Showing papers by "University of Electro-Communications published in 2010"

Unconventional s-wave superconductivity in Fe(Se,Te).

Abstract: The superconducting state is characterized by a pairing of electrons with a superconducting gap on the Fermi surface. In iron-based superconductors, an unconventional pairing state has been argued for theoretically. We used scanning tunneling microscopy on Fe(Se,Te) single crystals to image the quasi-particle scattering interference patterns in the superconducting state. By applying a magnetic field to break the time-reversal symmetry, the relative sign of the superconducting gap can be determined from the magnetic-field dependence of quasi-particle scattering amplitudes. Our results indicate that the sign is reversed between the hole and the electron Fermi-surface pockets ( s ± -wave), favoring the unconventional pairing mechanism associated with spin fluctuations.

Fault sensitivity analysis

Abstract: This paper proposes a new fault-based attack called the Fault Sensitivity Analysis (FSA) attack, which unlike most existing fault-based analyses including Differential Fault Analysis (DFA) does not use values of faulty ciphertexts. Fault sensitivity means the critical condition when a faulty output begins to exhibit some detectable characteristics, e.g., the clock frequency when fault operation begins to occur. We explain that the fault sensitivity exhibits sensitive-data dependency and can be used to retrieve the secret key. This paper presents two practical FSA attacks against two AES hardware implementations on SASEBO-R, PPRM1-AES and WDDL-AES. Different from previous work, we show that WDDL-AES is not perfectly secure against setup-time violation attacks. We also discuss a masking technique as a potential countermeasure against the proposed fault-based attack.

Strong-field rescattering physics: self—imaging of a molecule by its own electrons

Abstract: When an atom or molecule is exposed to a short intense laser pulse, electrons that were removed at an earlier time may be driven back by the oscillating electric field of the laser to recollide with the parent ion, to incur processes like high-order harmonic generation (HHG), high-energy above-threshold ionization (HATI) and nonsequential double ionization (NSDI). Over the years, a rescattering model (the three-step model) has been used to understand these strong field phenomena qualitatively, but not quantitatively. Recently we have established such a quantitative rescattering (QRS) theory. According to QRS, the yields for HHG, HATI and NSDI can be expressed as the product of a returning electron wave packet with various field-free electron–ion scattering cross sections, namely photo-recombination, elastic electron scattering and electron-impact ionization, respectively. The validity of QRS is first demonstrated by comparing with accurate numerical results from solving the time-dependent Schrodinger equation (TDSE) for atoms. It is then applied to atoms and molecules to explain recent experimental data. According to QRS, accurate field-free electron scattering and photoionization cross sections can be obtained from the HATI and HHG spectra, respectively. These cross sections are the conventional tools for studying the structure of a molecule; thus, QRS serves to provide the required theoretical foundation for the self-imaging of a molecule in strong fields by its own electrons. Since infrared lasers of duration of a few femtoseconds are readily available today, these results imply that they are suitable for probing the dynamics of molecules with temporal resolutions of a few femtoseconds.

A simple and faster branch-and-bound algorithm for finding a maximum clique

Abstract: This paper proposes new approximate coloring and other related techniques which markedly improve the run time of the branch-and-bound algorithm MCR (J. Global Optim., 37, 95–111, 2007), previously shown to be the fastest maximum-clique-finding algorithm for a large number of graphs. The algorithm obtained by introducing these new techniques in MCR is named MCS. It is shown that MCS is successful in reducing the search space quite efficiently with low overhead. Consequently, it is shown by extensive computational experiments that MCS is remarkably faster than MCR and other existing algorithms. It is faster than the other algorithms by an order of magnitude for several graphs. In particular, it is faster than MCR for difficult graphs of very high density and for very large and sparse graphs, even though MCS is not designed for any particular type of graphs. MCS can be faster than MCR by a factor of more than 100,000 for some extremely dense random graphs.

