arXiv subjects

T. Nakamura

Publications and source records attributed to T. Nakamura.

At least 145 records · Page 8Linked to original sources

arXiv2018

Giant Rashba splitting of quasi-1D surface states on Bi/InAs(110)-(2$\times$1)

Electronic states on the Bi/InAs(110)-(2$\times$1) surface and its spin-polarized structure are revealed by angle-resolved photoelectron spectroscopy (ARPES), spin-resolved ARPES, and density-functional-theory calculation. The surface state showed quasi-one-dimensional (Q1D) dispersion and a nearly metallic character; the top of the hole-like surface band is just below the Fermi level. The size of the Rashba parameter ($α_{\rm R}$) reached quite a large value ($\sim$5.5 eVÅ). The present result would provide a fertile playground for further studies of the exotic electronic phenomena in 1D or Q1D systems with the spin-split electronic states as well as for advanced spintronic devices.

cond-mat.mtrl-sci↗

arXiv2018

Efficient Edelstein effects in one-atom-layer Tl-Pb compound

We have investigated direct and inverse Edelstein effects in a one-atom-layer Tl-Pb compound with a large Rashba-type spin splitting. In spin pumping experiments at room temperature, spin-to-charge conversion voltage due to the inverse Edelstein effect is clearly observed in Py/Cu/Tl-Pb trilayer samples. To confirm efficient spin-charge interconversion in Tl-Pb compounds, the direct Edelstein effect is also studied in the same trilayer samples by measuring modulation of the effective magnetization damping in the Py layer via the charge-to-spin conversion in the Tl-Pb layer. Using both the results of direct and inverse Edelstein effects, the Edelstein length is estimated to be ~0.1 nm for Tl-Pb compounds.

cond-mat.mes-hall↗

arXiv2018

Spin-orbit interaction in Pt or Bi2Te3 nanoparticle-decorated graphene realized by a nanoneedle method

The introduction of spin-orbit interactions (SOIs) and the subsequent appearance of a two-dimensional (2D) topological phase are crucial for voltage-controlled and zero-emission energy spintronic devices. In contrast, graphene basically lacks SOIs due to the small mass of the carbon atom, and appropriate experimental reports for SOIs are rare. Here, we control small-amount (cover ratios < 8%) random decoration of heavy nanoparticles [platinum (Pt) or bismuth telluride (Bi2Te3)] onto mono-layer graphene by developing an original nanoneedle method. X-ray photoelectron spectra support low-damage and low-contamination decoration of the nanoparticles, suggesting the presence of Bi-C and Te-C coupling orbitals. In the samples, we find particle-density-dependent non-local resistance (RNL) peaks, which are attributed to the (inverse) spin Hall effect (SHE) arising from SOI with energies as large as about 30 meV. This is a larger value than in previous reports and supported by scanning tunneling spectroscopy. The present observation should lead to topological phases of graphene, which can be introduced by random decoration with controlled small amounts of heavy nanoparticles, and their applications.

cond-mat.mes-hall↗

arXiv2018

Atmospheric neutrino oscillation analysis with external constraints in Super-Kamiokande I-IV

An analysis of atmospheric neutrino data from all four run periods of \superk optimized for sensitivity to the neutrino mass hierarchy is presented. Confidence intervals for $Δm^2_{32}$, $\sin^2 θ_{23}$, $\sin^2 θ_{13}$ and $δ_{CP}$ are presented for normal neutrino mass hierarchy and inverted neutrino mass hierarchy hypotheses based on atmospheric neutrino data alone. Additional constraints from reactor data on $θ_{13}$ and from published binned T2K data on muon neutrino disappearance and electron neutrino appearance are added to the atmospheric neutrino fit to give enhanced constraints on the above parameters. Over the range of parameters allowed at 90% confidence level, the normal mass hierarchy is favored by between 91.5% and 94.5% based on the combined result.

hep-ex↗

arXiv2018

Search for Boosted Dark Matter Interacting With Electrons in Super-Kamiokande

A search for boosted dark matter using 161.9 kiloton-years of Super-Kamiokande IV data is presented. We search for an excess of elastically scattered electrons above the atmospheric neutrino background, with a visible energy between 100 MeV and 1 TeV, pointing back to the Galactic Center or the Sun. No such excess is observed. Limits on boosted dark matter event rates in multiple angular cones around the Galactic Center and Sun are calculated. Limits are also calculated for a baseline model of boosted dark matter produced from cold dark matter annihilation or decay.

hep-ex↗

arXiv2018

Gamma-ray Showers Observed at Ground Level in Coincidence With Downward Lightning Leaders

