arXiv subjects

T. Yoshida

Publications and source records attributed to T. Yoshida.

At least 73 records · Page 4Linked to original sources

arXiv2012

Self-Energy Effects on the Low- to High-Energy Electronic Structure of SrVO3

The correlated electronic structure of SrVO3 has been investigated by angle-resolved photoemission spectroscopy using in-situ prepared thin films. Pronounced features of band renormalization have been observed: a sharp kink ~60 meV below the Fermi level (EF) and a broad so-called "high-energy kink" ~0.3 eV below EF as in the high-Tc cuprates although SrVO3 does not show magnetic fluctuations. We have deduced the self-energy in a wide energy range by applying the Kramers-Kronig relation to the observed spectra. The obtained self-energy clearly shows a large energy scale of ~0.7 eV which is attributed to electron-electron interaction and gives rise to the ~0.3 eV "kink" in the band dispersion as well as the incoherent peak ~1.5eV below EF. The present analysis enables us to obtain consistent picture both for the incoherent spectra and the band renormalization.

cond-mat.str-el↗

arXiv2012

Search for Antihelium with the BESS-Polar Spectrometer

In two long-duration balloon flights over Antarctica, the BESS-Polar collaboration has searched for antihelium in the cosmic radiation with higher sensitivity than any reported investigation. BESS- Polar I flew in 2004, observing for 8.5 days. BESS-Polar II flew in 2007-2008, observing for 24.5 days. No antihelium candidate was found in BESS-Polar I data among 8.4\times 10^6 |Z| = 2 nuclei from 1.0 to 20 GV or in BESS-Polar II data among 4.0\times 10^7 |Z| = 2 nuclei from 1.0 to 14 GV. Assuming antihelium to have the same spectral shape as helium, a 95% confidence upper limit of 6.9 \times 10^-8 was determined by combining all the BESS data, including the two BESS-Polar flights. With no assumed antihelium spectrum and a weighted average of the lowest antihelium efficiencies from 1.6 to 14 GV, an upper limit of 1.0 \times 10^-7 was determined for the combined BESS-Polar data. These are the most stringent limits obtained to date.

astro-ph.CO↗

arXiv2011

Measurement of the cosmic-ray antiproton spectrum at solar minimum with a long-duration balloon flight over Antarctica

The energy spectrum of cosmic-ray antiprotons from 0.17 to 3.5 GeV has been measured using 7886 antiprotons detected by BESS-Polar II during a long-duration flight over Antarctica near solar minimum in December 2007 and January 2008. This shows good consistency with secondary antiproton calculations. Cosmologically primary antiprotons have been investigated by comparing measured and calculated antiproton spectra. BESS-Polar II data show no evidence of primary antiprotons from evaporation of primordial black holes.

astro-ph.HE↗

arXiv2011

Electronic reconstruction through the structural and magnetic transitions in detwinned NaFeAs

We use angle-resolved photoemission spectroscopy to study twinned and detwinned iron pnictide compound NaFeAs. Distinct signatures of electronic reconstruction are observed to occur at the structural (TS) and magnetic (TSDW) transitions. At TS, C4 rotational symmetry is broken in the form of an anisotropic shift of the orthogonal dxz and dyz bands. The magnitude of this orbital anisotropy rapidly develops to near completion upon approaching TSDW, at which temperature band folding occurs via the antiferromagnetic ordering wave vector. Interestingly, the anisotropic band shift onsetting at TS develops in such a way to enhance the nesting conditions in the C2 symmetric state, hence is intimately correlated with the long range collinear AFM order. Furthermore, the similar behaviors of the electronic reconstruction in NaFeAs and Ba(Fe1-xCox)2As2 suggests that this rapid development of large orbital anisotropy between TS and TSDW is likely a general feature of the electronic nematic phase in the iron pnictides, and the associated orbital fluctuations may play an important role in determining the ground state properties.

cond-mat.supr-con↗

arXiv2011

CANGAROO-III observation of TeV gamma rays from the unidentified gamma-ray source HESS J1614-518

We report the detection, with the CANGAROO-III imaging atmospheric Cherenkov telescope array, of a very high energy gamma-ray signal from the unidentified gamma-ray source HESS J1614-518, which was discovered in the H.E.S.S. Galactic plane survey. Diffuse gamma-ray emission was detected above 760 GeV at the 8.9 sigma level during an effective exposure of 54 hr from 2008 May to August. The spectrum can be represented by a power-law: 8.2+-2.2_{stat}+-2.5_{sys}x10^{-12}x (E/1TeV)^{-Gamma} cm^{-2} s^{-1} TeV^{-1} with a photon index Gamma of 2.4+-0.3_{stat}+-0.2_{sys}, which is compatible with that of the H.E.S.S. observations. By combining our result with multi-wavelength data, we discuss the possible counterparts for HESS J1614-518 and consider radiation mechanisms based on hadronic and leptonic processes for a supernova remnant, stellar winds from massive stars, and a pulsar wind nebula. Although a leptonic origin from a pulsar wind nebula driven by an unknown pulsar remains possible, hadronic-origin emission from an unknown supernova remnant is preferred.

