arXiv subjects

Yoichi Ando

Publications and source records attributed to Yoichi Ando.

At least 289 records · Page 16Linked to original sources

arXiv2000

Temperature- and magnetic-field-dependent thermal conductivity of pure and Zn-doped Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} single crystals

The thermal conductivity κof Bi-2212 is measured in pure and Zn-doped crystals as a function of temperature and magnetic field. The in-plane resistivity is also measured on the identical samples. Using these data, we make a crude estimate of the impurity-scattering rate Γof the pure and the Zn-doped crystals. Our measurement show that the "plateau" in the κ(H) profile is not observed in the majority of our Bi-2212 crystals, including one of the cleanest crystals available to date. The estimated values of Γfor the pure and Zn-doped samples allow us to compare the κ(H) data with the existing theories of the quasiparticle heat transport in d-wave superconductors under magnetic field. Our analysis indicates that a proper inclusion of the quasiparticle-vortex scattering, which is expected to play the key role in the peculiar behavior of the κ(H), is important for a quantitative understanding of the QP heat transport in the presence of the vortices.

cond-mat.supr-con↗

arXiv2000

Fermi Surface Topology of Bi_{2}Sr_{2}CaCu_{2}O_{8+d} at hν=33eV: hole or electron-like?

We present new results from Angle-Resolved Photoemission experiments (ARPES) on overdoped Bi_{2}Sr_{2}CaCu_{2}O_{8+d} (BSCCO) crystals. With greatly improved energy and momentum resolution, we clearly identify the existence of electron-like portions of the Fermi Surface (FS) near \bar{M} at hν=33eV. This is consistent with previously reported data and is robust against various FS crossing criteria. It is not due to an artifact induced from \vec{k}-dependent matrix element effects. We also present evidence for a breakage in the FS pointing to the possible existence of two types of electronic components.

cond-mat.supr-con↗

arXiv2000

Thermal Conductivity of Mg-doped CuGeO_3

The thermal conductivity $κ$ is measured in a series of Cu_1-xMg_xGeO_3 single crystals in magnetic fields up to 16 T. It has turned out that heat transport by spin excitations is coherent for lightly doped samples, in which the spin-Peierls (SP) transition exists, at temperatures well above the SP transition temperature T_SP. Depression of this spin heat transport appears below T^*, at which the spin gap locally opens. T^* is not modified with Mg-doping and that T^* for each Mg-doped sample remains as high as T_SP of pure CuGeO_3, in contrast to T_SP which is strongly suppressed with Mg-doping. The spin-gap opening enhances phonon part of the heat transport because of reduced scattering by the spin excitations, producing an unusual peak. This peak diminishes when the spin gap is suppressed both in magnetic fields and with the Mg-doping.

cond-mat.str-el↗

arXiv1999

The Hall effect in Zn-doped YBa_{2}Cu_{3}O_{7-δ} revisited: Hall angle and the pseudogap

The temperature dependence of the Hall coefficient is measured with a high accuracy in a series of Zn-doped YBCO_{6.78} crystals with the Zn concentration from 0% to 1.3%. We found that the cotangent of the Hall angle, cot(θ_H), starts to deviate upward from the T^2 dependence below T_0 (which is roughly 130 K), regardless of the Zn concentration. We discuss that this deviation is caused by the pseudogap; the direction of the deviation and its insensitivity to the Zn doping suggest that the pseudogap affects cot(θ_H) through a change in the effective mass, rather than through a change in the Hall scattering rate.

cond-mat.supr-con↗

arXiv1999

Charge transport along the c-axis in high-T_c cuprates

Using 61-T pulsed magnetic fields, the normal-state ρ_{ab} and ρ_{c} are measured in Bi-2201 system down to 0.66 K, and the coexistence of the "metallic" ρ_{ab} and the "semiconducting" ρ_{c}, usually called the charge confinement behavior, was confirmed to extend far below T_c. Recent measurement of the c-axis magnetoresistance under 16 T dc magnetic field in heavily underdoped Y-123 crystals revealed that the peculiar c-axis charge transport, and thus the charge confinement, is fundamentally related to the antiferromagnetic spin fluctuations.

cond-mat.supr-con↗

arXiv1999

Manifestations of the Charged Stripes in the Magnetoresistance of Heavily Underdoped YBa_2Cu_3O_{6+x}

