Microscopic Coexistence of Ferromagnetism and Superconductivity in Single-Crystal UCoGe

Abstract

Unambiguous evidence for the microscopic coexistence of ferromagnetism and superconductivity in UCoGe (T Curie $ 2:5 K and T SC $ 0:6 K) is reported from 59 Co nuclear quadrupole resonance (NQR). The 59 Co-NQR signal below 1 K indicates ferromagnetism throughout the sample volume, while the nuclear spin-lattice relaxation rate 1=T 1 in the ferromagnetic (FM) phase decreases below T SC due to the opening of the superconducting (SC) gap. The SC state is found to be inhomogeneous, suggestive of a self-induced vortex state, potentially realizable in a FM superconductor. In addition, the 59 Co-NQR spectrum around T Curie shows that the FM transition in UCoGe possesses a first-order character, which is consistent with the theoretical prediction that the low-temperature FM transition in itinerant magnets is generically of first-order.KEYWORDS: ferromagnetic superconductor, U-based heavy-fermion, UCoGe, nuclear quadrupole resonance DOI: 10.1143/JPSJ.79.023707After the discovery of superconductivity in UGe 2 under pressure, 1) the coexistence of superconductivity and ferromagnetism becomes one of the major topics in condensedmatter physics. This is because ferromagnetism and spinsinglet superconductivity are thought to be mutually exclusive. 2,3) In the presence of a large splitting between the majority and minority spin Fermi surfaces, as in a ferromagnetic (FM) state, more-exotic spin-triplet superconductivity is allowed, in which parallel spins pair within each spin Fermi surface. While FM superconductors such as UIr 4) and URhGe 5) have recently been demonstrated to occur experimentally, proof that the same charge carriers participate simultaneously in both phenomena has remained elusive.In 2007, new ambient-pressure ferromagnetic (FM) superconductor UCoGe was discovered by Huy et al.6) UCoGe is a weak ferromagnet with T Curie ¼ 3 K and the ordered moments s ¼ 0:03 B , and shows superconductivity at the transition temperature T SC ¼ 0:8 K, 6) highest among FM superconductors. In order to investigate the correlation between ferromagnetism and superconductivity, nuclear quadrupole resonance (NQR) measurements are ideally suited, since they provide microscopic information about the electronic and magnetic properties without applying external fields. In a magnetically ordered state, the NQR signal splits or shifts due to internal fields at the nuclear site, and the nuclear spin-lattice relaxation rate 1=T 1 provides siteselective information about the density of states at the Fermi level and thus about the superconducting (SC) gap structure. 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 mea...

Anomalous spin distribution in the superconducting ferromagnet UCoGe studied by polarized neutron diffraction

“…The small value of the Co moment in weak magnetic fields is in line with recent zero-field SR ͑Ref. 9͒ and 59 Co nuclear quadrupole resonance 24 measurements. Second, in a magnetic field a moment Co is induced on the Co site, oriented antiparallel to U but parallel to S U .…”

Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe

“…The single crystalline UCoGe was utilized for the measurement, which is the same sample reported previously 5,9 . The sample showed a large residual resistivity ratio (RRR) of approximately 30 along b axis.…”

“…Co NQR spectra from the ±5/2 ↔ ±7/2 transitions measured at 1.35 K and 0.9 K (< TCurie), and 0.1 K (< TSC) 9 . In the NQR spectrum at 1.35 K, small paramagnetic (PM) state signal is observed with the main FM-state signal.…”

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

“…UCoGe displays coexisting superconductivity and ferromagnetic order with the superconducting critical temperature (T c ∼ 0.8 K) below the Curie temperature (T C ∼ 3 K) [210]. NMR measurements show that the ferromagnetic transition in UCoGe is of first-order character [230]. The small saturated moment, μ sat ∼ 0.03μ B [210], and large ratio of the effective moment to saturated moment μ eff /μ sat , indicates the highly itinerant nature of the ferromagnetism.…”