arXiv subjects

Jin Nakamura

Publications and source records attributed to Jin Nakamura.

7 recordsLinked to original sources

arXiv2026

Short-Term Synaptic Plasticity Stabilizes Goal-Conditioned Dynamics in a PFC-Inspired Reservoir Model for Multistep Goal-Directed Action Planning

The prefrontal cortex (PFC) maintains goal information for action planning, but how recurrent circuits preserve it in an action-usable form over behavioral timescales remains unclear. Here we ask whether short-term synaptic plasticity (STP) can stabilize goal information as action-usable, goal-conditioned dynamics. We incorporated STP into a PFC-inspired reservoir computing model with basal-ganglia-inspired temporal-difference readout learning, and evaluated paired models with and without STP across 100 independently generated networks in a multistep goal-directed action-selection task with delayed execution. Goal identity was highly decodable during the delay even without STP, so STP was not required to form a linearly readable goal representation. Under state noise, however, success without STP fell from 75.8% to 49.5%, whereas the model with STP remained essentially unchanged (91.8% without noise versus 89.2% under noise; paired Cohen's dz=1.31). Time-resolved decoding, state-space separability, and action-value-difference analyses showed that STP preserved goal information as action-relevant goal-conditioned dynamics available at later action opportunities. Gain-matched and STP-state perturbation controls argued against a simple fixed recurrent-scaling explanation and supported online, history-dependent synaptic modulation. Effective-connectivity analyses showed delay-period goal-specific patterning that increased toward the later part of the trial with STP, where it should be read as goal- and task-state-conditioned patterning; effective connectivity without STP was time-invariant. A grid search identified a facilitation-dominant range of STP time constants associated with high success rates. These results suggest that STP supports robust goal-conditioned dynamics through dynamic modulation of goal-dependent effective recurrent connectivity.

q-bio.NC↗

arXiv2024

A concise 40 T pulse magnet for condensed matter experiments

There is a growing interest in using pulsed high magnetic field as a controlling parameter of physical phenomena in various scientific disciplines, such as condensed matter physics, particle physics, plasma physics, chemistry and biological studies. We devised a concise and portable pulsed magnetic field generator that produces a 40 T field with a pulse duration of 2 ms. It is assembled using only off-the-shelf components and a homemade coil that leverages small computers, Raspberry Pi, and Python codes. It allows for straightforward modification for general purposes. As working examples, we show representative applications in condensed matter experiments of magnetoresistance, magnetization, and magnetostriction measurements for graphite, NdNi$_{2}$P$_{2}$, and NdCo$_{2}$P$_{2}$, respectively, with the maximum magnetic field of 41 T and the lowest temperature of 4.2 K.

cond-mat.mtrl-sci↗

arXiv2023

Magnetic field induced valence change in Eu(Co$_{1-x}$Ni$_{x}$)$_{2}$P$_{2}$ up to 60 T

The solid solution 122 compounds, Eu(Co$_{1-x}$Ni$_{x}$)$_{2}$P$_{2}$, show valence transition between divalent state and intermediate valence states at Eu, which is firmly correlated to multiple degrees of freedom in the solid such as the isostructural transition between the collapsed tetragonal (cT) and uncollapsed tetragonal (ucT) structures, $3d$ magnetism, and the formation of P-P dimers. To gain insights into the correlated behavior, we investigate the effect of high magnetic fields on the samples of $x = 0.4$ and $0.5$ using magnetostriction and magnetization measurements up to 60 T. The samples are in the Eu valence fluctuating regime, where the possible structural transition from cT to ucT may be induced by the Eu valence change under the magnetic fields. For both samples, magnetostriction smoothly increases with increasing magnetic fields. The behavior is in good agreement with the calculated results using the interconfigurational fluctuation (ICF) model that describes the valence change. This indicates that $ΔL$ represents the change of the Eu valence state in these compounds. Magnetization curves for both compounds show good agreement with the ICF model at high magnetic fields. In contrast, in the low magnetic field region, magnetization curves do not agree with the ICF model. These results indicate that the Eu valence changes manifest themselves in the magnetization curves at high magnetic fields and that the magnetism of the $3d$ electrons manifests itself in the magnetization at low magnetic fields. Hence, we conclude that the valence change occurs within the Eu valence fluctuation regime coupled with the cT structure. Thereby, we believe that the transition to ucT structure which is firmly coupled with the divalent Eu state does not occur within the magnetic field range of the present study. (Continued)

cond-mat.str-el↗

arXiv2022

Design guidelines for the SPICE parameters of waveform-selective metasurfaces varying with the incident pulse width at a constant oscillation frequency

