G.-q. Zheng

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.

Unique Spin Dynamics and Unconventional Superconductivity in the Layered Heavy Fermion Compound CeIrIn5: NQR Evidence

We report measurements of the 115In nuclear spin-lattice relaxation rate ( 1/T1) between T = 0.09 and 100 K in the new heavy fermion (HF) compound CeIrIn5. At 0.4 < or = T< or = 100 K, 1/T1 is strongly T-dependent, which indicates that CeIrIn5 is much more itinerant than known Ce-based HFs. We find that 1/T1T, subtracting that for LaIrIn5, follows a (1 / T+straight theta)3/4 variation with straight theta = 8 K. We argue that this novel feature points to anisotropic, due to a layered crystal structure, spin fluctuations near a magnetic ordering. The bulk superconductivity sets in at 0.40 K below which the coherence peak is absent and 1/T1 follows a T3 variation, which suggests unconventional superconductivity with line-node gap.

Coexistence of superconductivity and antiferromagnetism in multilayered high-Tc superconductor HgBa2Ca4Cu 5Oy: Cu-NMR study

We report a coexistence of superconductivity and antiferromagnetism in five-layered compound HgBa 2 (Ta 4 Cu 5 O y (Hg-1245) with T c = 108 K, which is composed of two types of CuO 2 planes in a unit cell; three inner planes(IPs) and two outer planes (OPs). The Cu-NMR study has revealed that the optimally doped OP undergoes a superconducting (SC) transition at T c = 108 K, whereas the three underdoped IPs do an antiferromagnetic (AF) transition below T N ∼60 K with the Cu moments of ∼(0.3-0.4)μ B . Thus bulk superconductivity with a high value of T c =108 K and a static AF ordering at T N =60 K are realized in the alternating AF and SC layers. The AF-spin polarization at the IP is found to induce the Cu moments of ∼0.02μ B at the SC OP, which is the AF proximity effect into the SC OP.

Coexistence of Antiferromagnetism and Superconductivity near the Quantum Criticality of the Heavy-Fermion Compound CeRhIn5

We report a study on the interplay between antiferromagnetism (AFM) and superconductivity (SC) in a heavy-fermion compound CeRhIn5 under pressure P=1.75 GPa. The onset of the magnetic order is evidenced from a clear split of 115In nuclear quadrupole resonance spectrum due to the spontaneous internal field below the Néel temperature T(N)=2.5 K. Simultaneously, bulk SC below T(c)=2.0 K is demonstrated by the observation of the Meissner diamagnetism signal whose size is the same as in the exclusively superconducting phase. These results indicate that the AFM coexists homogeneously with the SC at a microscopic level.

Gapless magnetic and quasiparticle excitations due to the coexistence of antiferromagnetism and superconductivity in CeRhIn5: a study of 115In NQR under pressure.

We report systematic measurements of ac susceptibility, nuclear-quadrupole-resonance spectrum, and nuclear-spin-lattice-relaxation time (T1) on the pressure (P)-induced heavy-fermion superconductor CeRhIn5. The temperature (T) dependence of 1/T(1) at P=1.6 GPa has revealed that antiferromagnetism (AFM) and superconductivity (SC) coexist microscopically, exhibiting the respective transition at T(N)=2.8 K and T(MF)(c)=0.9 K. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below T(onset)(c)=2 K, but T(MF)(c)=0.9 K, followed by a T(1)T=const law. These results point to the unconventional characteristics of SC coexisting with AFM. We highlight that both of the results deserve theoretical work on the gapless nature in the low-lying excitation spectrum due to the coexistence of AFM and SC and the lack of the mean-field regime below T(onset)(c)=2 K.

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 .

New Superconducting and Magnetic Phases Emerge on the Magnetic Criticality in CeIn3

We report the discovery of new superconducting and novel magnetic phases in CeIn 3 on the verge of antiferromagnetism (AFM) under pressure ( P ) through the In-nuclear quadrupole resonance (NQR) measurements. We have found a P -induced phase separation of AFM and paramagnetism (PM) without any trace of a quantum phase transition in CeIn 3 . A new type of superconductivity (SC) was found, under P =2.28–2.5 GPa, to coexist with AFM. It is magnetically separated from PM where the heavy fermion SC takes place. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order magnetic phase transition might mediate attractive interaction to form Cooper pairs.

Exotic superconducting phase in CeCu2Si2 close to antiferromagnetism: A Cu-NQR study under hydrostatic pressure

We report Cu nuclear-quadrupole-resonance results under pressure on a homogeneous CeCu2Si2 (Tc = 0.65 K) that revealed critical magnetic fluctuations at the border to an antiferromagnetic phase. This exotic superconducting phase evolves into the typical heavy-fermion superconducting phase at minute pressures exceeding Pc ~ 0.2 GPa. The nuclear spin-lattice relaxation data in a pressure range 0.85 - 2.58 GPa are shown to be accountable by the SCR theory based on the nearly antiferromagnetic Fermi-liquid model. In 0<P<Pc ~ 0.2 GPa, by contrast, we conclude that the exotic superconducting phase manifests itself under the unconventional normal state where antiferromagnetic waves propagate over a long range without any trace of antiferromagnetic order and thereby the heavy-fermion state breaks up.

Magnetic Criticality and Unconventional Superconductivity in CeCoIn5: Study of 115In-Nuclear Quadrupole Resonance under Pressure

We report the systematic evolution of the superconducting (SC) characteristics of the heavy-fermion (HF) superconductor CeCoIn 5 via nuclear-quadrupole-resonance (NQR) measurement under pressure ( P ). The application of P significantly suppresses the nuclear spin–lattice relaxation rate 1/ T 1 that is dominated by antiferromagnetic (AFM) spin fluctuations (SFs) specific to a quantum critical point (QCP). It is demonstrated that the marked suppression of AFM SFs leads to a reduction in the SC energy gap or in the coupling strength of the Cooper pair. T c , nevertheless, increases with increasing P due to the increase in HF bandwidth. This is expected to make the lifetime of quasi-particles sufficiently long.

Carrier distribution and superconductivity in multilayer high-Tc cuprates proved by 63Cu NMR

The carrier distribution among the inequivalent CuO2planes in multilayer cuprates is investigated by63Cu Knight shift measurement with varying the carrier concentration, δ and the number of CuO2plane in a unit cell, n. We found that 70 (60)% of total carriers are doped selectively into two outer CuO2planes in n = 3 (4). This indicates resultantly that the inhomogeneity of carrier distribution increases with increasing δ and n in multilayer cuprates.