arXiv subjects

N. Kobayashi

Publications and source records attributed to N. Kobayashi.

At least 91 records · Page 5Linked to original sources

arXiv2009

Evolution of a pairing-induced pseudogap from the superconducting gap of (Bi,Pb)2Sr2CuO6

We have performed an ultrahigh-resolution angle-resolved photoemission spectroscopy study of slightly-overdoped (Bi,Pb)2Sr2CuO6 to elucidate the origin of pseudogap. By using a newly developed xenon-plasma light source, we determined the comprehensive momentum and temperature dependences of the superconducting gap and the pseudogap. We found that the antinodal pseudogap persists far above the superconducting transition temperature and is smoothly connected to the nodal gap. The characteristic temperature of the pseudogap scales well with the superconducting-gap size irrespective of the momentum location. The present experimental results point to the pairing origin of the pseudogap.

cond-mat.supr-con↗

arXiv2009

Gigantic Impact of Magnetic Impurity on Stripe Order in La$_{2-x}$Sr$_{x}$CuO$_4$ ($x$$\sim$1/8)

Precise neutron-scattering experiments on La$_{1.88-y}$Sr$_{0.12+y}$Cu$_{0.99}$R$_{0.01}$O$_4$ ($y$=0 for R=Cu, Zn and $y$=0.01 for R=Ga, Fe) were performed to examine the stability of stripe order in cuprate oxides through impurity substitution effects. For all the impurity-substituted samples a static charge-density-wave order similar to that reported in the low-temperature-tetragonal phase (LTT) was discovered in the low-temperature-orthorhombic (LTO) phase for the first time. Fe substitution significantly enhanced both the charge- and spin-density-wave orders. These results indicate that dynamical charge stripes can be stabilized into static ones around the Cu-site defects regardless of the type of crystal structure. Additionally, large magnetic moments at the Cu-site may induce a decrease in the Cu spin fluctuations around the defects, which in turn, enhances the static charge-density-wave order.

cond-mat.str-el↗

arXiv2009

$^{63/65}$Cu- and $^{35/37}$Cl-NMR Studies of Triplet Localization in the Quantum Spin System NH$_4$CuCl$_3$

$^{63/65}$Cu- and $^{35/37}$Cl-NMR experiments were performed to investigate triplet localization in the $S=1/2$ dimer compound NH$_4$CuCl$_3$, which shows magnetization plateaus at one-quarter and three-quarters of the saturation magnetization. In $^{63/65}$Cu-NMR experiments, signal from only the singlet Cu site was observed, because that from the triplet Cu site was invisible due to the strong spin fluctuation of onsite 3$d$-spins. We found that the temperature dependence of the shift of $^{63/65}$Cu-NMR spectra at the singlet Cu site deviated from that of macroscopic magnetization below T=6 K. This deviation is interpreted as the triplet localization in this system. From the $^{35/37}$Cl-NMR experiments at the 1/4-plateau phase, we found the two different temperature dependences of Cl-shift, namely the temperature dependence of one deviates below T=6 K from that of the macroscopic magnetization as observed in the $^{63/65}$Cu-NMR experiments, whereas the other corresponds well with that of the macroscopic magnetization in the entire experimental temperature region. We interpreted these dependences as reflecting the transferred hyperfine field at the Cl site located at a singlet site and at a triplet site, respectively. This result also indicates that the triplets are localized at low temperatures. $^{63/65}$Cu-NMR experiments performed at high magnetic fields between the one-quarter and three-quarters magnetization plateaus have revealed that the two differently oriented dimers in the unit cell are equally occupied by triplets, the fact of which limits the theoretical model on the periodic structure of the localized triplets.

cond-mat.stat-mech↗

arXiv2009

Possible Phase Transition Deep Inside the Hidden Order Phase of Ultraclean URu2Si2

To elucidate the underlying nature of the hidden order (HO) state in heavy-fermion compound URu2Si2, we measure electrical transport properties of ultraclean crystals in a high field/low temperature regime. Unlike previous studies, the present system with much less impurity scattering resolves a distinct anomaly of the Hall resistivity at H*=22.5 T well below the destruction field of the HO phase ~36 T. In addition, a novel quantum oscillation appears above a magnetic field slightly below H*. These results indicate an abrupt reconstruction of the Fermi surface, which implies a possible phase transition well within the HO phase caused by a band-dependent destruction of the HO parameter. The present results definitely indicate that the HO transition should be described by an itinerant electron picture.

cond-mat.str-el↗

arXiv2008

Colossal electroresistance and colossal magnetoresistive step in paramagnetic insulating phase of single crystalline bilayered manganite(La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$

