M. Yogi

Evidence for unconventional strong-coupling superconductivity in PrOs4Sb12: an Sb nuclear quadrupole resonance study.

We report Sb-NQR results which evidence a heavy-fermion (HF) behavior and an unconventional superconducting (SC) property in Pr(Os4Sb12 with 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 T0 approximately 10 K, associated with Pr3+(4f(2))-derived ground state. In the SC state, 1/T(1) shows neither a coherence peak just below T(c) K nor a T3-like power-law behavior observed for anisotropic HF superconductors with the line-node gap. The isotropic energy gap with its size Delta/k(B)=4.8 K seems to open up across T(c) below T(*) approximately 2.3 K. It is surprising that Pr(Os4Sb12 looks like an isotropic HF superconductor-it may indeed argue for Cooper pairing via quadrupolar fluctuations.

Evidence for a novel state of superconductivity in noncentrosymmetric CePt3Si: a 195Pt-NMR study.

We report on novel antiferromagnetic (AFM) and superconducting (SC) properties of noncentrosymmetric CePt3Si through measurements of the 195Pt nuclear spin-lattice relaxation rate 1/T(1). In the normal state, the temperature (T) dependence of 1/T(1) unraveled the existence of low-lying levels in crystal-electric-field multiplets and the formation of a heavy-fermion (HF) state. The coexistence of AFM and SC phases that emerge at T(N)=2.2 K and T(c)=0.75 K, respectively, takes place on a microscopic level. CePt3Si is the first HF superconductor that reveals a peak in 1/T(1) just below T(c) and, additionally, does not follow the T3 law that used to be reported for most unconventional HF superconductors. We remark that this unexpected SC characteristic may be related to the lack of an inversion center in its crystal structure.

Sb-NQR probe for superconducting properties in the Pr-based filled-skutterudite compound PrRu4Sb12

We report the electronic and superconducting properties in the Pr-based filled-skutterudite superconductor PrRu 4 Sb 1 2 with T c =1.3 K via the measurements of nuclear-quadrupole-resonance frequency ν Q and nuclear-spin-lattice-relaxation time T 1 of Sb nuclei. The temperature dependence of v Q has revealed the energy scheme of Pr 3 + crystal electric field that is consistent with an energy separation Δ C E F ∼70 K between the ground state and the first-excited state. In the normal state, the Korringa relation of (1/T 1 T) P r =const is valid, with [( 1/T 1 T) p r /(1/T 1 T) L a ] 1 / 2 ∼1.44, where (1/T 1 T) L a is for LaRu 4 Sb 1 2 . These results are understood in terms of a conventional Fermi-liquid picture in which the Pr-4f 2 state derives neither magnetic nor quadrupolar degrees of freedom at low temperatures. In the superconducting state, 1/T 1 shows a distinct coherence peak just below T c , followed by an exponential decrease with a value of 2Δ/k B T c =3.1. These results demonstrate that PrRu 4 Sb 1 2 is a typical weak-coupling s-wave superconductor, in strong contrast with the heavy-fermion superconductor PrOs 4 Sb 1 2 that is in an unconventional strong coupling regime. The present study on PrRu 4 Sb 1 2 highlights that the Pr-4f 2 -derived nonmagnetic doublet plays a key role in the unconventional electronic and superconducting properties in PrOs 4 Sb 1 2 .

Multiband superconductivity in filled-skutterudite compounds (Pr1- xLax)Os4Sb12 : An Sb nuclear-quadrupole-resonance study

We report on the systematic evolution of normal-state properties and superconducting characteristics in filled-skutterudite compounds (Pr 1- x La x )Os 4 Sb 12 determined using Sb nuclear-quadrupol...

Novel Phase Transition Near the Quantum Critical Point in the Filled-Skutterudite Compound CeOs4Sb12: An Sb-NQR Study

We report on a novel phase transition at T = 0.9 K in the Ce-based filled-skutterudite compound CeOs 4 Sb 12 via measurements of the nuclear-spin lattice relaxation rate 1/ T 1 and nuclear quadrupo...

