Eiji Hatta

Anisotropic Factor of Electrical Conductivity in p-Bi2Te3 Crystals

Temperature variation of electrical conductivity in two crystallographic directions (σ∥ and σ⟂) of p-Bi2Te3 crystals has been measured. The anisotropic factor, σ∥/σ⟂, shows an exponential form involving an activation energy. This can be interpreted as an effect of defects between the layers on the electronic density of states.

Electron tunneling experiments on skutterudite Co1−xFexSb3 semiconductors

Electron tunneling experiments were performed on p-Co1−xFexSb3–Al-oxide–Al junctions for x=0 and x=0.1 at 4.2 K. A U-shaped tunneling conductance curve obtained for polycrystalline p-CoSb3 clearly shows an energy-band gap of ∼50 meV. For p-Co0.9Fe0.1Sb3 skutterudites, a strong zero-bias conductance anomaly is observed. This anomaly may be due to a structural disorder arising from defects such as vacancies and interstitial Fe atoms.

Enhanced Orientation in Langmuir-Blodgett Films of Tetra-tert-butyl Phthalocyanines

We report the results of structural investigations on Langmuir (L) and Langmuir–Blodgett (LB) films of tetra-tert-butyl phthalocyanine (MTBPC) containing various transition metal ions. Techniques were developed for fabricating well-ordered L-monolayers using a flow-orientation method that dramatically enhances the molecular orientation of MTBPC in L-monolayers as compared to other techniques. The dilution procedure for developing solutions enables the formation of a well-defined single L-monolayer at the air–water interface. The tilt angles of MTBPCs in the L-monolayers are estimated by the limiting area technique. The tilting structure of LB films of MTBPCs on glass substrates as measured by X-ray diffraction (XRD) method is in good agreement with the estimate obtained by the limiting area technique on the corresponding L-monolayers. An annealing procedure significantly improves the ordering of the LB films as measured by XRD.

Phonon behaviors and electronic structures of the filled skutterudite YbyCo4Sb12 compounds: An electron tunneling study

Electron tunneling experiments were performed on YbyCo4Sb12–Al oxide–Al junctions for y=0–0.25 at 4.2 K. In the second derivative tunneling spectrum of CoSb3 compound (y=0), three peaks were observed at around 5, 20, and 33 mV, which are closely related to an optical phonon mode with a rigid rectangle, Sb–Sb bond bending and bond stretching, and a large Co atomic motion, respectively. Appearance of the strong peak at 7 mV observed in Yb-filled samples corresponds to a rattled phonon mode of Yb ions. The peak energy due to the Sb–Sb bonds is unchanged, whereas the one due to Co motions shifts to lower with increasing Yb concentration. This fact indicates that the filled Yb ions strongly interact to the host framework Co atoms, which were clearly observed in the change of tunneling conductance.

Inelastic electron tunneling across magnetically active interfaces in cuprate and manganite heterostructures modified by electromigration processes

We report a study of the electron tunneling transport in point-contact junctions formed by a sharp Ag tip and two different highly correlated oxides, namely, a magnetoresistive manganite La0.66Ca0.34MnO3 and a superconducting cuprate LaBa2Cu3O7−x. Strong chemical modifications of the oxide surface (supposedly, oxygen ion displacements) caused by applying high voltages to the junctions have been observed. This effect is believed to be responsible for an enormous growth of inelastic tunneling processes across a transition region that reveals itself in an overall V-shaped conductance background, with a strong temperature impact. The mechanism of the inelastic scattering is ascribed to charge transmission across magnetically active interfaces between two electrodes forming the junction. To support the latter statement, we have fabricated planar junctions between Cr and Ag films with an antiferromagnetic chromium oxide Cr2O3 as a potential barrier and at high-bias voltages have found an identical conductance t...

Tunneling Conductance of Al–Al Oxide–p‐Bi2Te3 Junctions

We measured tunneling conductance of Al-Al oxide-p-Bi 2 Te 3 tunnel junctions. A small increase of the tunneling conductance was observed in the energy gap region. The intensity of the increase of tunneling conductance decreased by annealing the p-Bi 2 Te 3 films. Therefore, it is considered that this increase in tunneling conductance is closely related to energy states caused by defects in p-Bi 2 Te 3 films. For the first time, energy states caused by defects are revealed by tunneling experiments in p-Bi 2 Te 3 films.

