arXiv subjects

T. Kobayashi

Publications and source records attributed to T. Kobayashi.

At least 37 records · Page 2Linked to original sources

arXiv2023

Role of hydrogen bonding in charge-ordered organic conductor $α$-(BEDT-TTF)$_2$I$_3$ probed by $^{127}$I nuclear quadrupole resonance

We present $^{127}$I nuclear quadrupole resonance spectra and nuclear relaxation of $α$-(BEDT-TTF)$_2$I$_3$ that undergoes a charge-ordering transition. Only one of the two I$_3$ anion sites shows a significant differentiation in the electric field gradients across the first-order transition. The charge modulation only in the BEDT-TTF layers can not reproduce; instead, an anion-donor interaction accompanied by hydrogen bonding is necessary. The dominating source for the nuclear relaxation is the local libration of the I$_3$ anions, but an anomalous peak is detected just below the transition, as observed by $^{13}$C NMR.

cond-mat.str-el↗

arXiv2023

Level Structures of $^{56,58}$Ca Cast Doubt on a doubly magic $^{60}$Ca

Gamma decays were observed in $^{56}$Ca and $^{58}$Ca following quasi-free one-proton knockout reactions from $^{57,59}$Sc beams at $\approx 200$ MeV/nucleon. For $^{56}$Ca, a $γ$ ray transition was measured to be 1456(12) keV, while for $^{58}$Ca an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the $2^+_1 \rightarrow 0^+_{gs}$ decays, and were compared to results from ab initio and conventional shell-model approaches. A shell-model calculation in a wide model space with a marginally modified effective nucleon-nucleon interaction depicts excellent agreement with experiment for $2^+_1$ level energies, two-neutron separation energies, and reaction cross sections, corroborating the formation of a new nuclear shell above the $N$ = 34 shell. Its constituents, the $0f_{5/2}$ and $0g_{9/2}$ orbitals, are almost degenerate. This degeneracy precludes the possibility for a doubly magic $^{60}$Ca and potentially drives the dripline of Ca isotopes to $^{70}$Ca or even beyond.

nucl-ex↗

arXiv2023

Silicon tracker array for RIB experiments at SAMURAI

This work describes a silicon tracker system developed for experiments with proton-rich radioactive ion beams at the SAMURAI superconducting spectrometer of RIBF at RIKEN. The system is designed for accurate angular reconstruction and atomic number identification of relativistic heavy ions and protons which are simultaneously produced in reactions motivated by studies of proton capture reactions of interest for nuclear astrophysics. The technical characteristics of the tracking array are described in detail as are its performance in two pilot experiments. The physics justification for such a system is also presented.

physics.ins-det↗

arXiv2023

Searching for universality of dineutron correlation at the surface of Borromean nuclei

The dineutron correlation is systematically studied in three different Borromean nuclei near the neutron dripline, 11Li, 14Be and 17B, via the (p, pn) knockout reaction measured at the RIBF facility in RIKEN. For the three nuclei, the correlation angle between the valence neutrons is found to be largest in the same range of intrinsic momenta, which can be associated to the nuclear surface. This result reinforces the prediction that the formation of the dineutron is universal in environments with low neutron density, such as the surface of neutron-rich Borromean nuclei.

nucl-ex↗

arXiv2023

Intruder configurations in $^{29}$Ne at the transition into the island of inversion: Detailed structure study of $^{28}$Ne

Detailed $γ$-ray spectroscopy of the exotic neon isotope $^{28}$Ne has been performed for the first time using the one-neutron removal reaction from $^{29}$Ne on a liquid hydrogen target at 240~MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for $^{28}$Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder $p$-wave strength providing evidence of the breakdown of the $N=20$ and $N=28$ shell gaps. Only a weak, possible $f$-wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large $p$-wave and small $f$-wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain.

