A02 Proposed Research Projects (2016-2017)

Paper | Original Paper


Takafumi Tomita, Shuta Nakajima, Ippei Danshita, Yosuke Takasu and Yoshiro Takahashi,
Observation of the Mott insulator to superfluid crossover of a driven-dissipative Bose-Hubbard system,
Science Advances 3, e1701513 (2017).

[Summary] Dissipation is ubiquitous in nature and plays a crucial role in quantum systems such as causing decoherence of quantum states. Recently, much attention has been paid to an intriguing possibility of dissipation as an efficient tool for the preparation and manipulation of quantum states. We report the realization of successful demonstration of a novel role of dissipation in a quantum phase transition using cold atoms. We realize an engineered dissipative Bose-Hubbard system by introducing a controllable strength of two-body inelastic collision via photoassociation for ultracold bosons in a three-dimensional optical lattice. In the dynamics subjected to a slow ramp-down of the optical lattice, we find that strong on-site dissipation favors the Mott insulating state: The melting of the Mott insulator is delayed, and the growth of the phase coherence is suppressed. The controllability of the dissipation is highlighted by quenching the dissipation, providing a novel method for investigating a quantum many-body state and its nonequilibrium dynamics.

Mateusz Borkowski, Alexei A. Buchachenko, Roman Ciuryło, Paul S. Julienne, Hirotaka Yamada, Yuu Kikuchi, Kakeru Takahashi, Yosuke Takasu, and Yoshiro Takahashi,
Beyond-Born-Oppenheimer effects in sub-kHz-precision photoassociation spectroscopy of ytterbium atoms,
Physical Review A 96, 063405 (2017).

[Summary] We present high-resolution two-color photoassociation spectroscopy of Bose-Einstein condensates of ytterbium atoms. The use of narrow Raman resonances and careful examination of systematic shifts enabled us to measure 13 bound-state energies for three isotopologues of the ground-state ytterbium molecule with standard uncertainties of the order of 500 Hz. The atomic interactions are modeled using an ab initio based mass-scaled Born-Oppenheimer potential whose long-range van der Waals parameters and total WKB phase are fitted to experimental data. We find that the quality of the fit of this model, of about 112.9 kHz (rms) can be significantly improved by adding the recently calculated beyond-Born-Oppenheimer (BBO) adiabatic corrections [J. J. Lutz and J. M. Hutson, J. Mol. Spectrosc. 330, 43 (2016)] and by partially treating the nonadiabatic effects using distance-dependent reduced masses. Our BBO interaction model represents the experimental data to within about 30.2 kHz on average, which is 3.7 times better than the “reference” Born-Oppenheimer model. We calculate the s-wave scattering lengths for bosonic isotopic pairs of ytterbium atoms with error bars over two orders of magnitude smaller than previous determinations. For example, the s-wave scattering length for Yb174 is +5.55812(50) nm.

Yosuke Takasu, Yoshiaki Fukushima, Yusuke Nakamura, and Yoshiro Takahashi,
Magnetoassociation of a Feshbach molecule and spin-orbit interaction between the ground and electronically,
Physical Review A 96, 023602 (2017).

[Summary] By preparing a cold-atom ensemble of mixtures of the ground 1S0 and metastable 3P2 states of ytterbiumatoms 171Yb, we successfully associate a Feshbach molecule 171Yb2 with one 171Yb atom in its electronically excited state and another one in the ground state, by sweeping a magnetic field across a Feshbach resonance. The atom-molecule conversion efficiency reaches about 50%, confirmed by a separate image of atoms and molecules with a Stern-Gerlach effect and an atom loss measurement. In addition, we successfully implement a spin-orbit coupling with a one-photon process between the 3P2 (pseudo-spin-up) and ground 1S0 (pseudo-spin-down) states of a Yb atom. As a benchmark, we observe a spin-momentum locking behavior at a large Rabi frequency. The achieved successful production of Feshbach molecules, along with the implementation of spin-orbital coupling between the 1S0 and 3P2 states, provides an important step towards the study of a topological superfluid.

Yumino Hayase, Takahiro Sakaue and Hiizu Nakanishi,
Compressive response and helix formation of a semiflexible polymer confined in a nanochannel,
Physical Review E 95, 052502/1-6 (2017).

[Summary] Configurations of a single semiflexible polymer is studied when it is pushed into a nanochannel in the case where the polymer persistence length lp is much longer than the channel diameter D:lp/D≫1. Using numerical simulations, we show that the polymer undergoes a sequence of recurring structural transitions upon longitudinal compression: random deflection along the channel, a helix going around the channel wall, double-fold random deflection, double-fold helix, etc. We find that the helix transition can be understood as buckling of deflection segments, and the initial helix formation takes place at very small compression with no appreciable weak compression regime of the random deflection polymer.

