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A01 Proposed Research Projects (2014-2015)

Paper | Original Paper

2017

Isala Dueramae, Shingo Fukuzawa, Naoki Shinyashiki, Shin Yagihara, *Rio Kita,
Dynamics of amyloid-like aggregation and gel formation of hen egg-white lysozyme in highly concentrated ethanol solution,
Journal of Biorheology 31, 21-28 (2017).

[Summary] We investigated the mechanisms of amyloidlike aggregation and gel formation in hen egg-white lysozyme (HEWL) in a mixed solvent comprising 90% v/v ethanol in water using dynamic light scattering (DLS) and circular dichroism CD. The mechanism of HEWL in ethanol aqueous solution is interpreted into three stages as: (I) denaturation of HEWL; (II) elongation of amyloid fibrils composed of β-sheet-rich HEWL by lateral aggregation; and (III) gel formation due to the creation of junctions in amyloid fibrils. The transformation of sol to gel can be confirmed by: (1) the oscillation behavior and the rapid increase in the intensity of scattered light; (2) the power-law behavior of the correlation function of scattered light g(2)(t); (3) the appearance of a long-time tail in the distribution function of the decay time G(τ); and (4) the beginning of the reduction in initial amplitude in g(2)(t). The gelation rate was strongly dependent on the protein concentration. The estimated rod length of the amyloid fibrils increased significantly over time. Scanning electron microscopy (SEM) performed on the formation of fibrils in the HEWL gels revealed that the structure was highly heterogeneous, with areas characterized by dense fiber networks interspersed with loose network areas.

Doreen Niether, Tsubasa Kawaguchi, Jana Hovancova, Kazuya Eguchi, Jan K. G. Dhont, *Rio Kita, and *Simone Wiegand,
Role of hydrogen bonding of cyclodextrin-drug complexes probed by thermodiffusion,
Langmuir 33, 8483-8492 (2017).

[Summary] Temperature gradient-induced migration of biomolecules, known as thermophoresis or thermodiffusion, changes upon ligand binding. In recent years, this effect has been used to determine protein−ligand binding constants. The mechanism through which thermodiffusive properties change when complexes are formed, however, is not understood. An important contribution to thermodiffusive properties originates from the thermal response of hydrogen bonds. Because there is a considerable difference between the degree of solvation of the protein−ligand complex and its isolated components, ligand-binding is accompanied by a significant change in hydration. The aim of the present work is therefore to investigate the role played by hydrogen bonding on the change in thermodiffusive behavior upon ligand-binding. As a model system, we use cyclodextrins (CDs) and acetylsalicylic acid (ASA), where quite a significant change in hydration is expected and where no conformational changes occur when a CD/ASA complex is formed in aqueous solution. Thermophoresis was investigated in the temperature range of 10−50 °C by infrared thermal diffusion forced Rayleigh scattering. Nuclear magnetic resonance measurements were performed at 25 °C to obtain information about the structure of the complexes. All CD/ASA complexes show a stronger affinity toward regions of lower temperature compared to the free CDs. We found that the temperature sensitivity of thermophoresis correlates with the 1-octanol/water partition coefficient. This observation not only establishes the relation between thermodiffusion and degree of hydrogen bonding but also opens the possibility to relate thermodiffusive properties of complexes to their partition coefficient, which cannot be determined otherwise. This concept is especially interesting for protein−ligand complexes where the protein undergoes a conformational change, different from the CD/ASA complexes, giving rise to additional changes in their hydrophilicity.

*Masayuki Hashisaka, Naoaki Hiyama, Takafumi Akiho, Koji Muraki, *Toshimasa Fujisawa,
Waveform measurement of charge- and spin-density wavepackets in a chiral Tomonaga–Luttinger liquid,
Nature Physics, (2017).

[Summary] In contrast to a free-electron system, a Tomonaga–Luttinger (TL) liquid in a one-dimensional (1D) electron system hosts charge and spin excitations as independent entities. When an electron is injected into a TL liquid, it transforms into charge- and spin-density wavepackets that propagate at different group velocities and move away from each other. This process, known as spin–charge separation, is the hallmark of TL physics. While spin–charge separation has been probed in momentum- or frequency-domain measurements in various 1D systems, waveforms of separated excitations, which are a direct manifestation of the TL behaviour, have been long awaited to be measured. Here, we present a waveform measurement for the pseudospin–charge separation process in a chiral TL liquid comprising quantum Hall edge channels. The charge- and pseudospin-density waveforms are captured by utilizing a spin-resolved sampling scope that records the spin-up or -down component of the excitations. This experimental technique provides full information for time evolution of the 1D electron system, including not only propagation of TL eigenmodes but also their decay in a practical device14.

