arXiv subjects

I. Terasaki

Publications and source records attributed to I. Terasaki.

53 records · Page 3Linked to original sources

arXiv2002

Transport properties and electronic states of the thermoelectric oxide NaCo_2O_4

Physical properties of the thermoelectric oxide NaCo_2O_4 are briefly reviewed. The high thermoelectric properties of this material are attributed to the substantially enhanced effective mass, which comes from the large entropy of Co$^{4+}$ in the low spin state. The large entropy confined in the CoO_2 block causes a spin-density-wave transition at 22 K upon Cu substitution for Co, which can be regarded as ``order from disorder''.

cond-mat.str-el↗

arXiv2002

Charge transport of Bi_{2-x}Pb_xSr_2ErCu_2O_8 single crystals near the insulator-superconductor transition

We prepared a set of Pb-substituted Bi_{2-x}Pb_xSr_2ErCu_2O_8 single crystals. The resistivity and the in-plane thermopower decrease with increasing x from 0 to 0.8, which indicates that the Pb substitution supplies holes in the CuO_2 plane. For x=0.8, a tiny resistivity drop (a trace of superconductivity) is seen near 70 K, suggesting that the doping level is close to the critical concentration. The c-axis dielectric constant also systematically changes with x, and suggests that the charge inhomogeneity is suppressed by the Pb substitution.

cond-mat.str-el↗

arXiv2002

In-plane anisotropy on the transport properties in the modulated Bi_2O_2-based conductors Bi-2212 and Bi-Sr-Co-O

We investigated the in-plane anisotropy on the resistivity and thermopower of the Bi_2Sr_2CaCu_2O_{8+δ} (Bi-2212) and Bi-Sr-Co-O (BiCo) single crystals. In Bi-2212, the b-axis resistivity is higher than the a-axis resistivity, and is expressed as a sum of the a-axis resistivity and an additional residual resistivity. A downward deviation due to pseudogap is observed below a characteristic temperature T^*, which is isotropic in the form of conductivity. These results suggest that the modulation structure along the b-axis works as an anisotropic scattering center, but does not affect the pseudogap formation. On the other hand, the anisotropy of the resistivity and the thermopower in Pb-doped BiCo is substantial, probably owing to the misfit structure between the hexagonal CoO_2 layer and the rock salt Bi_2O_2 layer. However, the anisotropy in the resistivity in Pb-free BiCo is very small, suggesting that the in-plane anisotropy is averaged by the modulation structure, whose direction is tilted by 45 deg from the a- and b-axes.

cond-mat.str-el↗

arXiv2002

Chemical potential shift in lightly-doped to overdoped Bi$_2$Sr$_2$Ca$_{1-x}${\it R}$_{x}$Cu$_2$O$_{8+y}$ ({\it R} = Pr, Er)

We have studied the chemical potential shift in the high-temperature superconductor Bi$_2$Sr$_2$Ca$_{1-x}${\it R}$_{x}$Cu$_2$O$_{8+y}$ ({\it R} = Pr, Er), where the hole concentration is varied from 0.025 to 0.17 per Cu, by precise measurements of core-level photoemission spectra. The result shows that the shift becomes slow in the underdoped region as in the case of La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO) but the effect is much weaker than in LSCO. The observed shift in the present system can be relatively well explained by numerical results on the doped two-dimensional Hubbard model, and suggests that the change of the electronic structure induced by hole doping is less influenced by stripe fluctuations than in LSCO.

cond-mat.supr-con↗

arXiv2002

Thermoelectric properties of the brownmillerite oxide Ca_{2-y}La_yCo_{2-x}Al_xO_5

We prepared the brownmillerite oxide Ca_{2-y}La_yCo_{2-x}Al_xO_5, and found that it was an n-type conductor. The thermopower and the resistivity of the single crystal are -90 microV/K and 68 mOhm cm along the ab direction at 440 K, which suggest relatively good thermoelectrical properties, compared with other transition-metal oxides. Their temperature dependences are of activation type, and the activation energies are 0.2 eV for the resistivity and 0.04 eV for the thermopower. These energies differ by one order in magnitude, which implies that a polaron dominates the charge transport. A sign of the thermopower of the polycrystals changes from negative to positive at 500 K, indicating that holes are excited thermally to decrease the magnitude of thermopower.

