Ferromagnetic Quantum Critical Fluctuations and Anomalous Coexistence of Ferromagnetism and Superconductivity in UCoGe Revealed by Co-NMR and NQR Studies

Abstract

Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies were carried out for the recently discovered UCoGe, in which the ferromagnetic and superconducting (SC) transitions are reported to occur at TCurie ∼ 3 K and TS ∼ 0.8 K (N. T. Huy et al., Phys. Rev. Lett. 99 (2007) 067006), in order to investigate the coexistence of ferromagnetism and superconductivity as well as the normal-state and SC properties from a microscopic point of view. From the nuclear spin-lattice relaxation rate 1/T1 and Knight-shift measurements, we confirm that ferromagnetic fluctuations that possess a quantum critical character are present above TCurie and also the occurrence of a ferromagnetic transition at 2.5 K in our polycrystalline sample. The magnetic fluctuations in the normal state show that UCoGe is an itinerant ferromagnet similar to ZrZn2 and YCo2. The onset SC transition is identified at TS ∼ 0.7 K, below which 1/T1 arising from 30% of the volume fraction starts to decrease due to the opening of the SC gap. This component of 1/T1, which follows a T 3 dependence in the temperature range 0.3 − 0.1 K, coexists with the magnetic components of 1/T1 showing a √ T dependence below TS. From the NQR measurements in the SC state, we suggest that the self-induced vortex state is realized in UCoGe. KEYWORDS: UCoGe, ferromagnetic superconductor, Co-NMR and NQR, coexistenceSince the discovery of superconductivity in ferromagnetic compounds under high pressure, 1, 2 the concept of the interplay between magnetism and superconductivity was changed, because ferromagnetism and superconductivity are considered to be mutually exclusive. The pressure studies in UGe 2 have shown that the superconducting (SC) transition temperature T S is higher in the pressure region where the ordered moments are enhanced, and that superconductivity is not observed when the ferromagnetism disappears in a further higher pressure region.3 These results suggest that ferromagnetism seems to enhance the superconductivity in UGe 2 . In addition, ambient-pressure ferromagnetic superconductivity was discovered in URhGe, where T Curie = 9.5 K and T S ∼ 0.25 K were reported. 4 The relation between the ferromagnetism and superconductivity observed in the U compounds is one of the attractive topics for the community studying strongly correlated electron systems.Quite recently, new ambient-pressure ferromagnetic superconductivity with T Curie = 3 K and T S ∼ 0.8 K was found in UCoGe.5

Field-Induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe

“…That is, the material is always in the superconducting mixed state without H c1 , which is also supported by recent NMR/NQR experiments and magnetization measurements in UCoGe [17,19,20]. Another possibility is that the minority-spin Fermi surface is not gapped and the majority-spin Fermi surface is only responsible for the superconductivity within the framework of spin-triplet state with equal-spin pairing.…”

59Co-NMR/Nuclear-Quadrupole-Resonance (NQR) Studies on Ferromagnetic Superconductor UCoGe

“…We used two samples for our experiments: one is the polycrystalline sample reported in literature 17) and the other is a 55 mg single crystal with 1.65 × 1.65 × 1.89 mm 3 dimension, grown by the Czochralski method in a tetra-arc furnace. 18,19) The FM transition temperature T Curie of the singlecrystal sample was evaluated to be 2.55±0.1 K from the Arrot plots shown in the inset of anisotropic behavior was observed with the easy axis along the c axis.…”

Microscopic Coexistence of Ferromagnetism and Superconductivity in Single-Crystal UCoGe

“…UCoGe is a FM superconductor suitable for NQR measurements, since it contains an NQR-active element of 59 Co. In the previous letter, we reported 59 Co-NQR studies in a polycrystalline UCoGe with T Curie ¼ 2:5 K and the SC onset temperature T onset SC ¼ 0:7 K. 7) We found inhomogeneous ferromagnetism below T Curie in the polycrystalline sample, from the observation of the FM and paramagnetic (PM) NQR spectra at lowest temperature. In addition, the SC anomaly was observed in 1=T 1 measured in both NQR spectra, suggesting that superconductivity exists both in the FM and PM states.…”

“…We found signals around 4 and 7 MHz, which could not be observed previously. 7) When the nuclear spin experiences an internal field H int , the Zeeman term…”