Enhancing the Superconducting Transition Temperature due to Strong-Coupling Effect under Antiferromagnetic Spin Fluctuations in CeRh1-xIrxIn5 : 115In-NQR Study

We report on systematic evolutions of antiferromagnetic (AFM) spin fluctuations and unconventional superconductivity (SC) in heavy-fermion (HF) compounds CeRh$_{1-x}$Ir$_{x}$In$_5$ via $^{115}$In nuclear-quadrupole-resonance (NQR) experiment. The measurements of nuclear spin-lattice relaxation rate $1/T_1$ have revealed the marked development of AFM spin fluctuations as a consequence of approaching an AFM ordered state with increasing Rh content. Concomitantly the superconducting transition temperature $T_{\rm c}$ and the energy gap $\Delta_0$ increase drastically from $T_{\rm c} = 0.4$ K and $2\Delta_0/k_{\rm B}T_{\rm c} = 5$ in CeIrIn$_5$ up to $T_{\rm c} = 1.2$ K and $2\Delta_0/k_{\rm B}T_{\rm c} = 8.3$ in CeRh$_{0.3}$Ir$_{0.7}$In$_5$, respectively. The present work suggests that the AFM spin fluctuations in close proximity to the AFM quantum critical point are indeed responsible for the onset of strong-coupling unconventional SC with the line node in the gap function in HF compounds.Comment: 4pages,5figures,to appear in Phys. Rev. Let

Planar CuO_2 hole density estimation in multilayered high-T_c cuprates

We report that planar CuO_2 hole densities in high-T_c cuprates are consistently determined by the Cu-NMR Knight shift. In single- and bi-layered cuprates, it is demonstrated that the spin part of the Knight shift K_s(300 K) at room temperature monotonically increases with the hole density $p$ from underdoped to overdoped regions, suggesting that the relationship of K_s(300 K) vs. p is a reliable measure to determine p. The validity of this K_s(300 K)-p relationship is confirmed by the investigation of the p-dependencies of hyperfine magnetic fields and of spin susceptibility for single- and bi-layered cuprates with tetragonal symmetry. Moreover, the analyses are compared with the NMR data on three-layered Ba_2Ca_2Cu_3O_6(F,O)_2, HgBa_2Ca_2Cu_3O_{8+delta}, and five-layered HgBa_2Ca_4Cu_5O_{12+delta}, which suggests the general applicability of the K_s(300 K)-p relationship to multilayered compounds with more than three CuO_2 planes. We remark that the measurement of K_s(300 K) enables us to separately estimate p for each CuO_2 plane in multilayered compounds, where doped hole carriers are inequivalent between outer CuO_2 planes and inner CuO_2 planes.Comment: 7 pages, 5 figures, 2 Tables, to be published in Physical Review

Novel Superconducting Phases in Copper Oxides and Iron-oxypnictides: NMR Studies

We reexamine the novel phase diagrams of antiferromagnetism (AFM) and high-Tc$superconductivity (HTSC) for a disorder-free CuO$_2$plane based on an evaluation of local hole density ($p$) by site-selective Cu-NMR studies on multilayered copper oxides. Multilayered systems provide us with the opportunity to research the characteristics of the disorder-free CuO$_2$plane. The site-selective NMR is the best and the only tool used to extract layer-dependent characteristics. Consequently, we have concluded that the uniform mixing of AFM and SC is a general property inherent to a single CuO$_2$plane in an underdoped regime of HTSC. The$T$=0 phase diagram of AFM constructed here is in quantitative agreement with the theories in a strong correlation regime which is unchanged even with mobile holes. This {\it Mott physics} plays a vital role for mediating the Cooper pairs to make$T_c$of HTSC very high. By contrast, we address from extensive NMR studies on electron-doped iron-oxypnictides La1111 compounds that the increase in$T_c$is not due to the development of AFM spin fluctuations, but because the structural parameters, such as the bond angle$\alpha$of the FeAs$_4$tetrahedron and the a-axis length, approach each optimum value. Based on these results, we propose that a stronger correlation in HTSC than in FeAs-based superconductors may make$T_c$ higher significantly.Comment: 5 pages, 4 figures, accepted for publication in J.Phys.Chem.Solids (2010

Advanced Science and Engineering of Correlated Electron Materials

In many solid-state materials, new phenomena evolve due to strong electron interactions. In this Lecture, review will be given on some advanced topics, including high temperature superconductivity, unconventional magnetism and dielectricity. Experimental techniques to study these physical properties will be introduced, and the physics behind the new phenomena will be discussed.大阪大学OpenCourseWare:大学院講義 (留学生向け特別プログラムから

Evolution of an Unconventional Superconducting State inside the Antiferromagnetic Phase of CeNiGe 3 under Pressure: A 73 Ge-Nuclear-Quadrupole-Resonance Study Since the discovery of the heavy-fermion (HF) super- conductor CeCu

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 Néel temperature T N exhibits its maximum at 8.5 K. The superconducting transition temperature T SC has been found to be enhanced with increasing T N , before reaching the quantum critical point at which the AFM order collapses. Above T SC , the AFM structure transits from an incommensurate spin-density-wave order to a commensurate AFM order at T $ 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