arXiv subjects

Yoichi Ando

Publications and source records attributed to Yoichi Ando.

At least 199 records · Page 11Linked to original sources

arXiv2006

Origin of the large thermoelectric power in oxygen-variable RBaCo_{2}O_{5+x} (R=Gd, Nd)

Thermoelectric properties of GdBaCo_{2}O_{5+x} and NdBaCo_{2}O_{5+x} single crystals have been studied upon continuous doping of CoO_2 planes with either electrons or holes. The thermoelectric response and the resistivity behavior reveal a hopping character of the transport in both compounds, providing the basis for understanding the recently found remarkable divergence of the Seebeck coefficient at x=0.5. The doping dependence of the thermoelectric power evinces that the configurational entropy of charge carriers, enhanced by their spin and orbital degeneracy, plays a key role in the origin of the large thermoelectric response in these correlated oxides.

cond-mat.str-el↗

arXiv2006

Crystal structure and high-field magnetism of La2CuO4

Neutron diffraction was used to determine the crystal structure and magnetic ordering pattern of a La2CuO4 single crystal, with and without applied magnetic field. A previously unreported, subtle monoclinic distortion of the crystal structure away from the orthorhombic space group Bmab was detected. The distortion is also present in lightly Sr-doped crystals. A refinement of the crystal structure shows that the deviation from orthorhombic symmetry is predominantly determined by displacements of the apical oxygen atoms. An in-plane magnetic field is observed to drive a continuous reorientation of the copper spins from the orthorhombic b-axis to the c-axis, directly confirming predictions based on prior magnetoresistance and Raman scattering experiments. A spin-flop transition induced by a c-axis oriented field previously reported for non-stoichiometric La2CuO4 is also observed, but the transition field (11.5 T) is significantly larger than that in the previous work.

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arXiv2006

Electronic Inhomogeneity and Breakdown of the Universal Thermal Conductivity in Cuprate Superconductors

We report systematic, high-precision measurements of the low-T (down to 70 mK) thermal conductivity κof YBa_{2}Cu_{3}O_{y}, La_{2-x}Sr_{x}CuO_{4} and Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}. Careful examinations of the Zn- and hole-doping dependences of the residual thermal conductivity κ_{0}/T, as well as the in-plane anisotropy of κ_{0}/T in Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}, 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, that are not considered in the standard perturbation theory for thermal conductivity, in the under- to optimally-doped regime.

cond-mat.supr-con↗

arXiv2005

Strong-coupling Effects in cuprate High-$T_{c}$ Superconductors by magnetooptical studies

Signatures of strong coupling effects in cuprate high-$T_{c}$ superconductors have been authenticated through a variety of spectroscopic probes. However, the microscopic nature of relevant excitations has not been agreed upon. Here we report on magneto-optical studies of the CuO$_{2}$ plane carrier dynamics in a prototypical high-$T_{c}$ superconductor YBa$%_{2} $Cu$_{3}$O$_{y}$ (YBCO). Infrared data are directly compared with earlier inelastic neutron scattering results by Dai \textit{et al}. [Nature (London) \textbf{406}, 965 (2000)] revealing a characteristic depression of the magnetic resonance in H $\parallel $ \textit{c} field less than 7 T. This analysis has allowed us to critically assess the role of magnetic degrees of freedom in producing strong coupling effects for YBCO system.

cond-mat.supr-con↗

arXiv2005

Infrared Hall effect in underdoped and optimally doped LSCO

We report a study of magneto-optical properties in the mid-infrared region of a series of La$_{2-x}$Sr$_x$CuO$_4$ samples with hole doping level ranging from severely underdoped ($x=0.03$)to optimally doped ($x=0.15$). The Faraday rotation and circular dichroism are measured in a magnetic field of 8 Tesla and in a temperature range between 30K and 300K. The doping and temperature dependence of infrared Hall angle is found to be understood within a simple Drude model. A significant increase of Hall frequency is observed when the hole doping level is reduced from optimal doping, which is compared with models of the pseudogap.

cond-mat.str-el↗

arXiv2005

Constituents of the "kink" in high-Tc cuprates

Applying the Kramers-Kronig consistent procedure, developed earlier, we investigate in details 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. 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 T* and sharpening below Tc, and is the main responsible for the formation of the kink in question.

cond-mat.supr-con↗

arXiv2005

Doping Evolution of the Underlying Fermi Surface in La2-xSrxCuO4

We have performed a systematic doping dependent study of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) (0.03$\leq x \leq$0.3) by angle-resolved photoemission spectroscopy. In the entire doping range, the underlying ``Fermi surface" determined from the low energy spectral weight approximately satisfies Luttinger's theorem, even down to the lightly-doped region. This is in strong contrast to the result on Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ (Na-CCOC), which shows a strong deviation from Luttinger's theorem. The differences between LSCO and Na-CCOC are correlated with the different behaviors of the chemical potential shift and spectral weight transfer induced by hole doping.