cond-mat.str-el↗

arXiv2002

Impurity-induced transition and impurity-enhanced thermopower in the thermoelectric oxide NaCo_{2-x}Cu_x$O_4

Various physical quantities are measured and analysed for the Cu-substituted thermoelectric oxide NaCo_{2-x}Cu_xO_4. As was previously known, the substituted Cu enhances the thermoelectric power, while it does not increase the resistivity significantly. The susceptibility and the electron specific-heat are substantially decreased with increasing x, which implies that the substituted Cu decreases the effective-mass enhancement. Through a quantitative comparison with the heavy fermion compounds and the valence fluctuation systems, we have found that the Cu substitution effectively increases the coupling between the conduction electron and the magnetic fluctuation. The Cu substitution induces a phase transition at 22 K that is very similar to a spin-density-wave transition.

cond-mat.str-el↗

arXiv2001

Anomalous Co-site substitution effects on the physical properties of the thermoelectric oxide NaCo2O4

We prepared polycrystalline samples of NaCo2-xMxO4 substituted for the Co site by other 3d or 4d elements (M=Mn,Cu, Zn, Ru, Rh, and Pd). While some of the elements (M=Mn,Ru and Rh) act as a strong scatterer to give rise to a rapid increase in resistivity, the others (M=Cu, Zn and Pd) do not deteriorate the electric conduction of the host. Most unusually, the Cu substitution significantly enhances the thermopower, which increases the figure of merit by several times at 100 K.

cond-mat.str-el↗

arXiv2001

Out-of-plane dielectric constant and insulator-superconductor transition in Bi_2Sr_2Dy_{1-x}Er_xCu_2O_8 single crystals

The out-of-plane dielectric constant of the parent insulator of the high-temperature superconductor Bi_2Sr_2(Dy,Er)Cu_2O_8 was measured and analysed from 80 to 300 K in the frequency range of 10^6-10^9 Hz. All the samples were found to show a fairly large value of 10-60, implying some kind of charge inhomogeneity in the CuO_2 plane. Considering that the superconducting sample Bi_2Sr_2(Ca,Pr)Cu_2O_8 also shows a similar dielectric constant, the charge inhomogeneity plays an important role in the insulator-superconductor transition.

cond-mat.str-el↗

arXiv2001

Thermal conductivity of the A_xBO_2 type layered oxides Na_0.77MnO_2 and LiCoO_2

We prepared polycrystalline samples of the A_xBO_2 type layered oxides Na_0.77MnO_2 and LiCoO_2, and measured thermal conductivity from 15 to 280 K. Na0.77MnO_2 shows a low thermal conductivity of 15 mW/cmK, while LiCoO_2 shows a much higher value of 40 mW/cmK at 280 K. By comparing them with the data of NaCo_2O_4, we have found that the low thermal conductivity of A_xBO_2 comes essentially from the A-ion deficiency, not from the layered structure itself.

cond-mat.str-el↗

arXiv2000

Low thermal conductivity of the layered oxide (Na,Ca)Co_2O_4: Another example of a phonon glass and an electron crystal

The thermal conductivity of polycrystalline samples of (Na,Ca)Co_2O_4 is found to be unusually low, 20 mW/cmK at 280 K. On the assumption of the Wiedemann-Franz law, the lattice thermal conductivity is estimated to be 18 mW/cmK at 280 K, and it does not change appreciably with the substitution of Ca for Na. A quantitative analysis has revealed that the phonon mean free path is comparable with the lattice parameters, where the point-defect scattering plays an important role. Electronically the same samples show a metallic conduction down to 4.2 K, which strongly suggests that NaCo_2O_4 exhibits a glass-like poor thermal conduction together with a metal-like good electrical conduction. The present study further suggests that a strongly correlated system with layered structure can act as a material of a phonon glass and an electron crystal.