Isala Dueramae, Masaru Yoneyama, Naoki Shinyashiki, Shin Yagihara, and *Rio Kita,
Self-assembly of acetylated dextran with various acetylation degrees in aqueous solutions: Studied by light scattering,
Carbohydrate Polymers 159, 171-177 (2017).

[Summary] Self-assembly of acetylated dextran (Ac-DEXs) was investigated with a modified dextran with acetic anhydride in the presence of pyridine. The effect of acetylation degree on solution properties has been investigated by static and dynamic light scattering (DLS). Molecular weight (Mw) and the radius of gyration (Rg) of dextran significantly increased with acetylation degree due to the aggregates formation. However, those aggregates noticeably reduced with further increase of acetylation degree. It suggested that the aggregates have shrinkage. The aggregate formation is clearly confirmed by DLS analysis in the presence of the bimodal relaxation distribution for Ac-DEXs. The hydrodynamic radius (Rh) of fast and slow mode is distinctly corresponded with single dextran macromolecules and aggregates, respectively. The Rh of aggregates varied slightly with increasing acetylation degree. The aggregates of Ac-DEXs represent hard spherical nanoparticles whereas the random coil structure is found in dextran. Formation of gel nanoparticles was monitored at the highest acetylated substitution.

2016

Kazuya Eguchi, Doreen Niether, Simone Wiegand, and *Rio Kita,
Thermophoresis of cyclic oligosaccharides in polar solvents,
European Physical Journal E 39, 86/1-8 (2016).

[Summary] Cyclodextrins are cyclic oligosaccharides which are interesting as drug delivery systems, because they can be used as containers for pharmaceutical substances. We studied the Ludwig-Soret effect of α-, β-, γ- and methyl-β-cyclodextrin in water and formamide by infrared thermal diffusion forced Rayleigh scattering (IR-TDFRS). In water the Soret coefficient, ST, of α-, β- and γ-cyclodextrin increases with increasing temperature and shows a sign change from negative to positive around T = 35 ◦C, while ST of methyl-β-cyclodextrin is positive in the entire investigated temperature. In formamide ST-values of all cyclodextrins coincide and show a slight decrease with temperature. We discuss the obtained results and relate the ST-values to the different hydrogen bonding capabilities of the cyclodextrins and the used solvents. It turns out that the change of ST with temperature correlates with the partition coefficient, log P, which indicates that more hydrophilic substances show a more pronounced temperature sensitivity of ST. Additionally we obtained a surprising result measuring the refractive index contrast factor with temperature, (∂n/∂T)c,p of cyclodextrins in formamide, which might be explained by a complex formation between cyclodextrins and formamide.

Kaito Sasaki, Rio Kita, *Naoki Shinyashiki, and Shin Yagihara,
Dielectric relaxation time of ice-Ih with different preparation,
Journal of Physical Chemistry B 120, 3950-3953 (2016).

[Summary] Dielectric relaxation process of ice-Ih has been investigated by many researchers. Pioneering studies focused on the temperature dependence of the dielectric relaxation time, τice, were reported by Auty in 1952 [Auty, R. P.; Cole, R. H. J. Chem. Phys. 1952, 120, 1309] and Johari in 1981 [Johari, G. P.; Whalley, E. J. Chem. Phys. 1981, 75, 1333]. However, the temperature dependences of τice found in these studies are not in agreement. While Auty et al. reported a single Arrhenius temperature dependence of τice for the entire 207−273 K temperature range, Johari et al. reported changes in the temperature dependence of ice at 230 and 140 K. In this study, the temperature dependence of τice is investigated by broadband dielectric spectroscopy for ice prepared by three different procedures. For all investigated ices, a dielectric relaxation process is observed, and τice decreases with increasing temperature. Temperature dependence of τice with rapid crystallization shows the same properties at temperatures down to 140 K as that reported by Johari et al. On the other hand, ice obtained by slow crystallization exhibits the same temperature dependence of τice as those reported by Auty et al. We suggest that the difference between the temperature dependences of τice found by Auty et al. and Johari et al. can be controlled by preparation conditions. That is, the growth rate of the ice crystal can affect τice because a slow growth speed of the ice crystal induces a smaller impurity content of ice, giving rise to an Arrhenius temperature dependence of τice.

