Toshiro Tani

X-Ray Study of Charge Density Wave Structure in 1T-TaS2

Positions of satellite reflections in charge density wave (CDW) phases of 1T-TaS 2 were measured by X-ray diffraction. In the commensurate (C-) phase, stacking of CDW layers is considerably disordered. On heating from the C-phase there appears a new incommensurate triclinic structure up to 280 K, where anomalies have been observed in various properties.

Room-Temperature Fluorescence Lifetime of Pseudoisocyanine (PIC) J Excitons with Various Aggregate Morphologies in Relation to Microcavity Polariton Formation

The results of room-temperature fluorescence lifetime measurements are reported for the excitation of J aggregates (Js) of pseudoisocyanine chloride (PIC-Cl) prepared in potassium polyvinyl sulfate (PVS) polymer thin films, their aqueous solutions, and NaCl aqueous solutions. Variations of the microscopic morphologies of the aggregates were investigated. The results show that fluorescence decay features correlated to the morphology change. The observed fluorescence lifetime and quantum efficiency of PIC J aggregates (PIC-Js) in a NaCl aqueous solution were 310 ps and 28%, respectively. The lifetime of the fibril-shaped macroaggregates prepared in PVS thin films was below the instrumental time resolution of 5 ps, and the efficiency decreased to below 3%. The results indicate that PIC-Js prepared with PVS polymers have an increased nonradiative contribution to the excitation deactivation process. In particular, macro-Js with isolated fibril-shaped structures revealed nonradiative pathway(s) that are closely associated to the specific packaging morphology of the constituent meso-Js. The possibility of a destructive effect on the formation of cavity-polaritons is also discussed.

Thermoelectric Power Observation of Nearly-Commensurate Charge-Density Wave Phase in 1T-TaS2

The temperature dependence of Seebeck coefficient of 1T-TaS 2 is observed in the nearly-commensurate charge-density wave phase with simultaneous measurement of resistivity. Seebeck coefficient is mainly negative and shows complicated behaviors with large hysteresis. At about 280 K in the heating process, we found distinctive anomalies both in Seebeck coefficient and in resistivity, which may correspond to the new phase transition found in the thermal expansion measurement recently. The possible electronic structures in the nearly-commensurate phase are discussed on the basis of the Seebeck effect observations.

Local reflection micro-spectroscopy of excitons in fibril-shaped molecular J-aggregates prepared in PVA thin films

Abstract Results on microscopic reflection imaging and spectroscopy of fibril-shaped J-aggregates of pseudoisocyanyne dyes prepared in thin film matrices of polyvinyle alcohol (PVA) are reported. We have found that formation of fibril-shaped structures with sub-micron widths is also possible in PVA. Despite the aggregation mechanism should be different from those in polyvinyl sulfate, reflection images and local spectroscopy show that the exciton–polariton characters are basically similar in the two matrices. Anomalous polarization dependence is also observed in PVA and seems rather strengthened.

Versatile Implementation in Angle-Resolved Optical Microscopy: Its Application to Local Spectrometry of Microcavities with PIC-J-Aggregates

Versatile novel implementations in microspectroscopy are developed, which can provide angle-resolved optical spectroscopy at local sample areas almost in diffraction limit. By selecting focus position of light flux incident within the back focal plane of the objective lens radially from the position of the optical axis of the microscope with employing off-centered pinhole, we can obtain parallel beam with oblique incidence and its angle tuning at the sample surface. In this paper, we describe our specific optical setup and its practical working principle in detail. We report, as a demonstration of its performance, our latest studies on optical properties of cavity polariton states in the so-called quantum microcavity structures, which contain molecular J-aggregates of pseudoisocyanine (PIC) dye as active working materials. By using the microscope technique, we obtain a fair amount of improvement in the linewidth observation of cavity polariton spectra.

Pressure Effect on the Impurity State and Impurity Conduction in Tellurium

Reflecting the anisotropic nature of the crystal [1, 2], the electronic structure of tellurium is quite peculiar. In particular, the top of the valence band is located in the vicinity of the H-point in the Brillouin zone and takes the form of camel-back shape because of the existence of the large kz-linear term[3], as is shown in Fig.3. The height of the maxima from the saddle point (H-point) is 2 meV and km is 2×106 cm−1 [4, −6]. Anzin et al. [7] found that this double-maximum structure disappeared under the hydrostatic pressure of 14 kbar by Shubnikov-de Haas experiment. This enables us to expect some conspicuous effect in the electronic properties under comparatively low hydrostatic pressure.

Electronic transport properties in 1T-TaS2

Abstract The temperature dependences of the Seebeck effect and transverse magnetoresistance are systematically observed on 1T-TaS 2 with various qualities. The Seebeck coefficient is positive in commensurate CDW state and its temperature dependence in 20 K ⪅ T ⪅ 60 K seems to indicate that the samples are classified into two groups: one is metallic and the other is non-metallic although they show all non-metallic behaviors in conductivity. In the non-metallic crystals we found a new anomaly at 20 K, as yet unknown. In the metallic crystals we observed small but negative magnetoresistance at 14.1 K, which suggests the variation of electronic states around this temperature. We discuss the sample dependence in relation to electronic localization and dimensionality.

Photophysics of Capped Nanocrystals and Molecular J-Aggregates

Novel functional materials based on novel principles are eagerly required in the field of photonic device technologies not only for faster processing performance but also for higher density of integration. The technological details are described elsewhere; however, we propose here simply that one way to tackle the problem: to create a strong coupling regime between the light and electrons directly within the functions of optoelectronic materials. In this sense the notion of excitons or exciton-polaritons, though well-understood, becomes significant in the new study of quantum dots in the regime of the size confinement effect.

Impurity States and Impurity Conduction in Tellurium under Hydrostatic Pressure

Under the hydrostatic pressure up to 4.7 kbar, the impurity states in undoped and Sb-doped single crystal tellurium are investigated by measuring the conductivities and Hall coefficients in the temperature region from 77 K to 1.4 K. The pressure dependences of the conductivity and Hall coefficient are different between undoped and Sb-doped crystal at low temperature. This indicates that there are two kinds of impurity states in tellurium, one is the impurity band-like states degenerate in the top of the valence band and the other is the ordinary acceptor states formed by group V impurities. Pressure dependences of these impurity states are discussed and in Sb-doped tellurium the possibility of the pressure-induced metal-insulator transition in impurity conduction is proposed.