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Journal Articles

Phase-change materials; Vibrational softening upon crystallization and its impact on thermal properties

Matsunaga, Toshiyuki*; Yamada, Noboru*; Kojima, Rie*; Shamoto, Shinichi; Sato, Masugu*; Tanida, Hajime*; Uruga, Tomoya*; Kohara, Shinji*; Takata, Masaki*; Zalden, P.*; et al.

Advanced Functional Materials, 21(12), p.2232 - 2239, 2011/06

 Times Cited Count:103 Percentile:95.79(Chemistry, Multidisciplinary)

Thermal properties of the amorphous and crystalline state of phase-change materials show remarkable differences such as higher thermal displacements and a more pronounced anharmonic behavior in the crystalline phase. These findings are related to the change of bonding upon crystallization.

Journal Articles

Structural basis for the fast phase change of Ge$$_{2}$$Sb$$_{2}$$Te$$_{5}$$; Ring statistics analogy between the crystal and amorphous states

Kohara, Shinji*; Kato, Kenichi*; Kimura, Shigeru*; Tanaka, Hitoshi*; Usuki, Takeshi*; Suzuya, Kentaro; Tanaka, Hiroshi*; Moritomo, Yutaka*; Matsunaga, Toshiyuki*; Yamada, Noboru*; et al.

Applied Physics Letters, 89(20), p.201910_1 - 201910_3, 2006/11

 Times Cited Count:183 Percentile:97.85(Physics, Applied)

The three-dimensional atomic configuration of amorphous Ge$$_{2}$$Sb$$_{2}$$Te$$_{5}$$ were derived by reverse Monte Carlo simulation with synchrotron-radiation X-ray diffraction data. The authors found that amorphous Ge$$_{2}$$Sb$$_{2}$$Te$$_{5}$$ can be regarded as "even-numbered ring structure", because the ring statistics is dominated by four- and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge-Ge homopolar bonds in amorphous GeTe constructs both odd- and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge$$_{2}$$Sb$$_{2}$$Te$$_{5}$$ is the key for the fast crystallization speed of the material.

Journal Articles

Structural study on optical recording materials Ge$$_{2}$$Sb$$_{2+x}$$Te$$_{5}$$ and GeBi$$_{2}$$Te$$_{4}$$

Shamoto, Shinichi; Yamada, Noboru*; Matsunaga, Toshiyuki*; Proffen, T.*

Physica B; Condensed Matter, 385-386(1), p.574 - 577, 2006/11

 Times Cited Count:4 Percentile:23.8(Physics, Condensed Matter)

Crystal structures of optical recording materials Ge$$_{2}$$Sb$$_{2+x}$$Te$$_{5}$$ and GeBi$$_{2}$$Te$$_{4}$$ have been studied in terms of the fast phase transition mechanism by both of real and reciprocal space Rietveld analyses of pulsed neutron powder diffraction data. These crystalline phases had large local lattice distortions. In addition, the crystallite size of GeBi$$_{2}$$Te$$_{4}$$ was very small (about 150${AA}$). These results suggest that the similarity of entropy between crystalline and amorphous phases plays an important role in terms of the fast phase transition mechanisms.

Journal Articles

Large displacement of germanium atoms in crystalline Ge$$_2$$Sb$$_2$$Te$$_5$$

Shamoto, Shinichi; Yamada, Noboru*; Matsunaga, Toshiyuki*; Proffen, T.*; Richardson, J. W.*; Chung, J.-H.*; Egami, Takeshi*

Applied Physics Letters, 86(8), p.081904_1 - 081904_3, 2005/02

 Times Cited Count:47 Percentile:82.45(Physics, Applied)

Local structure of NaCl-type crystalline Ge$$_2$$Sb$$_2$$Te$$_5$$ has been studied by the atomic pair distribution function analysis of pulsed neutron powder diffraction data. We have found the large displacement of germanium atoms in this crystalline phase. Usually, such a large lattice distortion has disadvantage for the electric conductivity. The electronic band structure, however, implies that this significant distortion in addition to the lattice defect at Na site reduces only thermal conductivity, while the highly ordered tellurium atoms at Cl site keep high electric conductivity.

Journal Articles

Structures of optical recording materials Ge$$_2$$Sb$$_{2+x}$$Te$$_5$$ studied by pulsed neutron diffraction

Shamoto, Shinichi; Taguchi, Tomitsugu; Matsunaga, Toshiyuki*; Yamada, Noboru*; Ishii, Kenji; Inami, Toshiya

Materials Research Society Symposium Proceedings, Vol.840, p.125 - 130, 2005/00

Metastable cubic Ge$$_2$$Sb$$_{2+x}$$Te$$_5$$ samples, which are widely used as optical recording materials, have been studied by both of pulsed neutron and X-ray powder diffraction measurements. The crystalline cubic phases are found to coexist with two types of amorphous phases, depending on their antimony contents. One has the same first sharp diffraction peak (FSDP) as the amorphous antimony at the higher Q position. Another has a new FSDP at a lower Q position. The origin of the peak shift is discussed in terms of their constituents.

Oral presentation

Isomer spectroscopy using actinide targets

Sugiyama, Koichi*; Go, Shintaro*; Tomimatsu, Taro*; Kai, Tamito*; Nagae, Daisuke*; Ishibashi, Yuichi*; Matsunaga, Sotaro*; Nagata, Yuto*; Nishibata, Hiroki*; Washiyama, Kohei*; et al.

no journal, , 

We have successfully performed in-beam gamma-ray spectroscopy using the isomer-scope technique to study excited-state structure of neutron-rich heavy-actinide nuclei. The neutron-rich heavy-actinide nuclei were produced in the multinucleon-transfer reactions with a $$^{248}$$Cm target and $$^{18}$$O projectiles accelerated with the JAEA tandem accelerator. Projectile-like scattered particles were detected with Si E-$$Delta$$E telescopes placed at the backward angle, and target-like scattered particles of isomers were caught by an annular aluminum plate placed at about 60-mm downstream from the target. Four Ge detectors and 4 LaBr detectors were placed at the periphery of the aluminum plate, and detected gamma rays from the isomers. Gamma rays emitted from the actinide isomers were successfully observed with a good sensitivity owing to the tungsten shield placed between the target and the detectors.

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