POSTER SESSION 4PO7: Substitution Effect on High Temperature Superconductors
Thursday, Feb. 24, 1:45 p.m. – 3:45 p.m., Hall D (GRB)
4PO7-1 Anisotropy and pair breaking effect of spinless Mg impurities in YBa2Cu3O7
J. Figueras, T. Puig, A.E. Carrillo, and X. Obrador, Institut de Ciencia de Materials de Barcelona, CSIC Campus U.A. Barcelona, 08193 Bellaterra, Spain
Presenting Author: X. Obradors
The analysis of the influence of spinless impurities in the normal and superconducting state is a subject of major interes in the context of the study of the consequences of the d-wave symmetry of the order parameter in superconducting cuprates. In the present work we report a study of the influence of a new spinless impurity with in-plane occupancy in YBa2Cu3O7: Mg ions. The anisotropy of the superconducting state has been determined by means of anisotropic magnetoresistivity measurements in quasi-single crystalline YBa2(Cu1-xMgx)3O7 melt textured samples. A comparison with the normal transport properties will be carried out. It is found that the pair breaking effect and mass anisotropy decrease associated to Mg doping is very similar to that of Zn ions, in spite of the strong dissimilarities in the electronic structure of both ions. The influence of different spinless impurities in the normal and superconducting state will be discussed.
4PO7-2 Structual and Magnetic Changes in Li Substituted YBa2Cu3Oy Determined by Neutron Powder Diffraction Measurements
F. Maury 1, M. Nicolas-Francillon 2, F. Bouréec 3, and I. Mirebeau 3. 1 Laboratoire des Solides Irradiés, Ecole Polytechnique, F-91128 Palaiseau Cédex, France. 2 Laboratoire de Physique de Solide, ESPCI, 10 rue Vauquelin, F-75231 Paris Cédex 05, France. 3 Laboratoire Léon Brillouin, Centre d’Etudes de Saclay, F-91191 Gif sur Yvette, France.
Presenting Author: F. Maury
The crystallographic structure of orthorhombic and tetragonal samples of lithium doped YBa2Cu3Oy has been determined by neutron powder diffraction measurements, with lambda = 1.225 Å. The experiment shows that when the samples are synthesized in oxygen, the lithium substitutes for the copper of the CuO2 planes. When the samples are synthesized in air, a small fraction (· 20%) of the lithium substitutes for the copper of the Cu-O- chains. This difference seems to be retained during the anneal in argon at 750°C, which transforms the crystallographic structure from orthorhombic to tetragonal. Moreover, the best fits to the diffraction spectra are obtained by assuming that each substitution of a Li for a Cu entails the loss of the neighbouring apical oxygen. This feature either does not seem to be modified by the transformation from orthorhombic to tetragonal sructure. It allows the best overall understanding of a number of experimental observations such as the measured magnetization as well as the cell parameter variations with the lithium content of the sample.
The magnetic structure of the tetragonal samples has been determined by neutron powder diffraction measurements with l = 4.738 Å. In YBa2Cu3-xLixO6+e , weak magnetic peaks are observed at low temperature, which can be indexed (1/2, 1/2, l) with integer l values. Peaks with half-integer l values are not seen. This shows that the structure is the same as that of pure YBa2Cu3O6. The Neel temperature, TN = 250-270 K for x = 0.09, is much lower than that of YBa2Cu3O6. This TN reduction is significantly stronger than in Zn substituted samples with equal substituted Cu and oxygen concentrations, whereas in orthorhombic supraconducting samples, the Tc reduction is significantly weaker. The ordered magnetic moment on the Cu2 sites in YBa2Cu2.91Li0.09O6.04, has been determined as a function of the temperature.
