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)｜ 藤堂研究室｜ 羽田野研究室 (セミナー)｜ 桂研究室

日時 | 場所 | 講演者（敬称略） | 講演題目 |
---|---|---|---|

6月28日15時 | 理学部1号館 447教室 | 森 貴司 | Ensemble equivalence in equilibrium statistical mechanics and beyond |

日時 | 講演者（敬称略） | 講演題目 |
---|---|---|

6月21日15時 | 鈴木貴文 | Detection scheme of non-Gaussian fluctuation in quantum conductors |

6月14日15時 | Cristian Enachescu (Alexandru Ioan Cuza University) | Spin crossover micro and nanoparticles. Theoretical and experimental investigations |

6月7日15時 | Per Arne Rikvold (Florida State University) | Model Spin-crossover Material with Short-range Antiferromagnetic and Long-range Ferromagnetic Interactions: Wang-L andau Calculation of Free-energy Landscapes |

5月31日15時 | 赤城裕(桂研) | Topological Excitations in Frustrated Magnets |

5月24日15時 | 島田尚(伊藤研) | Robustness of evolving open systems with mutual interactions |

5月17日15時 | 諏訪秀麿(藤堂研) | Multiple Gapless-Excitation Modes at Neel to Valence-Bond-Solid Transition |

4月19日15時 | 渡辺悠樹(東大物理工学科) | Filling constraints for spin-orbit coupled insulators in symmorphic and nonsymmorphic crystals |

過去のセミナー： 2015年度｜ 2014年度｜ 2013年度｜ 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

**日時：**6月28日15時より

**場所：**理学部1号館 447教室

**講演者：** 森貴司 (宮下研)

**講演タイトル：** Ensemble equivalence in equilibrium statistical mechanics and beyond

**講演要旨：**

Ensemble equivalence is one of the important results of equilibrium statistical mechanics, and it has been extensively studied until now. When the microcanonical entropy and the canonical free energy are related with each other via the Legendre transformation, it is said that the microcanonical ensemble is thermodynamically equivalent to the canonical ensemble. The ensemble equivalence in this sense has been rigorously proven for a large class of physical systems.

More recently, deeper levels of the ensemble equivalence were proposed, which are called the "macrostate equivalence" and the "measure equivalence". The macrostate equivalence means that the two ensembles give an identical prediction for measurements of macroscopic quantities. The measure equivalence means that the relative entropy per particle between the two ensembles vanishes in the thermodynamic limit. As far as we consider the microcanonical, canonical, and grandcanonical ensembles, it has been recently proven by Touchette [1] that these three definitions of ensemble equivalence (thermodynamic, macrostate, and measure) are identical.

If we go beyond the equilibrium statistical mechanics, the non-thermal ensembles which are different from the above equilibrium ensembles appear. Therefore, the ensemble equivalence between two general ensembles is an important problem, although it has not been explored yet. In this talk, we discuss this problem, and it is shown that the vanishing specific relative entropy implies the macrostate (partial) equivalence [2].

[1] H. Touchette, J. Stat. Phys. 159, 987 (2015)

[2] T. Mori, arXiv:1602.06182

**日時：**6月21日15時より

**場所：**理学部1号館 447教室

**講演者：** 鈴木貴文 (宮下研)

**講演タイトル：** Detection scheme of non-Gaussian fluctuation in quantum conductors

**講演要旨：**

Nonequilibrium current fluctuation through mesoscopic devices has been intensively studied in both classical and quantum regimes [1]. It is experimentally possible, in the classical regime, to count the number of the electrons which pass through a conductor, and the histogram is used to characterize the nonequilibrium transport [2]. The fluctuation theorem is tested also in the quantm regime [3].
In general, it requires a sophisticated scheme to measure the current fluctuation compared with the conductance measurement.
Recent experiments utilize on-chip detectors to accurately measure the current fluctuation [2]. The detector of the current
distribution can be understood as a tracer particle driven by non-Gaussian noise [4,5].
In this talk, we study the detection scheme of the non-Gaussian fluctuation in quantum conductors. We consider the quantum p
oint point coupled to a detector LC circuit, which is a simple and generic setup.

[1] J. P. Pekola, Nat. Phys. 11, 118 (2015).

