement between orbital and spin

講演要旨:
I would like to introduce an experiment on quantum coherence in the traversal of electrons through a hybrid system of a quantum dot and an AB interferometer. An attempt to calculate this was published long time ago [1], and some elaboration was done [2]. Their conclusion is the spin scattering on the dot results in partial coherence in the transport. We confirmed that experimentally [3] through the observation of the visibility modulation of AB oscillation with the spin state on the dot. However this publication subjected us to the criticism that what we observed in the experiment was merely the spin rotation on the dot. Of course there was rebuttal but we have not solved the problem experimentally yet. The problem may have some relation with spin decoherence in the two-impurity Kondo problem and also with flying qubit problem. I would be glad to have opinions or experimental ideas from the audience.

[1] H. Akera, Phys. Rev. B 47, 6835 (1993).
[2] J. Koenig and Y. Gefen, Phys. Rev. Lett. 86, 3855 (2001); Phys. Rev. B 65, 045316 (2002).
[3] H. Aikawa et al., Phys. Rev. Lett. 92, 176802 (2004).
[4] Z. Jiang et al. Phys. Rev. Lett. 93, 076802 (2004).

第17回

日時:11月12日15時より

場所:理1号館447号室

講演者:大越孝洋(東京大学大学院工学系研究科物理工学専攻今田研究室)

講演タイトル:Recent development on a quantum Monte Carlo method for strongly-correlated fermionic systems

講演要旨:
The quantum Monte Carlo method is a powerful tool for investigating strongly-correlated quantum systems, because it enables us to obtain unbiased accurate results within statistical errors. In spite of many successful applications to bosonic systems and quantum ferromagnets, it is usually difficult to apply it to fermionic systems or frustrated quantum anti-ferromagnets due to the notorious negative-sign problem. Therefore, the precise simulations of such systems is one of the most challenging goals in modern physics. However, a recently developed quantum Monte Carlo method which is called bold diagrammatic Monte Carlo has opened a new way to achieve this goal[1]. In this talk, I will review the bold diagramatic Monte Carlo and its successful application to a unitary Fermi gas which shows universal properties.

[1]K. Van. Houcke et al., Nature Phys. 8, 366 (2012).

第16回

日時:11月5日15時より

場所:理1号館447号室

講演者:辻直人(東大理青木研究室)

講演タイトル:Nonthermal fixed point in the Hubbard model

講演要旨:
A renewed interest in whether and how an isolated quantum many-body system thermalizes has been recently generated since its ideal realization in cold-atom systems. In particular, it is indicated by various theoretical studies that the system does not immediately thermalize but is often trapped by a nonthermal fixed point. For example, the fermionic Hubbard model shows prethermalization after an interaction quench, where local observables quickly arrive at the thermal values while the full m omentum distribution stays nonthermal for long time.

In this talk, I briefly overview our current understanding of thermalization in the Hubbard model. After that, I discuss our recent work on thermalization in the presence of a long-range order [1-3]. Due to classical fluctuations, prethermalization is prevented, and the transient dynamics is governed by a nonthermal critical point, which I discuss belongs to a universality class distinct from the conventional G inzburg-Landau theory.

[1] Tsuji, Eckstein, Werner, Phys. Rev. Lett. 110, 136404 (2013).
[2] Tsuji, Werner, Phys. Rev. B 88, 165115 (2013).
[3] Werner, Tsuji, Eckstein, Phys. Rev. B 86, 205101 (2012).

第15回

日時:10月29日15時より

場所:理1号館447号室

講演者:伊輿田英輝(東京大学大学院総合文化研究科沙川研究室)

講演タイトル:Purity of quantum single-particle generated from a quantum dot

講演要旨:
On-demand quantum single-particle generator is important resource to investigate fundamental aspects of quantum mechanics and quantum information processing. One of the promising system to generate single photon is cavity QED (quantum electrodynamics) systems, which is realized by trapping "atoms" in cavity photons. Solid-state cavity QED systems composed of semiconductor quantum dots (artificial atoms) and cavities have been attracting much attention since they are suitable for creating compact optical devices. Recently, the electron analog of the single-photon generator has been investigated. The most remarkable difference between photons and electrons is the quantum particle statistics. This difference is strikingly observed in Hong-Ou-Mandel interferometer. In addition, electrons interact each other via Coulomb repulsive interaction, while photons do not interact. Thus, further studies and applications utilizing these difference are expected.

