Kenji M. Kojima

Effect of Oxygen on Electrical Properties of Lead Phthalocyanine

Oxygen molecules were found to enter into lead phthalocyanine single crystals by thermal diffusion when they were annealed in an oxygen gas of atmospheric pressure above 230°C for more than ten hours. Diffusion constant is 7.2 ×10 -9 cm 2 sec -1 at 242°C with activation energy of 1.4 eV. Dark conductivity σ increased by three orders of magnitude at room temperature with the heat treatment. Correspondingly, activation energy of conductivity E defined by σ∝exp (- E / k T ) decreased from 0.85 to 0.54 eV. The measurement of thermoelectric power indicates that positively charge carriers are generated in the presence of oxygen. The experimental results are explained on the model that an oxygen molecule acts as a center of carrier generation by accepting an electron from a neighboring lead phthalocyanine molecule and releasing the hole-like carrier.

Isotropic photonic band gap and anisotropic structures in transmission spectra of two-dimensional fivefold and eightfold symmetric quasiperiodic photonic crystals

We measured and calculated the transmission spectra of two-dimensional quasiperiodic photonic crystals (PCs) based on a fivefold (Penrose) or eightfold (octagonal) symmetric quasiperiodic pattern. The photonic crystal consisted of dielectric cylindrical rods in air placed normal to the basal plane on vertices of tiles composing the quasiperiodic pattern. An isotropic photonic band gap (PBG) appeared in the TM mode, where electric fields were parallel to the rods, even when the real part of a dielectric constant of the rod was as small as 2.4. An isotropic PBG-like dip was seen in tiny Penrose and octagonal PCs with only six and nine rods, respectively. These results indicate that local multiple light scattering within the tiny PC plays an important role in the PBG formation. Besides the isotropic PBG, we found dips depending on the incident angle of the light. In this study, anisotropic structures were clearly observed in transmission spectra of quasiperiodic PCs. Based on rod-number and rod-arrangement dependence, it is thought that the shapes and positions of the anisotropic dips are determined by global multiple light scattering covering the whole system. In contrast to the isotropic PBG due to local light scattering, we could not find any PBGs due to global light scattering even though we studied transmission spectra of a huge Penrose PC with 466 rods.

Incommensurate Magnetic Ordering and Spin-Liquid-Like State in a Triangular Lattice BaVS3. Neutron Diffraction and Scattering Study.

We performed powder neutron diffraction and scattering experiments of the \(S = \frac{1}{2}\) triangular lattice system, BaVS 3 , and found magnetic reflections below T X ≃30 K. The propagation vector is determined to be incommensurate (0.226 0.226 0) in the hexagonal index. The ordered moment is roughly estimated to be ∼0.5 µ B /V. Although the elastic magnetic diffractions disappear above T X , low-energy inelastic scattering, which has an Q -ω dispersion similar to that below T X , was observed between T X and the metal-insulator transition point T MI ≃70 K, indicating the presence of a long-range and dynamic antiferromagnetic correlation, i.e., a spin-liquid-like state, between T X and T MI .

μSR studies of intercalated HfNCl superconductor

Abstract Muon spin relaxation measurements of the magnetic field penetration depth λ have been performed in a quasi two-dimensional (2-d) superconductor based on HfNCl intercalated with Li and organic molecules. The relaxation rate σ(T→0)∼0.5 μs −1 was obtained with transverse fields applied perpendicular to the aligned HfN planes. Using the relation σ∝λ −2 ∝n s2d /m ∗ (2-d superconducting carrier density/effective mass), we deduced the 2-d Fermi temperature TF2d. The ratio Tc/TF2d for the HfNCl-(Li) system is very close to those in two other quasi-2-d superconductors, YBa2Cu3O7 and (BEDT-TTF)2-X. Compared to the Kosterlitz–Thouless (KT) transition temperature TKT=TF2d/8 for the limit of strong coupling, Tcs of these systems are reduced by a factor of 3–4, indicating the importance of carrier overlapping and/or 3-d coupling.

Microwave and millimeter wave spectroscopy in the slightly hole-doped ladders of Sr14Cu24O41

We have measured the temperature and frequency dependence of the microwave and millimeter wave conductivity σ1(T,ω) along both the ladder (c-axis) and the leg (a-axis) directions in Sr14Cu24O41. Below a temperature T* ( ~ 170 K), we observed a stronger frequency dependence in σ1c(T,ω) than that in σ1a(T,ω), forming a small resonance peak developed between 30 GHz and 100 GHz. We also observed nonlinear dc conduction along the c-axis at rather low electric fields below T*. These results suggest some collective excitation contributes to the c-axis charge dynamics of the slightly hole-doped ladders of Sr14Cu24O41 below T*.

Magnetic Ground State of Pr0.89LaCe0.11CuO4+α-δ with Varied Oxygen Depletion Probed by Muon Spin Relaxation

The magnetic ground state of an electron-doped cuprate superconductor, Pr 1- x LaCe x CuO 4+α-δ ( x =0.11, α≃0.04), has been studied by means of muon spin rotation/relaxation (µSR) over a wide vari...

Magnetic Frustration in Iridium Spinel CompoundCuIr2S4

We demonstrate via a muon spin rotation experiment that the electronic ground state of the iridium spinel compound, CuIr

Superfluid density and volume fraction of static magnetism in stripe-stabilized La1.85−yEuySr0.15CuO4

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Magnetic phase diagram of hole-doped Ca2-xNaxCuO2Cl2 cuprate superconductor

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Molecular orbital excitations in cuprates: Resonant inelastic x-ray scattering studies

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