arXiv subjects

Soichi Tatsumi

Publications and source records attributed to Soichi Tatsumi.

5 recordsLinked to original sources

arXiv2026

Findings of sub-$T_\mathrm{g}$ endotherm in vapor-deposited ultrastable phenolphthalein glass

We have performed heat capacity measurements for physically vapor-deposited phenolphthalein glass using conventional and in-house high-sensitivity differential scanning calorimetry. As a result, we found that phenolphthalein forms an ultrastable glass when deposited at 313 K, about 0.86 times the ordinary glass transition temperature of 361 K. In addition, we observed a large endotherm (sub-$T_\mathrm{g}$ endotherm) between deposition and the ordinary glass transition temperature. From an enthalpy perspective, the integrated enthalpy of the sub-$T_\mathrm{g}$ endotherm increased with decreasing film thickness following an inverse-power dependence, whose exponent increased as the deposition rate decreased. In particular, the integrated enthalpy of the sub-$T_\mathrm{g}$ endotherm was comparable in magnitude to the enthalpy of fusion of crystalline phenolphthalein. To investigate the structural origin of the sub-$T_\mathrm{g}$ endotherm, we performed wide-angle X-ray diffraction and in situ atomic force microscopy. As a result, we found that while the stable structure is linked to an anisotropic amorphous structure consistent with past research on ultrastable glasses, the appearance of the sub-$T_\mathrm{g}$ endotherm is associated with surface morphological relaxation, as suggested by atomic force microscopy.

cond-mat.soft↗

arXiv2021

Towards the ideal glass transition by pinning in a dimer-polymer mixture

We use a mixture of a polymer and its dimer to control dynamics in a manner inspired by \emph{pinning} a fraction of the system. In our system of $α$-methyl styrene, where the polymer has a glass transition at higher temperature than the dimer, at intermediate temperatures, the polymer acts to "pin" the dimer. Within this temperature range, we use differential scanning calorimetry to infer a point-to-set length which we find to be profoundly influenced by the degree of pinning. We determine the dynamics of the system with dielectric spectroscopy and find that while the dynamics are very substantially slowed by the "pinning", the fragility exhibits only a small change relative to the precision of our measurements. This may indicate that in the approach we have used, fragility has a relatively weak dependence on quantities such as the point--to--set length. % than one might expect, However, an alternative explanation is that the dimer may act to \emph{plasticize} the polymer and thus open routes to relaxation that may be inaccessible to fully pinned systems.

cond-mat.soft↗

arXiv2020

Icosahedral order in liquid and glassy phases of cyclohexane

We performed all-atom molecular dynamics simulations for bulk cyclohexane and analysed the short- and medium-range structures in supercooled and glassy states by using the Voronoi tessellation technique. From the analyses of both the potential energy of the system and the radial distribution function of molecules, cyclohexane was found to be vitrified as the temperature decreased. Furthermore, the icosahedral-like structures are dominant at all temperatures and grow in a supercooled liquid, whereas the face-centred cubic structures do not grow when the temperature decreases. It was also ascertained that the icosahedral-like structure is more dominant than the full-icosahedral one. The network of the distorted icosahedron spreads throughout the system at low temperatures. Our simulation demonstrates the stability of the icosahedral structure even in a non-spherical molecule such as cyclohexane.

cond-mat.dis-nn↗

arXiv2018

The race to the bottom: approaching the ideal glass?

Key to resolving the scientific challenge of the glass transition is to understand the origin of the massive increase in viscosity of liquids cooled below their melting temperature (avoiding crystallisation). A number of competing and often mutually exclusive theoretical approaches have been advanced to describe this phenomenon. Some posit a bona fide thermodynamic phase to an "ideal glass", an amorphous state with exceptionally low entropy. Other approaches are built around the concept of the glass transition as a primarily dynamic phenomenon. These fundamentally different interpretations give equally good descriptions of the data available, so it is hard to determine which -- if any -- is correct. Recently however this situation has begun to change. A consensus has emerged that one powerful means to resolve this longstanding question is to approach the putative thermodynamic transition sufficiently closely, and a number of techniques have emerged to meet this challenge. Here we review the results of some of these new techniques and discuss the implications for the existence -- or otherwise -- of the thermodynamic transition to an ideal glass.

cond-mat.soft↗

arXiv2005

Observation of stable phase jump lines in convection of a twisted nematic

We report observations of stable, localized, line-like structures in the spatially periodic pattern formed by nematic electroconvection, along which the phase of the pattern jumps by $π$. With increasing electric voltage, these lines form a grid-like structure that goes over into a structure indistinguishable from the well known grid pattern. We present theoretical arguments that suggest that the twisted cell geometry we are using is indirectly stabilizing the phase jump lines, and that the PJL lattice is caused by an interaction of phase jump lines and a zig-zag instability of the surrounding pattern.

nlin.PS↗