nucl-ex↗

arXiv2023

Multiple Mechanisms in Proton-Induced Nucleon Removal at $\sim$100 MeV/Nucleon

We report on the first proton-induced single proton- and neutron-removal reactions from the neutron-deficient $^{14}$O nucleus with large Fermi-surface asymmetry $S_n-S_p$ = 18.6 MeV at $\sim$100 MeV/nucleon, a widely used energy regime for rare-isotope studies. The measured inclusive cross sections and parallel momentum distributions of the $^{13}$N and $^{13}$O residues are compared to the state-of-the-art reaction models, with nuclear structure inputs from many-body shell-model calculations. Our results provide the first quantitative contributions of multiple reaction mechanisms including the quasifree knockout, inelastic scattering and nucleon transfer processes. It is shown that the inelastic scattering and nucleon transfer, usually neglected at such energy regime, contribute about 50% and 30% to the loosely bound proton and deeply bound neutron removal, respectively. These multiple reaction mechanisms should be considered in analyses of inclusive one-nucleon removal cross sections measured at intermediate energies for quantitative investigation of single-particle strengths and correlations in atomic nuclei.

nucl-ex↗

arXiv2023

SuperFGD prototype time resolution studies

The SuperFGD will be a part of the ND280 near detector of the T2K and Hyper Kamiokande projects, that will help to reduce systematic uncertainties related with neutrino flux and cross-section modeling. The upgraded ND280 will be able to perform a full exclusive reconstruction of the final state from neutrino-nucleus interactions, including measurements of low momentum protons, pions and, for the first time, event-by event measurements of neutron kinematics. The time resolution defines the neutron energy resolution. We present the results of time resolution measurements made with the SuperFGD prototype that consists of 9216 plastic scintillator cubes (cube size is 1 cm$^3$) readout with 1728 wavelength-shifting fibers going along three orthogonal directions. We use data from the muon beam exposure at CERN. The time resolution of 0.97 ns was obtained for one readout channel after implementing the time calibration with a correction for the time-walk effect. The time resolution improves with energy deposited in a scintillator cube. Averaging two readout channels for one scintillator cube improves the time resolution to 0.68 ns which means that signals in different channels are not synchronous. Therefore the contribution from the time recording step of 2.5 ns is averaged as well. Averaging time values from N channels improves the time resolution by $\sim 1/\sqrt{N}$. Therefore a very good time resolution should be achievable for neutrons since neutron recoils hit typically several scintillator cubes and in addition produce larger amplitudes than muons. Measurements performed with a laser and a wide-bandwidth oscilloscope demonstrated that the time resolution obtained with the muon beam is not far from its expected limit. The intrinsic time resolution of one channel is 0.67 ns for signals of 56 photo-electron typical for minimum ionizing particles.

physics.ins-det↗

arXiv2023

Terahertz spin ratchet effect in magnetic metamaterials

We report on spin ratchet currents driven by terahertz radiation electric fields in a Co/Pt magnetic metamaterial formed by triangle-shaped holes forming an antidots lattice and subjected to an external magnetic field applied perpendicularly to the metal film plane. We show that for a radiation wavelength substantially larger than the period of the antidots array the radiation causes a polarization-independent spin-polarized ratchet current. The current is generated by the periodic asymmetric radiation intensity distribution caused by the near-field diffraction at the edges of the antidots, which induces spatially inhomogeneous periodic electron gas heating, and a phase-shifted periodic asymmetric electrostatic force. The developed microscopic theory shows that the magnetization of the Co/Pt film results in a spin ratchet current caused by both the anomalous Hall and the anomalous Nernst effects. Additionally, we observed a polarization-dependent trigonal spin photocurrent, which is caused by the scattering of electrons at the antidot boundaries resulting in a spin-polarized current due to the magnetization. Microscopic theory of these effects reveals that the trigonal photocurrent is generated at the boundaries of the triangle antidots, whereas the spin ratchet is generated due to the spatially periodic temperature gradient over the whole film. This difference causes substantially different hysteresis widths of these two currents.

cond-mat.mes-hall↗

arXiv2022

Dynamics of ethylene groups and hyperfine interactions between donor and anion molecules in $λ$-type organic conductors studied by $^{69,71}$Ga-NMR spectroscopy