Coherent transfer of photoassociated molecules into the rovibrational ground state

Abstract: We report on the direct conversion of laser-cooled $^{41}\mathrm{K}$ and $^{87}\mathrm{Rb}$ atoms into ultracold $^{41}\mathrm{K}^{87}\mathrm{Rb}$ molecules in the rovibrational ground state via photoassociation followed by stimulated Raman adiabatic passage. High-resolution spectroscopy based on the coherent transfer revealed the hyperfine structure of weakly bound molecules in an unexplored region. Our results show that a rovibrationally pure sample of ultracold ground-state molecules is achieved via the all-optical association of laser-cooled atoms, opening possibilities to coherently manipulate a wide variety of molecules.

Controlling phase separation of binary Bose-Einstein condensates via mixed-spin-channel Feshbach resonance

Abstract: We investigate controlled phase separation of a binary Bose-Einstein condensate in the proximity of a mixed-spin-channel Feshbach resonance in the $|F=1,{m}_{F}=+1\ensuremath{\rangle}$ and $|F=2,{m}_{F}=\ensuremath{-}1\ensuremath{\rangle}$ states of $^{87}\mathrm{Rb}$ at a magnetic field of 9.10 G. Phase separation occurs on the lower-magnetic-field side of the Feshbach resonance while the two components overlap on the higher-magnetic-field side. The Feshbach resonance curve of the scattering length is obtained from the shape of the atomic cloud by comparison with the numerical analysis of coupled Gross-Pitaevskii equations.

Measurement of Universal Thermodynamic Functions for a Unitary Fermi Gas

Abstract: Thermodynamic properties of matter generally depend on the details of interactions between its constituent parts. However, in a unitary Fermi gas where the scattering length diverges, thermodynamics is determined through universal functions that depend only on the particle density and temperature. By using only the general form of the equation of state and the equation of force balance, we measured the local internal energy of the trapped gas as a function of these parameters. Other universal functions, such as those corresponding to the Helmholtz free energy, chemical potential, and entropy, were calculated through general thermodynamic relations. The critical parameters were also determined at the superfluid transition temperature. These results apply to all strongly interacting fermionic systems, including neutron stars and nuclear matter.

A statistical study on the correlation between lower ionospheric perturbations as seen by subionospheric VLF/LF propagation and earthquakes

Abstract: [1] The subionospheric VLF/LF propagation is extensively used to investigate the lower ionospheric perturbation in possible association with earthquakes. An extensive period of data over 7 yr from January 2001 to December 2007 and a combination of different propagation paths in and around Japan are used to examine the statistical correlation between the VLF/LF propagation anomaly (average nighttime amplitude, dispersion, and nighttime fluctuation) and earthquakes with magnitude >6.0. It is then found that the propagation anomaly exceeding the 2σ (standard deviation) criterion indicating the presence of ionospheric perturbation is significantly correlated with earthquakes with shallow depth (<40 km). Finally, the mechanism of seismoionospheric perturbations is discussed.

Two-orbital model explains the higher transition temperature of the single-layer Hg-cuprate superconductor compared to that of the La-cuprate superconductor.

Abstract: In order to explore the reason why the single-layered cuprates, ${\mathrm{La}}_{2\ensuremath{-}x}(\mathrm{Sr}/\mathrm{Ba}{)}_{x}{\mathrm{CuO}}_{4}$ (${T}_{c}\ensuremath{\simeq}40\text{ }\text{ }\mathrm{K}$) and ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ (${T}_{c}\ensuremath{\simeq}90\text{ }\text{ }\mathrm{K}$) have such a significant difference in ${T}_{c}$, we study a two-orbital model that incorporates the ${d}_{{z}^{2}}$ orbital on top of the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital. It is found, with the fluctuation exchange approximation, that the ${d}_{{z}^{2}}$ orbital contribution to the Fermi surface, which is stronger in the La system, works against $d$-wave superconductivity, thereby dominating over the effect of the Fermi surface shape. The result resolves the long-standing contradiction between the theoretical results on Hubbard-type models and the experimental material dependence of ${T}_{c}$ in the cuprates.

