Showing papers by "Hiroshi Yamaguchi published in 2008"
TL;DR: In this paper, low-energy electron microscopy (LEEM) was used to measure the reflectivity of low energy electrons from graphitized graphitized silicon carbide (SiC) substrate.
Abstract: Low-energy electron microscopy (LEEM) was used to measure the reflectivity of low-energy electrons from graphitized $\mathrm{SiC}(0001)$. The reflectivity shows distinct quantized oscillations as a function of the electron energy and graphite thickness. Conduction bands in thin graphite films form discrete energy levels whose wave vectors are normal to the surface. Resonance of the incident electrons with these quantized conduction band states enhances electrons to transmit through the film into the $\mathrm{SiC}$ substrate, resulting in dips in the reflectivity. The dip positions are well explained using tight-binding and first-principles calculations. The graphite thickness distribution can be determined microscopically from LEEM reflectivity measurements.
324 citations
TL;DR: An approach to mechanical logic based on nanoelectromechanical systems that is a variation on the Parametron architecture is proposed and, as a first step towards a possible nanomechanical computer, both bit storage and bit flip operations are demonstrated.
Abstract: The Parametron was first proposed as a logic-processing system almost 50 years ago1. In this approach the two stable phases of an excited harmonic oscillator provide the basis for logic operations2,3,4,5,6. Computer architectures based on LC oscillators were developed for this approach, but high power consumption and difficulties with integration meant that the Parametron was rendered obsolete by the transistor. Here we propose an approach to mechanical logic based on nanoelectromechanical systems7,8,9 that is a variation on the Parametron architecture and, as a first step towards a possible nanomechanical computer10,11,12, we demonstrate both bit storage and bit flip operations.
302 citations
TL;DR: In this article, a superconducting quantum interference device (SQID) is proposed to detect minute variations in magnetic field. But it is not suitable for measuring displacements.
Abstract: Superconducting quantum interference devices, or SQUIDs as they are better known, are capable of detecting minute variations in magnetic field. Embedding a suspended beam into the structure of d.c. SQUID enables this sensitivity to be exploited for measuring displacements.
178 citations
TL;DR: In this article, the frequency response of an electromechanical oscillator was measured while being parametrically pumped by double frequency modulations to the effective spring constant via the piezoelectric effect.
Abstract: The frequency response of an electromechanical oscillator was measured while being parametrically pumped by double frequency modulations to the effective spring constant via the piezoelectric effect. A 13dB gain in the resonance amplitude was observed by increasing the pump power where further increase was limited by parametric excitation of the fundamental mode. Concurrently, the coherent amplification resulted in the quality factor of the resonance also being enhanced by ∼2.5 times. The on-chip degenerate piezoelectric parametric amplification demonstrated here could be implemented in nanoelectromechanical oscillators to bypass the detrimental effects of size minimization.
56 citations
TL;DR: In this article, the number of graphene layers grown on both the Si-terminated and Cterminated substrates can be determined microscopically by quantizing oscillations in the reflectivity between 0 and 7 eV.
Abstract: Low-energy electron microscopy (LEEM) has been used to measure reflectivity of low-energy electrons for graphene layers grown on Si-terminated 6H-SiC(0001) and 4H-SiC(0001) substrates and C-terminated 4H-SiC(000-1) substrates. We observe quantized oscillations in the reflectivity on all the substrates. The number of graphene layers grown on both the Si-terminated and C-terminated substrates can be determined microscopically as the number of dips in the reflectivity between 0 and 7 eV. We also find that the dips appear closer to the vacuum level on the C-terminated substrates than on the Si-terminated substrates. This could be explained by the differences in the work function and Fermi level position between the graphene layers grown on the Si-terminated and C-terminated substrates. [DOI: 10.1380/ejssnt.2008.107]
45 citations
TL;DR: In this article, the authors used a strained GaAs film grown on relaxed In0.1Ga0.9As buffer layers to increase the frequency of the fundamental mode by applying tensile strain.
Abstract: Micromechanical-beam resonators were fabricated using a strained GaAs film grown on relaxed In0.1Ga0.9As∕In0.1Al0.9As buffer layers. The natural frequency of the fundamental mode was increased 2.5–4 times by applying tensile strain, showing good agreement with the model calculation assuming strain of 0.35% along the beam. In addition, the Q factor of 19 000 was obtained for the best sample, which is one order of magnitude higher than that for the unstrained resonator. This technique can be widely applied for improving the performance of resonator-based micro-/nanoelectromechanical devices.
