連絡先： 森貴司(moriあっと spin.phys.s.u-tokyo.ac.jp), 鳩村拓矢(hatomura あっと 以下同様)

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

10月22日10時30分 | 理学部4号館1320教室 | Jurriaan Wouters (Utrecht University) | Exact ground states for interacting Kitaev chains |

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**日時：**10月22日10時30分より

**場所：**理学部4号館1320教室

**講演者：**Jurriaan Wouters (Utrecht University)

**講演タイトル：**Exact ground states for interacting Kitaev chains

**講演要旨：**

In attempts to better understand superconducting nanowires, many variations of the original Kitaev chain [A. Yu Kitaev, Phys.-Usp. 44, 131 (2001)] have been studied. This one-dimensional model of spinless fermions with hopping and p-wave superconducting pairing has gotten a lot of attention for being a possible building block for quantum computation.

I will discuss the specific Kitaev model, with an inhomogeneous, alternating chemical potential, without and with interactions. In the non-interacting case the model possesses exact strong zero modes and the two-fold degenerate ground state in a factorized form. The interacting model inherits the ground states from the non-interacting model. Also, an estimation of the gap can be given in the interacting case, that is verified by DMRG results.

**日時：**10月15日10時30分より

**場所：**理学部1号館447教室

**講演者：**大越 孝洋 (東大物工, 今田研)

**講演タイトル：**
Resummation of diagrammatic series with zero convergence radius for the unitary Fermi gas

**講演要旨：**

Feynman diagrams are powerful tools for studying various fields of physics. Still, the analysis usually involves approximations, because
only some types of diagrams or low-order diagrams are considered there.
However, the Monte Carlo method for unbiased sampling of Feynman
diagrams has been recently developed. On the other hand, the
diagrammatic series sometimes have zero radius of convergence. The
question is whether it is still possible to make accurate predictions by
summing up Feynman diagrams.

In this talk, we report high-precision results obtained by the
bold-line diagrammatic Monte Carlo method for the unitary Fermi gas with
zero convergence radius. We derive the large-order asymptotic behavior
of the diagrammatic series, and we give mathematical arguments and
numerical evidence for the resummability of the series by a specifically
designed conformal-Borel transformation that incorporates the
large-order behavior. Combining this new resummation method with
diagrammatic Monte Carlo evaluation up to order 9, we obtain new results
for the equation of state, which agree with the ultracold-atom
experimental data, except for the 4-th virial coefficient for which our
data point to the theoretically conjectured value. I will also report
our accurate results of Tan's contact.

Reference:

R. Rossi, T. Ohgoe, K. Van Houcke, and F. Werner, Phys. Rev. Lett. 121, 130405 (2018).

R. Rossi, T. Ohgoe, E. Kozik, N. Prokof'ev, B. Svistunov, K. Van Houcke,
and F. Werner, Phys. Rev. Lett. 121, 130406 (2018).

**日時：**7月9日11時より

**場所：**理学部4号館1320教室

**講演者：**中村 壮伸 (産総研)

**講演タイトル：**
Diffeomorphism invariance requirement on free-energy landscape (FEL) to describe reaction phenomena

**講演要旨：**

Free-energy landscape (FEL) is aimed to describe reaction phenomena for the system whose kinetics is dominated
by both energy and thermal fluctuation, such as biomolecules and glasses [1].

Traditional definition of FEL $F (z) = - T ln P (z)$ is represented via probability density function $P(z)$ as a function of reaction coordinate $z$.

We expect that FEL represents stable and transition states of the reaction as long as $z$ has ability to distinguish state.

However, the traditional definition cannot represent them in principle [2].

This is because FEL changes under diffeomorphism that conserves the ability to distinguish state.

We propose a new definition of FEL as a scalar under diffeomorphism by using diffusion tensor [3].

In this seminar, we explain the problem in the current definition, introduce new definition heuristically and show its properties.

