arXiv subjects

K. Tanaka

Publications and source records attributed to K. Tanaka.

At least 127 records · Page 7Linked to original sources

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

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↗

arXiv2009

Precise measurement of HFS of positronium

The ground state hyperfine splitting in positronium, $Δ_{\mathrm{HFS}}$, is sensitive to high order corrections of QED. A new calculation up to $O(α^3)$ has revealed a $3.9 σ$ discrepancy between the QED prediction and the experimental results. This discrepancy might either be due to systematic problems in the previous experiments or to contributions beyond the Standard Model. We propose an experiment to measure $Δ_{\mathrm{HFS}}$ employing new methods designed to remedy the systematic errors which may have affected the previous experiments. Our experiment will provide an independent check of the discrepancy. The measurement is in progress and a preliminary result of $Δ_{\mathrm{HFS}} = 203.399 \pm 0.029 \mathrm{GHz} (143 \mathrm{ppm})$ has been obtained. A measurement with a precision of O(1) ppm is expected within a few years.

hep-ex↗

arXiv2009

A Cooper pair light emitting diode

We demonstrate Cooper-pair's drastic enhancement effect on band-to-band radiative recombination in a semiconductor. Electron Cooper pairs injected from a superconducting electrode into an active layer by the proximity effect recombine with holes injected from a p-type electrode and dramatically accelerate the photon generation rates of a light emitting diode in the optical-fiber communication band. Cooper pairs are the condensation of electrons at a spin-singlet quantum state and this condensation leads to the observed enhancement of the electric-dipole transitions. Our results indicate the possibility to open up new interdisciplinary fields between superconductivity and optoelectronics.

cond-mat.other↗

arXiv2009

A momentum-dependent perspective on quasiparticle interference in Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}

Angle Resolved Photoemission Spectroscopy (ARPES) probes the momentum-space electronic structure of materials, and provides invaluable information about the high-temperature superconducting cuprates. Likewise, the cuprate real-space, inhomogeneous electronic structure is elucidated by Scanning Tunneling Spectroscopy (STS). Recently, STS has exploited quasiparticle interference (QPI) - wave-like electrons scattering off impurities to produce periodic interference patterns - to infer properties of the QP in momentum-space. Surprisingly, some interference peaks in Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} (Bi-2212) are absent beyond the antiferromagnetic (AF) zone boundary, implying the dominance of particular scattering process. Here, we show that ARPES sees no evidence of quasiparticle (QP) extinction: QP-like peaks are measured everywhere on the Fermi surface, evolving smoothly across the AF zone boundary. This apparent contradiction stems from different natures of single-particle (ARPES) and two-particle (STS) processes underlying these probes. Using a simple model, we demonstrate extinction of QPI without implying the loss of QP beyond the AF zone boundary.

cond-mat.supr-con↗

arXiv2009

Dependence of Band Renormalization Effect on the Number of Copper-oxide Layers in Tl-based Copper-oxide Superconductor using Angle-resolved Photoemission Spectroscopy

Here we report the first angle-resolved photoemission measurement on nearly optimally-doped multi-layer Tl-based superconducting cuprates (Tl-2212 and Tl-1223) and a comparison study to single layer (Tl-2201) compound. A "kink" in the band dispersion is found in all three compounds but exhibits different momentum dependence for the single layer and multi-layer compounds, reminiscent to that of Bi-based cuprates. This layer number dependent renormalization effect strongly implies that the spin resonance mode is unlikely responsible for the dramatic renormalization effect near the antinodal region.

cond-mat.supr-con↗

arXiv2009

Spectroscopy of 32Ne and the Island of Inversion

We report on the first spectroscopic study of the N=22 nucleus 32Ne at the newly completed RIKEN Radioactive Ion Beam Factory. A single gamma-ray line with an energy of 722(9) keV was observed in both inelastic scattering of a 226 MeV/u 32Ne beam on a Carbon target and proton removal from 33Na at 245 MeV/u. This transition is assigned to the de-excitation of the first J^pi = 2+ state in 32Ne to the 0+ ground state. Interpreted through comparison with state-of-the-art shell model calculations, the low excitation energy demonstrates that the Island of Inversion extends to at least N=22 for the Ne isotopes.