[2] B. Kueng et al., Phys. Rev. X 2, 011001 (2012).

[3] S. Nakamura et al., Phys. Rev. Lett. 104, 080602 (2010).

[4] Y. Utsumi et al., Phys. Rev. B 86, 075420 (2012).

[5] K. Kanazawa et al., Phys. Rev. Lett. 114, 090601 (2015).

**日時：**6月14日15時より

**場所：**理学部1号館 447教室

**講演者：**Cristian Enachescu (Alexandru Ioan Cuza University)

**講演タイトル：** Spin crossover micro and nanoparticles. Theoretical and experimental investigations

**講演要旨：**

Spin crossover molecular magnets[1] were extensively studied in recent years due to their potential applications in thermal and pressure sensors, optical displays, actuators or in data storage.
They are inorganic compounds, commutable between two states in thermodynamic competition: the low spin state (LS), stable at
low temperatures and the high spin state (HS), stable at high temperature. These two states have different optical, geometrical (volume), magnetic and vibrational properties. If the intermolecular interactions of elastic origin are higher than a threshold then the thermal switch between HS and LS states is accompanied by a thermal hysteresis; its width depends on the interaction strength while the transition temperatures are determined by the enthalpy vs. entropy ration in the HS and LS states. The LS and HS states commutation can also be triggered by irradiating the compound with an appropriate wavelength at low temperatures [2]. Under permanent light irradiation when temperature is changed back and forth, one is measuring the Light Induced Thermal Hysteresis (determined by the competition between the irradiation and the temperature dependent HS-LS nonradiative relaxation) [3]. These properties form the basis of
promising applications, such as memory devices, sensors or actuators.

In this communication we shall focus on some of the most recent experimental and theoretical investigations carried on spin crossover nanoparticles, such as thermal and relaxation properties in an environment [4] or the photoswitching at femtoseconds scale and subsequent studies on elastically driven cooperativity [5].

[1] M.A. Halcrow, Spin-crossover materials - properties and applications, in, John Wiley & Sons, Chichester, UK, 2013.

[2] S. Decurtins, P. Guetlich, C.P. Kohler, H. Spiering, A. Hauser, Light-induced excited spin state trapping in a transitio
n-metal complex: the hexa-1-propyltetrazole-iron (II) tetrafluoroborate spin-crossover system, Chem. Phys. Lett., 10 (1984)
1-4.

[3] C. Enachescu, R. Tanasa, A. Stancu, F. Varret, J. Linares, E. Codjovi, Phys. Rev. B, 72 (2005) 054413.

[4] R. Tanasa, J. Laisney, A. Stancu, M.L. Boillot, C. Enachescu, Hysteretic behavior of Fe(phen)(2)(NCS)(2) spin-transition
microparticles vs. the environment: A huge reversible component resolved by first order reversal curves, Appl. Phys. Lett.,
104 (2014).

[5] R. Bertoni, M. Lorenc, H. Cailleau, A. Tissot, J. Laisney, M.L. Boillot, L. Stoleriu, A. Stancu, C. Enachescu, E. Collet
, Elastically driven cooperative response of a molecular material impacted by a laser pulse, Nature Materials, 15 (2016) 606
-610.

**日時：**6月7日15時より

**場所：**理学部1号館 447教室

**講演者：**Per Arne Rikvold (Florida State University)

**講演タイトル：** Model Spin-crossover Material with Short-range Antiferromagnetic and Long-range Ferromagnetic Interactions: Wang-L andau Calculation of Free-energy Landscapes

**講演要旨：**

Spin-crossover materials present an interesting example of systems with competing interactions on different length scales. In these systems, the short-range interactions may be geometric or electronic in nature, while the long-range interactions are typically elastic.

Here I present a “toy model” of such a system: a square-lattice Ising model with nearest-neighbor antiferromagnetic and mean-field like long-range ferromagnetic interactions. Using standard importance sampling Monte Carlo (MC) methods, we have recently obtained rather complex phase diagrams for this model in several parameter regimes.

