We have observed Shubnikov-de Haas oscillations in FeSe. The Fermi surface deviates significantly from predictions of band-structure calculations and most likely consists of one electron and one hole thin cylinder. The carrier density is in the order
of 0.01 carriers/ Fe, an order-of-magnitude smaller than predicted. Effective Fermi energies as small as 3.6 meV are estimated. These findings call for elaborate theoretical investigations incorporating both electronic correlations and orbital ordering.
We have determined the Fermi surface in KOs$_2$O$_6$ ($T_c$ = 9.6 K and $B_{c2} sim$ 32 T) via de Haas-van Alphen (dHvA) oscillation measurements and a band structure calculation. We find effective masses up to 26(1) $m_e$ ($m_e$ is the free electron
mass), which are unusually heavy for compounds where the mass enhancement is mostly due to electron-phonon interactions. Orbit-resolved mass enhancement parameters $lambda_{dHvA}$ are large but fairly homogeneous, concentrated in the range 5 -- 8. We discuss origins of the large homogeneous mass enhancement in terms of rattling motion of the K ions.
