Resistivity and magnetoresistance of FeSe single crystals under helium-gas pressure

We present temperature-dependent in-plane resistivity measurements on FeSe single crystals under He-gas pressure up to 800 MPa and magnetic fields B≤10 T. A sharp phase transition anomaly is revealed at the tetragonal-to-orthorhombic transition at Tₛ slightly below 90 K. Tₛ becomes reduced with increasing pressure in a linear fashion at a rate dTₛ/dP-31 K/GPa. This is accompanied by a P-linear increase of the superconducting transition temperature at Tc~8.6 K with dTc/dP+5.8 K/GPa. Pressure studies of the normal-state resistivity highlight two distinctly different regimes: for T>Tₛ, i.e., in the tetragonal phase, the in-plane resistivity changes strongly with pressure. This contrasts with the state deep in the orthorhombic phase at TTₛ, preceding the superconducting transition. Here, a T-linear resistivity is observed the slope of which does not change with pressure, pointing against a spin-fluctuation origin of this term. Resistivity studies in varying magnetic fields both at ambient and finite pressure reveal clear changes of the magnetoresistance, Δρ∝B², upon cooling through Tₛ. Our data are consistent with a reconstruction of the Fermi surface accompanying the structural transition.