One question that cannot easily be answered by these methods, is that of the structure of the free-energy landscape in the space of the two macroscopic order parameters: magnetization and staggered magnetization. We have therefore developed a new method to directly obtain the density of states (DOS) of such a model, using the Wang-Landau MC (WLMC) method. I will give a brief introduction to the original WLMC method, and then describe the modifications we have made to obtain DOS constrained to specific values of the order parameters. I will show numerical free-energy landscapes for several parameter values and compare the results with phase diagrams previously obtained with importance sampling MC. The comparison raises some new questions about the nature of metastable phases in models with competing short-range and long-range interactions.

**日時：**5月31日15時より

**場所：**理学部1号館 447教室

**講演者：**赤城裕(桂研)

**講演タイトル：**Topological Excitations in Frustrated Magnets

**講演要旨：**

Topological defects play an important role in both conventional liquid crystals, such as nematic phase, and in the theory of two-dimensional quantum spin liquids [1]. However, relatively little is known about their role in spin version of nematic phase, namely quantum spin nematic phases which have no long-range dipole order and break only spin-rotational symmetry [2-5]. Moreover, most studies on such topological defects were analyzed in the continuum limit. Little is also known about the properties of topological defects on discrete lattice systems in microscopic models.

Then, we investigate such topological defects in these nontrivial states in a microscopic model. The model is the spin-1 bilinear biquadratic model in which such nontrivial states are stabilized on the triangular lattice [6-8]. Using homotopy analysis and numerical minimization of a variational wave function, we exhaustively examine what topological defects are in this model.

(1) We identify a new family of solitons at special SU(3) symmetric point. We also find that a soliton with higher topological charge spontaneously decays into “elementary” solitons with emergent interaction [9,10].

(2) In antiferro nematic phase with SU(2) symmetry [3-5], we find that \bar{C_0} type point defect spontaneously splits into two C_z type point defects, expanding the vortex core region [10].

(3) In antiferromagnetic 120°order, we clarify that the famous Z_2 vortex [11] has a preference of orientation in this model. As a nontrivial result, we also find the spin lengths are diminished near vortex core, depending on parameter region [10].

Reference:

[1] A. V. Chubukov, S. Sachdev, and T. Senthil, Nucl. Phys. B 426 [FS], 601 (1994).

[2] B. A. Ivanov, R. S. Khymyn, and A. K. Kolezhuk, Phys. Rev. Lett. 100, 047203 (2008).

[3] T. Grover and T. Senthil, Phys. Rev. Lett. 107, 077203 (2011).

[4] J. Takano and H. Tsunetsugu, J. Phys. Soc. Jpn. 80, 094707 (2011).

[5] C. Xu and A. W. W. Ludwig, Phys. Rev. Lett, 108, 047202 (2012).

[6] A. Lauchil, F. Mila, and K. Penc, Phys. Rev. Lett. 97, 087205 (2006).

[7] H. Tsunetsugu and M. Arikawa, J. Phys. Soc. Jpn. 75, 083701 (2006).

[8] A. Smerald and N. Shannon, Phys. Rev. B 88, 184430 (2013).

[9] H. T. Ueda, Y. Akagi, and N. Shannon, Phys. Rev. A. 93, 021606(R) (2016).

[10] Y. Akagi, H. T. Ueda, and N. Shannon, in preparation.

[11] H. Kawamura and S. Miyashita, J. Phys. Soc. Jpn. 53, 4138 (1984).

**日時：**5月24日15時より

**場所：**理学部1号館 447教室

**講演者：**島田尚氏（伊藤研）

**講演タイトル：**Multiple Gapless-Excitation Modes at Neel to Valence-Bond-Solid Transition

**講演要旨：**

A key and universal feature of various real complex systems such as ecosystems, reaction networks in living organisms, and s
ocial communities is its open and evolving nature. In those evolving open systems, their complexity emerges as a result of (
or at least persist) successive introductions of new elements. Therefore it is natural to ask how can a community or system,
which consists of lots of elements interacting each other, grow to more complex structure by adding new elements to it. To
tackle this general question, we have introduced a very simple graph dynamics model and found a novel type of mechanism of d
etermining the robustness of the system [1].

