Biomedical subjects

Yoichi Ando

Publications and source records attributed to Yoichi Ando.

At least 19 recordsLinked to original sources

PubMed2006

Constituents of the quasiparticle spectrum along the nodal direction of high-Tc cuprates.

Applying the Kramers-Kronig consistent procedure, developed earlier, we investigate in detail the formation of the quasiparticle spectrum along the nodal direction of high-Tc cuprates. The heavily discussed "70 meV kink" on the renormalized dispersion exhibits a strong temperature and doping dependence when purified from structural effects such as bilayer splitting, diffraction replicas, etc. This dependence is well understood in terms of fermionic and bosonic constituents of the self-energy. The latter follows the evolution of the spin-fluctuation spectrum, emerging below some doping dependent temperature and sharpening below Tc, and is mainly responsible for the formation of the kink in question.

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PubMed2006

High-energy spin excitations in the electron-doped superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) with T(c) = 21 K.

We use high-resolution inelastic neutron scattering to study the low-temperature magnetic excitations of the electron-doping superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) (T(c) = 21 +/- 1 K) over a wide energy range (4 meV < or = homega < or = 330 meV). The effect of electron doping is to cause a wave vector (Q) broadening in the low-energy (homega < or = 80 meV) commensurate spin fluctuations at (0.5, 0.5) and to suppress the intensity of spin-wave-like excitations at high energies (homega > or = 100 meV). This leads to a substantial redistribution in the spectrum of the local dynamical spin susceptibility chi''(omega), and reveals a new energy scale similar to that of the lightly hole-doped YB2Cu3O(6.353) (T(c) = 18 K).

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PubMed2006

Electronic inhomogeneity and breakdown of the universal thermal conductivity of cuprate superconductors.

We report systematic, high-precision measurements of the low-T (down to 70 mK) thermal conductivity kappa of YBa2Cu3O(y), La(2-x)Sr(x)CuO4, and Bi2Sr2CaCu2O(8+delta). Careful examinations of the Zn- and hole-doping dependences of the residual thermal conductivity kappa0/T, as well as the in-plane anisotropy of kappa0/T in Bi2Sr2CaCu2O(8+delta), indicate a breakdown of the universal thermal conductivity, a notable theoretical prediction for d-wave superconductors. Our results point to an important role of electronic inhomogeneities, which are not considered in the standard perturbation theory for thermal conductivity, in the underdoped to optimally doped regime.

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PubMed2005

Electron-hole asymmetry in GdBaCo2O(5+x): evidence for spin blockade of electron transport in a correlated electron system.

In RBaCo2O(5+x) compounds (R is rare earth), the variability of the oxygen content allows precise doping of CoO2 planes with both types of charge carriers. We study transport properties of doped GdBaCo2O(5+x) single crystals and find a remarkable asymmetry in the behavior of holes and electrons doped into a parent insulator GdBaCo2O(5.5). The doping dependencies of resistivity, Hall response, and thermoelectric power reveal that the doped holes greatly improve the conductivity, while the electron-doped samples always remain poorly conducting. This doping asymmetry provides strong evidence for a spin blockade of the electron transport in RBaCo2O(5+x).

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PubMed2005

Abrupt transition in quasiparticle dynamics at optimal doping in a cuprate superconductor system.

We report time-resolved measurements of the photoinduced change in reflectivity, DeltaR, in the Bi2Sr2Ca(1-y)Dy(y)Cu2O8+delta (BSCCO) system of cuprate superconductors as a function of hole concentration. We find that the kinetics of quasiparticle decay and the sign of DeltaR both change abruptly where the superconducting transition temperature T(c) is maximal. These coincident changes suggest that a sharp transition in quasiparticle dynamics takes place precisely at optimal doping in the BSCCO system.

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PubMed2005

Multiple bosonic mode coupling in the electron self-energy of (La2-xSrx)CuO4.

High resolution angle-resolved photoemission spectroscopy data along the (0,0)-(pi,pi) nodal direction with significantly improved statistics reveal fine structure in the electron self-energy of the underdoped (La2-xSrx)CuO4 samples in the normal state. Fine structure at energies of (40-46) meV and (58-63) meV, and possible fine structure at energies of (23-29) meV and (75-85) meV, have been identified. These observations indicate that, in (La2-xSrx)CuO4, more than one bosonic modes are involved in the coupling with electrons.

