Paper | Original Paper


Jim Gao, *Wei Guo, Satoshi Yui, Makoto Tsubota, and W. F. Vinen,
Dissipation in quantum turbulence in superfluid 4He above 1 K,
Physical Review B 97, 184518/1-10 (2018).

[Summary] There are two commonly discussed forms of quantum turbulence in superfluid 4He above 1 K: in one there isa random tangle of quantized vortex lines, existing in the presence of a nonturbulent normal fluid; in the secondthere is a coupled turbulent motion of the two fluids, often exhibiting quasiclassical characteristics on scaleslarger than the separation between the quantized vortex lines in the superfluid component. We study experimentally and numerically the decay of vortex tangle in thermal counterflow.

*Satoshi Yui, Makoto Tsubota, and Hiromichi Kobayashi,
Three-Dimensional Coupled Dynamics of the two-fluid model in superfluid 4He: Deformed velocity profile of normal fluid in thermal counterflow,
Physical Review Letters 120, 155301/1-4 (2018).

[Summary] The coupled dynamics of the two-fluid model of superfluid 4He is numerically studied for quantum turbulence of the thermal counterfow in a square channel. We combine the vortex filamentmodel of the superfluid and the Navier{Stokes equations of normal fluid. Simulations of the coupled dynamics show that the velocity profile of the normal fluid is deformed significantly by superfluid turbulence as the vortices become dense. This result is consistent with recently performed visualization experiments. We introduce a dimensionless parameter that characterizes the deformation of the velocity profile.

Keigo Nakamura, Tetsuya Hisanaga, Koichi Fujimoto, Keiji Nakajima and *Hirofumi Wada,
Plant-inspired pipettes,
Journal of the Royal Society Interface 15, 20170868 (2018).

[Summary] The female sex organ of the liverwort (Marchantia polymorpha) has a character- istic parasol-like form highly suitable for collecting water droplets containing sperm for fertilization. Motivated by this observation and using three- dimensional printing techniques, we develop a parasol-like rigid object that can grab, transport and release water droplets of a maximum size of about 1 cm. By combining experiments and scaling theory, we quantify the object’s fundamental wetting and fluid dynamical properties. We construct a stability phase diagram and suggest that it is largely insensitive to properties of liquids such as surface tension and viscosity. A simple scaling argument is developed to explain the phase boundary. Our study provides basic design rules of a simple pipette-like device with bubble-free capture and drop of liquids, which can be used in laboratory settings and has applications within soft robotics. Through systematic experimental investigations, we suggest the opti- mal design criteria of the liverwort-inspired object to achieve maximal pipetting performance. We also provide, based on our scalable model exper- iments, a biological implication for the mechanistic advantage of this structure in liverwort reproduction.

*Tomohiko G. Sano and Hirofumi Wada,
Snap-buckling in asymmetrically constrained elastic strips,
Physical Review E 97, 013002 (2018).

[Summary] When a flat elastic strip is compressed along its axis, it is bent in one of two possible directions via spontaneous symmetry breaking, forming a cylindrical arc. This is a phenomenon well known as Euler buckling. When this cylindrical section is pushed in the other direction, the bending direction can suddenly reverse. This instability is called “snap-through buckling” and is one of the elementary shape transitions in a prestressed thin structure. Combining experiments and theory, we study snap-buckling of an elastic strip with one end hinged and the other end clamped. These asymmetric boundary constraints break the intrinsic symmetry of the strip, generating mechanical behaviors, including largely hysteretic but reproducible force responses and switchlike discontinuous shape changes. We establish the set of exact analytical solutions to fully explain all our major experimental and numerical findings. Asymmetric boundary conditions arise naturally in diverse situations when a thin object is in contact with a solid surface at one end. The introduction of asymmetry through boundary conditions yields new insight into complex and programmable functionalities in material and industrial design.

Toshiaki Kanai, *Wei Guo, Makoto Tsubota,
Flows with fractional quantum circulation in Bose-Einstein condensates induced by nontopological phase defects,
Physical Review A 97, 013612/1-5 (2018).

Toshiaki Kanai, *Wei Guo, Makoto Tsubota, Flows with fractional quantum circulation in Bose-Einstein condensates induced by nontopological phase defects, Physical Review A 97, 013612/1-5 (2018).


*Fuyuki Nabeshima, Kosuke Nagasawa, and Atsutaka Maeda,
Superconducting fluctuations in FeSe0.5Te0.5 thin films probed via microwave spectroscopy,
Physical Review B97, 024504/1-6 (2018).

[Summary] We investigated the microwave conductivity spectrum of FeSe0.5Te0.5 epitaxial films on CaF2 in the vicinityof the superconducting transition. We observed the critical behavior of the superconducting fluctuations in thesefilms with a dimensional crossover from two dimensional (2D) to three dimensional (3D) as the film thicknessincreased. From the temperature dependence of the scaling parameters we conclude that the universality classof the superconducting transition in FeSe0.5Te0.5 is that of the 3D-XY model. The lower limit of the onsettemperature of the superconducting fluctuations T onset determined by our measurementswas 1.1Tc, suggesting that the superconducting fluctuations of FeSe0.5Te0.5 are at least as large as those of optimally and overdoped cuprates.

*G. N. Phan, Kousuke Nakayama, K. Sugawara, T. Sato, T. Urata, Yoichi Tanabe, Katsumi Tanigaki, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, and Takashi Takahashi,
Effects of strain on the electronic structure, superconductivity, and nematicity in FeSe studiedby angle-resolved photoemission spectroscopy,
Physical Review B95, 224507/1-6 (2017).

