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...

To gain insight into the unconventional superconductivity of Fe pnictides with no electron pockets, we measure the thermal conductivity κ and penetration depth λ in the heavily hole-doped regime of Ba 1−x K x Fe 2 As 2 . The residual thermal conductivity (κ/T ) T →0 K and T dependence of λ consistently indicate the fully gapped superconductivity at x = 0.76 and the (line) nodal superconductivity at higher hole concentrations. The magnitude of κ T T c | T →0 K and the slope of λ(T ) at low temperatures, both of which are determined by the properties of the low-energy excitations, exhibit a highly unusual nonmonotonic x dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range. We suggest that the observed x dependence is naturally explained by a doping crossover of the gap function between the s-wave states with and without sign reversal between -centered hole pockets.

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

The photoinduced ring-opening reaction of 1,3-cyclohexadiene (CHD) to produce 1,3,5-hexatriene (HT) plays an essential role in the photobiological synthesis of vitamin D3 in the skin. This reaction follows the Woodward–Hoffmann rule, and C5–C6 bond rupture via an electronically excited state occurs with conrotatory motion of the end CH2 groups. However, it is noted that the photoexcited S1(π,π*) state of CHD is not electronically correlated with the ground state of HT, and the reaction must proceed via nonadiabatic transitions. In the present study, we have clearly observed the nonadiabatic reaction pathway via the doubly excited state of CHD using ultrafast extreme UV photoelectron spectroscopy. The results indicate that the reaction occurs in only 68 fs and creates product vibrational coherence. Extensive computational simulations support the interpretation of experimental results and provide further insights into the electronic dynamics in this paradigmatic electrocyclic ring-opening reaction.

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