Kenji Yoshida

Gate-Tunable Large Negative Tunnel Magnetoresistance in Ni–C60–Ni Single Molecule Transistors

We have fabricated single C(60) molecule transistors with ferromagnetic Ni leads (FM-SMTs) by using an electrical break junction method and investigated their magnetotransport. The FM-SMTs exhibited clear gate-dependent hysteretic tunnel magnetoresistance (TMR) and the TMR values reached as high as -80%. The polarity of the TMR was found to be always negative over the entire bias range studied here. Density functional theory calculations show that hybridization between the Ni substrate states and the C(60) molecular orbitals generates an antiferromagnetic configuration in the local density of states near the Fermi level, which gives a reasonable explanation for the observed negative TMR.

Long-wavelength emission in photo-pumped GaAs1−xBix laser with low temperature dependence of lasing wavelength

This study demonstrates long-wavelength emission of up to 1204u2009nm in photo-pumped GaAs1−xBix lasers grown by molecular beam epitaxy under low temperature conditions. The characteristic temperature (T0) between 20 and 80u2009°C in the GaAs1−xBix lasers with Al0.3Ga0.7As electron blocking layer is approximately 100u2009K, which is larger than that of the typical 1.3-μm InGaAsP Fabry-Perot laser diodes (FP-LDs; T0u2009=u200966u2009K). The temperature coefficient of the lasing wavelength is approximately 40% of that of InGaAsP FP-LDs.

Terahertz Field Enhancement and Photon-Assisted Tunneling in Single-Molecule Transistors.

We have investigated the electron transport in single-C_{60}-molecule transistors under the illumination of intense monochromatic terahertz (THz) radiation. By employing an antenna structure with a sub-nm-wide gap, we concentrate THz radiation beyond the diffraction limit and focus it onto a single molecule. Photon-assisted tunneling (PAT) in the single molecule transistors is observed in both the weak-coupling and Kondo regimes. The THz power dependence of the PAT conductance indicates that when the incident THz intensity is a few tens of mW, the THz field induced at the molecule exceeds 100u2009 kV/cm, which is enhanced by a factor of ~10^{5} from the field in the free space.

A comprehensive quantification method for eicosanoids and related compounds by using liquid chromatography/mass spectrometry with high speed continuous ionization polarity switching

Fatty acids and related metabolites, comprising several hundreds of molecular species, are an important target in disease metabolomics, as they are involved in various mammalian pathologies and physiologies. Selected reaction monitoring (SRM) analysis, which is capable of monitoring hundreds of compounds in a single run, has been widely used for comprehensive quantification. However, it is difficult to monitor a large number of compounds with different ionization polarity, as polarity switching requires a sub-second period per cycle in classical mass spectrometers. In the present study, we developed and evaluated a comprehensive quantification method for eicosanoids and related compounds by using LC/MS with high-speed continuous ionization polarity switching. The new method employs a fast (30ms/cycle) continuous ionization polarity switching, and differentiates 137 targets either by chromatography or by SRM transition. Polarity switching did not affect the lower limits of quantification, which ranged similarly from 0.5 to 200pg on column. Lipid extracts from mouse tissues were analyzed by this method, and 65 targets were quantitatively detected in the brain, including 6 compounds analyzed in the positive ion mode. We demonstrated that a fast continuous ionization polarity switching enables the quantification of a wide variety of lipid mediator species without compromising the sensitivity and reliability.

Electrically pumped room-temperature operation of GaAs1−xBix laser diodes with low-temperature dependence of oscillation wavelength

Lasing oscillation at wavelengths up to 1045 nm at room temperature has been realized from GaAs1−xBix Fabry–Perot laser diodes (FP-LDs) by electrical injection, and the temperature characteristics of GaAs1−xBix FP-LDs are revealed for the first time. The characteristic temperature T0 of the GaAs0.97Bi0.03 FP-LD in the temperature range between 15 and 40 °C (T0 = 125 K) is similar to that reported for typical 0.98 µm InGaAs/GaAs LDs. The temperature coefficient of the lasing wavelength in GaAs0.97Bi0.03 FP-LDs is reduced to 0.17 nm/K, which is only 45% of that of GaAs FP-LDs.

Structural Stability of Ni Quantum Point Contacts under Electrical Stresses

We have investigated the structural stability of Ni quantum point contacts (QPCs) under electrical stresses by monitoring the junction conductance as a function of applied voltage. The histogram of the critical junction voltage, VC, at which there occurs one-by-one atom removal due to electromigration is found to have a peak at approximately 0.3 V, which is consistent with the surface diffusion potential of Ni. We have also shown that Ni QPCs are stable and can support extremely high current densities of over 1010 A/cm2, as long as the junction voltage is below the lower edge of the VC-histogram.

