Fermi surface in KFe₂As₂ determined via de Haas-van Alphen oscillation measurements

We have completely determined the Fermi surface in KFe₂As ₂ via de Haas-van Alphen (dHvA) measurements. Fundamental frequencies ε, α, ζ, and β are observed in KFe₂As ₂. The first one is attributed to a hole cylinder near the X point of the Brillouin zone while the others to hole cylinders at the Γ point. We also observe magnetic breakdown frequencies between α and ζ and suggest a plausible explanation for them. The experimental frequencies show deviations from frequencies predicted by band-structure calculations. Large effective masses up to 19 m_e for Bâ̂¥c have been found, m_e being the free-electron mass. The carrier number and Sommerfeld coefficient of the specific heat are estimated to be 1.01-1.03 holes per formula unit and 82-94 mJmol^-1 K^-2, respectively, which are consistent with the chemical stoichiometry and a direct measure of 93 mJmol^-1 K^-2. The Sommerfeld coefficient is about nine times enhanced over a band value, suggesting the importance of low-energy spin and/or orbital fluctuations, and places KFe₂As₂ among strongly correlated metals. We have also performed dHvA measurements on Ba _0.07K_0.93Fe₂As₂ and have observed the α and β frequencies.