Showing papers by "University of Electro-Communications published in 2008"

Unconventional Pairing Originating from the Disconnected Fermi Surfaces of Superconducting LaFeAsO 1-x F x

Abstract: For a newly discovered iron-based high ${T}_{c}$ superconductor ${\mathrm{LaFeAsO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$, we have constructed a minimal model, where inclusion of all five Fe $d$ bands is found to be necessary. The random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin-fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended $s$-wave pairing, while $d$-wave pairing can also be another candidate.

Magnetization vector manipulation by electric fields

Abstract: Conventional semiconductor devices use electric fields to control conductivity, a scalar quantity, for information processing. In magnetic materials, the direction of magnetization, a vector quantity, is of fundamental importance. In magnetic data storage, magnetization is manipulated with a current-generated magnetic field (Oersted-Ampere field), and spin current is being studied for use in non-volatile magnetic memories. To make control of magnetization fully compatible with semiconductor devices, it is highly desirable to control magnetization using electric fields. Conventionally, this is achieved by means of magnetostriction produced by mechanically generated strain through the use of piezoelectricity. Multiferroics have been widely studied in an alternative approach where ferroelectricity is combined with ferromagnetism. Magnetic-field control of electric polarization has been reported in these multiferroics using the magnetoelectric effect, but the inverse effect-direct electrical control of magnetization-has not so far been observed. Here we show that the manipulation of magnetization can be achieved solely by electric fields in a ferromagnetic semiconductor, (Ga,Mn)As. The magnetic anisotropy, which determines the magnetization direction, depends on the charge carrier (hole) concentration in (Ga,Mn)As. By applying an electric field using a metal-insulator-semiconductor structure, the hole concentration and, thereby, the magnetic anisotropy can be controlled, allowing manipulation of the magnetization direction.

Attribute-based encryption with partially hidden encryptor-specified access structures

Abstract: We propose attribute-based encryption schemes where encryptor-specified access structures (also called ciphertext policies) are hidden By using our schemes, an encryptor can encrypt data with a hidden access structure A decryptor obtains her secret key associated with her attributes from a trusted authority in advance and if the attributes associated with the decryptor's secret key do not satisfy the access structure associated with the encrypted data, the decryptor cannot decrypt the data or guess even what access structure was specified by the encryptor We prove security of our construction based on the Decisional Bilinear Diffie-Hellman assumption and the Decision Linear assumption In our security notion, even the legitimate decryptor cannot obtain the information about the access structure associated with the encrypted data more than the fact that she can decrypt the data

Accurate retrieval of structural information from laser-induced photoelectron and high-order harmonic spectra by few-cycle laser pulses.

Abstract: By analyzing accurate theoretical results from solving the time-dependent Schrodinger equation of atoms in few-cycle laser pulses, we established the general conclusion that laser-generated high-energy electron momentum spectra and high-order harmonic spectra can be used to extract accurate differential elastic scattering and photo-recombination cross sections of the target ion with free electrons, respectively. Since both electron scattering and photoionization (the inverse of photo-recombination) are the conventional means for interrogating the structure of atoms and molecules, this result implies that existing few-cycle infrared lasers can be implemented for ultrafast imaging of transient molecules with temporal resolution of a few femtoseconds.

D-Wave Collapse and Explosion of a Dipolar Bose-Einstein Condensate

Abstract: We investigate the collapse dynamics of a dipolar condensate of 52 Cr atoms when thes-wave scattering length characterizing the contact interaction is reduced below a critical value. A complex dynamics, involving an anisotropic, d-wave symmetric explosion of the condensate, is observed. The atom number decreases abruptly during the collapse. We find good agreement between our experimental results and those of a numerical simulation of the three-dimensional Gross-Pitaevskii equation, including contact and dipolar interactions as well as three-body losses. The simulation indicates that the collapse induces the formation of two vortex rings with opposite circulations.

Elliptic-Curve-Based Security Processor for RFID

Abstract: RFID (radio frequency identification) tags need to include security functions, yet at the same time their resources are extremely limited. Moreover, to provide privacy, authentication and protection against tracking of RFID tags without loosing the system scalability, a public-key based approach is inevitable, which is shown by M. Burmester et al. In this paper, we present an architecture of a state-of-the-art processor for RFID tags with an elliptic curve (EC) processor over GF(2163). It shows the plausibility of meeting both security and efficiency requirements even in a passive RFID tag. The proposed processor is able to perform EC scalar multiplications as well as general modular arithmetic (additions and multiplications) which are needed for the cryptographic protocols. As we work with large numbers, the register file is the most critical component in the architecture. By combining several techniques, we are able to reduce the number of registers from 9 to 6 resulting in EC processor of 10.1 K gates. To obtain an efficient modulo arithmetic, we introduce a redundant modular operation. Moreover the proposed architecture can support multiple cryptographic protocols. The synthesis results with a 0.13 um CMOS technology show that the gate area of the most compact version is 12.5 K gates.

