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Yoichi Ando

Publications and source records attributed to Yoichi Ando.

At least 37 records · Page 2Linked to original sources
PubMed2004

Thermal conductivity of Pr1.3-xLa0.7CexCuO4 single crystals and signatures of stripes in an electron-doped cuprate.

It was recently demonstrated that the anisotropic phonon heat transport behavior is a good probe of the stripe formation in La(2-x)Sr(x)CuO(4) (LSCO) [Phys. Rev. B 67, 104503 (2003)]]. Using this probe, we examined an electron-doped cuprate Pr(1.3-x)La(0.7)Ce(x)CuO(4) (PLCCO) and found that essentially the same features as those in LSCO are observed. Moreover, the in-plane resistivity rho(ab) of lightly doped PLCCO shows metallic behavior (drho(ab)/dT>0) in the Néel ordered state with a mobility comparable to that in LSCO. It is discussed that these peculiar properties in common with LSCO signify the existence of stripes in electron-doped cuprates.

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PubMed2004

Auditory evoked magnetic fields in relation to interaural cross-correlation of band-pass noise.

Auditory evoked magnetic fields of the human brain were analyzed in relation to the magnitude of the inter-aural cross-correlation (IACC). IACC of the stimuli was controlled by mixing diotic bandpass and dichotic independent bandpass noise in appropriate ratios. The auditory stimuli were binaurally delivered through plastic tubes and earpieces inserted into ear canals of the nine volunteers with normal hearing who took part in this study. All source signals had the same sound pressure level. Auditory evoked fields (AEFs) were recorded using a neuromagnetometer in a magnetically shielded room. Combinations of a reference stimulus (IACC=1.0) and test stimuli (IACC=0.2, 0.6, 0.85) were presented alternately at a constant interstimulus interval of 0.5 s and MEGs recorded. The results showed that the N1m latencies were not affected by IACC; however, the peak amplitude of N1m significantly decreased with increasing IACC.

Acoustic Stimulation↗
PubMed2003

Josephson plasmon and inhomogeneous superconducting state in La2-xSrxCuO4.

We report on the interlayer far infrared response for a series of La2-xSrxCuO4 crystals with 0.08<x<0.20 focusing on the survey of the Josephson plasmon resonance (JPR). The analysis of the JPR mode provides information on the local variation of the superfluid density within the CuO2 planes thus empowering one with a tool for "microscopy" on the superconducting condensate. Our results uncover the presence of regions with characteristic length of approximately 100-200 A within which superconductivity is strongly depressed or completely depleted. An examination of the doping trends suggests that development of superconducting inhomogeneities is triggered by the formation of the unidirectional spin density wave state in La2-xSrxCuO4 at x=1/8.

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PubMed2003

Signature of optimal doping in Hall-effect measurements on a high-temperature superconductor.

High-temperature superconductivity is achieved by doping copper oxide insulators with charge carriers. The density of carriers in conducting materials can be determined from measurements of the Hall voltage--the voltage transverse to the flow of the electrical current that is proportional to an applied magnetic field. In common metals, this proportionality (the Hall coefficient) is robustly temperature independent. This is in marked contrast to the behaviour seen in high-temperature superconductors when in the 'normal' (resistive) state; the departure from expected behaviour is a key signature of the unconventional nature of the normal state, the origin of which remains a central controversy in condensed matter physics. Here we report the evolution of the low-temperature Hall coefficient in the normal state as the carrier density is increased, from the onset of superconductivity and beyond (where superconductivity has been suppressed by a magnetic field). Surprisingly, the Hall coefficient does not vary monotonically with doping but rather exhibits a sharp change at the optimal doping level for superconductivity. This observation supports the idea that two competing ground states underlie the high-temperature superconducting phase.

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PubMed2003

Anisotropic electromagnetic response of lightly doped La2-xSrxCuO4 within the CuO2 planes.

Using infrared spectroscopy, we show that spin self-organization in untwinned La2-xSrxCuO4 (LSCO) crystals has profound consequences for the dynamical conductivity sigma(omega). The electronic response of CuO2 planes acquires significant anisotropy in the spin ordered state with enhancement of the conductivity along the direction of the diagonal spin stripes by up to a factor of 2. An examination of the anisotropic response indicates that the diagonal spin texture in weakly doped LSCO is also accompanied by the modulation of charge density. The electronic response of the charge stripes is found to be gapless consistent with the hypothesis of the metallic ground state. Our experiments directly show that the striped ordered systems reveal new degrees of freedom not present in ordinary one-dimensional conductors.

