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Register a new userEvidence for Unconventional Strong-coupling Superconductivity in PrOs_4Sb_12 : An Sb Nuclear Quadrupole Resonance (NQR) Study
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We report Sb-NQR results which evidence a heavy-fermion (HF) behavior and an unconventional superconducting (SC) property in the filled-skutterudite compound PrOs_4Sb_12 revealing a SC transition temperature T_c=1.85 K. The temperature (T) dependence of nuclear-spin-lattice-relaxation rate 1/T_1 and NQR frequency unravel a low-lying crystal-electric-field splitting below T_0~10 K, associated with Pr^3+ (4f^2)-derived ground state. The emergence of T_1T=const. behavior below T_F~4 K points to the formation of heavy-quasiparticle state. In the SC state, 1/T_1 shows neither a coherence peak nor a T^3like power-law behavior observed for HF superconductors to date. The isotropic energy-gap with a size of gap Delta/k_B=4.8 K begins to already open up at T^*~2.3 K without any coherence effect just below T_c=1.85 K. We highlight that the superconductivity in PrOs_4Sb_12, which is in an unconventional strong-coupling regime, differs from a conventional s-wave type and any unconventional ones with the line-node gap.
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We report on Sb nuclear-quadrupole-resonance (NQR) study in filled-skutterudite compounds (Pr_1-xLa_x)Os_4Sb_12. The Sb-NQR spectra have split into two sets, arising from different Sb_12 cages containing either Pr or La, which enables us to measure two kinds of nuclear spin-lattice relaxation time T_1^Pr and T_1^La. In the normal state, the temperature (T) dependence of 1/T^Pr_1T showed almost the same behavior as that for PrOs_4Sb_12 regardless of. In contrast, 1/T^La_1T markedly decreases with increasing La concentration. In the superconducting state for x=0.05 and 0.2, 1/T_1^Pr exponentially decreases down to T=0.7 K with no coherence peak below T_c as well as in PrOs_4Sb_12. A remarkable finding is that the residual density of states at the Fermi level below T_c is induced by La substitution for Pr. These results are proposed to be understood in terms of a multiband-superconductivity model that assumes a full gap for part of the FS and the presence of point nodes for a small 4f^2-derived FS inherent in PrOs_4Sb_12. For x=0.8 and 1,1/T^La_1 exhibits a coherence peak and the nodeless energy gap characteristic for weak-coupling s-wave superconductors. With increasing Pr content, T_c and the energy gap increases. The novel strong-coupling superconductivity in PrOs_4Sb_12 is inferred to be mediated by the local interaction between 4f^2-derived crystal-electric-field states with the electric quadrupole degree of freedom and conduction electrons. This coupling causes a mass enhancement of quasi-particles for a part of FS and induces a small FS, which is responsible for point nodes in the superconducting gap function. Note that the small FS does not play any primary role for the strong-coupling superconductivity in PrOs_4Sb_12.
We report the $^{121/123}$Sb-NMR/nuclear quadrupole resonance (NQR) measurements on the newly-discovered superconductor BaTi$_2$Sb$_2$O with a two-dimensional Ti$_2$O square-net layer formed with Ti$^{3+}$ (3$d^1$). NQR measurements revealed that the in-plane four-fold symmetry is broken at the Sb site below $T_{rm A} sim$ 40 K, without an internal field appearing at the Sb site. These exclude a spin-density wave (SDW)/ charge density wave (CDW) ordering with incommensurate correlations, but can be understood with the commensurate CDW ordering at $T_{rm A}$. The spin-lattice relaxation rate $1/T_1$, measured at the four-fold symmetry breaking site, decreases below superconducting (SC) transition temperature $T_{rm c}$, indicative of the microscopic coexistence of superconductivity and the CDW/SDW phase below $T_{rm A}$. Furthermore, $1/T_1$ of $^{121}$Sb-NQR shows a coherence peak just below $T_{rm c}$ and decreases exponentially at low temperatures. These results are in sharp contrast with those in cuprate and iron-based superconductors, and strongly suggest that its SC symmetry is classified to an ordinary s-wave state.
Pressure-induced superconductivity was recently discovered in the binary helimagnet CrAs. We report the results of measurements of nuclear quadrupole resonance for CrAs under pressure. In the vicinity of the critical pressure P_c between the helimagnetic (HM) and paramagnetic (PM) phases, a phase separation is observed. The large internal field remaining in the phase-separated HM state indicates that the HM phase disappears through a strong first-order transition. This indicates the absence of a quantum critical point in CrAs; however, the nuclear spin-lattice relaxation rate 1/T_1 reveals that substantial magnetic fluctuations are present in the PM state. The absence of a coherence effect in 1/T_1 in the superconducting state provides evidence that CrAs is the first Cr-based unconventional superconductor.
We present a systematic ^{115}In NQR study on the heavy fermion compounds CeRh_{1-x}Ir_xIn_5 (x=0.25, 0.35, 0.45, 0.5, 0.55 and 0.75). The results provide strong evidence for the microscopic coexistence of antiferromagnetic (AF) order and superconductivity (SC) in the range of 0.35 leq x leq 0.55. Specifically, for x=0.5, T_N is observed at 3 K with a subsequent onset of superconductivity at T_c=0.9 K. T_c reaches a maximum (0.94 K) at x=0.45 where T_N is found to be the highest (4.0 K). Detailed analysis of the measured spectra indicate that the same electrons participate in both SC and AF order. The nuclear spin-lattice relaxation rate 1/T_1 shows a broad peak at T_N and follows a T^3 variation below T_c, the latter property indicating unconventional SC as in CeIrIn_5 (T_c=0.4 K). We further find that, in the coexistence region, the T^3 dependence of 1/T_1 is replaced by a T-linear variation below Tsim 0.4 K, with the value frac{(T_1)_{T_c}}{(T_1)_{low-T}} increasing with decreasing x, likely due to low-lying magnetic excitations associated with the coexisting magnetism.
We report that a novel type of superconducting order parameter has been realized in the ferromagnetic states in UGe$_2$ via $^{73}$Ge nuclear-quadrupole-resonance (NQR) experiments performed under pressure ($P$). Measurements of the nuclear spin-lattice relaxation rate $(1/T_1)$ have revealed an unconventional nature of superconductivity such that the up-spin band is gapped with line nodes, but the down-spin band remains gapless at the Fermi level. This result is consistent with that of a ferromagnetic spin-pairing model in which Cooper pairs are formed among ferromagnetically polarized electrons. The present experiment has shed new light on a possible origin of ferromagnetic superconductivity, which is mediated by ferromagnetic spin-density fluctuations relevant to the first-order transition inside the ferromagnetic states.
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