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F. Zuo, S. Khizroev, V.N. Kopylov,
N.N. Kolesnikov. Pinning of Josephson vortices in single crystals of Tl2Ba2CuO6+d
superconductors.
Abstract: Magnetic hysteresis measurements are performed
on single crystals of Tl2Ba2CuO6+d
superconductors with field parallel to the ab-plane. Our results
show anomalously the Bean type critical state behavior with constant critical
current in this orientation. The penetration field Hp(T)
depends on temperature T according to Hp(T)=Hp(0)[1-(T/Tc)2],
in sharp contrast to Hp(T) with field in the c-axis
direction. For samples with high transition temperatures, the magnetization
undergoes abrupt changes as a function of field at low temperatures. We
propose that pinnings on the Josephson vortices in this direction are due
to structure defects in these crystals. A local variation in the tunneling
distance will introduce a finite pinning on Josephson vortices moving along
the ab-plane. The abrupt change in magnetization may be due to field
induced decoupling of the Josephson interaction between the layers.