Giant Coercivity in a One-Dimensional Cobalt-Radical Coordination Magnet

Abstract: A metal-radical polymer [Co(hfac)2·BPNN] showed a very large coercive field of 52 kOe (4.1 MA m-1) at 6 K, indicating that it is the hardest magnet ever reported. Above 10 K, a soft character appeared, owing to the fast dynamics of magnetization reorientation.

Algorithm 883: SparsePOP---A Sparse Semidefinite Programming Relaxation of Polynomial Optimization Problems

Abstract: SparsePOP is a Matlab implementation of the sparse semidefinite programming (SDP) relaxation method for approximating a global optimal solution of a polynomial optimization problem (POP) proposed by Waki et al. [2006]. The sparse SDP relaxation exploits a sparse structure of polynomials in POPs when applying “a hierarchy of LMI relaxations of increasing dimensions” Lasserre [2006]. The efficiency of SparsePOP to approximate optimal solutions of POPs is thus increased, and larger-scale POPs can be handled.

An Incremental Learning Method for Unconstrained Gaze Estimation

Abstract: This paper presents an online learning algorithm for appea- rance-based gaze estimation that allows free head movement in a casual desktop environment. Our method avoids the lengthy calibration stage using an incremental learning approach. Our system keeps running as a background process on the desktop PC and continuously updates the estimation parameters by taking user's operations on the PC monitor as input. To handle free head movement of a user, we propose a pose-based clustering approach that efficiently extends an appearance manifold model to handle the large variations of the head pose. The effectiveness of the proposed method is validated by quantitative performance evaluation with three users.

Experimental retrieval of target structure information from laser-induced rescattered photoelectron momentum distributions.

Abstract: We have measured two-dimensional photoelectron momentum spectra of Ne, Ar, and Xe generated by 800-nm, 100-fs laser pulses and succeeded in identifying the spectral ridge region (back-rescattered ridges) which marks the location of the returning electrons that have been backscattered at their maximum kinetic energies. We demonstrate that the structural information, in particular the differential elastic scattering cross sections of the target ion by free electrons, can be accurately extracted from the intensity distributions of photoelectrons on the ridges, thus effecting a first step toward laser-induced self-imaging of the target, with unprecedented spatial and temporal resolutions.

Augmented Stable Fuzzy Control for Flexible Robotic Arm Using LMI Approach and Neuro-Fuzzy State Space Modeling

Abstract: Designing the control strategy for a flexible robotic arm has long been considered a complex problem as it requires stabilizing the vibration simultaneously with the primary objective of position control. A stable state-feedback fuzzy controller is proposed here for such a flexible arm. The controller is designed on the basis of a neuro-fuzzy state-space model that is successfully trained using the experimental data acquired from a real robotic arm. The complex problem of solving stability conditions is taken care of by recasting them in the form of linear matrix inequalities and then solving them using a popular interior-point-based method. This asymptotically stable fuzzy controller is further augmented to provide enhanced transient performance along with maintaining the excellent steady-state performance shown by the stable control strategy. The controller hence designed has been successfully implemented for a real robotic arm to operate over a long angular range of 180 with several payload conditions and, for situations where the system is operated for a long range and with a large variation in payload conditions, it could successfully outperform the recently proposed proportional derivative and strain controller.

Effect of deformation temperature on microstructure evolution in aluminum alloy 2219 during hot ECAP

Abstract: The effect of deformation temperature on microstructure evolution during equal channel angular pressing (ECAP) was studied in a coarse-grained aluminum alloy 2219 in a wide temperature interval from 250 to 475 °C. The structural changes taking place during ECAP up to strains of 12 are classified into the following three stages irrespective of deformation temperatures: i.e. (1) an incubation period for formation of the embryos of deformation bands (DBs) at low strains; (2) development of large-scale DBs followed by grain fragmentation at moderate strains; (3) rapid development of new grain at high strains. Microstructure development in stages 1 and 2 is hardly influenced by temperature, while that in stage 3 is most significantly affected at higher temperature. An increase in the pressing temperature leads to decreasing the volume fraction of new grains and increasing the average grain size in stage 3. This can be attributed to relaxation of strain compatibility between grains due to frequent operation of dynamic recovery and grain boundary sliding at higher temperature. The mechanism of grain refinement is discussed in detail.

