SearcharxivSearch

arXiv subjects

Takeaki Yajima

Publications and source records attributed to Takeaki Yajima.

8 recordsLinked to original sources

Sensor Drift Compensation via Olfactory system and Reservoir Computing

Despite the promising applications of electronic noses (e-Noses) in medical diagnosis and industrial process control, sensor drift remains a critical challenge that degrades long-term sensing reliability by inducing gradual shifts in sensor responses. Conventional drift compensation methods are typically designed for batch learning and lack the ability to support continuous online learning in non-stationary environments. Although several online drift compensation methods have recently been proposed, they are mainly based on quasi-online mini-batch learning for distribution adaptation, while true sample-wise online learning without buffering remains largely unexplored. To address these issues, this paper proposes a sample-wise online drift compensation method based on spiking neural networks (SNNs) for feature adaptation and spiking reservoir computing (SRC) for classification. By exploiting spike-timing-dependent plasticity (STDP), the SNN self-organizes spatiotemporal attractor dynamics for label-free feature adaptation (STDP-FA), and the adapted features are classified by SRC with self-supervised adaptation driven by winner-take-all (WTA) competition. The proposed method addresses various concept drift patterns, including gradual drift, random drift, sensor failures, and abrupt changes. Simulations on a real-world sensor drift dataset demonstrate a clear improvement in classification accuracy over baseline methods.

eess.SP

CMOS-based area-and-power-efficient neuron and synapse circuits for time-domain analog spiking neural networks

Conventional neural structures tend to communicate through analog quantities such as currents or voltages, however, as CMOS devices shrink and supply voltages decrease, the dynamic range of voltage/current-domain analog circuits becomes narrower, the available margin becomes smaller, and noise immunity decreases. More than that, the use of operational amplifiers (op-amps) and continuous-time or clocked comparators in conventional designs leads to high energy consumption and large chip area, which would be detrimental to building spiking neural networks. In view of this, we propose a neural structure for generating and transmitting time-domain signals, including a neuron module, a synapse module, and two weight modules. The proposed neural structure is driven by a leakage current of MOS transistors and uses an inverter-based comparator to realize a firing function, thus providing higher energy and area efficiency compared to conventional designs. The proposed neural structure is fabricated using TSMC 65 nm CMOS technology. The proposed neuron and synapse occupy the area of 127 μm^{ 2} and 231 μm^{ 2}, respectively, while achieving millisecond time constants. Actual chip measurements show that the proposed structure implements the temporal signal communication function with millisecond time constants, which is a critical step toward hardware reservoir computing for human-computer interaction. Simulation results of the spiking-neural network for reservoir computing with the behavioral model of the proposed neural structure demonstrate the learning function.

cs.NE

Overlooked transportation anisotropies in d-band correlated rare-earth perovskite nickelates

Anisotropies in electronic transportations conventionally originate from the nature of low symmetries in crystal structures, and were not anticipated for perovskite oxides, the crystal asymmetricity of which is far below, e.g. van der Waals or topological crystal. Beyond conventional expectations, herein we demonstrate pronounced anisotropies in the inter-band coulomb repulsion dominated electronic transportation behaviors under low-dimensional confinement for the perovskite family of rare-earth nickelates (ReNiO3). From one aspect, imparting bi-axial interfacial strains upon various lattice planes results in extrinsic anisotropies in the abrupt orbital transitions of ReNiO3, and their metal to insulator transition behaviors that elevates the transition temperature beyond the existing merit. From the other aspect, the in-plane orbital entropy associated to the in-plane symmetry of the NiO6 octahedron within ReNiO3 causes intrinsic anisotropies for the gradually orbital transition with temperature to further improve their thermistor transportation properties. The present work unveils the overlooked role of the electronic orbital directionality within low dimensional correlated perovskites that can trigger anisotropic transportation behaviors, in spite of their relatively symmetric crystal structures. Establishing anisotropic transportations integrating the electron correlation and quantum confinement effects will bring in a new freedom for achieving further improvement in transportation properties of multi-functional perovskite oxides.

cond-mat.mtrl-sci

Entropy driven reverse-metal-to-insulator transition and delta-temperatural transports in metastable perovskites of correlated rare-earth nickelate