We report a significant decrease in the low-temperature resistance induced by the application of an electric current on the $ab$-plane in the paramagnetic insulating (PMI) state of (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$. A colossal electroresistance effect attaining -95% is observed at lower temperatures. A colossal magnetoresistive step appears near 5T at low temperatures below 10K, accompanied by an ultrasharp width of the insulator-metal transition. Injection of higher currents to the crystal causes a disappearance of the steplike transition. These findings have a close relationship with the presence of the short-range charge-ordered clusters pinned within the PMI matrix of the crystal studied.

cond-mat.mtrl-sci↗

arXiv2008

Optical probe of carrier doping by X-ray irradiation in organic dimer Mott insulator $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}]$Cl

We investigated the infrared optical spectra of an organic dimer Mott insulator $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl, which was irradiated with X-rays. We observed that the irradiation caused a large spectral weight transfer from the mid-infrared region, where interband transitions in the dimer and Mott-Hubbard bands take place, to a Drude part in a low-energy region; this caused the Mott gap to collapse. The increase of the Drude part indicates a carrier doping into the Mott insulator due to irradiation defects. The strong redistribution of the spectral weight demonstrates that the organic Mott insulator is very close to the phase border of the bandwidth-controlled Mott transition.

cond-mat.str-el↗

arXiv2008

Effect of pressure on steplike magnetostriction of single crystalline (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ bilayered manganite

e report the effect of pressure on the steplike magnetostriction of single crystalline bilayered manganite (LaPr)1.2Sr1.8Mn2O8, for our understandings of the ultrasharp nature of the field-induced first-order transition from a paramagnetic insulator to a ferromagnetic metal phase. The application of pressure suppresses a steplike transformation and causes a broad change in the magnetostriction. The injection of an electric current to the crystal also weakens the steplike variation in both the magnetostriction and magnetoresistance. The stabilization of ferromagnetic interaction or the delocalization of charge carriers is promoted with the applied pressure or applied current, resulting in the suppressed steplike behavior. Our findings suggest that the step phenomenon is closely related to the existence of localized carriers such as the short-range charge-orbital ordered clusters.

cond-mat.str-el↗

arXiv2008

The Subaru/XMM-Newton Deep Survey (SXDS) - II. Optical Imaging and Photometric Catalogs

We present multi-waveband optical imaging data obtained from observations of the Subaru/XMM-Newton Deep Survey (SXDS). The survey field, centered at R.A.=02:18:00, decl.=-05:00:00, has been the focus of a wide range of multi-wavelength observing programs spanning from X-ray to radio wavelengths. A large part of the optical imaging observations are carried out with Suprime-Cam on Subaru Telescope at Mauna Kea in the course of Subaru Telescope Observatory Projects. This paper describes our optical observations, data reduction and analysis procedures employed, and the characteristics of the data products. A total area of 1.22 sqdeg is covered in five contiguous sub-fields, each of which corresponds to a single Suprime-Cam field of view (34'x27'), in five broad-band filters B, V, Rc, i', z' to the depths of B=28.4, V=27.8, Rc=27.7, i'=27.7 and z'=26.6 (AB, 3-sigma, 2-arcsec aperture). The data are reduced and compiled into five multi-waveband photometric catalogs, separately for each Suprime-Cam pointing. The i'-band catalogs contain about 900,000 objects, making the SXDS catalogs one of the largest multi-waveband catalogs in corresponding depth and area coverage. The SXDS catalogs can be used for an extensive range of astronomical applications such as the number density of the Galactic halo stars to the large scale structures at the distant universe. The number counts of galaxies are derived and compared with those of existing deep extragalactic surveys. The optical data, the source catalogs, and configuration files used to create the catalogs are publicly available via the SXDS web page (http://www.naoj.org/Science/SubaruProject/SXDS/index.html)

astro-ph↗

arXiv2008

Mott transition and superconductivity in the strongly correlated organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}]$Br

The magnetic field effect on the phase diagram of the organic Mott system $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br in which the bandwidth was tuned by the substitution of deuterated molecules was studied by means of the resistivity measurements performed in magnetic fields. The lower critical point of the first-order Mott transition, which ended on the upper critical field $H_{\rm c2}$-temperature plane of the superconductivity, was determined experimentally in addition to the previously observed upper critical end point. The lower critical end point moved to a lower temperature with the suppression of $T_{\rm c}$ in magnetic fields and the Mott transition recognized so far as the $S$-shaped curve reached $T =$ 0 when $H > H_{\rm c2}$ in the end.

cond-mat.supr-con↗

arXiv2007

X-ray Irradiation-induced Carrier Doping Effects in Organic Dimer-Mott Insulators

