Yutaka Ishihara

The Effects of Temperature and Electric Field for the Photo-Generation of Free Carriers in Anthracene

In order to study the generation of free carriers in an anthracene crystal, the measurement of the pulsed photoconduction has been performed. The number of free holes and electrons produced in a crystal after a flash illumination has been found to depend both on the temperature and the electric field. The number of generated holes increases exponentially with the rise of the temperature. At the higher temperatures it shows a tendency of saturation. The temperature dependency is not so remarkable for electrons. The number of holes and electrons increases with the field strength; yet in the higher field region the tendency of the saturation appears. The mobility of holes at the room temperature is 0.8 cm 2 /V.s and decreases as the temperature is raised. The mobility of electrons at the room temperature is 0.4 cm 2 /V.s and its variation with temperature is small. A model of energy levels is proposed for the carrier generation.

Electrical transport properties of a quasi-one-dimensional Nb3Te4 single crystal

Abstract The electrical resistance of a linear chain metal Nb3Te4 were measured from 1.3 to 320 K. The residual resistance ratio R(300 K ) R(4.2 K ) is about 3. Nb3Te4 shows an anomaly in the resistivity vs temperature at about 80 K, suggesting an occurrence of a charge-density-wave transition. The transverse and longitudinal magnetoresistance at 4.2 K are proportional to the magnetic field in the range of 2–58 kOe. In the superconducting region close to the transition temperature Tc, the critical magnetic field Hc2 is proportional to δT=Tc−T. The angular dependence of Hc2 fits well with the fluxoid model of the Ginzburg-Landau theory. The ratio of the critical fields parallel and perpendicular to the chain direction is 4.8.

Thermoelectric power of quasi-one-dimensional Nb3X4 with X=S, Se andTe

Abstract The thermoelectric power of linear chain synthetic metals Nb 3 X 4 (with X = S , Se and Te ) was measured from 5 to 300 K. The thermopower is negative indicating a dominant transport by electrons. Common to three compounds, in lower temperature regions the thermopower rises linearly with temperature but soon saturates. With respect to Nb 3 S 4 and Nb 3 Se 4 we have found no special anomaly of the thermopower except for a little higher magnitude. With respect to Nb 3 Te 4 anomalies in the thermopower vs temperature appear at about 80 and 20 K which are explained in terms of the charge- density-wave phase transition from the simultaneous measurement of the resistivity and the observation of the electron diffraction patterns.

Superlattice structure of Nb3Te4 at low temperatures

Abstract A quasi-one-dimensional metal Nb3Te4 exhibits a superlattice structure below about 100 K. The phenomenon is revealed by electron diffraction for the first time. A clear domain pattern is observed by electron microscope images by both host lattice diffraction and superlattice diffraction beams. The wavevectors for the superlattice are described by q ∗ ± = 1 1.5a ∗ ± 3 7c ∗ with reciprocal lattice vectors. The cause of the superlattice generation is attributed to the charge-density-wave which couples strongly with lattice waves so that a deformation of the crystal lattice resulted.

Electrical conduction of a quasi-one dimensional Nb3S4 single crystal

Abstract With respect to single crystals of Nb 3 S 4 the electrical resistivity from 2.8 K to 300 K and the magnetoresistance at 4.2 K were measured. The resistivity is represented as a sum of a temperature independent and an intrinsic temperature dependent component. The temperature dependence of the intrinsic resistivity subjects to T 3 form between 7 and 50 K above which it becomes weaker than T 3 approaching a T linear from. This behaviour is discussed in terms of the electron-electron Umklapp scattering. The ratio of the resistivities perpendicular and parallel to the c -axis takes about 15 between room temperature and 50 K. The transverse magnetoresistance is proportional to the magnetic field. The longitudinal magnetoresistance is too small to be measured.

Electrical conduction and critical magnetic field for superconductivity of a Nb3Se4 single crystal

Abstract With respect to the quasi-one dimensionality of single crystals of Nb 3 Se 4 , the electrical resistivity from 1.3 to 320 K and the critical magnetic field for superconductivity are measured. The resistivity along the Nb-chain direction is represented as a sum of a temperature independent and an intrinsic temperature dependent term. The temperature dependence of the intrinsic resistivity subjects to T 3 form between 10 and 80 K above which it tends to a T linear form. The critical magnetic field is proportional to the temperature difference from the transition temperature. Its dependence is well fitted by the elliptical fluxoid model of Ginzburg-Landau theory. The ratio of the parallel and the perpendicular to the c -axis is 5.7.

Stripe and charge-density-wave domain structures of Nb3Te4

Abstract By means of electron microscopy, superlattice reflections from charge-density-wave domains were observed in a quasi-one-dimensional conductor Nb3Te4. Print-out effect was observed for the martensitic stripe domain structure at low temperature, suggesting the occurrence of radiation damage for electron beam acceleration voltage over 350 kV. By cooling the specimen, CDW and stripe domains appeared nearly simultaneously at 100 ± 5 K. By warming, however, the former disappeared at 105 ± 5 K, while the latter at 115–120 K.

Superconductivity in the metallic layered compound NbTe2

Abstract The layered-structure compound NbTe2 is one of the typical materials which lead to charge-density waves. However, much less information is available on the superconducting properties of this compound. Electrical resistivity and diamagnetic susceptibility have been measured for high-quality single crystals of NbTe2. The superconducting transition is observed at Tc of 0.5 K. The transition to the superconducting state depends strongly on the current density. Susceptibility χ ∥ = M H with applied magnetic fields parallel to the two-dimensional layers in the crystal is weak and the value of χ∥ is two times smaller than the magnitude for perfect diamagnetism in a magnetic field of 0.50 Oe. The susceptibility changes with external field, even less than 0.50 Oe at 0.40 K, which implies that the lower critical field Hc1 of NbTe2 is extremely small ( H c1

Electrical properties of lithium doped Nb3Te4

= 0.5 Oe ). The experimental results of NbTe2 are compared with the data of the other layered compound 2H-TaS2.

Electrical Properties of a Quasi-One-Dimensional Nb3Te4 Single Crystal

Abstract The resistivity and the thermopower of Li doped Nb 3 Te 4 were measured from 1.4 to 300 K. The superconducting upper critical field was also measured. With addition of Li, the resistive anomaly at about 100 K decreases, while that at about 40 K increases. With the increase of Li concentration, the magnitude of the thermopower at about 100 K becomes smaller. These results are discussed in terms of the modification of the shape of the Fermi surfaces.