A02 Proposed Research Projects (2014-2015)

Paper | Original Paper


*Kazuya Saito, Mafumi Hishida, and Yasuhisa Yamamura,
A Possible Critical Point for Nematic Order on the Basis of Landau Free Energy Having Dual Instabilities for Nano-Segregated Smectic Liquid Crystal”,
Soft Matter 11, 8493-8498 (2015).

[Summary] Landau expansion of free energy assuming dual instabilities for nano-segregated SmA phase is analyzed. In addition to known phase sequences (on cooling, disordered isotropic liquid => nematic phase!smectic phase, and disordered isotropic liquid => smectic phase), a new sequence (disordered isotropic liquid => density wave with subsidiary nematic order => smectic phase) and the existence of a critical point are demonstrated in case where the instability for density wave formation occurs at a higher temperature.

Toshiyuki Hosoya, Martin Miranda, Ryotaro Inoue and *Mikio Kozuma,
Injection locking of a high power ultraviolet laser diode for laser cooling of ytterbium atoms,
Review of Scientific Instruments 86, 073110/1-4 (2015).

[Summary] We developed a high-power laser system at a wavelength of 399 nm for laser cooling of ytterbium atoms with ultraviolet laser diodes. The system is composed of an external cavity laser diode providing frequency stabilized output at a power of 40 mW and another laser diode for amplifying the laser power up to 220 mW by injection locking. The systematic method for optimization of our injection locking can also be applied to high power light sources at any other wavelengths. Our system does not depend on complex nonlinear frequency-doubling and can be made compact, which will be useful for providing light sources for laser cooling experiments including transportable optical lattice clocks.

Martin Miranda, Ryotaro Inoue, Yuki Okuyama, Akimasa Nakamoto, and *Mikio Kozuma,
Site-resolved imaging of ytterbium atoms in a two-dimensional optical lattice,
Physical Review A 91, 063414/1-6 (2015).

[Summary] We report a high-resolution microscope system for imaging ultracold ytterbium atoms trapped in a two-dimensional optical lattice. By using the ultraviolet strong transition combined with a solid immersion lens and high-resolution optics, our system resolved individual sites in an optical lattice with a 544-nm spacing. Without any cooling mechanism during the imaging process, the deep potential required to contain the atoms was realized using a combination of a shallow ground-state and a deep excited-state potentials. The lifetime and limitations of this setup were studied in detail.

*Kazue Kudo and Yuki Kawaguchi,
Coarsening dynamics driven by vortex-antivortex annihilation in ferromagnetic Bose-Einstein condensates,
Physical Review A 91, 053609/1-8 (2015).

[Summary] In ferromagnetic Bose-Einstein condensates (BECs), the quadratic Zeeman effect controls magnetic anisotropy, which affects magnetic domain pattern formation. While the longitudinal magnetization is dominant (similar to the Ising model) for a negative quadratic Zeeman energy, the transverse magnetization is dominant (similar to the XY model) for a positive one. When the quadratic Zeeman energy is positive, the coarsening dynamics is driven by vortex-antivortex annihilation in the same way as the XY model. However, due to a superfluid flow of atoms, there exist several combinations of vortex-antivortex pairs in ferromagnetic BECs, which makes the coarsening dynamics more complicated than that of the XY model. We propose a revised domain growth law, which is based on the growth law of the two-dimensional XY model, for a two-dimensional ferromagnetic BEC with a positive quadratic Zeeman energy.

Mafumi Hishida, Asami Endo, Koyomi Nakazawa, Yasuhisa Yamamura, and *Kazuya Saito ,
Effect of n-alkanes on lipid bilayers depending on headgroups,
Chemistry and Physics of Lipids 188, 61-67 (2015).

