| We report for the first time extensive measurements of quantum oscillations in a Fe-based superconductor, LaFePO, (Tc ~7 K) using torque magnetometry at low temperatures (0.3K) and in static high magnetic fields (45 T). The Fermi surface is composed of quasi-two dimensional electron and hole cylinders; we identify four main frequencies (~1kT-2.4 kT) associated with electron pockets as well as several frequencies of smaller amplitude (0.743 kT -1.9 kT) associated to hole pockets representing a fraction varying between 2.8% to 9 % of basal plane area of the Brillouin zone. The corresponding effective masses range between 1.7 me and 2.1 me. Our results show that the Fermi surface is composed of nearly-nested electron and hole pockets in broad agreement with the band-structure predictions but with significant enhancement of the quasiparticle masses (2-3). The correspondence in the electron and hole Fermi surface areas provides firm experimental evidence that LaFePO, whilst unreconstructed, lies extremely close to a spin-density-wave instability, thus favouring models that invoke such a magnetic origin for high-temperature superconductivity in oxypnictides. |
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