Ultrafine Grain Formation in Ferritic Stainless Steel during Severe Plastic Deformation

Abstract: The development of submicrocrystalline structures in Fe-20 pct Cr ferritic stainless steel was studied in multidirectional forging to large total strains. The structural changes are characterized by the development of microshear bands in high density dislocation substructures. The multidirectional deformation promotes the multiple shearing, which results in the formation of a spatial net of mutually crossed microshear bands subdividing the original grains. The new grains with high-angle boundaries appear primarily at the microshear band intersections and subsequently along the bands. The fraction of ultrafine grains gradually increases with increasing the density of microshear bands as a result of continuous increase in misorientations among deformation subgrains during processing. An increase in the processing temperature can accelerate remarkably the kinetics of ultrafine grain evolution at large strains. The mechanism of strain-induced grain formation is discussed in detail.

Coherent optical frequency transfer over 50-km physical distance using a 120-km-long installed telecom fiber network.

Abstract: Optical frequency at 1542 nm was coherently transferred over a 120-km-long installed telecom fiber network between two cities (Tsukuba and Tokyo) in Japan separated by more than 50 km. The phase noise induced by the fiber length fluctuations was actively reduced by using a fiber stretcher and an acousto-optic modulator. The fractional frequency instability of the one-way transmitted light was reduced down to less than 8.0 x 10(-16) at an averaging time of 1s, which is limited by the theoretical limit deduced from the length and the intrinsic noise of the fiber.

Grasping force control of multi-fingered robot hand based on slip detection using tactile sensor

Abstract: To achieve a human like grasping with a multi- fingered robot hand, the grasping force should be controlled without using information from the grasped object such as its weight and friction coefficient. In this study, we propose a method for detecting the slip of a grasped object using the force output of Center of Pressure (CoP) tactile sensors. CoP sensors can measure the center position of a distributed load and the total load applied on the surface of the sensor, within 1 ms. These sensors are arranged on the fingers of the robot hand, and their effectiveness as slip detecting sensors is confirmed in tests of slip detection during grasping. Finally, we propose a method for controlling grasping force to resist tangential force applied to the grasped object using a feedback control system with the CoP sensor force output.

Control of domain wall position by electrical current in structured Co/Ni wire with perpendicular magnetic anisotropy

Abstract: We report the direct observation of the current-driven domain wall (DW) motion by magnetic force microscopy in a structured Co/Ni wire with perpendicular magnetic anisotropy. The wire has notches to define the DW position. It is demonstrated that single current pulses can precisely control the DW position from notch to notch with high DW velocity of 40 m/s.

Continuous static recrystallization in ultrafine-grained copper processed by multi-directional forging

Abstract: Annealing processes in copper, processed by multi-directional forging to strains of ɛ = 0.4–6.0 at 300 K, were studied at 503–573 K. Strain-induced ultrafine-grained copper shows mainly grain coarsening behaviour, which is categorized in three stages, i.e. (1) an incubation period, (2) a rapid and limited grain growth and finally (3) a classical (normal) grain growth. That is, in situ or continuous static recrystallization (cSRX) takes place in stages 1 and 2. The annealing characteristics and the mechanisms of cSRX are discussed with reference to those of conventional discontinuous SRX (dSRX).

Electronic Structure and Electron Correlation in LaFeAsO 1- x F x and LaFePO 1- x F x

Abstract: Photoemission spectroscopy is used to investigate the electronic structure of the newly discovered iron-based superconductors LaFeAsO 1- x F x and LaFePO 1- x F x . Line shapes of the Fe 2 p core-level spectra suggest an itinerant character of Fe 3 d electrons. The valence-band spectra are generally consistent with band-structure calculations except for the shifts of Fe 3 d -derived peaks toward the Fermi level. From the spectra taken in the Fe 3 p → 3 d core-absorption region, we have obtained the experimental Fe 3 d partial density of states, and explained it in terms of a band-structure calculation with a phenomenological self-energy correction, yielding a mass renormalization factor of \({\lesssim}2\).