We present a study of the in-plane and out-of-plane magnetoresistance (MR) in heavily-underdoped, antiferromagnetic YBa_2Cu_3O_{6+x}, which reveals a variety of striking features. The in-plane MR demonstrates a "d-wave"-like anisotropy upon rotating the magnetic field H within the ab plane. With decreasing temperature below 20-25 K the system acquires memory: exposing a crystal to the magnetic field results in a persistent in-plane resistivity anisotropy. The overall features can be explained by assuming that the CuO_2 planes contain a developed array of stripes accommodating the doped holes, and that the MR is associated with the field-induced topological ordering of the stripes.

cond-mat.supr-con↗

arXiv1999

Magnetoresistance Anomalies in Antiferromagnetic YBa_2Cu_3O_{6+x}: Fingerprints of Charged Stripes

We report novel features in the in-plane magnetoresistance (MR) of heavily underdoped YBa_2Cu_3O_{6+x}, which unveil a developed ``charged stripe'' structure in this system. One of the striking features is an anisotropy of the MR with a "d-wave" symmetry upon rotating the magnetic field H within the ab plane, which is caused by the rotation of the stripes with the external field. With decreasing temperature, a hysteresis shows up below ~20 K in the MR curve as a function of H and finally below 10 K the magnetic-field application produces a persistent change in the resistivity. This "memory effect" is caused by the freezing of the directionally-ordered stripes.

cond-mat.supr-con↗

arXiv1999

Resistive upper critical fields and irreversibility lines of optimally-doped high-T_c cuprates

We present the resistively-determined upper critical field H^ρ_{c2}(T) and the irreversibility lines H^ρ_{irr}(T) of various high-T_c cuprates, deduced from measurements in 61-T pulsed magnetic fields applied parallel to the c-axis. The SHAPE of both H^ρ_{c2}(T) and H^ρ_{irr}(T) depends monotonically on the anisotropy of the material and none of the samples show saturation of H^ρ(T) at low temperatures. The anomalous positive curvature, d^2 H^ρ/dT^2 > 0, is the strongest in materials with the largest normal-state anisotropy, regardless of whether anisotropy is varied by changing the carrier concentration or by comparing a variety of optimally-doped compounds.

cond-mat.supr-con↗

arXiv1999

Unusual Charge Localization in Zn-doped and Heavily Underdoped YBa2Cu3O7-d at Low Temperatures

The in-plane normal-state resistivity of Zn-doped YBa2Cu3O7-d and heavily underdoped pure YBCO single crystals is measured down to low temperatures under magnetic fields up to 18 T. We found that the temperature dependence of the normal-state rho_ab does not obey log(1/T) and tends not to diverge in the low temperature limit. The result suggests that the ``ground state'' of the normal state of YBCO is metallic.

cond-mat.supr-con↗

arXiv1999

Magnetoresistance in Heavily Underdoped YBa_2Cu_3O_{6+x}: Antiferromagnetic Correlations and Normal-State Transport

We report on a contrasting behavior of the in-plane and out-of-plane magnetoresistance (MR) in heavily underdoped antiferromagnetic (AF) YBa_2Cu_3O_{6+x} (x<0.37). The out-of-plane MR (I//c) is positive over most of the temperature range and shows a sharp increase, by about two orders of magnitude, upon cooling through the Neel temperature T_N. A contribution associated with the AF correlations is found to dominate the out-of-plane MR behavior for H//c from far above T_N, pointing to the key role of spin fluctuations in the out-of-plane transport. In contrast, the transverse in-plane MR (I//a(b);H//c) appears to be small and smooth through T_N, implying that the development of the AF order has little effect on the in-plane resistivity.

cond-mat.supr-con↗

arXiv1999

Specific-heat evidence for strong electron correlations in the thermoelectric material (Na,Ca)Co_{2}O_{4}

The specific heat of (Na,Ca)Co_{2}O_{4} is measured at low-temperatures to determine the magnitude of the electronic specific-heat coefficient γ, in an attempt to gain an insight into the origin of the unusually large thermoelectric power of this compound. It is found that γis as large as 48 mJ/molK^2, which is an order of magnitude larger than γof simple metals. This indicates that (Na,Ca)Co_{2}O_{4} is a strongly-correlated electron system, where the strong correlation probably comes from the low-dimensionality and the frustrated spin structure. We discuss how the large thermopower and its dependence on Ca doping can be understood with the strong electron correlations.

cond-mat.str-el↗

arXiv1999

Systematic Evolution of the Magnetotransport Properties of Bi_{2}Sr_{2-x}La_{x}CuO_{6} with Carrier Concentration

We report that it is possible to obtain a series of high-quality crystals of La-doped Bi-2201, of which the transport properties have been believed to be "dirtier" than those of other cuprates. In our crystals, the normal-state transport properties display behaviors which are in good accord with other cuprates; for example, in the underdoped region the in-plane resistivity ρ_{ab} shows the pseudogap feature and in the overdoped region the T dependence of ρ_{ab} changes to T^n with n > 1. The characteristic temperatures of the pseudogap deduced from the resistivity and the Hall coefficient data are presented.