41 citations
TL;DR: In this paper, the spin-state transition in perovskite Co oxide was observed in magnetic fields up to 53 T. The spin state transition is assisted by an internal field, originating from ferromagnetic exchange interaction between ions.
Abstract: High field magnetization and high-frequency electron spin resonance measurements on the perovskite Co oxide ${\text{Sr}}_{1\ensuremath{-}x}{\text{Y}}_{x}{\text{CoO}}_{3\ensuremath{-}\ensuremath{\delta}}$ ($x=0.22$ and 0.25) have been performed in magnetic fields up to 53 T. We have observed the spin-state transition, induced by the external magnetic fields, for the sample with $x=0.25$. A distinctive feature of this field-induced spin-state transition is that it is assisted by an internal field, originating from ferromagnetic exchange interaction between ${\text{Co}}^{3+}$ ions.
34 citations
32 citations
TL;DR: The spectral purity of an electromechanical resonance (quality factor Q) can be enhanced by more than one order of magnitude to ∼2.5×106 by parametrically exciting the fundamental mode via double frequency modulations to the spring constant mediated by the piezoelectric effect as mentioned in this paper.
Abstract: The spectral purity of an electromechanical resonance (quality factor Q) can be enhanced by more than one order of magnitude to ∼2.5×106 by parametrically exciting the fundamental mode via double frequency modulations to the spring constant mediated by the piezoelectric effect. We theoretically demonstrate that this Q enhancement is an intrinsic property of the parametric resonator by solving the equation of motion for this system. The Q enhancement demonstrated here results in smaller perturbations in the resonance (∼0.05Hz) becoming visible which enables the charge sensitivity of the electromechanical oscillator to improve by a factor equal to the Q enhancement.
31 citations
TL;DR: It is suggested that the anti-cholinergic effects of disopyramide can induce MG crisis and should therefore be carefully considered when disopyramside is used to treat AF in patients with MG.
Abstract: Class 1a anti-arrhythmic drugs are often used for the treatment of atrial fibrillation (AF), but it is not well known that myasthenia gravis (MG)-like symptoms can be generated by their anti-cholinergic effects. We had a patient with MG who developed symptomatic MG aggravation after AF treatment with disopyramide. Symptomatic MG aggravation was followed by Takotsubo-shaped cardiomyopathy, QT prolongation, and Torsades de Pointes. We suggest that the anti-cholinergic effects of disopyramide can induce MG crisis and should therefore be carefully considered when disopyramide is used to treat AF in patients with MG.
31 citations
TL;DR: In this paper, an electrometer based on field-effect transistors (FETs) was fabricated on a silicon-on-insulator substrate (SOI) for high charge sensitivity, 0.0013 e/√Hz at 1 Hz, at room temperature.
Abstract: An electrometer based on field-effect transistors (FETs) was fabricated on a silicon-on-insulator substrate (SOI). The electrometer has a nanometer-scale small channel and a capacitively coupled node, where single electrons are stored. We discuss the dependence of the charge sensitivity of the electrometer on its structure and on its operation condition and gives guides for achieving the higher charge sensitivity. The device optimization based on this dependence allows the demonstration of the electrometer with extremely high charge sensitivity, 0.0013 e/√Hz at 1 Hz, at room temperature.
TL;DR: In this paper, high-field electron spin resonance (ESR) measurements in magnetic fields up to about 53 T on the quasi-two-dimensional triangular-lattice antiferromagnet were reported.
Abstract: We report experimental results of high-field electron spin resonance (ESR) measurements in magnetic fields up to about 53 T on the quasi-two-dimensional triangular-lattice antiferromagnet ${\text{NiGa}}_{2}{\text{S}}_{4}$. From the temperature evolution of the ESR absorption linewidth, we find three distinct temperature regions: those (i) above 23 K, (ii) between 23 K and 8.5 K, and (iii) below 8.5 K. The linewidth is affected by the dynamics of ${Z}_{2}$ vortices below 23 K and one of the conventional spiral resonance modes explains well the frequency dependence of the ESR resonance fields far below 8.5 K. Furthermore, we found an anomaly in the magnetization curve around one third of the saturation magnetization for $H\ensuremath{\perp}c$, corresponding to softening of another resonance mode. These results suggest the occurrence of a ${Z}_{2}$ vortex-induced topological transition at 8.5 K.