Comparing to the classical condition for dynamical systems based on the linear stability, which allow each element to have a
t most only one strong interaction [2], the growth condition in our different class of system is looser (each element can ha
ve more than 10 interactions). However, dynamical systems are often used for modeling complex systems so it is important to
consider the relevance of our framework to dynamical systems. I will first show the condition to have an extinction of an el
ement in a certain type of coupled dynamical system reduces to our graph model [3]. The only condition one should consider d
uring this mapping is to make the interactions mutual. Therefore I next show how the bidirectional interactions change the r
obustness of the system.

[1] T. Shimada, Scientific Reports 4, 4082 (2014).

[2] M. R. Gardner and W. R. Ashby, Nature 228, 784 (1970).

[3] T. Shimada, Y. Murase, and N. Ito, Proceedings of the International Conference on Social Modeling and Simulation, plus E
conophysics Colloquium 2014, p. 99-109 (2015).

**日時：**5月17日15時より

**場所：**理学部1号館 447教室

**講演者：**諏訪秀麿(藤堂研)

**講演タイトル：**Multiple Gapless-Excitation Modes at Neel to Valence-Bond-Solid Transition

**講演要旨：**

Most continuous phase transitions are described by the Landau-Ginzburg-Wilson (LGW) paradigm where the fluctuation of an order parameter diverges together with a spontaneous symmetry breaking at a critical point. For more than a decade, the deconfined criticality has caught a great deal of attention as a highly non-trivial phase-transition point breaking the LGW paradigm[1]. The low-energy (or long-length-scale) physics will be described not by the original degrees of freedom manifest in the model Hamiltonian but by internal degrees of freedom emerging as fractional excitation. The existence or the stability of such a deconfined critical point has been debated for the Neel to the valence-bond-solid transition in the two-dimensional quantum spin systems, the three-dimensional non-compact CP$^1$ action, the loop, and the dimer models. The Monte Carlo simulations, as reported in the previous researches, suffer from the anomalous finite-size effect and the breakdown of the conventional finite-size scaling. It was recently proposed that this enigma can be resolved by introducing a critical scaling form with two divergent length scales, the correlation length and the spinon-deconfinement length (or the domain-wall thickness)[2].

We have studied the excitation energy around the deconfined critical point between the Neel and the valence-bond-solid phases in the two-dimensional quantum spin system (J-Q model) by means of the unbiased worldline quantum Monte Carlo method. The energy gaps are estimated by the generalized moment method capturing the asymptotic behavior of the imaginary-time correlation[3]. The transition point is located by the level spectroscopy using the lowest gaps, and the duality between the singlet and triplet excitations is found. We have shown that the multiple gapless sectors appear both in the singlet and triplet excitations at the critical point and the unique velocity governs the linear dispersions. Our results strongly support the fundamental excitation consists of deconfined spinons as predicted by the deconfined critical theory[4].

References:

[1] T. Senthil, et al., Science 303, 1490 (2004).

[2] H. Shao, et al., Science 352, 213 (2016).

[3] H. Suwa and S. Todo Phys. Rev. Lett. 115, 080601 (2015).

[4] H. Suwa, A. Sen, and A. W. Sandvik, in preparation.

**日時：**4月19日15時より

**場所：**理学部1号館 447教室

**講演者：**渡辺悠樹(東大物理工学科)

**講演タイトル：**Filling constraints for spin-orbit coupled insulators in symmorphic and nonsymmorphic crystals

**講演要旨：**

In this talk, we will discuss our new guiding principle that is useful to identify candidate materials for quantum spin
liquids and topological Dirac/Weyl semimetals, based on the electron filling and space group symmetry. To that end, we will
extend the previous Oshikawa-Hastings-Lieb-Schultz-Mattis theorem in two ways: (i) we allow for arbitrary spin-orbit coupling
and (ii) fully utilize the space group symmetry, not only the lattice translation symmetry, and find stronger conditions
when the space group is nonsymmorphic. We will also discuss a new type of topological crystalline insulator, which we call
'filling-enforced quantum band insulators'.

References:

[1] HW, H.C. Po, A. Vishwanath, and M. Zaletel, PNAS (2015) (arXiv:1505.04193).

[2] H.C. Po, HW, M. Zaletel, and A. Vishwanath, Science Advances (2016), in press (arXiv:1506.03816).

[3] HW, H.C. Po, M. Zaletel, and A. Vishwanath, arXiv:1603.05646.