Fermi Surface of Superconducting LaFePO Determined from Quantum Oscillations

We report extensive measurements of quantum oscillations in the normal state of the Fe-based superconductor LaFePO, ($T_c\sim 6\mathrm{K}$) using low temperature torque magnetometry and transport in high static magnetic fields (45 T). We find that the Fermi surface is in broad agreement with the band-structure calculations with the quasiparticle mass enhanced by a factor $\sim 2$. The quasi-two-dimensional Fermi surface consists of nearly nested electron and hole pockets, suggesting proximity to a spin or charge density wave instability.

Figure 1

(a) Torque measurements on LaFePO measured at $T=0.35\mathrm{K}$ and in magnetic fields up to 18 T for different magnetic field directions. The arrows indicate the position of $H_{c2}$. (b) Oscillatory part of the torque (dHvA) and (c) resistance (SdH) in magnetic fields up to 45 T. (d) Fourier transform spectra (field range 15–45 T) for two different magnetic field directions. (e) The angle dependence of the Fourier transform spectra for the field range 10–18 T (sample $B$).

Figure 2

(a) Angle dependence of all observed frequencies. Different symbols correspond to sample $A$ (open circles), sample $B$ (filled circles), and sample $C$ (diamonds). Possible harmonics of the main frequencies are shown by triangles and the dashed lines indicate their calculated location. Solid lines are fits to a $1/\cos \theta$ dependence. (b) Angle dependence of $F\cos \theta$. Solid lines are calculations for a simple cosine warped cylinder. (c) The temperature dependence of the Fourier amplitude for $\theta =32°$ (data are offset for clarity). Solid lines are fits to the Lifshitz-Kosevich formula [15]. The obtained values of the effective mass are listed in Table .

Figure 3

(a) Calculated dHvA frequencies versus angle for unshifted bands compared to experimental data (filled circles). (b) As in (a) but for shifted bands (see text). (c),(d) Two-dimensional (110) cuts of the Fermi surface and (e),(f) two-dimensional cuts in the $\Gamma XM$ plane (001) for unshifted and shifted bands, respectively. Solid black lines indicate the Brillouin zone. A possible interband nesting vector, $\mathbf{Q}\approx [\pi ,\pi ,0]$, between electron and hole bands is shown.