[Summary] One of central issues in iron-based superconductors is the role of structural change to the superconductingtransition temperature (Tc). Itwas found in FeSe that the lattice strain leads to a drastic increase in Tc, accompaniedby suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressivestrainedand strain-free FeSe, we experimentally show that the in-plane strain causes a marked change in theenergy overlap (Eh−e) between the hole and electron pockets in the normal state. The change in Eh−e modifiesthe Fermi-surface volume, leading to a change in Tc. Furthermore, the strength of nematicity is also found to becharacterized by Eh−e. These results suggest that the key to understanding the phase diagram is the fermiologyand interactions linked to the semimetallic band overlap.

*Masamichi Nakajima, K. Yanase, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, and Setsuko Tajima,
Gradual Fermi-surface modification in orbitally ordered state of FeSe revealedby optical spectroscopy,
Physical Review B95, 184502/1-5 (2017).

[Summary] We performed optical spectroscopy on a thin film of FeSe, in which orbital ordering shows up in theorthorhombic phase below Ts. The optical conductivity spectrum for FeSe exhibits no gap feature in the orbitallyordered state, in contrast to those for iron pnictides showing the gap opening in the magnetostructurally orderedphase. Instead,we observed a gradual suppression of the coherent carrier density with temperature. This highlightsa peculiar metallic state in FeSe that the Fermi surfaces are gradually modified below Ts. We also found ananomalous behavior of the optical phonon mode, indicating the intimate connection between the orbital andlattice degrees of freedom. The result implies the orbital origin of the structural transition.

Tomohiko G. Sano, Tetsuo Yamaguchi, and Hirofumi Wada,
Slip Morphology of Elastic Strips on Frictional Rigid Substrates,
Physical Review Letters 118, 178001 (2017).

[Summary] The morphology of an elastic strip subject to vertical compressive stress on a frictional rigid substrate isinvestigated by a combination of theory and experiment. We find a rich variety of morphologies, which—when the bending elasticity dominates over the effect of gravity—are classified into three distinct types ofstates: pinned, partially slipped, and completely slipped, depending on the magnitude of the vertical strainand the coefficient of static friction.We develop a theory of elastica under mixed clamped-hinged boundaryconditions combined with the Coulomb-Amontons friction law and find excellent quantitative agreementwith simulations and controlled physical experiments. We also discuss the effect of gravity in order tobridge the difference in the qualitative behaviors of stiff strips and flexible strings or ropes. Our study thuscomplements recent work on elastic rope coiling and takes a significant step towards establishing a unifiedunderstanding of how a thin elastic object interacts vertically with a solid surface.

*Yoshinori Imai, Yuichi Sawada, Fuyuki Nabeshima, Daisuke Asami, Masataka Kawai, Atsutaka Maeda,
Control of structural transiton in FeSe1-xTex thin films by changing substrate materials,
Scientific Reports 7, 46653/1-5 (2017).

[Summary] Iron chalcogenide superconductors FeSe1−xTex are important materials for investigating the relationbe-tween the superconductivity and the orbital and/or electronic nematic order, because the endmember material FeSe exhibits a structural transition without a magnetic phase transition. However,the phase separation occurs in the region of 0.1 ≤ x ≤ 0.4 for bulk samples, and it prevents the completeunderstanding of this system. Here, we report the successful fabrication of epitaxial thin films ofFeSe1−xTex with 0 ≤ x ≤ 0.7, which includes the phase-separation region, on LaAlO3 substrates viapulsed laser deposition. In the temperature dependences of differential resistivity for these films with0 ≤ x ≤ 0.3, the dip- or peak- anomalies, which are well-known to be originated from the structuraltransition in FeSebulk samples, are observed at the characteristic temperatures, T*. The dopingtemperature(x–T) phase diagram of FeSe1−xTex films clearly shows that T* decreases with increasingx, and that Tc suddenly changes at a certain Te content where T* disappears, which turns out to becommonly observed for both films on LaAlO3 and CaF2. These indicate the importance of controlling thestructural transition to achieve high Tc in iron chalcogenides.

Takaaki Iguchi, Takeyoshi Sugaya, and *Yukio Kawano,
Silicon-immersed terahertz plasmonic structures,
Applied Physics Letters 110, 151105-1-4 (2017).

[Summary] A Bull’s eye (BE) plasmonic structure exhibits a powerful property of highly enhanced light transmission through a sub-wavelength aperture. However, the BE structure in the terahertz (THz) region exhibits the problems of a relatively low THz-field enhancement factor and a large area with an increasing groove number. Here, we report on a design of a THz BE structure that utilizes a solid immersion method based on the high refractive index of a silicon (Si) material. By fabricating a deep-etched Si template covered with a gold film, we achieved a greatly enhanced THz transmission with an enhancement factor of up to 10^8 and also miniaturized the structure size by the factor of 11. These features demonstrate that the BE performance can be further improved by engineering dielectric materials.

*Che-hsiu Hsueh, Wen-Chin Wu, and Makoto Tsubota,
Quantum crystallography of Rydberg-dressed Bose gases on a square lattice,
Physical Review A 95, 013631/1-5 (2017).

[Summary] We numerically investigate the quantum crystallographic phases of a Rydberg-dressed Bose gas loaded on a square lattice by using the mean-field Gross–Pitaevskii model. For a relatively weak lattice confinement, the phases of ground state undergo amorphism, polycrystal, and polymorphism following the increase of the blockade radius, and if the confinement is stronger, a single crystal with a specific fillingfactor will be formed. In order to distinctively characterize these phases, the structure function is also studied. In such an anisotropic system, we report that the first diagonal element of the superfluid-fraction tensor should be a measurable quantity, and an anisotropy parameter can be defined.