4PO7-3 Magnetism induced by spinless Li impurities in YBa2Cu3O6+x
Julien Bobroff 1, Andrew MacFarlane 1, Henri Alloul 1, Philippe Mendels 1, Nicole Blanchard 1, Gaston Collin 2, and Jean-Francis Marucco 3. 1 LPS, UMR8502, Universite Paris-Sud, 91405 Orsay, France. 2 LLB,CEA CNRS, 91191 Gif sur Yvette, France. 3 LEMHE, UMR8647, Universite Paris-Sud, 91405 Orsay, France
Presenting Author: J. Bobroff
In-plane impurities substituted on Cu are very good probes of the properties of high Tc cuprates. We measure the effects of in-plane non magnetic Li+ impurities on Tc and local magnetism in YBaCuO. In the metallic regime, 89Y and 7Li NMR allow to show that the extra hole introduced by Li is not localized in its vicinity. The Tc depression due to Li+ is found identical to that of Zn2+. As both Li+ and Zn2+ are spinless but with different valences, these results establish the major influence of the spin perturbation with respect to the charge defect in cuprates. Furthermore, Li NMR allows to probe with an unpreceedented accuracy the local susceptibility of the impurity. It is found, as in the case of Zn, that Li induces local magnetic moments in its vicinity, which display a 1/(T+q ) temperature dependence. q increases with oxygen doping and reaches 300 K in the overdoped Y0.8Ca0.2Ba2Cu3O7. We attribute this dependence to a hole-doping dependent "Kondo like" effect. Such a Kondo effect for a spinless impurity is an indication that the non Fermi Liquid behavior of the CuO2 planes extends to the overdoped regime.
J. Bobroff et al., submitted to Phys.Rev.Lett. (cond-mat/9906152).
4PO7-4 Li substitution in YBCO: a µSR study from the AF to the SC state
P. Mendels 1, W.A. MacFarlane 1, J. Bobroff 1, N. Blanchard 1, G. Collin 2, J.F. Marucco 3, V. Guillen 3, and P.J.C. King 4. 1 Laboratoire de Physique des Solides, UMR 8502, Bat 510, UPS Orsay, 91405 Orsay, France. 2 Laboratoire Leon Brillouin, CEA Saclay, 91191 Gif sur Yvette, France. 3 L.E.M.H.E., Bat 410, UPS Orsay, 91405 Orsay, France. 4 ISIS-RAL, Chilton-Didcot, OX110QX, England.
Presenting Author: J. Bobroff
NMR experiments [1] have demonstrated that the substitutions of Li and Zn on planar Cu sites (both of which strongly depress Tc) have similar effects in the metallic phase of YBCO. In order to extend the latter insight beyond the metallic region, we have undertaken a µSR study over the whole range of oxygen contents for various concentrations of the Li substituants. Our major findings are:
(i) For the maximum oxygen content, the increase of the penetration depth is the same as for Zn for a given depression of Tc.
(ii) As for Zn, we find an intermediate phase between the superconducting and antiferromagnetic phases which freezes into a disordered magnetic state at low T (T<10K)
(iii) Unexpectedly, we find that the substitution of Cu2+ by Li+ in Pure YBCO6 does not yield as strong a doping effect as Y3+/Ca2+ substitution. Moreover, no freezing of the spin degree of freedom is found. ZFNMR studies on the same samples will also be presented.
[1] J. Bobroff et al., this conference.
4PO7-5 Study of the Electronic Structure of Zn-doped Y123 Ceramics
Jacques Soullard and Ilya G. Kaplan, Instituto de Fisica, P.O. Box 20-364, 01000 Mexico D.F., Mexico
Presenting Author: J. Soullard
It is well known that the substitution of Cu atoms by a small amount of Zn atoms in Y123 ceramics destroys completely the superconductivity.We carried out calculations,at the correlation level,of the electronic structure of CuO2 planes (where Zn substitutes Cu) using the methodology of the embedded cluster developed in [1].The calculations were performed at the Moller-Plesset perturbation theory level (MP2) for charged Cu4O12 and Cu2Zn2O12 clusters, the cluster charge being determined by the lattice charges. According to the obtained results, the positive charge on Zn ions is more than 3 times greater than that on Cu ions; this leads to a more negative charge on oxygens located in the neighborhood of Zn ions. The symmetry of the oxygen holes undergoes a drastic change near the Zn ions : their 2ps symetry becomes a mixed 2ps + 2pp symmetry.The symmetry of the holes on coppers does not change, its 3dx2-y2 character is preserved and the hole symmetry on zincs is found to be isotropic with a 4s character. These peculiarities of the electronic structure have to be taken into account in studies of the formation of hole pairs in Zn substituted cuprates.
[1] I.G. Kaplan, J. Soullard, J. Hernandez-Cobos and R. Pandey, J. Phys: Condensed Matter, 11, 1049 (1999).
4PO7-6 NMR studies of the local magnetism induced by irradiation defects in cuprates
Julien Bobroff 1, Henri Alloul 1, Florence Rullier-Albenque 2, and Pierre-Antoine Vieillefond 2. 1 LPS, UMR8502, 91405 Orsay, France. 2 LSI, UMR 7642 CEA/X/CNRS, Ecole Polytechnique 91128 Palaiseau, France.
