**Fig.6-8 T-dependence of spin-lattice relaxation rate (1/T _{1}) in superconducting PuRhGa_{5}**

No coherence peak appears just below T_{c}, and 1/T_{1} is proportional to T^{3} below T_{c}, indicating non-conventional superconductivity. The tetragonal crystal structure is shown in the inset.

**Fig.6-9 Two dimensional Fermi surfaces with d-wave superconducting gap **

In the non-conventional superconductor, the superconducting gap has nodes.

In uranium and transuranium compounds, many exotic magnetic and superconducting states due to strong correlation of electrons have been found. We have synthesized a single crystal of a new superconductor PuRhGa_{5} for the first time and clarified its exotic superconducting state by the nuclear magnetic resonance (NMR) measurements. In order to clarify the superconducting state, the spin-lattice relaxation rate (1/T_{1}) is particularly important. The BCS model theory for ordinary superconductors is well established, since measurements of (1/T_{1}) show the coherence peak just below T_{c} predicted by the BCS model.

In the present study, the nuclear quadrupole resonance of ^{69}Ga nuclei has been observed at zero field. Because of the zero field measurement, no distribution of superconducting order parameter due to the mixed state occurs. Fig.6-8 shows the T-dependence of 1/T_{1}. The important point here is that no coherence peak just below T_{c} is observed, 1/T_{1} decreasing with decreasing T in the superconducting state. This fact indicates that the superconducting state in this compound is a non-conventional one with an anisotropic superconducting gap. In ordinary superconductors, 1/T_{1} is proportional to exp (-Δ/T) below T_{c}. In contrast, T-dependence of 1/T_{1} shows a power law behavior: 1/T_{1}∝T^{n} in a non-conventional superconducting state. In PuRhGa_{5}, n is estimated as 3. The value of n reflects the type of anisotropy in the superconducting gap. From measurements of the upper critical field H_{C2}, it was revealed that the Fermi surface has a two dimensional nature. Combined with this fact, the observed T-dependence can be well reproduced by a model of two dimensional Fermi surfaces whose gap disappears at some points (nodes) (Fig.6-9) and a residual density of states due to defects.

As described above, we have identified an exotic superconducting state in a Pu-based new superconductor and now we will be investigating this origin of the non-conventional superconductivity.