Wei-Zhi Hu

Preparation of YBa2Cu4O8 single crystals in Y2O3 crucible using O2-HIP apparatus

Abstract YBa2Cu4O8 (Y-124) single crystals were grown by a self-flux method in a high-pressure gas mixture of 80% Ar–20% O2. The calcined powder mixture of Y–Ba–Cu–O at a ratio of Y:Ba:Cu=1:8:20 was charged in a Y2O3 crucible and put into an O2-HIP (hot isostatic pressing) apparatus. The heat-treatment at ∼1150°C under ∼2000 atm was carried out. YBa2Cu4O8 single crystals with typical size of ∼0.1×0.2×0.05 mm3 were obtained. The crystals exhibited a quite sharp superconducting transition at around 80 K. X-ray diffraction studies on the obtained crystals revealed that there are no stacking faults and the crystal has the lattice parameters similar to those reported for powder samples prepared in a high-pressure oxygen gas.

Photo-Induced Effect of the Electron-Spin Resonance on the Charge-Ordered State in Pr0.65Ca0.35MnO3

Electron-spin resonance (ESR) for the charge-ordered state in well-characterized Pr0.65Ca0.35MnO3 exhibited significant change of both the absorption profile and the effective spin susceptibility upon injection of laser light with photon energy of 1.17 eV provided by Nd-YAG laser. The increase of the effective spin susceptibility was clearly found out thanks to the injection of photons in the temperature range 90 K–80 K, which is below the transition temperature from the antiferromagnetic charge ordered state to the canted antiferromagnetic spin alternation state, TCAF∼125 K. The temperature dependence of the change of the ESR profile excludes the possibility of heating by laser light. The present result suggests that a kind of photo-induced insulator–metal transition may occur due to propagation of the delocalized carriers via probable double exchange interaction in the charge-ordering collapsed state created by the injection of photons.

ELECTRON SPIN RESONANCE IN PR0.65CA0.35MNO3

The magnetic behavior of the distorted perovskite manganese, Pr0.65Ca0.35MnO3 was studied by X-band electron spin resonance for powder samples. We observed the onset of the charge-ordered state at TCO∼215 K, the antiferromagnetic transition with the peak of the ESR linewidth, ΔHp−p at TAF∼180 K and the canted antiferromagnetic transition at TCAF∼125 K associated with the abrupt increase of both the effective magnetization and ΔHp−p. Below 90 K, the absorption intensity profile becomes weakened with decreasing temperature suggesting the existence of some kind of magnetic disorder below 90 K, which is responsible for a part of evidence of the existence of the spin-glass state as has been proposed by Yoshizawa et al., Phys. Rev. B52, 1689 (1996).

Observation of real space electronic state by laboratory X-ray diffraction in YBa2Cu3O6.9 and YBa2Cu3O6.0

Abstract The electron-density distribution in orthorhombic superconductor YBa 2 Cu 3 O 6.9 ( T c =91K) and tetragonal insulator YBa 2 Cu 3 O 6.0 was studied with laboratory powder X-ray diffraction. At 300K, there is an observable electron density distribution bridging between Cu and O along the a - and b -axes in the two-dimensional (2D) CuO 2 planes of YBa 2 Cu 3 O 6.9 ,which originates from the overlapping electronic orbitals from Cu 3d and O 2p σ states. The observed electron density forms the network of the -Cu-O-Cu- linkage in the 2D CuO 2 plane. The electron density profile at 11K remains similar to the density profile at 300K, which shows that the network of the electron-density distribution along the -Cu-O-Cu- interatomic linkage in the 2D CuO 2 plane is not due to the thermal vibration, but is intrinsic. On the contrary, no observable trace of the interatomic electron density is found in the 2D CuO 2 planes of YBa 2 Cu 3 O 6.0 , both Cu and O are rather ionic. The results provide a clue to the role of valence and/or bond state with Cu and O in 2D CuO 2 planes on the appearance of the metallic state leading to the onset of superconductivity.