astro-ph.HE↗

arXiv2011

Asymptotics for penalized additive B-spline regression

This paper is concerned with asymptotic theory for penalized spline estimator in bivariate additive model. The focus of this paper is put upon the penalized spline estimator obtained by the backfitting algorithm. The convergence of the algorithm as well as the uniqueness of its solution are shown. The asymptotic bias and variance of penalized spline estimator are derived by an efficient use of the asymptotic results for the penalized spline estimator in marginal univariate model. Asymptotic normality of estimator is also developed, by which an approximate confidence interval can be obtained. Some numerical experiments confirming theoretical results are provided.

math.ST↗

arXiv2011

An energy scale directly related to superconductivity in the high-$T_c$ cuprate superconductors: Universality from the Fermi arc picture

We have performed a temperature dependent angle-resolved photoemission spectroscopy (ARPES) study of the tri-layer high-$T_c$ cuprate superconductor (HTSC) Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ (Bi2223), and have shown that the \textquotedblleft effective\textquotedblright superconducting (SC) gap $Δ_{\rm{sc}}$ defined at the end point of the Fermi arc and the $T_c$ (= 110 K) approximately satisfies the weak-coupling BCS-relationship 2$Δ_{\rm{sc}}$ = 4.3$k_{\rm{B}}T_c$. Combining this result with previous ARPES results on single- and double-layer cuprates, we show that the relationship between 2$Δ_{\rm{sc}}$ = 4.3$k_{\rm{B}}T_c$ holds for various HTSCs. Furthermore, at $T$ $\sim$ $T_c$, the quasi-patricle width at the end point of the Fermi arc is found to coincide with $Δ_{\rm{sc}}$, consistent with the context of Planckian dissipation.

cond-mat.supr-con↗

arXiv2011

Angle-resolved photoemission spectroscopy study of PrFeAsO_{0.7}: Dependence of the electronic structure on the pnictogen height

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the iron-based superconductor PrFeAsO_{0.7} and examined the Fermi surfaces and band dispersions near the Fermi level. Heavily hole-doped electronic states have been observed due to the polar nature of the cleaved surfaces. Nevertheless, we have found that the ARPES spectra basically agree with band dispersions calculated in the local density approximation (LDA) if the bandwidth is reduced by a factor of ~2.5 and then the chemical potential is lowered by ~70 meV. Comparison with previous ARPES results on LaFePO reveals that the energy positions of the d_{3z^2-r^2}- and d_{yz,zx}-derived bands are considerably different between the two materials, which we attribute to the different pnictogen height as predicted by the LDA calculation.

cond-mat.supr-con↗

arXiv2011

Symplectic structure and monopole strength in 12C

The relation between the monopole transition strength and existence of cluster structure in the excited states is discussed based on an algebraic cluster model. The structure of $^{12}$C is studied with a 3$α$ model, and the wave function for the relative motions between $α$ clusters are described by the symplectic algebra $Sp(2,R)_z$, which corresponds to the linear combinations of $SU(3)$ states with different multiplicities. Introducing $Sp(2,R)_z$ algebra works well for reducing the number of the basis states, and it is also shown that states connected by the strong monopole transition are classified by a quantum number $Λ$ of the $Sp(2,R)_z$ algebra.

nucl-th↗

arXiv2010

Doping dependence of the $(π,π)$ shadow band in La-based cuprates studied by angle-resolved photoemission spectroscopy

The $(π,π)$ shadow band (SB) in La-based cuprate family (La214) was studied by angle-resolved photoemission spectroscopy (ARPES) over a wide doping range from $x=0.01$ to $x=0.25$. Unlike the well-studied case of the Bi-based cuprate family, an overall strong, monotonic doping dependence of the SB intensity at the Fermi level ($E_F$) was observed. In contrast to a previous report for the presence of the SB only close to $x=1/8$, we found it exists in a wide doping range, associated with a doping-independent $(π,π)$ wave vector but strongly doping-dependent intensity: It is the strongest at $x\sim 0.03$ and systematically diminishes as the doping increases until it becomes negligible in the overdoped regime. This SB with the observed doping dependence of intensity can in principle be caused by the antiferromagnetic fluctuations or a particular form of low-temperature orthorhombic lattice distortion known to persist up to $x\sim 0.21$ in the system, with both being weakened with increasing doping. However, a detailed binding energy dependent analysis of the SB at $x=0.07$ does not appear to support the former interpretation, leaving the latter as a more plausible candidate, despite a challenge in quantitatively linking the doping dependences of the SB intensity and the magnitude of the lattice distortion. Our finding highlights the necessity of a careful and global consideration of the inherent structural complications for correctly understanding the cuprate Fermiology and its microscopic implication.