We present the results of $^{69,71}$Ga-NMR measurements on an organic antiferromagnet $λ$-(BEDSe-TTF)$_2$GaCl$_4$ [BEDSe-TTF=bis(ethylenediseleno)tetrathiafulvalene], with comparison to reports on $λ$-(BETS)$_2$GaCl$_4$ [BETS=bis(ethylenedithio)tetraselenafulvalene] [T. Kobayashi et al., Phys. Rev. B 102, 235131 (2020)]. We found that the dynamics of two crystallographically independent ethylene groups induce two types of quadrupolar relaxation in the high-temperature region. As the ethylene motion freezes, hyperfine (HF) interactions develop between $π$ spin and Ga nuclear spin below 100 K, and thereby magnetic fluctuations of the $π$-spin system are detected even from the Ga site. The HF interaction in $λ$-(BETS)$_2$GaCl$_4$ was more than twice as large as in $λ$-(BEDSe-TTF)$_2$GaCl$_4$, implying that the short contacts between Cl atoms and the chalcogens of fulvalene part are essential for the transferred HF interaction. We propose that NMR using nuclei in anion layers is useful for studying interlayer interactions in organic conductors, which have not been studied experimentally. In addition, because the mechanism of the transferred HF interaction is considered to be the same as $π$-$d$ interaction in isostructural Fe-containing $λ$-type salts, our findings aid in the understanding of their physical properties.

cond-mat.str-el↗

arXiv2022

Isoscaling in central Sn+Sn collisions at 270 MeV/u

Experimental information on fragment emissions is important in understanding the dynamics of nuclear collisions and in the development of transport model simulating heavy-ion collisions. The composition of complex fragments emitted in the heavy-ion collisions can be explained by statistical models, which assume that thermal equilibrium is achieved at collision energies below 100 MeV/u. Our new experimental data together with theoretical analyses for light particles from Sn+Sn collisions at 270 MeV/u, suggest that the hypothesis of thermal equilibrium breaks down for particles emitted with high transfer momentum. To inspect the system's properties in such limit, the scaling features of the yield ratios of particles from two systems, a neutron-rich system of ${}^{132}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ and a nearly symmetric system of ${}^{108}\mathrm{Sn}+{}^{112}\mathrm{Sn}$, are examined in the framework of the statistical multifragmentation model and the antisymmetrized molecular dynamics model. The isoscaling from low energy particles agree with both models. However the observed breakdown of isoscaling for particles with high transverse momentum cannot be explained by the antisymmetrized molecular dynamics model.

nucl-ex↗

arXiv2022

Optimization of RF phase and beam loading distribution among RF stations in SuperKEKB

SuperKEKB is the e-/e+ collider which targets the world highest luminosity. In recent operation, SuperKEKB achieved a new world record of $4.71\times 10^{34} cm^{-2}s^{-1}$ for luminosity with the beam current of 1.4 A. In the future, the beam current will be increased further to aim at the design value of 3.6 A and much higher luminosity. The RF system consists of 38 cavities (30 klystron stations), which share the huge beam loading brought by high current beam with each other cavities. For beam stability and power efficiency, it is important to distribute the beam loading properly among RF cavities. It is equivalent to adjust the acceleration phase of each cavity. However, it is difficult to evaluate the acceleration phase using only the pickup signal. Therefore, we established a method to evaluate the beam loading balance among RF stations from the RF power measurement for each cavity, and to adjust the acceleration phase. This report introduces the method for evaluating and optimizing the beam loading (acceleration phase) among stations in SuperKEKB, which has a large number of RF stations, and its operation.

physics.acc-ph↗

arXiv2022

Antiferromagnetic ordering of organic Mott insulator $λ$-(BEDSe-TTF)$_2$GaCl$_4$

The band structure and magnetic properties of organic charge-transfer salt $λ$-(BEDSe-TTF)$_2$GaCl$_4$ (BEDSe-TTF: bis(ethylenediseleno)tetrathiafulvalene; abbreviated as $λ$-BEDSe) are investigated. The reported crystal structure is confirmed using X-ray diffraction measurements, and the transfer integrals are calculated. The degree of electron correlation $U/W$ ($U$: on-site Coulomb repulsion, $W$: bandwidth) of $λ$-BEDSe is larger than one and comparable to that of the isostructural Mott insulator $λ$-(ET)$_2$GaCl$_4$ (ET: bis(ethylenedithio)tetrathiafulvalene, abbreviated as $λ$-ET), whereas the $U/W$ of the superconducting salt $λ$-(BETS)$_2$GaCl$_4$ (BETS: bis(ethylenedithio)tetraselenafulvalene) is smaller than one. $^{13}$C-NMR and $μ$SR measurements revealed that $λ$-BEDSe undergoes an antiferromagnetic (AF) ordering below $T_{\rm N} = 22$~K. In the AF state, discrete $^{13}$C-NMR spectra with a remaining central peak are observed, indicating the commensurate AF spin structure also observed in $λ$-ET. The similarity between the structural and magnetic properties of $λ$-BEDSe and $λ$-ET suggests that both salts are in the same electronic phase, i.e., the physical properties of $λ$-BEDSe can be understood by the universal phase diagram of bandwidth-controlled $λ$-type organic conductors obtained by donor molecule substitution.