*Fuyuki Nabeshima, Yoshinori Imai, Ichiro Tsukada, and Atsutaka Maeda,
Growth and transport properties of FeSe/FeTe superlattice thin films,
Japanese Journal of Applied Physics 56, 020308 (2017).

[Summary] Superlattice thin films composed of iron chalcogenides, FeSe and FeTe, were grown via pulsed laser deposition. The X-ray diffraction patterns show clear satellite peaks demonstrating periodic stacking structures of FeSe and FeTe. The FeTe layers have the a-axis lengths identical to those of the FeSe layers, indicating that the FeTe layers are coherently strained to the underlying FeSe. The superlattice films show superconducting transition temperatures higher than FeSe, and more importantly the superconductivity emerged in several-unit-cell-thick layers. Our results demonstrate that the strained superlattice technique is a useful tool to control superconducting properties of Fe(Se,Te) thin films.


Yasuaki Morigaki, *Hirofumi Wada and *Yoshimi Tanaka,
Stretching an elastic loop: Crease, helicoid, and pop-out,
Physical Review Letters 117, 198003 (2016).

[Summary] Under geometric constraints, a thin structure can respond to an external loading in an unexpected way. A paper strip that is looped and pulled can be used for simple experimentation of such a process. Here, we study this seemingly very simple phenomenon in detail by combing experiments and theory. We identify the three types of shape transitions, i.e., crease, helicoid, and pop out, from a stretched loop, and classify them in terms of parameters characterizing a ribbon geometry. We establish a transition-type diagram by compiling our extensive experimental data. Numerical simulations based on the Kirchhoff rod theory and scaling argument reveal that the pop-out transition is governed by a single characteristic length ξ ∼ b2=h, where b and h are the ribbon’s width and thickness, respectively. We also reveal the key roles of other physical effects such as the anisotropy of the bending elasticity and plastic deformations upon the shape selection mechanisms of a constraint ribbon.

*Yuichi Sawada, Fuyuki Nabeshima, Daisuke Asami, Ryo Ogawa, Yoshinori Imai,
Transport properties of FeSe1-xTex thin films under magnetic fields up to 8 T,
Physica C 530, 27-30 (2016).

[Summary] We investigated transport properties under magnetic fields up to 8 T for FeSe1−x1−xTex thin films on CaF2. For x=0.2--0.4,x=0.2--0.4, where Tc’s are the highest, resistive broadening was observed, while the width of superconducting transition was almost the same with increasing magnetic fields in x=0--0.1,x=0--0.1, indicating differences in the nature of superconductivity between these two groups.

*Yoshinori Imai, Yuichi Sawada, Daisuke Asami, Fuyuki Nabeshima, Atsutaka Maeda,
Superconducting properties of FeSe1-xTex films with x = 0-0.4,
Physica C 530, 24-26 (2016).

[Summary] We investigate superconducting properties of FeSe1−x1−xTex films with x=0−0.4,x=0−0.4, which are around the optimal Te contents, on CaF2 substrates. From the results of electrical resistivity measurements, the values of Tc in films with x=0.3,0.4x=0.3,0.4 are about 22 K. These values are very close to that of the film with x=0.2x=0.2 which was reported to be the optimal composition in FeSe1−x1−xTex films on CaF2 substrates. In addition, a sudden suppression of Tc is observed in very narrow region, that is, 0.1 < x < 0.15. This behavior is different from the dome-shaped phase diagram that is familiar in iron-based superconductors.

Daichi Suzuki, Shunri Oda and *Yukio Kawano,
A flexible and wearable terahertz scanner,
Nature Photonics 10, 809-814 (2016).

[Summary] Imaging technologies based on terahertz (THz) waves have great potential for use in powerful non-invasive inspection methods. However, most real objects have various three-dimensional curvatures, and existing THz technologies often encounter difficulties in imaging such configurations, which limits the useful range of THz imaging applications. Here, we report the development of a flexible and wearable THz scanner based on carbon nanotubes (CNTs). We achieved room-temperature THz detection over a broad frequency band ranging from 0.14 THz to 39 THz and developed a portable THz scanner. Using this scanner, we performed THz imaging of samples concealed behind opaque objects, breakages and metal impurities inspection of a bent film, and multi-view scan of a syringe. Also we demonstrated a passive biometric THz scan of a human hand. Our results are expected to have considerable implications for non-destructive and non-contact inspections, such as medical examinations for the continuous monitoring of health conditions.

Daichi Matsumoto, Koji Fukudome and *Hirofumi Wada,
Two-dimensional fluid dynamics in a sharply bent channel: Laminar flow, separation bubble and vortex dynamics,
Physics of Fluids 28, 103608 (2016).

[Summary] Understanding the hydrodynamic properties of fluid flow in a curving pipe and channel is important for controlling the flow behavior in technologies and biome- chanics. The nature of the resulting flow in a bent pipe is extremely complicated because of the presence of a cross-stream secondary flow. In an attempt to disentangle this complexity, we investigate the fluid dynamics in a bent channel via the direct numerical simulation of the Navier-Stokes equation in two spatial dimensions. We exploit the absence of secondary flow from our model and systematically investigate the flow structure along the channel as a function of both the bend angle and Reynolds number of the laminar-to-turbulent regime. We numerically suggest a scaling relation between the shape of the separation bubble and the flow conduc- tance, and construct an integrated phase diagram.

