Theory of the transition at 0.2 K in Ni-doped Bi2Sr2CaCu2O8
A theory is put forward that the electronic phase transition at 0.2 K in Ni-doped Bi2Sr2CaCu2O8 is a result of the formation of a spin density wave in the system of Ni impurities. The driving force for the transition is the exchange interaction between the impurity spins and the spins of the conduction electrons. This creates a small gap at two of the four nodes of the superconducting gap. The effect is to reduce the thermal conductivity by a factor of 2, as observed.
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National Center for Theoretical Sciences, P.O. Box 2-131, Hsinchu, Taiwan 300, Republic of China and and Department of Physics and Applied Superconductivity Center, University of Wisconsin-Madison, 1150 University Avenue, Madison, Wisconsin 537.