cond-mat.str-el↗

arXiv2022

Interface superconductivity in FeSe thin films on SrTiO$_3$grown by the PLD technique

In this study, we fabricate 5-30-nm-thick films of FeSe/STO using pulsed laser deposition (PLD). The grown films exhibit superconductivity with an onset $T_{\mathrm{c}}$ that is much higher than that of bulk FeSe under ambient pressure. The observed $T_{\mathrm{c}}$ values are exceptionally high in terms of the strain vs. $T_{\mathrm{c}}$ relationship of the same material established so far. Furthermore, $T_{\mathrm{c}}$ increases as the film thickness decreases, except for films thinner than 10 nm. This thickness dependence of $T_{\mathrm{c}}$ is in good agreement with the results reported for films grown by molecular beam epitaxy (MBE) that exhibit interface superconductivity. These results indicate the realization of interface superconductivity in PLD-grown FeSe/STO. Furthermore, our PLD technique requires no post-annealing to realize interface superconductivity, which is different from MBE techniques. Because the PLD technique has the advantage that various interfaces can be fabricated easily by simply altering the target materials, our results open novel routes to study interface superconductivity toward higher $T_{\mathrm{c}}$ by systematic control of the interface.

cond-mat.supr-con↗

arXiv2022

Search for solar electron anti-neutrinos due to spin-flavor precession in the Sun with Super-Kamiokande-IV

Due to a very low production rate of electron anti-neutrinos ($\barν_e$) via nuclear fusion in the Sun, a flux of solar $\barν_e$ is unexpected. An appearance of $\barν_e$ in solar neutrino flux opens a new window for the new physics beyond the standard model. In particular, a spin-flavor precession process is expected to convert an electron neutrino into an electron anti-neutrino (${ν_e\to\barν_e}$) when neutrino has a finite magnetic moment. In this work, we have searched for solar $\barν_e$ in the Super-Kamiokande experiment, using neutron tagging to identify their inverse beta decay signature. We identified 78 $\barν_e$ candidates for neutrino energies of 9.3 to 17.3 MeV in 2970.1 live days with a fiducial volume of 22.5 kiloton water (183.0 kton$\cdot$year exposure). The energy spectrum has been consistent with background predictions and we thus derived a 90% confidence level upper limit of ${4.7\times10^{-4}}$ on the $ν_e\to\barν_e$ conversion probability in the Sun. We used this result to evaluate the sensitivity of future experiments, notably the Super-Kamiokande Gadolinium (SK-Gd) upgrade.

hep-ex↗

arXiv2022

First application of mass measurement with the Rare-RI Ring reveals the solar r-process abundance trend at A=122 and A=123

The Rare-RI Ring (R3) is a recently commissioned cyclotron-like storage ring mass spectrometer dedicated to mass measurements of exotic nuclei far from stability at Radioactive Isotope Beam Factory (RIBF) in RIKEN. The first application of mass measurement using the R3 mass spectrometer at RIBF is reported. Rare isotopes produced at RIBF, $^{127}$Sn, $^{126}$In, $^{125}$Cd, $^{124}$Ag, $^{123}$Pd, were injected in R3. Masses of $^{126}$In, $^{125}$Cd, and $^{123}$Pd were measured whereby the mass uncertainty of $^{123}$Pd was improved. This is the first reported measurement with a new storage ring mass spectrometery technique realized at a heavy-ion cyclotron and employing individual injection of the pre-identified rare nuclei. The latter is essential for the future mass measurements of the rarest isotopes produced at RIBF. The impact of the new $^{123}$Pd result on the solar $r$-process abundances in a neutron star merger event is investigated by performing reaction network calculations of 20 trajectories with varying electron fraction $Y_e$. It is found that the neutron capture cross section on $^{123}$Pd increases by a factor of 2.2 and $β$-delayed neutron emission probability, $P_\mathrm{1n}$, of $^{123}$Rh increases by 14\%. The neutron capture cross section on $^{122}$Pd decreases by a factor of 2.6 leading to pileup of material at $A=122$, thus reproducing the trend of the solar $r$-process abundances. The trend of the two-neutron separation energies (S$_\mathrm{2n}$) was investigated for the Pd isotopic chain. The new mass measurement with improved uncertainty excludes large changes of the S$_\mathrm{2n}$ value at $N=77$. Such large increase of the S$_\mathrm{2n}$ values before $N=82$ was proposed as an alternative to the quenching of the $N=82$ shell gap to reproduce $r$-process abundances in the mass region of $A=112-124$.