Switching magnetic vortex core by a single nanosecond current pulse

Abstract: In a ferromagnetic nanodisk, the magnetization tends to swirl around in the plane of the disk and can point either up or down at the center of this magnetic vortex. This binary state can be useful for information storage. It is demonstrated that a single nanosecond current pulse can switch the core polarity. This method also provides the precise control of the core direction, which constitutes fundamental technology for realizing a vortex core memory.

Diode-pumped 65 fs Kerr-lens mode-locked Yb(3+):Lu(2)O(3) and nondoped Y(2)O(3) combined ceramic laser.

Abstract: Diode-pumped Kerr-lens mode-locked laser operation of Yb3+:Lu2O3 and nondoped Y2O3 combined ceramics has been achieved; 65 fs pulses with an average power of 320 mW under 5W of pump power were obtained at the center wavelength of 1032 nm. The spectral bandwidth and the time bandwidth product were 18.9 and 0.345 nm, respectively. To our knowledge, this is the first demonstration of a Kerr-lens mode-locked laser operation based on Yb3+:Lu2O3 ceramic.

Large-Angle Electron Diffraction Structure in Laser-Induced Rescattering from Rare Gases

Abstract: We have measured full momentum images of electrons rescattered from Xe, Kr, and Ar following the liberation of the electrons from these atoms by short, intense laser pulses. At high momenta the spectra show angular structure (diffraction) which is very target dependent and in good agreement with calculated differential cross sections for the scattering of free electrons from the corresponding ionic cores.

Extraction of the species-dependent dipole amplitude and phase from high-order harmonic spectra in rare-gas atoms

Abstract: Based on high-order harmonic generation (HHG) spectra obtained from solving the time-dependent Schr\"odinger equation for atoms, we established quantitatively that the HHG yield can be expressed as the product of a returning electron wave packet and photorecombination cross sections, and the shape of the returning wave packet is shown to be largely independent of the species. By comparing the HHG spectra generated from different targets under identical laser pulses, accurate structural information, including the phase of the recombination amplitude, can be retrieved. This result opens up the possibility of studying the target structure of complex systems, including their time evolution, from the HHG spectra generated by short laser pulses.

Compact electron beam ion trap for spectroscopy of moderate charge state ions.

Abstract: A compact electron beam ion trap (EBIT) has been constructed for spectroscopic studies of moderate charge state ions. The electron beam energy range of the present EBIT is 100–1000eV, for which it is rather difficult to operate an ordinary EBIT which used to be designed for operation with higher electron energy (∼10keV or more). To cut down the running costs, a superconducting wire with a high critical temperature is used for the central magnet so that it can be operated without liquid helium. The performance of the compact EBIT has been investigated through visible spectroscopy of highly charged krypton and iron ions.

Exchange Coupling and Energy-Level Crossing in a Magnetic Chain [Dy2Cu2]n Evaluated by High-Frequency Electron Paramagnetic Resonance

Abstract: A 4f−3d heterometallic polymeric coordination compound [{Dy(hfac)2(CH3OH)}2{Cu(dmg)(Hdmg)}2]n ([Dy2Cu2]n; H2dmg = dimethylglyoxime; Hhfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) was synthesized, and the X-ray crystallographic analysis shows that the structure is isomorphous to the known ferrimagnetic [Gd2Cu2]n polymer A centrosymmetric diamond-arrayed ferrimagnetic unit involving the oximate bridge, Dy−O−N−Cu, is repeated to form a discrete chain An onset of the frequency dependence was found in the ac magnetic susceptibility measurements down to 18 K Low-temperature magnetization measurements on [Dy2Cu2]n exhibited magnetic hysteresis with magnetization steps To examine the energy level structure and the exchange coupling between the Dy and Cu ions, high-frequency EPR (HF-EPR) spectra of a polycrystalline sample of [Dy2Cu2]n were recorded at various frequencies (347−5254 GHz) and temperatures We analyzed the spectra by treating the Dy moments as Ising spins and built an exchange-coupling model

Octave-spanning Raman comb with carrier envelope offset control.

Abstract: We report that adiabatic manipulation of a Raman process allows us to produce an optical-frequency comb from single-frequency lasers. We realize an octave-spanning Raman comb with carrier-envelope-offset frequency control, by using dual-frequency laser radiation locked on a single laser cavity and simultaneously its second harmonic. It is shown that in both the temporal and spectral domains, locking the dual frequencies on a single laser cavity provides control of the carrier-envelope-offset frequency at integer multiples of the free spectral range of the laser cavity.