Akinori Umeno
University of Tokyo
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Featured researches published by Akinori Umeno.
Nano Letters | 2013
Kenji Yoshida; Ikutaro Hamada; Shuichi Sakata; Akinori Umeno; Masaru Tsukada; Kazuhiko Hirakawa
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.
Applied Physics Letters | 2009
Akinori Umeno; Kazuhiko Hirakawa
We have investigated the electromigration process at gold nanojunctions as small as several tens of atoms. Junction conductance showed successive drops by one conductance quantum, corresponding to one-by-one removal of gold atoms, only when the junction voltage exceeded certain critical values. The peak position in the histogram of the observed critical voltages agreed with the activation energies for surface diffusion of gold atoms. This fact indicates that the elementary process of electromigration in such small junctions is the self-diffusion of metal atoms driven by microscopic kinetic energy transfer from a single conduction electron to a single metal atom.
Applied Physics Letters | 2005
Akinori Umeno; Kazuhiko Hirakawa
Fabrication of nanometer-separated gold junctions has been performed using “iodine tincture,” a medical liquid known as a disinfectant, as an etching/deposition electrolyte. In the gold-dissolved iodine tincture, gold electrodes were grown or eroded slowly enough to form quantum point contacts in an atomic scale. The resistance evolution during the electrochemical deposition showed plateaus at integer multiples of the resistance quantum, (2e2∕h)−1, at room temperature (e: the elementary charge, h: the Planck constant). Iodine tincture is a commercially available common material, which makes the fabrication process to be simple and cost effective. Moreover, in contrast to the conventional electrochemical approaches, this method is free from highly toxic cyanide compounds or extraordinarily strong acids.
IEEE Transactions on Magnetics | 2001
Akinori Umeno; Hiroko Kotani; Masakazu Iwasaka; Shoogo Ueno
The effect of magnetic fields on the orientation of adherent cells was investigated quantitatively using a long-term magnetic field exposure system at 8 T. Smooth muscle cells, which are classified as adherent cells, were passaged and dispersed in Dulbeccos Modified Eagles Medium (DMEM). A cell culture flask was filled to capacity with the smooth muscle cell suspension, and placed in the center of a superconducting magnets bore. The smooth muscle cell culture was incubated for 3 days, and exhibited randomly oriented patterns of rod-like shaped cells without magnetic field exposure. In contrast, the smooth muscle cell culture exposed to 8 Tesla (T) magnetic fields for 70 hours changed into a one-directionally oriented cell culture. To quantify the orientational order, an orientational order parameter for a 2-dimensional system was introduced by adapting the 3-dimensional order parameter. The degree of orientation of the culture with 8 T magnetic field exposure for 70 hours was 0.72, while the degree of orientation without exposure was 0.21. The results indicated that the magnetic orientational order of smooth muscle cells was lower than that of floating cells.
Applied Physics Express | 2010
Kenji Yoshida; Akinori Umeno; Shuichi Sakata; Kazuhiko Hirakawa
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.
Japanese Journal of Applied Physics | 2009
Kenji Yoshida; Akinori Umeno; Shuichi Sakata; Kazuhiko Hirakawa
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.
IEEE\/ASME Journal of Microelectromechanical Systems | 2007
Murat Gel; Tadashi Ishida; Tetsuo Akasaka; Akinori Umeno; Koji Araki; Kaz Hirakawa; Hiroyuki Fujita
We report fabrication and characterization of a mechanically controlled quantum point contact with on-chip electrostatic actuators. Two metallic tips separated with a nano gap are integrated with a micro mechanism, which can be actuated by applied voltage. The electrostatic actuation of the micro mechanism allows the control of separation gap at nanometer level. The gap variation at nanometer level is calibrated with the devices operated in transmission electron microscope. Furthermore, performance of the device is demonstrated by measuring conductance quantization while separating the tips in contact as well as electrical measurements by using tips coated with self assembled monolayers (1-4 benzene dithiol) and conjugated Ru complex
Applied Physics Express | 2010
Shuichi Sakata; Akinori Umeno; Kenji Yoshida; Kazuhiko Hirakawa
We investigated the critical voltage for electromigration (EM) in atomic-scale Cu nanojunctions by using EM spectroscopy. The critical voltage was determined to be 0.35 V from the peak of the obtained EM spectrum, which is close to the atom diffusion potential for clean Cu surfaces. It was also demonstrated that ballistic Cu nanojunctions can support current densities on the order of 10 GA/cm2, as long as the junctions are biased below the critical voltage. The results suggest that both high current densities and high EM reliabilities can be achieved when the dimension of the metal interconnects is reduced to several tens of atoms.
international interconnect technology conference | 2010
Kazuhiko Hirakawa; Akinori Umeno; Kenji Yoshida; Shuichi Sakata
We have investigated electromigration process at metal nanojunctions as small as several tens of atoms and found that the elementary process of electromigration in such nanojunctions is the self-diffusion of metal atoms driven by microscopic kinetic energy transfer from single conduction electrons to single metal atoms. We have also shown that metal nanojunctions are stable and can support extremely high current densities of over 1010 A/cm2, as long as the junction voltage is below the critical value.
PHYSICS OF SEMICONDUCTORS: 28th International Conference on the Physics of Semiconductors - ICPS 2006 | 2007
Tetsuo Akasaka; Akinori Umeno; Su Heon Hong; Kazuhiko Hirakawa; Koji Araki
Self‐assembled network of gold nanoparticles connected by molecules on SiO2/Si surfaces over macroscopic distances was prepared by alternately dipping substrates into gold nanoparticle‐ and molecule‐solutions. A significant conductivity was observed even after 1 cycle of deposition, suggesting that a conductive pathway was formed between the electrodes through the gold nanoparticle‐molecule network. Furthermore, it should be noted that the present unique fabrication method is compatible with planar transistor geometry.