Photo-Induced Effect on the Charge-Ordered State with Canted Antiferromagnetic Spin Order in Pr0.65Ca0.35MnO3

Electron-spin resonance (ESR) and powder X-ray diffraction were studied for the charge- ordered (CO) state in well-characterized Pr0.65Ca0.35MnO3 (Space group: Pbnm at 296 K). Upon injection of near infrared cw-YAG laser light with the photon energy of 1.17 eV, the remarkable change of the spin-resonance profile and the prominent decrease of the intensity of superlattice reflections coming from the charge-orderd state were observed below the transition temperature from the antiferromagnetic CO state to the canted antiferromagnetic spin alternation ground state, TCAF ~125 K. The present result is from neither heating by laser irradiation nor phase segregation due to X-ray irradiation, and eventually provides a structural evidence of collapse the CO state by photon injection. We suspect that the photo-induced insulator-metal transition occurs together with collapse of the CO state.

Preparation and Characterization of YBa2Cu4O8 Single Crystal Grown in Y2O3 Crucible Using O2-Hip Apparatus

YBa2Cu4O8 (Y-124) single crystals were grown by a self-flux method in a high-pressure gas (~2000 atm) mixture of 80% Ar – 20% O2 using an HIP (hot isostatic pressing) apparatus. To minimize contamination from a crucible during crystal growth, we used a crucible consisting of Y2O3. The calcined powder at a cation ratio of Y: Ba: Cu = 1: 8: 20 was heated at 1150 °C and then slowly cooled to 1050 °C in 30 h. Y-124 single crystals with typical size of ~ 0.1 x 0.2 x 0.05 mm3 were obtained. Large crystals with ~ mm size could be occasionally picked up. T c of the obtained crystals was 78 – 81 K. Detailed structural analysis using 4-circle single crystal diffractometer revealed that the displacement of the apical oxygen atom in c-direction is smaller than that in previous reports for crystals grown in Al2O3, implying less cation substitution in the Cu-site. Electrical resistivity along a-, b- and c-directions was measured by a four probe method. Considerable enhancement in the c-axis conductivity was observed in a temperature range between ~200 K and T c, indicating the onset of three-dimensional conductivity at low temperatures.

Comparative Study on Electron-Density Distribution of Superconducting YBa2Cu3O6.9 and Non-Superconducting YBa2Cu3O6.0

The charge-density distribution ρ(r) (= |ϕ(r)|2 wave function) of non-superconducting YBa2Cu3O6.0 was determined, in comparison with that of superconducting YBa2Cu3O6.9, by a Maximum-Entropy Method (MEM) analysis using a laboratory X-ray powder diffraction method. The ρ(r) map of YBa2Cu3O6.0 indicates that the bonding electron density (Cu 3dx 2 -y 2 - O 2p σ ) in the CuO2 planes is rather small, in contrast to that of YBa2Cu3O6.9. The rather ionic feature of YBa2Cu3O6.0 seem common in non-superconducting cuprates such as (Sr0.15,Ca0.85)CuO2 and PrBa2Cu3O6.87. Furthermore, we also report on the ρ(r) of superconducting YBa2Cu4O8 with one-dimensional Cu2O2 double chains.

Charge density distribution in alkaline-earth cuprates with one- or two-dimensional arrangements of CuO

Abstract We report the charge density distribution ϱ(r) ( = |φ(r)| 2 ; φ(r)wave function) of Ca 2 CuO 3 with the one-dimensional Cu O chain and (Sr 0.15 , Ca 0.85 )CuO 2 with the two-dimensional CuO 2 plane, in comparison with that of superconducting YBa 2 Cu 3 O 6.9 with both the Cu O atomic arrangements. ϱ(r) was obtained by the Maximum-Entropy Method (MEM) analysis using laboratory X-ray powder diffraction data. The ϱ(r) in (Ca,Sr)CuO 2 is almost spherical, indicating that ionic features are dominant in the CuO 2 planes. This contrasts with the case of superconducting YBa 2 Cu 3 O 6.9 , which shows remarkable overlapping between Cu 3d and O 2p orbitals. On the contrary, the ϱ(r) in Ca 2 CuO 3 indicates substantial overlapping between Cu and O only along the single chain direction and no interaction between each Cu O chain.