[Summary] Phase behavior and structural properties were examined for bilayers of phospholipids with different headgroups (DMPC, DMPS and DMPE) while adding n-alkanes to study effect of flexible additives. Change in the temperatures of main transition of the lipid/alkane mixtures against the length of added alkanes depends largely on the headgroup. Theoretical analysis of the change of the temperature of transition indicates that the headgroup dependence is dominantly originated in the strong dependence of total enthalpy on the headgroups. The results of X-ray diffraction show that the enthalpic stabilization due to enhanced packing of lipid molecules by alkanes in the gel phase causes the headgroup-dependent change in the phase transition behavior. The enhanced packing in the gel phase also lead to easy emergence of the subgel phase with very short relaxation times at room temperature in the DMPE-based bilayers.

*Daisuke Takahashi, Michikazu Kobayashi, and Muneto Nitta,
Nambu-Goldstone Modes Propagating along Topological Defects: Kelvin and Ripple Modes from Small to Large Systems,
Physical Review B 91, 184501-1-19 (2015).

[Summary] Nambu-Goldstone modes associated with (topological) defects such as vortices and domain walls in (super)fluids are known to possess quadratic/noninteger dispersion relations in finite/infinite-size systems. Here, we report interpolating formulas connecting the dispersion relations in finite- and infinite-size systems for Kelvin modes along a quantum vortex and ripplons on a domain wall in superfluids. Our method can provide not only the dispersion relations but also the explicit forms of quasiparticle wave functions ( u,v ). We find a completeagreement between the analytical formulas and numerical simulations. All these formulas are derived in a fully analytical way, and hence not empirical ones. We also discuss common structures in the derivation of these formulas and speculate on the general procedure.

*Wataru Kurebayashi, Sho Shirasaka, and Hiroya Nakao,
A criterion for timescale decomposition of external inputs for generalized phase reduction of limit-cycle oscillators,
Nonlinear Theory and Its Applications (IEICE) 6, 171-180 (2015).

[Summary] The phase reduction method is a dimension reduction method for weakly driven limit-cycle oscillators, which has played an important role in the theoretical analysis of synchronization phenomena. Recently, we proposed a generalization of the phase reduction method [W. Kurebayashi et al., Phys. Rev. Lett. 111, 2013]. This generalized phase reduction method can robustly predict the dynamics of strongly driven oscillators, for which the conventional phase reduction method fails. In this generalized method, the external input to the oscillator should be properly decomposed into a slowly varying component and remaining weak fluctuations. In this paper, we propose a simple criterion for timescale decomposition of the external input, which gives accurate prediction of the phase dynamics and enables us to systematically apply the generalized phase reduction method to a general class of limit-cycle oscillators. The validity of the criterion is confirmed by numerical simulations.

Takuya Yanagimachi, Mafumi HIshida, Yasuhisa Yamamura, and Kazuya Saito,
Ultraslow oscillation of nematic disclination after abrupt switching of DC voltage,
Journal of the Physical Society of Japan 84, 033601/1-4 (2015).

[Summary] A slow damped oscillation of nematic disclination was observed after abrupt (step-wise) change of DC electric field. The oscillation was not observed under high frequency AC field. These results imply that molecular reorientation in-fluence disclination dynamics after abrupt switching. An effective model, different from the elastic theory, was proposed to analyze the results.

*Yoji Kawamura, Hiroya Nakao,
Phase description of oscillatory convection with a spatially translational mode,
Physica D: Nonlinear Phenomena 295-296, 11–29 (2015).

[Summary] We formulate a theory for the phase description of oscillatory convection in a cylindrical Hele–Shaw cell that is laterally periodic. This system possesses spatial translational symmetry in the lateral direction owing to the cylindrical shape as well as temporal translational symmetry. Oscillatory convection in this system is described by a limit-torus solution that possesses two phase modes; one is a spatial phase and the other is a temporal phase. The spatial and temporal phases indicate the “position” and “oscillation” of the convection, respectively. The theory developed in this paper can be considered as a phase reduction method for limit-torus solutions in infinite-dimensional dynamical systems, namely, limit-torus solutions to partial differential equations representing oscillatory convection with a spatially translational mode. We derive the phase sensitivity functions for spatial and temporal phases; these functions quantify the phase responses of the oscillatory convection to weak perturbations applied at each spatial point. Using the phase sensitivity functions, we characterize the spatiotemporal phase responses of oscillatory convection to weak spatial stimuli and analyze the spatiotemporal phase synchronization between weakly coupled systems of oscillatory convection.


