Naohito Tsujii

Anisotropy of superconductivity from MgB2 single crystals

Magnesium diboride (MgB2) single crystals, with a maximum size of 0.5×0.5×0.02 mm3, were grown by the vapor transport method in a sealed molybdenum crucible. A superconducting transition with the onset temperature of 38.6 K was confirmed by both transport and magnetization measurements. The upper critical field anisotropy ratio, Hc2∥ab(0)/Hc2∥c(0), was estimated to be 2.6 from the magnetic field-temperature phase diagram for MgB2 single crystals.The discovery of superconductor in magnesium diboride MgB2 with high Tc (39 K) has raised some challenging issues; whether this new superconductor resembles a high temperature cuprate superconductor(HTS) or a low temperature metallic superconductor; which superconducting mechanism, a phonon- mediated BCS or a hole superconducting mechanism or other new exotic mechanism may account for this superconductivity; and how about its future for applications. In order to clarify the above questions, experiments using the single crystal sample are urgently required. Here we have first succeeded in obtaining the single crystal of this new MgB2 superconductivity, and performed its electrical resistance and magnetization measurements. Their experiments show that the electronic and magnetic properties depend on the crystallographic direction. Our results indicate that the single crystal MgB2 superconductor shows anisotropic superconducting properties and thus can provide scientific basis for the research of its superconducting mechanism and its applications.

Universality in heavy fermion systems with general degeneracy.

We discuss the relation between the T2 coefficient of electrical resistivity A and the T-linear specific-heat coefficient gamma for heavy-fermion systems with general N, where N is the degeneracy of quasiparticles. A set of experimental data reveals that the Kadowaki-Woods relation, A/gamma2=1 x 10(-5) muOmega cm(K mol/mJ)2, collapses remarkably for large-N systems, although this relation has been regarded to be commonly applicable to the Fermi liquids. Instead, based on the Fermi-liquid theory we propose a new relation, A /gamma2=1 x 10(-5) with A =A/1/2N(N-1) and gamma =gamma/1/2N(N-1). This new relation exhibits an excellent agreement with the data for the whole range of degenerate heavy fermions.

High Thermoelectric Power Factor in a Carrier-Doped Magnetic Semiconductor CuFeS2

The chalcopyrite CuFeS2 is a natural magnetic semiconductor, where Fe spins order antiferromagnetically at TN = 853 K. We investigate the thermoelectric properties of the carrier-doped chalcopyrite alloys, Cu1-xFe1+xS2 with x = 0.03 and 0.05, and Zn0.03Cu0.97FeS2. All systems showed characteristic behavior of n-type degenerate semiconductors. The large Seebeck coefficient with a high carrier-density indicates enhanced carrier mass of m* = 3–6m0, where m0 is the electron mass. Consequently, the thermoelectric power factor exceeds 1×10-3 W K-2 m-1 at 400 K. We propose that utilizing magnetic semiconductor can be a new effective strategy to obtain enhanced values of the power factor.

Thermoelectricity Generation and Electron–Magnon Scattering in a Natural Chalcopyrite Mineral from a Deep‐Sea Hydrothermal Vent

Current high-performance thermoelectric materials require elaborate doping and synthesis procedures, particularly in regard to the artificial structure, and the underlying thermoelectric mechanisms are still poorly understood. Here, we report that a natural chalcopyrite mineral, Cu1+x Fe1-x S2 , obtained from a deep-sea hydrothermal vent can directly generate thermoelectricity. The resistivity displayed an excellent semiconducting character, and a large thermoelectric power and high power factor were found in the low x region. Notably, electron-magnon scattering and a large effective mass was detected in this region, thus suggesting that the strong coupling of doped carriers and antiferromagnetic spins resulted in the natural enhancement of thermoelectric properties during mineralization reactions. The present findings demonstrate the feasibility of thermoelectric energy generation and electron/hole carrier modulation with natural materials that are abundant in the Earths crust.

Field-induced magnetic ordering in the quantum spin system KCuCl3

KCuCl 3 is a three-dimensional coupled spin-dimer system and has a singlet ground state with an excitation gap Δ/k B =31 K. High-field magnetization measurements for KCuCl 3 have been performed in static magnetic fields ofup to 30 T and in pulsed magnetic fields of up to 60 T. The entire magnetization curve including the saturation region was obtained at T= 1.3 K. From the analysis of the magnetization curve, it was found that the exchange parameters determined from the dispersion relations of the magnetic excitations should be reduced, which suggests the importance of the renormalization effect in the magnetic excitations. The field-induced magnetic ordering accompanied by the cusplike minimum of the magnetization was observed, as in the isomorphous compound TlCuCl 3 . The phase boundary was almost independent of the field direction, and is represented by the power law. These results are consistent with the magnon Bose-Einstein condensation picture for field-induced magnetic ordering.

Deviation from the Kadowaki-Woods relation in Yb-based intermediate-valence systems

The T2-coefficient of the electrical resistivity, A, is compared with the electronic specific heat coefficient, γ, for a number of Yb-based compounds. It is revealed that many systems, including YbCuAl, YbInCu4, YbAl3, and YbCu5, show A/γ2 values close to 0.4 × 10−6 μΩ cm mol2 K2 mJ−2, which are remarkably small compared to those obtained from an expression known as the Kadowaki–Woods relation: A/γ2 = 1.0 × 10−5 μΩcm mol2 K2 mJ−2. Empirically, the compounds with the smaller A/γ2 values appear to show weak intersite magnetic correlation and/or to have almost fully degenerate (J = 5/2 or 7/2) ground states.

Magnetic properties of Mn- and Eu-doped ZnS nanocrystals

The magnetic properties of Mn- and Eu-doped ZnS nanocrystals have been investigated. The results suggest that most of the Mn ions are coordinated on the surface of the nanocrystal, and the amount of Mn doped inside the nanocrystal does not exceed one per nanocrystal. Low-temperature magnetization shows the absence of antiferromagnetic interaction between Mn ions, in contrast to the bulk materials. For Eu-doped nanocrystal, the magnetic susceptibility data reveal the coexistence of Eu2+ and Eu3+ ions, though the starting material contains only Eu3+ ions.

Synthesis, Characterization, and Magnetic Properties of γ-NaxCoO2 (0.70≤x≤0.84)

Powder Na

Electric and Magnetic Properties of Mn‐ and Fe‐Doped CuInS2 Compounds

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Ultra low thermal conductivity of disordered layered p-type bismuth telluride

CoO