*Takuya Saito and Takahiro Sakaue,
Complementary mode analyses between sub-and superdiffusion,
Physical Review E 95, 042143 (2017).

[Summary] Several subdiffusive stochastic processes in nature, e.g., the motion of a tagged monomer in polymers, the height fluctuation of interfaces, particle dynamics in single-file diffusion, etc., can be described rigorously or approximately by the superposition of various modes whose relaxation times are broadly distributed. In this paper, we propose a mode analysis generating superdiffusion, which is paired with or complementary to subdiffusion. The key point in our discussion lies in the identification of a pair of conjugated variables, which undergo sub- and superdiffusion, respectively. We provide a simple interpretation for the sub- and superdiffusion duality for these variables using the language of polymer physics. The analysis also suggests the usefulness of looking at the force fluctuation in experiments, where a polymer is driven by a constant velocity.

*Takahiro Sakaue, Jean-Charles Walter, Enrico Carlon and Carlo Vanderzande,
Non-Markovian dynamics of reaction coordinate in polymer folding,
Soft Matter 13, 3174-3181 (2017).

[Summary] We develop a theoretical description of the critical zipping dynamics of a self-folding polymer. We use tension propagation theory and the formalism of the generalized Langevin equation applied to a polymer that contains two complementary parts which can bind to each other. At the critical temperature, the (un)zipping is unbiased and the two strands open and close as a zipper. The number of broken base pairs n(t) displays a subdiffusive motion characterized by a variance growing as 〈Δn2(t)〉 ∼ tα with α < 1 at long times. Our theory provides an estimate of both the asymptotic anomalous exponent α and of the subleading correction term, which are both in excellent agreement with numerical simulations. The results indicate that the tension propagation theory captures the relevant features of the dynamics and shed some new insights on related polymer problems characterized by anomalous dynamical behavior.

*Kazuya Saito, Takaaki Ikeda, Yasuhisa Yamamura, Hideki Saitoh, Mafumi Hishida, Yutaro Kobayashi, Takeshi Fujita, and Junji Ichikawa,
Cell-quintupling: Structural phase transition in a molecular crystal, bis(trans-4-butylcyclohexyl)methanol,
The Journal of Chemical Physics 146, 074503 (2017).

[Summary] Astructural phase transition at 151.6Kof the title compound [bis(trans-4–butylcyclohexyl)methanol] is examined by X-ray diffraction crystallography, Fourier-transform infrared spectroscopy, and adiabatic calorimetry. A general consideration on possible superstructures indicates that a single modulation wave is sufficient to drive this cell-quintupling transition. The entropy of transition determined calorimetrically indicates that two conformations are dominant in the room-temperature phase in contrast to the fivefold disorder expected from the structure of the low-temperature phase.

*Kazuya Saito, Mafumi Hishida, Kent Koike, Shigenori Nagatomo, and Yasuhisa Yamamura,
X-ray study of molecular association in alcohols having bulkysubstituents,
Chemical Physics Letters 673, 74-77 (2017).

[Summary] The formation of globular associate on cooling, which was previously claimed on the basis of small dielectric constant, is supported through measuring X-ray scattering from dicyclohexylmethanol (DCHM). Radial distribution function of the DCHM molecules exhibits strong temperature dependence between 65 ℃ and 130 ℃ in contrast to a little change in that of tricyclohexylmethanol (TCHM), which is similar to DCHM but essentially non-associating in experimental conditions (95–160℃).

*Takahiro Sakaue, Takuya Saito,
Active diffusion of model chromosomal loci driven by athermal noise,
Soft Matter 13, 81-87 (2017).

[Summary] Active diffusion, i.e., fluctuating dynamics driven by athermal noise, is found in various out-of-equilibrium systems. Here we discuss the nature of the active diffusion of tagged monomers in a flexible polymer. A scaling argument based on the notion of tension propagation clarifies how the polymeric effect is reflected in the anomalous diffusion exponent, which may be of relevance to the dynamics of chromosomal loci in living cells.


*Takahiro Sakaue,
Dynamics of polymer translocation: a short review with an introduction of weakly-driven regime,
Polymers 8, 424:1-12 (2016).