cond-mat.supr-con↗

arXiv2010

The General Antiparticle Spectrometer (GAPS) - Hunt for dark matter using low-energy antideuterons

The GAPS experiment is foreseen to carry out a dark matter search using a novel detection approach to detect low-energy cosmic-ray antideuterons. The theoretically predicted antideuteron flux resulting from secondary interactions of primary cosmic rays with the interstellar medium is very low. So far not a single cosmic antideuteron has been detected by any experiment, but well-motivated theories beyond the standard model of particle physics, e.g., supersymmetry or universal extra dimensions, contain viable dark matter candidates, which could led to a significant enhancement of the antideuteron flux due to self-annihilation of the dark matter particles.This flux contribution is believed to be especially large at small energies, which leads to a high discovery potential for GAPS. GAPS is designed to achieve its goals via a series of ultra-long duration balloon flights at high altitude in Antarctica, starting in 2014. The detector itself will consist of 13 planes of Si(Li) solid state detectors and a time of flight system. The low-energy antideuterons (< 0.3 GeV/n) will be slowed down in the Si(Li) material, replace a shell electron, and form an excited exotic atom. The atom will be deexcited by characteristic x-ray transitions and will end its life by forming an annihilation pion star. This unique event structure will allow for nearly background free detection. To prove the performance of the different detector components at stratospheric altitudes, a prototype flight will be conducted in 2011 from Taiki, Japan.

astro-ph.IM↗

arXiv2010

Two- and Three-Dimensional Fermi Surfaces and their Nesting Properties in Superconducting BaFe2(As1-xPx)2

We have studied the three-dimensional shapes of the Fermi surfaces (FSs) of BaFe2(As1-xPx)2 (x=0.38), where superconductivity is induced by isovalent P substitution, by angle-resolved photoemission spectroscopy. Moderately strong electron mass enhancement has been identified for both the electron and hole FSs. Among two observed hole FSs, the nearly two-dimensional one shows good nesting with the outer two-dimensional electron FS, but its orbital character is different from the outer electron FS. The three-dimensional hole FS shows poor nesting with the electron FSs. The present results suggest that the three-dimensionality and the difference in the orbital character weaken FS nesting, leaving partial nesting among the outer electron FSs of dxy character and/or that within the three-dimensional hole FS, which may lead to the nodal superconductivity.

cond-mat.supr-con↗

arXiv2010

Fermi surfaces and quasi-particle band dispersions of the iron pnictides superconductor KFe2As2 observed by angle-resolved photoemission spectroscopy

We have performed an angle-resolved photoemission study of the iron pnictide superconductor KFe2As2 with Tc 4 K. Most of the observed Fermi surfaces show almost two-dimensional shapes, while one of the quasi-particle bands near the Fermi level has a strong dispersion along the kz direction, consistent with the result of a band-structure calculation. However, hole Fermi surfaces αand ζare smaller than those predicted by the calculation while other Fermi surfaces are larger. These observations are consistent with the result of a de Haas-van Alphen study and a theoretical prediction on inter-band scattering, possibly indicating many body effects on the electronic structure.

cond-mat.str-el↗

arXiv2010

Zn-impurity effects on quasi-particle scattering in La2-xSrxCuO4 studied by angle-resolved photoemission spectroscopy

Angle-resolved photoemission measurements were performed on Zn-doped La2-xSrxCuO4 (LSCO) to investigate the effects of Zn impurities on the low energy electronic structure. The Zn-impurity-induced increase in the quasi-particle (QP) width in momentum distribution curves (MDC) is approximately isotropic on the entire Fermi surface and energy-independent near the Fermi level (EF). The increase in the MDC width is consistent with the increase in the residual resistivity due to the Zn impurities if we assume the carrier number to be 1-x for x=0.17 and the Zn impurity to be a potential scatterer close to the unitarity limit. For x=0.03, the residual resistivity is found to be higher than that expected from the MDC width, and the effects of antifferomagnetic fluctuations induced around the Zn impurities are discussed. The leading edges of the spectra near (pi,0) for x=0.17 are shifted toward higher energies relative to EF with Zn substitution, indicating a reduction of the superconducting gap.