[1] D. Frenkel, “Understanding Molecular Simulation From Algorithms to Applications” Academic Press, 2nd ed. (2001), C. Chipot and A. Pohorille “Free Energy Calculations ?Theory and Applications in Chemistry and Biology” Springer (2007)

[2] D. Frenkel,“Simulations: The dark side”, Eur. Phys. J. Plus (2013) 128: 10

[3] T. Nakamura, “Diffeomorphism invariance requirement on free-energy landscape to describe reaction phenomena” arXiv:1803.09034 (2018)

**日時：**6月25日10時30分より

**場所：**理学部4号館1320教室

**講演者：**川畑 幸平 (東大, 上田研)

**講演タイトル：**
Topological Unification of Time-Reversal and Particle-Hole Symmetries in Non-Hermitian Physics

**講演要旨：**

Topological phases of matter have been widely explored in equilibrium closed systems, but richer properties appear in non-equilibrium open systems that are effectively described by non-Hermitian Hamiltonians [1-3]. While several properties unique to non-Hermitian topological systems were uncovered [4,5], the fundamental role of symmetry in non-Hermitian physics has yet to be understood. In particular, it has remained unclear how symmetry protects non-Hermitian topological phases. In this work [6], we show that time-reversal and particle-hole symmetries are topologically equivalent in the complex energy plane and hence unified in non-Hermitian physics. A striking consequence of this symmetry unification is the emergence of nonequilibrium topological phases of matter that are absent in Hermitian systems and hence unique to non-Hermitian systems. We illustrate this by presenting a non-Hermitian counterpart of the Majorana chain in an insulator with time-reversal symmetry and that of the quantum spin Hall insulator in a superconductor with particle-hole symmetry. Our work establishes the fundamental symmetry principle in non-Hermitian physics and paves the way toward a unified framework for non-equilibrium topological phases of matter.

[1] C. M. Bender and S. Boettcher, Phys. Rev. Lett. 80, 5243 (1998).

[2] K. Kawabata, Y. Ashida, and M. Ueda, Phys. Rev. Lett. 119, 190401 (2017).

[3] R. El-Ganainy et al., Nat. Phys. 14, 11 (2018).

[4] H. Shen, B. Zhen, and L. Fu, Phys. Rev. Lett. 120, 146402 (2018).

[5] M. A. Bandres et al., Science 358, eaar4005 (2018).

[6] K. Kawabata, S. Higashikawa, Z. Gong, Y. Ashida, and M. Ueda, arXiv: 1804.04676.

**日時：**6月18日10時30分より

**場所：**理学部1号館447教室

**講演者：**池田 達彦 (東大物性研, 常次研)

**講演タイトル：**
Floquet-theoretical formulation and analysis of high-harmonic generation in solids

**講演要旨：**

Illuminated by an intense laser pulse, gases emit the high harmonics of the input laser frequency. Such high-harmonic generation (HHG) is a typical nonlinear optical process and has been the basis for the attosecond physics. Recently, HHG has attracted renewed attention owing to its successful observations in bulk solids [1], and its mechanism in solids is under active debate.

In this talk, we will present a theoretical formulation [2] of HHG in terms of the Floquet-Bloch wave function incorporating the periodicities in time and space. On the basis of this formulation, we analyze the HHG spectrum in a simple model. In particular, we discuss the mechanism and the behavior of the plateaus in the HHG spectrum.

[1] S. Ghimire et al., Nature Phys. 7, 138 (2011).

[2] T. N. Ikeda, K. Chinzei, H. Tsunetsugu, in preparation.

**日時：**5月28日10時30分より

**場所：**理学部4号館1320教室

**講演者：**布能 謙 (理研)

**講演タイトル：**
Thermodynamic costs of shortcuts to adiabaticity

**講演要旨：**

Control techniques known as shortcuts to adiabaticity (STA) have
attracted considerable interest in various fields such as quantum optics,
quantum information and quantum thermodynamics since they mimic the
quantum adiabatic dynamics of the system and suppress excitations even
for arbitrary fast driving. In this talk, we discuss the thermodynamic
cost of implementing STA, which arises as a natural question from both
fundamental and practical point of view. We study the work done by the
auxiliary control field which implements STA, and derive a fundamental
inequality that relates nonequilibrium work fluctuations with the operation time
needed to complete the task.

K. Funo, J.-N. Zhang, C. Chatou, K. Kim, M. Ueda and A. del Campo, PRL 118, 100602 (2017).

**日時：**5月21日10時30分より

**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 nanoparticles. Thermal transition, photoexcitation and relaxation explained by elastic models.

**講演要旨：**

See this PDF file

**日時：**5月14日10時30分より

**場所：**理学部4号館1320教室

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

**講演タイトル：**
Fluctuations in uniaxial ferromagnetic films driven by slowly oscillating magnetic fields with constant bias

**講演要旨：**

Ferromagnets perturbed by time-dependent magnetic fields are technologically important examples of nonequilibrium physics. Using kinetic Monte Carlo simulations and droplet theory of magnetization reversal, we explain recent experiments [1] on ordered, uniaxial ferromagnetic films forced by oscillating fields. The associated dynamic phase transition (DPT) at a critical oscillation period is known to be in the equilibrium Ising universality class. Consistent with the experiments, we find that the response to a constant bias field for oscillation periods above the critical period of the DPT differs radically from the field response of an equilibrium ferromagnet above its critical temperature: instead of a wide, central susceptibility maximum, “sidebands” are observed. This effect is independent of the system size for large systems, ruling out critical behavior associated with a phase transition. Rather, it is a stochastic-resonance phenomenon that has no counterpart in the corresponding thermodynamic phase transition, providing a reminder that the equivalence of the DPT to an equilibrium phase transition is limited to the critical region near the critical period and zero bias [2].

