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 Impurities-Doped YBa2(Cu1-xMx)3O7 (M=Zn and Ni)

The 90 K superconductor, YBa 2 Cu 3 O δ (YBCO), has proved to be highly adaptable chemically as it can accommodate a wide variety of cationic and anionic substitutions. Indeed, the majority of the chemical elements, excluding noble gases and actinides, have been reported to substitute to some extent into the YBCO structure. This review covers the literature on such substitutions and their effect on the properties of YBCO. Reported solubility limits are given, together with crystal symmetry and trends in unit cell parameters with dopant concentration. The dopant site is considered; this is additionally complex in the case of copper substitution because of the two distinct copper sites in the crystal structure. The effect of the dopant on the critical temperature, T c , is reviewed; the literature is often contradictory due to the dual effects of variable oxygen content and the nature of the dopant. Preparation methods appear to have an effect on solubility limits, crystal symmetry and T c , Also, the methods used to determine solubility limits are often imprecise which can lead to contradictions. The magnetic properties of doped materials are reviewed; for some dopants, particularly the magnetic lanthanides, antiferromagnetism and superconductivity co-exist. The related RBa 2 Cu 3 O δ phases (R = lanthanide), their structure, properties and behaviour on doping are reviewed in a similar way. For the larger rare earths, the related systems R 1+ x Ba 2- x CU 3 O δ , are reviewed; as x increases, the transition temperature decreases and compositions R 1.5 Ba 1.5 Cu 3 O δ are semiconducting. The upper and lower solubility limit changes with R, and for R = Dy, the upper limit is x = 0 composition, LaBa 2 Cu 3 O δ , cannot be prepared in air since substitution of La onto the Ba site occurs, forming the Ba-deficient solid solutions. The discovery of superconductivity above liquid nitrogen temperatures in YBa 2 Cu 3 O 7 , has generated a vast quantity of research; to date, > 20 000 papers have appeared on the 123 materials (source: BIDS). This review, therefore, cannot be comprehensive, but attempts to highlight important substitutions, their effect on the properties of YBCO and any contradictions in the literature.

Crystal chemical substitutions and doping of YBa2CU3Ox and related superconductors

NMR studies of high Tc superconductors

Crystal structures of tetragonal YBa2Cu2.7Co0.3O7-γ and tetragonal YBa2Cu3O7-γ are determined by X-ray and TOF neutron powder diffractometries. Structural parameters are obtained by the Rietveld profile refinement method. In YBa2Cu2.7Co0.3O7-γ', cobalt atoms preferentially occupy one of the copper sites at 1a(0 0 0) rather than the 2g(0 0 z) site with z~0.36. The lattice constants are a = 3.8644(2)A and c = 11.7881(9)A for the quenched YBa2Cu3O6.15 and a = 3.8752(2)A and c = 11.6851(9)A for the furnace-cooled YBa2Cu2.7Co0.3O6.81.

Crystal Structure of Tetragonal YBa_2Cu_ Co_δO_ (δ=0, 0.3)

The site preference of Cu substituted Zn and Ni in YBa2Cu3O7−γ

It has been shown that the superconducting transition temperature, T c , of M substituted YBa 2 (Cu 1- x M x ) 3 O 7-δ is markedly enhanced by low-temperature annealing in high-pressure oxygen gas, being essentially the same as that of the undoped one up to x =0035, in cases of M=Fe and Co, while in cases of M=Zn and Ni, T c is almost unchanged by the annealing Since Co replaces only Cu1 and Fe replaces both Cu1 and Cu2, the results indicate that the superconductivity is affected little by the magnetic impurities substituting both the two Cu sites, as in the case of the Y substitutions with rare earth elements, while it is crucially affected by Zn The result may prove that the Cooper pairs do not run on copper ions in this material

Superconductivity Unresponsive to Magnetic Substituents in YBa2(Cu1-xMx)3O7-δ

From the studies of the Cu substituted YBa 2 (Cu 1−x M x ) 3 O 7−y (M=Fe, Co, Ni, Zn) systems, it is concluded that the pair-breaking of the Abrikosov and Gor'kov type by magnetic impurities is absent, or very small if it exists, both on the C(1) and Cu(2) sites. The Hall coefficient is measured for the systems. There is a certain relation between T c and n H in the substituted materials with Fe and Co, while no relation is deduced from the Ni- and Zn-substituted ones.

Tc-suppression and hole concentration in Cu substituted YBa2(Cu1−xMx)3O7−δ with Fe, Co, Ni, Zn

Nuclear resonance spectra with the internal field of about 90 kOe were observed in zero external magnetic field in YBa 2 (Cu 1- x Fe x ) 3 O 7- y with x ≥0.15, just above the concentration x where superconductivity disappears. As the resonance width and the e 2 q Q satellite patterns are similar to the NMR spectrum associated with antiferromagnetic Cu(2) moments in the tetragonal YBa 2 Cu 3 O δ (\(\delta\lesssim6.4\)), the observed spectrum indicates that almost the same antiferromagnetic ordering with the pure YBa 2 Cu 3 O δ (\(\delta\lesssim6.4\)) also appears in the YBa 2 (Cu 1- x Fe x ) 3 O 7- y system when the superconductivity ceases with the substitution of Fe atoms.

Observation of the Antiferromagnetic Ordering of Cu(2) Sites in YBa2(Cu1-xFex)3O7-y

The detailed characterization of YBa2(Cu1-xCox)3Oy has been made for various x values. The critical temperature is found to be constant in the small x region and then decrease linearly with x and abruptly vanish at a critical concentration. This result indicates a very different role of magnetic impurities in this compound compared with conventional superconductors. Two transition stages, which were independent of Co concentration, were observed in the ac susceptibility. This is concluded to be an intrinsic character of the Co substitution system. The results are discussed in comparison with other substitution systems.

The characterization of YBa2(Cu1−xCox)3Oy

The dc magnetic susceptibility of YBa 2 (Cu 1- x M x ) 3 O y with M=Fe and Co ( x <0.08) annealed in high-pressure oxygen gas was measured in the normal state. The annealing was done in flowing oxygen gas of about 6 kg/cm 2 for 24 hours at low temperatures (350–450°C) after an ordinary annealing in air. Although the O 2 annealing markedly enhances the superconducting transition temperature, the measured susceptibilities of Fe and Co substituents were essentially unchanged. The dc magnetic susceptibilities of the air- and O 2 -annealed samples with Ni ( x <0.08) were also measured and found to be the same. In this compound, the Ni atom was found to have a magnetic moment close to that of the Ni 2+ ion ( S =1, P eff =2.83 µ B ).

Naoki Kawaji

Papers

Superconductivity Unresponsive to Magnetic Substituents in YBa2(Cu1-xMx)3O7-δ

Tc-suppression and hole concentration in Cu substituted YBa2(Cu1−xMx)3O7−δ with Fe, Co, Ni, Zn

Observation of the Antiferromagnetic Ordering of Cu(2) Sites in YBa2(Cu1-xFex)3O7-y

The characterization of YBa2(Cu1−xCox)3Oy

Magnetic Properties of O2-Annealed YBa2(Cu1-xMx)3Oy with M=Fe, Co and Ni