In this study, we numerically demonstrate how the response of recently reported circuit-based metasurfaces is characterized by their circuit parameters. These metasurfaces, which include a set of four diodes as a full wave rectifier, are capable of sensing different waves even at the same frequency in response to the incident waveform, or more specifically the pulse width. This study reveals the relationship between the electromagnetic response of such waveform-selective metasurfaces and the SPICE parameters of the diodes used. First, we show that reducing a parasitic capacitive component of the diodes is important for realization of waveform-selective metasurfaces in a higher frequency regime. Second, we report that the operating power level is closely related to the saturation current and the breakdown voltage of the diodes. Moreover, the operating power range is found to be broadened by introducing an additional resistor into the inside of the diode bridge. Our study is expected to provide design guidelines for circuit-based waveform-selective metasurfaces to select/fabricate optimal diodes and enhance the waveform-selective performance at the target frequency and power level.

physics.app-ph↗

arXiv2004

Holes in the valence band of superconducting boron-doped diamond film studied by soft X-ray absorption and emission spectroscopy

Carbon- and boron-2$p$ states of superconducting and non-superconducting boron-doped diamond samples are measured using soft X-ray emission and absorption spectroscopy. For the superconducting sample, a large density of hole states is observed in the valence band in addition to the states in the impurity band. The hole states in the valence band is located at about 1.3 eV below the valence band maximum regardless of the doping level, which cannot be interpreted within a simple rigid band model. Present experimental results, combined with the first principles calculations, suggest that superconductivity is to be attributed to the holes in the valence band.

cond-mat.supr-con↗

arXiv2004

Electronic structures of B-2p and C-2p of boron-doped diamond film by soft X-ray absorption and emission spectroscopy

X-ray absorption (XAS) and emission (XES) spectroscopy near B-K and C-K edges have been performed on metallic (~1at%B, B-diamond) and semiconducting (~0.1at%B and N, BN-diamond) doped-diamond films. Both B-K XAS and XES spectra shows metallic partial density of state (PDOS) with the Fermi energy of 185.3 eV, and there is no apparent boron-concentration dependence in contrast to the different electric property. In C-K XAS spectrum of B-diamond, the impurity state ascribed to boron is clearly observed near the Fermi level. The Fermi energy is found to be almost same with the top of the valence band of non-doped diamond, E_V, 283.9 eV. C-K XAS of BN-diamond shows both the B-induced shallow level and N-induced deep-and-broad levels as the in-gap states, in which the shallow level is in good agreement with the activation energy (E_a=0.37 eV) estimated from the temperature dependence of the conductivity, namely the change in C-2p PDOS of impurity-induced metallization is directly observed. The electric property of this diamond is mainly ascribed to the electronic structure of C-2p near the Fermi level. The observed XES spectra are compared with the DVX-alpha cluster calculation. The DVX-alpha result supports the strong hybridization between B-2p and C-2p observed in XAS and XES spectra, and suggests that the small amount of borons (<1at%) in diamond occupy the substitutional site rather than interstitial site.

cond-mat.supr-con↗

arXiv2001

Electronic Structure of B-2p State in AlB2 Single Crystal: Direct Observation ofpσand pπDensity of States

X-ray emission (XES) and absorption (XAS) spectra near the B-$K$ edge were measured on single-crystalline AlB$_2$ compound which is an isostructural diboride of superconducting MgB$_2$. The partial density of states (PDOS) of B-2$pσ$ and $pπ$ orbitals were derived from the polarization dependence of XES and XAS spectra. There are considerable amounts of PDOS near the Fermi energy in AlB$_2$ similarly to that in MgB$_2$, but there are almost no PDOS in $pσ$ orbitals of AlB$_2$ near the Fermi energy, i.e., a pseudo-gap in $pσ$ state and a broad metallic state in $pπ$ state are observed. The present result indirectly supports scenarios that the $pσ$ holes play an important role in the occurrence of superconductivity in MgB$_2$. The overall features of PDOS were found to be in good agreement with the result of band calculation of AlB$_2$, but a small discrepancy in the Fermi energy is observed, which is attributed to the Al vacancy in the compounds, i.e., the estimated concentration is Al$_{0.93}$B$_2$.

cond-mat.supr-con↗