研究内容

　While the conventional BCS-type superconductors show the superconducting gap opened over the whole Fermi surfaces, anisotropic superconductors are generally characterized by zero in the gap (either point or line nodes) which break the symmetry of the crystal lattice. Identification of the gap-node structure therefore is of fundamental importance in understanding anisotropic superconductivity and its pairing mechanism. Regarding this issue, it has recently been pointed out that the zero-energy density of states (ZEDOS) in the superconducting mixed state exhibits a characteristic oscillation with respect to the angle between the magnetic field and the nodal directions. Experimentally, the angular dependent ZEDOS can be probed by the thermal conductivity or the specific heat measurements in rotating magnetic field at low temperatures. While the former measurement has high sensitivity to the nodal quasiparticles relevant to the phenomenon, the latter experiment has an advantage over the former in the point that the interpretation of the result is more straightforward. We have performed the angle-resolved specific heat measurements in the superconducting mixed state of the anisotropic superconductor CeCoIn₅ (T_c=2.3 K), which is considered to be a d-wave superconductor, and observed the characteristic angular oscillation in the basal plane.

Fig. 1. Specific heat of CeCoIn₅ measured at 0.38 K in a magnetic field of 3 T rotated within the (001) plane. The fourfold angular oscillation comes from the gap-node structure of d-wave superconductivity. The field angle is measured with respect to the [100] direction.

　Figure 1 shows an example of the specific heat data for a single crystal of CeCoIn₅, obtained at 0.38 K. The magnetic field of 3 T is rotated in the (001) plane of the tetragonal crystal, and the field angle is measured with respect to the [100] direction. As can be seen from the figure, the specific heat shows a characteristic fourfold angular oscillation in the c-plane, with the minima (maxima) appearing along the [100] ([110]) directions. The solid line is a fit to a sinusoidal function ΔC_H/C_H=A₄cos(4θ), where C_H denotes the field dependent part of the specific heat. We have done the similar measurements at various temperatures and fields, and found that the relative amplitude A₄ of the angular oscillation persists to low field region H/H_c2～0.18, where H_c2 denotes the upper critical field. We confirmed that A₄ is a decreasing function of magnetic field, and rapidly decreases by raising temperatures up to ～0.5 T_c. These results strongly suggest that the observed angular oscillation comes from the ZEDOS of nodal quasiparticles of d-wave superconductivity. From the angular position of the specific-heat minima([100]directions), we may conclude that the gap symmetry of CeCoIn₅ is most lilely to be of d_xy type.

Reference
H. Aoki et al.: J. Phys. Condens. Matter 16 (2004) L13.

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