Kazuhisa Washio, Ryo Nakazawa, Masayuki Hashisaka, Koji Muraki, Y. Tokura, and Toshimasa Fujisawa,
Long-lived binary tunneling spectrum in the quantum Hall Tomonaga-Luttinger liquid,
Physical Review B 93, 075304 (2016).

[Summary] The existence of long-lived non-equilibrium states without showing thermalization, which has previously been demonstrated in time evolution of ultracold atoms, suggests the possibility of their spatial analogue in transport behavior of interacting electrons in solid-state systems. Here we report long-lived non-equilibrium states in one-dimensional edge channels in the integer quantum Hall regime. An indirect heating scheme in a counterpropagating configuration is employed to generate a non-trivial binary spectrum consisting of high- and low-temperature components. This unusual spectrum is sustained even after travelling 5 - 10 μm, much longer than the length for electronic relaxation (about 0.1 μm), without showing significant thermalization. This observation is consistent with the integrable model of Tomonaga-Luttinger liquid. The long-lived spectrum implies that the system is well described by non-interacting plasmons, which are attractive for carrying information for a long distance.

Yasutaka Ohnishi, Rio Kita, Kazuyoshi Tsuchiya and *Satoru Iwamori,
Optical characteristics of poly(tetrafluoroethylene) thin film prepared by a vacuum evaporation,
Japanese Journal of Applied Physics 55, 02BB04/1-5 (2016).

[Summary] Poly(tetrafluoroethylene) (PTFE) thin films were deposited onto a glass slide substrate by a heat-resistance type vacuum evaporation apparatus due to changing the evaporation conditions. Transparency of the PTFE thin films prepared by the vacuum evaporation depended on the deposition conditions, i.e., temperatures of the basket, and distance between the evaporation source and substrate. To elucidate relationship between the molecular structure and transparency of the PTFE thin film prepared by the vacuum evaporation, chemical structures, crystallinity and thermophysical property were investigated. The chemical bonding state of the PTFE thin film prepared by the vacuum evaporation was almost the same as that of the pristine PTFE, however, the crystalinity was different. Although the pristine PTFE was crystal structure, the transparent evaporated thin film was estimated to be microcrystal structure. In addition, endothermic peaks in a differential scanning calorimeter (DSC) spectrum of the PTFE thin film were different from that of the pristine PTFE. These endothermic peaks of the PTFE thin film prepared by the vacuum evaporation shifted lower temperature compared to the pristine PTFE, which suggests that molecular weight of the PTFE thin film prepared by the vacuum evaporation decreased compared with that of the pristine PTFE.

2015

D. Imanaka, S. Sharmin, Masayuki Hashisaka, Koji Muraki, and *Toshimasa Fujisawa,
Exchange-induced spin blockade in a two-electron double quantum dot,
Physical Review Letters 115, 176802 (2015).

[Summary] We have experimentally identified the exchange-induced spin blockade in a GaAs double quantum dot. The transport is suppressed only when the eigenstates are well-defined singlet and triplet states, and thus sensitive to dynamic nuclear-spin polarization that causes singlet-triplet mixing. This gives rise to unusual current spectra, such as a sharp current dip and an asymmetric current profile near the triplet resonance of a double quantum dot. Numerical simulations suggest that the current dip is a signature of identical nuclear-spin polarization in the two dots, which is attractive for coherent spin manipulations in a material with nuclear spins.

J. C. H. Chen, Y. Sato, R. Kosaka, Masayuki Hashisaka, Koji Muraki, and Toshimasa Fujisawa,
Enhanced electron-phonon coupling for a semiconductor charge qubit in a surface phonon cavity,
Scientific Reports 5, 15176 (2015).

[Summary] Electron-phonon coupling is a major decoherence mechanism, which often causes scattering and energy dissipation in semiconductor electronic systems. However, this electron-phonon coupling may be used in a positive way for reaching the strong or ultra-strong coupling regime in an acoustic version of the cavity quantum electrodynamic system. Here we propose and demonstrate a phonon cavity for surface acoustic waves, which is made of periodic metal fingers that constitute Bragg reflectors on a GaAs/AlGaAs heterostructure. Phonon band gap and cavity phonon modes are identified by frequency, time and spatially resolved measurements of the piezoelectric potential. Tunneling spectroscopy on a double quantum dot indicates the enhancement of phonon assisted transitions in a charge qubit. This encourages studying of acoustic cavity quantum electrodynamics with surface phonons.

