UCoGe is one of the ferromagnetic superconductors that show a superconducting transition inside ferromagnetism, and it is thought that the quantum criticality of ferromagnetism is related to superconductivity. In fact, it is known that even if the ferromagnetism disappears by the application of hydrostatic pressure of about 1 GPa (10,000 atmospheric pressure), the superconducting phase still exists and that superconductivity is slightly enhanced near the disappearance of ferromagnetism. This is one of the proofs of superconductivity induced by the ferromagnetic quantum critical point (QCP). On the other hand, our past nuclear quadrupole resonance (NQR) has revealed that the ferromagnetic transition is actually a weak first-order phase transition. At intermediate pressure, it was observed that ferromagnetism and paramagnetism undergo a superconducting transition with phase separation, and attention was paid to how the two superconducting states coexist in the sample.
In this study, we performed 59 Co NQR on a UCoGe single crystal sample at 0.3 GPa where this phase separation is observed even in the superconducting state. When we measured the nuclear spin-lattice relaxation rate (1 / T 1 ), the anomaly of superconductivity was clearly observed at the paramagnetic site, that is, the superconducting gap was opened, but in ferromagnetism, it was below the superconducting transition temperature. However, no clear abnormality was observed. In addition, the results of the NQR spectrum (see the figure below) showed that the proportion of paramagnetic sites in the sample increases with superconductivity, that is, the appearance of superconductivity may slightly suppress ferromagnetism. Even in the case of ferromagnetic superconductors, the superconducting condensation energy decreases under the internal magnetic field due to ferromagnetism, so paramagnetic is considered to be more advantageous for the manifestation of superconductivity in the phase-separated state.
This research is collaborative research with Nagoya University and Tohoku University Institute for Materials Research. The results are published as a letter in the Journal of the Physical Society of Japan. Click here for the preprint.
Figure: Temperature dependence of the UCoGe 59 Co NQR spectrum at 0.3 GPa . (a) When excited by a strong high frequency magnetic field. The paramagnetic signal weakened with the advent of ferromagnetism, but in the superconducting state it turned relatively stronger. (b) When excited by a weak high-frequency magnetic field. The paramagnetic signal was more clearly observed, while the ferromagnetic signal was barely visible. This difference means that the optimal RF field is different at the two sites.
Article information
Manago, M; Kitagawa, S; Ishida, K; Deguchi, K; Sato, N K; Yamamura, T