*Michikazu Kobayashi, and Muneto Nitta,
Nonrelativistic Nambu-Goldstone Modes Associated with Spontaneously Broken Space-Time and Internal Symmetries,
Physical Review Letters 113, 120403/1-5 (2014).

[Summary] We show that a momentum operator of a translational symmetry may not commute with an internal symmetry operator in the presence of a topological soliton in nonrelativistic theories. As a striking consequence, there appears a coupled Nambu-Goldstone mode with a quadratic dispersion consisting of translational and internal zero modes in the vicinity of a domain wall in an O(3) σ model, a magnetic domain wall in ferromagnets with an easy axis.

Mafumi Hishida, Yasuhisa Yamamura, and *Kazuya Saito,
Salt effects on lamellar repeat distance depending on head groups of neutrally charged lipids,
Langmuir 30, 10583-10589 (2014).

[Summary] Change in lamellar repeat distances of neutrally charged lipids upon addition of monovalent salts was measured with small-angle X-ray scattering for combinations of two lipids (PC and PE lipids) and six salts. Large dependence on lipid head group is observed in addition to those on added cation and anion. The ion and lipid dependences have little correlation with measured surface potentials of lipid membranes. These results indicate that the lamellar swelling by salt is not explained through balance among interactions considered previously (van der Waals interaction, electrostatic repulsion emerged by ion binding, etc.). It is suggested that effect of water structure, which is affected by not only ions but also lipid itself, should be taken into account for understanding membrane−membrane interactions, as in the Hofmeister effect.

Koyomi Nakazawa, Mafumi Hishida, Shigenori Nagatomo, Yasuhisa Yamamura, and *Kazuya Saito,
Interplay between phase transition of DPPC bilayer and photoisomerization of doped stilbene molecule,
Chemistry Letters 43, 1352-1354 (2014).

[Summary] Cis-trans conformational change of incorporated stilbene in lipid bilayer and the phase-transition behavior of the lipid show strong correlation to each other: Phase-transition temperature is lower with cis-stilbene than with trans-stilbene, whereas photoisomerization rate of stilbene is affected by the phase of the lipid.

*Michikazu Kobayashi, and Muneto Nitta,
Nonrelativistic Nambu-Goldstone modes propagating along a Skyrmion line,
Physical Review D 90, 025010/1-9 (2014).

[Summary] We study Nambu-Goldstone (NG) modes or gapless modes propagating along a Skyrmion (lump) line in a relativistic and nonrelativistic O(3) sigma model, the latter of which describes isotropic Heisenberg ferromagnets. We show for the nonrelativistic case that there appear two coupled gapless modes with a quadratic dispersion. In addition to the well-known Kelvin mode consisting of two translational zero modes transverse to the Skyrmion line, we show that the other consists of a magnon and dilaton, that is, a NG mode for a spontaneously broken internal U(1) symmetry and a quasi-NG mode for a spontaneously broken scale symmetry of the equation of motion. We find that the commutation relations of Noether charges admit a central extension between the dilatation and phase rotation, in addition to the one between two translations found recently. The counting rule is consistent with the Nielsen-Chadha and Watanabe-Brauner relations only when we take into account quasi-NG modes.

*Wataru Kurebayashi, Tsubasa Ishii, Mikio Hasegawa, and Hiroya Nakao,
Design and control of noise-induced synchronization patterns,
EPL (Europhysics Letters) 107, 10009/1-6 (2014).

[Summary] We propose a method for controlling synchronization patterns of limit-cycle oscillators by common noisy inputs, i.e., by utilizing noise-induced synchronization. Various synchronization patterns, including fully synchronized and clustered states, can be realized by using linear filters that generate appropriate common noisy signals from given noise. The optimal linear filter can be determined from the linear phase response property of the oscillators and the power spectrum of the given noise. The validity of the proposed method is confirmed by numerical simulations.