Measurement of Universal Thermodynamic Functions for a Unitary Fermi Gas

Abstract: Thermodynamic properties of matter generally depend on the details of interactions between its constituent parts. However, in a unitary Fermi gas where the scattering length diverges, thermodynamics is determined through universal functions that depend only on the particle density and temperature. By using only the general form of the equation of state and the equation of force balance, we measured the local internal energy of the trapped gas as a function of these parameters. Other universal functions, such as those corresponding to the Helmholtz free energy, chemical potential, and entropy, were calculated through general thermodynamic relations. The critical parameters were also determined at the superfluid transition temperature. These results apply to all strongly interacting fermionic systems, including neutron stars and nuclear matter.

Accurate UWB Radar Three-Dimensional Imaging Algorithm for a Complex Boundary Without Range Point Connections

Abstract: Ultrawide-band pulse radars have immeasurable potential for a high-range-resolution imaging in the near field and can be used for noncontact measurement of industrial products with specular or precision surfaces, such as reflector antenna or aircraft fuselage, or identifying and locating the human body in security systems. In our previous work, we developed a stable and high-speed 3-D imaging algorithm, Envelope, which is based on the principle that a target boundary can be expressed as inner or outer envelopes of spheres, which are determined using antenna location and observed ranges. Although Envelope produces a high-resolution image for a simple shape target that may include edges, it requires an exact connection for observed ranges to maintain the imaging quality. For complex shapes or multiple targets, this connection becomes a difficult task because each antenna receives multiple echoes from many scattering points on the target surface. This paper proposes a novel imaging algorithm without range point connection to accomplish high-quality and flexible 3-D imaging for various target shapes. The algorithm uses an accurate estimation for the direction of arrival using signal amplitudes and realizes direct mapping from observed ranges to target points. Several comparative studies of conventional algorithms clarify that our proposed method accomplishes accurate and reliable 3-D imaging even for complex or multiple boundaries.

Sensitization of Titanium Dioxide Photoanodes with Cadmium Selenide Quantum Dots Prepared by SILAR: Photoelectrochemical and Carrier Dynamics Studies

Abstract: Quantum dot sensitized solar cells have the potentiality to emerge as viable third-generation photovoltaic devices. In these cells, the photoanode is constituted by a nanostructured oxide layer sensitized to the visible by suitable quantum dots (QDs). Here, we report on some aspects of the sensitization of titanium dioxide nanoparticulate electrodes with CdSe QDs deposited by the so-called successive ionic layer adsorption and reaction (SILAR) method. Parallel SILAR experiments with nanoparticulate and rutile single crystal electrodes indicate that an almost full coverage of the titanium dioxide surface is achieved from the initial stages of the process. However, photoelectrochemical experiments done with both SILAR and colloidal CdSe QD sensitized electrodes indicate a faster recombination in the former case. Furthermore, a study of the ultrafast carrier dynamics of CdSe QDs by means of the near-field heterodyne detection transient grating method reveals that the rate of the photogenerated electron injec...

Bidirectionalizing graph transformations

Abstract: Bidirectional transformations provide a novel mechanism for synchronizing and maintaining the consistency of information between input and output. Despite many promising results on bidirectional transformations, these have been limited to the context of relational or XML (tree-like) databases. We challenge the problem of bidirectional transformations within the context of graphs, by proposing a formal definition of a well-behaved bidirectional semantics for UnCAL, i.e., a graph algebra for the known UnQL graph query language. The key to our successful formalization is full utilization of both the recursive and bulk semantics of structural recursion on graphs. We carefully refine the existing forward evaluation of structural recursion so that it can produce sufficient trace information for later backward evaluation. We use the trace information for backward evaluation to reflect in-place updates and deletions on the view to the source, and adopt the universal resolving algorithm for inverse computation and the narrowing technique to tackle the difficult problem with insertion. We prove our bidirectional evaluation is well-behaved. Our current implementation is available online and confirms the usefulness of our approach with nontrivial applications.

Control of Multiple Magnetic Domain Walls by Current in a Co/Ni Nano-Wire

Abstract: All-electrical control and local detection of multiple magnetic domain walls in perpendicularly magnetized Co/Ni nano-wires were demonstrated. A series of domain walls was reproducibly shifted in the same direction by the current, keeping the distance between the walls almost the same. Furthermore, the walls can be shifted back and forth depending on the direction of the pulsed currents.