The metal to insulator transition (MIT) in Mott-Hubbard systems is one of the most important discoveries in condensed matter physics, and results in abrupt orbital transitions from the insulating to metallic phases by elevating temperature across a critical point (TMIT). Although the MIT was previously expected to be mainly driven by the orbital Coulomb repulsion energy, the entropy contribution to the orbital free energy that also determines the relative stability of the metallic and insulating phases was largely overlooked. Herein, we demonstrate an orbital-entropy dominated reversible electronic phase transition in the metastable perovskite family of correlated rare-earth nicklates (ReNiO3), in addition to their previously known MIT driven by orbital Coulomb energies. In reverse to MIT, the resistivity of ReNiO3 abruptly increases by 2-3 orders by elevating T across another critical point (TR-MIT) below TMIT, and such transition is named as reverse-metal to insulator transition (R-MIT). Combining the afterwards exponentially decreasing resistivity in the insulating phase of ReNiO3 at further temperature elevation, a distinguished delta-temperatural transport character is established, which is potentially applicable for locking the working temperatures range for electric devices. The TR-MIT is shown to be enhanced via reducing the compositional complexity and size of Re or imparting bi-axial compressive strains, and meanwhile the transition sharpness of delta-temperatural transport is reduced. Our discovery indicates that temperature range for a thermodynamically stable insulating phase of ReNiO3 is in between of TR-MIT and TMIT, while a new conductive phase with high orbital entropy is formed by further descending temperature below TR-MIT.

physics.app-ph

A micrometer-thick oxide film with high thermoelectric performance at temperature ranging from 20-400 K

Thermoelectric (TE) materials achieve localised conversion between thermal and electric energies, and the conversion efficiency is determined by a figure of merit zT. Up to date, two-dimensional electron gas (2DEG) related TE materials hold the records for zT near room-temperature. A sharp increase in zT up to ~2.0 was observed previously for superlattice materials such as PbSeTe, Bi2Te3/Sb2Te3 and SrNb0.2Ti0.8O3/SrTiO3, when the thicknesses of these TE materials were spatially confine within sub-nanometre scale. The two-dimensional confinement of carriers enlarges the density of states near the Fermi energy3-6 and triggers electron phonon coupling. This overcomes the conventional σ-S trade-off to more independently improve S, and thereby further increases thermoelectric power factors (PF=S2σ). Nevertheless, practical applications of the present 2DEG materials for high power energy conversions are impeded by the prerequisite of spatial confinement, as the amount of TE material is insufficient. Here, we report similar TE properties to 2DEGs but achieved in SrNb0.2Ti0.8O3 films with thickness within sub-micrometer scale by regulating interfacial and lattice polarizations. High power factor (up to 103 μWcm-1K-2) and zT value (up to 1.6) were observed for the film materials near room-temperature and below. Even reckon in the thickness of the substrate, an integrated power factor of both film and substrate approaching to be 102 μWcm-1K-2 was achieved in a 2 μm-thick SrNb0.2Ti0.8O3 film grown on a 100 μm-thick SrTiO3 substrate. The dependence of high TE performances on size-confinement is reduced by ~103 compared to the conventional 2DEG-related TE materials. As-grown oxide films are less toxic and not dependent on large amounts of heavy elements, potentially paving the way towards applications in localised refrigeration and electric power generations.

cond-mat.mtrl-sci

Hot electron transport in a strongly correlated transition metal oxide

Oxide heterointerfaces are ideal for investigating strong correlation effects to electron transport, relevant for oxide-electronics. Using hot-electrons, we probe electron transport perpendicular to the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO)- Nb-doped SrTiO$_3$ (Nb:STO) interface and find the characteristic hot-electron attenuation length in LSMO to be 1.48 $\pm$ 0.10 unit cells (u.c.) at -1.9 V, increasing to 2.02 $\pm$ 0.16 u.c. at -1.3 V at room temperature. Theoretical analysis of this energy dispersion reveals the dominance of electron-electron and polaron scattering. Direct visualization of the local electron transport shows different transmission at the terraces and at the step-edges.

cond-mat.mtrl-sci

Charge writing at the LaAlO3/SrTiO3 surface

Biased conducting-tip atomic force microscopy (AFM) has been shown to write and erase nanoscale metallic lines at the LaAlO3/SrTiO3 interface. Using various AFM modes, we show the mechanism of conductivity switching is the writing of surface charge. These charges are stably deposited on a wide range of LaAlO3 thicknesses, including bulk crystals. A strong asymmetry with writing polarity was found for 1 and 2 unit cells of LaAlO3, providing experimental evidence for a theoretically predicted built-in potential.

cond-mat.mtrl-sci

Termination Control of the Interface Dipole in La$_{0.7}$Sr$_{0.3}$MnO$_3$/Nb:SrTiO$_3$ (001) Schottky Junctions

In order to investigate the interface termination dependence of perovskite band alignments, we have studied the Schottky barrier height at La$_{0.7}$Sr$_{0.3}$MnO$_3$/Nb:SrTiO$_3$ (001) heterointerfaces. As the Nb:SrTiO$_3$ semiconductor was varied from TiO$_2$ termination to SrO termination by variable insertion of a SrMnO$_3$ layer, a large systematic increase in the Schottky barrier height was observed. This can be ascribed to the evolution of the interface dipole induced to screen the polar discontinuity at the interface, which gives a large internal degree of freedom for tuning band diagrams in oxides.

cond-mat.str-el