*Hiroya Nakao, Tatsuo Yanagita, Yoji Kawamura,
Phase-Reduction Approach to Synchronization of Spatiotemporal Rhythms in Reaction-Diffusion Systems,
Physical Review X 4, 021032/1-23 (2014).

[Summary] Reaction-diffusion systems can describe a wide class of rhythmic spatiotemporal patterns observed in chemical and biological systems, such as circulating pulses on a ring, oscillating spots, target waves, and rotating spirals. These rhythmic dynamics can be considered limit cycles of reaction-diffusion systems. However, the conventional phase-reduction theory, which provides a simple unified framework for analyzing synchronization properties of limit-cycle oscillators subjected to weak forcing, has mostly been restricted to low-dimensional dynamical systems. Here, we develop a phase-reduction theory for stable limit-cycle solutions of reaction-diffusion systems with infinite-dimensional state space. By generalizing the notion of isochrons to functional space, the phase-sensitivity function—a fundamental quantity for phase reduction—is derived. For illustration, several rhythmic dynamics of the FitzHugh-Nagumo model of excitable media are considered. Nontrivial phase-response properties and synchronization dynamics are revealed, reflecting their complex spatiotemporal organization. Our theory will provide a general basis for the analysis and control of spatiotemporal rhythms in various reaction-diffusion systems.

Michikazu Kobayashi, and *Eiji Nakano, and Muneto Nitta,
Color Magnetism in Non-Abelian Vortex Matter,
Journal of High Energy Physics 6, 130/1-12 (2014).

[Summary] We propose color magnetism as a generalization of the ordinary Heisenberg (anti-)ferro magnets on a triangular lattice. Vortex matter consisting of an Abrikosov lattice of non-Abelian vortices with color magnetic fluxes shows a color ferro or anti-ferro magnetism, depending on the interaction among the vortex sites. A prime example is a non-Abelian vortex lattice in rotating dense quark matter, showing a color ferromagnetism. We show that the low-energy effective theory for the vortex lattice system in the color ferromagnetic phase is described by a 3+1 dimensional.

*Mafumi Hishida, Koichiro Tanaka, Yasuhisa Yamamura, and Kazuya Saito,
Cooperativity between water and lipids in lamellar to inverted-hexagonal phase transition,
Journal of the Physical Society of Japan 83, 044801 (2014).

[Summary] It has been unclear whether the role of water in the self-assembly of soft materials and biomolecules is influential or water is just a background medium. Here we investigate the correlation between hydration state of lipid membrane and structural phase transition of the membrane between lamellar and inverted-hexagonal phases, as an intermediate process of membrane fusion, by using the complementary techniques of X-ray scattering and terahertz (THz) spectroscopy. By comparing two lipid species, our results indicate that the structural changes of the lipid membrane depend on the behavior of the surrounding water, especially in the second hydration layer, in addition to the molecular shape of lipids. The water behaves differently at each membrane surface owing to the different hydrophilicities of the lipid head groups.

Masahiro Kazama, *Wataru Kurebayashi, Takahiro Tsuchida, Yuta Minoshima, Mikio Hasegawa, Koji Kimura, and Hiroya Nakao,
Enhancement of noise correlation for noise-induced synchronization of limit-cycle oscillators by threshold filtering,
NOLTA, IEICE 5, 157-171 (2014).

[Summary] Nonlinear oscillators driven by correlated noisy signals can synchronize without di- rect mutual interactions. Here we show that correlation between noisy signals can be enhanced by applying a threshold filter, and the filtered signals can be used to improve noise-induced synchronization. We derive analytical expressions for the correlation coefficient between the filtered signals, and, using simple examples, we demonstrate that the correlation can actually be enhanced and the synchronization can be improved by the threshold filtering in some cases.