The FFLO State in Unconventional Superconductors

Recent experiments suggest that inhomogeneous superconductivity (the FFLO state) exists in organic¹, pnictide², and heavy Fermion³ superconductors. The FFLO state is a type of superconducting pair density wave, although unlike the proposed pair density wave in cuprates, the FFLO state is accompanied by a magnetized charge density wave (CDW) of the left over (un-condensed) quasiparticles. We will review the data from each of the families of superconductors above and present new data for the Ba_xK_1-xFe₂As₂ family of pnictide superconductors via measurements of the rf penetration depth using a tunnel diode oscillator, down to 40 mK. We will also discuss early results of experiments to measure the q-vector of a CDW in organic conductors with x-rays. This CDW is known to compete with the superconducting state rather than coexist with the FFLO state referred to above. Additionally, this technique may be a way to measure the q-vector of the FFLO state directly. Understanding these competing and intertwined orders in unconventional superconductors will further our knowledge of quantum mechanical ground states in electronic materials.

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