We report X-ray irradiation-induced carrier doping effects on the electrical conductivity in the organic dimer-Mott insulators $κ$-(ET)$_{2}$$X$ with $X =$ Cu[N(CN)$_{2}$]Cl and Cu$_{2}$(CN)$_{3}$. For $κ$-(ET)$_{2}$Cu[N(CN)$_{2}$]Cl, we have observed a large decrease of the resistivity by 40 % with the irradiation at 300 K and the metal-like temperature dependence down to about 50 K. The irradiation-induced defects expected at the donor molecule sites might cause a local imbalance of the charge transfer in the crystal. Such molecular defects result in the effective doping of carriers into the half-filled dimer-Mott insulators.

cond-mat.str-el↗

arXiv2007

Adaptive Optics Imaging of Lyman Break Galaxies as Progenitors of Spheroids in the Local Universe

In order to reveal the stellar mass distribution of z~3 galaxies, we are conducting deep imaging observations of U-dropout Lyman Break Galaxies (LBGs) with Adaptive Optics (AO) systems in K-band, which corresponds to rest-frame V-band of z~3 galaxies. The results of the Subaru intensive-program observations with AO36/NGS/IRCS indicate that 1) the K-band peaks of some of the LBGs brighter than K=22.0 mag show significant offset from those in the optical images, 2) the z~3 Mv* LBGs and serendipitously observed Distant Red Galaxies (DRGs) have flat profiles similar to disk galaxies in the local universe (i.e., Sersic with n<2), and 3) the surface stellar mass densities of the Mv* LBGs are 3-6 times larger than those of disk galaxies at z=0-1. Considering the lack of n>2 systems among the luminous z~3 LBGs and DRGs, and their strong spatial clustering, we infer that the dense n<2 disk-like structures evolve into the n>2 spheroids of nearby galaxies through relaxations due to major merger events.

astro-ph↗

arXiv2007

Adaptive Optics Rest-Frame V-band Imaging of Lyman Break Galaxies at z~3: High-surface Density Disk-like Galaxies ?

In order to reveal the rest-frame V-band morphology of galaxies at z~3, we conducted AO-assisted K-band imaging observations of z~3 LBGs with Mv*-0.5 to Mv*+3.0 mag. LBGs brighter than Mv* have larger rHL (0.40") than the fainter LBGs (0.23") on average, and there is no bright LBGs with a small rHL. The LBGs brighter than Mv* have red rest-frame U-V colors (average of 0.2 mag) and most of the fainter LBGs show blue rest-frame U-V colors (average of -0.4 mag). The K-band peaks of some of the LBGs brighter than K=22.0 mag show significant shift from those in the optical images. The images of all but one of the LBGs with K<21.5 mag are fitted well with Sersic profile with n index less than 2, similar to disk galaxies in the local universe. Assuming that the LBGs have a disk-shape, we compared their size-luminosity and size-stellar mass relation with those of z=0 and z=1 disk galaxies. The LBGs are brighter than z=0 and z=1 disk galaxies at the same effective radius. The rest-frame V-band surface brightness of the LBGs are 2.2-2.9 mag and 1.2-1.9 mag brighter than the disk galaxies at z=0 and z=1, respectively. The size-stellar mass relation of the LBGs shows that the effective radii of the LBGs do not depend on their stellar mass. For the LBGs brighter than Mv*, the average surface stellar mass density is 3-6 times larger than those of the z=0 and z=1 disk galaxies. We also examine the profiles of the serendipitously observed DRGs. They are also fitted with the Sersic profiles with n<2. The implications of the dominance of n<2 population among galaxies at z~3 and the presence of the high surface stellar mass density disk systems are discussed.

astro-ph↗

arXiv2007

NMR Study on the Vortex Slush Phase in Organic Superconductor κ-(BEDT-TTF)2Cu(NCS)2

The vortex state in a single crystal of the layered organic superconductor κ-(BEDT-TTF2Cu(NCS)2, where BEDT-TTF (or ET) is bis(ethylenedithio)tetrathiafulvalene, was studied by 1H-NMR. Under a low field region around 0.75 T, the vortex glass-liquid transition was demonstrated by a diverging of the longitudinal nuclear spin relaxation rate and peak-broadening in spectra. Under a high field region near the upper critical field Hc2(0)~7 T, the curvature of nuclear spin relaxation curves showed a drastic change at the temperature where the emergence of the quantum vortex slush state was reported. The mechanism in this curvature change was discussed in terms of the fluctuating field produced by fragments of vortex glass.