Bénard–von Kármán Vortex Street in a Bose-Einstein Condensate

Abstract: Vortex shedding from an obstacle potential moving in a Bose-Einstein condensate is investigated. Long-lived alternately aligned vortex pairs are found to form in the wake, which is similar to the Benard-von Karman vortex street in classical viscous fluids. Various patterns of vortex shedding are systematically studied and the drag force on the obstacle is calculated. It is shown that the phenomenon can be observed in a trapped system.

Current status of seismo-electromagnetics for short-term earthquake prediction

Abstract: Short-term (timescale of hours, days and weeks) earthquake (EQ) prediction is of essential importance to mitigate EQ disasters. Short-term EQ prediction has so far been based on seismic measurement...

Fibrous CdS/CdSe quantum dot co-sensitized solar cells based on ordered TiO2 nanotube arrays

Abstract: A new kind of fibrous quantum dot sensitized solar cell has been designed and fabricated by using CdS and CdSe co-sensitized TiO(2) nanotubes on Ti wire as the photoanode and highly active Cu(2)S as the counter electrode. By optimizing the CdSe deposition time and the length of the nanotube, a power conversion efficiency of 3.18% has been obtained under AM 1.5 illumination (100 mW cm(-2)). The potential application of this kind of solar cell has also been discussed in this paper.

Direct Correlation between Ultrafast Injection and Photoanode Performance in Quantum Dot Sensitized Solar Cells

Abstract: The performance of quantum dot (QD) sensitized solar cells depends mainly on both electron injection from the QDs to the oxide matrix and recombination rates. Here we show a direct correlation between ultrafast carrier dynamics and photoanode (and complete solar cell) performance. TiO2 nanoparticulate electrodes sensitized with colloidal CdSe QDs are prepared by either direct or linker-assisted adsorption (using cysteine, p-mercaptobenzoic acid, and mercaptopropionic acid). These electrodes are examined by ultrafast carrier dynamics, photopotential, and incident photon-to-current efficiency measurements to unravel factors controlling the efficiency in a closed solar cell. Subpicosecond time-resolved measurements are carried out by means of a lens-free heterodyne transient grating technique. In general, faster electron injection is observed for QDs directly adsorbed on TiO2, which correlates with a better cell performance. Otherwise, increasingly faster electron injection is obtained as QD size decreases, ...

Nonuniversal Efimov atom-dimer resonances in a three-component mixture of 6Li.

Abstract: We observed an enhanced atom-dimer loss due to the existence of Efimov states in a three-component mixture of $^{6}\mathrm{Li}$ atoms. We measured the magnetic-field dependence of the atom-dimer loss in the mixture of atoms in state $|1⟩$ and dimers formed in states $|2⟩$ and $|3⟩$, and found two peaks corresponding to the degeneracy points of the energy levels of $|23⟩$ dimers and the ground and first excited Efimov trimers. We found that the locations of these peaks disagree with universal theory predictions, in a way that cannot be explained by nonuniversal two-body properties. We constructed theoretical models that characterize the nonuniversal three-body physics of three-component $^{6}\mathrm{Li}$ atoms in the low-energy domain.

Quantum Kelvin-Helmholtz instability in phase-separated two-component Bose-Einstein condensates

Abstract: We theoretically study the Kelvin-Helmholtz instability in phase-separated two-component Bose-Einstein condensates using the Gross-Pitaevskii and Bogoliubov-de Gennes models. A flat interface between the two condensates is shown to deform into sawtooth or Stokes-type waves, leading to the formation of singly quantized vortices on the peaks and troughs of the waves. This scenario of interface instability in quantum fluids is quite different from that in classical fluids.

Load-based covert channels between Xen virtual machines

Abstract: Multiple virtual machines on a single virtual machine monitor are isolated from each other. A malicious user on one virtual machine usually cannot relay secret data to other virtual machines without using explicit communication media such as shared files or a network. However, this isolation is threatened by communication in which CPU load is used as a covert channel. Unfortunately, this threat has not been fully understood or evaluated. In this study, we quantitatively evaluate the threat of CPU-based covert channels between virtual machines on the Xen hypervisor. We have developed CCCV, a system that creates a covert channel and communicates data secretly using CPU loads. CCCV consists of two user processes, a sender and a receiver. The sender runs on one virtual machine, and the receiver runs on another virtual machine on the same hypervisor. We measured the bandwidth and communication accuracy of the covert channel. CCCV communicated 64-bit data with a 100% success rate in an ideal environment, and with a success rate of over 90% in an environment where Web servers are processing requests on other virtual machines.