However, semiconductor quantum dots are strongly influenced by environmental noise sources. Thus, when designing a solid-state single-photon source, we should consider environmental noise. Especially, pure dephasing is caused by fluctuation of energy levels in quantum dots. In addition in the case of electron generator, dephasing due to Coulomb interaction should be taken into account. The performance of such single-particle generator is characterized by the collection efficiency and the indistinguishability (purity). If the particles are affected by noise strongly, they loses their purity and do not interfere.

In this talk, we discuss purity of single-photon and single-electron generated from a quantum dot. We analytically derive the density matrix and purity of single-particle, by using input-output formalism and Keldysh Green function. The one-particle excitation spectra suggests single-particle is mixed by environmental noise or Coulomb repulsion and this is directly confirmed by calculating single-particles' purity.

第14回

日時:10月22日15時より

場所:理1号館447号室

講演者:羽田野直道

講演タイトル:Arrow of time in open quantum systems

講演要旨:
We explain the emergence of the arrow of time in open quantum systems in the following two steps. First, the Schroedinger equation of open quantum systems, despite its time-reversal symmetry, can have solutions that break the symmetry [1]. For example, the solution of the "resonant state" takes the form in which particles trapped in the potential around the origin leak into the infinity and hence the survival probability in the trapping potential decays exponentially in time. The solution has a complex eigenvalue but does not contradict the Hermiticity of the Hamiltonian in the Hilbert space because the corresponding eigenfunction nonetheless holds because the exponential blowup of the eigenfunction is counterbalanced by the exponential decay in time.
We note that the "resonant state" that breaks the time-reversal symmetry is always accompanied by its time-reversal state, which we refer to as the "anti-resonant" state. The pair of the resonant and anti-resonant states thereby recover the time-reversal symmetry of the Schroedinger equation. The anti-resonant state takes the form in which particles flow into the trapping potential around the origin and hence the probability in the potential grows up exponentially in time.
We realize from the first step that the universe must be selecting the resonant state and discarding the anti-resonant state. The next step is to reveal what chooses the resonant state instead of the anti-resonant state. We give an answer to it on the basis of our new discussion on an open tight-binding system. We first show that the equation which produces the resonant and anti-resonant states takes the form of a quadratic eigenvalue problem [3]. Using this argument, we can expand the time-evolution operator exp(-iHt) only in terms of the discrete (point-spectrum) eigenstates including the resonant and anti-resonant states. The essential point here is that the expansion does not have a background integral. The expansion inevitably selects the resonant states for t>0 and the anti-resonant states for t<0.
This tells us that our choice of the initial condition or the terminal condition brakes the time-reversal symmetry. If we look at the time evolution for t>0 starting from an initial condition, we necessarily select the resonant states and discard the anti-resonant state. If we look at the time evolution for t<0 ending in a terminal condition, we necessarily select the anti-resonant state instead.
The study has been carried out under collaboration with Drs. Tomio Petrosky, Gonzalo Ordonez, Shachar Klaiman, Hiroaki Nakamura and Keita Sasada.

[1] N. Hatano, K. Sasada, H. Nakamura and T. Petrosky, Some properties of the resonant state in quantum mechanics and its computation, Prog. Theor. Phys. 119 (2008) 187.
[2] N. Hatano and G. Ordonez, Resonant-state expansion in open quantum systems without back ground integrals, unpublished.
[3] S. Klaiman and N. Hatano, Resonance theory for discrete models: methodology and isolated resonaces, J. Chem. Phys. 134 (2011) 154111.

