Yuji Zenitani

Superconductivity at 39 K in magnesium diboride

In the light of the tremendous progress that has been made in raising the transition temperature of the copper oxide superconductors (for a review, see ref. 1), it is natural to wonder how high the transition temperature, Tc, can be pushed in other classes of materials. At present, the highest reported values of Tc for non-copper-oxide bulk superconductivity are 33 K in electron-doped CsxRbyC60 (ref. 2), and 30 K in Ba1-xKxBiO3 (ref. 3). (Hole-doped C60 was recently found to be superconducting with a Tc as high as 52 K, although the nature of the experiment meant that the supercurrents were confined to the surface of the C60 crystal, rather than probing the bulk.) Here we report the discovery of bulk superconductivity in magnesium diboride, MgB2. Magnetization and resistivity measurements establish a transition temperature of 39 K, which we believe to be the highest yet determined for a non-copper-oxide bulk superconductor.

Superconductivity at 18 K in Yttrium Sesquicarbide System, Y2C3

In this study, we investigated superconductivity at 18 K in an yttrium sesquicarbide system, Y 2 C 3 , by a magnetization measurement, although this material with a maximum T c of 11.5 K has alread...

Superconductivity in MgB 2

We review superconductivity in MgB 2 in terms of crystal and electronic structure, electron-phonon coupling, two-gap superconductivity and application. Finally, we introduce the development of new superconducting materials in related compounds.

Nd Ion Doping Effects on the Multipolar Interactions in CeB6

The Nd ion doping effects on the multipolar interactions in CeB 6 were studied. At high magnetic fields, the antiferro-quadrupolar (AFQ) phase II is not significantly affected by Nd doping. This may be because the Nd ion is in the paramagnetic state. However, with decreasing magnetic field, the Nd doping effect becomes apparent. This may be because Nd–Nd interaction dominates the long-range ordering at low magnetic fields. At low magnetic fields, just after T Q and T N coincide with each other when Nd ion is doped, new phases, namely, phases IIIA and V appear. Phase IIIA is the AF magnetic state where all types of quadrupolar interaction are small. The nature of phase V seems to change largely at x ∼0.5. For x 0.5, the AF magnetic state where both Ce and Nd ions contribute may be realized.

Novel approaches to crystallize materials with narrow liquidus lines: application to spin ladder compound La4+4nCu8+2nO14+8n (n=2,3) and high-Tc cuprate Bi-2223

Abstract We propose two novel approaches, a modified slow cooling (MSC) method and an improved traveling solvent floating zone (I-TSFZ) method, to crystallize materials with narrow liquidus lines. The MSC method uses a flux-poor starting composition to attain sufficient supersaturation, and involves a rapid cooling from high temperature to a proper crystallization temperature to avoid the formation of unwanted phases. The I-TSFZ method uses a very slow growth velocity to grow crystals under small supersaturation, and a steep temperature gradient to precisely control the growth temperature. The MSC method and the I-TSFZ method have been successfully applied to grow spin ladder compounds La 4+4 n Cu 8+2 n O 14+8 n ( n =2,3) and a high- T c cuprate Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ , respectively.

Superconducting energy gap in YNi2B2C

Abstract The first tunneling measurements of the superconducting energy gap in YNi2B2C have been carried out. Observed energy gap value is typically 2Δp-p = 5.3±0.1 meV from the peak-to-peak amplitude in dI/dV, while 2Δ = 3.7±0.1 meV is determined from the standard analysis using the BCS theory. Since the Tc measured from the tunnel junction is 12 K, the ratio 2Δ/kBTc = 3.5±0.1 is obtained. This is in good agreement with the BCS prediction, indicating that the superconducting mechanism in this compound is a conventional type.

Hybridization-induced superconductivity from electron repulsion on a tetramer lattice having a disconnected Fermi surface

Plaquette lattices with each unit cell containing multiple atoms are good candidates for disconnected Fermi surfaces, which are shown by Kuroki and Arita to be favorable for spin-flucutation-mediated superconductivity from electron repulsion. Here we find an interesting example in a tetramer lattice where the structure within each unit cell dominates the nodal structure of the gap function. We trace its reason to the way in which a Cooper pair is formed across the hybridized molecular orbitals, where we still end up with a T c much higher than usual.

Magnetic and superconducting phase diagram in oxybromite cuprate Ca2-xNaxCuO2Br2

A comprehensive magnetic and superconducting phase diagram determined by muon spin rotation/relaxation (μSR)(μSR) is presented for Ca2-xNaxCuO2Br2Ca2-xNaxCuO2Br2 which has apical bromine atoms. Evidence for antiferromagnetic (AF) order in lightly doped samples and that for quasi-static spin glass (SG)-like state in moderately doped ones are obtained by ZF-μμSR at low temperatures. While the phase diagram is qualitatively similar to that in typical 2-1-4 cuprates including Ca2-xNaxCuO2Cl2Ca2-xNaxCuO2Cl2 and La2-xSrxCuO4La2-xSrxCuO4, it exhibits a slight shift (expansion) over the x axis.

Crystal growth and transport properties of spin ladder compounds La4+4nCu8+2nO14+8n (n=2, 3)

Here we summarize recent progress on the study of spin ladder compounds, La 2 Cu 2 O 5 (four-leg) and La 8 Cu 7 O 19 (five-leg), n = 2 and 3 members of the homologous series La 4+4n Cu 8+2n O 14+8n . Following the successful crystal growth of La 2 Cu 2 O 5 , we grew a pure-phase five-leg system La 8 Cu 7 O 19 by a modified slow cooling method. The room-temperature resistivity of the as-grown La 8 Cu 7 O 19 crystal was 1.9 Ω cm, which is much lower than that of the La 2 Cu 2 O 5 crystals (1.7 x 10 3 Ω cm). A significant drop in the resistivity (11.0 mΩ cm at 300 K) was observed upon hole doping by means of high oxygen-pressure annealing, although no transition was observed from the semiconducting behavior. In addition, a new crystal growth method to enhance hole doping level of La 2 Cu 2 O 5 is shown.

Trial of hole-doping to 4-and 5-leg ladder compounds

Abstract The electrical resistivity and magnetic susceptibility measurements have been made for the spin- 1 2 4- and 5-leg ladder compounds, La 8 Cu 8 O 20 and La 8 Cu 7 O 19 , respectively. Hole-doping has been attempted by means of high oxygen pressure annealing and substitution of Li +1 for Cu +2 , La 8 Cu 8− x Li x O 20 . For La 8 Cu 8 O 20 and La 8 Cu 7 O 19 , the electrical resistivities exhibit semiconductive behaviors, and the magnetic susceptibilities have a broad peak at around 150K. For the annealed samples under high oxygen pressure, the electrical resistivity becomes more conductive than that for as-sintered samples. The Li-substituted samples La 8 Cu 8− x Li x O 20 with x =0.4, 0.8 and 1.2 also shows the semiconductive behavior, but it becomes more conductive with increasing the Li concentration x . The magnetic susceptibility shows the broad peak at around 150K for as-sintered sample, and spin-glass like behavior was observed at around 10K. This transition is shifted to low temperature with increasing Li concentration.