Takahisa Tokumoto

Persistent photoexcited conducting states in functionalized pentacene

We report a study of long-lived photoexcited states in single crystals of functionalized forms of pentacene: the 6,13-bis(triisopropylsilylethynyl)pentacene and the 6,13-bis(triethylsilylethynyl)pentacene. These materials are organic semiconductors with band gaps in the range ≈1eV. The relaxation rate of these states is thermally activated, as determined from time-and temperature-dependent measurements of the photoconductivity after illumination, and these states can be produced in a stable population at temperatures below 150K. Consequently, after the low-temperature illumination, thermally stimulated current (TSC) is observed in the dark current with an increasing temperature. Analysis of the TSC signal allows estimation of energies associated with the excited states. Possible mechanisms for the current associated with the photoexcited states are discussed, and preliminary iodine-doped studies of the material are also presented.

Debye relaxation in high magnetic fields

Dielectric relaxation is universal in characterizing polar liquids and solids, insulators, and semiconductors, and the theoretical models are well developed. However, in high magnetic fields, previously unknown aspects of dielectric relaxation can be revealed and exploited. Here, we report low-temperature dielectric relaxation measurements in lightly doped silicon in high dc magnetic fields

Spin Exchange Interaction in Substituted Copper Phthalocyanine Crystalline Thin Films.

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Partial Breakdown of the Field-induced Superconductivity by Dynamical Spin Reversal

both parallel and perpendicular to the applied ac electric field

Low electrical conductivity threshold and crystalline morphology of single-walled carbon nanotubes – high density polyethylene nanocomposites characterized by SEM, Raman spectroscopy and AFM

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Vortex Dynamics and the Fulde-Ferrell-Larkin-Ovchinnikov State in a Magnetic-Field-Induced Organic Superconductor

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Electric and thermoelectric transport probes of metal–insulator and two-band magnetotransport behavior in graphite

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Persistent photo-excited conducting states in functionalized pentacene

, we observe a temperature and magnetic-field-dependent dielectric dispersion

Temperature- and field-dependent energy transfer in CdSe nanocrystal aggregates studied by magneto-photoluminescence spectroscopy

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Cyclotron decay time of a two-dimensional electron gas from 0.4 to 100 K

characteristic of conventional Debye relaxation where the free-carrier concentration is dependent on thermal dopant ionization, magnetic freeze-out, and/or magnetic localization effects. However, for