Non-monotonic pressure evolution of the upper critical field in superconducting FeSe
Abstract
The pressure dependence of the upper critical field, Hc2,c, of single crystalline FeSe was studied using measurements of the inter-plane resistivity, c in magnetic fields parallel to tetragonal c-axis. Hc2,c(T) curves obtained under hydrostatic pressures up to 1.56 GPa, the range over which the superconducting transition temperature, Tc, of FeSe exhibits a non-monotonic dependence with local maximum at p1≈ 0.8 GPa and local minimum at p2≈ 1.2 GPa. The slope of the upper critical field at Tc, (dHc2,c/dT)Tc, also exhibits a non-monotonic pressure dependence with distinct changes at p1 and p2. For p<p1 the slope can be described within multi-band orbital model. For both p1<p <p2 and p>p2 the slope is in good quantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov-de Haas measurements. This finding indicates that Fermi surface changes are responsible for the local minimum of Tc(p) at p2≈ 1.2 GPa.
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