Bursts of gamma ray showers have been observed in coincidence with downward propagating negative leaders in lightning flashes by the Telescope Array Surface Detector (TASD). The TASD is a 700~square kilometer cosmic ray observatory located in southwestern Utah, U.S.A. In data collected between 2014 and 2016, correlated observations showing the structure and temporal development of three shower-producing flashes were obtained with a 3D lightning mapping array, and electric field change measurements were obtained for an additional seven flashes, in both cases co-located with the TASD. National Lightning Detection Network (NLDN) information was also used throughout. The showers arrived in a sequence of 2--5 short-duration ($\le$10~$μ$s) bursts over time intervals of several hundred microseconds, and originated at an altitude of $\simeq$3--5 kilometers above ground level during the first 1--2 ms of downward negative leader breakdown at the beginning of cloud-to-ground lightning flashes. The shower footprints, associated waveforms and the effect of atmospheric propagation indicate that the showers consist primarily of downward-beamed gamma radiation. This has been supported by GEANT simulation studies, which indicate primary source fluxes of $\simeq$$10^{12}$--$10^{14}$ photons for $16^{\circ}$ half-angle beams. We conclude that the showers are terrestrial gamma-ray flashes (TGFs), similar to those observed by satellites, but that the ground-based observations are more representative of the temporal source activity and are also more sensitive than satellite observations, which detect only the most powerful TGFs.

physics.ao-ph↗

arXiv2018

Depth of Ultra High Energy Cosmic Ray Induced Air Shower Maxima Measured by the Telescope Array Black Rock and Long Ridge FADC Fluorescence Detectors and Surface Array in Hybrid Mode

The Telescope Array observatory utilizes fluorescence detectors and surface detectors to observe air showers produced by ultra high energy cosmic rays in the Earth's atmosphere. Cosmic ray events observed in this way are termed hybrid data. The depth of air shower maximum is related to the mass of the primary particle that generates the shower. This paper reports on shower maxima data collected over 8.5 years using the Black Rock Mesa and Long Ridge fluorescence detectors in conjunction with the array of surface detectors. We compare the means and standard deviations of the observed $X_{\mathrm{max}}$ distributions with Monte Carlo $X_{\mathrm{max}}$ distributions of unmixed protons, helium, nitrogen, and iron, all generated using the QGSJet~II-04 hadronic model. We also perform an unbinned maximum likelihood test of the observed data, which is subjected to variable systematic shifting of the data $X_{\mathrm{max}}$ distributions to allow us to test the full distributions, and compare them to the Monte Carlo to see which elements are not compatible with the observed data. For all energy bins, QGSJet~II-04 protons are found to be compatible with Telescope Array hybrid data at the 95% confidence level after some systematic $X_{\mathrm{max}}$ shifting of the data. Three other QGSJet~II-04 elements are found to be compatible using the same test procedure in an energy range limited to the highest energies where data statistics are sparse.

astro-ph.HE↗

arXiv2018

Study of muons from ultra-high energy cosmic ray air showers measured with the Telescope Array experiment

One of the uncertainties in interpretation of ultra-high energy cosmic ray (UHECR) data comes from the hadronic interaction models used for air shower Monte Carlo (MC) simulations. The number of muons observed at the ground from UHECR-induced air showers is expected to depend upon the hadronic interaction model. One may therefore test the hadronic interaction models by comparing the measured number of muons with the MC prediction. In this paper, we present the results of studies of muon densities in UHE extensive air showers obtained by analyzing the signal of surface detector stations which should have high $\it{muon \, purity}$. The muon purity of a station will depend on both the inclination of the shower and the relative position of the station. In 7 years' data from the Telescope Array experiment, we find that the number of particles observed for signals with an expected muon purity of $\sim$65% at a lateral distance of 2000 m from the shower core is $1.72 \pm 0.10{\rm (stat.)} \pm 0.37 {\rm (syst.)}$ times larger than the MC prediction value using the QGSJET II-03 model for proton-induced showers. A similar effect is also seen in comparisons with other hadronic models such as QGSJET II-04, which shows a $1.67 \pm 0.10 \pm 0.36$ excess. We also studied the dependence of these excesses on lateral distances and found a slower decrease of the lateral distribution of muons in the data as compared to the MC, causing larger discrepancy at larger lateral distances.

astro-ph.HE↗

arXiv2018

Search for Neutrinos in Super-Kamiokande associated with the GW170817 neutron-star merger