nucl-ex↗

arXiv2009

Coherent quasi-particles-to-incoherent hole-carriers crossover in underdoped cuprates

In underdoped cuprates, only a portion of the Fermi surface survives as Fermi arcs due to pseudogap opening. In hole-doped La$_{2}$CuO$_4$, we have deduced the "coherence temperature" $T_{coh}$ of quasi-particles on the Fermi arc above which the broadened leading edge position in angle-integrated photoemission spectra is shifted away from the Fermi level and the quasi-particle concept starts to lose its meaning. $T_{coh}$ is found to rapidly increase with hole doping, an opposite behavior to the pseudogap temperature $T^*$. The superconducting dome is thus located below both $T^*$ and $T_{coh}$, indicating that the superconductivity emerges out of the coherent Fermionic quasi-particles on the Fermi arc. $T_{coh}$ remains small in the underdoped region, indicating that incoherent charge carriers originating from the Fermi arc are responsible for the apparently metallic transport at high temperatures.

cond-mat.supr-con↗

arXiv2008

Abrikosov flux-lines in two-band superconductors with mixed dimensionality

We study vortex structure in a two-band superconductor, in which one band is ballistic and quasi-two-dimensional (2D), and the other is diffusive and three-dimensional (3D). A circular cell approximation of the vortex lattice within the quasiclassical theory of superconductivity is applied to a recently developed model appropriate for such a two-band system [Tanaka et al 2006 Phys. Rev. B 73, 220501(R); Tanaka et al 2007 Phys. Rev. B 75, 214512]. We assume that superconductivity in the 3D diffusive band is "weak", i.e., mostly induced, as is the case in MgB$_2$. Hybridization with the "weak" 3D diffusive band has significant and intriguing influence on the electronic structure of the "strong" 2D ballistic band. In particular, the Coulomb repulsion and the diffusivity in the "weak" band enhance suppression of the order parameter and enlargement of the vortex core by magnetic field in the "strong" band, resulting in reduced critical temperature and field. Moreover, increased diffusivity in the "weak" band can result in an upward curvature of the upper critical field near the transition temperature. A particularly interesting feature found in our model is the appearance of additional bound states at the gap edge in the "strong" ballistic band, which are absent in the single-band case. Furthermore, coupling with the "weak" diffusive band leads to reduced band gaps and van Hove singularities of energy bands of the vortex lattice in the "strong" ballistic band. We find these intriguing features for parameter values appropriate for MgB$_2$.

cond-mat.supr-con↗

arXiv2008

Universal versus Material-Dependent Two-Gap Behaviors in the High-Tc Cuprates: Angle-Resolved Photoemission Study of La2-xSrxCuO4

We have investigated the doping and temperature dependences of the pseudogap/superconducting gap in the single-layer cuprate La$_{2-x}$Sr$_x$CuO$_4$ by angle-resolved photoemission spectroscopy. The results clearly exhibit two distinct energy and temperature scales, namely, the gap around ($π$,0) of magnitude $Δ^*$ and the gap around the node characterized by the d-wave order parameter $Δ_0$, like the double-layer cuprate Bi2212. In comparison with Bi2212 having higher $T_c$'s, $Δ_0$ is smaller, while $Δ^*$ and $T^*$ are similar. This result suggests that $Δ^*$ and $T^*$ are approximately material-independent properties of a single CuO$_2$ plane, in contrast the material-dependent $Δ_0$, representing the pairing strength.

cond-mat.str-el↗

arXiv2008

Measurement of cosmic-ray low-energy antiproton spectrum with the first BESS-Polar Antarctic flight

The BESS-Polar spectrometer had its first successful balloon flight over Antarctica in December 2004. During the 8.5-day long-duration flight, almost 0.9 billion events were recorded and 1,520 antiprotons were detected in the energy range 0.1-4.2 GeV. In this paper, we report the antiproton spectrum obtained, discuss the origin of cosmic-ray antiprotons, and use antiprotons to probe the effect of charge sign dependent drift in the solar modulation.