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arXiv2005

Magnetic and Charge Correlations in La{2-x-y}Nd_ySr_xCuO_4: Raman Scattering Study

Two aspects in connection with the magnetic properties of La_{2-x-y}Nd_ySr_xCuO_4 single crystals are discussed. The first is related to long wavelength magnetic excitations in x = 0, 0.01, and 0.03 La_{2-x}Sr_xCuO_4 detwinned crystals as a function of doping, temperature and magnetic field. Two magnetic modes were observed within the AF region of the phase diagram. The one at lower energies was identified with the spin-wave gap induced by the antisymmetric DM interaction and its anisotropic properties in magnetic field could be well explained using a canonical form of the spin Hamiltonian. A new finding was a magnetic field induced mode whose dynamics allowed us to discover a spin ordered state outside the AF order which was shown to persist in a 9 T field as high as 100 K above the Néel temperature T_N for x = 0.01. For these single magnon excitations we map out the Raman selection rules in magnetic fields and demonstrate that their temperature dependent spectral weight is peaked at the Néel temperature. The second aspect is related to phononic and magnetic Raman scattering in La_{2-x-y}Nd_ySr_xCuO_4 with three doping concentrations: x = 1/8, y = 0; x = 1/8, y = 0.4; and x = 0.01, y = 0. We observed that around 1/8 Sr doping and independent of Nd concentration there exists substantial disorder in the tilt pattern of the CuO_6 octahedra in both the orthorhombic and tetragonal phases which persist down to 10 K and are coupled to bond disorder in the cation layers. The weak magnitude of existing charge/spin modulations in the Nd doped structure did not allow us to detect specific Raman signatures on lattice dynamics or two-magnon scattering around 2200 cm-1.

cond-mat.str-el↗

arXiv2005

Electron-Hole Asymmetry in GdBaCo_{2}O_{5+x}: Evidence for Spin Blockade of Electron Transport in a Correlated Electron System

In RBaCo_{2}O_{5+x} compounds (R is rare earth) variability of the oxygen content allows precise doping of CoO_2 planes with both types of charge carriers. We study transport properties of doped GdBaCo_{2}O_{5+x} single crystals and find a remarkable asymmetry in the behavior of holes and electrons doped into a parent insulator GdBaCo_{2}O_{5.5}. Doping dependences 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 RBaCo_{2}O_{5+x}.

cond-mat.str-el↗

arXiv2005

Constant effective mass across the phase diagram of high-T$_{c}$ cuprates

We investigate the hole dynamics in two prototypical high temperature superconducting systems: La$_{2-x}$Sr$_{x}$CuO$_{4}$ and YBa$_{2}$Cu$_{3}% $O$_{y}$ using a combination of DC transport and infrared spectroscopy. By exploring the effective spectral weight obtained with optics in conjunction with DC Hall results we find that the transition to the Mott insulating state in these systems is of the "vanishing carrier number" type since we observe no substantial enhancement of the mass as one proceeds to undoped phases. Further, the effective mass remains constant across the entire underdoped regime of the phase diagram. We discuss the implications of these results for the understanding of both transport phenomena and pairing mechanism in high-T$_{c}$ systems.

cond-mat.str-el↗

arXiv2005

Electrodynamics of the Nodal Metal in Weakly Doped High-$T_{c}$ Cuprates

We report on the detailed analysis of the infrared (IR) conductivity of two prototypical high-$T_{c}$ systems YBa$_{2}$Cu$_{3}$O$_{y}$ and La$_{2-x}$Sr$%_{x}$CuO$_{4}$ throughout the complex phase diagram of these compounds. Our focus in this work is to thoroughly document the electromagnetic response of the nodal metal state which is initiated with only few holes doped in parent antiferromagnetic systems and extends up to the pseudogap boundary in the phase diagram. The key signature of the nodal metal is the two-component conductivity: the Drude mode at low energies followed by a resonance in mid-IR. The Drude component can be attributed to the response of coherent quasiparticles residing on the Fermi arcs detected in photoemission experiments. The microscopic origin of the mid-IR band is yet to be understood. A combination of transport and IR data uncovers fingerprints of the Fermi liquid behavior in the response of the nodal metal. The comprehensive nature of the data sets presented in this work allows us to critically re-evaluate common approaches to the interpretation of the optical data. Specifically we re-examine the role of magnetic excitations in generating electronic self energy effects through the analysis of the IR data in high magnetic field.