[Summary] As emphasized in a recent review (by V.V. Palyulin, T. Ala-Nissila, R. Metzler), theoretical understanding of the unbiased polymer translocation lags behind that of the (strongly) driven translocation. Here, we suggest the introduction of a weakly-driven regime, as described by the linear response theory to the unbiased regime, which is followed by the strongly-driven regime beyond the onset of nonlinear response. This provides a concise crossover scenario, bridging the unbiased to strongly-driven regimes.

Kyohei Shitara and *Takahiro Sakaue,
Shear modulus of structured electro-rheological fluid mixtures,
Physical Review E 93, 052603/1-7 (2016).

[Summary] Some immiscible blends under a strong electric field often exhibit periodic structures, bridging the gap between two electrodes. Upon shear, the structures tilt, and exhibit an elastic response which is mostly governed by the electric energy. Assuming a two-dimensional stripe structure, we calculate the Maxwell stress, and derive an expression for the shear modulus, demonstrating how it depends on the external electric field, the composition, and the dielectric properties of the blend. We also suggest the notion of effective interfacial tension, which renormalizes the effect of the electric field. This leads to a simple derivation of the scaling law for the selection of the wavelength of the structure formed under an electric field.

*Takahiro Sakaue, and Chihiro H. Nakajima,
Miscibility Phase Diagram of Ring Polymer Blends: A Topological Effect,
Physical Review E 93, 042502/1-9 (2016).

[Summary] The miscibility of polymer blends, a classical problem in polymer science, may be altered, if one or both of the component do not have chain ends. Based on the idea of topological volume, we propose a mean-field theory to clarify how the topological constraints in ring polymers affect the phase behavior of the blends. While the large enhancement of the miscibility is expected for ring-linear polymer blends, the opposite trend toward demixing, albeit comparatively weak, is predicted for ring-ring polymer blends. Scaling formulas for the shift of critical point for both cases are derived. We discuss the valid range of the present theory, and the crossover to the linear polymer blends behaviors, which is expected for short chains. These analyses put forward a view that the topological constraints could be represented as an effective excluded-volume effects, in which the topological length plays a role of the screening factor.

Ahmed Khorshid, Susan Amin, Zhiyue Zhang, Takahiro Sakaue, *Walter Reisner,
Non-Equilibrium Dynamics of Nanochannel Confined DNA,
Macromolecules 49(5), 1933–1940 (2016).

[Summary] We show that the dynamic non-equilibrium segmental concentration profile of a single nanochannel confined DNA molecule can be described via a partial differential evolution equation based on nonlinear diffusion, using an approach analogous to that used in the description of many-molecule systems such as polymer solutions. This equation can describe the segmental concentration profile of a single polymer along the nanochannel as a function of time for chain behavior ranging from states of high compression to equilibrium. In particular, to demonstrate the generality of our approach, we show that our model can describe two distinct types of experimental behavior generated via a sliding bead assay, symmetric relaxation resulting from free expansion of the polymer after compression, and the evolution of DNA concentration “shock waves” as a molecule is driven from equilibrium to a compressed state.

*Kazuya Saito, Yasuhisa Yamamura, Yohei Miwa, and Shoichi Kutsumizu,
A structural model of the chiral Im3m cubic phase,
Physical Chemistry Chemical Physics 18, 3280-3284 (2016).

[Summary] Assuming the twisted arrangement of rodlike molecules as the origin of the chirality as in the existing model, a new model of the molecular arrangement in the cubic "Im3m" phase is proposed. The adoption of a basic structure different from that assumed in the existing model resolves most difficulties of the model including the random placement of defects concerning the sense of twist.

Mafumi Hishida, Ryuta Yanagisawa, Hatsuho Usuda, Yasuhisa Yamamura, and *Kazuya Saito,
Communication: Rigidification of a lipid bilayer by an incorporated n-alkane,
Journal of Chemical Physics 144, 041103 (2016).

[Summary] Towards a greater understanding of the effects of organic molecules in biomembranes, the effects of a flexible alkyl chain on the morphologies of phospholipid vesicles are investigated. Vesicles composed of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) and tetradecane (TD) rupture during cooling from the liquid–crystalline phase to the gel phase. A model calculation based on the size-dependent rupture probability indicates the bending rigidity of the bilayer in the gel phase is more than 10 times higher than that without TD, resulting in the rupture arising from elastic stress. The rigidification is caused by the denser molecular packing in the hydrophobic region by TD. There is little change of the rigidity in the liquid–crystalline phase. Additionally, the rigidification produces a characteristic morphology of the ternary giant vesicles including TD. Reported thermal behaviors imply that molecules with a linear and long alkyl chain, such as trans fatty acids, universally exhibit a similar effect, in contrast to rigid and bulky molecules, such as cholesterol.