A04 Proposed Research Projects (2016-2017)

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.

*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.