nucl-ex↗

arXiv2022

Border of the Island of Inversion: Unbound states in $^{29}$Ne

The nucleus $^{29}$Ne is situated at the border of the island of inversion. Despite significant efforts, no bound low-lying intruder $f_{7/2}$-state, which would place $^{29}$Ne firmly inside the island of inversion, has yet been observed. Here, the first investigation of unbound states of $^{29}$Ne is reported. The states were populated in $^{30}\mathrm{Ne}(p,pn)$ and $^{30}\mathrm{Na}(p,2p)$ reactions at a beam energy of around $230$ MeV/nucleon, and analyzed in terms of their resonance properties, partial cross sections and momentum distributions. The momentum distributions are compared to calculations using the eikonal, direct reaction model, allowing $\ell$-assignments for the observed states. The lowest-lying resonance at an excitation energy of 1.48(4) MeV shows clear signs of a significant $\ell$=3-component, giving first evidence for $f_{7/2}$ single particle strength in $^{29}$Ne. The excitation energies and strengths of the observed states are compared to shell-model calculations using the sdpf-u-mix interaction

nucl-ex↗

arXiv2022

A First Glimpse at the Shell Structure beyond $^{54}$Ca: Spectroscopy of $^{55}$K, $^{55}$Ca, and $^{57}$Ca

States in the $N=35$ and 37 isotopes $^{55,57}$Ca have been populated by direct proton-induced nucleon removal reactions from $^{56,58}$Sc and $^{56}$Ca beams at the RIBF. In addition, the $(p,2p)$ quasi-free single-proton removal reaction from $^{56}$Ca was studied. Excited states in $^{55}$K, $^{55}$Ca, and $^{57}$Ca were established for the first time via in-beam $γ$-ray spectroscopy. Results for the proton and neutron removal reactions from $^{56}$Ca to states in $^{55}$K and $^{55}$Ca for the level energies, excited state lifetimes, and exclusive cross sections agree well with state-of-the-art theoretical calculations using different approaches. The observation of a short-lived state in $^{57}$Ca suggests a transition in the calcium isotopic chain from single-particle dominated states at $N=35$ to collective excitations at $N=37$.

nucl-ex↗

arXiv2021

First Gadolinium Loading to Super-Kamiokande

In order to improve Super-Kamiokande's neutron detection efficiency and to thereby increase its sensitivity to the diffuse supernova neutrino background flux, 13 tons of $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ (gadolinium sulfate octahydrate) was dissolved into the detector's otherwise ultrapure water from July 14 to August 17, 2020, marking the start of the SK-Gd phase of operations. During the loading, water was continuously recirculated at a rate of 60 m$^3$/h, extracting water from the top of the detector and mixing it with concentrated $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ solution to create a 0.02% solution of the Gd compound before injecting it into the bottom of the detector. A clear boundary between the Gd-loaded and pure water was maintained through the loading, enabling monitoring of the loading itself and the spatial uniformity of the Gd concentration over the 35 days it took to reach the top of the detector. During the subsequent commissioning the recirculation rate was increased to 120 m$^3$/h, resulting in a constant and uniform distribution of Gd throughout the detector and water transparency equivalent to that of previous pure-water operation periods. Using an Am-Be neutron calibration source the mean neutron capture time was measured to be $115\pm1$ $μ$s, which corresponds to a Gd concentration of $111\pm2$ ppm, as expected for this level of Gd loading. This paper describes changes made to the water circulation system for this detector upgrade, the Gd loading procedure, detector commissioning, and the first neutron calibration measurements in SK-Gd.

physics.ins-det↗