Toshihito Umegaki

Investigation of electronic transport in carbon nanotubes using Green’s-function method

We have investigated the fundamental transport characteristics in carbon nanotubes (CNTs) in order to realize a high-frequency device. In the present analysis, the electron and hole densities are excited at each lead and propagate as a wave from the left contact to the right contact in the CNTs. First, we applied Green’s-function method formulation to CNTs with arbitrary chirality. We then calculated the basic conductive characteristics in the CNTs. In the metallic and intrinsic CNTs, the assumption of a linearly varying potential distribution is valid because the electron and hole densities satisfy the charge neutrality condition, and their distributions become uniform. We evaluated the I-V characteristics of semiconductive and metallic CNTs. Based on the obtained results, we can control the differential conductance from a negative value to a value of several times the conductance quantum by means of chirality, Fermi level, and bias voltage. We evaluated the GV∕I-V characteristics of the zigzag CNT and f...

Electronic transport in carbon nanotubes using the transfer-matrix method

We have studied the basic conductive characteristics in carbon nanotubes (CNTs) for the purpose of application to a high-frequency device. In the analysis, the current flow in the CNTs is viewed as the quantum transport of electronic waves. First, we analyzed the dispersion relations of electronic waves in the CNTs based on the linear combination of atomic orbitals expansion method. In addition, we investigated the current-density distributions around the circumference of the CNTs and the current-voltage characteristics by using the transfer-matrix method. As a result, the current distributions were found to be significantly controlled by both the chirality of the CNTs and the position of the current sources around the circumference. Based on these results, we propose herein a ferrite device that acts as a filter in the terahertz frequency domain. In this device, the high-frequency current flowing on the CNTs may excite and receive directly the spin waves in the ferrite film beneath them, and the operatin...

Piezoelectric characterization of Pb(Zr,Ti)O3 thin films deposited on metal foil substrates by dip coating

We fabricated the piezoelectric bimorphs composed of Pb(Zr,Ti)O3 (PZT) thin films on metal foil substrates. To efficiently inexpensively manufacture piezoelectric bimorphs with high flexibility, 1.2-µm-thick PZT thin films were directly deposited on both surfaces of 10- and 20-µm-thick bare stainless-steel (SS) foil substrates by dip coating with a sol–gel solution. We confirmed that the PZT thin films deposited on the SS foil substrates at 500 °C or above have polycrystalline perovskite structures and the measured relative dielectric constant and dielectric loss were 323–420 and 0.12–0.17, respectively. The PZT bimorphs were demonstrated by comparing the displacements of the cantilever specimens driven by single- and double-side PZT thin films on the SS foil substrates under the same applied voltage. We characterized the piezoelectric properties of the PZT bimorphs and the calculated their piezoelectric coefficient |e 31,f| to be 0.3–0.7 C/m2.