Publications associated with Quantum Matter in High Magnetic Fields

Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor CaKFe4As4

Physical Review B American Physical Society 101 (2020) 134502

M Bristow, W Knafo, P Reiss, W Meier, PC Canfield, SJ Blundell, A Coldea

<p>The upper critical field of multiband superconductors is an important quantity that can reveal details about&nbsp;the nature of the superconducting pairing. Here we experimentally map out the complete upper-critical-field&nbsp;phase diagram of a stoichiometric superconductor, CaKFe4As4, up to 90 T for different orientations of the&nbsp;magnetic field and at temperatures down to 4.2K. The upper critical fields are extremely large, reaching&nbsp;values close to &sim;3 Tc at the lowest temperature, and the anisotropy decreases dramatically with temperature,&nbsp;leading to essentially isotropic superconductivity at 4.2K. We find that the temperature dependence of the&nbsp;upper critical field can be well described by a two-band model in the clean limit with band-coupling parameters&nbsp;favoring intraband over interband interactions. The large Pauli paramagnetic effects together with the presence&nbsp;of the shallow bands is consistent with the stabilization of an FFLO state at low temperatures in this clean&nbsp;superconductor.</p>

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