第13回

日時:10月15日15時より

場所:理1号館447号室

講演者:森貴司

講演タイトル:Nonadditivity in quasi-equilibrium states in a local model

講演要旨:
Additivity is a fundamental concept of thermodynamics and statistical physics. Roughly speaking, if the total amount of energy is given by sum of internal energies of the macroscopic sybsystems, the system is said to be additive. Additivity ensures the convexity or concavity of thermodynamic functions and the ensemble equivalence.
A short-range interacting system is unlikely to be nonadditive since the interaction energy between subsystems is typically very small compared to the bulk energy. In this talk, however, we present a short-range interacting model on the two-dimensional lattice without additivity [1]. This model is referred to as the "elastic spin model", which was originally introduced as a theoretical model of spin-crossover transitions [2]. The essential point is the separation of several timescales, and hence, strictly speaking, nonadditivity emerges when the system is not in genuine thermal equilibrium but in a long-lived "quasi-equilibrium" state, which is described by equilibrium statistical mechanics of a nonlocal effective Hamiltonian.
An interesting feature is that the elastic spin model is nonadditive but, nevertheless, extensive. Nonadditivity and extensivity are directly confirmed by the work measurements in the thermodynamic process to divide the system into two macroscopic subsystems. We also find that several statistical ensembles are not equivalent; e.g. negative specific heats are observed numerically.

[1] T. Mori, Phys. Rev. Lett. 111, 020601 (2013)
[2] M. Nishino, K. Boukheddaden, Y. Konishi, and S. Miyashita, Phys. Rev. Lett. 98, 247203 (2007)

第12回

日時:7月9日15時より

場所:理1号館447号室

講演者:Pierre-Marie Billangeon (Macroscopic Quantum Coherence Team - RIKEN, Smart Energy Research Laboratories - NEC Corporation)

講演タイトル:Dicke superradiance in circuit QED

講演要旨:
The Dicke model describes the interaction of an ensemble of $N$ two-level atoms with one or various bosonic modes: it is a well known paradigmatic illustration of field-matter interaction and cooperative effects in quantum systems. In the thermodynamic limit $(N\to\infty)$, this system exhibits a phase transition by varying the coupling strength $\lambda$ between each atom and the radiation field from a normal phase $(\lambda<\lambda_c)$ to a superradiant phase $(\lambda>\lambda_c)$ with a spontaneously broken parity $(\mathbb{Z}_2)$ symmetry. Hepp & Lieb rigorously justified its existence at finite temperature by calculating its thermodynamic properties (Ann. Phys. 76, 360 (1973)), thus revealing an analogy with the Ising model of a ferromagnet. A calculation based on the more intuitive coherent state representation appeared subsequently (Wang & Hioe - Phys. Rev. A 7, 831 (1973)). However, even though the result of Hepp & Lieb is mathematically correct, Rzazewski et al. (Phys. Rev. Lett. 35, 432 (1975)) pointed out that a self-consistent description based on a gauge-invariant formulation of the problem did not allow the existence of the superradiant phase in atomic systems (the so-called no-go theorem for superradiant phase transition - SPT - in cavity QED). We could derive a similar result in the case of the dissipative harmonic oscillator using the method of Wang & Hioe, and we are wondering if this simple example can allow to clarify the role of the anharmonicity of the artificial atoms involved in the superradiant phase transitions. The field of Josephson qubits is a promising candidate in the quest for an integrable and scalable hardware for quantum information processing purposes. Besides that, these systems provide a very flexible test bed to validate some intricate predictions of quantum mechanics: circuit QED allowed to reproduce on-chip many experiments done in atomic systems, by coupling an artificial atom to the electromagnetic field confined in a 1D cavity. The possiblity to observe the SPT in circuit QED is presently under debate (Nataf & Ciuti - Nature Commun. 1, 72 (2010), Viehmann, von Delft and Marquardt - Phys. Rev. Lett. 107, 113602 (2011)): like in cavity QED, a self-consistent description seems to be the critical point. In order to have a better understanding of how these devices will behave on a large scale, a more rigorous description of the circuits found in circuit QED appears to be useful.

