Tadataka Watanabe

Gap function with point nodes in borocarbide superconductor YNi2B2C

To determine the superconducting gap function of YNi2B2C, the c-axis thermal conductivity kappa(zz) was measured in H rotated in various directions. The angular variation of kappa(zz) in H rotated within the ab plane shows a peculiar fourfold oscillation with narrow cusps. The amplitude of this fourfold oscillation becomes very small when H is rotated conically around the c axis with a tilt angle of 45 degrees. These results provide the first compelling evidence that the gap function has point nodes located along the a and b axes. This unprecedented gap structure challenges the current view on the pairing mechanism.

High-field state of the flux-line lattice in the unconventional superconductor CeCoIn5

Ultrasound velocity measurements of the unconventional superconductor CeCoIn_5 with extremely large Pauli paramagnetic susceptibility reveal an unusual structural transformation of the flux line lattice (FLL) in the vicinity of the upper critical field. The transition field coincides with that at which heat capacity measurements reveal a second order phase transition. The lowering of the sound velocity at the transition is consistent with the collapse of the FLL tilt modulus and a crossover to quasi two-dimensional FLL pinning. These results provide a strong evidence that the high field state is the Fulde-Ferrel-Larkin-Ovchinikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicular to the vortices.

Superconducting gap function in antiferromagnetic heavy-fermion UPd2Al3 probed by angle-resolved magnetothermal transport measurements

The superconducting gap structure of heavy fermion UPd_2Al_3, in which unconventional superconductivity coexists with antiferromagnetic (AF) order with atomic size local moments, was investigated by the thermal conductivity measurements in a magnetic field rotating in various directions relative to the crystal axes. The results provide strong evidence that the gap function \Delta(k) has a single line node orthogonal to the c-axis located at the AF Brillouin zone boundary, while \Delta(k) is isotropic within the basal plane. The determined nodal structure is compatible with the resonance peak in the dynamical susceptibility observed in neutron inelastic scattering experiments. Based on these results, we conclude that the superconducting pairing function of UPd_2Al_3 is most likely to be d-wave with a form \Delta(k)=\Delta_0 cos(k_zc)

Enhanced quasiparticle heat conduction in the multigap superconductor Lu2Fe3Si5 .

Thermal transport measurements have been made on the Fe-based superconductor Lu2Fe3Si5 (T(c) ∼ 6  K) down to a very low temperature T(c)/120. The field and temperature dependences of the thermal conductivity confirm the multigap superconductivity with fully opened gaps on the whole Fermi surfaces. In comparison to MgB2, Lu2Fe3Si5 reveals a remarkably enhanced quasiparticle heat conduction in the mixed state. The results can be interpreted as a consequence of the unequal weight of the Fe 3d-electron character among the distinct bands.

Emergence of highly degenerate excited states in the frustrated magnet MgCr2O4.

High degeneracy in ground states leads to the generation of exotic zero-energy modes, a representative example of which is the formation of molecular spin-liquid-like fluctuations in a frustrated magnet. Here we present single-crystal inelastic neutron scattering results for the frustrated magnet MgCr(2)O(4), which show that a common set of finite-energy molecular spin excitation modes is sustained in both the liquid-like phase above magnetic ordering temperature T(N) and an ordered phase with an extremely complex magnetic structure below T(N). Based on this finding, we propose the concept of high degeneracy in excited states, which promotes local resonant elementary excitations. This concept is expected to have ramifications on our understanding of excitations in many complex systems, including not only spin but also atomic liquids, complex order systems, and amorphous systems.

Disorder-sensitive superconductivity in the doped iron silicide superconductor (Lu1―xRx)2Fe3Si5 (R = Sc, Y, and Dy)

We studied effect of non-magnetic and magnetic impurities on superconductivity in Lu2Fe3Si5 by small amount substitution of the Lu site, which investigated structural, magnetic, and electrical properties of non-magnetic (Lu1−xScx)2Fe3Si5, (Lu1−xYx)2Fe3Si5, and magnetic (Lu1−xDyx)2Fe3Si5. The rapid depression of Tc by non-magnetic impurities in accordance with the increase of residual resistivity reveals the strong pair breaking dominated by disorder. We provide compelling evidence for the sign reversal of the superconducting order parameter in Lu2Fe3Si5.

Pressure dependence of the magnetic properties of MnPS3

The pressure dependence of the magnetic properties of two-dimensional transition metal phosphorus trichalcogenides MnPS 3 has been investigated. In this compound, the lattice constant c which includes the van der Waals gap is reported to decrease rather rapidly with pressure, compared with the lattice constants a and b. The magnitude of the interlayer exchange interaction can be increased by reducing the van der Waals gap with applying a pressure. While the effective magnetic moment is almost independent of the pressure and close to spin only value of Mn 2+ ion, the Neel temperature T N and the spin flop magnetic field HF show the pressure dependence. The increase rate of TN and the decrease rate of HF are 13 K/GPa and 4.0 × 103 Oe/GPa, respectively. These results are well explained by the increase of the dimensionality and the decrease of the anisotropy.

Novel Elastic Instabilities within Antiferromagnetically Ordered Phase in the Orbitally-Frustrated Spinel GeCo2O4

Ultrasound velocity measurements of the orbitally-frustrated GeCo

Gap symmetry of superconductivity in UPd2Al3

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Observation of elastic anomalies driven by coexisting dynamical spin Jahn-Teller effect and dynamical molecular-spin state in the paramagnetic phase of frustrated MgCr2O4

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