Importance of Moisture Control in Formation of Nanogap Electrodes by Electrical Break Junction Method

We have systematically investigated the fabrication of nanogap electrodes of Ni by the electrical break junction (EBJ) method under various environmental conditions. When EBJ was performed in the atmosphere, the anode side of the Ni electrodes was seriously damaged. The damaged region was analyzed by Auger electron spectroscopy and was identified to be Ni oxides formed during EBJ process by anodic oxidation via atmospherically-derived moisture adsorbed on the metal surfaces. When the EBJ process was performed in an evacuated environment, nanogap electrodes with atomic-order spacing were reproducibly fabricated even at room temperature.

Electron transport in endohedral metallofullerene Ce@C82 single-molecule transistors

We have investigated the electron transport in endohedral metallofullerene Ce@C82 single-molecule transistors (SMTs) together with that in reference C84 SMTs. The vibrational modes (bending and stretching) of the encapsulated single Ce atom in the C82 cage appear in Coulomb stability diagrams for the single-electron tunneling through Ce@C82 molecules, demonstrating the single-atom sensitivity of the transport measurements. When a bias voltage larger than 100u2009mV is applied across the source/drain electrodes, large hysteretic behavior is observed in the current-voltage (I-V) characteristics. At the same time, the pattern in the Coulomb stability diagram is changed. No such hysteretic behavior is observed in the I-V curves of hollow-cage C84 SMTs, even when the bias voltage exceeds 500u2009mV. This hysteretic change in the I-V characteristics is induced by a nanomechanical change in the configuration of the Ce@C82 molecule in the nanogap electrode due to the electric dipole that exists in Ce@C82.

Stage-specific modification of G protein beta subunits in rat placenta.

We previously analysed the plasma membrane proteins of rat placenta and prepared a database of 150 plasma membrane proteins, expressed in a stage-specific manner, utilizing two-dimensional gel electrophoresis (2D/E) [Mol. Cell. Endocrinol. 115(1995)149]. In this study, we focused on the proteins, tentatively named psL-I (MW 36.2 kDa, pI 5.3) and psL-II (35.9 kDa, 5.3), which were expressed mainly in late pregnancy. Close to psL-I and psL-II on 2D/E gels, we also recognized more abundant proteins [psC-I (36.2 kDa, 5.4) and psC-II (35.9 kDa, 5.4), respectively] arranged side by side with the same MW but different pI. Expression of psL-I and psL-II was detected only in junctional zone of placenta, whereas psC-I and psC-II were expressed in both labyrinth and junctional zones. In addition, psL-I and psL-II began to increase on day 16 of pregnancy and peaked at term, whereas expression of psC-I and psC-II was relatively constant. The analysis of these four proteins (psL-I, psL-II, psC-I and psC-II) by preparative 2D/E, peptide mapping, amino acid sequence and mass spectrometry (MALDI-TOF-MS) revealed that psC-I was a G protein beta1 subunit, and psC-II was a beta2 subunit, and showed that psL-I and psL-II were molecular modified forms of psC-I and psC-II, respectively. Expression of these G protein beta subunits (psL-I, psL-II, psC-I and psC-II) was also observed in rat choriocarcinoma cells, Rcho-1 cells. Expression of psC-I and psC-II was much higher than those of psL-I and psL-II, and their level was relatively constant regardless of the stage of differentiation in vitro. Interestingly, expression of psL-I and psL-II gradually increased in association with the differentiation. Since the expression of beta1 and beta2 subunit proteins and their mRNAs was constant during the process of differentiation in Rcho-1 cells, the expression of these lower pI forms of G protein subunits (psL-I and psL-II) was thought to be post-translationally regulated. In conclusion, there are modified forms of G protein beta1 and beta2 subunits, in the placenta and Rcho-1 cells, which are expressed in a pregnancy-stage or differentiation stage specific manner.

Stochastic resonance in bistable atomic switches

We have investigated the conductance of bistable gold atomic switches as a function of periodic input voltages mixed with a random noise. With increasing noise amplitude, the atomic switches biased below the threshold voltage for conductance switching start exhibiting switching in conductance between two stable states. Clear synchronization between the input and output signals is observed when an optimized noise amplitude is mixed with the periodic input voltage, even when the atomic switches are driven by an input voltage as low as approximately 10% of the threshold voltage. The observed behavior can be explained in terms of the stochastic resonance. The results presented here indicate that utilization of noise can dramatically reduce the operation voltage of metal atomic switches.