We report the results of a neutrino search in Super-Kamiokande for coincident signals with the first detected gravitational wave produced by a binary neutron star merger, GW170817, which was followed by a short gamma-ray burst, GRB170817A, and a kilonova/macronova. We searched for coincident neutrino events in the range from 3.5 MeV to $\sim$100 PeV, in a time window $\pm$500 seconds around the gravitational wave detection time, as well as during a 14-day period after the detection. No significant neutrino signal was observed for either time window. We calculated 90% confidence level upper limits on the neutrino fluence for GW170817. From the upward-going-muon events in the energy region above 1.6 GeV, the neutrino fluence limit is $16.0^{+0.7}_{-0.6}$ ($21.3^{+1.1}_{-0.8}$) cm$^{-2}$ for muon neutrinos (muon antineutrinos), with an error range of $\pm5^{\circ}$ around the zenith angle of NGC4993, and the energy spectrum is under the assumption of an index of $-2$. The fluence limit for neutrino energies less than 100 MeV, for which the emission mechanism would be different than for higher-energy neutrinos, is also calculated. It is $6.6 \times 10^7$ cm$^{-2}$ for anti-electron neutrinos under the assumption of a Fermi-Dirac spectrum with average energy of 20 MeV.

astro-ph.HE↗

arXiv2018

The Cosmic-Ray Energy Spectrum between 2 PeV and 2 EeV Observed with the TALE detector in monocular mode

We report on a measurement of the cosmic ray energy spectrum by the Telescope Array Low-Energy Extension (TALE) air fluorescence detector. The TALE air fluorescence detector is also sensitive to the Cherenkov light produced by shower particles. Low energy cosmic rays, in the PeV energy range, are detectable by TALE as "Cherenkov Events". Using these events, we measure the energy spectrum from a low energy of $\sim 2$ PeV to an energy greater than 100 PeV. Above 100 PeV TALE can detect cosmic rays using air fluorescence. This allows for the extension of the measurement to energies greater than a few EeV. In this paper, we will describe the detector, explain the technique, and present results from a measurement of the spectrum using $\sim 1000$ hours of observation. The observed spectrum shows a clear steepening near $10^{17.1}$ eV, along with an ankle-like structure at $10^{16.2}$ eV. These features present important constraints on galactic cosmic rays origin and propagation models. The feature at $10^{17.1}$ eV may also mark the end of the galactic cosmic rays flux and the start of the transition to extra-galactic sources.

astro-ph.HE↗

arXiv2018

Evidence of Intermediate-Scale Energy Spectrum Anisotropy of Cosmic Rays E$\geq$10$^{19.2}$ eV with the Telescope Array Surface Detector

An intermediate-scale energy spectrum anisotropy has been found in the arrival directions of ultra-high energy cosmic rays of energies above $10^{19.2}$ eV in the northern hemisphere, using 7 years of data from the Telescope Array surface detector. A relative energy distribution test is done comparing events inside oversampled spherical caps of equal exposure, to those outside, using the Poisson likelihood ratio. The center of maximum significance is at $9^h$$16^m$, $45^{\circ}$. and has a deficit of events with energies $10^{19.2}$$\leq$$E$$<$$10^{19.75}$ eV and an excess for $E$$\geq$$10^{19.75}$ eV. The post-trial probability of this energy anisotropy, appearing by chance anywhere on an isotropic sky, is found by Monte Carlo simulation to be $9$$\times$$10^{-5}$ ($3.74$$σ_{global}$).

astro-ph.HE↗

arXiv2018

Search for an excess of events in the Super-Kamiokande detector in the directions of the astrophysical neutrinos reported by the IceCube Collaboration

We present the results of a search in the Super-Kamiokande (SK) detector for excesses of neutrinos with energies above a few GeV that are in the direction of the track events reported in IceCube. Data from all SK phases (SK-I through SK-IV) were used, spanning a period from April 1996 to April 2016 and corresponding to an exposure of 225 kilotonne-years . We considered the 14 IceCube track events from a data set with 1347 livetime days taken from 2010 to 2014. We use Poisson counting to determine if there is an excess of neutrinos detected in SK in a 10 degree search cone (5 degrees for the highest energy data set) around the reconstructed direction of the IceCube event. No significant excess was found in any of the search directions we examined. We also looked for coincidences with a recently reported IceCube multiplet event. No events were detected within a $\pm$ 500 s time window around the first detected event, and no significant excess was seen from that direction over the lifetime of SK.

astro-ph.HE↗

arXiv2017

Construction of KAGRA: an Underground Gravitational Wave Observatory

Major construction and initial-phase operation of a second-generation gravitational-wave detector KAGRA has been completed. The entire 3-km detector is installed underground in a mine in order to be isolated from background seismic vibrations on the surface. This allows us to achieve a good sensitivity at low frequencies and high stability of the detector. Bare-bones equipment for the interferometer operation has been installed and the first test run was accomplished in March and April of 2016 with a rather simple configuration. The initial configuration of KAGRA is named {\it iKAGRA}. In this paper, we summarize the construction of KAGRA, including the study of the advantages and challenges of building an underground detector and the operation of the iKAGRA interferometer together with the geophysics interferometer that has been constructed in the same tunnel.

