Anisotropic superconductors between types I and II

Physical Review B: Condensed Matter and Materials Physics. 2019. Vol. 99. No. 2. Article 024515.

Teixeira Saraiva T., Vagov A., Axt V. M., Aguiar J. A., Shanenko A.

Self-duality or matching between the magnetic and the condensate coherence lengths is a fundamental property

of isotropic superconductors at the critical Bogomolnyi point (B point). The self-dual state of the condensate

is infinitely degenerate, which is the core reason for the sharp transition between the superconductivity types in

the nearest vicinity of the critical temperature Tc. Below Tc nonlocal interactions in the condensate remove

the degeneracy, which leads to the appearance of a finite intertype (IT) domain between types I and II. This

domain exhibits the mixed state with exotic field-condensate configurations and nonstandard magnetic response,

which cannot be understood within the dichotomy of the conventional superconductivity types. At a first glance,

this picture does not apply to an anisotropic system because no spatial matching between the condensate and

magnetic field can be generally expected for direction-dependent characteristic lengths. However, contrary

to these expectations, here we demonstrate that anisotropic superconductors follow the same scenario of the

interchange between types I and II. In anisotropic materials the IT domain is governed by the B point of the

effective isotropic model obtained by the appropriate scaling transformation of the initial anisotropic formalism.

This transformation depends on the direction of the applied magnetic field, and thus the superconductivity type

of strongly anisotropic materials can be dependent on this direction.

Keywords: Intertype superconductors, anisotropy