astro-ph↗

arXiv2008

A bright, dust-obscured, millimeter-selected galaxy beyond the Bullet Cluster (1E0657-56)

Deep 1.1 mm continuum observations of 1E0657-56 (the "Bullet Cluster") taken with the millimeter-wavelength camera AzTEC on the 10-m Atacama Submillimeter Telescope Experiment (ASTE), have revealed an extremely bright (S$_{\rm{1.1mm}}=15.9$ mJy) unresolved source. This source, MMJ065837-5557.0, lies close to a maximum in the density of underlying mass-distribution, towards the larger of the two interacting clusters as traced by the weak-lensing analysis of Clowe et al. 2006. Using optical--IR colours we argue that MMJ065837-5557.0 lies at a redshift of $z = 2.7 \pm 0.2$. A lensing-derived mass-model for the Bullet Cluster shows a critical-line (caustic) of magnification within a few arcsecs of the AzTEC source, sufficient to amplify the intrinsic millimeter-wavelength flux of the AzTEC galaxy by a factor of $\gg 20$. After subtraction of the foreground cluster emission at 1.1mm due to the Sunyaev-Zel'dovich effect, and correcting for the magnification, the rest-frame FIR luminosity of MMJ065837-5557.0 is $\le 10^{12} \rm L_{\odot}$, characteristic of a luminous infrared galaxy (LIRG). We explore various scenarios to explain the colors, morphologies and positional offsets between the potential optical and IR counterparts, and their relationship with MMJ065837-5557.0. Until higher-resolution and more sensitive (sub)millimeter observations are available, the detection of background galaxies close to the caustics of massive lensing clusters offers the only opportunity to study this intrinsically faint millimeter-galaxy population.

astro-ph↗

arXiv2008

Tracing star formation in galaxies with molecular line and continuum observations

We report our recent progress on extragalactic spectroscopic and continuum observations, including HCN(J=1-0), HCO$^+$(J=1-0), and CN(N=1-0) imaging surveys of local Seyfert and starburst galaxies using the Nobeyama Millimeter Array, high-J CO observations (J=3-2 observations using the Atacama Submillimeter Telescope Experiment (ASTE) and J=2-1 observations with the Submillimeter Array) of galaxies, and $λ$ 1.1 mm continuum observations of high-z violent starburst galaxies using the bolometer camera AzTEC mounted on ASTE.

astro-ph↗

arXiv2008

ASTE CO(3--2) Observations of the Southern Barred Spiral Galaxy NGC 986: a Large Gaseous Bar Filled with Dense Molecular Medium

We present CO(3-2) emission observations toward the 3'x3' (or 20x20kpc at a distance of 23Mpc) region of the southern barred spiral galaxy NGC 986 using the Atacama Submillimeter Telescope Experiment (ASTE). This effort is a part of our on-going extragalactic CO(3-2) imaging project ADIoS (ASTE Dense gas Imaging of Spiral galaxies). Our CO(3-2) image revealed the presence of a large (the major axis is 14 kpc in total length) gaseous bar filled with dense molecular medium along the dark lanes observed in optical images. This is the largest ``dense-gas rich bar'' known to date. The dense gas bar discovered in NGC 986 could be a huge reservoir of possible ``fuel'' for future starbursts in the central region, and we suggest that the star formation in the central region of NGC 986 could still be in a growing phase. We found a good spatial coincidence between the overall distributions of dense molecular gas traced by CO(3-2) and the massive star formation depicted by H$α$. The global CO(3-2) luminosity $L'_{\rm CO(3-2)}$ of NGC 986 was determined to be $(5.4 \pm 1.1) \times 10^8$ K km s$^{-1}$ pc$^2$. The CO(3-2)/CO(1-0) integrated intensity ratio was found to be 0.60 +/- 0.13 at a spatial resolution of 44'' or 5 kpc, and a CO(3-2)/CO(2-1) ratio was 0.67 +/- 0.14 at a beam size of ~25'' or ~2.8 kpc. These line ratios suggest moderate excitation conditions of CO lines ($n_{\rm H_2} \sim 10^{3-4}$ cm$^{-3}$) in the central a few kpc region of NGC 986.