gr-qc↗

arXiv2017

A Measurement of the Tau Neutrino Cross Section in Atmospheric Neutrino Oscillations with Super-Kamiokande

Using 5,326 days of atmospheric neutrino data, a search for atmospheric tau neutrino appearance has been performed in the Super-Kamiokande experiment. Super-Kamiokande measures the tau normalization to be 1.47$\pm$0.32 under the assumption of normal neutrino hierarchy, relative to the expectation of unity with neutrino oscillation. The result excludes the hypothesis of no-tau-appearance with a significance level of 4.6$σ$. The inclusive charged-current tau neutrino cross section averaged by the tau neutrino flux at Super-Kamiokande is measured to be $(0.94\pm0.20)\times 10^{-38}$ cm$^{2}$. The measurement is consistent with the Standard Model prediction, agreeing to within 1.5$σ$.

hep-ex↗

arXiv2017

Spin-polarized quasi 1D state with finite bandgap on the Bi/InSb(001) surface

One-dimensional (1D) electronic states were discovered on 1D surface atomic structure of Bi fabricated on semiconductor InSb(001) substrates by angle-resolved photoelectron spectroscopy (ARPES). The 1D state showed steep, Dirac-cone-like dispersion along the 1D atomic structure with a finite direct bandgap opening as large as 150 meV. Moreover, spin-resolved ARPES revealed the spin polarization of the 1D unoccupied states as well as that of the occupied states, the orientation of which inverted depending on the wave vector direction parallel to the 1D array on the surface. These results reveal that a spin-polarized quasi-1D carrier was realized on the surface of 1D Bi with highly efficient backscattering suppression, showing promise for use in future spintronic and energy-saving devices.

cond-mat.mtrl-sci↗

arXiv2017

The status of KAGRA underground cryogenic gravitational wave telescope

KAGRA is a 3-km interferometric gravitational wave telescope located in the Kamioka mine in Japan. It is the first km-class gravitational wave telescope constructed underground to reduce seismic noise, and the first km-class telescope to use cryogenic cooling of test masses to reduce thermal noise. The construction of the infrastructure to house the interferometer in the tunnel, and the initial phase operation of the interferometer with a simple 3-km Michelson configuration have been completed. The first cryogenic operation is expected in 2018, and the observing runs with a full interferometer are expected in 2020s. The basic interferometer configuration and the current status of KAGRA are described.

gr-qc↗

arXiv2017

Search for Anisotropy in the Ultra High Energy Cosmic Ray Spectrum using the Telescope Array Surface Detector

The Telescope Array (TA) experiment is located in the western desert of Utah, USA, and observes ultra high energy cosmic rays (UHECRs) in the Northern hemisphere. At the highest energies, $E>10$~EeV, the shape of cosmic ray energy spectrum may carry an imprint of the source density distribution along the line of sight different in different directions of the sky. In this study, we search for such directional variations in the shape of the energy spectrum using events observed with the Telescope Array's surface detector. We divide the TA field of view into two nearly equal-exposure regions: the "on-source" region which we define as $\pm 30^\circ$ of the supergalactic plane containing mostly nearby structures, and the complementary "off-source" region where the sources are further away on average. We compare the UHECR spectra in these regions by fitting them to the broken power law and comparing the resulting parameters. We find that the off-source spectrum has an earlier break at highest energies. The chance probability to obtain such or larger difference in statistically equivalent distributions is estimated as $6.2\pm1.1\times10^{-4}$ ($3.2σ$) by a Monte-Carlo simulation. The observed difference in spectra is in a reasonable quantitative agreement with a simplified model that assumes that the UHECR sources trace the galaxy distribution from the 2MRS catalogue, primary particles are protons and the magnetic deflections can be neglected.

astro-ph.HE↗

arXiv2017

Three-dimensional band structure of LaSb and CeSb:Absence of band inversion

We have performed angle-resolved photoemission spectroscopy (ARPES) of LaSb and CeSb, a candidate of topological insulator. Using soft-x-ray photons, we have accurately determined the three-dimensional bulk band structure and revealed that the band inversion at the Brillouin-zone corner - a prerequisite for realizing topological-insulator phase - is absent in both LaSb and CeSb. Moreover, unlike the ARPES data obtained with soft-x-ray photons, those with vacuum ultraviolet (VUV) photons were found to suffer significant $k_z$ broadening. These results suggest that LaSb and CeSb are topologically trivial semimetals, and unusual Dirac-cone-like states observed with VUV photons are not of the topological origin.

cond-mat.mtrl-sci↗