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PubMed2003

Anisotropic magnetoresistance in lightly doped La(2)-(x)Sr(x)CuO(4): impact of antiphase domain boundaries on the electron transport.

Detailed behavior of the magnetoresistance (MR) is studied in lightly doped antiferromagnetic La(1.99)Sr(0.01)CuO(4), where, thanks to the weak-ferromagnetic moment due to spin canting, the antiferromagnetic (AF) domain structure can be manipulated by the magnetic field. The MR behavior demonstrates that CuO(2) planes indeed contain antiphase AF-domain boundaries in which charges are confined, forming antiphase stripes. The data suggest that a high magnetic field turns the antiphase stripes into in-phase stripes, and the latter appear to give better conduction than the former, which challenges the notion that the antiphase character of stripes facilitates charge motion.

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PubMed2003

Ising-like spin anisotropy and competing antiferromagnetic-ferromagnetic orders in GdBaCo2O5.5 single crystals.

In RBaCo2O5+x compounds (R is rare earth), a ferromagnetic-antiferromagnetic competition is accompanied by a giant magnetoresistance. We study the magnetization of detwinned GdBaCo2O5.5 single crystals and find a remarkable uniaxial anisotropy of Co3+ spins which is tightly linked with the chain oxygen ordering in GdO0.5 planes. Reflecting the underlying oxygen order, CoO2 planes also develop a spin-state order consisting of Co3+ ions in alternating rows of S=1 and S=0 states. The magnetic structure appears to be composed of weakly coupled ferromagnetic ladders with Ising-like moments, which gives a simple picture for magnetotransport phenomena.

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PubMed2003

Intrinsic tunneling spectra of Bi2(Sr(2-x)Lax)CuO(6+delta).

We have measured intrinsic-tunneling spectra of a single CuO-layer La-doped Bi2(Sr(2-x)Lax)CuO(6+delta) (Bi2201-Lax). Despite a difference of a factor of 3 in the optimal superconducting critical temperatures for Bi2201-La0.4 and Bi2212 (32 and 95 K, respectively) and different spectral energy scales, we find that the pseudogap vanishes at a similar characteristic temperature T* approximately 230-300 K for both compounds. We find also that, in Bi2201-Lax, pseudogap humps are seen as sharp peaks and, in fact, even dominate the intrinsic spectra.

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PubMed2003

Magnetic-field-induced localization of quasiparticles in underdoped La(2-x)SrxCuO4 single crystals.

Magnetic-field-induced ordering of electrons around vortices is a striking phenomenon recently found in high-T(c) cuprates. To identify its consequence in the quasiparticle dynamics, the magnetic-field (H) dependence of the low-temperature thermal conductivity kappa of La(2-x)SrxCuO4 crystals is studied for a wide doping range. It is found that the behavior of kappa(H) in the subkelvin region changes drastically across optimum doping, and the data for underdoped samples are indicative of unusual magnetic-field-induced localization of quasiparticles; this localization phenomenon is probably responsible for the unusual "insulating normal state" under high magnetic fields.

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PubMed2003

Textural properties corresponding to visual perception based on the correlation mechanism in the visual system.

We present a set of texture parameters that correspond to perceptual properties of visual texture. For machine vision or a computer interface, it is important that the computational measurements of texture correspond well to the perceptual properties. To understand the mechanism of our visual system, it is important to know how we extract or characterize information for texture perception. In this study, we show that the autocorrelation function (ACF) analysis provides useful measures for representing three salient perceptual properties of texture: contrast, coarseness, and regularity. The validity of the ACF analysis was examined by comparing the calculated factors to the subjective scores collected for various kinds of natural textures. The effectiveness of the analysis depends on the structure of the estimated ACF. When a texture has a harmonic structure, the estimated ACF has periodical peaks corresponding to the periods of the texture. Both perceived coarseness and regularity are strongly related to these peaks in the ACF. However, the estimated ACF does not have a periodical structure when the texture is random. In this case, the texture coarseness and regularity are represented by the decay rate of the ACF.

Adult↗
PubMed2003

Dependence of upper critical field and pairing strength on doping in cuprates.

We have determined the upper critical field Hc2 as a function of hole concentration in bismuth-based cuprates by measuring the voltage induced by vortex flow in a driving temperature gradient (the Nernst effect), in magnetic fields up to 45 tesla. We found that Hc2 decreased steeply as doping increased, in both single and bilayer cuprates. This relationship implies that the Cooper pairing potential displays a trend opposite to that of the superfluid density versus doping. The coherence length of the pairs xi(0) closely tracks the gap measured by photoemission. We discuss implications for understanding the doping dependence of the critical temperature Tc0.