cond-mat.str-el↗

arXiv2010

Mass renormalization in the band width-controlled Mott-Hubbard systems SrVO3 and CaVO3 studied by angle-resolved photoemission spectroscopy

Ca1-xSrxVO3 is a Mott-Hubbard-type correlated electron system whose bandwidth can be varied by the V-O-V bond angle, but the actual effect of bandwidth control on the electronic structure has been controversial in previous photoemission experiments. In this work, band dispersions and Fermi surfaces of SrVO3 and CaVO3 are studied by angle-resolved photoemission spectroscopy. Near the Fermi level (EF), three bands forming cylindricalFermi surfaces derived from the three V 3d t2g orbitals have been observed. The observed band widths for both compounds are almost half of those predicted by local-density-approximation band-structure calculation, confirming mass renormalization caused by electron correlation. It has been clearly demonstrated that the width of the d band in CaVO3 is narrower than that in SrVO3, qualitatively consistent with the result of band-structure calculation. Roles of the orthorhombic lattice distortion and electron correlation in the observed band narrowing are discussed.

cond-mat.str-el↗

arXiv2010

Evolution of the electronic structure from the insulator to the superconductor in Bi2Sr2-xLax(Ca,Y)Cu2O8+d

La-doped and Y-doped Bi2Sr2CaCu2O8+d (Bi2212) compounds Bi2Sr2-xLax(Ca,Y)Cu2O8+d, which range from the insulator to the deeply underdoped superconductor, have been studied by angle-resolved photoemission spectroscopy. We have observed that the lower Hubbard band (LHB) of the parent insulator is gradually shifted upward with doping without significantly changing the band dispersions, which implies a downward shift of the chemical potential with hole doping. This behaviour is analogous to Bi2Sr2-xLaxCuO6+d (Bi2201) and Ca2-xNaxCuO2Cl2 (Na-CCOC) but is different from La2-xSrxCuO4 (LSCO), where the LHB stays well below the chemical potential and does not move while its intensity quickly diminishes in the underdoped region.

cond-mat.supr-con↗

arXiv2010

Neutrino oscillation and expected event rate of supernova neutrinos in adiabatic explosion model

We study how the influence of the shock wave appears in neutrino oscillations and the neutrino spectrum using density profile of adiabatic explosion model of a core-collapse supernova which is calculated in an implicit Lagrangian code for general relativistic spherical hydrodynamics. We calculate expected event rates of neutrino detection at SK and SNO for various theta_{13} values and both normal and inverted hierarchies. The predicted event rates of bar{nu}_e and nu_e depend on the mixing angle theta_{13} for the inverted and normal hierarchies, respectively, and the influence of the shock appears for about 2 - 8 s when sin^2 2 theta_{13} is larger than 10^{-3}. These neutrino signals for the shock propagation is decreased by < 30 % for bar{nu}_e in inverted (SK) or by < 15 % for nu_e in normal hierarchy (SNO) compared with the case without shock. The obtained ratio of the total event for high-energy neutrinos (20 MeV < E_{nu} < 60 MeV) to low-energy neutrinos (5 MeV < E_{nu} < 20 MeV) is consistent with the previous studies in schematic semi-analytic or other hydrodynamic models of the shock propagation. The time dependence of the calculated ratio of the event rates of high-energy to low-energy neutrinos is a very useful observable which is sensitive to theta_{13} and hierarchies. Namely, time-dependent ratio shows clearer signal of the shock propagation that exhibits remarkable decrease by at most factor \sim 2 for bar{nu}_e in inverted (SK), whereas it exhibits smaller change by \sim 10 % for nu_e in normal hierarchy (SNO). Observing time-dependent high-energy to low-energy ratio of the neutrino events thus would provide a piece of very useful information to constrain theta_{13} and mass hierarchy, and eventually help understanding the propagation how the shock wave propagates inside the star.

astro-ph.HE↗

arXiv2010

Characteristic electronic structure and its doping evolution in lightly-doped to underdoped YBa2Cu3Oy

We have performed an angle resolved photoemission spectroscopy (ARPES) study of lightly-doped to underdoped YBa2Cu3Oy (YBCO) untwinned single crystals and a core-level x-ray photoemission spectroscopy (XPS) study of YBCO single and polycrystals. In the zone diagonal (nodal) direction, dispersive quasi-particle (QP) features crossing the Fermi level were observed down to the hole concentration of ~ 4%, which explains the metallic transport of the lightly-doped YBCO. The chemical potential shift estimated from XPS was more rapid than in the Bi2212 cuprates. Upon hole doping, very rapid spectral weight transfer from high binding energies to the QP feature, even faster than La2-xSrxCuO4, was observed.

cond-mat.supr-con↗