cond-mat.str-el↗

arXiv1999

Anomalous charge transport of the parent antiferromagnet Bi2Sr2RCu2O8

The resistivity and the thermopower for Bi2Sr2Ca{1-x}RxCu2O8 single crystals are measured and analyzed with a special interest in the parent antiferromagnet insulators. Above room temperature, the parent insulators show an electric conduction confined in the CuO2 plane and a decreasing thermopower with temperature, which are very similar to those for high-$T_c$ cuprates. These data strongly suggest that essential anomalies of high-Tc cuprates inheres in the parent insulators.

cond-mat.supr-con↗

arXiv1999

Anisotropic dielectric constant of the parent antiferromagnet Bi2Sr2MCu2O8 (M=Dy, Y and Er) single crystals

The anisotropic dielectric constants of the parent antiferromagnet Bi2Sr2MCu2O8 (M=Dy, Y and Er) single crystals were measured from 80 to 300 K. The in-plane dielectric constant is found to be very huge (10^4-10^5). This suggests a remnant of the Fremi surface of the parent antiferromagnet. The out-of-plane dielectric constant is 50-200, which is three orders of magnitude smaller than the in-plane one. A significant anomaly is that a similar out-of-plane dielectric constant is observed in superconducting samples.

cond-mat.supr-con↗

arXiv1999

Transport Properties in (Na,Ca)Co_2O_4 Ceramics

The resistivity and thermopower of polycrystalline (Na,Ca)Co_2O_4 were measured and analyzed. Both the quantities increase with x, suggesting that the carrier density is decreased by the substitutions of Ca^{2+} for Na^{+}. Considering that the temperature dependence of the resistivity show a characteristic change with x, the conduction mechanism is unlikely to come from a simple electron-phonon scattering. As a reference for NaCo_2O_4, single crystals of a two-dimensional Co oxide (Bi,Pb)_2M_3Co_2O_9 (M=Sr and Ba) were studied. The Pb substitution decreases the resistivity, leaving the thermopower nearly intact.

cond-mat.str-el↗

arXiv1999

Na-site substitution effects of the thermoelectric properties of NaCo_2O_4

The resistivity and thermopower of Na$_{1+x}$Co$_2$O$_4$ and Na$_{1.1-x}$Ca$_x$Co$_2$O$_4$ are measured and analyzed. In Na$_{1+x}$Co$_2$O$_4$, whereas the resistivity increases with $x$, the thermopower is nearly independent of $x$. This suggests that the excess Na is unlikely to supply carriers, and decreases effective conduction paths in the sample. In Na$_{1.1-x}$Ca$_x$Co$_2$O$_4$, the resistivity and the thermopower increase with $x$, and the Ca$^{2+}$ substitution for Na$^+$ reduces the majority carriers in NaCo$_2$O$_4$. This means that they are holes, which is consistent with the positive sign of the thermopower. Strong correlation in this compound is evidenced by the peculiar temperature dependence of the resistivity.

cond-mat.str-el↗

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

Anisotropic resistivity of the antiferromagnetic insulator Bi_2Sr_2ErCu_2O_8

Anisotropic resistivities of Bi_2Sr_2Ca_{1-x}Er_xCu_2O_8 single crystals were measured and analyzed from 4.2 to 500 K with special interest in the parent antiferromagnetic insulator of x=1.0. Although the resistivity is semiconducting along both the in- and out-of-plane directions, the temperature dependence is found to be significantly different. As a result, the resistivity ratio for x=1.0 takes a broad maximum near room temperature. The electric conduction in parent antiferromagnetic insulators is different from other semiconductors, and is as unconventional as that in high-temperature superconductors.

cond-mat.supr-con↗

arXiv1999

Large Thermopower in a Layered Oxide NaCo_2O_4

A transition-metal oxide NaCo_2O_4 is a layered oxide in which CoO_2 and Na alternately stack along the c axis. Recently we have found that this compound shows unusually large thermopower with low resistivity, which is comparable to those of Bi_2Te_3. The negative transverse magnetoresistance and the strongly temperature-dependent Hall coefficient suggest that electron correlation dominates the conduction mechanism in NaCo_2O_4.

cond-mat.mtrl-sci↗