Y. Kitaoka

Spin-triplet superconductivity in Sr2RuO4 identified by 17O Knight shift

Superconductivity — one of the best understood many-body problems in physics — has again become a challenge following the discovery of unconventional superconducting materials: these include heavy-fermion, organic and the high-transition-temperature copper oxide superconductors. In conventional superconductors, the electrons form superconducting Cooper pairs in a spin-singlet state, which has zero total spin (S = 0). In principle, Cooper pairs can also form in a spin-triplet state (S = 1), analogous to the spin-triplet ‘p-wave’ state of paired neutral fermions in superfluid 3He (ref. 4). At present, the heavy-fermion compound UPt3 is the only known spin-triplet superconductor,, although the layered oxide superconductor Sr2RuO4 (ref. 7) is believed, on theoretical grounds, to be a promising candidate. The most direct means of identifying the spin state of Cooper pairs is from measurements of their spin susceptibility, which can be determined by the Knight shift (as probed by nuclear magnetic resonance (NMR)). Here we report Knight-shift measurements of Sr2RuO2 using 17O NMR. Our results show no change in spin susceptibility on passing through the superconducting transition temperature, which provides the definitive identification of Sr2RuO4 as a spin-triplet superconductor.

Cu NMR and NQR studies of impurities-doped YBa2(Cu1-xMx)3O7 (M=Zn and Ni)

The normal and superconducting properties in impurity-doped YBa 2 (Cu 1- x M x ) 3 O 7 (M=Zn and Ni) have been investigated by 63 Cu NMR and NQR techniques. The nonmagnetic Zn-doping into the CuO 2 plane in YBa 2 Cu 3 O 7 (YBCO 7 ) causes a local collapse of antiferromagnetic (AF) spin correlation near Zn impurities and induces a gapless superconductivity with a finite density of states at the Fermi surface. On the other hand, in the case of Ni doping, there is no appreciable modification except for a small decrease of T c , although the Ni spins behave actually as local moments. The gapless feature in Zn-doped YBCO 7 is shown to be consistently interpreted within the framework of the d -wave model with gap zeros of lines at the Fermi surface as argued in heavy fermion superconductors. It is pointed out that the AF spin fluctuation-induced superconductivity is applicable to high- T c cuprates.

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 strong-coupling s-wave superconductivity in MgB2: (11)B NMR Study.

We have investigated a gap structure in a newly discovered superconductor, MgB2, through measurement of the (11)B nuclear spin-lattice relaxation rate, (11)(1/T(1)). (11)(1/T(1)) is proportional to the temperature (T) in the normal state, and decreases exponentially in the superconducting (SC) state, revealing a tiny coherence peak just below T(c). The T dependence of 1/T(1) in the SC state can be accounted for by an s-wave SC model with a large gap size of 2Delta/k(B)T(c) approximately 5 which suggests it is in a strong-coupling regime.

Cu NMR and NQR Studies of High-Tc Superconductor La2-xSrxCuO4.

Results of extensive Cu NMR and NQR studies in the superconducting La 2- x Sr x CuO 4 with x =0.075∼0.24 have been presented. The rapid decrease of the nuclear spin-lattice relaxation rate, 1/ T 1 , and the spin Knight shift below T c confirm microscopically the existence of the superconducting phase with lower T c than 38 K for 0.10 ≤ x ≤0.24. For x =0.10, magnetic anomalies emerge at low- T in some small parts of the sample, e.g . T -independent T 1 and broadening of spectrum, having some relevance to the magnetic order in La 2- x Ba x CuO 4 with x =0.125. For x >0.15, the T 1 T =const. behavior and the existence of the residual spin Knight shift far below T c have revealed that the superconductivity is of a gapless type with a finite density of states at the Fermi level caused by the mixture of different structures, i.e. tetragonal and orthorhombic.

Gapless superconductivity in Zn-doped YBa2Cu3O7 studied by Cu NMR and NQR: Possibility of d-wave superconductivity in high-Tc oxides

Abstract The superconducting properties of Zn-doped YBa2Cu3O7 have been investigated by the 63Cu-NQR and -NMR technique. Both the Knight shift K and the nuclear-spin-lattice relaxation time T1 were measured by using the oriented powder. The residual spin Knight shift of 63Cu at 4.2 K increases appreciably with increasing Zn-content from x=0.01 to 0.02. Correspondingly, the T1T=constant law holds at low temperatures far below Tc=79 K and 68 K for x=0.01 and 0.02 respectively, providing a clear sign of gapless superconductivity with finite density of states at the Fermi level, whereas the behavior of rapid decrease of 1/T1 just below Tc is almost unchanged even though Zn impurities are doped into the CuO2 plane. The NMR results in the Zn-doped and undoped YBa2Cu3O7 can be well interpreted by the combined effect of the gap zeros of lines at the Fermi surface originating from a d-wave pairing and the residual density of states at the Fermi level induced by non-magnetic Zn-impurities and by some imperfection of the crystal existing inevitably even in undoped YBa2Cu3O7.

Nuclear Relaxation and Knight Shift Studies of Copper in YBa2Cu3O7-y

The nuclear relaxation rate 1/ T 1 and the Knight shift have been studied for boththe Cu–O plane and chain sites by Cu nuclear quadrupole (NQR) and magnetic resonance (NMR) techniques. Below T c =92 K, 1/ T 1 for the plane site decreases markedly without the enhancement just below T c characteristic for the BCS superconductor, and the Knight shift also shows a more rapid reduction than the BCS prediction, giving evidence of a dominant singlet pairing. In contrast, 1/ T 1 for the chain site shows a weak temperature dependence below T c . This difference of the 1/ T 1 behavior for the plane and chain Cu sites is attributed to the difference of the relaxation processes, i.e., the magnetic and quadrupole relaxation processes, respectively. From both the behaviors of 1/ T and the Knight shift at the plane site, it is pointed out that the Cooper pair may be of a d-type formed by a strong coupling with a large energy gap.

Uniform mixing of high-Tc superconductivity and antiferromagnetism on a single CuO2 plane of a Hg-based five-layered cuprate.

We report a site selective Cu-NMR study on underdoped Hg-based five-layered high-Tc cuprate HgBa2Ca4CU5O(12+delta) with a Tc = 72 K. Antiferromagnetism (AFM) has been found to take place at T(N) = 290 K, exhibiting a large antiferromagnetic moment of 0.67-0.69 microB at three inner planes (IP). This value is comparable to the values reported for nondoped cuprates, suggesting that the IP may be in a nearly nondoped regime. Most surprisingly, the AFM order is also detected with M(AFM)(OP) = 0.1 microB even at two outer planes (OP) that are responsible for the onset of superconductivity (SC). The high-Tc SC at Tc = 72 K can uniformly coexist on a microscopic level with the AFM at OPs. This is the first microscopic evidence for the uniform mixed phase of AFM and SC on a single CuO2 plane in a simple environment without any vortex lattice and/or stripe order.

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.

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.