TL;DR: In this article, an on-chip cooling protocol for a micro-mechanical resonator by employing a superconducting flux qubit is described. But the authors do not specify the response time of passive back-action of the resonator's displacement.
Abstract: Using a semi-classical approach, we describe an on-chip cooling protocol for a micro-mechanical resonator by employing a superconducting flux qubit. A Lorentz force, generated by the passive back-action of the resonator's displacement, can cool down the thermal motion of the mechanical resonator by applying an appropriate microwave drive to the qubit. We show that this on-chip cooling protocol, with well-controlled cooling power and a tunable response time of passive back-action, can be highly efficient. With feasible experimental parameters, the effective mode temperature of a resonator could be cooled down by several orders of magnitude.
TL;DR: Time-resolved Kerr rotation spectroscopy under the radio frequency field to depolarize dynamic nuclear polarization reveals the intrinsic spin-relaxation time $({T}_{2}^{\ensuremath{\ast}})$ and $g$ factor of two-dimensional electrons in a quantum Hall system as mentioned in this paper.
Abstract: Time-resolved Kerr rotation spectroscopy under the radio frequency field to depolarize dynamic nuclear polarization reveals the intrinsic spin-relaxation time $({T}_{2}^{\ensuremath{\ast}})$ and $g$ factor of two-dimensional electrons in a quantum Hall system Out-of-plane magnetic field increases the spin coherence drastically through the Landau level quantization ${T}_{2}^{\ensuremath{\ast}}$ is enhanced strongly around odd filling factors where a quantum Hall ferromagnet is formed Collapse of spin coherence and appearance of an anomalous Kerr signal observed around $\ensuremath{
u}=1$ are discussed in the relation to the formation of Skyrmions
TL;DR: The in-plane conductance of individual graphene nanoislands thermally grown on SiC substrate was successfully measured using an integrated nanogap probe without lithographic patterning and the size dependence of the conductance suggests that the band gap opening is due to the lateral confinement effect.
Abstract: The in-plane conductance of individual graphene nanoislands thermally grown on SiC substrate was successfully measured using an integrated nanogap probe without lithographic patterning. A Pt nanogap electrode with a 30 nm gap integrated on the cantilever tip of a scanning probe microscope enables us to image a conductance map of graphene nanoislands with nanometer resolution. Single- and double-layer graphene islands are clearly distinguished in the conductance image. The size dependence of the conductance of the nanoislands suggests that the band gap opening is due to the lateral confinement effect.
TL;DR: It is demonstrated how magnetically coupling a nanomechanical resonator to a double quantum dot confining two electrons can enable the manipulation of a single electron spin and the readout of the resonator's natural frequency.
Abstract: We demonstrate how magnetically coupling a nanomechanical resonator to a double quantum dot confining two electrons can enable the manipulation of a single electron spin and the readout of the resonator's natural frequency. When the Larmor frequency matches the resonator frequency, the electron spin in one of the dots can be selectively and coherently flipped by the magnetized oscillator. By simultaneously measuring the charge state of the two-electron double quantum dots, this transition can be detected thus enabling the natural frequency and displacement of the mechanical oscillator to be determined.
TL;DR: In this paper, the damping of a GaAs microcantilever strongly depends on the temperature and it is minimized at around 50 K. This temperature dependence is mainly caused by the thermoelastic damping effect, which is generated by the strain-induced temperature gradient in the GaAs cantilever.
Abstract: Damping of the mechanical motion is one of the factors that limit the sensitivity of mechanical resonators. For the development of high-sensitivity transducers, it is important to understand the damping mechanism of the resonators and minimize the damping effect. Here, we show that the damping of a GaAs microcantilever strongly depends on the temperature and it is minimized at around 50 K. This temperature dependence of the damping is mainly caused by the thermoelastic damping effect, which is generated by the strain-induced temperature gradient in the GaAs cantilever. The minimization of the mechanical damping at around 50 K is due to the suppression of the thermal expansion of GaAs at this temperature, which eliminates the temperature gradient in the cantilever. This result indicates that the sensitivity of GaAs resonators can be improved by adjusting the operating temperature. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, a 3D alignment with 10nm order accuracy in 3D electron-beam (EB) lithography has been achieved by means of highly accurate rotation control and mark location using the transmission electron signal.