[1] P. Riego, P. Vavassori, and A. Berger, Phys. Rev. Lett. 118, 117202 (2017).

[2] G. M. Buendia and P. A. Rikvold, Phys. Rev. B 96, 134306 (2017).

**日時：**5月7日10時30分より

**場所：**理学部4号館1320教室

**講演者：**栂 裕太 (NIMS)

**講演タイトル：**
Coercivity analysis based on atomistic spin model for rare-earth permanent magnet

**講演要旨：**

The focus of rare-earth permanent magnets research, particularly Nd2Fe14B magnet, is to increase the coercive field Hc and to improve its temperature dependency. The Hc has many reasons why it is much smaller than the anisotropy magnetic field: previous research has mainly studied the influence of grain boundaries magnetism and of dipole-dipole interactions based on continuum approximation models [1]. On the other hands, recent first-principles calculations have been proposing that changes in atomic-scale magnetic properties (e.g., magnetic anisotropy, exchange coupling, etc.) near the interfaces significantly affect the coercivity [2].

To examine how the magnetic properties in atomic scale and thermal fluctuations affect the coercivity, we have constructed an atomistic classical Heisenberg model using many parameters from first-principles calculations [3]. By comparing the atomistic model and the continuum approximation model via magnetic anisotropy and domain wall energy, we clarify that the coercivity depends on the orientation of the permanent magnet. This orientation dependence is attributed to the layered structure of Nd atoms and weak exchange couplings between Nd and Fe atoms [4].

Additionally, in this talk, we introduce our attempts on the coercivity analysis by free energy landscape calculation using replica-exchange Wang-Landau method [5] which is a massive parallelization technique for multicanonical Monte Carlo methods.

[1] R. Dittrich, et al., J. Mag. Mag. Mater. 250, 12 (2002).

[2] Y. Tatetsu, S. Tsuneyuki, and Y. Gohda, Phys. Rev. Appl. 6, 064029 (2016).

[3] Y. Toga, et al., Phys. Rev. B 94, 174433 (2016); 94, 219901 (2016).

[4] Y. Toga, M. Nishino, S. Miyashita, T. Miyake, A. Sakuma, arXiv:1804.05824

[5] T. Vogel, et. al., Phys. Rev. Lett. 110, 210603 (2013).

**日時：**4月23日10時30分より

**場所：**理学部1号館447教室

**講演者：**中川 大也 (理研, 上田研)

**講演タイトル：**
Topological classification of gapless Floquet states

**講演要旨：**

When a quantum system is defined on lattice, the discrete spatial translational symmetry leads to a periodic structure in momentum space, i.e. the Brillouin zone. The topology of the Brillouin zone imposes various constraints on realizable band structures. For example, the Nielsen-Ninomiya theorem [1,2] shows that a single chiral fermion cannot be realized in lattice systems. On the other hand, in periodically driven quantum systems (called Floquet systems), the time translational symmetry is also discrete, and therefore the (quasi-)energy has a ``Brillouin zone". Surprisingly, this periodic structure of the energy space enables exotic band structures that are prohibited in static systems, such as the single chiral fermion [3,4]. In this talk, we first show that the exotic band structures are deeply related to topology of unitary time-evolution operators. Next, based on a topological classification of unitary time-evolution operators with symmetries, we demonstrate that a wide range of lattice-prohibited band structures are realizable using topological Floquet unitary operators. Specifically, we show that all gapless surface states of topological insulators and superconductors in ten Altland-Zirnbauer symmetry classes are realizable as bulk quasi-energy spectrum in Floquet systems.

References:

[1] H. Nielsen and M. Ninomiya, Nucl. Phys. B 185, 20 (1981).

[2] H. Nielsen and M. Ninomiya, Nucl. Phys. B 185, 173 (1981).

[3] T. Kitagawa et al., Phys. Rev. B 82, 235114 (2010).

[4] S. Higashikawa, M. Nakagawa, and M. Ueda, in preparation.