T. Takabatake

Enhancement of superconductivity in Bi2Sr2CaCu2O8+δ

Abstract We found that T c of Bi 2 Sr 2 CaCu 2 O 8+δ increases from 80 to 90 K by hydrogen absorption without a significant depression of the superconducting volume fraction. The enhancement of T c may be induced by the decrease in hole concentration which is in excess for the superconductivity in the host compound. For the 110 K phase of Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+δ , however, no enhancement of T c was achieved upon hydrogenation.

Gap suppression in CeNiSn under hydrostatic pressure

Abstract The pressure response of valence-fluctuating compound CeNiSn with an energy gap has been studied by resistivity measurements up to 24 kbar. A phenomelogical analysis with a simple activation law shows that the energy gap decreases linearly with pressure from 0.6 meV at a rate of -0.02 meV/kbar. The gap suppression does not lead to the transition to the magnetically ordered state, contrary to the case of CeNi 1− x Cu x Sn. Application of pressure manifests the presence of Kondo effect in the quasilogarithmic increase in the resistivity above 100 K.

Hydrogen intercalation in some superconducting copper oxides

Abstract Effect of hydrogen absorption on superconducting and transport properties has been studied on YBa 2 Cu 3 O 6.9 , Bi 2 Sr 2 CaCu 2 O 8+y and Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+y . We found that the introduction of hydrogen into the system decreases one hole carrier per one hydrogen. The variation of T c with hydrogen concentration strongly depends on the preferential site occupation of hydrogen. Our results support the model that T c is largely determined by the concentration of mobile holes on the CuO 2 plane.

Thermoelectric and transport properties of CeBiPt and LaBiPt

The electrical resistivity ρ, Hall coefficient RH, thermoelectric power S, and thermal conductivity κ have been measured on an antiferromagnet CeBiPt (TN=1 K) and the nonmagnetic analogue LaBiPt, which crystallize in the cubic MgAgAs-type structure. The large values |RH|∼1 cm3/C and ρ∼1.2 mΩ cm at 300 K indicate that both compounds are low-carrier concentration semimetals. The strong temperature dependences of ρ, S, and RH for CeBiPt contrast with the weak temperature dependences for LaBiPt. For CeBiPt, S(T) has a positive maximum of 120 μV/K at 100 K. However, the sign of RH(T) changes from positive to negative with increasing temperature at 170 K, above which S(T) of CeBiPt becomes smaller than LaBiPt. These observations suggest strong dependence of mobility for hole and electron carriers in CeBiPt. In this system, κ(T) is dominated by the phonon contribution, which is consistent with a small carrier concentration. Furthermore, the combination of S, ρ, and κ allows us to estimate the figure of merit for...

Nuclear magnetic relaxation of 1H in high-Tc superconductor YBa2Cu3O6.9Hx (x = 0.18 and 0.63)

Abstract Spin-lattice relaxation time T1 (at 29 and 58 MHz) and T1ϱ of 1H have been measured in YBa2Cu3O6.9Hx (x = 0.18 and 0.63) between 300 and 77 K. In the fresh sample with x = 0.18, T1 has a minimum near 260 K, which is attributed to the diffusion of hydrogen in the oxygen vacancies in the Cu(1)-O plane with an activation energy of Ea = 0.29 eV. For the sample with x = 0.63, another diffusion mode with Ea = 0.25 eV exists at lower temperature around 167 K. This mode might be due to the diffusion in the interstitial site in the Y-plane. But all of these modes disappeared in the temperature dependence of T1 after keeping the samples in ampoules in three months, and no evidence of diffusion was found either in T1ϱ measurements. The temperature dependence of T1 for x = 0.18 showed an abrupt change at T c ≊ 90 K , suggesting the effect of superconductivity.

Low-temperature thermoelectric properties of Yb14MSb11 (M = Mn, Zn)

We have synthesized Yb14MnSb11 and Yb14ZnSb11 to study the thermoelectric properties at low temperature (4<T<300 K). The materials were characterized by means of susceptibility (χ), resistivity (ρ), thermopower (S) and thermal conductivity (κ) measurements. The κ(T) is dominated by phonon heat transport, and the Jahn–Teller effect in the MnSb4 tetrahedra does not make a significant contribution to the reduction of the thermal conductivity at intermediate temperatures and above. A sharp minimum in κ(T) for M = Mn occurs in the ferromagnetic transition at TC = 54 K. The thermopower for Yb14MnSb11 is positive and increases upon heating, while that for Yb14ZnSb11 exhibits a negative peak at 60 K, before increasing and becoming positive. These different behaviors support the assertion that the Yb ions are in a divalent state in the former and in a valence fluctuating state in the latter.

Anisotropic transport and magnetic properties of frustrated CeRhSn

Abstract The resistivity ρ, magnetic susceptibility χ, and specific heat C were measured for a single crystal of the valence-fluctuating hexagonal system CeRhSn. Strong anisotropy in both the resistivity (ρa>ρc) and magnetic susceptibility (χc>χa) was found. A hump appears in ρc(T) below 7 K , where both C/T and χ exhibit strong upturns. Power-law behaviors χc∝T−1.1 and χa∝T−0.35 were found below 4 and 16 K , respectively, down to 0.4 K . These results suggest the presence of unquenched moments in the quasi-Kagome lattice.

Bulk-sensitive high-resolution ce 3d–4f resonance photoemission study of CeNiSn and CePdSn

Abstract High-resolution Ce 3d–4f resonance photoemission (RPES) spectra highly reflecting bulk 4f states have been measured for a Kondo semiconductor CeNiSn and its reference material CePdSn. An intensity of f 0 final states is markedly weaker for CeNiSn than for CePdSn. We also report on photon-energy dependence of the RPES spectra. Comparison between the RPES spectra of CeNiSn and La 3d–4f RPES spectra of LaNiSn shows that the rare-earth 5d enhancement is significantly weaker than Ce 4f resonance enhancement. The high-resolution 3d–4f RPES spectra clarify the difference of the bulk electronic states near the Fermi level between CeNiSn and CePdSn.

Interplay of low-energy phonons and magnetic excitations in the Kondo insulator YbB12

Peculiarities in the lattice dynamics of the Kondo insulator Y bB(12) have been studied by inelastic neutron scattering. Selected phonon modes were traced above and below the temperature region (T ~ 50 K) where the gap opens in the electron density of states. The intensities of some low-energy modes exhibit an anomalous temperature dependence for q vectors close to the Brillouin zone boundary, suggesting a renormalization of the phonon eigenvectors. This effect is thought to arise from a coupling with magnetic excitations of the same symmetry, which exist at nearby energies. It is argued that this magnetovibrational coupling may in turn play a role in the steep temperature crossover existing in Y bB(12) between the low-temperature (Kondo insulator) and high-temperature (incoherent spin-fluctuation) regimes, which is rapidly suppressed by lighter Zr substitution.

X-ray Absorption and Inverse Photoemission Study of Y1-xCaxTiO3

We have performed the O 1 s x-ray absorption (XAS) and resonant inverse photoemission spectroscopy (RIPES) in Mott insulator YTiO 3 , band insulator CaTiO 3 and Y 0.61 Ca 0.39 TiO 3 with a metal–in...