Naoaki Hiyama, *Masayuki Hashisaka, and Toshimasa Fujisawa,
An edge magnetoplasmon Mach-Zehnder interferometer,
Applied Physics Letters 107, 143101 (2015).

[Summary] We report an edge-magnetoplasmon (EMP) Mach-Zehnder (MZ) interferometer in a quantum Hall system. The MZ interferometer, which is based on the interference of two EMP beams traveling in chiral one-dimensional edge channels, is constructed by tailoring edge channels with functional devices such as splitters and delay lines. We measured 1 GHz EMP beams transmitted through the interferometer while tuning the phase evolution along two interference paths using tunable delay lines. Clear interference patterns as a function of the phase difference ensure the MZ interference. Moreover, the MZ interferometry is applied to evaluate the EMP transport through an attenuator interposed in one of the paths. This technique will be useful for investigating the functionalities of devices in plasmonics.

*Takahiro Hatano,
Friction laws from dimensional-analysis point of view.,
Geophysical Journal International 202, 2159-2162 (2015).

[Summary] Friction laws, which are a key to the understanding of the diversity of earthquakes, are considered theoretically. Using dimensional analysis, the logarithmic dependence of the friction coefficient on the slip velocity and the state variable is derived without any knowledge of the underlying physical processes on the frictional surface. This is based on a simple assumption that the friction coefficient is expressed as the difference from a reference state. Therefore, the functional form of the rate and state dependent friction law itself does not necessarily mean that thermal activation processes dominate friction. It is also shown that if there are two (or more) state variables having the same dimension, we need not assume the logarithmic dependence on the state variables.

Kousaku Maeda, Naoki Shinyashiki, Shin Yagihara, Simone Wiegand, and *Rio Kita,
Ludwig-Soret effect of aqueous solutions of ethylene glycol oligomers, crown ethers, and glycerol: Temperature, molecular weight, and hydrogen bond effect,
Journal of Chemical Physics 143, 124504/1-7 (2015).

[Summary] The thermal diffusion, also called the Ludwig-Soret effect, of aqueous solutions of ethylene glycol oligomers, crown ethers, and glycerol is investigated as a function of temperature by thermal diffusion forced Rayleigh scattering. The Soret coefficient, S_T, and the thermal diffusion coefficient, D_T, show a linear temperature dependence for all studied compounds in the investigated temperature range. The magnitudes and the slopes of S_T and D_T vary with the chemical structure of the solute molecules. All studied molecules contain ether and/or hydroxyl groups, which can act as acceptor or donor to form hydrogen bonds, respectively. By introducing the number of donor and acceptor sites of each solute molecule, we can express their hydrogen bond capability. S_T and D_T can be described by an empirical equation depending on the difference of donor minus acceptor sites and the molecular weight of the solute molecule.

S. K. Kundu, S. Choe, K. Sasaki, Rio Kita, N. Shinyashiki, and *S. Yagihara,
Relaxation dynamics of liposomes in an aqueous solution,
Physical Chemistry Chemical Physics 17, 18449-18455 (2015).

[Summary] The gel–liquid crystal phase transition has been studied by the temperature and frequency dependent dielectric relaxation behavior of liposomes in an aqueous solution (40 g L^{-1} DPPC–water mixture). Four relaxation processes were observed in the frequency range from 40 Hz to 30 GHz which were ascribed to different molecular mechanisms, related to the structural units of the system. The gel–liquid crystal phase transition was also described very accurately from the temperature-dependent dielectric relaxation strength, relaxation time and symmetric shape parameter of the relaxation functions obtained from the fitting procedure. Relaxation process 3, obtained from the dielectric fitting procedure, was confirmed by dielectric modulus analysis. A comparison of the lipid membrane with non-biological systems like liquid crystals was performed. It was determined that the lipid membrane has a ferroelectric liquid crystal like behavior. Process 3 is comparable to the soft mode relaxation process observed in ferroelectric liquid crystals which was detected close to the smectic-C*–smectic-A phase transition. Differential scanning calorimetry was also used to confirm the gel–liquid crystal phase transition of this mixture.