Abstract: Three-dimensional (3D) alignment with 10nm order accuracy in 3D electron-beam (EB) lithography has been achieved by means of highly accurate rotation control and mark location using the transmission electron signal Accurately aligned EB writing from various directions on micron order resists blocks on a small substrate provides great structural flexibility in the creation of 3D nanostructures As a demonstration of the accuracy, a 3D hydrogen silsesquioxane nanostructure was made by 3D EB writing and two-step development using different developers Moreover, a 3D poly(methyl methacrylate) nanostructure was made by repeated 3D EB writing and development to demonstrate 3D nanofabrication with great structural flexibility
TL;DR: In this paper, a 3D nanostructure in poly(methyl methacrylate) (PMMA) was constructed using electron-beam (EB) writing and development, which demonstrates the great structural flexibility obtainable in EB technique.
Abstract: Nanofabrication of three-dimensional (3D) structures with a high aspect ratio has been achieved using electron-beam (EB) lithography Although electron scattering, or the proximity effect, on the remaining parts in positive resist is generally serious for repeated EB writing from different 3D directions, we can largely avoid the effect by adding appropriate surrounding buffer regions, which are cut off by the following EB writing and development This enables us to make a 3D nanostructure in poly(methyl methacrylate) (PMMA), which demonstrates the great structural flexibility obtainable in our EB technique
01 Mar 2008
TL;DR: In this paper, the first measurement of spatially resolved in-plane conductance of few-layer (one or two-layer) graphene grown on a SiC substrate, measured using an integrated nanogap probe was reported.
Abstract: We report the first measurement of spatially resolved in-plane conductance of few-layer (one- or two-layer) graphene grown on a SiC substrate, measured using an integrated nanogap probe. The morphology and number of layers of the thermally grown graphene were confirmed by in-situ observation using low energy electron microscopy (LEEM). The gap current (conductance) images were measured using an integrated nanogap probe with a 30-nm-gap on a conventional SPM system in vacuum. Island shapes with a typical width of 30 nm were clearly observed in the conductance image. Single- and double-layer graphene islands could be clearly distinguished, because the conductance of double-layer graphene is about four times that of single-layer graphene. The layer number of few-layer graphene has been successfully estimated from the electrical transport measurement using the integrated nanogap probe.
TL;DR: In this article, the authors investigated the height dependence of Young's modulus for carbon nanopillars grown by focused ion-beam-induced chemical vapor deposition (FIB-CVD) using phenanthrene gas as a source material.
Abstract: We investigated the height dependence of the Young's modulus for carbon nanopillars grown by focused-ion-beam-induced chemical vapor deposition (FIB-CVD) using phenanthrene gas as a source material Carbon nanopillars of different heights were grown by FIB-CVD at various growth times and a fixed ion-beam focal point The growth heights ranged from 36 to 356 µm The diameters at the bottoms of the pillars were nearly the same, 100 nm, and it increased as the growth progressed Young's modulus of the carbon nanopillars was evaluated from resonant frequency for mechanical vibration using uniform and nonuniform models Young's moduli differed for different growth heights for both of the models
02 Jul 2008
TL;DR: In this paper, an electrometer based on field-effect transistors (FETs) was fabricated on a silicon-on-insulator substrate (SOI) for high charge sensitivity, 0.0013 e/√Hz at 1 Hz, at room temperature.
Abstract: An electrometer based on field-effect transistors (FETs) was fabricated on a silicon-on-insulator substrate (SOI). The electrometer has a nanometer-scale small channel and a capacitively coupled node, where single electrons are stored. We discuss the dependence of the charge sensitivity of the electrometer on its structure and on its operation condition and gives guides for achieving the higher charge sensitivity. The device optimization based on this dependence allows the demonstration of the electrometer with extremely high charge sensitivity, 0.0013 e/√Hz at 1 Hz, at room temperature.
TL;DR: Tsubaki et al. as mentioned in this paper investigated the magnetic field distribution around CNTs current path by magnetic force microscopy (MFM) under the consideration of the magnetic properties of magnetically coated tip of MFM, tip heights, current directions, and background magnetic field, etc.