cond-mat.str-el↗

arXiv2006

Diffraction-limited 3 um Spectroscopy of IRAS 04296+3429 and IRAS 05341+0852: Spatial Extent of Hydrocarbon Dust Emission and Dust Evolutionary Sequence

We present 3 um spectroscopy of the carbon-rich proto-planetary nebulae IRAS 04296+3429 and IRAS 05341+0852 conducted with the adaptive optics system at the Subaru Telescope. We utilize the nearly diffraction-limited spectroscopy to probe the spatial extent of the hydrocarbon dust emitting zone. We find a hydrocarbon emission core extending up to 100--160 mas from the center of IRAS 04296+3429, corresponding to a physical diameter of 400--640 AU, assuming a distance of 4 kpc. On the other hand, we find that IRAS 05341+0852 is not spatially resolved with this instrumentation. The physical extent of these proto-planetary nebulae, along with the reanalyzed data of IRAS 22272+5435 published previously, suggests a correlation between the physical extent of the hydrocarbon dust emission and the spectral evolution of the aliphatic to aromatic features in these post-AGB stars. These measurements represent the first direct test of the proposed chemical synthesis route of carbonaceous dust in the circumstellar environment of evolved stars.

astro-ph↗

arXiv2006

Bulk and surface low-energy excitations in YBa2Cu3O7-d studied by high-resolution angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission spectroscopy on YBa2Cu3O7-delta (Y123; delta = 0.06; Tc = 92 K). By accurately determining the Fermi surface and energy band dispersion, we solve long-standing controversial issues as to the anomalous electronic states of Y-based high-Tc cuprates. We unambiguously identified surface-bilayer-derived bonding and antibonding bands, together with their bulk counterparts. The surface bands are highly overdoped (hole concentration x = 0.29), showing no evidence for the gap opening or the dispersion anomaly in the antinodal region, while the bulk bands show a clear dx2-y2-wave superconducting gap and the Bogoliubov quasiparticle-like behavior with a characteristic energy scale of 50-60 meV indicative of a strong electron-boson coupling in the superconducting state. All these results suggest that the metallic and superconducting states coexist at the adjacent bilayer of Y123 surface.

cond-mat.supr-con↗

arXiv2006

Cu-NMR study on the disordered quantum spin magnet with the Bose-glass ground state

Cu-NMR study has been performed on the disordered spin-gap system Tl1-xKxCuCl3 In the high-field H > HC=Δ/μ_B, where Δis the spin-gap, the hyperfine field becomes extremely inhomogeneous at low temperatures due to the field-induced magnetic order, indicating that the ordered spin state must be different from the pure TlCuCl3. In the low field H < HC, a saturating behavior in the longitudinal nuclear spin relaxation rate 1/T1 was observed at low temperatures, indicating existence of the magnetic ground state proposed to be Bose-glass phase by Fisher.

cond-mat.str-el↗

arXiv2006

Magnetic-field effects on the charge-spin stripe order in La-214 high-Tc cuprates

Magnetic-field effects on the charge-spin stripe order in La-214 high-Tc cuprates have been investigated from measurements of the in-plane electrical-resistivity, Rho_ab_. In La_2-x_Ba_x_CuO_4_ with x=0.10 and La_2-x_Sr_x_CuO_4_ with x=0.115 where the incommensurate charge peaks are weak and unobservable in zero field in elastic neutron-scattering measurements, respectively, the normal-state value of Rho_ab_ at low temperatures markedly increases with increasing field up to 27 T. For La_2-x_Ba_x_CuO_4_ with x=0.11 and Zn-substituted La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ with x=0.115 and y=0.02 where the charge stripe order is fairly stabilized in zero field, on the other hand, the increase in Rho_ab_ with increasing field is negligibly small. In conclusion, when the charge-spin stripe order is not fully stable in zero field, magnetic field operates to stabilize the charge-spin stripe order. The value of Rho_ab_ increases with increasing field depending on the stability of the charge stripe order.

cond-mat.supr-con↗

arXiv2006

Shadow bands in single-layered Bi_2Sr_2CuO_6 studied by angle-resolved photoemission spectroscopy

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the origin of shadow band. We found that the shadow band is exactly the c(2x2) replica of the main band irrespective of the carrier concentration and its intensity is invariable with respect to temperature, doping, and substitution constituents of block layers. This result rules out the possibility of antiferromagnetic correlation and supports the structural origin of shadow band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the existence of the c(2x2) shadow band, demonstrating that the shadow band is not a unique feature of Bi-based cuprates. We conclude that the shadow band is related to the orthorhombic distortion at the crystal surface.

cond-mat.supr-con↗