*Ichiro Tsukada, Ataru Ichinose, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda,
Origin of lattice compression of FeS1-xTex thin films on CaF2 substrates.,
AIP Advances 6, 095314/1-7 (2016).

[Summary] Microstructure of FeSe1-xTex thin films near the interface to CaF2 is investigated by means of transmission electron microscopy (TEM) and energy-dispersive X-ray analysis (EDX). TEM observation at the initial crystal-growth stage reveals that marked lattice compression occurs along the in-plane direction in the films with Se-rich composition, while the a-axis length of FeTe remains as its original value of bulk crystal. Subsequent EDX analysis demonstrates substantial diffusion of Se into the CaF2 substrate. Such diffusion is not prominent for Te. Thus, the formation of Se-deficient layer at the initial growth stage on CaF2 is concluded to be the main reason of the lattice compression in FeSe1-xTex thin films.

Hirofumi Wada,
Structural mechanics and helical geometry of thin elastic composites,
Soft Matter 12, 7386-7397 (2016).

[Summary] Helices are ubiquitous in nature, and helical shape transition is often observed in residually stressed bodies, such as composites, wherein materials with different mechanical properties are glued firmly together to form a whole body. Inspired by a variety of biological examples, the basic physical mechanism responsible for the emergence of twisting and bending in such thin composite structures has been extensively studied. Here, we propose a simplified analytical model wherein a slender membrane tube undergoes a helical transition driven by the contraction of an elastic ribbon bound to the membrane surface. We analytically predict the curvature and twist of an emergent helix as functions of differential strains and elastic moduli, which are confirmed by our numerical simulations. Our results may help understand shapes observed in different biological systems, such as spiral bacteria, and could be applied to novel designs of soft machines and robots.

Xiangying Deng, Shunri Oda, and *Yukio Kawano,
Frequency Selective, High Transmission Spiral Terahertz Plasmonic Antennas,
Journal of Modeling and Simulation of Antennas and Propagation​​ 2, 1-6 (2016).

[Summary] Terahertz is long been studied and has potential in various applications. However, lack of efficient and sensitive detector is still an obstacle in the development of terahertz technology. The utilization of antennas can largely enhance sensitivity of terahertz detector. A bull’s eye plasmonic antenna has a great potential to sub-wavelength terahertz technologies because of its unique property of local electric-field concentration. However, a conventional bull’s eye antenna has a problem of a narrow band, limiting its ability for multi-frequency detection. Here we propose two terahertz plasmonic antennas for enabling arbitrary frequency selection: the split-joint bull’s eye structure and the spiral bull’s eye structure. We show that both of the two antennas can achieve wide-band transmission without large sacrifice on overall transmission. The frequency band can be arbitrarily tuned either by varying excitation directions or by adjusting structure parameters. These advantageous features will open up a door to frequency-selective terahertz antennas and their applications to multi-frequency investigations.

*Hideyuki Takahashi, Yoshinori Imai, Atsutaka Maeda,
Low-temperature-compatible tunneling-current-assisted scanning microwave microscope utilizing a rigid coaxial resonator,
Review of Scientific Instruments 87, 063706/1-6 (2016).

[Summary] We present a design for a tunneling-current-assisted scanning near-field microwave microscope. For stable operation at cryogenic temperatures, making a small and rigid microwave probe is important. Our coaxial resonator probe has a length of approximately 30 mm and can fit inside the 2-in. bore of a superconducting magnet. The probe design includes an insulating joint, which separates DC and microwave signals without degrading the quality factor. By applying the SMM to the imaging of an electrically inhomogeneous superconductor, we obtain the spatial distribution of the microwave response with a spatial resolution of approximately 200 nm. Furthermore, we present an analysis of our SMM probe based on a simple lumped-element circuit model along with the near-field microwave measurements of silicon wafers having different conductivities.

*Yuichi Sawada, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda,
Investigation of Transport Properties for FeSe1-xTex Thin Films under Magnetic Fields.,
Journal of the Physical Society of Japan 85, 073703/1-4 (2016).

[Summary] We investigated the transport properties under magnetic fields of up to 9 T for FeSe1−xTex thin films on CaF2. Measurements of the temperature dependence of the electrical resistivity revealed that for x = 0.2–0.4, where Tc is the highest, the width of the superconducting transition increased with increasing magnetic field, while the width was almost the same with increasing magnetic field for x = 0–0.1. In addition, the temperature dependence of the Hall coefficient drastically changed between x = 0.1 and 0.2 at low temperatures. These results indicate that clear differences in the nature of the superconductivity and electronic structure exist between x = 0–0.1 and x ≥ 0.2.

*Shinichi Ikawa, and Makoto Tsubota,
Coflow turbulence of superfluid 4He in a square channel: Vortices trapped on a cylindrical attractor,
Physical Review B 93, 184508/1-8 (2016).

[Summary] We perform a numerical simulation of the dynamics of quantized vortices produced by coflow in a squarechannel using the vortex filament model. Unlike the situation in thermal counterflow, where the superfluidvelocity vs and normal-fluid velocity vn flow in opposite directions, in coflow, vs and vn flow in the samedirection. Quantum turbulence in thermal counterflow has been long studied theoretically and experimentally,and its various features have been revealed. In recent years, an experiment on quantum turbulence in coflow hasbeen performed to observe different features of thermal counterflow. By supposing that vs is uniform and vntakes the Hagen-Poiseuille profile, which is different from the experiment where vn is thought to be turbulent,we calculate the coflow turbulence. Vortices preferentially accumulate on the surface of a cylinder for vs  vn bymutual friction; namely, the coflow turbulence has an attractor. How strongly the vortices are attracted depends onthe temperature and velocity. The length of the vortices increases as the vortices protruding from the cylindricalattractor continue to wrap around it. As the vortices become dense on the attractor, they spread toward its interiorby their repulsive interaction. Then, the superfluid velocity profile induced by the vortices gradually mimics thenormal-fluid velocity profile. This is an indication of velocity matching, which is an important feature of coflowturbulence.