Takahiro Hatano, Clément Narteau and Peter Shebalin,
Common dependence on stress for the statistics of granular avalanches and earthquakes,
Scientific Reports 5, 12280 (2015).

[Summary] Both earthquake size-distributions and aftershock decay rates obey power laws. Recent studies have demonstrated the sensibility of their parameters to faulting properties such as focal mechanism, rupture speed or fault complexity. The faulting style dependence may be related to the magnitude of the differential stress, but no model so far has been able to reproduce this behaviour. Here we investigate the statistical properties of avalanches in a dissipative, bimodal particulate system under slow shear. We find that the event size-distribution obeys a power law only in the proximity of a critical volume fraction, whereas power-law aftershock decay rates are observed at all volume fractions accessible in the model. Then, we show that both the exponent of the event size-distribution and the time delay before the onset of the power-law aftershock decay rate are decreasing functions of the shear stress. These results are consistent with recent seismological observations of earthquake size-distribution and aftershock statistics.

Péter Ván, Noa Mitsui, Takahiro Hatano,
Non-equilibrium thermodynamical framework for rate- and state-dependent friction,
Periodica Polytechnica Civil Engineering 59, 583-589 (2015).

[Summary] Rate- and state-dependent friction laws for velocity-step and healing are analysed from a thermodynamic point of view. Assuming a logarithmic deviation from steady-state, a unification of the classical Dieterich and Ruina models of rock friction is proposed.

Kazuhiro Kishi, Masashi Kawaguchi, Hitoshi Miura, *Masahide Sato and Makio Uwaha,
Relation between the Step Pattern and the Velocity of the Moving Linear Adatom Source,
e-Journal of Surface Science and Nanotechnology 13, 269-274 (2015).

[Summary] During deposition of Ga atoms on a Si(111) vicinal face, a step on the vicinal face shows a comb-like pattern.Keeping the formation of comb-like pattern in mind, we carry out phase field simulations, in which a linear adatom source advances in front of a step. The comb-like pattern consisting of straight finger-like intrusions is formed when the source velocity is smaller than a critical value determined by the step anisotropy. Initially, the straight step is unstable and step wandering is induced by the asymmetry of the surface diffusion field. The amplitude of step fluctuations increases with time and an array of short intrusions is formed. Shorter intrusions cannot catch sufficient adatoms, so that coarsening of the pattern occurs. When the adatom source moves slowly, the intrusions grow long and the step shows a regular comb-like pattern. By the coarsening, the distance between intrusions is several times larger than the initial value. The pattern is metastable for a rapid change in the source velocity. When the adatom source moves fast, the intrusions cannot keep up with the adatom source and an irregular pattern is formed. When the strength of crystal anisotropy is weak, splitting of the tips of intrusions frequently occurs and the step shows an irregular seaweed-like pattern. With a strong crystal anisotropy, the step shows a dendrite pattern.

Ken H. Nagai, Yutaka Sumino, Raul Montagne, Igor S. Aranson, and *Hugues Chaté,
Collective motion of self-propelled particles with memory,
Physical Review Letters 114, 168001/1-6 (2015).

[Summary] We show that memory, in the form of underdamped angular dynamics, is a crucial ingredient for the collective properties of self-propelled particles. Using Vicsek-style models with an Ornstein-Uhlenbeck process acting on angular velocity, we uncover a rich variety of collective phases not observed in usual overdamped systems, including vortex lattices and active foams. In a model with strictly nematic interactions the smectic arrangement of Vicsek waves giving rise to global polar order is observed. We also provide a calculation of the effective interaction between vortices in the case where a telegraphic noise process is at play, explaining thus the emergence and structure of the vortex lattices observed here and in motility assay experiments.

Youhei Kanatsu and *Masahide Sato,
Crystallization of Brownian particles in a pyramidal pit by a uniform external force,
Journal of the Physics Society of Japan 84, 044601/1-6 (2015).

[Summary] We carry out Brownian dynamics simulations and study the crystallization of particles in an inverse pyramidal-shaped container induced by an external force. Owing to the side walls of the container, the face-centered cubic (fcc) structure is mainly formed. In the bulk, both disordered solidlike particles and hexagonal close-packed (hcp) structured particles are hardly formed. These two types of particle appear near the central axis of the container. Their numbers increase with increasing strength of the external force.