Multiband Superconductivity in Heavy Fermion Compound CePt3Si without Inversion Symmetry: An NMR Study on a High-Quality Single Crystal

We report on novel superconducting characteristics of the heavy fermion (HF) superconductor CePt 3 Si without inversion symmetry through 195 Pt-NMR study on a single crystal with T c = 0.46 K that is lower than T c ∼0.75 K for polycrystals. We show that the intrinsic superconducting characteristics inherent to CePt 3 Si can be understood in terms of the unconventional strong-coupling state with a line–node gap below T c = 0.46 K. The mystery about the sample dependence of T c is explained by the fact that more or less polycrystals and single crystals inevitably contain some disordered domains, which exhibit a conventional BCS s -wave superconductivity (SC) below 0.8 K. In contrast, the Neel temperature T N ∼2.2 K is present regardless of the quality of samples, revealing that the Fermi surface responsible for SC differ from that for the antiferromagnetic order. These unusual characteristics of CePt 3 Si can be also described by a multiband model; in the homogeneous domains, the coherent HF bands are respo...

Sb-NQR Probe for Multipole Degree of Freedom in the First Pr-Based Heavy-Fermion Superconductor PrOs4Sb12

We report 121,123 Sb nuclear quadrupole resonance (NQR) measurements in the filled-skutterudite superconductor PrOs 4 Sb 12 in the temperature range of 0.05–30 K. The electric field gradients (EFG)...

Unconventional superconductivity and magnetism in

Abstract CePt 3 Si is a novel ternary compound exhibiting antiferromagnetic order at T N ≈ 2.2 K and superconductivity (SC) at T c ≈ 0.75 K . Large values of H c 2 ′ ≈ - 8.5 T / K and H c 2 ( 0 ) ≈ 5 T indicate Cooper pairs formed out of heavy quasiparticles. The mass enhancement originates from Kondo interaction with a characteristic temperature T K ≈ 8 K . NMR and μ SR measurements evidence coexistence of SC and long-range magnetic order on a microscopic scale. Moreover, CePt 3 Si is the first heavy fermion SC without an inversion symmetry. This gives rise to a novel type of NMR relaxation rate 1 / T 1 which is very unique and never reported before for other heavy fermion superconductors. Studies of Si/Ge substitution allow us to establish a phase diagram.

Magnetic Field Evolution of a Novel Phase Transition in CeOs4Sb12: 121Sb NMR Study

We report on the evolution of a novel phase transition under magnetic field ( H ) in the filled skutterudite compound CeOs 4 Sb 12 probed by 121 Sb NMR. Measurements of nuclear spin–lattice relaxation rate 121 (1/ T 1 ) for the oriented powder with H ∥[100] have revealed that antiferromagnetic (AFM) spin fluctuations due to the proximity of a quantum critical point at H =0 are markedly suppressed by the application of H , which is consistent with the result indicating that the AFM order at T N =0.9 K with a very small moment of 0.07µ B collapses, exceeding H =1 T. As H increases further, a large reduction in 1/ T 1 T was observed below the temperature T 0 , which varies as a dome against H with a maximum of 1.8 K at H ∼6 T. The values of T 1 T =const. well below T 0 decrease with increasing H , indicating that the residual density of states depends on H . It is suggested that the field-induced novel transition is associated with the freezing of multipole degree of freedom.

Unconventional superconductivity and magnetism in CePt3Si1-xGex

Abstract CePt 3 Si is a novel ternary compound exhibiting antiferromagnetic order at T N ≈ 2.2 K and superconductivity (SC) at T c ≈ 0.75 K . Large values of H c 2 ′ ≈ - 8.5 T / K and H c 2 ( 0 ) ≈ 5 T indicate Cooper pairs formed out of heavy quasiparticles. The mass enhancement originates from Kondo interaction with a characteristic temperature T K ≈ 8 K . NMR and μ SR measurements evidence coexistence of SC and long-range magnetic order on a microscopic scale. Moreover, CePt 3 Si is the first heavy fermion SC without an inversion symmetry. This gives rise to a novel type of NMR relaxation rate 1 / T 1 which is very unique and never reported before for other heavy fermion superconductors. Studies of Si/Ge substitution allow us to establish a phase diagram.