第11回

日時:7月5日15時より

場所:理1号館447号室

講演者:鷲尾巧(東京大学大学院 新領域創成科学研究科 特任准教授)

講演タイトル:筋収縮の数理モデルとその心臓シミュレーションへの応用について

講演要旨:
 筋収縮の根本であるサルコメア内アクチン-ミオシン系の素反応を表す数理モ デルは現在に至っても確立されておらず、生理学の分野では多様な実験事実を再 現できるように現象論的な立場から構成された複雑な常微分方程式系が採用され ることが多い。しかし、このようなアプローチを通して、骨格筋の収縮、心臓の 拍動、昆虫の羽ばたきなど生物の多様な運動に共通する本質的な原理に近づくこ とは困難であると考えられる。
 本講演では、統計力学的法則に従う確率的状態遷移モデルを分子の集合からな るサルコメアモデル内の個々の分子に適用し、多様な実験事実を単一のシンプル なモデルからモンテカルロ法により再現しようとする取り組みについて紹介する。 さらに、上記モンテカルロ法に基づくサルコメアモデルを連続体力学と結び付け 心拍動を再現するマルチスケール解析を通して、ミオシン分子の特性に由来する 未解明の現象を説明しようとする試みについても紹介する。

第10回

日時:7月2日15時より

場所:理1号館447号室

講演者:荒畑恵美子

講演タイトル:Two-particle current from Superfluid Fermi Gases in the BCS-BEC Crossover

講演要旨:
In recent years, the crossover from the BCS-type superfluid to the Bose-Einstein condensation (BEC) of tightly-bound molecules including the unitary gas as an intermediate regime have been realized using a tunable pairing interaction associated with a Feshbach resonance. In the BCS-BEC crossover it will be important to reveal on how and whether the particles are interacting. In this presentation, we propose that two-particle (double photoemission) current (DPE current) is a powerful technics to provide direct insight into the pair-correlations. The DPE was originally studied in conventional and unconventional superconductor both theoretically and experimentally. In this type of experiment, a pair of fermions is ejected from superconductors. We derive a general expression for DPE current from superfluid Fermi gases in the BCS-BEC crossover. Using these results, we show DPE current as a function of energy and momentum transfers, and identify the contributions of the condensed pair components and uncorrelated pair states. We also show the angular distributions of DPE current and the possibility of distinguishing between weakly bound Cooper pairs and tightly-bound molecules.

第9回

日時:6月25日15時より

場所:理1号館447号室

講演者:島田尚

講演タイトル:On the "state" of the market and its dynamics

講演要旨:
Time series with large number of degree of freedom are typical experimental results we have about complex systems. However such data are often non-stationary and noisy, hence the analysis tends to be very difficult. As an example of approach to such problems, I will introduce our recent study on financial data. From a clustering analysis of correlation coefficients between the stock returns in the past 20 years, we define typical "states" of the market. We can also see that the "states" changes rather abruptly and intermittently.

M. C. Munix, TS, et al., "Identifying States of a Financial Market" SCIENTIFIC REPORTS vol. 2 (2012) 644

第8回

日時:6月20日15時より

場所:理4号館1320号室

講演者:Per Arne Rikvold (Department of Physics, Florida State University)

講演タイトル:Phase separation in a hyperbolic plane

講演要旨:
I report a preliminary numerical study by kinetic Monte Carlo simulation of the dynamics of phase separation following a quench from high to low temperature in a system with a single, conserved, scalar order parameter (a kinetic Ising ferromagnet) confined to a hyperbolic lattice. The results are compared with simulations of the same system on two different, Euclidean lattices, in which cases we observe power-law domain growth with an exponent near the theoretically known value of 1/3. For the hyperbolic lattice we observe much slower domain growth, consistent to within our current accuracy with power-law growth with a much smaller exponent near 0.13.
The talk will start with an introduction to non-Euclidean lattices and their mapping to the Euclidean plane.

