75 As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe2As2 under pressure. At P = 4.7 and 10.8 kbar, the temperature dependences of nuclear-spin-lattice relaxation rate (1/T1) measured in the tetragonal phase show no coherence peak just below Tc(P ) and decrease with decreasing temperature. The superconductivity is gapless at P = 4.7 kbar but evolves to that with multiple gaps at P = 10.8 kbar. We find that the superconductivity appears near a quantum critical point under pressures in the range 4.7 kbar ≤ P ≤ 10.8 kbar. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.

We report systematic 75 As-NQR and 139 La-NMR studies on nickel-pnictide superconductors LaNiAsO1−xFx (x=0, 0.06, 0.10 and 0.12). The spin lattice relaxation rate 1/T1 decreases below Tc with a well-defined coherence peak and follows an exponential decay at low temperatures. This result indicates that the superconducting gap is fully opened, and is strikingly different from that observed in iron-pnictide analogs. In the normal state, 1/T1T is constant in the temperature range Tc ∼4 K≤ T ≤10 K for all compounds and up to T =250 K for x=0 and 0.06, which indicates weak electron correlations and is also different from the iron analog. We argue that the differences between the iron and nickel pnictides arise from the different electronic band structure. Our results highlight the importance of the peculiar Fermi-surface topology in iron-pnictides. 74.25.Nf, 74.90.+n

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We present extensive 75 As NMR and NQR data on the superconducting arsenides PrFeAs 0.89 F 0.11 (T c =45 K), LaFeAsO 0.92 F 0.08 (T c =27 K), LiFeAs (T c = 17 K) and Ba 0.72 K 0.28 Fe 2 As 2 (T c = 31.5 K) single crystal, and compare with the nickel analog LaNiAsO 0.9 F 0.1 (T c =4.0 K) . In contrast to LaNiAsO 0.9 F 0.1 where the superconducting gap is shown to be isotropic, the spin lattice relaxation rate 1/T 1 in the Fe-arsenides decreases below T c with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below T c and shows a step-wise T variation as well. These results indicate spinsinglet superconductivity with multiple gaps in the Fe-arsenides. The Fe antiferromagnetic spin fluctuations are anisotropic and weaker compared to underdoped copper-oxides or cobalt-oxide superconductors, while there is no significant electron correlations in LaNiAsO 0.9 F 0.1 . We will discuss the implications of these results and highlight the importance of the Fermi surface topology.

75 As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe 2 As 2 under pressure. At P = 4.7 and 10.8 kbar, the temperature dependence of nuclear-spin-lattice relaxation rate (1/T 1 ) measured at tetragonal phase show no coherence peak just below T c and decrease with decreasing temperature. The superconductivity is of gapless at P = 4.7 kbar but evolves to multiple gaps at P = 10.8 kbar. We find that the superconductivity appears near a quantum critical point. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.KEYWORDS: CaFe 2 As 2 , superconductivity, quantum critical point, NMR/NQR, pressureSince the discovery of high-transition temperature (T c ) superconductivity (SC) in LaFeAsO 1−x F x at T c = 26 K, 1) iron pnictides have become one of the most fascinating research areas in condensed-matter physics. The carrier-doping suppresses structural and magnetic phase transitions in undoped ReFeAsO and SC appears.1) The high-T c SC has also been found in K-doped BaFe 2 As 2 2) and in LiFeAs. 3, 4) Previous nuclear-magnetic resonance (NMR)/nuclear-quadrupole resonance (NQR) measurements on ReFeAsOF, Ba 0.7 K 0.3 Fe 2 As 2 , and Li x FeAs consistently found multiple gap SC. [5][6][7][8] The multiple gap feature is believed to be relevant to their multiple electronic band structure. 9) On the other hand, the mechanism of the Cooper pair formation in iron pnictide is still under debate.The discoveries of pressure(P)-induced SC in RFe 2 As 2 (R = Ca, Sr, and Ba) provide a new route to investigate SC in iron pnictide.10-13) In RFe 2 As 2 , they also show structural transition from a tetragonal (tetra.) to orthorhombic (orth.) structure accompanied by antiferromagnetic order.14-16) In BaFe 2 As 2 and SrFe 2 As 2 , both phase transitions are suppressed by pressure, and SC was found around P c ∼ 50 kbar, with T c ∼ 30 K. 10,13) On the other hand, SC in CaFe 2 As 2 has been observed with lower P c ∼ 5 kbar and T c (P) ∼ 10K.11, 12) The most strikingly different feature in CaFe 2 As 2 is the occurrence of another structural phase transition under P. Above P ∼ 5 kbar, high temperature (T ) tetra. phase changes to a collapsed tetragonal (c-tetra.) phase with drastic reduction in both the unit cell volume (5%) and the c/a ratio (11%).17, 18) Notably, when tetra. phase collapses, P-induced SC disappears. 19)Here we report results of zero-field (ZF) NMR/NQR study in CaFe 2 As 2 under P. P-induced SC in the tetra. phase is successfully confirmed. The T dependences of nuclear spinlattice relaxation rate (1/T 1 ) show no coherence peak just below T c (P). Below T c (P), the T dependences of 1/T 1 indicate the unconventional SC. Present results indicate that electron correlations near a quantum critical point (QCP) play a vital role in inducing unconventional SC. * E-mail address: kawasaki@science.okay...