*Kazuya Fujimoto, Makoto Tsubota,
Direct and inverse cascades of spin-wave turbulence in spin-1 ferromagnetic spinor Bose-Einstein condensates,
Physical Review A 93, 033620/1-10 (2016).

[Summary] We theoretically and numerically study spin wave turbulence in spin-1 ferromagnetic spinor Bose-Einstein condensates, finding direct and inverse cascades with power-law behavior. To derive these power exponents analytically, the conventional weak wave turbulence theory is applied to the spin-1 spinor Gross-Pitaevskiiequation. Thus we obtain the −7/3 and −5/3 power laws in the transverse spin correlation function for the directand inverse cascades, respectively.

*John J. Molina, Kotaro Otomura, Hayato Shiba, Hideki Kobayashi, Masaki Sano, and Ryoichi Yamamoto,
Rheological evaluation of colloidal dispersions using the smoothed profile method: formulation and applications,
Journal of Fluid Mechanics 792, 590-619 (2016).

[Summary] The smoothed profile method is extended to study the rheological behaviour of colloidal dispersions under shear flow by using the Lees–Edwards boundary conditions. We start with a reformulation of the smoothed profile method, a direct numerical simulation method for colloidal dispersions, so that it can be used with the Lees–Edwards boundary condition, under steady or oscillatory-shear flow. By this reformulation, all the resultant physical quantities, including local and total shear stresses, become available through direct calculation. Three simple rheological simulations are then performed for (1) a spherical particle, (2) a rigid bead chain and (3) a collision of two spherical particles under shear flow. Quantitative validity of these simulations is examined by comparing the viscosity with that obtained from theory and Stokesian dynamics calculations. Finally, we consider the shear-thinning behaviour of concentrated colloidal dispersions.

*Yuji Hirokane, Yasuhide Tomioka, Yoshinori Imai, Atsutaka Maeda, Yoshinori Onose,
Longitudinal and transverse thermoelectric transport in MnSi,
Physical Review B 93, 014436/1-5 (2016).

[Summary] We have investigated the longitudinal and transverse thermoelectric phenomena (Seebeck and Nernst effects) in a single crystal of intermetallic compound MnSi. The Seebeck coefficient is largely decreased (≈50%) by magnetic field around the helimagnetic transition temperature, which implies the large entropy release at the magnetic transition. We have discerned the anomalous and normal components of Nernst signal dependent on the magnetization and the magnetic field, respectively. For the anomalous Nernst effect, the intrinsic mechanism induced by the Berry phase of electrons seems dominant over the extrinsic mechanisms induced by scattering at least in the high temperature (T≥15 K) and resistive (ρ>4 μΩcm) region. The Nernst and Seebeck signals show little change in the course of the transition to skyrmion crystal.

Shugo Yasuda and Ryoichi Yamamoto,
Synchronized molecular-dynamics simulation for the thermal lubrication of a polymeric liquid between parallel plates,
Computers & Fluids 124, 185 (2016).


*Satoshi Yui, Kazuya Fujimoto, Makoto Tsubota ,
Logarithmic velocity profile of quantum turbulence of superfluid 4He,
Physical Review B 92, 224513/1-5 (2015).

[Summary] The logarithmic velocity profile is the most important statistical law of classical turbulence affected by channelwalls. This paper demonstrates numerically that the logarithmic velocity profile of a superfluid flow appears inquantum turbulence under pure normal flow in a channel. We investigated the configuration and dynamics of aninhomogeneous vortex tangle affected by the walls, and found the characteristic behavior of the log-law.

Michiel Laleman, Marco Baiesi, Boris P. Belotserkovskii, Takahiro Sakaue, Jean-Charles Walter,and *Enrico Carlon,
Torque-Induced Rotational Dynamics in Polymers: Torsional Blobs and Thinning,
Macromolecules 49, 405-414 (2015).

[Summary] By using the blob theory and computer simulations, we investigate the properties of a linear polymer performing a stationary rotational motion around a long impenetrable rod. In particular, in the simulations the rotation is induced by a torque applied to the end of the polymer that is tethered to the rod. Three different regimes are found, in close analogy with the case of polymers pulled by a constant force at one end. For low torques the polymer rotates maintaining its equilibrium conformation. At intermediate torques the polymer assumes a trumpet shape, being composed by blobs of increasing size. At even larger torques the polymer is partially wrapped around the rod. We derive several scaling relations between various quantities as angular velocity, elongation, and torque. The analytical predictions match the simulation data well. Interestingly, we find a “thinning” regime where the torque has a very weak (logarithmic) dependence on the angular velocity. We discuss the origin of this behavior, which has no counterpart in polymers pulled by an applied force.

Kristopher Erickson, Xiaowei He, A. Alec Talin, Bernice Mills, Robert H. Hauge, Takashi Iguchi, Naoki Fujimura, Yukio Kawano, *Junichiro Kono, *François Léonard,
Figure of Merit for Carbon Nanotube Photothermoelectric Detectors,
ACS Nano 9, 11618–11627 (2015).