第7回

日時:6月18日15時より

場所:理1号館447号室

講演者:Cristian Enachescu (Faculty of Physics and CARPATH Center, "Al. I. Cuza" University of Iasi, Romania)

講演タイトル:First Order Reversal Curves Method for Hysteresis: Application on Spin Crossover Compounds

講演要旨:
Hysteresis is a complex phenomenon determined by the lag that can be observed between the input parameter, i.e. the applied magnetic field in magnetism and the output parameter, i.e. the magnetic moment of the sample. A distinction has to be made between the rate dependent hystereses, which appear only as an effect of lag vanishes for low-frequency measurements and rate independent hystereses which do not change if the field rate in the measurements is modified in a wide range of values. The rate independent hysteresis can be linked with the existence in the system of entities with metastable states. Each such entity has its individual hysteresis loop that depends on the particle's shape, anisotropy, volume, etc. It is characterized by a free energy function that has for a definite domain of the input parameter two minima separated by a maximum. The behavior of an ensemble of particles will also display hysteresis that will be controlled not only by the hysteretic properties of each isolated particle but also by the interactions between particles.
The First Order Reversal Curves (FORC) method is a general, model-independent technique which provides a sensitive characterization of the interactions and domain behavior in materials with hysteresis with applications in physics, geology and technology. The FORCs are a specific class of minor hysteresis loops, for which the sweeping process of the input parameter is reversed once from one of the branches of the major hysteresis loop. This method allows a direct determination of a two-dimensional distribution (FORC diagram), usually of individual hysteresis width and interactions between domains.
A special area of interest in recent years was the application of an FORC technique as a tool to understand the hysteretic behavior of spin crossover molecular magnets, materials that do show a complex nonlinear behavior. The spin crossover materials display in the two stable states (low spin and high spin) different magnetic properties (diamagnetic and paramagnetic), but do not illustrate a classical magnetic hysteresis. Nevertheless, due to elastic interactions, some of these compounds show a complex nonlinear behavior including temperature, pressure, and light-induced thermal hysteresis. We have applied the FORC diagram method for the thermal hysteresis of spin crossover materials and have shown that the diagram can be interpreted in terms of distributions of physical parameters such as the energy gap between the states, or interactions between like-spin domains. The FORC method applied on diluted spin crossover materials has suggested that distributions of internal stresses and domain sizeincrease with dilution. In addition, we present experimental FORC data for rate dependent light induced hysteresis and for the pressure hysteresis. Finally, we discuss the models for all hysteresis and their correlations with experiments.

References
[1] R. Tanasa, C. Enachescu, A. Stancu, F. Varret, et al., Phys. Rev. B71, 014431 (2005)
[2] C. Enachescu, R. Tanasa, A. Stancu, F. Varret, et al., Phys. Rev. B72, 054413 (2005)
[3] C. Enachescu, L. Stoleriu, A. Stancu, A. Hauser, Phys. Rev. Lett, 102, 257204 (2009)
[4] A. Tissot, C. Enachescu, M.L. Boillot, J. Mater. Chem.,22, 20451 (2012)
[5] P. Chakraborty, C. Enachescu, A. Hauser, Eur. J. Inorg. Chem, 5-6, 770 (2013)

第6回

日時:6月4日15時より

場所:理1号館447号室

講演者:羽田野直道

講演タイトル:Complete and bi-orthogonal basis for resonant states

講演要旨:
In open quantum systems, the bound states together with the integration over the scattering states form a complete orthonormal basis set. In order to take account of resonant states, we would deform the integration contour in the complex wave-number plane. This approach has a significant drawback; there would not be a unique expansion of an arbitrary state with respect to resonant states because one would never get rid of the deformed background integral from the basis set.

We here report a complete and bi-orthonormal basis set which consists of all discrete states (the resonant states, the anti-resonant states, the bound states and the anti-bound states) but does not contain the background integral at all. The key ingredient is the quadratic eigenvalue problem. Using the approach of the effective Hamiltonian, we reformulate the Schroedinger equation of an open quantum system in terms of a quadratic eigenvalue problem of a non-Hermitian closed system. This gives a complete and bi-orthonormal basis set of all discrete states under a specific measure.

This is a collaboration with Shachar Klaiman, Gonzalo Ordonez and Kanabu Nawa.

