Kunisuke Asayama

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.

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.

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 ...

NMR and NQR Studies of Magnetism and Superconductivity in the Antiferromagnetic Heavy Fermion Superconductors UM2Al3 (M=Ni and Pd)

Both magnetic and superconducting characteristics of the new heavy fermion superconductors (HFS), UNi 2 Al 3 and UPd 2 Al 3 have been investigated extensively by means of 27 Al NMR and NQR. Systematic studies of the nuclear-spin-lattice relaxation time T 1 , the Knight shift and the spectrum of 27 Al have revealed different types of magnetic fluctuations and spin structure between these two compounds. UNi 2 Al 3 shows a band-type itinerant antiferromagnetic behavior, while UPd 2 Al 3 belongs to a heavy fermion antiferromagnet with an ordinary size of U-derived moment. In the superconducting state, the relaxation behavior in UPd 2 Al 3 is similar to those observed in the other HFS reported so far, providing an evidence for an unconventional anisotropic superconductivity with line of gap zeros on the Fermi surface as well. Knight shift has been found to decrease considerably below T c , showing that the pseudo spin is well defined, not affected by the impurity scattering.

Antiferromagnetic spin correlation and superconductivity in La2−xSrxCuO4: CuNQR study

Abstract We have measured the nuclear spin-lattice relaxation time T 1 of 63 Cu in La 2− x Sr x CuO 4 in the concentration range of x = 0.075–0.15 approaching the magnetic phase boundary around x = 0.05. ( T 1 T ) -1 is found to be markedly enhanced with decreasing Sr content and temperature due to the development of the AF spin correlation among Cu spins. It is shown that ( T 1 T ) -1 follows a T -dependence of the form C /( T +θ) associated with the Curie-Weiss law of the staggered susceptibility χ Q ( T ) at the zone boundary ( Q = ( π / α , π / α )). An important finding is that the Weiss temperature θ is decreased from 75 K (x = 0.15) to 20 K (x = 0.07 5) with decreasing Sr content, which causes the large enhancement of ( T 1 T ) -1 and accompanies the decreases of T c , becoming 0 K at θ = 0 K for x = 0.05, whereas the Curie constant, C , slightly increases.

d-Wave Superconductivity in Antiferromagnetic Heavy-Fermion Compound UPd2Al3 –Evidence from27Al NMR/NQR Studies–

The nuclear-spin-lattice relaxation rate, 27 (1/ T 1 ), of the high-quality polycrystalline UPd 2 Al 3 with a record T c =2 K and the narrowest Al NQR linewidth to date of 12 kHz has been measured in zero magnetic field by the NQR technique. 27 (1/ T 1 ) below T c has been found to obey the T 3 - law at least down to 0.18 K, giving strong evidence that the energy gap vanishes along lines on the Fermi surface. From the analysis of 27 Al Knight shift below T c , it is shown that the anisotropic residual Knight shift originates from the f -electron susceptibility in the antiferromagnetic state, whereas the isotropic reduction of the shift below T c is due to a singlet pairing nature of the superconductivity. Both the results of 27 (1/ T 1 ) and 27 Al Knight shift in UPd 2 Al 3 can be reproduced consistently by a d-wave pairing model characterized by lines of vanishing gap on the Fermi surface.

Cu NQR Study of the Spin Dynamics in High-TcSuperconductor La2-xSrxCuO4

The nuclear spin-lattice relaxation time T 1 of 63 Cu in La 2- x Sr x CuO 4 was measured in the concentration range of x =0.075∼0.15 near the magnetic phase boundary around x =0.05. At low Sr concentration and low temperature, ( T 1 T ) -1 was found to increase markedly with a development of strong antiferromagnetic ( AF ) spin correlation among Cu spins. In normal state, ( T 1 T ) -1 exhibits a temperature dependence of C /( T +θ) associated with the Curie-Weiss law of the staggered susceptibility χ Q ( T ) at the zone boundary, Q =(π/ a , π/ a ), above 60 K and approaches a nearly constant value in a narrow T -region just above T c . T c decreases with decreasing θ, becoming zero at x =0.05 when θ is extrapolated to zero. The rapid decrease of 1/ T 1 below T c has commonly been observed irrespectively of further development of the AF correlation with decreasing Sr content.

Nuclear Magnetic Resonance In Co Alloys

NMR satellite lines were observed at frequencies lower than main line in Co alloys with dilute impurities, It is concluded that these satellites are due to the Co nuclei located at the nearest neighbor sites of the impurities and that the changes of magnetic moments are well localizes around the impurities. An equation is derived which can represent the frequencies of satellites. Some physical interpretations are given for the equation.