[Summary] Carbon nanotubes (CNTs) have emerged as promising materials for visible, infrared, and terahertz photodetectors. Further development of these photodetectors requires a fundamental understanding of the mechanisms that govern their behavior as well as the establishment of figures of merit for technology applications. Recently, a number of CNT detectors have been shown to operate based on the photothermoelectric effect. Here we present a figure of merit for these detectors, which includes the properties of the material and the device. In addition, we use a suite of experimental characterization methods for the thorough analysis of the electrical, thermoelectric, electrothermal, and photothermal properties of the CNT thin-film devices. Our measurements determine the quantities that enter the figure of merit and allow us to establish a path toward future performance improvements.

*Takuya Saito, *Takahiro Sakaue,
Driven anomalous diffusion: An example from polymer stretching,
Physical Review E 92, 012601/1-13 (2015).

[Summary] The way tension propagates along a chain is a key to govern many anomalous dynamics in macromolecular systems. After introducing the weak and the strong force regimes of the tension propagation, we focus on the latter, in which the dynamical fluctuations of a segment in a long polymer during its stretching process is investigated. We show that the response, i.e., average drift, is anomalous, which is characterized by the nonlinear memory kernel, and its relation to the fluctuation is nontrivial. These features are discussed on the basis of the generalized Langevin equation, in which the role of the temporal change in spring constant due to the stress hardening is pinpointed. We carried out the molecular dynamics simulation, which supports our theory.

Chunyu Shih, John J. Molina, and Ryoichi Yamamoto,
Dynamic polarisation of a charged colloid in an oscillating electric field,
Molecular Physics 113, 2511 (2015).

C. Patrick Royall, Jens Eggers, *Akira Furukawa, and Hajime Tanaka,
Probing colloidal gels at multiple length scales: The role of hydrodynamics,
Physical Review Letters 114, 258302 (2015).

[Summary] Comparing experiments to numerical simulations, with hydrodynamic interactions switched off, we demonstrate the crucial role of the solvent for gelation. Hydrodynamic interactions suppress the formation of larger local equilibrium structures of closed geometry, and instead lead to the formation of highly anisotropic threads, which promote an open gel network. We confirm these results with simulations which include hydrodynamics. Based on three-point correlations, we propose a scale-resolved quantitative measure for the anisotropy of the gel structure. We find a strong discrepancy for interparticle distances just under twice the particle diameter between systems with and without hydrodynamics, quantifying the role of hydrodynamics from a structural point of view.

Gregory Lecrivain, Giacomo Petrucci, *Uwe Hampel and Ryoichi Yamamoto,
Attachment of solid elongated particles on the surface of a stationary gas bubble,
International Journal of Multiphase Flow 71, 83-93 (2015).

[Summary] The flotation process essentially relies on the attachment of solid particles on the surface of gas bubbles immersed inwater. The present study seeks to investigate the effect of the particle shape on the attachment mechanism. Using an in-house optical micro-bubble sensor the approach, the sliding and the adhesion of micron milled glass fibres on the surface of a stationary air bubble immersed in stagnant water is thoroughly investigated. The translational and rotational velocities were measured for fibres of various aspect ratios. The results are compared with a theoretical model and with experimental data obtained with spherical glass beads.

*Kazuya Fujimoto and Makoto Tsubota,
Bogoliubov-wave turbulence in Bose-Einstein condensates,
Physical Review A 91, 053620/1-12 (2015).

[Summary] We theoretically and numerically study Bogoliubov-wave turbulence in three-dimensional atomic Bose-Einstein condensates with the Gross-Pitaevskii equation, investigating three spectra for the macroscopic wavefunction, the density distribution, and the Bogoliubov-wave distribution. In this turbulence, Bogoliubov wavesplay an important role in the behavior of these spectra, so that we call it Bogoliubov-wave turbulence. In aprevious study [Proment et al., Phys. Rev. A 80, 051603(R) (2009)], a −3/2 power law in the spectrum forthe macroscopic wave function was suggested by using weak wave turbulence theory, but we find that another−7/2 power law appears in both theoretical and numerical calculations.

*Satoshi Yui and Makoto Tsubota,
Counterflow quantum turbulence of He-II in a square channel: numerical analysis with nonuniform flows of the normal fluid,
Physical Review B 91, 184504/1-12 (2015).

[Summary] We perform a numerical analysis of counterflow quantum turbulence of superfluid 4He with nonuniform flows by the vortex filament model. The resulting tangle of quantized vortices makes a characteristic nonlinear oscillation to form inhomogeneous quantum turbulence. We investigated the detail of the state and compare them with some typical experimental results.

Elisa Zemma, Makoto Tsubota, and Javier Luzuriaga,
Possible Visualization of a Superfluid Vortex Loop Attached to an Oscillating Beam,
Journal of Low Temperature Physics 179, 310-319 (2015).

[Summary] Visualization using tracer particles is a relatively new tool available for thestudy of superfluid turbulence and flow, which is applied here to oscillating objectssubmerged in the liquid. We report observations of a structure seen in videos takenfrom outside a cryostat filled with superfluid helium at 2 K, which is possibly a vortexloop attached to an oscillator. The feature, which has the shape of an incomplete arch,is visualized due to the presence of solid H2 tracer particles and is attached to a beamoscillating at 38 Hz in the liquid. It has been recorded in videos taken at 240 frames persecond, fast enough to take ∼6 images per period. This makes it possible to follow thestructure, and to see that it is not rigid. It moves with respect to the oscillator, and itsdisplacement is in phase with the velocity of the moving beam. Analyzing the motion,we come to the conclusion that we may be observing a superfluid vortex attached tothe beam and decorated by the hydrogen particles. An alternative model, consideringa solid hydrogen filament, has also been analyzed, but the observed phase betweenthe movement of the beam and the filamentary structure is better explained by thesuperfluid vortex hypothesis.

