Nonmonotonic pressure evolution of the upper critical field in superconducting FeSe

The pressure dependence of the upper critical field, Hc2,c, of single crystalline FeSe was studied using measurements of the interplane resistivity, ρc, 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 nonmonotonic 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,0.28em0ex(dHc2,c/dT)_Tc, also exhibits a nonmonotonic pressure dependence with distinct changes at p₁ and p₂. For p<p₁ the slope can be described within a multiband orbital model. For both p₁ p₂ the slope is in good semiquantitative 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.