arXiv subjects

Takao Watanabe

Publications and source records attributed to Takao Watanabe.

26 records · Page 2Linked to original sources

arXiv2014

Unscaling Superconducting Parameters with $T_c$ for Bi-2212 and Bi-2223: A Magnetotransport Study in the Superconductive Fluctuation Regime

To investigate the origin of the enhanced $T_c$ ($\approx$ 110 K) of the trilayer cuprate superconductor Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10+δ}$ (Bi-2223), we have performed systematic magnetoresistance (MR) measurements on this superconductor, as well as on the bilayer superconductor, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi-2212). The in-plane coherence length, $ξ_{ab}$, and the specific-heat jump, $ΔC$, have been estimated using the theory of renormalized superconductive fluctuations, and the doping dependence of these parameters has been qualitatively explained using the Fermi arc approach. A detailed comparison of the superconducting parameters with $T_c$ for these compounds suggests that an additional superconducting condensation energy exists, due to an increase in the number of stacking CuO$_{2}$ planes in a unit cell.

cond-mat.supr-con↗

arXiv2014

Doping Dependencies of Onset Temperatures for the Pseudogap and Superconductive Fluctuation in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$, Studied from both In-Plane and Out-of-Plane Magnetoresistance Measurements

To investigate the relationship between the pseudogap and superconductivity, we measured both the in-plane ($ρ_{ab}$) and out-of-plane ($ρ_c$) resistivity for oxygen-controlled Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ single crystals subject to magnetic fields (parallel to the $c$ axis) of up to 17.5 T. The onset temperature for the superconductive fluctuation, $T_{scf}$, is determined by the large positive in-plane magnetoresistance (MR) and negative out-of-plane MR observed near $T_c$, whereas the pseudogap opening temperature $T^*$ is determined by the semiconductive upturn of the zero-field $ρ_c$. $T_{scf}$ was found to scale roughly as $T_c$, with a decreasing temperature interval between them upon doping. On the other hand, $T^*$ starts out much higher than $T_{scf}$ but decreases monotonically upon doping; finally, at a heavily overdoped state, it is not observed above $T_{scf}$. These results imply that the pseudogap is not a simple precursor of superconductivity, but that further study is needed to determine whether or not $T^*$ exists below $T_{scf}$ in the heavily overdoped state.

cond-mat.supr-con↗

arXiv2012

Effects of Annealing under Tellurium Vapor for Fe$_{1.03}$Te$_{0.8}$Se$_{0.2}$ Single Crystals

In this study, the effects of annealing under tellurium vapor have been investigated for Fe$_{1.03}$Te$_{0.8}$Se$_{0.2}$ single crystals. The as-grown crystal is not superconducting. After annealing it at 400 $^\circ$C for 250 h in tellurium (Te) vapor, it shows metallic resistivity temperature dependence below 150 K and a sharp superconducting transition at 13 K. The sample also shows a full magnetic shielding effect at low temperatures. The improvement in superconductivity has been attributed to the removal of excess Fe, which is inevitably incorporated in the as-grown crystals, by the annealing.

cond-mat.supr-con↗

arXiv2010

Pseudogap Phase Boundary in Overdoped Bi_2Sr_2CaCu_2O_8 Studied by Measuring Out-of-plane Resistivity under the Magnetic Fields

The characteristic pseudogap temperature T* in Bi2Sr2CaCu2O8 system has been systematically evaluated as a function of doping, especially focusing on its overdoped region, by measuring the out-of-plane resistivity under the magnetic fields. Overdoped samples have been prepared by annealing TSFZ-grown Bi2Sr2CaCu2O8 single crystals under the high oxygen pressures (990 kgf/cm2). At a zero field, the out-of-plane resistivity showed a metallic behavior down to Tc (= 62 K), while under the magnetic fields of over 3 T,it showed typical upturn behavior from around 65 K upon decreasing temperature. This result suggests that the pseudogap and superconductivity are different phenomena.

cond-mat.supr-con↗

arXiv2004

Crystal growth and anisotropic transport properties of high-Tc superconductors Bi2Sr2Can-1CunO2n+4+d (n = 2, 3)

Large high-quality single crystals of Bi2Sr2Can-1CunO2n+4+d (n = 2, 3) were successfully grown using an improved traveling solvent floating zone (TSFZ) method, which features a slow growth rate and steep temperature gradient along the melting zone. By measuring anisotropic resistivities and susceptibilities of Bi2Sr2CaCu2O8+d, the characteristic pseudogap temperature T* was studied as a function of doping. The T* suggest that the pseudogap is not simply a precursor of high-Tc superconductivity, but that the pseudogap and the superconducting gap compete with each other. The anisotropic resistivities of Bi2Sr2Ca2Cu3O10+d were also measured, revealing that the Tc remains fixed in the overdoped region while anisotropy decreases continuously. This anomalous behavior will be discussed in terms of the inequivalent hole doping, which occurs between two inequivalent CuO2 planes in the triple-layer system.

cond-mat.supr-con↗

arXiv2002

Doping Dependence of Anisotropic Resistivities in Trilayered Superconductor Bi2Sr2Ca2Cu3O10+delta (Bi-2223)

The doping dependence of the themopower, in-plane resistivity rho_ab(T), out-of-plane resistivity rho_c(T), and susceptibility has been systematically measured for high-quality single crystal Bi2Sr2Ca2Cu3O10+delta. We found that the transition temperature Tc and pseudogap formation temperature T_rho_c*, below which rho_c shows a typical upturn, do not change from their optimum values in the "overdoped" region, even though doping actually proceeds. This suggests that, in overdoped region, the bulk $T_c$ is determined by the always underdoped inner plane, which have a large superconducting gap, while the carriers are mostly doped in the outer planes, which have a large phase stiffness.

cond-mat.supr-con↗

arXiv2002

In-plane Anisotropy on Resistivity and Thermopower in The Misfit Layered Oxide Bi2-xPbxSr2Co2Oy

We investigated the in-plane anisotropy on the resistivity and thermopower of Bi2-xPbxSr2Co2Oy single crystals, which have a misfit structure between the hexagonal CoO2 layer and the rock salt Bi2Sr2O4 layer. The resistivity and thermopower show significantly large anisotropy, which exceeds two at maximum. This anisotropy would come from the anisotropic pseudogap formation enhanced by the misfit structure. The thermopower changes with Pb doping to take a maximum at x=0.4. The misfit structure improves the thermoelectric properties through chemical pressure. The power factor is as large as 9 muW/cmK2 at 100 K for x=0.6, which is the highest value for thermoelectric oxides at 100 K.

cond-mat.str-el↗

arXiv2000

Pseudogap in Bi2Sr2CaCu2O8+d studied by measuring anisotropic susceptibilities and out-of-plane transport

We find in the Bi2Sr2CaCu2O8+d system that the characteristic temperatures T*_chi [below which the uniform susceptibilities chi_ab(T) (H perpendicular to c) and chi_c(T)(H parallel to c) decrease] and T*_rho_c (below which the out-of-plane resistivity rho_c(T) shows typical upturn) coincide for all doping levels. We attribute the T dependence of chi's and rho_c to the anomalous (pseudogapped) density-of-states (DOS) in high-Tc cuprates. Furthermore, the anisotropy in the T dependence of chi's is universal, i.e., chi_c proportional to1.6chi_ab, showing that there is only a single T-dependent component in the chi's. This implies that the Curie-like behavior (dchi/dT<0) observed in overdoped samples is also caused by a DOS effect.

cond-mat.supr-con↗