*Masayuki Hashisaka, Tomoaki Ota, Koji Muraki, and Toshimasa Fujisawa,
Shot-noise evidence of fractional quasiparticle creation in a local fractional quantum Hall state,
Physical Review Letters 114, 056802/1-5 (2015).

[Summary] We experimentally identify fractional quasiparticle creation in a tunneling process through a local fractional quantum Hall (FQH) state. The local FQH state is prepared in a low-density region near a quantum point contact in an integer quantum Hall (IQH) system. Shot-noise measurements reveal a clear transition from elementary-charge tunneling at low bias to fractional-charge tunneling at high bias. The fractional shot noise is proportional to T1(1−T1) over a wide range of T1, where T1 is the transmission probability of the IQH edge channel. This binomial distribution indicates that fractional quasiparticles emerge from the IQH state to be transmitted through the local FQH state. The study of this tunneling process enables us to elucidate the dynamics of Laughlin quasiparticles in FQH systems.

*Hironori Hoshino and Shin Nakamura,
Effective temperature of nonequilibrium dense matter in holography,
Physical Review D 91, 026009 /1-10 (2015).

[Summary] We study properties of effective temperature of nonequilibrium steady states by using the anti–de Sitter spacetime/conformal field theory (AdS/CFT ) correspondence. We consider nonequilibrium systems with a constant flow of current along an electric field, in which the current is carried by both the doped charges and those pair created by the electric field. We find that the effective temperature agrees with that of the Langevin systems if we take the limit where the pair creation is negligible. The effect of pair creation raises the effective temperature whereas the current by the doped charges contributes to lower the effective temperature in a wide range of the holographic models.

Masashi Kawaguchi, Hitoshi Miura, Kazuhiro Kishi, Masahide Sato, and *Makio Uwaha,
Period of a comblike pattern controlled by atom supply and noise,
Physical Review E 91, 012409/1-9 (2015).

[Summary] Pattern formation of a step on a growing crystal surface induced by a straight line source of atoms, which is escaping from the step at a velocity Vp, is studied with the use of a phase field model. From a straight step, fluctuations of the most unstable wavelength λmax grow. Competition of intrusions leads to coarsening of the pattern, and survived intrusions grow exponentially. With sufficient strength of the crystal anisotropy, a regular comblike pattern appears. This peculiar step pattern is similar to that observed on a Ga-deposited Si(111) surface. The final period of the intrusions, Λ, is determined when the exponential growth ends. The period depends on the strength Fu of a current noise in diffusion as Λ∼λmax|lnFu|: such a logarithmic dependence is confirmed for the first time. A nonmonotonic Vp dependence of Λ indicates that the comblike pattern with a small Vp is related to an unstable growth mode of the free needle growth in a channel. The pattern is stabilized by the guiding linear source.

2014

*Norio Kumada, Preden Roulleau, Benoit Roche, Masayuki Hashisaka, Hiroki Hibino, Ivana Petković, and Christian Glattli,
Resonant edge magnetoplasmons and their decay in graphene,
Physical Review Letters 113, 266601 (2014).

[Summary] We investigate resonant edge magnetoplasmons (EMPs) and their decay in graphene by high-frequency electronic measurements. From EMP resonances in disk shaped graphene, we show that the dispersion relation of EMPs is nonlinear due to interactions, giving rise to the intrinsic decay of EMP wave packets. We also identify extrinsic dissipation mechanisms due to interaction with localized states in bulk graphene from the decay time of EMP wave packets. We indicate that, owing to the linear band structure and the sharp edge potential, EMP dissipation in graphene can be lower than that in GaAs systems.

Kousaku Maeda, Naoki Shinyashiki, Shin Yagihara, Simone Wiegand, *Rio Kita,
How does thermodiffusion of aqueous solutions depend on concentration and hydrophobicity?,
European Physical Journal E 37, 94(6pages) (2014).

[Summary] The thermal diffusion of aqueous solutions of mono-, di-ethylene glycols, poly(ethylene glycol), methanol, and glycerol is investigated systematically as a function of concentration using the Thermal Diffusion Forced Rayleigh Scattering (TDFRS). For all investigated binary mixtures, the Soret coefficient, ST , decays with increasing concentration of the non-aqueous component showing two regions. For aqueous solution of ethylene glycol, at a very low solute content the decay is steep, while it becomes less steep for higher solute concentration. All mixtures show a sign change of ST with concentration. The sign change concentration is discussed with respect to chemical structures of solute molecules and the partition coefficient, log p . It turns out that the number of hydroxyl groups plays an important role. For the investigated aqueous mixtures, we find empirical linear relations between the sign change concentration and the ratio of the number of hydroxyl groups to the number of carbon atoms as well as the partition coefficient, log p.

