Speaker
Alex Gurevich (Florida State U)
Description
Low carrier densities and short coherence lengths in
semi-metallic ferropnictide superconductors can result in
exotic behaviors at strong magnetic fields due to the
interplay of multiband superconductivity, unconventional
pairing symmetry and Zeeman and orbital pairbreaking. In
this talk I discuss how these effects manifest
themselves in the anomalous temperature dependence of the
upper critical field Hc2(T) and the Fulde-Ferrel-
Larkin-Ovchinnikov (FFLO) transition in clean anisotropic
multband superconductors. In particular, if the
magnetic field H it is tilted away from the symmetry axis,
the wave vector Q of the FFLO oscillations is not
parallel to H, which may result in fractional vortices. The
crystalline anisotropy and the s± pairing symmetry
with the sign change of the order parameter on different
sheets of the Fermi surface can significantly increase
the orbitally-limited Hc2(T) and facilitate the FFLO
transition in multiband ferropnictides. In these materials a
small shift of the chemical potential upon doping can result
in the Lifshits transition due to emerging pockets
of the Fermi surface, which in turn trigger the FFLO
instability even for moderate values of the Maki parameter
in the main bands. It is argued that the FFLO state may be
more common than is usually assumed based on the
observed shapes of HC2(T), so specific heat or torque
magnetometry may be required to reveal the FFLO state.