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PubMed2005

Magic doping fractions for high-temperature superconductors.

We report hole-doping dependence of the in-plane resistivity rho(ab) in a cuprate superconductor La(2-x)Sr(x)CuO4, carefully examined using a series of high-quality single crystals. Our detailed measurements find a tendency towards charge ordering at particular rational hole-doping fractions of 1/16, 3/32, 1/8, and 3/16. This observation appears to suggest a specific form of charge order and is most consistent with the recent theoretical prediction of the checkerboard-type ordering of the Cooper pairs at rational doping fractions x = (2m+1)/2n, with integers m and n.

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PubMed2005

Coherence and superconductivity in coupled one-dimensional chains: a case study of YBa2Cu3Oy.

We report the infrared (IR) response of Cu-O chains in the high-T(c) superconductor YBa(2)Cu(3)O(y) over the doping range spanning y=6.28-6.75. We find evidence for a power law scaling at mid-IR frequencies consistent with predictions for Tomonaga-Luttinger liquid, thus supporting the notion of one-dimensional transport in the chains. We analyze the role of coupling to the CuO2 planes in establishing metallicity and superconductivity in disordered chain fragments.

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PubMed2005

Effects of the degree of fluctuation on subjective preference for a 1 Hz flickering light.

Humans are believed to have a preferred amount of stimulus variation in their perceptual environment. Here, paired comparison tests were conducted to examine whether the fluctuation of a flickering light improves subjective preference. Sine-wave and bandpass noise acted as the light source. We have previously shown that the preferred temporal frequency of a flickering light without any fluctuation is approximately 1 Hz (Soeta et al 2002 Journal of the Optical Society of America A 19 289 - 294). This was used as the center frequency of the light source. The bandwidth was set at 1, 2, 4, 8, and 16 Hz, to control the amplitude of the first peak of the autocorrelation function, phi1. Results show that the preferred phi1 of a flickering light is 0.46.

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PubMed2004

Electronic phase diagram of high-Tc cuprate superconductors from a mapping of the in-plane resistivity curvature.

We propose that resistivity curvature mapping (RCM) based on the in-plane resistivity data is a useful way to objectively draw electronic phase diagrams of high-Tc cuprates, where various crossovers are important. In particular, the pseudogap crossover line can be conveniently determined by RCM. We show experimental phase diagrams obtained by RCM for Bi2Sr2-zLazCuO6+delta, La2-xSrxCuO4, and YBa2Cu3Oy, and demonstrate the universal nature of the pseudogap crossover. Intriguingly, the electronic crossover near optimum doping depicted by RCM appears to occur rather abruptly, suggesting that the quantum-critical regime, if it exists, must be very narrow.

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PubMed2004

Metal-to-insulator crossover in YBa2Cu3Oy probed by low-temperature quasiparticle heat transport.

It was recently demonstrated that in La2-xSrxCuO4 the magnetic-field (H) dependence of the low-temperature thermal conductivity kappa up to 16 T reflects whether the normal state under high magnetic field is a metal or an insulator. We measure the H dependence of kappa in YBa(2)Cu(3)O(y) (YBCO) at subkelvin temperatures for a wide doping range, and find that at low doping the kappa(H) behavior signifies the change in the ground state in this system as well. Surprisingly, the critical doping is found to be located deeply inside the underdoped region, about the hole doping of 0.07 hole/Cu; this critical doping is apparently related to the stripe correlations as revealed by the in-plane resistivity anisotropy.

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PubMed2004

A universal scaling relation in high-temperature superconductors.

Since the discovery of superconductivity at elevated temperatures in the copper oxide materials there has been a considerable effort to find universal trends and correlations amongst physical quantities, as a clue to the origin of the superconductivity. One of the earliest patterns that emerged was the linear scaling of the superfluid density (rho(s)) with the superconducting transition temperature (T(c)), which marks the onset of phase coherence. This is referred to as the Uemura relation, and it works reasonably well for the underdoped materials. It does not, however, describe optimally doped (where T(c) is a maximum) or overdoped materials. Similarly, an attempt to scale the superfluid density with the d.c. conductivity (sigma(dc)) was only partially successful. Here we report a simple scaling relation (rho(s) proportional, variant sigma(dc)T(c), with sigma(dc) measured at approximately T(c)) that holds for all tested high-T(c) materials. It holds regardless of doping level, nature of dopant (electrons versus holes), crystal structure and type of disorder, and direction (parallel or perpendicular to the copper-oxygen planes).