Andrew J. Dunleavy, Karoline Wiesner, Ryoichi Yamamoto, and *C. Patrick Royall,
Mutual information reveals multiple structural relaxation mechanisms in a model glassformer,
Nature Comm. 6, 6089/1-8 (2015).

[Summary] Among the key challenges to our understanding of solidification in the glass transition is thatit is accompanied by little apparent change in structure. Recently, geometric motifs have beenidentified in glassy liquids, but a causal link between these motifs and solidification remainselusive. One ‘smoking gun’ for such a link would be identical scaling of structural and dynamiclengthscales on approaching the glass transition, but this is highly controversial. Here weintroduce an information theoretic approach to determine correlations in displacement forparticle relaxation encoded in the initial configuration of a glass-forming liquid. We uncovertwo populations of particles, one inclined to relax quickly, the other slowly. Each population iscorrelated with local density and geometric motifs. Our analysis further reveals a dynamiclengthscale similar to that associated with structural properties, which may resolve thediscrepancy between structural and dynamic lengthscales.


Ahmed Khorshid, Philip Zimny, David Tetreault-La Roche, Geremia Massarelli, *Takahiro Sakaue, and *Walter Reisner,
Dynamic Compression of Single Nanochannel Confined DNA via a Nanodozer Assay,
Physical Review Letters 113, 268104:1-5 (2014).

[Summary] We show that a single DNA molecule confined and extended in a nanochannel can be dynamically compressed by sliding a permeable gasket at a fixed velocity relative to the stationary polymer. The gasket is realized experimentally by optically trapping a nanosphere inside a nanochannel. The trapped bead acts like a ``nanodozer," directly applying compressive forces to the molecule without requirement of chemical attachment. Remarkably, these strongly non-equilibrium measurements can be quantified via a simple nonlinear convective-diffusion formalism and yield insights into the local blob statistics, allowing us to conclude that the compressed nanochannel-confined chain exhibits mean-field behaviour.

*Miho Yanagisawa, Shinpei Nigorikawa, Takahiro Sakaue, Kei Fujiwara, and Masayuki Tokita,
Multiple patterns of polymer gels in microspheres due to the interplay among phase separation, wetting, and gelation,
Proc. Natl. Acad. Sci. USA 111, 15894-15899 (2014).

[Summary] We investigate how microdroplet confinement affects pattern formation of a polymer blend in the liquid-and-gel coexisting phase, wherein interactions between the droplet surface and the polymers regulate wettability of the gelation polymer. The complete and partial wetting of the polymers produces two stable states: hollow microspheres and hemisphere microgels. In addition, gelation during phase separation produces various shapes as trapped states. The relation between capsule thickness and droplet size is changed by the dynamical coupling. Furthermore, multiple patterns with spherical asymmetric shapes are produced by the partial wetting and shape deformation along the phase boundaries between the sol/gel phases. These findings reveal a complex pattern formation arising from the interplay among the interfacial tensions, gel elasticity, and wetting in microspheres.

Shugo Yasuda and Ryoichi Yamamoto,
Synchronized molecular dynamics simulation via macroscopic heat and momentum transfer: an application to polymer lubrication,
Phys. Rev. X 4, 041011/1-10 (2014).

[Summary] A synchronized molecular-dynamics simulation via macroscopic heat and momentum transfer isproposed to model the nonisothermal flow behaviors of complex fluids. In this method, the moleculardynamicssimulations are assigned to small fluid elements to calculate the local stresses and temperaturesand are synchronized at certain time intervals to satisfy the macroscopic heat- and momentum-transportequations. This method is applied to the lubrication of a polymeric liquid composed of short chains of tenbeads between parallel plates.

Adnan Hamid, John J. Molina, and Ryoichi Yamamoto,
Direct Numerical Simulations of Sedimenting Spherical Particles at Finite Reynolds Number,
RSC Advances 4, 53681-53693 (2014).

[Summary] We performed direct numerical simulations to investigate the inertial effects on the static and dynamic properties of a sedimenting suspension over a wide range of volume fractions from 0.01 to 0.4. The microstructure analysis at the high Re revealed that at Re=1 inertial forces have significant effects and these create a deficiency of particles around a given particle, which is more pronounced in the direction of gravity than in the perpendicular direction. Moreover, at Re=10, strong inertial forces generated a significant deficit of particles in both directions, which decreased velocity fluctuations and particle diffusion in both directions.

Shugo Yasuda and Ryoichi Yamamoto,
Multiscale simulation for thermo-hydrodynamic lubrication of a polymeric liquid between parallel plates,
Molecular Simulation 41, 1002 (2014).

Akira Furukawa, Davide Marenduzzo, and Michael E. Cates,
Activity-induced clustering in model dumbbell swimmers: The role of hydrodynamic interactions,
Physical Review E 90, 22303 (2014).

