Vortex structure in the boson-fermion model

It has recently been observed in high-temperature superconductors (HTSC) that the vortex charge strongly depends on doping. The sign of the charge is negative (positive) for overdoped (underdoped) compounds and its value is much larger than estimated from the BCS theory. We show that these results can be explained within the boson-fermion model. We demonstrate that the effective vortex charge is determined predominantly by concentration of the tightly bound electron pairs and sign of the vortex charge depends on the position of the bosonic level with respect to the Fermi energy.