Takanori Kida

Upper Critical Fields of the 11-System Iron-Chalcogenide Superconductor FeSe0:25Te0:75

We have performed electrical resistivity measurements of a polycrystalline sample of FeSe 0.25 Te 0.75 , which exhibits superconductivity at T c ∼14 K, in magnetic fields up to 55 T to determine the upper critical field µ 0 H c2 . In this compound, very large slopes of µ 0 H c2 at the onset, the mid-point, the zero-resistivity temperatures on superconductivity are determined to be -13.7, -10.1, and -6.9 T/K, respectively. The observed µ 0 H c2 ( T )s of this compound are considerably smaller than those expected from the Werthamer–Helfand–Hohenberg model, manifesting the Pauli limiting behavior. These results suggest that this compound shows the spin-singlet pairing in the superconducting state.

Weak Superconducting Fluctuations and Small Anisotropy of the Upper Critical Fields in an Fe1:05Te0:85Se0:15 Single Crystal

We have investigated the temperature dependence of the resistive upper critical fields [µ 0 H c2 ( T )] for an Fe 1.05 Te 0.85 Se 0.15 single crystal, which exhibits superconductivity at T c ∼14 K, in magnetic fields of up to 55 T. Two-dimensional feature and superconducting fluctuations of the samples are found to be weak, because the resistive broadening effect on applied magnetic fields of up to 14 T is small. The Pauli paramagnetic effect is obviously evidenced by the strong suppression of the µ 0 H c2 a b ( T ) ( H ∥ a b ) curve and nearly isotropic µ 0 H c2 (0) ≈47 T is seen for both H ∥ a b and H ∥ c . This fact is almost identical to the results of Fe 1+ y Te 0.6 Se 0.4 single crystals reported previously. We have discussed that the small anisotropy of the upper critical field at low temperatures in Fe 1+ y (Te,Se) systems against the variation of the Te/Se ratio.

Systematic Study on Fluorine-Doping Dependence of Superconducting and Normal State Properties in LaFePO1-xFx

We have investigated the fluorine-doping dependence of lattice constants, transports and specific heat for polycrystalline LaFePO 1- x F x . F doping slightly and monotonically decreases the in-plane lattice parameter. In the normal state, electrical resistivity at low temperature is proportional to the square of temperature and the electronic specific heat coefficient has large value, indicating the existence of moderate electron–electron correlation in this system. Hall coefficient has large magnitude, and shows large temperature dependence, indicating the low carrier density and multiple carriers in this system. Temperature dependence of the upper critical field suggests that the system is a two gap superconductor. The F-doping dependence of these properties in this system are very weak, while in the FeAs system (LaFeAsO), the F doping induces the large changes in electronic properties. This difference is probably due to the different F-doping dependence of the lattice in these two systems. It has been...

Low-lying electronic states of the ferrous high-spin (S = 2) heme in deoxy-Mb and deoxy-Hb studied by highly-sensitive multi-frequency EPR

The low-lying electronic states of the ferrous high-spin heme in deoxy-myoglobin (deoxy-Mb) and deoxy-hemoglobin (deoxy-Hb) were probed by multi-frequency electron paramagnetic resonance (MFEPR) spectroscopy. An unexpected broad EPR signal was measured at the zero magnetic field using cavity resonators at 34-122 GHz that could not be simulated using any parameter sets for the S=2 spin Hamiltonian assuming spin quintet states in the (5)B(2) ground state. Furthermore, we have observed novel, broad EPR signals measured at 70-220 GHz and 1.5K using a single pass transmission probe. These signals are attributed to the ferrous high-spin heme in deoxy-Mb and deoxy-Hb. The resonant peaks shifted to a higher magnetic field with increasing frequency. The energy level separation between the ground singlet and the first excited state at the zero magnetic field was directly estimated to be 3.5 cm(-1) for deoxy-Hb. For deoxy-Mb, the first two excited singlet states are separated by 3.3 cm(-1) and 6.5 cm(-1), respectively, from the ground state. The energy gap at the zero magnetic field is directly derived from our MFEPR for deoxy-Mb and deoxy-Hb and strongly supports the theoretical analyses based on the Mössbauer and magnetic circular dichroism experiments.

High Magnetic Field Study on Giant Negative Magnetoresistance in the Molecular Conductor TPP[Cr(Pc)(CN)2]2

We investigated the magnetic and transport properties of the phthalocyanine molecular conductor TPP[Cr(Pc)(CN)2]2 in magnetic fields of up to 54 T. We observed giant negative magnetoresistance which hardly depends on the magnetic field direction owing to the isotropic nature confirmed by electron spin resonance measurements. The magnitude of magnetoresistance [|ρ(μ0H)/ρ(0 T) − 1|] is proportional to the square of magnetization as observed in the case of the spin scattering process, while the proportionality coefficient increases with decreasing the temperature. The magnetization does not saturate even at 53 T, indicating the existence of the large antiferromagnetic exchange interactions between the localized spins. In spite of this antiferromagnetic exchange interaction and low dimensionality, a convex magnetization curve was observed in the low temperature and high magnetic field range. To reproduce this magnetization curve, we proposed a model taking into account the antiferromagnetic exchange interacti...

Unconventional critical behavior in the weak ferromagnet BaIrO3

We report the results of ac/dc magnetization study of the critical behavior in a weak ferromagnet BaIrO3. The critical temperature has been determined to be Tc=182.8 K by the ac susceptibility measurement. We have analyzed our dc magnetization data near Tc with the help of the modified Arrott plot method. The critical exponents have been determined to be β=0.82±0.03, γ=1.03±0.03, and δ=2.20±0.01, which roughly obeys δ=γ/β+1. Using these values of Tc, β, and γ, we find that each magnetization-field-temperature (M-H-T) behavior below and above Tc obeys a scaling relation, following a single equation of state in which M/|e|β is uniquely related to H/|e|(γ+β) where e≡(T-Tc)/Tc. These findings demonstrate that the ferromagnetic transition in BaIrO3 belongs to a new universality class of the second-order phase transition.

Magnetic properties of a quasi-two-dimensional S=12 Heisenberg antiferromagnet with distorted square lattice

We successfully synthesize single crystals of the verdazyl radical

Haldane State Formed by Oxygen Molecules Encapsulated in Single-Walled Carbon Nanotubes

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Direct observation of spin-quadrupolar excitations in Sr2CoGe2 O7 by high-field electron spin resonance

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Crystal Structure of the Spin 1/2 Honeycomb-Lattice Antiferromagnet Cu2(pymca)3(ClO4)

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