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S. Ouazi (doctorant), J. Bobroff, H. Alloul

Collaborations : G. Collin, LLB-CEA ; M.H. Julien, C. Berthier, LCMI, Grenoble

We have found that a foreign atom introduced in a high Tc superconductor induces a “magnetic wave” in its vicinity, much as a stone thrown in a sea of superconducting electrons. This wave was detected through the use of nuclear magnetic resonance, a technique widely used for body imaging in hospitals (called MRI). Surprisingly, such a magnetic wave exists even though the inserted impurity atom (Zinc) is non magnetic. This wave had already been observed in the high temperature metallic state of these compounds, revealing that the electronsare strongly correlated to each other. But it is the first time that this magnetic wave is followed through the superconducting transition. As the system becomes superconducting, the wave remains but changes shape. New electronic phenomena occur near the impurity, whose effect on the charge have also been observed by Scanning Tunneling Microscopy by the group of J.C.Davis (Cornell University). The shape and size of this wave gives a new way to get a better understanding of the exotic nature of these superconductors. Its existence reveals not only the fact that electrons may have both a magnetic and a superconducting response, but also stresses the peculiar symmetry and nature of the superconducting state, whose origin is not yet understood.

figure : In a high Tc superconductor, a non magnetic impurity (the black sphere) induces a magnetic wave even when the sample is in the superconducting state.

References :

[1] “Correlation length in cuprate superconductors deduced from impurity-induced magnetization » S. Ouazi, J. Bobroff, H. Alloul, W.A. MacFarlane, Phys. Rev. B 70, 104515 (2004)

[2]"Impurity induced Local Magnetism and Density of States in the superconducting state of YBa2Cu3O7" S. Ouazi, J. Bobroff, H. Alloul, M. LeTacon, N. Blanchard, G. Collin, M.H. Julien, W.A. MacFarlane, Phys. Rev. Lett. 96, 127005 (2006)