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PubMed2002

Magnetic shape-memory effects in a crystal.

Magnetic fields affect the motion of electrons and the orientation of spins in solids, but are thought to have little impact on crystal structure, particularly in compounds with low magnetic susceptibility, such as antiferromagnets. Here we describe an unexpected magnetic effect on crystal shape, in which the direction of the crystal's axes are swapped and the shape changes when a magnetic field is applied; this in turn induces curious memory effects in resistivity and magnetic susceptibility. Ironically, this phenomenon occurs in one of the most well-studied two-dimensional antiferromagnets, La(2-x)Sr(x)CuO(4).

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PubMed2002

Magnetoresistance of untwinned YBa(2)Cu(3)O(y) single crystals in a wide range of doping: anomalous hole-doping dependence of the coherence length.

Magnetoresistance (MR) in the a-axis resistivity of untwinned YBa(2)Cu(3)O(y) single crystals is measured for a wide range of doping ( y = 6.45-7.0). The y dependence of the in-plane coherence length xi(ab) estimated from the fluctuation magnetoconductance indicates that the superconductivity is anomalously weakened in the 60-K phase; this observation, together with the Hall coefficient and the a-axis thermopower data which suggest the hole doping to be 12% for y approximately equal to 6.65, gives evidence that the origin of the 60-K plateau is the 1/8 anomaly. At high temperatures, the normal-state MR data show signatures of the Zeeman effect on the pseudogap in underdoped samples.

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PubMed2002

Novel anisotropy in the superconducting gap structure of Bi2Sr2CaCu2O(8+delta) probed by quasiparticle heat transport.

Since the nature of pairing interactions is manifested in the superconducting gap symmetry, the exact gap structure, particularly any deviation from the simple d(x(2)-y(2)) symmetry, would help in elucidating the pairing mechanism in high- T(c) cuprates. Anisotropic heat transport measurement in Bi(2)Sr(2)CaCu(2)O(8+delta) reveals that the quasiparticle populations are different for the two nodal directions and thus the gap structure must be uniquely anisotropic, suggesting that pairing is governed by interactions with a rather complicated anisotropy. Intriguingly, it is found that the "plateau" in the magnetic-field dependence of the thermal conductivity is observed only in the b-axis transport.

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PubMed2002

Autocorrelation analyses of magnetoencephalographic alpha waves in relation to subjective preference for a flickering light.

Human cortical responses corresponding to the subjective preference for a flickering light of varying period were investigated. Paired-comparison tests were performed to examine the subjective preference for a flickering light, and MEG was recorded during presentations of the most preferred and less preferred flickering lights alternately. Results showed that the effective duration of the autocorrelation function, tau(e), which represents a repetitive feature of the MEG alpha waves, becomes longer during the preferred condition. This reveals that the brain repeats a similar rhythm under preferred conditions.

Adult↗
PubMed2002

Electromagnetic response of static and fluctuating stripes in cuprate superconductors.

Using infrared spectroscopy, we found that changes in the in-plane charge dynamics attributable to static stripe order in La(1.275)Nd(0.6)Sr(0.125)CuO(4) or superconductivity in La(1.875)Sr(0.125)CuO(4) are confined to energies smaller than 100 cm(-1). An absorption peak in the low- omega conductivity of the Nd-doped compound is suggestive of localization effects due to the reduced dimensionality of static charge stripes. Neither superconductivity nor static stripe ordering has a noticeable effect on the depression of the scattering rate at omega<1000 cm(-1) characteristic of the pseudogap state in other classes of moderately doped cuprates.

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PubMed2002

Electrical resistivity anisotropy from self-organized one dimensionality in high-temperature superconductors.

We investigate the manifestation of stripes in the in-plane resistivity anisotropy in untwinned single crystals of La2-xSrxCuO4 ( x = 0.02-0.04) and YBa(2)Cu(3)O(y) ( y = 6.35-7.0). It is found that both systems show strongly temperature-dependent in-plane anisotropy in the lightly hole-doped region and that the anisotropy in YBa(2)Cu(3)O(y) grows with decreasing y below approximately 6.60 despite the decreasing orthorhombicity, which gives most direct evidence that electrons self-organize into a macroscopically anisotropic state. The transport is found to be easier along the direction of the spin stripes already reported, demonstrating that the stripes are intrinsically conducting in cuprates.

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