cond-mat.supr-con↗

arXiv2005

Multiple Bosonic Mode Coupling in Electron Self-Energy of (La_2-xSr_x)CuO_4

High resolution angle-resolved photoemission spectroscopy data along the (0,0)-($π$,$π$) nodal direction with significantly improved statistics reveal fine structure in the electron self-energy of the underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ samples in the normal state. Fine structure at energies of (40$\sim$46) meV and (58$\sim$63)meV, and possible fine structure at energies of (23$\sim$29)meV and (75$\sim$85)meV, have been identified. These observations indicate that, in LSCO, more than one bosonic modes are involved in the coupling with electrons.

cond-mat.str-el↗

arXiv2005

Large magneto-thermal effect and the spin-phonon coupling in a parent insulating cuprate Pr_{1.3}La_{0.7}CuO_4

The magnetic-field (H) dependence of the thermal conductivity κof Pr_{1.3}La_{0.7}CuO_4 is found to show a pronounced minimum for in-plane fields at low temperature, which is best attributed to the scattering of phonons by free spins that are seen by a Schottky-type specific heat and a Curie-Weiss susceptibility. Besides pointing to a strong spin-phonon coupling in cuprates, the present result demonstrates that the H-dependence of the phonon heat transport should not be naively neglected when discussing the κ(H) behavior of cuprates, since the Schottky anomaly is ubiquitously found in cuprates at any doping.

cond-mat.supr-con↗

arXiv2005

Strongly nonlinear magnetization above $T_c$ in $\rm Bi_2Sr_2CaCu_2O_{8+δ}$

Using high-resolution magnetometry we have investigated in detail the magnetization $M$ above the critical temperature $T_c$ in $\rm Bi_2Sr_2CaCu_2O_{8+δ}$. In a broad range of temperature $T$ above $T_c$, we find that $M(T,H)$ is strongly non-linear in the field $H$. We show that as $T\to T_c$, the susceptibility $χ(T,H)$ diverges to very large values ($χ\to$ -1) if measured in weak $H$. In addition, $M(H)$ displays an anomalous non-analytic form $M\sim H^{1/δ}$ in weak fields with a strongly $T$-dependent exponent $δ(T)$. These features strongly support the proposal that, above $T_c$, the pair condensate survives to support significant London rigidity.

cond-mat.supr-con↗

arXiv2005

Low-temperature nodal-quasiparticle transport in lightly doped YBa_{2}Cu_{3}O_{y} near the edge of the superconducting doping regime

In-plane transport properties of nonsuperconducting YBa_{2}Cu_{3}O_{y} (y = 6.35) are measured using high-quality untwinned single crystals. We find that both the a- and b-axis resistivities show log(1/T) divergence down to 80 mK, and accordingly the thermal conductivity data indicate that the nodal quasiparticles are progressively localized with lowering temperature. Hence, both the charge and heat transport data do not support the existence of a "thermal metal" in nonsuperconducting YBa_{2}Cu_{3}O_{y}, as opposed to a recent report by Sutherland {\it et al.} [Phys. Rev. Lett. {\bf 94}, 147004 (2005)]. Besides, the present data demonstrate that the peculiar log(1/T) resistivity divergence of cuprate is {\it not} a property associated with high-magnetic fields.

cond-mat.supr-con↗

arXiv2005

Infrared signatures of hole and spin stripes in La{2-x}Sr{x}CuO{4}

We investigate the hole and lattice dynamics in a prototypical high temperature superconducting system La{2-x}Sr{x}CuO{4} using infrared spectroscopy. By exploring the anisotropy in the electronic response of CuO2 planes we show that our results support the notion of stripes. Nevertheless, charge ordering effects are not apparent in the phonon spectra. All crystals show only the expected infrared active modes for orthorhombic phases without evidence for additional peaks that may be indicative of static charge ordering. Strong electron-phonon interaction manifests itself through the Fano lineshape of several phonon modes. This analysis reveals anisotropic electron-phonon coupling across the phase diagram, including superconducting crystals. Due to the ubiquity of the CuO2 plane, these results may have implications for other high Tc superconductors.

cond-mat.supr-con↗

arXiv2005

Coherence and superconductivity in coupled one-dimensional chains: a case study of YBa$_{2}$Cu$_{3}$O$_{y}$

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 CuO$_{2}$ planes in establishing metallicity and superconductivity in disordered chain fragments.

cond-mat.supr-con↗

arXiv2005

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

We report time-resolved measurements of the photoinduced change in reflectivity, Delta R, in the Bi2Sr2Ca(1-y)Dy(y)Cu2O(8+delta) (BSCCO) system of cuprate superconductors as a function of hole concentration. We find that the kinetics of quasiparticle decay and the sign of Delta R both change abruptly where the superconducting transition temperature Tc is maximal. These coincident changes suggest that a sharp transition in quasiparticle dynamics takes place precisely at optimal doping in the BSCCO system.

cond-mat.supr-con↗