Lev N. Bulaevskii, Shi-Zeng Lin
Dissipation in current carrying superconductors is caused by motion of vortices due to the Lorentz force. In usual superconductors this motion is suppressed by defects of the crystal lattice which are present naturally or introduced artificially. They pin the vortices resulting in low voltage in the I-V characteristics below a critical current. Above this current, in the flux flow state, vortices move almost freely. Increase of the critical current is highly desirable for applications, such as power lines, magnets and motors. Here we show that in magnetic superconductor ErNi2B2C below the Neel temperature a novel polaronic mechanism of pinning comes into play and becomes predominant. The nonuniform magnetic field induced by vortices polarizes Er magnetic moments. Thus vortices can move by carrying nonuniform polarization like polarons do, or release nonuniform part of polarization and move freely as in the flux flow state. As the polarization lowers the system energy, the second option is realized only at currents exceeding a critical one. When the magnetic relaxation time is long, vortices move very slowly at low currents and then jump to higher velocity at the critical current. In the I-V characteristics such a polaron dissociation is identified as a depinning transition. The polaron-like pinning with high critical current may be tailored in other magnetic superconductors, for example (RE)Ba2Cu3O7 (RE is rare earth magnetic ion).
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http://arxiv.org/abs/1204.6348
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