第5回

日時:5月28日15時より

場所:理1号館447号室

講演者:池田達彦(東大理物理上田研究室)

講演タイトル:The second law of pure state thermodynamics

講演要旨:
Non-equilibrium dynamics in an isolated quantum system has recently attracted much attention. In particular, it has been understood that phenomena, which look like thermalization, occur in such a system even though the system is described by a single pure quantum state and it evolves in time according to unitary evolution. In this talk, we show the second law of thermodynamics for such a system, which states that the diagonal entropy [1] increases every time we perform an operation on the system [2]. We also show that the diagonal entropy after an operation involves a universal constant, \gamma-1), where \gamma is Euler's constant, due to the quantum coherence between many-body eigenstates. We also verify the universal quantum correction by numerical calculations. We examine the quench dynamics of hard-core Bosons by exact diagonalization.

[1] A. Polkovnikov, Annals of Physics 326, 486 (2011).
[2] T. N. Ikeda, N. Sakumichi, A. Polkovnikov, and M. Ueda, arXiv:1303.5471.

第4回

日時:5月21日15時より

場所:理1号館447号室

講演者:伊藤伸泰

講演タイトル:Molecular dynamics simulation for gas-liquid transition

講演要旨:
 Results of molecular dynamics simulation studies on thermal and nonequilibrium dynamic properties of gas-liquid transition are shown. Ising universality was reconfirmed by equilibrium simulation[1].
Gas bubble nucleation in liquid phase was analyzed[2,3] and pool boiling was reproduced[4].

[1] H. Watanabe, N. Ito and C.-K. Hu, J. Chem. Phys. 136 (2012) 204102
[2] H. Inaoka, S. Yukawa and N. Ito, Physica A389 (2010) 2500
[3] H. Inaoka, S. Yukawa and N. Ito, Physica A391 (2012) 423
[4] H. Inaoka and N. Ito, "Numerical simulation of pool boiling of a Lennard-Jones liquid", to appear in Physica A (2013).

第3回

日時:5月7日16時30分より

場所:理1号館447号室

講演者:宮下精二

講演タイトル:多体型の定常状態

講演要旨:
力学的な運動をする系において熱平衡状態がどのように実現されるのか、またどのように緩和するのかについて、最近の数値計算[1,2]を詳解し、関連の話題を考察する。

[1] F Jin, T Neuhaus, K Michielsen, S Miyashita, M A Novotny, M I Katsnelson and H De Raedt, Equilibration and thermalization of classical systems, New Journal of Physics 15 (2013) 033009.
[2] Fengping Jin, K Michielsen, M A Novotny, S Miyashita, Shengjun Yuan, H De Raedt, Quantum decoherence scaling with bath size: Importance of dynamics, connectivity, and randomness, Physical Review A 87, 022117 (2013).

第2回

日時:4月23日15時より

場所:理1号館447号室

講演者:Eric Vincent (Service de Physique de l'Etat Condense (SPEC), CEA-Saclay and Triangle de la Physique, FCS Campus Paris Saclay, France)

講演タイトル:Superspin glass state in interacting magnetic nanoparticle systems

講演要旨:
Interacting magnetic nanoparticles display a wide variety of magnetic behaviors that are now being gathered in the emerging field of "supermagnetism". We have investigated how the out-of-equilibrium dynamics in the disordered superspin glass (SSG) state of a frozen ferrofluid sample is affected by the orientation of the magnetic nanoparticles ("texturation").

Via magnetization relaxation experiments at low temperatures, we were able to estimate dynamic superspin correlation lengths for both textured and non-textured samples. The comparison with simulations and experiments on spin glasses shows that the dynamic correlations in SSG's appear to develop in a way reminiscent to those in spin glasses, at time/length scales which are intermediate between simulations and experiments on spin glasses.

S. Nakamae, C. Crauste-Thibierge, D. L'Hôte, E. Vincent, E. Dubois, V. Dupuis and R. Perzynski, Applied Physics Letters 101, 242409 (2012).

第1回

日時:4月16日15時より

場所:理1号館447号室

講演者:新M1

講演タイトル:研究紹介

講演要旨:
N/A