[Summary] Using a fluid-particle dynamics approach, we numerically study the effects of hydrodynamic interactions on the collective dynamics of active suspensions within a simple model for bacterial motility: each microorganism is modeled as a stroke-averaged dumbbell swimmer with prescribed dipolar force pairs. Using both simulations and qualitative arguments, we show that, when the separation between swimmers is comparable to their size, the swimmers' motions are strongly affected by activity-induced hydrodynamic forces. To further understand these effects, we investigate semidilute suspensions of swimmers in the presence of thermal fluctuations. A direct comparison between simulations with and without hydrodynamic interactions shows these to enhance the dynamic clustering at a relatively small volume fraction; with our chosen model the key ingredient for this clustering behavior is hydrodynamic trapping of one swimmer by another, induced by the active forces. Furthermore, the density dependence of the motility (of both the translational and rotational motions) exhibits distinctly different behaviors with and without hydrodynamic interactions; we argue that this is linked to the clustering tendency. Our study illustrates the fact that hydrodynamic interactions not only affect kinetic pathways in active suspensions, but also cause major changes in their steady state properties.

Ken-ichi Mizuochi, Hiizu Nakanishi, and *Takahiro Sakaue,
Dynamical scaling of polymerized membranes,
Europhysics Letters 107, 38003:p1 -p6 (2014).

[Summary] Monte Carlo simulations have been performed to analyze the sub-diffusion dynamics of a tagged monomer in self-avoiding polymerized membranes in the flat phase. By decomposing the mean square displacement into the out-of-plane (//) and the in-plane (⊥) components, weobtain good data collapse with two distinctive diffusion exponents 2 \alpha_{//} = 0.36 ± 0.01 and 2 \alpha_{⊥} =0.21 ± 0.01, and the roughness exponents \zeta_{//} = 0.6 ± 0.05 and \zeta_{⊥ }= 0.25 ± 0.05, respectively foreach component. Their values are consistent with the relation from the rotational symmetry. We derive the generalized Langevin equations to describe the sub-diffusional behaviors of a tagged monomer in the intermediate time regime where the collective effect of internal modes in the membrane dominate the dynamics to produce negative memory kernels with a power law. Wealso briefly discuss how the long-range hydrodynamic interactions alter the exponents.

*Kazuya Fujimoto, Makoto Tsubota,
Spin-superflow turbulence in spin-1 ferromagnetic spinor Bose-Einstien condensates,
Physical Review A 90, 013629/1-7 (2014).

[Summary] Spin-superflow turbulence (SST) in spin-1 ferromagnetic spinor Bose-Einstein condensates is theoretically and numerically studied. We found that the -5/3 and -7/3 power laws appear in spectra of the superflow kinetic and the spin-dependent interaction energy, respectively.

Adnan Hamid, John J. Molina, and Ryoichi Yamamoto,
Simulation studies of microstructure of colloids in sedimentation,
Molecular Simulation 41, 968 (2014).

Chunyu Shih and Ryoichi Yamamoto,
Dynamic electrophoresis of charged colloids in an oscillating electric field,
Physical Review E 89, 062317-1-11 (2014).

[Summary] The dynamics of charged colloids in an electrolyte solution is studied using direct numerical simulations via the smoothed profile method. We calculated the complex electrophoretic mobility μ∗(ω) of the charged colloidsunder an oscillating electric field of frequency ω. We show the existence of three dynamically distinct regimes, determined by the momentum diffusion and ionic diffusion time scales. The present results agree well with approximate theories based on the cell model in dilute suspensions; however, systematic deviations between the simulation results and theoretical predictions are observed as the volume fraction of colloids is increased, similarto the case of constant electric fields.

Ai Nakatsuji, *Makoto Tsubota, and Hideo Yano,
Statistics of vortex loops emitted from quantum turbulence driven by an oscillating sphere,
Physical Review B 89, 174520/1-7 (2014).

[Summary] We perform numerical simulation of quantum turbulence at zero temperature generated by an oscillating sphere. In this simulation, we injected vortices on the sphere to generate turbulence. Although we prepare injected vortex loops of identical length, they are extended by the oscillating sphere to form a tangle through numerous reconnections. The resulting tangle around the sphere is anisotropic and affected by the oscillation. The vortex tangle continues to emit vortex loops, which leave the sphere. The statistics of emitted loops differ significantly from those of the original injected vortices. First, the sizes of the emitted loops are widely distributed, ranging from smaller to much larger than the size of the initial injected loop. Second, the propagation direction of the emitted loops exhibits anisotropy: Small loops move away almost isotropically but large ones do so anisotropically along the oscillation direction of the sphere. Thus, the oscillating object stirs the initial injected vortices to reproduce a group of vortices with different statistics. Such physics is compared with the experiments of vibrating objects.

Xiaowei He, Naoki Fujimura, J. Meagan Lloyd, Kristopher J. Erickson, A. Alec Talin, Qi Zhang, Weilu Gao, Qijia Jiang, Yukio Kawano, Robert H. Hauge, *François Léonard and *Junichiro Kono,
Carbon Nanotube Terahertz Detector,
Nano Letters 14, 3953–3958 (2014).

[Summary] Terahertz (THz) technologies are promising for diverse areas such as medicine, bioengineering, astronomy, environmental monitoring, and communications. However, despite decades of worldwide efforts, the THz region of the electromagnetic spectrum still continues to be elusive for solid state technology. Here, we report on the development of a powerless, compact, broadband, flexible, large-area, and polarization-sensitive carbon nanotube THz detector that works at room temperature. The detector is sensitive throughout the entire range of the THz technology gap, with responsivities as high as ∼2.5 V/W and polarization ratios as high as ∼5:1. Complete thermoelectric and opto-thermal characterization together unambiguously reveal the photothermoelectric origin of the THz photosignal, triggered by plasmonic absorption and collective antenna effects, and suggest that judicious design of thermal management and quantum engineering of Seebeck coefficients will lead to further enhancement of device performance.