Guo Qing Zheng

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.

Unconventional Superconductivity and Electron Correlations in the Cobalt Oxyhydrate Na0:35CoO2 yH2O from Nuclear Quadrupole Resonance

We report a careful 59Co nuclear quadrupolar resonance measurement on the recently discovered cobalt oxyhydrate Na0.35CoO2.yH(2)O superconductor from T=40 K down to 0.2 K. We find that in the normal state the spin-lattice relaxation rate 1/T(1) follows a Curie-Weiss type temperature (T) variation, 1/T(1)T=C/(T-theta), with theta=-42 K, suggesting two-dimensional antiferromagnetic spin correlations. Below T(c)=3.9 K, 1/T(1) decreases with no coherence peak and follows a T(n) dependence with n approximately 2.2 down to approximately 2.0 K but crosses over to a 1/T(1) proportional to T variation below T=1.4 K, which suggests non-s-wave superconductivity. The data in the superconducting state are most consistent with the existence of line nodes in the gap function.

Local hole distribution in the CuO2 plane of high-Tc Cu-oxides studied by Cu and oxygen NQR/NMR

The hole numbers at Cu and oxygen sites in the CuO 2 plane in a wide variety of high- T c materials have been extracted from the measured nuclear quadrupole frequency, ν Q . The hole number in the Cu-3 d x 2 - y 2 orbit, n x 2 - y 2 , is found to be more than 0.85 in La 2- x Sr x CuO 4 ( x =0.075∼0.24), while decreases to 0.74 in Tl 2 Ba 2 Ca 2 Cu 3 O 10 . Thus estimated n x 2 - y 2 is supported by the result that the orbital Knight shift along the c -axis, K c o r b , increases in proportion to n x 2 - y 2 . In contrast to the decrease of n x 2 - y 2 , the hole number in the O-2 p σ orbit, n p σ , increases in going from the La- to the Tl-systems. It has been found that T c is determined by two parameters about holes, i.e., n x 2 - y 2 +2 n p σ and n x 2 - y 2 /2 n p σ . The former tells the microscopic \lq\lqdoping level” of the CuO 2 plane, while a smaller n x 2 - y 2 /2 n p σ results in a weaker antiferromagnetic spin fluctuations of low energy. It has been found that not only a proper n x 2 - y 2 +2 ...

63Cu NQR/NMR Study of Zn-Substituted YBa2 Cu4 O8 –Effect of Impurity on Spin-Gap-Like Behavior–

The nuclear-spin-lattice relaxation rate, 1/ T 1 , and the Knight shift, K a b , of 63 Cu in the plane have been measured for YBa 2 ( Cu 1- x Zn x ) 4 O 8 ( x =0.01 and 0.02) in order to unravel th...

Spin Correlation in High-T c Cuprate HgBa2Ca2Cu3O8+δ with T c=133 K –An Origin of T c-Enhancement Evidenced by 63Cu-NMR Study–

The Knight shift, the nuclear spin-lattice relaxation rate, 1/ T 1 , and the transverse relaxation rate, 1/ T 2G , of 63 Cu have been separately measured for the square and the pyramidal CuO 2 planes in HgBa 2 Ca 2 Cu 3 O 8+δ (Hg1223) with the highest transition temperature, T c =133 K to date. From the quantitative analysis of 1/ T 1 and 1/ T 2G , it is shown that the product of the staggered susceptibility and the characteristic energy of spin fluctuations around a zone boundary, χ Q Γ Q , for the square CuO 2 plane site is larger than that for YBa 2 Cu 3 O 7 (YBCO 7 ), although that for the pyramidal CuO 2 plane site is almost the same. This enhancement of χ Q Γ Q for the square site is considered to be responsible for the higher T c in Hg1223 than in YBCO 7 .

17O and 63Cu NMR Investigations of High-Tc SuperconductorLa1.85Sr0.15CuO4 with Tc=38 K

17 O and 63 Cu NMR measurements have been made in L 1.85 Sr 0.15 CuO 4 with T c =38 K. Both 17 O Knight shift and quadrupole tensor were determined from the analyses of the 17 O spectrum of oriented powder. The quadrupole frequencies of the planar oxygen and Cu nuclei are consistently interpreted by the analyses of Hanzawa et al . made in YBa 2 Cu 3 O 7 with the electric field gradients due to the on-site holes at Cu and O sites. Then the weight factor of the Cu-3 d ( x 2 - y 2 ) and 2 p } σ wave functions are deduced, the latter having the larger weight. From the Knight shifts vs the bulk susceptibility plots, the hyperfine coupling constants for 17 O and 63 Cu were almost the same as those in YBa 2 Cu 3 O 7 . Both shifts decrease significantly below T c having residual parts of 63 K ab =0.38% and 17 K c =0.02% at 4.2 K, which were larger than those in YBa 2 Cu 3 O 7 .

NMR study of local hole distribution, spin fluctuation and superconductivity in Tl2Ba2Ca2Cu3O10

Abstract 63Cu, 17O and 205Tl NMR have been performed in the high-Tc superconductor Tl2Ba2Ca2Cu3O10 whose Tc(max) is 127 K. The hole densities at Cu and oxygen sites in the CuO2 plane have been extracted from the nuclear quadrupole frequency νQ. The striking feature is that the Cu holes are significantly transferred to oxygen site due to strong hybridization between Cu and oxygen. From an analysis of T1 and T2G, it has been found that the spectral weight of the spin fluctuation is transferred to higher energy compared to YBa2Cu3O7, while the magnetic correlation length ξ does not differ much. Thus, it is suggested that the higher Tc is due to higher characteristic energy of spin fluctuations, i.e. the superconductivity is spin fluctuation mediated. The superconducting properties are consistently explained by a d-wave superconductivity model with a finite density of states (DOS) at the Fermi level. We show that the disorder of the Ca/TlO layer caused by the partial inter-substitution of Tl and Ca is responsible for the potential scattering to produce such a DOS. It is found that if such a potential scattering were absent, Tc would go up to 132 K which is quite close to the record Tc realized in the Hg based compound.

Anisotropic Spin Fluctuations in Heavy-Fermion Superconductor CeCoIn5: In-NQR and Co-NMR Studies

We report In-NQR and Co-NMR experiments of CeCoIn 5 that undergoes a superconducting transition with a record high T c = 2.3 K to date among heavy-fermion superconductors. At zero magnetic field, a...

Characteristics of the spin fluctuation in Tl2Ba2Ca2Cu3O10

We report a 63 Cu and 17 O NMR study in the high- T c superconductor Tl 2 Ba 2 Ca 2 Cu 3 O 10 with T c (max)=127 K. The results of T 1 and T 2 G reveal that the characteristic energy of the antiferromagnetic spin fluctuation Γ Q is higher than that in YBa 2 Cu 3 O 7 , while the magnetic correlation lengths ξ do not differ much from each other. The high T c in this compound is thus related to large Γ Q , suggesting that the superconductivity is spin fluctuation mediated.

Coexistence of antiferromagnetic order and unconventional superconductivity in heavy-fermion CeRh1-xIrxIn5 compounds: Nuclear quadrupole resonance studies

We present a systematic