Abstract: Recently carbon nanotubes (CNTs) are reported to be able to generate large magnetic field because of their nanometer-size-diameter [K. Tsubaki, H. Yamaguchi, J. Phys. C 38 (2006) 49]. The magnetic fields around CNTs current path are investigated by magnetic force microscopy (MFM). Under the consideration of the magnetic properties of magnetically coated tip of MFM, tip heights, current directions, and background magnetic field, etc., the magnetic field distribution are analyzed. The distribution of the magnetic field generated by the CNTs current is found to be asymmetric, and its distribution anomaly is found to be a kind of hysteresis effect of the MFM cantilever materials.
TL;DR: In this paper, an electron-emission cathode with a Si/SiO2/Si structure using metal-oxide-semiconductor field effect transistor (MOSFET) fabrication technology was fabricated.
Abstract: We fabricated an electron-emission cathode with a Si/SiO2/Si structure using metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication technology. Electrons travel from an electron-source Si layer to a thin polycrystalline Si (poly-Si) through a thin SiO2 and some of them with high energy, i.e., hot electrons, are emitted from the poly-Si surface. By utilizing an electron-inversion layer as the electron source, high efficiency and stability of electron emission were achieved. Material decomposition at a depth of a few nanometers was also demonstrated using a low-energy-electron irradiation from the cathode operated in a low vacuum condition.
25 Sep 2008
TL;DR: In this paper, the effect of trion formation processes via trion excited states on spin orientation in quantum dots was discussed and the degree of circular polarization of the luminescence emitted from a negative trion strongly depends on the excitation energy at a specific bias voltage.
Abstract: Spin selective optical excitation using polarizationresolved photoluminescence spectroscopy is demonstrated in a single charge-tunable GaAs quantum dot. The degree of circular polarization of the luminescence emitted from a negative trion strongly depends on the excitation energy at a specific bias voltage. We discuss the effect of the trion formation processes via trion excited states on spin orientation in quantum dots. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Proceedings Article•
02 Jul 2008TL;DR: In this paper, an electrometer based on field-effect transistors (FETs) was fabricated on a silicon-on-insulator substrate (SOI) for high charge sensitivity, 0.0013 e/√Hz at 1 Hz, at room temperature.
Abstract: An electrometer based on field-effect transistors (FETs) was fabricated on a silicon-on-insulator substrate (SOI). The electrometer has a nanometer-scale small channel and a capacitively coupled node, where single electrons are stored. We discuss the dependence of the charge sensitivity of the electrometer on its structure and on its operation condition and gives guides for achieving the higher charge sensitivity. The device optimization based on this dependence allows the demonstration of the electrometer with extremely high charge sensitivity, 0.0013 e/√Hz at 1 Hz, at room temperature.
TL;DR: In this paper, the magnetic properties of a micro-cantilever integrating a GaMnAs Hall bar have been investigated and it was shown that the magnetic field dependence of its mechanical resonance frequency below the Curie temperature is remarkably different from that above the curie temperature.
Abstract: To study the magnetization properties of a micron-sized GnMnAs film, we characterize the mechanical properties of a micro-cantilever integrating a GaMnAs Hall bar. The magnetic field dependence of its mechanical resonance frequency below the Curie temperature is remarkably different from that above the Curie temperature. The frequency shift, which reflects the field-induced Mn spin rotation, is mainly caused by the magnetoelastic coupling. This result demonstrates that a micromechanical cantilever is a powerful tool for investigating the magnetic properties of micrometer-sized ferromagnetic materials. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
23 Jun 2008
TL;DR: In this article, the authors have fabricated micro-electromechanical parametric resonators based on the piezoelectricity in GaAs/AlGaAs modulation-doped heterostructures.
Abstract: Piezoelectricity has been playing an important role in recent micro- and nano-electromechanical devices, where it makes possible a highly efficient transaction between mechanical motion and electric signals. Compound semiconductors are known as piezoelectric materials and highly functional micro- and nano-electromechanical devices can therefore be integrated with electronic and optical devices. We have fabricated micro-electromechanical parametric resonators based on the piezoelectricity in GaAs/AlGaAs modulation-doped heterostructures. Three important electromechanical operations - the actuation and the detection of mechanical motion, as well as the resonant frequency modulation - have all been successfully achieved on chip using a piezoelectricity for the first time. In addition, the so-called parametric resonance, which has been shown to have benefits, such like signal amplification and force-sensitivity improvement, was also shown to be electrically excited using the functionality of resonant frequency modulation.