Evaluation of metal-Bi 2 Te 3 contacts by electron tunneling spectroscopy

Understanding the states of the metal electrode-semiconductor junctions is important for improving the performance of the thermoelectric devices. Tunneling spectroscopy has been applied to the evaluation of metal-semiconductor Schottky junctions. In this study, tunneling spectroscopy is applied to understanding the states of a metal/Bi/sub 2/Te/sub 3/ or a metal/Bi/sub 2/Se/sub 3/ junction. Bi/sub 2/Te/sub 3/ and Bi/sub 2/Se/sub 3/ single crystals were prepared by Bridgman method. The metal contacts were made on the cleavage surface of Bi/sub 2/Te/sub 3/ or Bi/sub 2/Se/sub 3/ by the thermal evaporation in the vacuum of 10/sup -3/ Pa. All tunnel conductance were measured at 4.2 K. Mg/, Al/ and Ag/Bi/sub 2/Te/sub 3/ junctions exhibit V-shaped tunnel conductance curves which indicate that these contacts ate the Schottky barrier. A Au/Bi/sub 2/Te/sub 3/ junction shows a flat conductance characteristic, so that we expect to be an ohmic contact formed in this junction. It becomes clear that the Schottky barrier is formed on the Mg/, Al/, or Ag/-Bi/sub 2/Te/sub 3/ junction. In the case of Au/ or Al/Bi/sub 2/Se/sub 3/ is also formed the Schottky barrier. Since Bi/sub 2/Te/sub 3/ and Bi/sub 2/Se/sub 3/, which are narrow energy gap semiconductors, form the Schottky barriers with those metals, the characterization of metal-semiconductor contacts is important to improve the performance of thermoelectric devices.

Evidence for Strong Electron-Magnon Coupling in Gadolinium

Tunneling spectroscopy measurements on normal state Gd–Gd oxide–Al junctions have been performed. Small deviations from the overall parabolic dependence of conductance on voltage were revealed and analyzed using a theoretical approach, taking into account interactions of tunneling electrons with elementary excitations in the oxide layer and metal electrodes. These processes have been studied by calculating the even and odd conductances that characterize the symmetrical (emission of oxide phonons) and asymmetrical (self-energy effects in electrodes) processes, respectively. It has been found that in gadolinium, one of the simplest magnetic metals known, an interaction of electrons with magnetic excitations is as important as the electron-phonon contribution. This result explains the difference between the calculated electron-phonon coupling parameters and the experimentally derived electron mass enhancement factors in magnetic rare-earth metals.

Electron tunneling experiments on spin glass Ni1−xMnx films

Systematic tunneling measurements have been performed on AlAl oxide/Ni1−xMnx junctions to study the one-particle density of states in spin glass Ni1−xMnx (x = 0.24–0.35) films where the lattice and spin disorders coexist. The strong zero bias conductance anomaly indicating the metal-insulator transition was observed at a critical concentration xc ≈ 0.24. This is the direct evidence that the variation of the tunneling conductance is largely due to the magnetic disorder.

Tunneling through a narrow‐gap semiconductor with different conduction‐ and valence‐band effective masses

We have calculated tunneling conductance in metal–narrow‐gap‐semiconductor (NGS)–metal tunnel junctions. Flietner’s two‐band model is used to describe the dispersion relation within the energy gap in an isotropic NGS with different conduction‐ and valence‐band edge effective masses. The results are compared with the tunneling conductance calculated by Kane’s two‐band model, which has been commonly used to describe the tunneling characteristics through the energy gap in semiconductors. These results propose that the tunneling conductance in the tunnel junctions in which a narrow gap semiconductor of largely different conduction‐ and valence‐band effective masses is used as a tunneling barrier can exhibit quite a different behavior, especially in the region of the midgap, from the tunneling conductance described by Kane’s two‐band model.