Low-Cost Flexible Nano-Sulfide/Carbon Composite Counter Electrode for Quantum-Dot-Sensitized Solar Cell

Abstract: Cu2S nanocrystal particles were in situ deposited on graphite paper to prepare nano-sulfide/carbon composite counter electrode for CdS/CdSe quantum-dot-sensitized solar cell (QDSC). By optimization of deposition time, photovoltaic conversion efficiency up to 3.08% was obtained. In the meantime, this composite counter electrode was superior to the commonly used Pt, Au and carbon counter electrodes. Electrochemical impedance spectra further confirmed that low charge transfer resistance at counter electrode/electrolyte interface was responsible for this, implied the potential application of this composite counter electrode in high-efficiency QDSC.

Gradually reconfiguring virtual network topologies based on estimated traffic matrices

Abstract: Traffic matrix is essential to traffic engineering (TE) methods. Because it is difficult to monitor traffic matrices directly, several methods for estimating them from link loads have been proposed. However, estimated traffic matrix includes estimation errors which degrade the performance of TE significantly. In this paper, we propose a method that reduces estimation errors while reconfiguring the virtual network topology (VNT) by cooperating with the VNT reconfiguration. In our method, the VNT reconfiguration is divided into multiple stages instead of reconfiguring the suitable VNT at once. By dividing the VNT reconfiguration into multiple stages, our traffic matrix estimation method calibrates and reduces the estimation errors in each stage by using information monitored in prior stages. We also investigate the effectiveness of our proposal using simulations. The results show that our method can improve the accuracy of the traffic matrix estimation and achieve an adequate VNT as is the case with the reconfiguration using the actual traffic matrices.

Integration of posture and rhythmic motion controls in quadrupedal dynamic walking using phase modulations based on leg loading/unloading

Abstract: In this paper, we intend to show the basis of a general legged locomotion controller with the ability to integrate both posture and rhythmic motion controls and shift continuously from one control method to the other according to the walking speed. The rhythmic motion of each leg in the sagittal plane is generated by a single leg controller which controls the swing-to-stance and stance-to-swing phase transitions using respectively leg loading and unloading information. Since rolling motion induced by inverted pendulum motion during the two-legged stance phases results in the transfer of the load between the contralateral legs, leg loading/unloading involves posture information in the frontal plane. As a result of the phase modulations based on leg loading/unloading, rhythmic motion of each leg is achieved and inter-leg coordination (resulting in a gait) emerges, even without explicit coordination amongst the leg controllers, allowing to realize dynamic walking in the low- to medium-speed range. We show that the proposed method has resistance ability against lateral perturbations to some extent, but that an additional ascending coordination mechanism between ipsilateral legs is necessary to withstand perturbations decreasing the rolling motion amplitude. Even without stepping reflex using vestibular information, our control system, relying on phase modulations based on leg loading/unloading and the ascending coordination mechanism between ipsilateral legs, enables low speed dynamic walking on uneven terrain with long cyclic period, which was not realized in our former studies. Details of trajectory generation, movies of simulations and movies of preliminary experiments using a real robot are available at: http://robotics.mech.kit.ac.jp/kotetsu/ .

167 W, power scalable ytterbium-doped photonic bandgap fiber amplifier at 1178nm

Abstract: An ytterbium-doped photonic bandgap fiber amplifier operating at the long wavelength edge of the ytterbium gain band is investigated for high power amplification. The spectral filtering effect of the photonic bandgap efficiently suppresses amplified spontaneous emission at the conventional ytterbium gain wavelengths and thus enables high power amplification at 1178 nm. A record output power of 167 W, a slope efficiency of 61% and 15 dB saturated gain at 1178 nm have been demonstrated using the ytterbium-doped photonic bandgap fiber.