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PubMed2004

Magnetic order in lightly doped La2-xSrxCuO4.

We study long wavelength magnetic excitations in lightly doped La2-xSrxCuO4 (x</=0.03) detwinned crystals. The lowest energy magnetic anisotropy induced gap can be understood in terms of the antisymmetric spin interaction inside the antiferromagnetic (AF) phase. The second magnetic resonance, analyzed in terms of in-plane spin anisotropy, shows unconventional behavior within the AF state; it led to the discovery of collective spin excitations pertaining to a field induced magnetically ordered state. This state persists in a 9 T field to more than 100 K above the Néel temperature in x=0.01.

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PubMed2004

Manifestation of the magnetic resonance mode in the nodal quasiparticle lifetime of the superconducting cuprates.

Studying the nodal quasiparticles in superconducting cuprates by photoemission with highly improved momentum resolution, we show that a new "kink" feature in the scattering rate is a key to uncover the nature of electron correlations in these compounds. Our data provide evidence that the main doping independent contribution to the scattering can be well understood in terms of the conventional Fermi liquid model, while the additional doping dependent contribution has a magnetic origin. This sheds doubt on applicability of a phonon-mediated pairing mechanism to high-temperature superconductors.

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PubMed2004

Quantum phase transitions in the cuprate superconductor Bi2Sr2-xLaxCuO6+delta.

To elucidate a quantum phase transition (QPT) in Bi(2)Sr(2-x)La(x)CuO(6+delta), we measure charge and heat transport properties at very low temperatures and examine the following characteristics for a wide range of doping: normal-state resistivity anisotropy under 58 T, temperature dependence of the in-plane thermal conductivity kappa(ab), and the magnetic-field dependence of kappa(ab). It turns out that all of them show signatures of a QPT at the 1/8 hole doping. Together with the recent normal-state Hall measurements under 58 T that signified the existence of a QPT at optimum doping, the present results indicate that there are two QPTs in the superconducting doping regime of this material.

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PubMed2004

Spin-flop transition and the anisotropic magnetoresistance of Pr(1.3-x)La(0.7)CexCuO4: unexpectedly strong spin-charge coupling in the electron-doped cuprates.

We use transport and neutron-scattering measurements to show that a magnetic-field-induced transition from noncollinear to collinear spin arrangement in adjacent CuO2 planes of lightly electron-doped Pr(1.3-x)La(0.7)CexCuO4 (x=0.01) crystals affects significantly both the in-plane and out-of-plane resistivity. In the high-field collinear state, the magnetoresistance (MR) does not saturate but exhibits an intriguing fourfold-symmetric angular dependence, oscillating from being positive at B//[100] to being negative at B//[110]. The observed MR of more than 30% at low temperatures induced by a modest modification of the spin structure indicates an unexpectedly strong spin-charge coupling in electron-doped cuprates.

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PubMed2004

Evolution of the Hall coefficient and the peculiar electronic structure of the cuprate superconductors.

Although the Hall coefficient R(H) is an informative transport property of metals and semiconductors, its meaning in the cuprate superconductors has been ambiguous because of its unusual characteristics. Here we show that a systematic study of R(H) in La2-xSrxCuO4 single crystals over a wide doping range establishes a qualitative understanding of its peculiar evolution, which turns out to reflect a two-component nature of the electronic structure caused by an unusual development of the Fermi surface recently uncovered by photoemission experiments.

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PubMed2004

Local ordering in the pseudogap state of the high-Tc superconductor Bi2Sr2CaCu2O(8+delta).

We report atomic-scale characterization of the pseudogap state in a high-Tc superconductor, Bi2Sr2CaCu2O(8+delta). The electronic states at low energies within the pseudogap exhibit spatial modulations having an energy-independent incommensurate periodicity. These patterns, which are oriented along the copper-oxygen bond directions, appear to be a consequence of an electronic ordering phenomenon, the observation of which correlates with the pseudogap in the density of electronic states. Our results provide a stringent test for various ordering scenarios in the cuprates, which have been central in the debate on the nature of the pseudogap and the complex electronic phase diagram of these compounds.

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