We report $^{31}$P- and $^{75}$As-NMR studies on (Ca$_4$Al$_2$O$_{6}$)Fe$_2$(As$_{1-x}$P$_x$)$_2$ with an isovalent substitution of P for As. We present the novel evolution of emergent phases that the nodeless superconductivity (SC) in 0$le x le$0.4
and the nodal one around $x$=1 are intimately separated by the onset of a commensurate stripe-type antiferromagnetic (AFM) order in 0.5$le x le$ 0.95, as an isovalent substitution of P for As decreases a pnictogen height $h_{Pn}$ measured from the Fe plane. It is demonstrated that the AFM order takes place under a condition of 1.32AA$le h_{Pn} le$1.42AA, which is also the case for other Fe-pnictides with the Fe$^{2+}$ state in (Fe$Pn$)$^{-}$ layers. This novel phase evolution with the variation in $h_{Pn}$ points to the importance of electron correlation for the emergence of SC as well as AFM order.
We report on magnetic characteristics in four-layered high-T_c superconductors Ba_2Ca_3Cu_4O_8(F_yO_{1-y})_2 with apical fluorine through Cu- and F-NMR measurements. The substitution of oxygen for fluorine at the apical site increases the carrier den
sity (N_h) and T_c from 55 K up to 102 K. The NMR measurements reveal that antiferromagnetic order, which can uniformly coexist with superconductivity, exists up to N_h = 0.15, which is somewhat smaller than N_h = 0.17 being the quantum critical point (QCP) for five-layered compounds. The fact that the QCP for the four-layered compounds moves to a region of lower carrier density than for five-layered ones ensures that the decrease in the number of CuO_2 layers makes an interlayer magnetic coupling weaker.
We report a $^{73}$Ge nuclear-quadrupole-resonance (NQR) study on novel evolution of unconventional superconductivity in antiferromagnetic (AFM) CeNiGe$_3$. The measurements of the $^{73}$Ge-NQR spectrum and the nuclear spin-lattice relaxation rate (
$1/T_1$) have revealed that the unconventional superconductivity evolves inside a commensurate AFM phase around the pressure ($P$) where N{e}el temperature $T_{rm N}$ exhibits its maximum at 8.5 K. The superconducting transition temperature $T_{rm SC}$ has been found to be enhanced with increasing $T_{rm N}$, before reaching the quantum critical point at which the AFM order collapses. Above $T_{rm SC}$, the AFM structure transits from an incommensurate spin-density-wave order to a commensurate AFM order at $Tsim 2$ K, accompanied by a longitudinal spin-density fluctuation. With regard to heavy-fermion compounds, these novel phenomena have hitherto never been reported in the $P$-$T$ phase diagram.
We report on the pressure-induced unconventional superconductivity in the heavy-fermion antiferromagnet CeIn3 by means of nuclear-quadrupole-resonance (NQR) studies conducted under a high pressure. The temperature and pressure dependences of the NQR
spectra have revealed a first-order quantum-phase transition (QPT) from an AFM to PM at a critical pressure Pc=2.46 GPa. Despite the lack of an AFM quantum critical point in the P-T phase diagram, we highlight the fact that the unconventional SC occurs in both phases of the AFM and PM. The nuclear spin-lattice relaxation rate 1/T1 in the AFM phase have provided evidence for the uniformly coexisting AFM+SC phase. In the HF-PM phase where AFM fluctuations are not developed, 1/T1 decreases without the coherence peak just below Tc, followed by a power-law like T dependence that indicates an unconventional SC with a line-node gap. Remarkably, Tc has a peak around Pc in the HF-PM phase as well as in the AFM phase. In other words, an SC dome exists with a maximum value of Tc = 230 mK around Pc, indicating that the origin of the pressure-induced HF SC in CeIn3 is not relevant to AFM spin fluctuations but to the emergence of the first-order QPT in CeIn3. When the AFM critical temperature is suppressed at the termination point of the first-order QPT, Pc = 2.46 GPa, the diverging AFM spin-density fluctuations emerge at the critical point from the AFM to PM. The results with CeIn3 leading to a new type of quantum criticality deserve further theoretical investigations.
We report Cu- and F-NMR studies on a four-layered high-temperature superconductor Ba2Ca3Cu4O8F2(0234F(2.0)) with apical fluorine (F-1), an undoped 55 K-superconductor with a nominal Cu2+ valence on average. We reveal that this compound exhibits the a
ntiferromagnetism (AFM) with a Neel temperature TN=100 K despite being a Tc= 55 K-superconductor. Through a comparison with a related tri-layered cuprate Ba2Ca2Cu3O6F2 (0223F(2.0)), it is demonstrated that electrons are transferred from the inner plane (IP) to the outer plane (OP) in 0234F(2.0) and 0223F(2.0), confirming the self-doped high-temperature superconductivity (HTSC) having electron and hole doping in a single compound. Remarlably, uniform mixing of AFM and HTSC takes place in both the electron-doped OPs and the hole-doped IPs in 0234F(2.0).