cond-mat.supr-con↗

arXiv1999

A re-examination of the electronic structure of Bi_2Sr_2CaCu_2O_{8+d} and Bi_2Sr_2Cu_1O_{6+d} - An electron-like Fermi Surface and the absence of flat bands at $E_F$

We present a re-examination of the electronic structure and Fermi Surface (FS) of Bi-Sr-Ca-Cu-O (BSCCO) as obtained from angle-resolved photoemission experiments. By applying a stricter set of FS crossing criteria as well as by varying the incident photon energy outside the usual range, we have found very different behavior from that previously observed. In particular we have found an electron-like FS centered around the Gamma point, and the flat bands at E_F near the M point of the zone are absent. These results are robust over a large range of dopings and from single to double layer samples.

cond-mat.str-el↗

arXiv1999

Vortex phase transformations probed by the local ac response of Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} single crystals with various doping

The linear ac response of the vortex system is measured locally in Bi-2212 single crystals at various doping, using a miniature two-coil mutual-inductance technique. It was found that a step-like change in the local ac response takes place exactly at the first-order transition (FOT) temperature T_{FOT}(H) determined by a global dc magnetization measurement. The T_{FOT}(H) line in the H-T phase diagram becomes steeper with increasing doping. In the higher-field region where the FOT is not observed, the local ac response still shows a broadened but distinct feature, which can be interpreted to mark the growth of a short-range order in the vortex system.

cond-mat.supr-con↗

arXiv1998

Normal-state magnetotransport in La_{1.905}Ba_{0.095}CuO_{4} single crystals

The normal-state magnetotransport properties of La_{2-x}Ba_{x}CuO_{4} single crystals with x=0.095 are measured; at this composition, a structural transition to a low-temperature-tetragonal (LTT) phase occurs without suppression of superconductivity. None of the measured properties (in-plane and out-of-plane resistivity, magnetoresistance, and Hall coefficient) shows any sudden change at the LTT phase transition, indicating that the occurrence of the LTT phase does not necessarily cause an immediate change in the electronic state such as the charge-stripe stabilization.

cond-mat.supr-con↗

arXiv1998

Non-Universal Power Law of the "Hall Scattering Rate" in a Single-Layer Cuprate Bi_{2}Sr_{2-x}La_{x}CuO_{6}

In-plane resistivity ρ_{ab}, Hall coefficient, and magnetoresistance (MR) are measured in a series of high-quality Bi_{2}Sr_{2-x}La_{x}CuO_{6} crystals with various carrier concentrations, from underdope to overdope. Our crystals show the highest T_c (33 K) and the smallest residual resistivity ever reported for Bi-2201 at optimum doping. It is found that the temperature dependence of the Hall angle obeys a power law T^n with n systematically decreasing with increasing doping, which questions the universality of the Fermi-liquid-like T^2 dependence of the "Hall scattering rate". In particular, the Hall angle of the optimally-doped sample changes as T^{1.7}, not as T^2, while ρ_{ab} shows a good T-linear behavior. The systematics of the MR indicates an increasing role of spin scattering in underdoped samples.

cond-mat.supr-con↗

arXiv1998

Charge Localization from Local Destruction of Antiferromagnetic Correlation in Zn-doped YBa2Cu3O7-d

The in-plane normal-state resistivity of Zn-doped YBa2Cu3O7-d single crystals is measured down to low temperatures by suppressing superconductivity with magnetic fields up to 18 T. Substitution of Cu with Zn in the CuO2 planes is found to induce carrier localization at low temperatures in "clean" samples with kF l > 5, where the mean free path l is larger than the electron wave length and thus localization is not normally expected. The destruction of the local antiferromagnetic correlation among Cu spins by Zn is discussed to be the possible origin of this unusual charge localization.

cond-mat.supr-con↗

arXiv1998

Thermal Conductivity of the Spin Peierls Compound CuGeO_3

The thermal conductivity of the Spin-Peierls (SP) compound CuGeO_3 was measured in magnetic fields up to 16 T. Above the SP transition, the heat transport due to spin excitations causes a peak at around 22 K, while below the transition the spin excitations rapidly diminish and the heat transport is dominated by phonons; however, the main scattering process of the phonons is with spin excitations, which demonstrates itself in an unusual peak in the thermal conductivity at about 5.5 K. This low-temperature peak is strongly suppressed with magnetic fields in excess of 12.5 T.

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