Presenting Author: H. Alloul
Irradiation by 2.5 MeV electrons allows to introduce in-plane defects in cuprates which induce a Tc depression and residual resistivity similar to that induced by Cu substitutions, such as Zn [1]. These defects are thought to be mainly O and Cu vacancies, although no definite local experimental evidences are presently available. As NMR has already shown that non magnetic Zn induces magnetic moments in its vicinity, it is important to understand whether the irradiation defects have a similar influence. We have performed 89Y and 17O measurements on electron irradiated YBCO samples and Zn substituted samples with various oxygen contents. The 17O NMR broadening and the Tc depressions are found quantitatively similar at optimal doping. Thus irradiation defects seem to induce the same kind of magnetic influence in the CuO2 planes as Zn. Detailed quantitative comparisons for different oxygen dopings will be provided. Cu vacancies display both charge and spin defect with respect to the Cu2+ host while Zn2+ is only a spin less defect. The present results seem therefore to indicate that the large impurity scattering in cuprates should be attributed to their spinless nature rather than to a charge effect.
[1] A. Legris et al , Journal de Physique (France) I3, 1605, 1993.
4PO7-7 Zinc doping effect on microstructure in YBCO
A. Li, X.N. Ying, X.S. Xu, Q.M. Zhang, Y.P. Zhu, and Y.N. Yang, National Laboratory of Solid State Microstructures and Physics Department of Nanjing University, Nanjing 210093, P.R. China
Presenting Author: Y.N. Wang
Internal friction measurement and X-ray diffraction (XRD) experiment were conducted on a series of zinc substituted YBCO ceramics in the temperature range 90-270K. In the internal friction spectrum a thermally activated relaxation peak near 100K, whose intensity shows an unambiguous tendency of decreasing upon zinc doping, was observed at kilohertz frequencies. The relaxation mechanism for this peak is interpreted in terms of jumping of apical oxygen atoms that may be related to the dynamic tilting of CuO5 pyramids. Furthermore, the onset and development of CuO5 tilting upon cooling should be manifested in the continuous or discontinuous subtle change of lattice parameters which was detected in our XRD results. The average temperature derivatives of lattice parameters, originating from both thermal expansion and subtle change of lattice parameter, decrease with increasing zinc concentration in the similar way as the intensity of the relaxation peak and the superconducting transition temperature. The zinc doping effect is discussed in favor of the strong and expanded nature of the disturbance on electronic state [1] that is essential for tilting mechanism of CuO5 pyramids.
[1] R.P. Gupta and M. Gupta, Phys. Rev. B59, 3381 (1999).
4PO7-8 Inelastic neutron scattering peak in the normal state of Zn substituted YBCO7
Nejat Bulut, Dept. of Mathematics, Koc University, Istinye, 80860 Istanbul, Turkey
Presenting Author: N. Bulut
The effects of the nonmagnetic impurities on the Q=(p ,p ) spin-fluctuation spectrum is calculated in the normal state using the framework of the two-dimensional Hubbard model. It is shown that the scattering of the spin-fluctuations by the impurities with momentum transfers near 2kF could lead to a peak in the spin-fluctuation spectrum at 2m where mu is the chemical potential. The results are compared with the neutron scattering data on Zn substituted YBCO7.
4PO7-9 Impurity Effect in High-Tc Superconductor
Masahiro Goto and Shin-ichi Uchida, Department of Superconductivity, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
Presenting Author: M. Goto
In-plane resistivity, Hall effect, and magnetic susceptibility measurement have been carried out on the single crystals of YBa2Cu3O7-y and La2-xSrxCuO4 with various kinds of impurity. In the underdoped regime, the effect of divalent impurities, such as Zn2+, Mg2+, is characterized by an extremely large component both in resistivity and Hall angle and the appearance of a magnetic moment. On the other hand, in the system substituted the monovalent/trivalent ion, such as Li+ and Al3+, the residual resistivity is not so large, but the induced magnetic moment is larger than that in the divalent impurity substituted system. This is due to the excess holes/electrons which are localized near the impurities and contribute to the magnetic moment. The reduction of residual resistivity may be explained by the behavior of the resistivity, which shows the parallel shift of the dependence of temperature as the dopant increases.