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 |
---|---|---|---|

5月31日15時 | 理学部1号館 | 赤城裕(桂研) | Topological Excitations in Frustrated Magnets |

日時 | 講演者（敬称略） | 講演題目 |
---|---|---|

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：**5月31日15時より

**場所：**理学部1号館

**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

**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

**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

**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.