astro-ph↗

arXiv2008

Doping evolution of the electronic structure in the single-layer cuprates Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$: Comparison with other single-layer cuprates

We have performed angle-resolved photoemission and core-level x-ray photoemission studies of the single-layer cuprate Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ (Bi2201) and revealed the doping evolution of the electronic structure from the lightly-doped to optimally-doped regions. We have observed the formation of the dispersive quasi-particle band, evolution of the Fermi ``arc'' into the Fermi surface and the shift of the chemical potential with hole doping as in other cuprates. The doping evolution in Bi2201 is similar to that in Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_2$ (Na-CCOC), where a rapid chemical potential shift toward the lower Hubbard band of the parent insulator has been observed, but is quite different from that in La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), where the chemical potential does not shift, yet the dispersive band and the Fermi arc/surface are formed around the Fermi level already in the lightly-doped region. The (underlying) Fermi surface shape and band dispersions are quantitatively analyzed using tight-binding fit, and the deduced next-nearest-neighbor hopping integral $t'$ also confirm the similarity to Na-CCOC and the difference from LSCO.

cond-mat.supr-con↗

arXiv2008

Extracting the spectral function of the cuprates by a full two-dimensional analysis: Angle-resolved photoemission spectra of Bi2Sr2CuO6

Recently, angle-resolved photoemission spectroscopy (ARPES) has revealed a dispersion anomaly at high binding energy near 0.3-0.5eV in various families of the high-temperature superconductors. For further studies of this anomaly we present a new two-dimensional fitting-scheme and apply it to high-statistics ARPES data of the strongly-overdoped Bi2Sr2CuO6 cuprate superconductor. The procedure allows us to extract theself-energy in an extended energy and momentum range. It is found that the spectral function of Bi2Sr2CuO6 can be parameterized using a small set of tight-binding parameters and a weakly-momentum-dependent self-energy up to 0.7 eV in binding energy and over the entire first Brillouin zone. Moreover the analysis gives an estimate of the momentum dependence of the matrix element, a quantity, which is often neglected in ARPES analyses.

cond-mat.supr-con↗

arXiv2008

Abrupt Onset of Second Energy Gap at Superconducting Transition of Underdoped Bi2212

The superconducting gap - an energy scale tied to the superconducting phenomena-opens on the Fermi surface at the superconducting transition temperature (TC) in conventional BCS superconductors. Quite differently, in underdoped high-TC superconducting cuprates, a pseudogap, whose relation to the superconducting gap remains a mystery, develops well above TC. Whether the pseudogap is a distinct phenomenon or the incoherent continuation of the superconducting gap above TC is one of the central questions in high-TC research. While some experimental evidence suggests they are distinct, this issue is still under intense debate. A crucial piece of evidence to firmly establish this two-gap picture is still missing: a direct and unambiguous observation of a single-particle gap tied to the superconducting transition as function of temperature. Here we report the discovery of such an energy gap in underdoped Bi2212 in the momentum space region overlooked in previous measurements. Near the diagonal of Cu-O bond direction (nodal direction), we found a gap which opens at TC and exhibits a canonical (BCS-like) temperature dependence accompanied by the appearance of the so-called Bogoliubov quasiparticles, a classical signature of superconductivity. This is in sharp contrast to the pseudogap near the Cu-O bond direction (antinodal region) measured in earlier experiments. The emerging two-gap phenomenon points to a picture of richer quantum configurations in high temperature superconductors.

cond-mat.supr-con↗

arXiv2007

Polaron Coherence Condensation as the Mechanism for Colossal Magnetoresistance in Layered Manganites

Angle-resolved photoemission spectroscopy data for the bilayer manganite La1.2Sr1.8Mn2O7 show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La1.2Sr1.8Mn2O7 can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction.

cond-mat.str-el↗