Jun Akimitsu

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 in the Ladder Material Sr0.4Ca13.6Cu24O41.84

We have observed superconductivity in the ladder material Sr 0.4 Ca 13.6 Cu 24 O 41.84 under pressures of 3 GPa and 4.5 GPa by means of electrical measurements. The superconducting transition temperatures T c (onset) are 12 K and 9 K at 3 and 4.5 GPa, respectively. The superconducting volume fraction was obtained to be about 5% from magnetization measurement under 3.5 GPa at 4.2 K, indicating the bulk nature of the superconductivity in this system.

Unconventional superconductivity in Y5Rh6Sn18 probed by muon spin relaxation.

Conventional superconductors are robust diamagnets that expel magnetic fields through the Meissner effect. It would therefore be unexpected if a superconducting ground state would support spontaneous magnetics fields. Such broken time-reversal symmetry states have been suggested for the high—temperature superconductors, but their identification remains experimentally controversial. We present magnetization, heat capacity, zero field and transverse field muon spin relaxation experiments on the recently discovered caged type superconductor Y5Rh6Sn18 ( TC= 3.0 K). The electronic heat capacity of Y5Rh6Sn18 shows a T3 dependence below Tc indicating an anisotropic superconducting gap with a point node. This result is in sharp contrast to that observed in the isostructural Lu5Rh6Sn18 which is a strong coupling s—wave superconductor. The temperature dependence of the deduced superfluid in density Y5Rh6Sn18 is consistent with a BCS s—wave gap function, while the zero-field muon spin relaxation measurements strongly evidences unconventional superconductivity through a spontaneous appearance of an internal magnetic field below the superconducting transition temperature, signifying that the superconducting state is categorized by the broken time-reversal symmetry.

Superconductivity in the Bi−Sr−Cu−O system

A superconducting transition with an onset temperature near 8 K was observed for the Bi-Sr-Cu-O system through electrical resistivity and magnetic susceptibility measurements. The superconductivity is of a bulk nature and reproducible.

Superconductivity in the Nd-Sr-Ce-Cu-O System

A new high-Tc superconductor Nd-Sr-Ce-Cu-O system has been found by the resistivity and magnetization measurements. The highest onset transition temperature is about 28 K. The superconducting phase may be described as (Nd1-xSrx-yCey)2CuO4-δ.

Evidence for strong-coupling s-wave superconductivity in MgB2: (11)B NMR Study.

We have investigated a gap structure in a newly discovered superconductor, MgB2, through measurement of the (11)B nuclear spin-lattice relaxation rate, (11)(1/T(1)). (11)(1/T(1)) is proportional to the temperature (T) in the normal state, and decreases exponentially in the superconducting (SC) state, revealing a tiny coherence peak just below T(c). The T dependence of 1/T(1) in the SC state can be accounted for by an s-wave SC model with a large gap size of 2Delta/k(B)T(c) approximately 5 which suggests it is in a strong-coupling regime.

A New Family of Superconducting Copper Oxides: (Ln1-xCex)2(Ba1-yLny)2Cu3O10-δ (Ln: Nd, Sm, EU)

A new family of high- T C superconductors (Ln 1- x Ce x ) 2 (Ba 1- y Ln y ) 2 Cu 3 O 10-δ (Ln: Nd, Sm, EU) has been found by resistivity and magnetization measurements. The highest onset transition temperature is about 43 K. The structure is proposed on the basis of electron diffractions and Rietveld analysis of powder X-ray diffraction data.

Spin dynamics of the spin-ladder dimer-chain material Sr14Cu24O41

We have performed inelastic neutron scattering on a single crystal sample of Sr14Cu24O41 to study the spin dynamics of the Cu2O3 spin ladder layers, and CuO2 chains. Data collected with incident energies of 50 meV, 200 meV, 350 meV and 500 meV are best fitted with a dispersion with a spin gap of 32.5+/-0.1 meV and a maximum of 193.52.4 meV, consistent with a coupling along the ladders, J|| = 130 meV and a rung coupling J^=72 meV. We find that excitations with an energy transfer of approximately 11.5 meV can be described solely in terms of a dimer chain with an antiferromagnetic intra-dimer coupling, J1 = 11.2 meV, between next-nearest-neighbour Cu ions and a ferromagnetic inter-dimer coupling, J2 = -1.1 meV. The dimers are separated by two Cu ions providing a periodicity for the dimer chain of five units.

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...

Observation of Orbital Ordering in K2CuF4

Direct observation of orbital ordering in the ferromagnet K 2 CuF 4 has been made by means of the polarized neutron diffraction technique. The observed interference terms between the aspherical contribution of the spin density distribution and the nuclear contribution from the displaced F - ions have been satisfactorily explained in terms of the orbital ordering model proposed by Khomskii and Kugel.