Mamoru Fujine, *Masahide Sato, Tetsuya Toyooka, Hiroyasu Katsuno, Yoshihisa Suzuki, and Tsutomu Sawada,
Crystallization of Brownian particles in thin systems constrained by walls,
Physical Review E 90, 032404/1-7 (2014).

[Summary] Keeping formation of a colloidal crystal by a centrifugal force in mind, we carry out Brownian dynamics simulations in thin systems and study ordering of particles induced by an external force. During solidification, the two-dimensional ordering along walls initially occurs. Then, the ordered particles on the walls act as substrates, and crystallization proceeds into bulk. When the external force is weak, the close-packed face of the crystal structure is parallel to the bottom wall. The direction of the close-packed face depends on the strength of the external force: The close-packed face becomes parallel to the side walls with a strong external force.

Masakazu Yamagishi, Narii Watase, Masayuki Hashisaka, Koji Muraki, and *Toshimasa Fujisawa,
Spin-dependent tunneling rates for electrostatically defined GaAs quantum dot,
Physical Review B 90, 035306 (2014).

[Summary] The tunneling rates for spin-up and -down electrons are investigated for a GaAs quantum dot in an in-plane magnetic field by using a real-time single-electron counting scheme with a nearby charge detector. An extremely small spin-polarized current on the order of attoamperes is analyzed with the spin and energy dependences of the tunneling rates. Fully spin-polarized current is obtained when only a spin-up Zeeman sublevel is located in the transport window. When both Zeeman sublevels are allowed to contribute to the transport, we find that the tunneling rate for spin-up electrons is considerably higher than that for spin-down electrons. This partially spin-polarized current can be explained by the exchange-enhanced spin splitting in low-density regions near the tunneling barriers.

*Norio Kumada, Romain Dubourget, Ken’ichi Sasaki, Shinichi Tanabe, Hiroki Hibino, Hiroshi Kamata, Masayuki Hashisaka, Koji Muraki, Toshimasa Fujisawa,
Plasmon transport and its guiding in graphene,
New Journal of Physics 16, 063055 (2014).

[Summary] Transport of plasmons in graphene has been investigated by time-resolved electrical measurements. We demonstrate that the velocity v (or the refractive index ∝ −v^1) and the characteristic impedance Z of the plasmon mode can be tuned through the carrier density. By exploiting the Z tunability, we present a gate-defined plasmonic waveguide. An important advantage of the gate-defined waveguide is dynamical switching of guiding characteristics with the gate voltages. One can tailor the patterns of gate electrodes to define two output waveguides branching off from a source waveguide, and the output waveguide can be switched by changing the gate voltages. Indeed, we show the routing in a Y-shaped channel: the path for the plasmon transmission can be selected by tuning Z of each path. These results can be well reproduced by simulation, encouraging the design of graphene-based plasmonic devices.

*Masayuki Hashisaka, Tomoaki Ota, Masakazu Yamagishi, Toshimasa Fujisawa and Koji Muraki,
Cross-correlation measurement of quantum shot noise using homemade transimpedance amplifiers,
Review of Scientific Instruments 85, 054704 (2014).

[Summary] We report a cross-correlation measurement system, based on a new approach, which can be used to measure shot noise in a mesoscopic conductor at milliKelvin temperatures. In contrast to other measurement systems in which high-speed low-noise voltage amplifiers are commonly used, our system employs homemade transimpedance amplifiers (TAs). The low input impedance of the TAs signifi-cantly reduces the crosstalk caused by unavoidable parasitic capacitance between wires. The TAs are designed to have a flat gain over a frequency band from 2 kHz to 1 MHz. Low-noise performance is attained by installing the TAs at a 4 K stage of a dilution refrigerator. Our system thus fulfills the technical requirements for cross-correlation measurements: low noise floor, high frequency band, and negligible crosstalk between two signal lines. Using our system, shot noise generated at a quantum point contact embedded in a quantum Hall system is measured. The good agreement between the obtained shot-noise data and theoretical predictions demonstrates the accuracy of the measurements.